No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction
Abstract
The Cadiz Contourite Channel is the largest and most prominent contourite channel in the middle slope of the Gulf of Cadiz, and is known to channelise the southern branch of the Lower Core of Mediterranean Outflow Water (MOW) as it flows westwards from the Gibraltar Gateway. The channel lies in water depth between 650 and 1500 m, is 150 km long, 2-12 km wide, up to 120 m deep, and broadly s-shaped in plan view. It has several associated subparallel marginal channels and shorter spillover channel segments. Its geometry is controlled by the interaction of a strong bottom current with the seafloor morphology, affected by neotectonic deformation and diapiric intrusion. Bottom photographs and dredge hauls reveal a channel floor shaped by high-energy flow, in places with bare rock, boulders and gravel, and elsewhere covered with sandy contourites. The rocky substrate and derived clasts are formed of authigenic iron-rich carbonates, testifying the high degree of fluid escape from adjacent diapiric ridges and mud volcanoes. The sandy substrate shows a wide range of current-induced bedforms including small, straight-crested ripples, large sinuous sand waves and dunes (wavelength 3.5-5 m, height 0.3-0.9 m), weak surface lineation on sands, and aligned gravel stringers and deep erosive scours around large boulders. Bedform orientation indicates flows directed to the south/south-west (main channel) and west (spillover channel), which can be related to MOW bottom currents, and current velocities that vary between about 0.2 and 0.8 m s(-1), even in the same channel location. However, current vane orientation was clearly responding, at least in part, to tidal effects and periodicity in the Gulf of Cadiz at the time the photographs were taken. Maximum current velocities are achieved by a combination of barotropic and internal tides (probably generated at the continental slope) that reinforce the normal MOW flow. In addition, meteorologically-induced internal waves with periods shorter than tidal ones may exert an even greater influence on current intensity, especially when they occur at times of sudden changes of meteorological forcing. This effect further influences MOW variability. In all cases, the funnelling effect of the Cadiz Channel amplifies tidal or meteorologically-induced bottom currents.
... 3. Marine sediments are subject to environmental stresses which are not found in the subaerial environment, e.g. tides, surface waves and internal waves (Mestdagh et al., 2020;Stow et al., 2013b;Urgeles and Camerlenghi, 2013). ...
... The resulting configuration of drifts varies from sheeted to mounded, and both depositional and erosional features are classified also based on location within the slopes, morphology and evolution (Faugères et al., 1999;Rebesco et al., 2014;Stow et al., 2002). Interest in contourite depositional systems has increased simultaneously with their use as palaeoceanographic indicators (Camerlenghi et al., 1997;Llave et al., 2007a;Miramontes et al., 2019;Rebesco et al., 1996;Voelker et al., 2014), palaeoclimatic recorders (Grützner et al., 2003), and for their economic importance in the frame of hydrocarbon exploration (Brackenridge et al., 2013;Fonnesu et al., 2020;Sansom, 2018;Stow et al., 2013b;Viana and Rebesco, 2007). Recent studies also demonstrated the key role of bottom currents in the spreading and accumulation of microplastics in oceanic sediments . ...
... Zitellini et al., 2001Zitellini et al., , 2009Gutscher et al., 2002;Gràcia et al., 2003b;Gutscher, 2004;Terrinha et al., 2009) This study confirms the fact that the whole area is under a compressive deformation regime, with numerous active structures having high tsunamigenic potential (Gràcia et al., 2003b(Gràcia et al., , 2003aMartínez-Loriente et al., 2018Terrinha et al., 2009Terrinha et al., , 2003Zitellini et al., 2004). During The MOW is also responsible for the third key feature commonly associated with the Alentejo basin, which is the emplacement of one of the most well-known contourite depositional systems worldwide Hernández-Molina et al., 2016a, 2014bMulder et al., 2003;Stow et al., 2013b). The contourite system dominates the sediment distribution of the upper slope along the entire Gulf of Cadiz until the Alentejo basin, with stratigraphic architecture and grain sizes reflecting the changes in the MOW hydrodynamics through time. ...
... Ce manuscrit s'organise en 5 chapitres : Preu et al., 2013;Droghei et al., 2016;Mestdagh et al., 2020). Les tourbillons et les ondes internes caractéristiques de ces interfaces (Reid et al., 1977) peuvent être à l'origine de l'érosion ou de la resuspension des sédiments du fond (e.g., Rebesco et al., 2014;Hanebuth et al., 2015;Droghei et al., 2016;De Castro et al., 2020); Les courants de marée (les courants tidaux) jouent un rôle important sur la sédimentation profonde, particulièrement dans les zones proches des canyons sous-marins et des chenaux contouritiques (e.g., Shepard, 1976;Rebesco et al., 2014;Stow et al., 2013a). ...
... Eanes atteint toutefois 50 km de longueur et 0,8 à 1,7 km de largeur (Kenyon and Belderson, 1973;Habgood et al., 2003;Hernández-Molina et al., 2006;Hanquiez et al., 2007). Les (Nelson et al., 1993Baraza et al., 1999;Mulder et al., 2002Mulder et al., , 2003Habgood et al., 2003;Hernández-Molina et al., 2003Llave et al., 2007aLlave et al., , b, 2011García et al., 2009García et al., , 2016García et al., , 2020Hanquiez et al., 2010;Stow et al., 2013a). ...
... L'interaction entre la forte activité de la MOW et la pente continentale a généré neuf chenaux contouritiques dont cinq ont des dimensions pouvant atteindre 150 km de longueur, 12 km de largeur et 0,8 km de profondeur : les chenaux de Cadix, de Guadalquivir, de Huelva, de Diego Cao et de Gusano (Kenyon and Belderson, 1973;Llave et al., 2001;Hernández-Molina et al., 2003Stow et al., 2013a). Ces cinq chenaux majeurs, caractérisés par un aspect méandriforme, ont une orientation générale nord-est sud-ouest. ...
La sédimentation de la pente moyenne du Golfe de Cadix est fortement influencée par l’écoulement d’un courant de fond d’origine méditerranéenne : la Mediterranean Outflow Water (MOW). La compréhension de la relation entre l’écoulement de la MOW, l’Atlantic Meridional Overturning Circulation (AMOC) et les variations climatiques en Atlantique Nord représente un enjeu majeur et complexe. Ces travaux de thèse, en étudiant les caractéristiques sédimentaires des sédiments des drifts contouritiques du Golfe de Cadix, corps sédimentaires propices à l’étude de la variabilité des courants de fond, visent à comprendre la dynamique de la MOW depuis le Pléistocène inférieur. Cette étude s’appuie essentiellement sur des archives sédimentaires prélevées lors de l’Expédition Integrated Ocean Drilling Program 339 « Mediterranean Outflow » (2011-2012) et valorise également des carottes sédimentaires collectées durant la mission océanographique CADISAR (2001). Ce travail a permis d’identifier les sources des minéraux argileux composant les dépôts du système contouritique de Cadix (apports fluviatiles du Guadalquivir et poussières éoliennes nord-africaines) et mis en évidence les rôles de la circulation océanique de surface et l’écoulement de la MOW, dans la distribution des minéraux argileux dans le Golfe de Cadix. Les assemblages ostracodiques, la minéralogie des argiles ainsi que les mesures granulométriques, effectués à haute résolution sur des archives sédimentaires prélevées à différentes bathymétries, ont permis d’affiner le schéma de circulation de la MOW dans le système contouritique de Cadix pour le dernier cycle climatique. Des séries sédimentaires plus anciennes couvrant l’intervalle des Stades Isotopiques Marins (MIS) 12-11 ont montré que le modèle de fonctionnement général établi demeurait valable mais d’intensité moindre, pour l’analogue climatique le plus récent. Enfin les mesures granulométriques et des cortèges argileux réalisées à haute résolution sur une longue série sédimentaire du drift de Faro, mises en âges par une stratigraphie isotopique inédite, ont permis de déterminer trois phases de construction du drift de Faro ainsi que de reconstruire la dynamique de la MOW depuis le Pléistocène inférieur (1,2 Ma).
... Our study area is influenced by the shallower NACW, with associated current velocities of about 0.2 m/s, and by the upper (M5 and M4) MOW branches (Figure 1c), characterized by currents ranging from 0.3 to 0.4 m/s [37]. The density interface between both water bodies is located between 400 and 500 m in depth [34]. ...
... M5 flows through the zone of contourite deposits north of the diapiric ridges, reaching speeds of up to 0.34 m/s, while the M4 core crosses Cádiz ridge through the intermediate valleys and is channelled through the Huelva channel at speeds up to 0.37 m/s. These strong current velocities allow the uninterrupted sediment transports that restrain the long-lasting formation of bioturbation features [37,59]. Therefore, in these bottoms, sand and coarse sediments predominate in which, depending on the textural characteristics of the grain (size, density, and shape) and the characteristics of the current (speed, depth, temperature), they can originate different bedforms [9] (Figure 13). ...
... However, in deep environments it is not clear yet what the origin of these sedimentary structures may be. Stow et al. [37] suggest that they may be related to tidal currents, which are of great importance in the Gulf of Cádiz [69]. ...
The dense and deep water flow that leaves the Mediterranean Sea to the Atlantic flows through the upper and middle slope of the Gulf of Cádiz as a powerful bottom stream that models and interacts with bathymetry. The detailed analysis of underwater images, obtained with a photogrammetric sled in the central area of the upper and middle slope of the Gulf of Cádiz, together with multibeam bathymetry and oceanographic and sediment types data, has allowed conducting a detailed study of the seafloor microtopography and the predominant oceanographic dynamics in the study area. Different fine-scale spatial bedforms were identified, such as ripples, dunes, burrows, mounds, obstacle marks, rock bottoms, and low-roughness bottoms using underwater images. Besides, a geostatistical study of the different video transects studied was carried out and allowed us to differentiate three types of bottoms depending on the processes that affect their microtopography.
... Detailed sedimentologic analysis has provided some criteria for discriminating facies-scale bottom current deposits from turbidites or other deepwater pelagic deposits. Specific contourite sedimentary facies have been described by a number of studies (e.g., Stow and Lovell, 1979;Stow, 1982;Stow and Holbrook, 1984;Stow and Piper, 1984;Pickering et al., 1989;Faugères and Stow, 1993;Gao et al., 1998;Faugères et al., 1999;Stow et al., 2002aStow et al., , 2013aStow et al., , 2013bRebesco, 2005;Llave et al., 2006;Øvrebø et al., 2006;Shanmugam, 2006Shanmugam, , 2012Shanmugam, 2014;Stow and Faugères, 2008;Stow et al., 2013aStow et al., , 2013bRebesco et al., 2014;de Castro et al., 2020de Castro et al., , 2021ade Castro et al., , 2021bStow and Smillie, 2020;Hüneke et al., 2021;Rodrigues et al., 2022a). Recent research proposes new diagnostic criteria for contouritic sedimentary facies (Alonso et al., 2016;Brackenridge et al., 2018;Rodríguez-Tovar and Hernández-Molina, 2018;de Castro et al., 2020de Castro et al., , 2021ade Castro et al., , 2021bStow and Smillie, 2020;Yu et al., 2020;Hüneke et al., 2021;Rodríguez-Tovar, 2022) based on statistical analyses (as Principal Component Analysis, PCA) of texture, microfacies, ichnological features and geochemical data (X-ray fluorescence, XRF). ...
... Detailed sedimentologic analysis has provided some criteria for discriminating facies-scale bottom current deposits from turbidites or other deepwater pelagic deposits. Specific contourite sedimentary facies have been described by a number of studies (e.g., Stow and Lovell, 1979;Stow, 1982;Stow and Holbrook, 1984;Stow and Piper, 1984;Pickering et al., 1989;Faugères and Stow, 1993;Gao et al., 1998;Faugères et al., 1999;Stow et al., 2002aStow et al., , 2013aStow et al., , 2013bRebesco, 2005;Llave et al., 2006;Øvrebø et al., 2006;Shanmugam, 2006Shanmugam, , 2012Shanmugam, 2014;Stow and Faugères, 2008;Stow et al., 2013aStow et al., , 2013bRebesco et al., 2014;de Castro et al., 2020de Castro et al., , 2021ade Castro et al., , 2021bStow and Smillie, 2020;Hüneke et al., 2021;Rodrigues et al., 2022a). Recent research proposes new diagnostic criteria for contouritic sedimentary facies (Alonso et al., 2016;Brackenridge et al., 2018;Rodríguez-Tovar and Hernández-Molina, 2018;de Castro et al., 2020de Castro et al., , 2021ade Castro et al., , 2021bStow and Smillie, 2020;Yu et al., 2020;Hüneke et al., 2021;Rodríguez-Tovar, 2022) based on statistical analyses (as Principal Component Analysis, PCA) of texture, microfacies, ichnological features and geochemical data (X-ray fluorescence, XRF). ...
Bottom current deposits (contourites) form in association with modern-day or ancient oceanic gateways. A paucity of examples in the ancient record and the lack of consensus on diagnostic criteria for differentiating them from other deepwater deposits limit our understanding of how they may record past global oceanic circulation, tectonic events and gateway evolution. This work describes an exceptional example of Eocene to middle Miocene deep-marine deposits located both onshore and offshore deepwater environments around the island of Cyprus. Multidisciplinary approaches were used to discriminate contourite facies associations, propose a sedimentary model, and interpret the relations with regional tectonics and the evolution of the nearby Indian Gateway. Contourite deposits appear in late Eocene to middle Miocene intervals interstratified with pelagic/hemipelagic sediments, turbidites and mass-transport deposits (MTDs). These deepwater deposits developed along a slope basin located on the upper plate of an active margin, evolving from a wide basin formed during a period of tectonic quiescent into a series of shallowing-upward, segmented sub-basins affected by compressional stress. The present study proposes a sedimentary model in which two contourite depositional systems developed: first in the Eocene (dominated by finer-grained contourites), and then during the latest Oligocene to middle Miocene (dominated by coarser-grained contourites). Both systems were buried by extensive marl deposits and record the respective influence of deep (circulating NW) and intermediate (circulating SE) water masses. The long-term evolution of the contourites reflects tectonic events that enhanced subduction processes south of Cyprus as well as exchange between the Neotethys Ocean and the Indian and Atlantic Oceans —until the final closure of the Indian Gateway by the end of the middle Miocene, when a new circulation pattern was established with the formation of the Mediterranean Sea.
The contourites described here represent bi-gradational sequences that normally form in association with contouritic drifts, sometimes having the asymmetric top-cut sequence characteristics of plastered drifts and contourite terraces. The coarser (sandy) contourites, formed from the latest Oligocene to middle Miocene, consist of three packages associated with compressive and flexural phases. They pertain to I) Chattian (late Oligocene); II) Aquitanian/Burdigalian (early Miocene) and III) Langhian (middle Miocene). Evidence of enhanced bottom current episodes occurs toward the top of these packages before they are buried by later dominant marl deposits. The sandy contourites thus formed during the compressive phases, whereas the predominately finer-grained units formed during later flexural phases. The intermittent turbidites and MTDs (developed during compressional phases in combination with pelagic/hemipelagic sediments) represent the sediment supply for the contourite deposits after their winnowing and / or reworking. Our research found that the diagnostic criteria for discriminating ancient bottom current deposits from other deepwater deposits are related primarily to variations in sedimentary processes, current behaviour and its velocity, sedimentation rates and paleoenvironmental conditions. This highlights the importance of primary sedimentary structures, microfacies and ichnological features in making determinations at the sedimentary facies scale. Due to their common occurrence, sedimentary thickness (30-40 m), potential porosity and permeability, sandy contourites can form deepwater reservoirs for energy geosciences.
