Article
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The role of cohesive mud has been shown to be effective in turbulence dampening and flow transformation of sediment gravity flows (Baas et al., 2009(Baas et al., , 2011(Baas et al., , 2016aSumner et al., 2009;Baker & Baas, 2020). Sedimentologists are often uncertain as to whether the cohesive mud was from original flow (fluvial input) or muddy substrate erosion along the submarine routing system (Shan et al., 2019a(Shan et al., , 2019bPatacci et al., 2020). Because organic matter is prone to be absorbed by fine-grained sediments through chemical binding or physical sealing (Mayer, 1993;1994;Keil et al., 1997;Gordon & Goñi, 2004), the organic carbon amount and cohesive mud content show a good linear relationship (R 2 = 0.70, Fig. 8C). ...
... Submarine failure might be initiated by earthquakes, overloading, sea-level change and cyclic wave loading (Girardclos et al., 2007;Shan et al., 2019aShan et al., , 2019b. The 1929 Grand Banks earthquakes generated submarine failure, which in turn triggered very thick debris flows (high yield strength debris flow) (Piper et al., 1999). ...
Article
Full-text available
Hyperpycnal flows are important agents for transporting detrital sediments from rivers to oceans. Previous studies often assumed that the deposits of flood-controlled delta fronts would be dominated by graded sand with common hummocky cross stratification. This study documents, for the first time, hybrid event beds (HEBs) and plumite deposits inside the succession of a modern delta front. This delta in the Taiwan Strait is fed by the Choshui River, which is considered a highly efficient sediment transport system with individual floods with extremely high suspended sediment concentrations. The deltaic event beds recorded in the core were interpreted as hyperpycnal flow-generated HEBs and turbidites triggered by hypopycnal flows. AMS 14C dating, grain size analysis and measurements of stable isotopic composition of organic matter were conducted to delineate the depositional process of the recognized beds. The internal subdivisions of the HEBs were differentiated mainly based on sedimentary textures, including cohesive mud content, sand content and sorting. The disorganized portion (H3) appears internally chaotic and contains large rafted substrate clasts but also displays an upward increase in dispersed cohesive mud from 35% (H3a) to 50% (H3b). In contrast, massive H1 divisions are characterized by much lower cohesive mud of ca. 8%. The vertical arrangement between depositional facies allows the discrimination of three HEBs types. The stable carbon isotopic composition of the organic matter reveals that the cohesive mud in each division of the HEBs was sourced from marine substrate, rather than supplied by the original hyperpycnal flows. Therefore, the HEBs are generated by energetic hyperpycnal flows, which can delaminate the muddy sea-floor and incorporate large quantities of substrate fragments. The bulking of erosional hyperpycnal turbulent flow forces flow transformation and generates a more cohesive flow with a turbulent, dense head and trailing bipartite debris flow. The occurrence of HEBs in modern shallow delta front settings is also an important novelty, implying a high density of hyperpycnal flows at the mouth of sediment-laden rivers as well as the crucial impact of substrate entrainment on the development of HEBs.
... In theory, seafloor instability triggers soft sediment redeposition as MTDs, and if the redeposition is fully developed, they ultimately may evolve into turbidity flow (Shan et al., 2019). Compared with MTDs, turbidites are distributed more broadly within marine environments, and this is more likely to control the distribution of GH, as indicated by field exploration findings. ...
Article
Fine-grained gas hydrate (GH) reservoirs are extensively studied worldwide, among which the Shenhu Area (located on the northern slope of the South China Sea) is a world-class GH exploration area. However, the lithology, physical properties, and depositional origins of the fine-grained GH reservoirs are not well known. Understanding how sediment grain-size parameters affect the fine-grained GH reservoir quality could provide and important breakthrough for reservoir evaluation. Eight cores, recovered from various expeditions of the Guangzhou Marine Geological Survey, can be combined with 2D/3D seismic data to provide a rare opportunity to systematically investigate the grain-size characteristics of the GH reservoir, as well as the surrounding sediments. A combination of lithology, grain size characteristics (mean size, sorting, skewness, kurtosis), high-resolution seismic features, and associated bivariate and cluster analysis results support the identification of two distinct intervals of fine-grained sediments that were deposited by different sedimentary processes. There is a relatively higher content of coarser silt in the lower interval than in the upper interval, and their boundary depths are highly consistent with those of the GH-bearing layer and the overlying non-GH-bearing layer. With respect to the unconsolidated GH-bearing sediments from Well G, both the porosity (52%–64%) and sorting coefficient (1.68–2.2) have limited variation, while high GH saturation (>30%) occurs at the top layer. The positive correlation between saturation and the coarsest one-percentile grain size (R=0.55) reveals that an increase in the coarse fraction/particle size favours the development of a larger pore throat diameter and improves the initial permeability and reservoir properties. The seismic features and cross-plots of the coarsest one-percentile and median values indicate that the lower thin-bedded fine-grained sediments with hydrate may be fine-grained turbidite complexes, including channels/levees/lobes and mass transport deposits. If this is the case, then it may be inferred that turbidite sediments provide good reservoir physical properties, favourable for GH formation and accumulation. These insights into the relationship between the Quaternary fine-grained turbidites and GH saturation may promote a clearer understanding of the characteristics and development of fine-grained GH reservoirs globally, including in the Shenhu Area of the South China Sea.
Article
Deposits of sediment gravity flows in the Aberystwyth Grits Group (Silurian, west Wales, United Kingdom) display evidence that sole marks are suitable for reconstructing depositional processes and environments in deep-marine sedimentary successions. Based on drone imagery, 3D laser scanning, high-resolution sedimentary logging, and detailed descriptions of sole marks, an outcrop 1600 m long between the villages of Aberarth and Llannon was subdivided into seven lithological units, representing: a) mudstone-poor, coarse-grained and thick-bedded submarine channel fills, dominated by the deposits of erosive high-density turbidity currents with flute marks; b) mudstone-rich levee deposits with thin-bedded, fine-grained sandstones formed by low-density turbidity currents that scoured the bed to form flute marks; c) channel–lobe transition-zone deposits, dominated by thick beds, formed by weakly erosive, coarse-grained hybrid events, with pronounced mudstone-rich or sandstone-dominated debritic divisions and groove marks below basal turbiditic divisions, and with subordinate amounts of turbidites and debris-flow deposits; d) tabular, medium- to thick-bedded turbiditic sandstones with flute marks and mixed sandstone–mudstone hybrid event beds mainly with groove marks, interpreted as submarine lobe-axis (or off-axis) deposits; and e) tabular, thin- to medium-bedded, fine-grained, mainly turbiditic sandstones mostly with flute marks, formed in a lobe-fringe environment. Both lobe environments also comprised turbidites with low-amplitude bed waves and large ripples, which are interpreted to represent transient-turbulent flows. The strong relationship between flute marks and turbidites agrees with earlier predictions that turbulent shear flows are essential for the formation of flute marks. Moreover, the observation as part of this study that debris-flow deposits are exclusively associated with groove marks signifies that clay-charged, laminar flows are carriers for tools that are in continuous contact with the bed. A new process model for hybrid event beds, informed by the dominance of tool marks, in particular grooves, below the basal sand division (H1 division of Haughton et al. 2009) and by the rapid change from turbidites in the channel to hybrid event beds in the channel–lobe transition zone, is proposed. This model incorporates profound erosion of clay in the channel by the head of a high-density turbidity current and subsequent transformation of the head into a debris flow following rapid lateral flow expansion at the mouth of the channel. This debris flow forms the groove marks below the H1 division in hybrid event beds. A temporal increase in cohesivity in the body of the hybrid event is used to explain the generation of the H1, H2, and H3 divisions (sensuHaughton et al. 2009) on top of the groove surfaces, involving a combination of longitudinal segregation of bedload and vertical segregation of suspension load. This study thus demonstrates that sole marks can be an integral part of sedimentological studies at different scales, well beyond their traditional use as indicators of paleoflow direction or orientation.
Article
A number of open-air Paleolithic sites have been found and excavated in South China, and are characterized by Red Clay deposits. A detailed knowledge of the Red Clays is crucial for better understanding the archaeological significance of the sites and the paleoenvironments during hominin occupation. However, the origin and provenance of the Red Clays is debatable. Here, three sediment sequences from two Paleolithic sites (Wuyashan and Sandinggai) on fluvial terraces in north Hunan Province, China, were investigated using grain-size analysis and end-member modeling. The spatial distribution and grain-size characteristics of the Red Clays reveal their aeolian origin. The end-member modeling demonstrates that the Red Clays consist of two end-members (EM1 and EM2 with mode sizes of 5.6 and 11.2 µm, respectively). The south-eastward-fining trend indicates that the EM1 component (fine dust) was transported from North China. We deduce that the EM2 component (coarse dust) is mainly from the nearby desiccated fluvial systems of the Yangtze River. The change in grain size with depth in the Red Clay sections may be caused by chemical weathering. The aeolian origin of the Red Clays may imply that the highlands occupied by hominins in the study region were not inundated with floods.
Article
Full-text available
Submarine or sub-lacustrine lobe deposits are important reservoirs, but the fan fringe deposits form heterogeneities within deep water fan deposits. Fan fringe facies records the complex sediment gravity flow types. By understanding of the bed types and flow mechanisms, we can identify the fan fringe deposit, which aids in the reconstruction of deep water fan and reservoir evaluations. The Jiucaiyuanzi and Dalongkou sections in the West Bogda Mountains preserve well-exposed 536-m and 171-m thick successions, respectively, of a deep water lacustrine depositional system from the Middle Permian Lucaogou Formation. Bed types of the Lucaogou Formation include high-density turbidite, low-density turbidite, incomplete Bouma-type turbidite, hybrid event beds, and slump deposits. The Lucaogou Formation is interpreted here as a fan fringe facies due to the thin bed thickness that characterize turbidites and hybrid event beds, as well as the predominance of the isolated sheet architecture. Previous studies suggest that these deposits were considered as deposited in a deep water setting due to the absence of wave-related structures. The presence of abundant mud clasts in massive medium-coarse grained sandstone beds reflects the significant erosional capability and interactions between high-density turbidity currents and lake floor. The fan fringe facies here contains amalgamated and thick-bedded homolithic facies (~ 30%) and thin-bedded heterolithic facies (~ 70%). The examination of the bed type is of wider significance for facies prediction and reservoir heterogeneity in the sub-lacustrine fan fringe facies.