In summary, this work demonstrates the role of plate tectonics and oceanic gateways in driving the paleo-oceanic circulation that, in turn, controls sedimentary processes and shapes the morphology of oceanic basins and continental margins. It also allows for comparison with other present-day and ancient continental margin deposits. Future high-resolution approaches and analyses of other geological settings could help resolve the sedimentary architectures of similar deepwater systems in terms of episodic tectonic processes —involving compressive-flexural stress variations. They control the Earth’s surface environment (sea-level, climate and oceanic circulation) over time by influencing sediment supply, packages of strata and types of contourite deposits.
... Attempts to better understand the sedimentological features of modern contourite channel systems by studying cores were generally unsuccessful because the sediment proved too unconsolidated for acquisition (Expedition 339 Scientists, 2012;Brackenridge et al., 2018;. As a result, the information from these features is generally limited to bathymetric remote sensing and direct imaging (Stow et al., , 2013. Observations from the ancient rock record are predominantly based on seismic data, cores and logs (Hern andez- Gong et al., 2017). ...
... Contourite channels associated with lateral drifts and terraces have been recognized to contain large volumes of well-sorted sands in contourite depositional systems (Viana, 2008;Mutti & Carminatti, 2011;Stow et al., 2013;Hern andez-Molina et al., 2016;Brackenridge et al., 2018;. These systems however were, as of yet, not well-understood, and the inter-channel and down-channel facies distribution had not yet been documented. ...
Despite the rise in published evidence of deep‐marine bottom current processes and associated deposits there are still very few documented outcrop examples. Herein are reported results of a contourite channel system related to the late Miocene palaeo‐Mediterranean Outflow Water in the Rifian Corridor, Morocco. This work aims to unravel the sedimentary evolution and facies distribution based on the study of large morphological features related to contourite channels and their subsequent sandstone dominated infill sequences. It was found that the channel evolution and facies distribution are related to spatiotemporal changes in flow characteristics of the palaeo‐Mediterranean Outflow Water. The recognized channel facies distribution correlates well with previously established bedform stability diagrams. Erosion and upper‐stage flow regime bedforms are associated with the most vigorous bottom currents, generally related to its core. Laterally, following the decrease in flow velocity towards the adjacent drift, bedforms comprise dunes, lower‐stage plane bedforms and more heterolithic facies. Similar facies changes are also observed down‐channel, related to a decrease in flow velocities resulting from turbulent mixing of water masses, associated decreases in density gradients and the subsequent deceleration due to gravity. Results of this work have been used to propose a three‐dimensional facies model for channelized sandy contourites. This study contributes to understanding contourite systems, thus benefitting oceanographic and climatic reconstructions as well as aiding the predictability of contourite channel systems for industry geoscience applications.
... In such settings, eddies can temporally increase water velocities by an order of magnitude (Sheen et al., 2012;Gardner et al., 2017), resulting in winnowing, erosion and redistribution of sediment. This can result in the formation of extensive sand sheets, such as the Falkland Sand Sheet in the South Atlantic (Nicholson and Stow, 2019) and the Cadiz Sand Sheet in the eastern North Atlantic (Stow et al., 2013a(Stow et al., , 2013b. ...
... We call this feature the Ryukyu Sand Sheet (Fig. 2). It covers an area (including the gravels) of at least 35,000 km 2 , which places it as one of the largest known sand sheets anywhere in the deep ocean, comparable in size with the Falkland Sand Sheet in the SW Atlantic (30,000 km 2 , Nicholson and Stow, 2019), and considerably larger than the Cadiz Sand Sheet (Stow et al., 2013a(Stow et al., , 2013bBrackenridge et al., 2018) and the Barra Sand Sheet (Stow et al., 2002c) principally by the action of strong bottom currents (Table 2). ...
Contourites are well-known from many continental margins under the influence of bottom currents but have been little reported from the Pacific Ocean. This paper documents a new area of contourite-controlled sedimentation in the NW Pacific Ocean, which we call the Ryukyu Sand Sheet. This contourite sand sheet has an area of around 35,000 km² and extends from the narrow island shelves to over 1500 m water depth. It comprises mainly moderate to well-sorted fine-grained sands, with current ripples and giant sediment waves and is also associated with small drifts. It is formed under the influence of three principal current systems – the Kuroshio Current, the Kuroshio Countercurrent and the Ryukyu Current. The interaction of these currents with each other and with a complex seafloor topography, spawns a series of meso-scale gyres, eddies and vortices that shape the seafloor and lead to deposition of an extensive sandy substrate, locally with gravels and exposed seafloor. Strong surface currents, as well as deep-water thermohaline circulation, both influence the depositional and erosional processes of deep-sea sediments. The role of the modern Kuroshio Current in this context supports earlier work that proposed an ancestral Kuroshio Current for the deposition of Miocene contourites onshore Japan. Sediment supply to the Ryukyu Sand Sheet is by a mixed process of seafloor polishing and sand spillover that involves combined oceanographic and gravitational processes.
... Del mismo modo, se han comparado las imágenes obtenidas por el ROV en el canal Gil Eanes (ROV-13) con las descritas por Stow et al. (2013), en el entorno del canal de Cádiz. De esta forma se encuentran similitudes con las formas de fondo generadas en un canal contornítico secundario que discurre subparalelo al canal principal de Cádiz, donde las formas más frecuentes sobre el substrato arenoso son dunas y ripples con ausencia de fangos. ...
... Pero dicho flujo no circula por la totalidad de la sección del canal, sino que su intensidad varía significativamente a medida que nos alejamos del flujo principal, llegando a observar sedimentos arenosos e incluso fangosos en el ROV-8. La contornita arenosa registrada en el testigo posee valores medios de tamaño de grano de 0.180 mm, lo que junto con las estructuras descritas permite inferir el rango de velocidad de corriente que actúa en el lugar de muestreo 2013). El flujo que circula por el canal de corriente profunda freestanding, Fs 3 , se encontraría dentro de un rango de 0.2 a 0.6 m/s. ...
... S3 to S6) suggest that they are furrows formed by contour currents [cf. (48)(49)(50)(51)(52)(53)]. Contour-current furrows are produced when high-velocity along-slope currents erode into underlying sediments, scouring depressions aligned with the flow direction (54,55). The merging of several furrows on interpreted horizons 3 and 4 ( Fig. 4E and fig. ...
Efforts to understand how Pleistocene climate changes were translated into fluctuations in ice sheet extent and volume are limited by a lack of consensus about the glacial history of the North Sea. Here, we use high-resolution 3D seismic data to interpret the landforms and sediments of the central North Sea in unprecedented detail. In contrast to previous interpretations of multiple extensive early glaciations, our data suggest that grounded ice extended across the central North Sea only once, from western Norway, during the Early Pleistocene. This ice sheet advance, which probably occurred ~1.1 million years ago, deposited an up to 120-meter-thick layer of till across >10,000 square kilometers of the central basin. During the rest of the Early Pleistocene, elliptical pockmarks and elongate contour-current furrows show that the central basin was instead scoured by along-slope currents. These findings constrain the extent of ice sheets before and during the Mid-Pleistocene Transition and reconcile marine and terrestrial evidence for glaciation in northwest Europe.
... IODP Exp. 339 in the Gulf of Cadiz (Stow et al., 2013) clearly highlighted the MOW as a key actor in the genesis of several contourite depositional systems (CDS) along the eastern Atlantic continental margin. The work of Hernández-Molina et al. (2006 in the Gulf of Cadiz, Hanebuth et al. (2015) at the Galicia Bank, Hernández-Molina et al. (2011) and Collart et al. (2018) at the Ortegal CDS, Ercilla et al. (2008), Van Rooij et al. (2010) and Liu et al. (2020) at the Le Danois CDS and Delivet et al. (2016) at the Goban Spur, respectively presented from a proximal to a distal MOW, delivered new insights into the temporal and lateral variability of this intermediate water mass along the NW European margin and its effects on local bottom-current dynamics. ...
Small-scale contourite drift is an important component of continental margins that can record information about
complex oceanographic processes. The Belgica Mound Drift is one example of a small-scale contourite drift. It is
formed under the influence of cold-water coral (CWC) mounds and represents one of the most distal contouritic
expressions influenced by the Mediterranean Outflow Water (MOW) in the NE Atlantic Ocean. Three distinct
evolutionary stages have been identified from new high-resolution pseudo-3D reflection seismic data, each
associated with a significant change in paleoceanography, affecting both bottom-current intensity and sediment
input. The pre-drift stage (Pliocene–Early Pleistocene) corresponds to the regional RD1 erosive event, which was
caused by the reintroduction of the MOW in the Porcupine Seabight, creating a distinct paleotopography that will
influence all ensuing sedimentary processes. The second stage (Early Pleistocene–Middle Pleistocene) is the
contourite drift inception in two distinct centres of growth, strongly steered by topographic obstacles such as the
CWC mounds. During the third and final stage (Middle Pleistocene–present day), the contourite drift is developed
under a more stable but less dynamic environment, characterised by more continuous and mounded aggradational
stratification. The final stage of the contourite drift is related to the Middle Pleistocene Transition, with a
spatially variable reduction in the MOW-related bottom currents and sediment input. The spatial and temporal
evolution of this drift shows that its present-day morphology is controlled by the location of initial growth.
Evolving moat morphology indicates that the intensity of the bottom currents generally increases during the drift
evolution.
This research presents a crucial paradigm for advancing our knowledge of elucidating the complexities of
smaller-sized contourite systems in diverse oceanic environments.
... They are particularly abundant in the deep sea. Submarine furrows recorded the depositional processes and sedimentary history of both modern and ancient deepsea environments (Flood, 1983;Poppe et al., 2002;Lastras et al., 2007;Biscara et al., 2010;Kilhams et al., 2011;Stow et al., 2013). Hence, a detailed research on submarine furrows could provide important information on changes of associated flows (Flood, 1994;Lobo et al., 2011;Rebesco et al., 2014). ...
... The shape is similar to that of sediment waves, although the genesis is mixed, with fluid-flow and gravitational processes also being involved. The effect of the bottom current as an erosive agent in the formation of large contourite channels has been observed in the GoC (i.e., Hernández-Molina et al., 2006;Stow et al., 2013;García et al., 2009). Both fluid expulsion and bottom current processes act in the vicinity of the St. Petersburg MV, conditioning the existence and shape of these channels. ...
... Other facies models have been focusing on contour and bottom currents' deposits that can develop at the exit of a strait or seaway (e.g. Hernández-Molina et al. 2006;Stow et al. 2009Stow et al. , 2013. ...
Straits and seaways are the key features that connect oceans, seas and lakes. They regulate water, sediment and biota exchanges, and influence local and global climate. A good understanding of the dynamic evolution of straits and seaways is therefore fundamental to accurately reconstruct the palaeogeography, tectonics, palaeoecology and stratigraphy of interconnected basins, the climate dynamics of Earth's past as well as to exploit both conventional and renewable energy resources.
This volume provides a comprehensive collection of articles dealing with both ancient and modern case studies, bringing together different but complementary disciplines, such as marine geology, process sedimentology and stratigraphy.
With the contents encompassing the evolution, geomorphology, stratigraphy, sedimentology, oceanography and palaeogeography of straits and seaways, and their influence on climate, the book will be of interest to Earth scientists working in many fields.
... Key results of the analysis of the Arenazzolo Formation presented here are in line with established facies models of contourites. First of all, traction structures like cross-lamination and planar lamination (as observed in FA3) have been recognized in many contourites (Shanmugam et al., 1993;Mutti & Carminati, 2012), and ripples, lineations and other bedforms have been recognized in modern-day bottom current systems (Stow et al., 2013;Rebesco et al., 2014;Esentia et al., 2018). Continuous accumulation of sand, uninterrupted by phases of sediment starvation, and the relatively hostile late Messinian environments likely prevented bioturbation (which is often used to interpret contourites) from destroying traction structures and explains why they are well-preserved. ...
The evolution of marine gateways and sea straits exerts major control on bottom current depositional systems. A well‐known interval in geological history characterized by frequent changes in marine connectivity is the Messinian Salinity Crisis (5.97 to 5.33 Ma) when the Mediterranean allegedly experienced major (>1 km) sea‐level drawdown followed by a catastrophic marine replenishment at the base of the Zanclean. Controversy exists around the timing and mode of this event as unambiguous flood deposits have so far never been drilled or recognized in outcrops. In the Sicilian Caltanissetta Basin (Italy), the Messinian/Zanclean boundary is directly underlain by the Arenazzolo Formation. This 5 to 7 m thick sandy sedimentary interval may reveal a genetic link with the abrupt refilling of the Mediterranean, but at present a detailed study to understand its origin is lacking. In this work, the Arenazzolo Formation at Eraclea Minoa has been studied by a multi‐method approach, employing detailed facies description, grain‐size analyses, petrographic analyses and palaeocurrent analyses. Palaeogeographic reconstructions and facies associations show that the Arenazzolo Formation sands were deposited on the northern flank of the Gela thrust front by persistent bottom currents, flowing parallel to the regional slope physiography, during a transgression. It is hypothesized that these currents are associated to the active circulation of surface and intermediate water masses coeval with a terminal Messinian flood, when basin margins overtopped and a reconnection between western and eastern Mediterranean was created. The Arenazzolo Formation is a unique example of a contouritic deposit formed by bottom currents that establish during the reconnection of major isolated water bodies.
... Other facies models have been focusing on contour and bottom currents' deposits that can develop at the exit of a strait or seaway (e.g. Hernández-Molina et al. 2006;Stow et al. 2009Stow et al. , 2013. ...
Straits and seaways are connections between basins, key areas for exchange and transfer of water, heat, biota and sediments, and they can influence regional and global climate. A better understanding of straits/seaway dynamics, their evolution and preserved deposits is therefore fundamental to reconstruct the paleoecology, paleogeography and stratigraphy of interconnected basins. Straits and seaways are also important for understanding climate dynamics in Earth's past, and safely exploit energy resources (fossil and renewable). This Special Publication aims at improving the knowledge on this topic, and at providing a comprehensive collection of state-of-the-art works bringing together complementary disciplines. Even though studies on modern and ancient straits and seaways exist, they are not necessarily conceived towards a better geological understanding of these features as depositional systems in their own right. In this article we emphasize the geological importance of straits and seaways, summarizing the content and key findings of the contributions on this topic. The articles included in this volume explore four main research themes related to straits and seaways: (a) occurrence and classification; (b) morphological features, facies and stratigraphic variability, sedimentary processes and dynamics; (c) tectonic and climatic controls, their feedback with climate changes; (d) palaeogeographic reconstructions and preservation of associated deposits.
... Vertical profiles from the 3D seismic data reveal high-amplitude reflections (HARs) with Root Mean Square (RMS) extractions clearly evidencing high-amplitude anomalies within these moats, particularly where they bend (Fig. 4). Such anomalies might indicate coarser sediment along the moats, as evoked for other contourite channels and moats within CDS known to contain vast quantities of well-sorted sands 21,26,29,41,42 . Present day contourite channels in the Gulf of Cadiz 21 and late Miocene channels exposed in Morocco 42,43 confirm that extensive sandy deposits typify these features. ...
Numerous bottom current-controlled depositional and erosional features, which together form Contourite Depositional Systems (CDS), have been recognized in deep-water settings over the past decade. Most of these systems are described based on two-dimensional (2D) seismic data, whereas only a few CDS have been characterised from high-resolution 3D data. Here we document a newly identified CDS that formed during the Paleocene within the Morondava Basin, offshore west Madagascar, through analysis of a depth-migrated 3D seismic survey, enhanced by the implementation of seismic attributes. Three seismic units (SU) mark the main evolutionary stages of the CDS: (a) the onset (SU1), (b) drift growth (SU2), and (c) burial (SU3) stages. The growth stage documents lateral upslope migration of a mounded drift and its associated moat. The increasing, long-term influence of bottom currents along the foot of the slope occurred simultaneously with plate tectonic, climatic and oceanographic changes. Evidence amassed from the CDS highly erosive bounding discontinuities, internal discontinuities, and moat architecture all indicate the intermittent behaviour of the currents over shorter time frames during its formation. Drift deposits form under the influence of weaker currents, while discontinuities appear to record the most vigorous currents, producing the large-scale morphology of the system.
... They form sediment bodies of various morphologies and dimensions and are associated with erosive features, such as moats and furrows (Faugères & Stow, 2008;Faugères et al., 1999;Rebesco et al., 2014;Stow et al., 2002). The northern part of the Gulf of Cádiz is considered a natural laboratory for contourite research given the presence of an extensive contourite depositional system created due to the interaction of the Mediterranean Outflow Water (MOW) with the seafloor (palaeo)-topography (Brackenridge et al., 2018;Gonthier et al., 1984;Hernández-Molina et al., 2002, 2006Llave et al., 2011Llave et al., , 2007bStow et al., 2013). The MOW contourite depositional system has been and continues to be studied intensively, along with the discovery and characterisation of numerous other small-scale contourite drifts (Chen et al., 2014;Hanebuth et al., 2015;Miramontes et al., 2016;Tallobre et al., 2016). ...
X‐ray fluorescence, grain‐size and oxygen and carbon stable isotope measurements of a 33 m long piston core, recovered from the Pen Duick drift located at the foot of the prominent Pen Duick Escarpment (Atlantic Moroccan margin), are combined to decipher past oceanographic conditions. The data indicate that, similar to the northern Gulf of Cádiz, the Azores Front exerts a major control on the palaeoclimatology of the region. Contrasting to the northern Gulf of Cádiz, where Mediterranean Outflow Water is the main water mass at similar water depths, the palaeoceanography of the studied area is mostly influenced by the amount of Antarctic Intermediate Water advected from the south. The density contrast between the Antarctic Intermediate Water and the overlying North Atlantic Central Water determined the strength of the prevailing internal tides and corresponding high current speeds, which drastically impacted the sedimentary record. The most notable impact is the presence of a 7.8 kyr condensed section (30.5‐22.7 ka BP). The formation of the Pen Duick sediment drift was not just controlled by the strength of the bottom currents and the intensity of the internal tides, but also by the amount of (aeolian) sediment supplied to the region. Although variable, drift‐growth phases seem to mainly occur during colder periods of the last glacial, i.e. Heinrich and Dansgaard‐Oeschger events during the Marine Isotope Stage 3 and the late Marine Isotope Stage 2. These periods, characterised by increased aeolian dust supply and higher bottom currents, coincide with a phase of prolific cold‐water coral growth and enhanced coral mound formation as recorded in numerous cores obtained from the southern Gulf of Cádiz. This implies that both records (on and off mound cores) are pivotal to provide the complete picture of the palaeo‐climatic and palaeo‐oceanographic conditions in the region.
... This implies that the velocity of the bottom current is high in the palaeo-channel. Besides, the sandy facies along the contourite channels are associated with sandy bed forms that includes large sandy dunes (Stow et al. 2013;Brackenridge et al. 2018;. ...
This study entails the characterization of the depositional environment of the Hupo Basin shelf. By means of sedimentary structure analysis, grain size, textures, sediment color, as well as optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) ages, a sediment core (19ESDP-101, 120 m) has been interpreted to be the product of shallow-water contourites. The shallow-water (165 m) contourite deposits observed in the core are divided into seven sedimentary facies grouped into four facies associations (FA): FA1) contourite drift, FA2) contourite drift/channel transition, FA3) contourite channel/drift transition, and FA4) contourite channel. FA1 resulted from the interaction between hemipelagic fallout, low-density gravity flow, and sedimentation under low velocity bottom currents. Compared to FA1, both FA 2 and FA3 are indicative of higher velocity bottom currents, owing to their relative increase in grain size and the presence of subtle indicators of bed-load transport. FA 4 portrayed massive to slightly bedded sand, representing a contourite channel environment with high-energy conditions. Fluctuations in the bottom current activity, related to the intensification and deceleration of the North Korean Cold Current (NKCC), have caused fluctuations between contouritic and hemipelagic-dominated periods. The vertical sedimentary facies stacking patterns observed at the Hupo Basin site suggested that, over time, the depositional processes changed at the site where the core was optained. The facies association stacking pattern suggests the lateral migration of the contourite depositional system and continuous flow of the NKCC. Our findings and interpretations can serve as a much needed reference for shallow-water contourite recognition in modern environments. Moreover, our proposed model can be used to more accurately interpret shallow-water contourite deposits.
... In contrast, suspension dominated sedimentation follows the density field, which may be on the right hand side, or alternate between bends, depending on whether the velocity and density fields are coupled or uncoupled (Cossu et al., 2015;Davarpanah Jazi et al., 2020). These hypotheses have only been validated against contourite channels (sensu Akhmetzhanov et al., 2007;Stow et al., 2013) at ocean gateways (Cossu et al., 2015), and examples of deep-ocean channels (sensu Peakall and Sumner, 2015), specifically the North Atlantic Mid-Ocean Channel (NAMOC) and the North Pole NP-28 Channel, which are not associated with prominent fan deposits (Cossu et al., 2015;Boggild and Mosher, 2021). ...
Models of the sedimentary architecture of submarine channel-levee systems and their formative flow processes are predominantly based on studies from low latitude settings. Here, we integrate high-resolution seismic reflection, bathymetry and GLORIA side scan data to document the architecture and interpret the formative processes of a series of ultra-high latitude (72–76°N) submarine channel-levee systems that feed lobe complexes off the Greenland margin. We demonstrate that the sedimentary architecture of the channel-fills are dominated by vertical or near-vertical sediment accumulation, reflecting the lack of, or very limited nature of, lateral migration over time. All the Greenland channel-levee systems show significant cross-sectional asymmetry, and a peak sinuosity of 1.38, on a low gradient slope (∼0.3°). The bounding external levees are very thick (∼200 m) and wide relative to low latitude systems. Comparison of these channel-levee systems with other examples reveals that these characteristics appear to be common to systems in high and ultra-high latitudes, suggesting latitudinal controls in the sedimentary architecture of submarine channel-levee systems. The differences between high- and low-latitude systems is likely due to the interplay of physical forcing (i.e., Coriolis force) and climatic factors that control sediment calibre and flow type, both of which are latitudinally dependent. Several formative mechanisms for supressing the initial phase of lateral migration and subsequent asymmetrical development are proposed, including:i) rapid channel aggradation, (ii) Coriolis forcing causing preferred deposition on the right-hand side of the channel, and iii) variance in flow properties, with traction- and suspension-dominated flows deposited on opposing sides of the channel. We argue that a high latitudinal location of larger channel-levee systems may result in the dominance of vertical stacking of channels, the construction of large external levees, and the development of a low sinuosity planform.
... Specific contourite sedimentary facies have been described by a number of studies (e.g., Stow and Lovell, 1979;Stow, 1982;Stow and Holbrook, 1984;Stow and Piper, 1984;Pickering et al., 1989;Faugères and Stow, 1993;Stow et al., 1996Stow et al., , 2002Stow et al., , 2013aGao et al., 1998;Faugères et al., 1999;Rebesco, 2005;Llave et al., 2006;Øvrebø et al., 2006;Shanmugam, 2006Shanmugam, , 2012Shanmugam, , 2013Stow and Faugères, 2008;Rebesco et al., 2014;H€ uneke et al., 2020;de Castro et al., 2020de Castro et al., , 2021aH€ uneke et al., 2020;Stow and Smillie, 2020;de Weger et al., 2021). ...
Along-slope bottom currents and a series of secondary oceanographic processes interact at different scales to form sedimentary deposits referred to as contourite and mixed (turbidite-contourite) depositional systems. The recent proliferation of both academic and industry research on deep-marine sedimentation documents significant advances in the understanding of these systems, but most nonspecialists remain unaware of the features in question and how they form. Contourites and mixed depositional systems represent a major domain of continental margin and adjacent abyssal plain sedimentation in many of the world’s oceans. They also appear in Paleozoic, Mesozoic and Cenozoic stratigraphic sections. The growing interest in these systems has led to a refined but still evolving understanding of them. In addition to resolving their exact origins and evolutionary trajectories, research must also continue to ascertain their role in deep-sea ecosystems, geological hazards, environmental policy and economic development. Key gaps in understanding persist regarding their formation, their function in oceanographic systems and their evolution over time. This chapter summarizes current conceptual paradigms for contourite and mixed depositional systems, lists global geographic examples of these systems and discusses their identification and interpretation in terms of diagnostic features as they appear in 2D and 3D seismic datasets and at sedimentary facies scale. This chapter also considers the role that bottom currents play in shaping the seafloor and controlling the sedimentary stacking patterns of deepwater sedimentary successions. The growing interest in, and implications of, contourite and mixed depositional systems demonstrates that these systems represent significant deep-marine sedimentary environments. Combined efforts of researchers, industry partners and policy-makers can help advance understanding and responsible stewardship of deepwater depositional systems.
... All these distinctive sedimentary features indicate that these sands are contourites. Similar features of sandy contourites have been documented at the Gulf of Cadiz (Nelson et al., 1993;Stow et al., 2013) and the lower slope of the Faroe-Shetland Channel (Masson et al., 2010). ...
Complex inter-relationships between alongslope and downslope sediment dispersion exist on glaciated continental margins and vary widely along continental margins depending on sediment supply and bottom current strength. In eastern Canada, proglacial sedimentation rates are relatively high on the SW Grand Banks Slope compared to the sediment starved SE Grand Banks margin, but relatively low compared to the glacially dominated Scotian margin off eastern Canada. As on other parts of the Canadian margin, its late Quaternary sediment architecture has been constructed by interacting alongslope and downslope processes. These include sediment transported by downslope glacial meltwater discharge, alongslope bottom currents and ice-rafting. Based on the analysis of sediment cores going back to 24 ka (Heinrich event 2), this study investigates fine-grained sedimentary facies and the development of variable depositional patterns on the glacially influenced SW Grand Banks Slope off Newfoundland (eastern Canada). Both turbidites and contourites show stratification, but differ in internal structure, the presence of IRD, and the nature of their upper and lower boundaries. Sandy contourites are mostly massive, occurring either as lenses or as part of the ideal bi-gradational vertical sequence with mottled silt-mud. Glacial silty contourites have distinct rhythmic laminae with the long axis of IRD parallel to bedding. Regional scale thickness variations allow discrimination between hemipelagites and muddy contourites. Depositional architecture is built through temporal and spatial coupling of the diverse sedimentary processes. During the last glacial maximum and early deglaciation, turbidity currents fed either by meltwater or oceanographic processes flowed in canyons, and a contourite depositional system developed between the canyons. The two systems interacted on inter-canyon ridges, where contourite sedimentation was not completely overwhelmed by energetic turbidity currents. In the Holocene, alongslope processes became dominant, building a drift with clearly variable thickness, in part related to seabed morphology. A conceptual model is proposed to present the key elements of depositional processes in this depositional system, and a similar evolutionary history can be expected on other distal glacial margins.
... Camerlenghi et al., 1997;Sagnotti et al., 2001;Lucchi et al., 2002a,b;Grützner et al., 2003Grützner et al., , 2005Villa et al., 2003;Amblas et al., 2006), hydrocarbon exploration (e.g. Viana et al., 2007;Brackenridge et al., 2013;Shanmugam, 2013;Stow et al., 2013) and geological hazard assessments (Laberg & Camerlenghi, 2008). ...
Submarine canyons globally incise 11 % of the seafloor on continental slopes, and provide conduits for sediment and associated particle transport from the shallow marine realm to the deep-sea. Turbidity currents are thought to be the primary mechanism of sediment transport. An individual turbidity current can transport more than ten times the annual sediment for all the world’s rivers at speeds of up to 20 ms−1 . These fast flow velocities coupled with high sediment concentration mean turbidity currents can be destructive; posing a threat to subsea infrastructure. Understanding the frequency, timing and magnitude of turbidity currents (and other mechanisms of sediment transport) is therefore integral to developing our knowledge of particle flux to the deep sea, as well as for geohazard assessments. Despite this, very few measurements of turbidity currents exist, and only over the past decade have technological advancements, such as the use of acoustic Doppler current profilers (ADCPs), been deployed to directly monitor flows in detail. However, deployments are typically of short duration (a few months), use instruments that record at low temporal resolution, or are located distal to the flow source, therefore missing parts of an annual or longer cycles of turbidity current activity. To quantify the frequency and timing of sediment transport in submarine canyons this thesis focuses on unprecedented temporally and spatially extensive geophysical monitoring datasets, specifically using ADCPs in physiographically-diverse settings to show that: (1) Turbidity current activity is primarily controlled by sediment supply. Once sediment is available only minor triggering mechanism are required to initiate a flow; (2) Periods of enhanced turbidity current activity can be anticipated and flow probability is predictable where both sediment supply and triggering mechanism can be quantified; and (3) Even in systems of turbidity current inactivity, the focusing of bottom currents and internal submarine canyon tides are capable of transporting particles to the deep-sea. These findings highlight the highly active and complex nature of sediment transport in submarine canyons.
... Whereas mixed terrigenous-biogenic contourite drifts, notably from the North Atlantic and Mediterranean, are commonly characterized by rather high sedimentation rates (e.g. 10-115 cm/ky or even higher: Robinson and McCave, 1994;Stow et al., 2013;Rebesco et al., 2014;Miramontes et al., 2016;Gauchery et al., 2021b), this is not the case for the Ioffe Drift and Rio Grande Rise where they are generally lower than 1 cm/ky (Fig. 16), i.e. also significantly lower than in the Santa Catarina -São Paulo Plateau area (see Fig. 9). ...
Several drifts of different types and sizes are identified near the northern exit of the Vema Channel, within the projects Neogene-Quaternary contourites of the Central and South Atlantic and Lateral sedimentation in the deep ocean (on the examples from the Central and South-Western Atlantic). Herein, we discuss new results of a multidisciplinary study of sixteen sediment cores and high-resolution sub-bottom (seismoacoustic) profiles, to address the impact of Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW) passing through the Vema gateway on contourite accumulation and erosion. To the west of the northern exit from the Vema Channel, an anticyclonic gyre of LCDW is instrumental in the development of contourite drifts and sediment waves. The contourite origin of generally silty terrigenous sediments in the Santa Catarina Plateau – São Paulo Plateau area is ascertained by both morpho-seismic and sedimentary characteristics. Sedimentary features include: a lack of primary sedimentary structures and pervasive bioturbation; sharp erosional contacts, local hiatuses and stiff mud horizons; some sandy/silty layers and indistinct bedding; mostly fine grain-size, very poor sorting and distinctive bi-gradational sequences; a high degree of correlation between the content of sortable silt (SS) in the total < 63 μm size fraction and SS mean sizes in all eight cores studied. Biostratigraphy, oxygen isotope records and ten new accelerator mass-spectrometry (AMS) ¹⁴C dates reveal the mid to late Quaternary age of this mainly terrigenous contourite depositional system (CDS).