Article
Full-text available
Submarine or sub-lacustrine lobe deposits are important reservoirs, but the fan fringe deposits form heterogeneities within deep water fan deposits. The identification of fan fringe deposits can aid in the reconstruction of deep water fan and reservoir evaluations. Fan fringe facies records the complex sediment gravity flow types. By understanding of the bed types and flow mechanisms, we can identify the fan fringe deposit. The Jiucaiyuanzi and Dalonggou sections in the West Bogda Mountains preserve well-exposed 536-m and 171-m thick successions, respectively, of a deep water lacustrine depositional system from the Middle Permian Lucaogou Formation. Bed types of the Lucaogou Formation include high-density turbidite, low-density turbidite, incomplete Bouma-type turbidite, hybrid event beds, and slump deposits. The Lucaogou Formation is interpreted here as a fan fringe facies due to the thin bed thickness that characterize turbidites and hybrid event beds, as well as the predominance of the isolated sheet architecture. Previous studies suggest that these deposits were considered as deposited in a deep water setting due to the absence of wave-related structures. The presence of abundant mud clasts in massive medium-coarse grained sandstone beds reflects the significant erosional capability and interactions between high-density turbidity currents and lake floor. The fan fringe facies here contains amalgamated and thick-bedded homolithic facies (~30%) and thin-bedded heterolithic facies (~70%). The fan fringe facies forms heterogeneities. The examination of the bed type is of wider significance for facies prediction and reservoir heterogeneity in the sub-lacustrine fan fringe facies.
Article
Full-text available
The East China Sea Shelf has an unusually wide and low gradient shelf, supplied from sediment‐charged rivers and large river delta systems, with bottom currents sweeping the sea floor and located in the path of strong typhoons. Sediment gravity flow deposits, including four hybrid event beds and a high density turbidite, are identified in a core from the mid‐shelf of the East China Sea. The hybrid event beds typically comprise three or two internal divisions from the base to the top: (i) H1, H3 and H5; or (ii) H3 and H5. Radiocarbon ages of the hybrid event beds were in the range of 3821 to 8526 yr BP. Based on correlation with surrounding cores, the hybrid events may have happened at any time between 1930 yr and 3890 yr BP. The δ13C values in hybrid event beds together with bathymetry data suggest local erosion on the shelf. The average δ13C value for the H1 division is similar to the H3 division in the hybrid event beds, implying that the organic matter in the H1 and H3 divisions may come from the same source area. Cross‐plots of (La/Lu)UCC versus (La/Y)UCC, (La/Y)UCC versus (Gd/Lu)UCC, (La/Yb)UCC versus (Gd/Yb)UCC and (La/Yb)UCC versus (Sm/Nd)UCC of the five units reveal that the sediment source of the four hybrid event beds and a turbidite was ultimately primarily from Korean rivers. Partial transformation from a moderate‐strength debris flow with the additional role of erosional bulking can explain occurrences of hybrid event beds on the East China Sea Shelf. The data indicate that hybrid sediment gravity flow deposits were sourced from intra‐shelf failures and subsequently transformed and deposited as hybrid event beds. The study shows that hybrid sediment gravity flows and turbidity currents may not necessarily indicate proximity to a major fluvial or deltaic system and that intra‐shelf sedimentation can be a sediment source. It is unlikely that the debris flows and turbidity currents were triggered by a hyperpycnal flow or tsunami, because both can carry continental and/or coastal signals which have not been recognized in the core. Typhoons are the probable triggering mechanism. This article is protected by copyright. All rights reserved.