By contrast, the dominantly calcareous Ioffe Drift, overlying the Florianopolis Fracture Zone (FFZ) ridge to the northeast of the Vema Channel, is far from any source of terrigenous material, and accumulated in an area of low biological productivity. The overall asymmetric geometry, mainly lenticular, upward-convex seismic units separated by erosional unconformities, reflection truncation, small-scale moats dissecting the drift surface collectively indicate its contourite origin. The interpretation of the drift as the CDS is supported by sediment characteristics including common hiatuses corresponding in some cases to erosional contacts, pervasive bioturbation, generally poor sediment sorting, and more or less well-developed bi-gradational sequences. The erosion and deposition in the drift area are mostly controlled by the main LCDW branch following northeastward along the FFZ. Extensive erosion by bottom currents has created numerous hiatuses and markedly reduced the thickness of drift's sediments. The stratigraphic record from the Ioffe Drift sediment cores reveals an Upper Pliocene – Quaternary succession. The results of detailed analyses of six cores from the Ioffe Drift area, compared with two cores and the upper part of DSDP Site 516 from the Rio Grande Rise, notably the use of a SS analogue, provide new information on calcareous biogenic contourites deposited in the pelagic realm of the South Atlantic.
... The higher density of the Mediterranean outflow today compared with the ENACW forced the MOW to rapidly descends into the Atlantic to reach its equilibrium depth between 500 and 1500 m, above the North Atlantic Deep Water (NADW) (Ambar and Howe, 1979;Sanchez-Leal et al., 2017) (Fig. 1). The fast flow of the MOW along the southern margin of Iberia interacts with bottom sediments to form the Gulf of Cadiz contourite system (Hernandez-Molina et al., 2016;Llave et al., 2001;Stow et al., 2013b). ...
The study of planktic and benthic δ¹⁸O at site U1389 in the Gulf of Cadiz allowed us to reconstruct climate variability during the penultimate glacial period at an unprecedented millennial scale resolution. Hereby, a sequence of interstadial-stadial episodes similar to the Dansgaard-Oeschger events recorded during Marine Isotope Stage (MIS)3, were recognized in MIS6. After a detailed correlation with millennial-scale variability recorded along the Iberian margin and the North Atlantic we were able to link the millennial changes next to the Iberian Peninsula with Antarctic climate variability due to the interhemispheric seesaw response of the Atlantic circulation. The straight coupling at site U1389 of a) the planktic δ¹⁸O that reflects climate change in the Atlantic with b) the benthic δ¹⁸O, which records the temperature and oxygen isotope composition of the mixture of Mediterranean Outflow and Atlantic intermediate water, indicates that millennial climate variability also had a strong impact on the eastern Mediterranean. The detailed analysis of the benthic δ¹³C and the fine sand content in the sediments from site U1389 led us to recognize the response of the Mediterranean overflow water (MOW) to millennial scale changes in Mediterranean overturning circulation during the penultimate glaciation. Both Mediterranean Overturning Circulation and MOW strength increased at times of cool and arid climates and weakened during warm and more humid episodes, similar to what has been described for MIS3. The tuning of MOW weakening events occurring at times of precession minima with eastern Mediterranean sapropels and enhanced Asian monsoon allowed the elaboration of a new chronology for MIS6. It was compared with the Epica Dome C time scale, which was also based on the tuning of atmospheric δ¹⁸O with Asian Monsoons speleothem records.
Mediterranean overturning during MIS6 was also strongly affected by freshwater perturbations occurring at times of insolation maxima, leading to the formation of the “glacial sapropel” S6. Two major drops in seawater δ¹⁸O centered at 175 and 150 ky were observed near the entrance of the Mediterranean that were related to meltwater released from the ice sheets, coinciding with periods of insolation maxima. The entry of this meltwater anomaly through the Strait of Gibraltar is clearly registered in the western and eastern Mediterranean, especially at the onset of sapropel S6, suggesting that it was a major component of the freshwater that initiated the buoyancy gain preceding deep water stagnation.
... Deep-water bedforms (e.g., sediment waves and isolated scour depressions) are usually caused by the turbidity currents, bottom currents (e.g., tidal currents, internal waves/tides, and contour currents), fluid escape, or the combination of the above-mentioned processes (Wynn and Stow, 2002;Stow et al., 2013;Pohl et al., 2019;Lo Iacono et al., 2020;Maestrelli et al., 2020). Turbidity currents are generally formed through failures of soft sediments or hyperpycnal flows from rivers (Talling, 2014;Paull et al., 2018). ...
Dendritic canyon system is the most widespread type of submarine canyons along continental margins and plays important roles in sediment delivery and deep-water ecosystem. Based on high-resolution multibeam bathymetric and seismic reflection data, this study investigates a slope-confined dendritic canyon system in the Dongsha Slope, South China Sea. The dendritic canyon system between water depths of 550–2200 m consists of one main canyon (C2) and three V-shaped tributaries (C1, C3, and C4), in association with clearly visible tectonic deformation (uplifting and faulting) at the upper segment. The main canyon is characterized by a concave-up longitudinal profile, while the tributaries show convex-up and/or linear profiles, probably indicating that they are affected by tectonic deformation to variable degrees. The alignments and locations of tributaries C3 and C4 are determined by the NE-SW trending faults, while the distribution of the tributary C1 and the upper segment of C2 is less affected by faults, which also indicates that tectonic controls on the dendritic canyon system vary spatially among tributaries. Widespread NE-SW oriented sediment waves and southeast-facing scarps and isolated scour depressions at canyon banks highlight the significant influence of northwestward-propagating internal tide/wave on the morpho-sedimentary processes. The internal tides/waves within canyons not only flushed canyon floor, but also contributed to the asymmetric distribution of sidewall gullies and failures. The obliquely shoaling internal waves, combined with other hydrodynamics (e.g., Kuroshio Current and mesoscale eddies), could significantly intensify along-slope bottom currents and erode the NE-SW-oriented trough and tributaries. Under the significant erosion of ocean currents, some of the sidewall gullies, troughs and sub-tributaries may finally evolve into new tributaries. This study reveals the coupling relationship between slope-confined dendritic canyon system and tectonic/oceanographic processes, which can shed light on the slope/canyon evolution of similar settings worldwide.
... >10 m-2 km long, ~1 m-1.5 km wide and ~ 1-100 m deep (Viekman et al., 1992;Stow et al., 2009). Both of these features are usually narrower and less incised than submarine channels ( Fig. 9; Rebesco et al., 2014) and may be elongated according to the turbidity or bottom current direction (as they result from the effect of these processes on the seafloor; Stow et al., 2013). In some cases, subcircular to oval scours are attributed to vertical spouts of water and dynamic bottom current flows (Stoker et al., 2003), or from their interaction with seafloor irregularities (Lobo et al., 2011). ...
Interactions between along-slope bottom currents and down-slope turbidity flows can create a myriad of features and deposits. Despite numerous efforts to differentiate contourites from turbidites and mixed features, reliable diagnostic criteria are still lacking from the stratigraphic and sedimentological viewpoints. The main aim of this study is to develop criteria to differentiate mixed, along-slope-, and down-slope-generated elements from other deep-water deposits across bathymetric, seismic and sediment core data.
Mixed (turbidite-contourite) systems can be placed in three main groups based on their location, dimensions, elongation, lateral migration, spatial and temporal variability: 1) turbidite-dominated mixed systems, 2) synchronous systems, and 3) contourite-dominated mixed systems. The persistence of bottom currents —in addition to their velocity, direction, and hydrodynamic fluctuations— is responsible for entraining and redistributing fine-grained particles, carried in suspension by coeval turbidity flows, and reworking previously deposited sediments. Changes in turbidity current velocity, frequency, and duration condition the provision of sediments and development of turbidites along mixed systems. Several preliminary models are also being proposed in this study, in order to enhance our understanding of the lateral and vertical distribution of mixed systems across the sedimentary record.
Interactions between along- and down-slope processes may be synchronous, asynchronous or passive. Synchronous interactions typically occur within the same physiographic setting and the two processes interact coevally in space and time. Asynchronous interactions are also common across the modern and ancient sedimentary records, as bottom currents sweep across the deep-water environments during breaks of the turbidity flows. Passive interactions occur along the distal margins of mixed systems, or when the two processes occur near each other but do not cross over in time. Further controlling factors are held to be influential in the evolution of mixed systems at the short- to long-term; varying degrees of confinement, sediment supply or climatic fluctuations can generate cyclic stacking patterns and affect their overall dimensions. Accordingly, mixed systems feature more complex geometries than previously believed, as interactions may generate new secondary processes and features. Such systems form potential plays and may become future targets for energy geosciences and other research fields.
... Sedimentary basins may experience episodic oceanic connections via a strait during their evolution: during their opening and consequent "marine" flooding or during their closure when the connection with the ocean is lost. These straits can be located along overall complex tectonic settings, i.e. at the frontiers of tectonic plates like the Strait of Gibraltar located between the Eurasian and the African plates (Stow et al., 2013) or the Messina Strait between Calabria and Sicily (Bignami et al., 1990;Androsov et al., 1993;Brandt et al., 1997;Ferranti et al., 2008;Longhitano, 2013;Longhitano et al., 2014;Longhitano, 2018aLonghitano, , 2018b. These water passages can be sills which are highly sensitive to climate, sea-level variations and tectonics. ...
Straits are crossed by marine currents that are amplified due to constrictions. These nearshore high-velocity flows are problematic for offshore infrastructures (bridge pillars, cables, pipelines etc), but constitute an interesting carbon-free energy source. Many modern straits are dominated by tidal currents which flow axially, with reversal directions and phase difference between the two interlinked basins. These tidal currents interplay with: sediment sources (including in situ carbonate production and deltas), tectonic activity, and inherited lowstand features, all shaping the seafloor into complex geomorphologies. Previous studies have highlighted a common tidal strait depositional model with a strait-center zone in erosion and on each side a dune-bedded strait zone with 3D and 2D tidal dunes and tidal ripples. Even if the internal waves associated with and generated by the straits are widely documented, the effects of the internal waves on the seafloor need to be further investigated. The aim of this study is to unravel the combined effects of the tidal currents and the internal tides on current pattern and on the morphosedimentary features.
We present a strait example based on an interdisciplinary approach using high-resolution geophysical and oceanographical data to better constrain the hydrodynamics and the processes acting on the seafloor. We focus on the Rion Strait in Greece which controls the connection between the Corinth Gulf and the Mediterranean Sea. Based on high-resolution multibeam bathymetry (MBES) over an area of 211 km², we identify and quantify different morphologies by extracting bathymetric swath profiles. These results are integrated with currents data (ADCP) and CTD profiles. In addition, we use high-resolution chirp sub-bottom profiles and sparker seismic reflection profiles to document the morphology and internal architectures of the sedimentary deposits and the erosional features in the strait bottom. To complete this dataset, we analyzed Sea Surface Temperatures (SST) from satellite sensors.
The Rion Strait displays complex bathymetric features without tidal dunes. At the excepted depositional location of tidal dunes, we identify an erosive area with a pool and crest morphology. This new example completes the tidal strait depositional model by a re-localization of erosion, bypass and deposition in an asymmetric strait swept by baroclinic currents. This example illustrates the key role of internal tides in straits located between a confined deep basin and an open sea.
... Although average bottom currents are relatively weak in the western South China Sea, below 20 cm/s, they can be very variable and reach a maximum velocity close to 80 cm/s ( Figure S9 in Supporting Information S1; Yang et al., 2019). These periods of intense circulation could be responsible for the observed seafloor erosion (Stow et al., 2013). ...
The genesis of submarine channels is often controlled by gravity flows, but they can also be formed by oceanographic processes. Using multibeam bathymetric and two-dimensional seismic data from the western South China Sea, this work reveals how pockmarks ultimately form channels under the effect of bottom currents and gravity processes. We demonstrate that alongslope and across-slope channels were initiated by pockmark trains on the seafloor. Discrete pockmarks were elongated due to the erosion of gravity processes and bottom currents, and later coalesced to form immature channels with irregular thalwegs. These gradually evolved into mature channels with continuous overbanks and smooth thalwegs. Submarine channel evolution was significantly influenced by seafloor topography since the Late Miocene. The evolutionary model documented here is key to understanding how channels are formed in deep-water environments.
... The present study especially focuses on the Faro elongated and separated mounded Drift, the Huelva mounded Drift and the large sheeted drift located southeast of the Guadalquivir Bank (Fig.1B). These drift drilled by the Integrated Ocean Drilling Program (IODP) Expedition 339 D.A.V. Stow et al., 2013).A further thirteen piston cores from the CADISAR cruise collected over the channel/ridge and overflowsedimentary lobe sectors complemented this work. ...
Despite major advances in our understanding of the interactions between bottom currents and sedimentary deposits over the last forty years, few studies have focused on the nature of fine particles in contourite depositional systems (CDS). XRD analyses of marine sediments can be used to improve our understanding of fine-grained sediment sources and settling processes. This work presents a detailed sedimentological study of sediment cores collected over the middle slope of the Gulf of Cadiz as part of Integrated Ocean Drilling Program Expedition 339 and the 2001 CADISAR cruise. We performed high-resolution clay mineral analyses to reconstruct the pathways of fine-grained particles from their sources to their deposition along the contourite depositional system of the Gulf of Cadiz (source-to-sink approach). The clay mineral associations reflect the major contribution of the Guadalquivir River and North African rivers/dusts to fine particles settling over the middle slope. Our results suggest that size segregation deposition processes along the path of the Mediterranean Outflow Water (MOW) are responsible for the distinct clay mineral associations between sites located under the upper MOW and the lower MOW. We observed changes of sedimentation rates over the contourite depositional system throughout the last 25 kyrs. We propose that these changes are due to temporal variations in the vertical distribution of the upper and the lower MOW whose concentrations in suspended particulate matter are drastically different. Sea-level and large scale atmospheric changes (e.g., ITCZ migration) over this period induced major variations in the distance of river mouths to the Gulf of Cadiz CDS, and in the amount of Northwest African dust delivered to this depositional system, respectively. Climate changes therefore modified fine particle sources and pathways, which considerably influenced clay minerals settling in the middle slope of the Gulf of Cadiz since the Last Glacial Maximum.
... The density output represents the sum of the value of all kernel surfaces overlaying individual raster cells. The line-based KD calculation was adapted from the quartic kernel function for point densities (Silverman 1986), being greater at the line and zero at the limit of the radius distance. The KD calculations are applied to the full sample to show overall patterns of landslide distribution, thus not accounting for further subclass analysis based on size or location. ...
Submarine landslides are a ubiquitous geohazard in the marine environment and occur at multiple scales. Increasing efforts have been made during the last decade to catalogue and categorise submarine landslides in comprehensive databases, aiming to better understand their preconditioning and trigger factors. Using the recently compiled, open-access MAGICLAND dataset, we investigate the distribution and morphometric trends of submarine landslides observed in seven distinct geomorphologic domains offshore west and southwest Iberia. Higher densities of submarine landslides occur on the proximal regions of the south and southwestern margins of the study area. These regions are located adjacent to or coincident with higher density areas and clusters of earthquake epicentres. Submarine canyons are another major location for collapses which are particularly abundant at canyon mouths. However, significant numbers occur within all domains with pronounced relief, including distal regions hundreds of kilometres away from the foot of the continental slope. Landslide size range is inversely proportional to their spacing and frequency, a tendency observed within each domain on the whole study area. Positive correlations were obtained between the parameters analysed, but relationships between unidimensional parameters such as length and width exhibit lower correlation coefficients. Correlations between 2D and 3D parameters such as area and volume are stronger, supporting similar findings by other studies. The relationships obtained are, however, variable across domains, and the correlation values are influenced by the seafloor geomorphology. This work brings new insights on submarine landslide distribution in the understudied west and southwest Iberian continental margin, complements previous inventories made for nearby regions, and provides valuable data with wider applications for submarine landslide databases.