Conference Paper
Full-text available
The accumulation of organic-rich mudstones was largely associated to low energy depositional environments with anoxic bottom waters, where mudstone deposition was mainly related to gradual and continuous mud fallout from dilute buoyant plumes. Based on this, basin inner zones were considered as unsuitable environments for source rock deposition due to the generally low concentration of OM resulting from both low inputs and production, adding an intense OM degradation during its transit time through the water column. Nevertheless, recent detailed sedimentological analyses in a variety of unconventional shales have revealed that the participation of fallout processes is probably subordinated to other still poorly known depositional processes, opening a new paradigm for source rocks origin. In Argentina, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation is composed of organic-rich mudstones and carbonates dominated by type II kerogen representing South America's main unconventional reservoir. New cores and excellent outcrops provide a great opportunity to study the depositional history of this unit. These deposits were previously interpreted as accumulated by fallout deposition in a quiet and anoxic deep marine environment. However, recent studies revealed that the Vaca Muerta Formation is a highly heterogeneous stratigraphic unit accumulated by different and poorly know depositional processes. In fact, the formation displays distinct lithofacies alternating at centimeter to millimeter scale having variable organic matter content (up to 14% TOC), features that influence the reservoir quality and performance. High resolution sedimentological analysis were performed on relatively uncompacted intervals preserved in early diagenetic calcareous concretions collected from the basal deposits of the Vaca Muerta Formation in basinal settings. Evidences found in concretions suggest a deposition related to fluid mud flows instead of the classic model of "normal fallout". The triggered mechanisms for the origin of the recognized fluid mud flow deposits are mainly associated to direct river discharges during flood events. Each flood event would be capable of generating quasi-steady muddy hyperpycnal flows that may be sustained for days, weeks, or even months. These long-lasting events would be able to transfer significant volumes of organic matter and fine-grained sediments for long distances towards distal basinal settings. The erosion capacity of muddy hyperpycnal flows enables the incorporation of intrabasinal components (e.g. marine microfossils, carbonate mud, type II OM) which are transported together with the primary extrabasinal sedimentary load (e.g. detrital mud, micas, plant debris). The rapid and direct basinward transfer of OM by hyperpycnal flows would have avoided its dilution and degradation in coastal marine environments. Finally, the arrival of extinguishing hyperpycnal flows to the basin inner zones would have provided a fast deposition and burial of the OM, favoring its long term preservation. Therefore, muddy hyperpycnites would have a great potential for the accumulation of type II-III source rocks. The future understanding of the complexity of fluid mud flows and their internal stacking pattern will be crucial to identify long-term exploitable intervals in unconventional oil/gas plays.
Article
Full-text available
The erosion and deposition dynamics of fine sediment in a highly turbid estuarine channel were successfully surveyed during the period from August 29 to September 12, 2009 using an echo sounder in combination with a high-resolution acoustic Doppler current profiler. Field measurements were conducted focusing on the tide driven dynamics of suspended sediment concentration (SSC), and fluid mud at the upstream of the macrotidal Chikugo river estuary during semidiurnal and fortnightly tidal cycles. Morphological evolution was observed especially during the spring tide over a period of two weeks. The elevation of the channel bed was stable during neap tide, but it underwent fluctuations when the spring tide occurred owing to the increase in the velocity and shear stress. Two days of time lag were observed between the maximum SSC and peak tidal flow, which resulted in the asymmetry between neap-to-spring and spring-to-neap transitions. During the spring tide, a hysteresis loop was observed between shear stress and SSC, and its direction was different during flood and ebb tides. Although both fine sediments and flocs were dominant during flood tides, only fine sediments were noticed during ebb tides. Hence, the net elevation change in the bed was positive, and sedimentation took place during the semilunar tidal cycle. Finally, a bed of consolidated mud was deposited on the initial bed, and the height of the channel bed increased by 0.9 m during the two-week period. The observed hysteretic effect between shear stress and SSC during the spring tides, and the asymmetrical neap-spring-neap tidal cycle influenced the near-bed sediment dynamics of the channel, and led to the formation of a fluid mud layer at the bottom of the river.
Article
Full-text available
This paper reexamines the late Pleistocene to early Holocene transgressive succession (ca. 19.0-8.0 cal. k.y. B.P.) beneath the modern Changjiang delta plain, one of the world's great rivers, by means of a detailed process-oriented investigation of the sedimentary facies and stratigraphic architecture in two newly drilled cores (ZK01 and ZK02), supplemented by correlation with previous work and comparison with the modern deltaic deposits. Results suggest that the study interval, which has previously been considered to have accumulated in an estuary, instead represents a back-stepping delta. The target succession shows a general fining- and muddier-seaward trend for both the channel-bottom and adjacent tidal- or point-bar sediments, from coarse sand and gravels of the purely fluvial channel, through tide-influenced fluvial-channel deposits dominated by medium to coarse sand, to fine-grained sandy or muddy deposits of the (terminal) distributary channels, and finally to prodeltaic mud; the sand-dominated deposits that characterize transgressive estuary mouths are not present. The prodeltaic and delta-front deposits of the paleo- and modern Changjiang deltas display strong similarities, including the pervasive interbedding of tideand wave-generated fluid-mud deposits and muddy tidal-bar deposits, which are punctuated by terminal-distributary-channel facies that are not expected in an estuary. In more detail, three superimposed successions are recognized, each of which displays an upward-shallowing trend indicating a progradational nature, which is also indicated by the presence of seaward-dipping seismic reflectors in the correlative seaward part of this stratigraphic interval. The three successions show an overall retrogressive, fining-upward trend as a result of stepwise transgression like that seen in the Mississippi and Ganges- Brahmaputra River deltas. This architecture indicates that the large sediment supply of the Changjiang was able to keep pace with or exceed the rate of relative sea-level rise at certain times and locations during the later part of the postglacial transgression. We also propose that the maximum flooding surface occurred at ca. 9.0 cal. k.y. B.P., which is earlier than previously thought; this is also a reflection of the high rate of sediment input of the Changjiang, which causes a turnaround from net transgression to net progradation to occur when the rate of sea-level rise is higher. This study shows that we should not just simply interpret any transgressive succession in an incised valley as estuarine. This study provides significant new insights into the interpretation and sequence-stratigraphic reconstruction of ancient deltaic deposits, and it advances our understanding of the nature of tide-dominated delta successions.
Article
Full-text available
Hybrid event beds comprising both clean and mud-rich sandstone are important components of many deep-water systems and reflect the passage of turbulent sediment gravity flows with zones of clay-damped or suppressed turbulence. ‘Behind-outcrop’ cores from the Pennsylvanian deep-water Ross Sandstone Formation reveal hybrid event beds with a wide range of expression in terms of relative abundance, character and inferred origin. Muddy hybrid event beds first appear in the underlying Clare Shale Formation where they are interpreted as the distal runout of the wakes to flows which deposited most of their sand up-dip before transforming to fluid muds. These are overlain by unusually thick (up to 4.4 m), coarse sandy hybrid event beds (89% of the lowermost Ross Formation by thickness) that record deposition from outsized flows in which transformations were driven by both substrate entrainment in the body of the flow and clay fractionation in the wake. A switch to dominantly fine-grained sand was accompanied initially by the arrest of turbulence-damped, mud-rich flows with evidence for transitional flow conditions and thick fluid-mud caps. The mid and upper Ross Formation contain metre-scale bed sets of hybrid event beds (21 to 14%, respectively) in: (i) upward-sandying bed set associations immediately beneath amalgamated sheet or channel elements; (ii) stacked thick-bedded and thin-bedded hybrid event bed-dominated bed sets; (iii) associations of hybrid event bed-dominated bed sets alternating with conventional turbidites; and (iv) rare outsized hybrid event beds. Hybrid event bed dominance in the lower Ross Formation may reflect significant initial disequilibrium, a bias towards large volume flows in distal sectors of the basin, extensive mud-draped slopes and greater drop heights promoting erosion. Higher in the formation, hybrid event beds record local perturbations related to channel switching, lobe relocations and extension of channels across the fan surface. The Ross Sandstone Formation confirms that hybrid event beds can form in a variety of ways, even in the same system, and that different flow transformation mechanisms may operate even during the passage of a single flow. This article is protected by copyright. All rights reserved.
Article
Full-text available
The Tropical Cyclone Best Track Dataset issued by the China Meteorological Administration (CMA), for the period from 1949 to 2013, was analyzed, and the typhoons threatening the Jiangsu coast were categorized into three different classes in terms of track, including typhoons making straight landfall, typhoons active in offshore areas, and typhoons moving northward after landfall. On the basis of the 65-year dataset, the typhoon parameters of these three categories, including the central pressure and the maximum wind speed, were investigated. Statistical analysis suggested that the minimum central pressure increased northward and shoreward gradually. The relationship between the maximum wind speed and the central pressure was established through second-degree polynomial fitting. Considering typhoons No. 1210, No. 0012, and No. 9711 as the basic typhoons, ten hypothetical cyclones with typical tracks and minimum central pressure occurring during the period from 1949 to 2013 were designed, providing the driving conditions for numerical simulation of typhoon-induced storm surges along the Jiangsu coast.
Article
Full-text available
The origin of massive sands in turbidite successions has commonly been attributed to the rapid dumping of sand due to flow unsteadiness in collapsing, single surge-type, high-density turbidity currents. The general applicability of this model is questioned, and it is proposed that rapid deposition of massive sands also occurrs due to non-uniformity in prolonged, quasi-steady high-density turbidity currents. -from Authors
Article
Full-text available
The identification of "organoporosity" (microscale and nanoscale pores within organic matter in shales), its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering three-dimensional images of the pores, such as by argon-ion milling and/or field emission scanning electron microscopy, have all been well documented and discussed for unconventional gas shales. However, other types of pores exist within shales that can be important to storage and migration of gas (and oil), and other technologies are available for their identification and imaging. The different pore types found in the Barnett and Woodford shales are described and classified in this article. Copyright © 2011. The American Association of Petroleum Geologists. All rights reserved.
Article
Full-text available
Recent studies of marine shelf sediment dispersal show that wave-enhanced sediment-gravity flows are widespread phenomena and can transport large volumes of fluid mud rapidly across low-gradient shelves. Flow evolution is controlled by sediment supply, seabed gradient, and spatial distribution of wave energy at the seabed. Using existing flow models, we predict that such flows in mud-dominated sediments will develop a three-part microstratigraphy produced by changing flow conditions, beginning with wave-induced turbulent resuspension, then development of a wave-enhanced sediment-gravity flow, prior to lutocline collapse and suspension settling. Petrographic examination of modern flow deposits collected from the Eel Shelf reveals that resultant beds possess a microstratigraphy consistent with our hypothesis: a silt-rich basal subunit with curved ripple laminae, abruptly overlain by a subunit composed of continuous intercalated silt/clay laminae, and an upper clay-rich drape. Analyses of beds from ancient mud-rich outer-shelf and basinal successions (Cleveland Ironstone, Jurassic, UK, and Mowry Shale, Cretaceous, United States) show that they too contain beds with this three-part organization, suggesting that such flows were active in these ancient settings too. Recognition of these microstructures in these ancient mud-dominated successions demonstrates that sediment in these settings was commonly reworked and transported advectively downslope by high-energy events, contrasting with previous interpretations of these units that deposition was dominated by quiescent suspension settling. Identification of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identified in other shale-dominated successions.