... Sedimentary processes building up continental margins basically have a combined action of downslope movements driven by gravity (i.e.: submarine mass-movements and turbidity currents) and alongslope transport and sedimentation forced by bottom currents, which form contourite depositional systems (CDS -Hernández- Hernández-Molina et al., 2016a). CDS are typically associated with contourite drift (i.e., sediment deposited or substantially reworked by the action of persistent bottom currents - Stow et al., 2002) in the form of a coarsely elongated and/or mounded deposit made up of graded bioturbated sedimentary sequences Gonthier et al., 1984;Stow et al., 2008;Rebesco et al., 2014;Esentia et al., 2018) and often impressive mud wave fields, a moat, contouritic channels and a range of sedimentary bedforms (Stow et al., 2013;Hernández-Molina et al., 2009). Contourite drifts can be considered meso-scale morpho-sedimentary landforms that can occur in many different shapes and sizes, ranging from extensive sheeted drifts to small-scale patch drifts (Esentia et al., 2018) and can reach up to 2 km in height and hundreds of km in length (Rebesco et al., 2014). ...
Submarine geomorphology, the study of landforms and processes within the submarine domain, is a young discipline that owes its birth to technological achievements that made it possible to explore the underwater sphere of our Earth system. Submarine domains represent over 70% of Earth's surface, i.e. the largest geomorphic system on our planet (more than twice the size of what we can observe on Earth's land surface). From the middle of the last century onwards, technological advances have led to more and more high-performance acoustic equipment and robotic underwater systems, enabling us to depict and investigate, in ever greater detail, parts of the ocean floor long thought to be unfathomable. The present chapter gives an overview of the extent to which technological progress has strongly determined the way in which the study of landscapes and landforms within the submarine domain is approached, creating substantial differences to approaches used in classical studies of geomorphology. Main drivers of seafloor geomorphic changes are introduced to provide a representative summary of the variety of landforms generated by the action of a range of tectonic, sedimentary, and bio-geochemical processes, including the impact of human activity. The chapter concludes with a brief discussion on the relevance of the applied value of submarine geomorphological research, its new trends, and the key contribution it is providing to confirming the importance of geomorphology to the full range of Earth system sciences and environment-related topics.
... At the same time, downslope transport, channeled through submarine canyons, occurs frequently. Extensive turbidite intervals -intercalated between contouritic layers and often reworked by bottom currents -have been identified in several Pleistocene and Pliocene sediments in this area (Stow et al., 2013). ...
... Contourite depositional systems (CDSs) are very common along many continental margins, with prominent examples occurring along the western South Atlantic margin (Hernández-Molina et al., 2016), the Gulf of Cadiz (Stow et al., 2013), the Gulf Stream (Reynolds et al., 1999), North Atlantic margin (Akhmetzhanov et al., 2007), western Mediterranean Sea and the Antarctic Peninsula Pacific Margin (Uenzelmann-Neben, 2006). The dynamic processes driven by downslope and alongslope currents play a significant role in the construction and shaping of continental margins (Mulder et al., 2013). ...
Contourites are globally-distributed features along continental margins where deep-water masses come across the seafloor; however, the genesis and metal-sources of marine mineral deposits associated with contourite deposits remain largely unknown. Abundant Fe-rich nodules were recovered along the continental margin of the South China Sea, where extensive cold-seep systems were also discovered. We measured synthetically geochemical elements, δ¹³C, Sr and Nd isotopic data and organic proxies in Fe-oxy-hydroxide deposits from the upper and lower slope. Fe-rich nodules recovered from the upper slope represent the effect of an integrated diagenetic-hydrogenetic formation environment, in which fluid venting from deep-seated hydrocarbon reservoirs and deep-water erosion and chemistry could play an important role. In contrast, another kind of ferromanganese nodule forming along the lower slope and predominantly hydrogenetic is connected to a distinct deep-water flow from the North Pacific. These results give us a new insight into the mineralization processes associated with Fe oxidation-reduction processes on continental margins affected by contourite systems with abundant hydrocarbon seep, bottom water, and terrigenous inputs.
... Here, we used the line feature delimiting the landslides as input. The line KD was adapted from the quartic kernel function for point densities (Silverman 1986), being greater at the line and zero at the limit of the radius distance, which was set at 30 km. The nearest neighbour tests the clustering or overdispersion of points. ...
... Contourites are routinely observed in modern deep-sea environments, and the mixed turbidite-contourite systems (MTCSs) composed of both turbidites and contourites are the norm on many continental margins (Mulder et al., 2006(Mulder et al., , 2008Hernández-Molina and Stow, 2008;Martín-Chivelet et al., 2008;Sansom, 2018;Thiéblemont et al., 2019). Although the MTCS has been widely reported in modern deepsea environments worldwide, such as the Gulf of Cadiz (Mulder et al., 2006;Marchès et al., 2010;Brackenridge et al., 2013;Stow et al., 2013;Hernández-Molina et al., 2014;Alonso et al., 2016), the Gulf of Mexico (Shanmugam et al., 1993a;Shanmugam, 2016) and the South China Sea (Gong et al., 2013(Gong et al., , 2015(Gong et al., , 2016Li et al., 2013), there is still much controversy over the criteria by which contourites, especially sandy contourites, can be recognised and distinguished from turbidites (Faugères and Stow, 1993;Rebesco, 2005;Martín-Chivelet et al., 2008;Shanmugam, 2008;Stow and Faugères, 2008;Rebesco et al., 2014;Stow and Smillie, 2020). The lack of commonly accepted facies-scale criteria, particularly for sandy bottom current deposits, is unfortunate because their proper recognition from the rock record would allow significant insights into the palaeoclimatical and palaeoceanographical processes (Knutz, 2008;Hovikoski et al., 2020). ...
Accurately identifying contourites and analysing the interaction between turbidity flows and bottom currents from the mixed turbidite-contourite systems (MTCSs) in the deep-sea environments are still challenging. Reports on ancient MTCSs are scarce and most of them remain contentious. This study describes at various scales the sedimentological and sedimentary geological characteristics of the Middle Ordovician Yingtaogou Formation, which is identified as a MTCS, from the western margin of the North China Craton (NCC). Twelve typical lithofacies are recognised in the Yingtaogou Formation, which can be grouped into four facies associations interpreted to reflect various deposition from turbidity flow, contour current, debris flow, and hemipelagic settling in the slope–abyssal plain environment. The evidence for contourites is four-fold. Firstly, the palaeocurrent direction based on flute casts on turbidite soles is almost perpendicular to that indicated by traction current structures. Secondly, it is possible to observe a variety of traction current structures (large-scale cross-bedding, cross-lamination, wavy bedding, lenticular bedding, parallel bedding, and horizontal lamination), which are commonly found in reverse and bi-gradational sedimentary sequences at various scales. Thirdly, internal erosive surfaces are widely developed, which reflect the fluid winnowing and velocity fluctuations. Fourthly, these contourites are characterised by an abundance of reverse grading and bi-gradational sequences at various scales. In addition, the contourites identified in this study generally lack bioturbational structures, which is probably related to the late migration and radiation of the Ordovician deep-sea benthos. The sedimentary processes, products, and controlling factors of the interacting down- and along-slope currents were further identified, and a bioturbation-free facies model of contourites and a depositional model of the MTCS were established. This study may contribute to establishing the sedimentological criteria for recognition of ancient contourites and a better understanding of the interaction and spatiotemporal evolution of down- and along-slope sedimentary processes, as well as to a deeper comprehension of the Middle Ordovician palaeoceanographical environment in the western margin of the NCC.
... ; Hern andez-Molina et al., 2006Molina et al., , 2014bStow et al., 2013; Brackenridge et al., 2018;Lozano et al., 2020). The dominance of vertical biogenic structures in F5 supports the occurrence of energetic environmental conditions. ...
Idealized facies of bottom current deposits (contourites) have been established for fine‐grained contourite drifts in modern deep‐marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine‐grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non‐existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo‐Mediterranean Outflow Water. Foraminifera assemblages were used for age‐constraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain‐size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo‐Mediterranean Outflow Water, which is mainly driven by precession and millennial‐scale climate variations. Tides substantially modulated the palaeo‐Mediterranean Outflow Water on a sub‐annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process‐based interpretation of deep‐water bottom current deposits.
... Binary phase diagrams are used to illustrate how two environmental conditions (e.g., grain size and flow velocity) combine to control a range of sedimentary growth morphologies (bedforms) and have been used within clastic sedimentology ever since proposed by Harms et al. (1982). Different phase diagrams have been compiled for many different clastic environments, from rivers (Lapotre et al., 2017;Baas, et al., 2016;Southard and Boguchwal, 1990) to the deep ocean (Stow et al., 2013). Recently, apparently disruptive parameters such as biological and mineralogical cohesion have been investigated, constrained, and provided a third axis within the phase diagram system, rather than a truly confounding barrier to their use (Schindler et al., 2015). ...
Ancient and recent terrestrial carbonate-precipitating systems are characterised by a heterogeneous array of deposits volumetrically dominated by calcite. In these environments, calcite precipitates display an extraordinary morphological diversity, from single crystal rhombohedral prisms, to blocky crystalline encrustations, or spherulitic to dendritic aggregates. Despite many decades of thorough descriptive and interpretative work on these fabrics, relating calcite micro-morphology with sedimentary hydrogeochemical conditions remains a challenge. Environmental interpretations have been hampered by the fact that calcite morphogenesis results from the complex interaction between different physico-chemical parameters which often act simultaneously (e.g., carbonate mineral supersaturation, Mg/Ca ratio of the parental fluid, organic and inorganic additives). To try to experimentally address the sedimentological causes of calcite morphogenesis, an experimental approach yielding a first attempt at a calcite growth-form phase diagram is presented here. The initial aim was to account for the carbonate products experimentally nucleated in alkaline, saline lake settings. These are the result of at least two competing calcite precipitation ‘driving forces’ that affect morphogenesis: the calcite supersaturation level of the parental fluid, and the concentration of microbial-derived organic molecules (alginic acid). A key finding of this study is that common naturally-occurring calcite products such as calcite floating rafts, rhombohedral prismatic forms, di-pyramid calcite crystals, spherulitic calcite grains, or vertically stacked spheroidal calcite aggregates, can be related to specific hydrogeochemical contexts, and their physical transitions pinpointed in a phase diagram. By exploring binary or ternary responses to forcing in morphological phase-space, links between calcite growth forms and (palaeo)environmental conditions can be determined. This provides a truly process-oriented means of navigating questions around carbonate precipitate morphogenesis for the future.
... Binary phase diagrams are used to illustrate how two environmental conditions (e.g., grain size and flow velocity) combine to control a range of sedimentary growth morphologies (bedforms) and have been used within clastic sedimentology ever since proposed by Harms et al. (1982). Different phase diagrams have been compiled for many different clastic environments, from rivers (Lapotre et al., 2017;Baas, et al., 2016;Southard and Boguchwal, 6 1990) to the deep ocean (Stow et al., 2013). Recently, apparently disruptive parameters such as biological and mineralogical cohesion have been investigated, constrained, and provided a third axis within the phase diagram system, rather than a truly confounding barrier to their use (Schindler et al., 2015). ...
Ancient and recent terrestrial carbonate-precipitating systems are characterised by a heterogeneous array of deposits volumetrically dominated by calcite. In these environments, calcite precipitates display an extraordinary morphological diversity, from single crystal rhombohedral prisms, to blocky crystalline encrustations, or spherulitic to dendritic aggregates. Despite many decades of thorough descriptive and interpretative work on these fabrics, relating calcite micro-morphology with sedimentary hydrogeochemical conditions remains a challenge. Environmental interpretations have been hampered by the fact that calcite morphogenesis results from the complex interaction between different physico-chemical parameters which often act simultaneously (e.g., carbonate mineral supersaturation, Mg/Ca ratio of the parental fluid, organic and inorganic additives). To try to experimentally address the sedimentological causes of calcite morphogenesis, an experimental approach yielding a first attempt at a calcite growth-form phase diagram is presented here. The initial aim was to account for the carbonate products experimentally nucleated in alkaline, saline lake settings. These are the result of at least two competing calcite precipitation ‘driving forces’ that affect morphogenesis: the calcite supersaturation level of the parental fluid, and the concentration of microbial-derived organic molecules (alginic acid). A key finding of this study is that common naturally-occurring calcite products such as calcite floating rafts, rhombohedral prismatic forms, di-pyramid calcite crystals, spherulitic calcite grains, or vertically stacked spheroidal calcite aggregates, can be related to specific hydrogeochemical contexts, and their physical transitions pinpointed in a phase diagram. By exploring binary or ternary responses to forcing in morphological phase-space, links between calcite growth forms and (palaeo)environmental conditions can be determined. This provides a truly process-oriented means of navigating questions around carbonate precipitate morphogenesis for the future.
... Large-scale cross-stratification and widespread regional discontinuities, especially at the base of the drift are often associated with contourite drifts (e.g. Knutz, 2008;Stow et al., 2013). In addition, a contourite depositional system (CDS) is defined as the association of various drifts and related erosional features (see Hernández-Molina et al., 2003;Rebesco and Camerlenghi, 2008;Rebesco and Stow, 2001). ...
Canyons, channels, and gullies are incised features on the surface of the earth that act as subaqueous conduits for transferring sediment from source areas to basins. For several decades, these subaqueous sediment conduits have played an important role as storage for petroleum resources. An understanding of the architecture, morphometry, and development of these subaqueous sediment conduits in a general depositional environment will help maximize their potential. This study utilized 3D seismic reflection data obtained from various ancient subaqueous sediment conduits from fluvial, lacustrine, deltaic, and deep marine depositional environments from the Norwegian Barents Sea, offshore Israel, onshore China, and offshore New Zealand. The study aims to: 1) investigate their seismic stratigraphic architectures; 2) investigate their morphometric elements; 3) evaluate their sedimentary processes and paleoenvironments; and 4) compare their similarities and differences as well as global comparison with previous works. The results showed that isolated bright amplitudes with incised surfaces were characteristic of these subaqueous sediment conduits, while most of the conduits had morphometric elements, such as thalwegs, walls (including terraces), overspill points or levees, and flanks. These architectural and morphometric elements showed that gravity flow is dominant in subaqueous sediment conduits in the lacustrine and marine environments, and traction currents were observed in the subaqueous sediment conduits in the fluvial environments, as well as river-dominated deltas (minor marine influences). Furthermore, the subaqueous sediment conduits were similar with respect to the existence of incised surfaces and morphometric elements; however, their sedimentary processes, tectonics, climates (including latitudinal controls), and paleoenvironments were different. This research demonstrated that a comprehensive study of all depositional environments and the utilization of cutting-edge techniques for advanced seismic interpretations (quantitative seismic geomorphology, in particular) are necessary to appropriately interpret the sedimentary processes and paleoenvironments that lead to improved petroleum resources utilization.
... Stow and Faugères (2008) proposed that incomplete bi-gradational sequences are also common in contourite drifts, suggesting the terms "base-cut" (intervals C3-C5), "top-cut" (intervals C1-C3) or "base and top cut" (interval C3). These complete and incomplete sequences were identified during the IODP 339 expedition in the Gulf of Cadiz (Stow et al., 2013;, 2016. Alonso et al. (2016) and de Castro et al. (2020a) described the base-cut contourite sequences in areas near contourite channels and moats, that is, in a channel-drift transition zone. ...