Article
Full-text available
Gravity-driven flows on the seafloor are the largest, yet least well understood, sediment transport agents on Earth. Recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types, many hundreds of kilometers from paleocoastlines. These sedimentary deposits often defy conventional turbidite or debrite interpretations, having a character suggestive of deposition from flows with transient turbulent-laminar rheologies. In the Wilcox Formation (Gulf of Mexico), inferred transitional flow deposits have distinctive stratigraphic stacking patterns, from fine-grained debrites to coarser grained turbidites. The vertical sequence of beds is here inferred to reflect the longitudinal bed distribution in response to lobe progradation, and demonstrates a transition from well-mixed turbulent flow, to progressively more rheologically stratified flow, and eventually to fully laminar flow. The progressive development of internal rheological boundaries resulted in a high-concentration but fluidal basal layer, and an upper quasi-laminar layer with an overriding sheared dilute turbidity current. The long runout of the flows is linked to their high silt and clay content; it is most likely flow expansion at the channel-lobe transition that drives flow transformation. This process-based model may be applicable to many deep-water settings and provides a framework within which to interpret the stratigraphic and spatial distribution of these complex deposits.
Article
Full-text available
Deep-water deposits consisting mainly of massive sand are commonly identified as deposits of turbidity currents (i.e., turbidites). Speculation has risen in recent years as to whether some of these massive sandy deposits could have instead been deposited by debris flows. This possibility is explored here by examining the flow mechanics of sand-rich subaqueous gravity flows by means of laboratory experiments. In these experiments, sandy gravity flows were generated when well-mixed slurries of sand, clay, and water were released into a tank filled with tap water and allowed to flow under gravity over a slope that declined from 4.6° to 0.0°. The observed flow mechanics and resulting depositional features were strongly tied to the "coherence" of the debris flows (i.e., the ability of the slurry to resist being eroded and broken apart by the shear and pressure undergone by the flow). Low water content and high clay content resulted in strongly coherent debris flows, whereas high water content and low clay content resulted in weakly coherent flows. As little as 0.7 to 5 wt% of bentonite clay or 7 to 25 wt% of kaolinite clay at water contents ranging from 25 to 40 wt% was required to generate coherent gravity flows. Weakly coherent and moderately coherent flows produced significant, low-concentration subsidiary turbidity currents, and their deposits developed coarse-tail grading, water-escape structures, and minor increases in thickness at the base of the slope. Strongly coherent debris flows commonly hydroplaned and generated only minor subsidiary turbidity currents. Their deposits were structureless and ungraded, commonly showing tension cracks, compression ridges, water-escape structures, detached slide blocks, and a significant increase in thickness at the base of the slope. Application of distorted geometric scaling suggests that many aspects of these experiments appropriately scale up to the field scale of natural submarine debris flows.
Article
Full-text available
Hydrographic data and water samples for suspended matter were taken in the distal mud area in the East China Sea in winter 1997 before a winter storm, right after the storm, and 14 days later. Based on the field hydrographic data and the concentrations of total suspended matter, the effect of the winter storm on the sediment dynamic processes in the mud area was studied. The results show that the tidal currents dominate the hydrodynamic regime in the study area under calm winter weather with certain stratification of the water column. The winter storm caused strong mixing of the water column and destroyed its stratified structure; however, the high concentration of total suspended sediment (TSM) did not appear simultaneously with the well-mixed water column after the storm, as we expected, but 12 hours later. The storm-induced high TSM in the mud area seems to lag in time the occurrence of the storm in the study area. Analysis indicates that the storm cannot resuspend the surface sediment at station 111, but it could resuspend the surface sediment at shallower water depth outside of the mud area and increase the TSM in waters, which can be carried out by the Yellow Sea Coastal Current and transported to the mud area at station 111. The high TSM observed during the winter stormy period on the shelf might not be caused by the local resuspension.
Article
Full-text available
The relationship between submarine sediment gravity flows and the character of their deposits is poorly understood. Annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment-laden flows. Decelerating fast (>3 m/s) flows with fixed sand content (10 vol%) and variable mud content (0-17 vol%) resulted in only four deposit types. Clean sand with a mud cap that resembled a turbidity current deposit (turbidite) formed if the flow was turbulent when deposition began, or if the muddy fluid had insufficient strength to suspend the sand. The clean sand could contain structures if mud content was low (<6%) and the deceleration period was >300 s. Ungraded muddy sand with a mud cap that resembled a debris-flow deposit (debrite) formed if the flow became laminar before sand could deposit. Clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late-stage settling of sand from a muddy suspension. These deposits resemble enigmatic submarine flow deposits called linked debrite-turbidites. The experiments provide a basis for inferring flow type from deposit character for submarine sediment-laden flows.