The purpose of this study is to differentiate and characterise contouritic sands in two different locations with variable sediment compositions (siliciclastic and bioclastic) based on a multiproxy approach that includes the analysis of sedimentary texture, semi-quantitative geochemistry, microfacies and ichnological information, as well as a Principal Component Analysis (PCA) applied to geochemical and sedimentary data on sediment cores. The integration of sedimentological analyses and the PCA permits also to the differentiation between fine-grained deep-water deposits such as hemipelagites, muddy contourites and hyperpycnites. A depositional model is proposed here, based on geochemical and sedimentological characteristics of the Holocene-highstand Mozambique Channel upper slope sands, and glacial-lowstand Corsica Contourite Depositional System middle slope sands. The upper continental slope of the Mozambican margin is characterised by siliciclastic sandy contourites, muddy contourites and muddy hyperpycnites. Mozambique siliciclastic sandy contourites constitute large accumulations of well-sorted very fine to coarse sand with evidences of strong winnowing and reworking under high-energy conditions. The sedimentary facies represents highstand contourite sands and shows a reversely-graded trend. The contourite drift on the Pianosa ridge (eastern flank of the Corsica Trough) consists of bioclastic sandy and muddy contourites and hemipelagites. Bioclastic sandy contourites are made up of shallow-marine winnowed bioclasts with a reversely- and normally-graded trend and represent lowstand contourite sands. The PCA in the two environments —showing a distinctive geochemical signal— allows for differentiation of the contourite deposits. In siliciclastic sands, reworking is marked by an accumulation of Si, Zr, and Sr in fine- to medium-grained sands. In bioclastic sands, reworking is less evident but it is characterised by accumulations of Ca and Sr. The reworking and winnowing bottom current effects are also observed at the microfacies scale. Both types of contourite deposits show evidences of intermittent depositional conditions depending on the ichnodiversity, distribution and abundance of trace fossils. This work is useful to discriminate similar fine-grained deposits in different continental margins which would contribute to a better understanding of sedimentary deposits and processes in deep-marine environments.
Submarine landslides that occur on continental margins can reshape the seafloor morphology and form steep slide scarps. Although the emplacement processes of submarine landslides have been well studied, the development of post-slide deposition has long been overlooked. Here, we use high-resolution multibeam bathymetric and three-dimensional seismic data from the northern South China Sea margin to decode how slide scarps affect bottom currents and generate associated bottom current-related features. Distinct features have been identified within the slide scars, such as sediments waves and drifts. In parallel, multiple longitudinal negative topographies are distinguished along the scarps, undercutting into underlying strata with width and depth up to 240 m and 20 m, respectively. These incisional structures are interpreted as moats, formed following the emplacement of landslides, and running along the foot of scarps that parallel to contours. The origin of these moats could thus represent an interaction between slide scars and contourite deposition. We suggested that escarpments act as morphological barriers, regionally affecting the velocity and direction of bottom currents, leading to an intensification of their velocities and their erosive ability. Furthermore, we propose a co-evolution model of submarine landslides and related moats, which elucidates and emphasizes the significance of scarps in moat initiation and evolution. Our results demonstrate that the interaction between escarpments and bottom current flow can lead to topographic remodeling and sedimentary processes on varying continental margins.
High-resolution records from International Ocean Discovery Program (IODP) sites U1386 and U1387 drilled during IODP Expedition 339 into the Faro Drift, made it possible to assess the impact of intensifications of the upper core (MOWU) of the Mediterranean Outflow Water (MOW) and of changes in sediment supply on the sedimentation in the northern Gulf of Cadíz since the Middle Pleistocene. This work focuses on the comparison of records covering Marine Isotope Stage (MIS) 2–1 and MIS 12–11, in order to investigate the behaviour and circulation regime of the MOWU over two climatic cycles of similar astronomical configurations and their associated deglaciation. The analysis of facies established on the basis of grain-size, XRF core-scanning, and carbonate content revealed contourite beds formed by the MOWU during MIS 11 and MIS 1 and deglaciations (deglaciation V and I). Contourite sequences show that MOWU velocity at the seabed was higher during MIS 2–1 than during MIS 12–11, and that sediment supply was different between these two climatic cycles. While overall low during MIS 12–11, MOWU intensity increased during deglaciationV and MIS 11 and preceded large ice rafted events and cooling in the North Atlantic Ocean. As a major element of the MOW, MOWU strengthening during deglaciation V likely contributed to higher heat and moisture transport towards the high latitudes inducing a slight increase of calving and size of boreal ice sheets. The MOW-derived injection of heat and salt in the North Atlantic Ocean during deglaciation V might have contributed, through reactivation of the upper AMOC, to the switch of the Atlantic thermohaline circulation from a glacial to an interglacial mode.
The Eirik Drift, off southern Greenland, is one of a series of contourite deposits in the northern North Atlantic that record changes in the strength and location of western boundary currents in the region. To date however, the sedimentary facies, and particularly the variation in facies across this drift, have received relatively little investigation. Here, we present an analysis of the sedimentary facies observed within a transect of cores from the crest to toe of the Eirik Drift from late Pleistocene to Holocene. The Holocene sequence consists of muddy contourites with high sedimentation rates at the drift toe, and a condensed sequence of sandy contourites on the upper drift flanks, consistent with winnowing under strong bottom currents on the upper drift and deposition under a low velocity, sediment-laden current at the drift toe. We interpret this to be a combined result of episodic, high-energy benthic storm events associated with the East Greenland Current (EGC) on the upper drift and more continuous, lower velocity Deep Western Boundary Current (DWBC) on the drift flanks. The deglacial interval is represented by muddy contourites across the drift, with evidence for decreasing current activity (both EGC and DWBC) and more widespread ice-rafted deposition from the Bolling-Allerod into the Younger Dryas. Palaeocurrent data from this interval show two separate current directions at the crest of the drift, suggesting temporary, local detachment of the DWBC or EGC, linked to temporal variation in current strength. The late glacial interval consists of glaciomarine hemipelagites and muddy contourites, with evidence for a higher degree of current influence at shallower depths, consistent with a moderate EGC and weak DWBC. This is the first time that the EGC is recognised as having a significant role in sedimentation on the Eirik Drift.
The shallow-marine turbidite fans in the Upper Miocene Huangliu Formation of the Yinggehai Basin in the northwestern South China Sea (SCS) provide an excellent opportunity to understand their sedimentary processes in a shelf depositional environment. The down-slope gravity flow processes and along-slope bottom-current reworking processes of shallow-marine turbidite fans were interpreted by using seismic, well logging, core, petrographic, geochemical, and petrophysical data. Several depositional elements were identified in the shallow-marine turbidite fans, namely, channel-fill high-density turbidites (HDTs), channel-fill low-density turbidites (LDTs) and associated frontal splays, sand-rich/mud-rich lobe deposits, and bottom-current reworked channel-fill/lobe deposits. Deep U-shaped (or V-shaped) seismic reflections and low root-mean-square (RMS) amplitudes characterize the channel-fill HDTs that consist of massive fine-grained sandstones with mud clasts. The channel-fill LDTs, characterized by V-shaped or worm-shaped reflections, mostly consist of normally graded, laminated and rippled, very fine-grained sandstones. Frontal splays are generally associated with channel-fill LDTs. The sand-rich lobe deposits show continuous high-amplitude sheet-like reflections and consist of HDTs and LDTs, whereas the mud-rich lobe deposits show continuous moderate-amplitude reflections and consist of muddy debrites. The bottom-current reworked sandstones (BCRSs), which comprise well-sorted, fine-grained sandstones with traction-current structures, are usually located in the upper parts of thick sandbodies. The variability of depositional elements from large-scale channel-fill HDTs with strong basal erosion in fan-1 to small-scale channel-fill LDTs in fan-2 is closely linked with sea-level fluctuations that result in variable gravity-flow energy and sediment input. However, the reoccurrence of large-scale channel-fill HDTs in fan-3 at sea-level highstands may possibly be attributed to enhanced sediment input from the source areas. Down-slope flow transformation from turbidity flows into muddy debris flows within an individual channel-lobe complex (CLC) resulted in a dramatic increase in clay content and resultant decreasing reservoir quality from the channel-fill HDTs to the mud-rich lobe deposits, because muddy sediments are incorporated into the precursor turbidity flows and turbulence is suppressed. Additionally, it is suggested that the widely developed traction-current structures and tidal signatures (double mud layers, mud-draped ripples, discrete wavy bedding, internal truncation surface, and convex-up laminae) are the products of reworking by internal waves and -tides. During periods of sea-level highstands, the upper parts of gravity-flow sandstones would undergo bottom-current reworking, thus resulting in the retransportation of muddy fines and the formation of reworked sandstones with traction-current structures and tidal signatures. In this study, a combination of traction-current structures, tidal signatures, vertical sequences showing sharp upper contacts and non-gradational upper contacts, and trace elements is considered to be convincing diagnostic criteria in distinguishing reworked sandstones from gravity-flow sandstones. The representative bottom-current reworked sandstones should be preferable hydrocarbon targets in further exploration because of their better reservoir properties compared with gravity-flow sandstones. This research offers some insight into gravity-flow processes and bottom-current reworking processes in a shallow marine environment.
The divine teacher-student relationship among three generations of Indian geoscientists (1940s-2020s): a remarkable story of knowledge transfer from T. N. Muthuswami Ayer or "TNM" (a crystallographer and mineralogist) through A. Parthasarathy (an engineering geologist and quantitative sedimentologist), to G. Shanmugam (a process sedimentologist and petroleum geologist) and beyond
The missing link in the plate boundary between Eurasia and Africa in the central Atlantic is presented and discussed. A set of almost linear and sub parallel dextral strike–slip faults, the SWIM1 Faults, that form a narrow band of deformation over a length of 600 km coincident with a small circle centred on the pole of rotation of Africa with respect to Eurasia, was mapped using a new swath bathymetry compilation available in the area offshore SW Portugal. These faults connect the Gloria Fault to the Rif–Tell Fault Zone, two segments of the plate boundary between Africa and Eurasia. The SWIM faults cut across the Gulf of Cadiz, in the Atlantic Ocean, where the 1755 Great Lisbon earthquake, M ~ 8.5–8.7, and tsunami were generated, providing a new insight on its source location.
Two sand-rich channelized depositional systems, formed by strong contour currents, were studied west of the Faeroe Bank Channel and in the Gulf of Cadiz. Both are areas beyond the exit of constrictions where water overflows from the Norwegian Sea and the Mediterranean Sea, respectively. West of the Faeroe Bank, newly mapped channels are developed mainly under the influence of a geostrophic current and are characterized by significant lateral migration, which determines the marked cross-sectional asymmetry and the architecture of the deposits. The pathways of the Mediterranean Undercurrent in the Gulf of Cadiz are complex, with the greater proportion flowing under geostrophic conditions along a terrace but with some of the denser water becoming ageostrophic and descending downslope owing to gravity. A series of 'peel-off' channels is formed, with the largest one, Gil Eanes, being about 40 km long. Most of the channel fills consist of medium-coarse sand. Levees are mainly silts with a higher sand content in the vicinity of the channel. Both depositional systems have a variety of contourite sand channels, which in most respects are remarkably similar. In both cases there are stretches where the flow is ageostrophic, with water descending downslope for as much as 400 m before resuming geostrophic flow at deeper levels. In each case the main pathway of the densest water is the shallowest and several branches turn off to the left of this main pathway before bending to the right under the influence of Coriolis forces. In both cases there are channel fills of medium-coarse sand, probably cross-bedded, and up to 200 ms thick. Sheets of sand with a thickness of a few metres to a few tens of metres are common. Similarities to turbidite channels are the aggradational nature of some channel floors and the flanking muddy or silty sediment waves. Contourite channel depositional complexes are distinguished from turbiditic ones by their coarsening-up rather than fining-up sand units, the asymmetry in channel architecture, the presence of regional unconformities, and the distribution pattern with well-marked boundaries of current-derived deposits.
Thus far, the subject of deep-marine sands emplaced by baro-clinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petro-leum exploration. Internal waves are gravity waves that oscil-late along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans re-veal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16– 164 ft]) than surface waves (<2 m [6.56 ft]), (2) longer wave-lengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s –1 [1.64–6.56 ft s –1 ]) than surface waves (25 cm s –1 [10 in. s –1 ]). Maximum speeds of 48 cm s –1 (19 in. s –1) for baroclinic cur-rents were measured on guyots. However, core-based sedi-mentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and A U T H O R
The contourite drifts off southwest Iberia that formed as a result of the interaction of the Mediterranean Outflow Water (MOW) with the continental middle slope were studied in the Algarve margin using multichannel reflection seismic lines, oil-wells, piston cores, and a bathymetric compilation of four datasets. The seismostratigraphic interpretation of a dense array of oil industry seismic and stratigraphic correlation allowed the identification of five seismic units of Early Pliocene through Holocene in the Faro and Albufeira drifts and four correlative seismic units in the Lagos and Sagres drifts and three in the Portimão drift. A three-phased evolutionary model for the contourite formation is proposed. Firstly, a precursory phase of Pliocene age made up of sheeted drifts represents an initial phase of deposition under bottom-current activity that is correlated with the first stages of an enhanced MOW at about 3.5 Ma. Secondly, the building up phase of Early Pleistocene age is related to a strengthening of the MOW close to the onset of the Northern Hemisphere glaciations at about 2.6 Ma during which were deposited low-mounded drifts. Thirdly, the growing phase from Middle-Pleistocene through Holocene suggests the presence of a stronger MOW since the beginning of the Middle-Pleistocene Transition at about 1.3–1.0 Ma, accounting for the deposition of mounded drifts and formation of the Álvares Cabral moat. Seismostratigraphic interpretation and isochron maps allowed for the establishment of the main oceanographic, climatic, morphologic and tectonic factors that controlled the drifts deposition: i) the Pliocene and Quaternary MOW circulation forced by climate changes; ii) the sea bottom topography inherited from the Late Miocene, mainly shaped by the Portimão, Lagos and Bartolomeu Dias canyons system; iii) the interaction between along-slope and down-slope processes since the Pliocene; and iv) Pliocene–Quaternary fault-activity and diapirism.
Deep marine currents are strongly influenced by climatic changes. They also deposit, rework, and sort sediment, and can generate kilometer-scale sedimentary bodies (drifts). These drifts are made of thoroughly bioturbated, stacked sedimentary sequences called contourites [Gonthier et al., 1984]. As a consequence, change in the direction or intensity of currents can be recorded in the sediments.
The importance of contour currents in shaping and building continental margins has long been accepted. Their economic implications and the stratigraphic framework in which they are developed remain largely unknown. Data retrieved from sidescan sonar images, seismic profiles and their attribute maps, as well as sea-floor coring, boreholes and the few known outcrops around the world, suggest that bottom currents can locally develop large deposits of relatively coarse-grained sediments. Accumulation of coarse-grained deposits under the influence of bottom currents requires sediment availability, a geologically persistent strong circulation regime and a favourable physiographic setting both for enhancing the currents and for hosting the sediments. The hydrocarbon exploration of oceanic depositional systems demands a better understanding of the role of bottom currents and their implications for petroleum systems such as reservoir and sealing rocks. Such understanding implies additional alternatives for the definition of exploration targets and prospect risk reduction. Correlating seismic anomalies from 3D mapping with core and well logging data reveals the depositional geometry and sedimentological characteristics of coarse-grained contourites. Fine-grained drifts can locally and regionally develop large and thick accumulations, which have an important seal potential for trapping hydrocarbon.
The role of internal waves in the sediment dynamics of the northwestern Alboran continental slope was investigated in a selected area around the Guadiaro submarine canyon. Nepheloid layer distribution was identified using closely spaced CTD/transmissometer profiles collected during two hydrographic surveys. A well-defined pattern of suspended sediment distribution consisting of surface, intermediate, and near-bottom nepheloid layers was found. Intermediate and bottom nepheloid layers were always observed within the canyon and on the adjacent continental slope, spanning mainly from 200 to 500 m depth. In addition, a current meter with a turbidimeter was deployed in the lower section of the Guadiaro Canyon at 600 m depth, 25 meters above the seafloor. Time series analysis indicated that the currents, temperature, and turbidity within the canyon fluctuate mainly at semidiurnal tidal frequencies, suggesting the presence of semidiurnal internal tides affecting the near-bottom suspended sediment concentration along the canyon axis. High-resolution bathymetry from the study area was used to evaluate the internal wave reflection conditions at semidiurnal tidal frequency for the entire continental slope region. Critical slope conditions were reached on the upper continental slope and along the canyon axis, coinciding with the region in which nepheloid layers were observed. This region also coincides with a zone of erosion on the upper continental slope of the study area previously identified by Hernández-Molina [1993]. These results indicate that the generation of intermediate and bottom nepheloid layers, as well as the erosion and shaping of the northwestern Alborán continental slope, may result from the interaction of internal waves and the seafloor morphology.
A new morphosedimentary map of the Gulf of Cadiz is presented, showing the contourite depositional system on the gulf's middle slope. This map is constructed from a broad database provided by the Spanish Research Council and the U.S. Naval Research Laboratory. Our map shows that this contourite depositional system comprises five morphosedimentary sectors: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) contourite deposition; and (5) submarine canyons. The Gulf of Cadiz contourite depositional system stems directly from the interaction between Mediterranean Outflow Water and the seafloor; its morphosedimentary sectors are clearly related to the systematic deceleration of the Mediterranean Outflow Water´s westward branches, bathymetric stress on the margin, and the Coriolis force. The slope's depositional system can be considered as a mixed contourite and turbidite system, i.e., a detached combined drift and fan.
Fast currents, high concentrations of suspended sediment and grooved mud
beds are associated with erosion in frequent abyssal storms where a fast
deep mean flow is augmented (or reversed) by intense intermittent
currents. This occurs about 5 km below the Gulf Stream or its rings. The
waning phase of a storm results in development of bedforms and rapid
deposition of a mud blanket. Several other regions of the world ocean
display evidence of abyssal storm activity.
Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the sedimentary system located in the Gulf of Cadiz which is under the influence of a strong current, the Mediterranean Outflow Water (MOW). When it comes out from the Strait of Gibraltar, the MOW is either channelled along major or secondary channels, or spills over a sedimentary levee. Frequent earthquakes and the constant current shearing generate widespread sediment deformation and instability of contourite deposits. Secondary channels can form by retrogression following an initial failure. At their mouth, sediment accumulates in the form of small sandy contourite lobes. These observations suggest that the Gulf of Cadiz system shares many similarities with channel-levee complexes formed by turbidity current activity. The main difference is that, in the Gulf of Cadiz, the main process is a strongly flowing saline current which locally interacts with gravity processes.
Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s−1 to <0·5 m s−1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope-oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave-covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well-sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.
https://scholar.google.de/scholar?hl=de&as_sdt=0%2C5&q=zenk+On+the+Mediterranean+outflow+west+of+Gibraltar&btnG=
A buried mixed contourite-turbidite system has been identified in the Pliocene succession of the Algarve basin in the northern Gulf of Cadiz. This margin is currently dominated by the Mediterranean Outflow Water and associated contourite deposition. Analysis of seismic data along with careful geographical and oceanographic reconstructions of the margin show the gradual move from a turbidite-dominated towards a contourite-dominated margin, and the subsequent 'birth' of an impressive elongate mounded contourite drift system- the Faro-Albufeira drifts. The contourite drift can be distinguished from down-slope (turbiditic and mass transport deposits) based on the acoustic character, distribution analysis and through careful margin reconstruction. In the earliest Pliocene, Seismic Unit P1 has been interpreted as a dominant down-slope (most likely turbidite) system sourced mainly from the northeast. There is clear evidence of contourite reworking at Seismic Unit P2, where upslope progradation and a sheeted morphology are observed. High amplitude reflections are thought to be a result of more vigorous bottom currents in the early Upper Pliocene that were capable to redistributing coarser sediments. However, in the northeast of the study area a thick sequence of chaotic seismic facies has been interpreted as mass transport deposits sourced from the north indicate that the bottom currents were unable to dominate over the entire margin due to high down-slope clastic influx. Semi-transparent Seismic Unit P3 indicates that the Upper Pliocene initially experienced a reduction in bottom current intensity; however upslope progradation shows that a mixed system was maintained. Above the Base Quaternary Discontinuity (ca. 2.6 Ma), highly erosive discontinuities and high amplitude seismic reflections are evidence of pronounced intensification of the Mediterranean Outflow Water and a move to a fully contourite-dominated slope. Mixed turbidite-contourite systems such as the one identified in the Algarve Basin could provide impressive petroleum potential where downslope clastics are winnowed and reworked by bottom water currents to leave good reservoir properties. Here, we present a conceptual model for sheeted drifts as hydrocarbon reservoirs in the subsurface. (C) 2013 Published by Elsevier Ltd.
A point-by-point reply is here made to the discussion put forward by G. Shanmugam of our recent paper on internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China. We readily acknowledge that some issues raised by that author are pertinent. However, others are inconsequential, marginal, disputable, or redundant. The available evidence clearly supports our interpretation that the Middle Ordovician Xujiajuan Formation was deposited in a deep-water setting. Furthermore, we uphold that an internal-wave and internal-tide interpretation for these deposits is plausible in view of the occurrence of bidirectional cross-bedding and unidirectional cross-bedding dipping upslope in a deep-water environment. Also, we consider the request to provide conclusive evidence for the existence of a pycnocline in our stratigraphic record as unreasonable—it is self-evident that these data are currently not available, and the absence of proof is not proof of the contrary.
This chapter reviews the historical context and describes the different contourite facies recognized. The contourite facies models are outlined and the interpretation of contourite sequences is discussed. Some hybrid contourite facies are considered and criteria for the distinction between contourites and associated facies in deep-water systems are focused on. The problems and controversy surrounding the recognition of fossil contourites in the ancient records arealso described. The range of recognized contourite facies, including the facies groups, are tabulated. . Muddy contourites are homogeneous and appear as thick featureless units. Silty contourites have a larger silt-sized component and potential for revealing some internal structure. Gravel-rich contourites and gravel-bearing contourites are common in drifts at high latitudes as a result of input from ice-rafted material. The facies models for both muddy and sandy contourites are originally based on data from many examples of contourites. Composite contourite facies model showing grain-size variation through the standard mud-silt-sand contourite sequence, linked to variation in contour-current velocity is diagrammatically represented.
The Mediterranean outflow water (MOW) paleocirculation during the last 50,000 years has been inferred from the grain-size distribution of contourite beds in core MD99-2341 from the Gulf of Cadiz (Southern Iberian Margin–Atlantic Ocean). Three main contourite facies are described. Their vertical succession defines two contourite sequences that reveal past variations of the MOW bottom-current velocity. A comparison of contourite sequences and the planktonic δ18O record of core MD99-2341 with the δ18O record from Greenland Ice Core GISP2 show a close correlation of sea-surface water conditions and deep-sea contouritic sedimentation in the Gulf of Cadiz with Northern Hemisphere climate variability on millennial timescales. A high MOW velocity prevailed during Dansgaard-Oeschger stadials, Heinrich events and the Younger Dryas cold climatic interval. The MOW velocity was comparatively low during the warm Dansgaard-Oeschger interstadials, Bølling-Allerød and the Early Holocene. Rapid sea-level fluctuations on the order of 35 m during Marine Oxygen Isotope Stage 3 are considered to have exerted limiting controls on the MOW volume transport and thus positively modulated the MOW behaviour during the last 50 kyr.
This paper provides both an introduction to and summary for the Atlas of Contourite Systems that has been compiled as part of the International Geological Correlation Project - IGCP 432. Following the seminal works of George Wust on the physical oceanography of bottom currents, and Charley Hollister on contourite sediments, a series of significant advances have been made over the past few decades. While accepting that ideas and terms must remain flexible as our knowledge base continues to increase, we present a consensus view on terminology and definitions of bottom currents, contourites and drifts. Both thermohaline and wind-driven circulation, influenced by Coriolis Force and molded by topography, contribute to the oceanic system of bottom currents. These semi-permanent currents show significant variability in time and space, marked by periodic benthic storm events in areas of high surface kinetic energy.
Six different drift types are recognized in the ocean basins and margins at depths greater than about 300 m: (i) contourite sheet drifts; (ii) elongate mounded drifts; (iii) channel related drifts; (iv) confined drifts; (v) infill drifts; and (vi) modified drift-turbidite systems. In addition to this overall geometry, their chief seismic characteristics include: a uniform reflector pattern that reflects long-term stability, drift-wide erosional discontinuities caused by periodic changes in bottom current regime, and stacked broadly lenticular seismic depositional units showing oblique to downcurrent migration. At a smaller scale, a variety of seismic facies can be recognized that are here related to bottom current intensity. A model for seismic facies cyclicity (alternating transparent/reflector zones) is further elaborated, and linked to bottom current/climate change. Both erosional features and depositional bedforms are diagnostic of bottom current systems and velocities.
Many different contourite facies are now known to exist, encompassing all compositional types. We propose here a Cl-5 notation for the standard contourite facies sequence, which can be interpreted in terms of fluctuation in bottom current velocity and/or sediment supply. Several proxies can be utilized to decode contourite successions in terms of current fluctuation. Gravel lag and shale chip contourites, as well as erosional discontinuities are indicative of still greater velocities. There are a small but growing number of land-based examples of fossil contourites, based on careful analysis using the recommended three-stage approach to interpretation. Debate still surrounds the recognition and interpretation of bottom current reworked turbidites.
The northern margin of the Gulf of Cadiz is swept by Mediterranean Outflow Water between about 500 and 1000 m water depth. This warm, saline, thermohaline, bottom current attains velocities in excess of 1 m s-1 through the narrow and relatively shallow Gibraltar gateway, and then descends and slows as it moves towards the north and west around the Iberian margin. It was established in its present form in the latest Miocene, following tectonic re-opening of the Gibraltar gateway, and has since helped to sculpt the slope region in conjunction with downslope processes and diapiric intrusion. The principal area of contourite deposition, up to 600 m in thickness, is the Faro-Albufeira drift complex in a mid-slope setting some 30 km south of Faro. This comprises an elongate low-mounded drift (Faro-Albufeira) and adjacent broad sheeted drifts (Faro and Bartolomeu Dias Planaltos), flanked and partly dissected by deep, erosional, bottom-current channels and buried channels. The seismic character is one of progradational-aggradational depositional units with laterally extensive sub-parallel reflectors, widespread discontinuities and a large-scale cyclicity in seismic facies. The upper 10 m of cored section comprises muddy, silty and sandy contourites of mixed terrigenous and biogenic composition, that show small-scale cyclicity in grain size and associated sedimentary features. Rates of accumulation varied from < 1 to 14.5 cm ka-1 (cores), and 3.5 to 29.5 cm ka-1 (seismics). The large and small-scale cyclicity noted can be related to fluctuation in bottom current velocity related to climate and sea-level changes, although the precise correlation between these events remains uncertain.
Surficial sediment analyses, bottom photographs and current-meter data, coupled with studies of sidescan and 3.5 kHz echodata clearly show the imprint of a strong, seasonally-affected, slope current on the Hebrides shelf and slope. The present day shelf sediments are in fact relict, coarse-grained material (gravel with boulders) of Pleistocene glacial deri-vation, which have been modified during the Holocene by winnowing, sea-floor polishing and transport of the finer fraction across the shelf to the northwest. On the outer shelf and upper slope the sharp change from gravel to sand is marked by a physiographic-textural boundary, herein termed the sandline, which occurs at a depth of between 170 m and 300 m. Further down the continental margin on the lower slope, the lower limit of the sand-rich facies is marked by the mudline. This typically occurs at a depth of around 1200 m, and marks the depth of substantially increased clay-sized material. Above the mudline the long-term (Holocene to present), time-integrated signature of bottom current flow has resulted in the relatively slow accumulation of a mid-slope sandy contourite deposit, the Barra contourite sand sheet. This covers an area of 1000-1500 km2 with an estimated sand volume of 30 000 m3. Below the mudline the clay-rich deposits represent a hemipelagic drape with only minor bottom current influence and intense reworking by benthic organisms.
Deep-water reservoirs consisting of turbiditic sandstones moderately to heavily reworked by bottom currents are common in canyon- and trough-filling deep-water (bathyal) Palaeocene-Eocene sequences of the Campos Basin, offshore southeastern Brazil. A number of wells with conventional logs, together with cores, provided the database for the study. Seismic data provide additional support, but low resolution and noise hamper detailed analysis. The sandstones presenting better reservoir quality in these sequences are interpreted as being deposited by turbidity currents, as suggested by the dominance of unstratified normally graded sandstones, with grain sizes ranging from fine to coarse sand, and low clay-matrix content. Sandstones interpreted as bottom-current deposits (mid-water contourites) form poor-quality reservoirs, or baffles and barriers. These rocks are commonly moderately to heavily bioturbated, with variable, frequently high, clay-matrix content. Common trace fossils include Planolites, Palaeophycus and Zoophycos. Locally, these sandstones show faint horizontal stratification and planar cross-stratification. Contourites with thickness ranging from a few decimetres to several metres occur intercalated with turbiditic sandstones. Because they present distinct reservoir qualities, the mapping of the limits between turbidites and contourites is critical for adequate reservoir characterization. Most of this mapping has been performed using well information, constrained by outcrop analogues. The currents responsible for reworking turbiditic sands are interpreted to be deviated geostrophic currents, with velocity enhanced in narrow canyons and troughs.
The Faro Drift is an elongate sediment body (50 km long, 300 m thick) that parallels the northern margin of the Gulf of Cadiz south of Portugal. On the basis of location, morphology and seismic character of the drift together with bottom photographs, sediment distribution and the known regional oceanography, we can be certain that the Faro Drift was constructed by bottom currents related to the deep outflow of Mediterranean water. Detailed study of a closely-spaced suite of cores has therefore allowed characterization of the sediments into three contourite facies: sands and silts, mottled silts and muds, and homogeneous muds. The first is equivalent to the sandy contourites and the two others to the muddy contourites of earlier studies. These three facies are arranged in irregular vertical 'sequences' that reflect long-term variations in bottom current velocity. They are distinctly different from typical fine-grained turbidite and hemipelagite facies.
A wide variety of bedforms, both depositional and erosional in origin, has been recognized on the deep seafloor and attributed to the influence of bottom currents. These range in scale from those visible in bottom photographs (centimeter to decimeter), to those recorded with seafloor bathymetric imaging (meter to kilometer). In many cases it has been possible to provide some quantifi cation of substrate grain size and flow velocity responsible for each bedform type. We have synthesized both our own and published data in order to present a bedform-velocity matrix, which facilitates the estimation of bottom current velocity based on bedform type. Despite imperfections, we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables, pipelines, and other seafloor installations.
Internal wave activity and induced mixing phenomena in the Portimao Canyon area (southern Portuguese coast) are analysed. Observations of current velocities and CTD measurements were taken during a 24-h period at two fixed stations located along the main axis of the canyon. The station nearest shore was located close to the canyon head and the other station was 5km seaward. These basic measurements are complemented with vertical sections of temperature, acquired by XBT casts, along the main axis of the canyon, and also with time series of temperatures recorded by a thermistor chain moored on the continental shelf, near the canyon head. The time sequences of velocities and density profiles recorded at the fixed stations were analysed using a procedure based on empirical orthogonal function (EOF) analysis and dynamical modes decomposition (DMD) techniques. By this method, two different sources for internal wave activity are identified: a clear internal tide signal on the one hand, and a shorter than tidal period internal waves which exhibit greater current velocities and density oscillation amplitudes than internal tide, on the other. It is suggested that these shorter period internal waves are responsible for the vertical mixing affecting the water column over the slope and shelf waters in this region. Finally, it is also suggested that the evacuation of these mixed water masses formed around the continental shelf break may be related to a core of relatively cold water flowing over the continental slope toward west.