Article
Full-text available
The Tilje Formation (Early Jurassic; 120-300 m thick) consists predominantly of heterolithic deposits and is thought to have accumulated in tide-dominated estuarine and deltaic environments in an active rift setting. Anomalously thick (>0.5 cm) and internally structureless mudstone layers, which are interpreted to represent fluid-mud deposits, are widespread and occur in three different environmental settings: (1) in the basal part of upward-fining tidal-fluvial channels where they generate upward-sanding successions: (2) in the deposits of mouth bars and terminal distributary channels where they are associated with the coarsest sands and the least-bioturbated sediments, suggesting deposition during tidally modulated river floods; and (3) in delta-front successions where they immediately overlie thick, wave-generated storm beds, suggesting that these fluid-mud deposits result from wave resuspension of previously deposited mud. These observations provide criteria for the recognition of ancient fluid-muds and for interpreting their origin. The tectonic setting may be responsible for their abundance.
Conference Paper
The accumulation of organic-rich mudstones was largely associated to low energy depositional environments with anoxic bottom waters, where mudstone deposition was mainly related to gradual and continuous mud fallout from dilute buoyant plumes. Based on this, basin inner zones were considered as unsuitable environments for source rock deposition due to the generally low concentration of OM resulting from both low inputs and production, adding an intense OM degradation during its transit time through the water column. Nevertheless, recent detailed sedimentological analyses in a variety of unconventional shales have revealed that the participation of fallout processes is probably subordinated to other still poorly known depositional processes, opening a new paradigm for source rocks origin. In Argentina, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation is composed of organic-rich mudstones and carbonates dominated by type II kerogen representing South America's main unconventional reservoir. New cores and excellent outcrops provide a great opportunity to study the depositional history of this unit. These deposits were previously interpreted as accumulated by fallout deposition in a quiet and anoxic deep marine environment. However, recent studies revealed that the Vaca Muerta Formation is a highly heterogeneous stratigraphic unit accumulated by different and poorly know depositional processes. In fact, the formation displays distinct lithofacies alternating at centimeter to millimeter scale having variable organic matter content (up to 14% TOC), features that influence the reservoir quality and performance. High resolution sedimentological analysis were performed on relatively uncompacted intervals preserved in early diagenetic calcareous concretions collected from the basal deposits of the Vaca Muerta Formation in basinal settings. Evidences found in concretions suggest a deposition related to fluid mud flows instead of the classic model of "normal fallout". The triggered mechanisms for the origin of the recognized fluid mud flow deposits are mainly associated to direct river discharges during flood events. Each flood event would be capable of generating quasi-steady muddy hyperpycnal flows that may be sustained for days, weeks, or even months. These long-lasting events would be able to transfer significant volumes of organic matter and fine-grained sediments for long distances towards distal basinal settings. The erosion capacity of muddy hyperpycnal flows enables the incorporation of intrabasinal components (e.g. marine microfossils, carbonate mud, type II OM) which are transported together with the primary extrabasinal sedimentary load (e.g. detrital 10
Article
We investigated grain size, clay mineralogy, elemental geochemistry, organic composition, and AMS ¹⁴C dating ages in Core KX12-3 from the middle Okinawa Trough in order to better understand the sediment provenances and transport processes and also the forcing mechanisms behind their variations over the last 18.5 ky. The geochemical-mineralogical indices reveal three notable phases of change in the sediment provenance of the core, which are related to the development of the Kuroshio Current coupled with sea-level fluctuations, the position shift of the mouths of large rivers, the magnitude of tidal bottom stress, and the East Asian monsoon intensity. The sea-level lowstand deposits of Interval 1 (18.5-9.3 ka) formed from mixed sediments originating mostly from Chinese rivers (the paleo-Huanghe and the paleo-Changjiang) as well as from materials eroded from seafloor by the strong tidal stress. Thereafter during Interval 2 (9.3-8.7 ka), a significant sea-level rise, the moderate tidal bottom stress, and a gradual landward retreat of river mouths led to obviously decreased sediment supplies from both seafloor erosion and mainland China rivers (especially the paleo-Huanghe) into to the study area. After this transitional period, the dominant sediment provenance during the mid-to-late Holocene (Interval 3) changed to the Changjiang and seafloor erosion, resulting in lower detrital linear sedimentation rate and finer grain size. At the same time, some fine-grained materials from Taiwan may have been transported northward to the study area by the Kuroshio Current, as evidenced by higher values of chlorite/kaolinite ratio and chemical index of illite, and lower TOC/TN values. In addition, a prominent decline in chlorite/kaolinite values that occurred at 5.0-3.5 ka may have been linked to a suppression of the Kuroshio Current, probably related to the Pulleniatina minimum event and/or the late Holocene Neoglacial cold event commonly found in the northwestern Pacific.