Measurements of currents, temperatures and salinities were made in March 1970 and 1971 on the north side of the Gulf of Cadiz in the Mediterranean undercurrent. Observations were concentrated in two areas. In the first the undercurrent is free from the sea bed and flows as a free jet although in contact with the continental slope on its northerly side, whilst in the second it is in direct contact with the sea bed and influenced by the topography of a submarine valley. These measurements demonstrate the mesoscale variability of the undercurrent, and the interruption of the flow in the first area by the arrival of denser near-bottom water and an eddy-like motion in the overlying water.
Records of benthic foraminiferal assemblage variations and benthic δ13C along 12 sediment cores from the western Iberian Margin, between 36° and 42°N at water depths from 820 to 3580m, are used to monitor fluctuations of the Mediterranean Outflow Water (MOW) during the past 30ka. The chronostratigraphy of the cores is based on planktonic δ18O records, 14C AMS-dating, and the recognition of Heinrich Events H1 through H4. Increased abundances of suspension feeding benthic foraminifers, denoted as ’Epibenthos Group‘, closely match areas where the recent MOW core layers impinge on the continental slope at 800 and 1300m water depth, and near-bottom current velocities are enhanced. Elevated ‘Epibenthos Group’ abundances, increased benthic δ13C, and sedimentological evidence for winnowing and erosion are found in glacial sections up to the earliest Termination I in cores at water depths between 1600 and 2200m off southern Portugal. The combined evidence reveals enhanced current activity at these depths due to a deep glacial MOW. The MOW advection at the Portuguese margin during the last Glacial was about 700m deeper than today, conceivably forced by increased MOW density due to higher salinity and colder temperatures of Mediterranean waters. The deep MOW current gradually decreased in strength and shoaled to 1300m water depth during the Termination and early Holocene. A shallow MOW core layer became active with the onset of Termination I at depths between 600 and 1000m. Both the shallow and deep MOW current culminated during the Younger Dryas period. The present flow pattern with two MOW core layers centred at 800 and 1300m water depth was established between 7.5 and 5.5ka.
The Pliocene—Quaternary pattern of contourite deposits on the eastern Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the Mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late Miocene opening of the Strait of Gibraltar. Coincident with the northwestward decrease in undercurrent speeds from the Strait there is the following northwestward gradation of sediment facies associations: (1) upper slope facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium-fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. Compared to this, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Further northwestward, sediment drift grades to biogenous silt near the Faro Drift at the Portuguese border. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean undercurrent, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies.
This paper provides for the first time a detailed vertical and spatial representation of Quaternary evolution of the contourite depositional system (CDS) in the Gulf of Cadiz, based on the results of careful morphological, structural and stratigraphic analyses using high-resolution seismic reflection profiles as well as oil company borehole data, and piston and gravity cores. Different drifts observed on the stratigraphic architecture allow us to propose a regional Quaternary evolution for the whole system, in which three major stages can be identified. (1) In the Early Pleistocene to Mid-Pleistocene, the CDS was mainly dominated by depositional processes, where the upper and lower cores of the Mediterranean Outflow Water (MOW) generated the mounded elongated Cadiz-Faro-Albufeira drift in the transition between the middle and upper slope, and the equivalent Huelva-Guadalquivir drift on the middle slope. During this stage the main erosive features were established close to the Strait of Gibraltar. (2) In the Mid-Pleistocene to Late Pleistocene, two important changes in the CDS took place. One occurred at the transition between the middle and upper slope, related to a change in the upper branch of the MOW, when a mixed drift began to develop, burying the eastern part of the Cadiz-Faro-Albufeira mounded elongated and separated drift. The second change is observed on the central area of the middle slope, related to the lower branch of the MOW, where a large contourite channel (the Guadalquivir channel) progressively eroded the western part of the mounded Huelva-Guadalquivir drift. Laterally an extensive sheeted drift buried the previous mounded deposits. (3) In the Late Pleistocene to Holocene, in the northern area of the CDS, a plastered drift started to be developed in the transitional zone between the upper and middle slope. On the middle slope, the mounded elongated Huelva-Guadalquivir drift was not developed and more erosive processes became dominant as the lower core of the MOW intensified. In the sector close to the Straits of Gibraltar, a field of broad seabed forms was generated. These three evolutionary stages have been controlled by tectonics, including recent diapiric movement, Guadalquivir Bank uplift, and reactivation along several fault systems and anticline-syncline structures. Tectonics has been a key factor in the sea-floor morphological changes, which has caused new pathways for the core and branches of the MOW, and consequently has produced the contourite stratigraphic and architectural changes. Superimposed on these tectonic changes, both climatic and eustatic changes during the Quaternary (but especially from the Mid-Pleistocene) have controlled the development of vertical contourite stratigraphy. The general conclusion of this study is that the contourite depositional system of the Gulf of Cadiz has changed from a dominantly depositional system to a dominantly erosive one during the Quaternary.
High-quality 2D and 3D seismic data were analysed to investigate the stratigraphic organization of the sedimentary deposits and the impact of the palaeocirculation of the SW Atlantic Ocean in the construction of the Santos Basin slope from late Palaeogene to Recent time. Seven seismic sequences were identified based on their external geometry, internal seismic pattern and seismic boundaries. A correlation between these sequences, the glacio-eustatic curves and the major climatic-palaeoceanographic events was attempted. The base of the studied sequences is the c. 28 Ma intra-Oligocene Rupelian-Chatian unconformity. Six other seismic horizons corresponding to the sequence boundaries were mapped ranging from the late Oligocene to the Pliocene. Variations in the bottom-current intensity were characterized by the seismic pattern of each sequence and by the evolution of margin physiography. A 100 km long channel-like gutter, the Santos Channel, was excavated at the foot of an intra-slope escarpment. It was the locus of major flow circulation until the middle Miocene, when the margin physiography reorganization transferred the main axis of deep current action downslope and excavated the São Paulo Channel at the foot of the present continental slope. Two major contourite drifts were accumulated in response to the different combinations of bottom-current axis position and slope physiography. Those drifts constitute the Santos Drift System. Conversely to the present-day circulation pattern, with southward flow above the slope dominated by the western boundary Brazil Current, both at surface and deep waters, the geological record indicates that the palaeocirculation in the Santos Basin was marked by the opposite sense of circulation of surface waters (southward palaeo Brazil Current) and of the intermediate to deep waters (northward Southern Ocean Current). The study indicates that periods of relative sea-level rise to highstands correspond to increase in drift accumulation whereas during lowstands slope drift sedimentation is reduced.
Mediterranean Water contributes significantly to the hydrological properties and circulation of the intermediate layers of the North Atlantic, yet the temporal variability of the Mediterranean Undercurrent is still not well understood, even at intra-annual time scales. The present study addresses the evolution of the temperature field in the layers of Mediterranean Water off south Portugal during one year (July 1993–July 1994), obtained with an almost weekly repetition of a high-resolution expendable bathythermograph (XBT) section in the vicinity of Portimão Canyon, during the AMUSE experiment. The information obtained with this set of data was complemented by velocity observations of the Mediterranean Undercurrent with RAFOS floats launched simultaneously with the XBT lines. The typical structure of the Mediterranean Undercurrent along the slope of the Gulf of Cadiz, consisting of a westward jet associated with the cores of the thermohaline structure, was observed in almost 60% of the 24 repetitions of the XBT section. The results show that this typical structure in the velocity was not always accompanied by a typical thermal structure. Anomalous behaviour of the Undercurrent – southward or southwestward flow or local recirculation – occurred only in winter and corresponded to some anomalies in the thermal field associated with the Mediterranean Water. The occurrence of warm features offshore of and separated from the main body of warm water found against the slope off south Portugal seemed to be connected with episodes of changes in direction and speed of the Undercurrent. Several scales of time variability within the Mediterranean Water layers were identified ranging from a week to a month. A possible seasonal trend in the temperature of these layers was apparent in the XBT data and seemed also to be found in time series obtained with moored current meters.
The Gulf of Cadiz is situated geologically at the Gibraltar Arc, the westernmost arc of the Alpine-Himalayan orogenic belt. Based on extensive previous studies that include swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism, heat flow probes, and underwater photography surveys, more than 500 polymetallic nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived carbonate chimneys, slabs, and crusts. Nodules show a wide range of sizes, densities, weights and morphologies. Nodules are composed of multiple millimetre-thick layers of Fe and Mn oxyhydroxides surrounding a nucleus composed of Early-Middle Miocene plastic marls, which were expulsed from underlying units by fluid venting. Nodules show a high mean abundance of Fe (39.03%), moderate Mn (5.84%), and low contents of trace metals and REEs compared to the average content of deep-sea polymetallic nodules. They display fast growth rates (av. 2,500 mm Myr-1) which are probably the main cause for the low contents of transition metals. The oxide layers contain both bacterial-derived hydrocarbons and aromatic hydrocarbons such as phenanthrene, characteristic of mature hydrocarbons. We propose both diagenetic and hydrogenous processes for nodule, beneath and on the seabed, as consequence of alternating episodes of burial and exhumation. Diagenetic processes beneath the seabed are fuelled by deep-seated hydrocarbon seeps probably through microbial-mediated anaerobic oxidation of hydrocarbons. On the other hand, hydrogenous nodule growth on the seafloor is controlled by mineral precipitation from the Mediterranean Outflow Waters.
During the 2000 and 2001 R/V Cornide de Saavedra cruises a vast field of hydrocarbon-derived carbonate chimneys was discovered and sampled along the Cádiz Contourite Channel and Guadalquivir Diapiric Ridge. Photographs taken on board using an underwater camera revealed a spectacularly high density of pipe-like chimneys lying on the sea floor, though some of them were also found protruding from muddy sediment. The major axes of the fallen chimneys, which can be as long as 1m, exhibit a regular spatial distribution showing a conspicuous NW-SE trend. Furthermore, their basal morphology shows a characteristic angular breakage associated to a flexo-traction process. Both the uniform distribution of the major axis and the basal morphology of the chimneys imply that the cause of the rupture could be related either to strong bottom sea currents or to inertia forces associated with oscillatory ground motion due to seismic shaking. The preferential orientation of the broken chimneys, their mechanical properties, the geological and topographical conditions of the area in relation to seismic ground motion amplification and the flow speed of the Mediterranean Outflow Water (MOW) along the Cádiz Contourite Channel, suggest that the most probable cause of the chimneys’ rupture was an earthquake. The minimum magnitude of this seismic event is estimated to be more than 7.0, the causative fault being located directly under the chimney field or as far as 30–60km away. The chimneys’ rupture took place between ca 5ka to Present.
Extensive mud volcanism, mud diapirism and carbonate chimneys related to hydrocarbon-rich fluid venting are observed throughout the Spanish–Portuguese margin of the Gulf of Cádiz. All the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome/accretionary complex units which were emplaced in the Late Miocene in response to NW-directed convergence between the African and Eurasian plates. The study area was investigated by multibeam echo-sounder, high and ultra-high resolution seismic profiling, dredging and coring. The structures observed on multibeam bathymetry, at water depths between 500 and 1300 m, are dominated by elongate mud ridges, mud cones, mud volcanoes and crater-like collapse structures ranging in relief from 50 to 300 m and size from 0.8 to 2 km in diameter. The main morphotectonic features, named the Guadalquivir Diapiric Ridge (GDR) and the Cádiz Diapiric Ridge (CDR), are longitudinally shaped diapirs which trend NE–SW and consist of lower–middle Miocene plastic marly clays. The GDR field and the TASYO field, which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development, are described in this paper. The GDR field is characterised by numerous, single, sub-circular mud volcanoes and mud cones. The single mud volcanoes are cone-shaped features with relatively gentle slopes of 3°–6°, consisting of several generations of mud breccia deposition with indications of gas-saturation, degassing structures and the presence of H2S. The mud cones have asymmetric profiles with steep slopes of up to 25° and contain large surficial deposits of hydrocarbon-derived carbonate chimneys and slabs. The TASYO field is characterised by an extensive concentration of small, sub-circular depressions, oval and multi-cone mud volcanoes and large sediment slides. Mud volcanoes in this area are characterised by moderate slopes (8°–12°), have bathymetric relief ranging from 100 to 190 m and consist of sulphide-rich mud breccia, calcite chimneys, carbonate crusts and chemosynthetic fauna (Pogonophora tube worms). We propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic/accretionary wedge. This stress results in the uplifting and squeezing plastic marly clay deposits. Additionally, the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon-enriched fluids to migrate.
High-resolution grain size and magnetic susceptibility records from the eastern Gulf of Cadiz (site MD99-2339; 1170m water depth) reveal contourites formed by the Mediterranean Outflow (MOW) during the last 47kyr BP. Oscillations in the MOW's intensity occurred in phase with Greenland temperature variations with a stronger outflow during northern hemisphere coolings such as Dansgaard-Oeschger stadials, Heinrich events, and the Younger Dryas. Benthic δ13C data implies the Western Mediterranean Deep Water as one of the main sources feeding the outflow current, while differential changes in the properties of
the Mediterranean source and entrained North Atlantic Central Water largely control the MOW's strength. Detailed studies for Heinrich events 1, 4 and 5 show that the flow strength peaked only when subtropical surface waters prevailed in the eastern Gulf of Cadiz, while incursions of icebergs and subpolar surface water were not favorable for the MOW's intensification. As the MOW was strong when the Atlantic's thermohaline circulation (THC) was weakened, the heat and salt injected by the MOW into the intermediate North Atlantic waters might have preconditioned the THC to switch from the stadial to the interstadial mode.
Internal waves occur nearly ubiquitously in lakes and oceans yet their sedimentary records remain largely unrecognized. Waves propagate at the interface between fluids of different densities. Surface waves propagate at the interface between air and water, which is a strong density gradient. Internal waves propagate along weaker gradients (pycnoclines) within density-stratified fluids, behaving similarly to surface waves but typically at lower frequencies and larger amplitudes. Internal waves that occur at tidal frequencies are called internal tides; they are very common on the outer continental shelf and slope, and are generated as the surface tides move stratified water up and down a sloping surface. Large internal solitary waves known as solitons are ubiquitous wherever strong currents and stratification occur in the vicinity of irregular topography. These waves can force short-period, strong bottom-current pulses and may trigger upslope-surging vortex cores of dense fluid (boluses) that can induce mobilization of bottom sediments.
The north-east Atlantic continental margin displays a wide range of sediment transport systems with both along-slope and down-slope processes. Off most of the north-west African margin, south of 26°N, upwelling produces elevated accumulation rates, although there is little fluvial input. This area is subject to infrequent but large-scale mass movements, giving rise to debris flows and turbidity currents. The turbidity currents traverse the slope and deposit thick layers on the abyssal plains, while debris flows deposit on the continental slope and rise. From the Atlas Mountains northwards to 56°N, the margin is less prone to mass movements, but is cut by a large number of canyons, which also funnel turbidity currents to the abyssal plains. The presence of a lithospheric plate boundary off SW Iberia is believed to have led to high rates of sediment transport to the deep sea. Even larger quantities of coarse sediments have fed the canyons and abyssal plains in the Bay of Biscay as a result of drainage from melting icecaps. Bottom currents have built sediment waves off the African and Iberian margins, and created erosional furrows south of the Canaries. The Mediterranean outflow is a particularly strong bottom current near the Straits of Gibraltar, depositing sand waves and mud waves in the Gulf of Cadiz. North of 56°N, the margin is heavily influenced by glacial and glaciomarine processes active during glacial times, which built glacial trough-mouth fans, such as the North Sea Fan, and left iceberg scour marks on the upper slope and shelf. Over a long period, especially during interglacials, this part of the margin has been greatly affected by along-slope currents, with less effect by turbidity currents than on the lower latitude margins. Large-scale mass movements are again a prominent feature, particularly off Norway and the Faeroes. Some of these mass movements have occurred during the Holocene, although high glacial sedimentation rates may have contributed to the instability.