Article
Hybrid event beds, comprising clay-poor and clay-rich sandstone, are abundant in Maastrichtian-aged sandstones of the Springar Formation in the north-west Vøring Basin, Norwegian Sea. This study focuses on an interval, informally referred to as the Lower Sandstone, which has been penetrated in five wells that are distributed along a 140 km downstream transect. Systematic variations in bed style within this stratigraphic interval are used to infer variation in flow behaviour in relatively proximal and distal settings, although individual beds were not correlated. The Lower Sandstone shows an overall reduction of total thickness, bed amalgamation, sand to mud ratio and grain size in distal wells. Turbidites dominated by clay-poor sandstone are at their most common in relatively proximal wells, whereas hybrid event beds are at their most common in distal wells. Hybrid event beds typically comprise a basal clay-poor sandstone (non-stratified or stratified) overlain by banded sandstone, with clay-rich non-stratified sandstone at the bed top. The dominant type of clay-poor sandstone at the base of these beds varies spatially; non-stratified sandstone is thickest and most common proximally, whereas stratified sandstones become dominant in distal wells. Stratified and banded sandstone record progressive deposition of the hybrid event bed. Thus, the facies succession within hybrid event beds records the longitudinal heterogeneity of flow behaviour within the depositional boundary layer; this layer changed from non-cohesive at the front, through a region of transitional behaviour (fluctuating non- cohesive and cohesive flow), to cohesive behaviour at the rear. Further, spatial variation in the dominant type of clay-poor sandstone at the bed base suggests that the front of the flow remained non-cohesive, and evolved from high-concentration and turbulence-suppressed to increasingly turbulent flow; this is thought to occur in response to deposition and declining sediment fall out. This research may be applicable to other hybrid event bed prone systems, and emphasises the dynamic nature of hybrid flows.
Article
The texture of clastic sediments reflects the process of deposition of these sediments. Sample point patterns representing the variations in a deposit of two parameters (C, an approximation of the maximum grain size, and M, the median) are characteristic of the depositional agent. The lutite content of the samples is shown on the patterns by percentage lines. These CM patterns are generally sharply defined and vary considerably with the type of depositional agent. The CM patterns are a geologic tool which can be used to analyze the deposition of recent sediments and to reconstruct the conditions of deposition of ancient sediments. They can particularly help sedimentation studies aiming at finding stratigraphic traps by definition of permeability trends.
Article
The discovery of turbidites represents perhaps themajor genuine advance of sedimentology during the twentieth century. Turbidites are the deposits of turbidity currents and were originally related to the gravitational instability and re-sedimentation of previously accumulated shallow water sediments into deep waters. As these flows originate and entirely evolve within a marine or lacustrine basin, their associated deposits are here termed intrabasinal turbidites. Controversially, increasing evidences support that turbidity currents can also be originated by the direct discharge of sediment–water mixtures by rivers in flood (hyperpycnal flows). Since these flows are originated in the continent, their associated deposits are here termed extrabasinal turbidites. Deposits related to these two different turbidity currents are often confused in the literature although they display diagnostic features that allow a clear differentiation between them. Intrabasinal turbidites are mostly related to surge-like (unsteady) flows that initiate from a cohesive debris flow that accelerates along the slope and evolves into a granular and finally a turbulent flow. Its flow behavior results on the accumulation of normally graded beds and bedsets that lacks terrestrial phytodetritus and lofting rhythmites. Extrabasinal turbidites, on the contrary, are deposits related to fully turbulent flows having interstitial freshwater and sustained by a relatively dense and long-lived river discharge. According to the grain size of suspended materials, hyperpycnal flows can be muddy or sandy. Sandy hyperpycnal flows (with or without associated bedload) often accumulate sandy to gravelly composite beds in prodelta to inner basin areas. Their typical deposits show sharp to gradual internal facies changes and recurrence, with abundant plant remains. In marine waters, the density reversal induced by freshwater results in the accumulation of lofting rhythmites at flow margin areas. Muddy hyperpycnal flows are loaded by a turbulent suspension dominantly composed of a mixture of silt and clay-sized particles (b62.5 μm) of varying compositions. Since the suspended sediment concentration does not substantially decrease in waning flows, muddy hyperpycnal flows will be not affected by lofting, and the flow will remain attached to the sea bottom until its final accumulation. Typical deposits compose cm to dm-thick graded shale beds disposed over an erosive base with displaced marine microfossils and dispersed plant remains.
Article
Three primary controls on deep-sea sedimentation can be identified: sedimentary supply, tectonics and sea-level fluctuations as well as a number of secondary controls. Five major sections present details of: processes; facies: modern and ancient: modern deep-sea environments; ancient deep-sea systems recognition; and, ancient deep-sea systems: examples and controls. -after Author
Article
Detailed petrographic studies of shales show that they consist of a wide range of components, including a wide spectrum of composite particles that were contributed to the precursor muds in the form of high-water-content suspended floccules, bedload floccules, rip-up intraclasts, pedogenic aggregates, and fully lithified shale clasts. Experimental studies show that shale clasts of sand to silt size (shale lithics) can survive hundreds to thousands of kilometers of bedload transport. Observations of modern river and shelf muds reveal the common presence of shale lithics in these sediments, and suggest that a significant portion of ancient shale formations could potentially consist of reworked shale lithics and not, as commonly assumed, of primary composite particles such as clay floccules and organo-minerallic aggregates. Identification of shale lithics in the rock record presents challenges, but careful petrographic examination (using SEM and ion-milled samples) and case studies will help to develop robust criteria for recognition. The presented observations have manifold implications for the interpretation of many aspects of shales: mud transport and accumulation, sediment compaction and basin-fill modeling, and geochemical proxies. They emphasize the essential need for petrographic examination of shale samples before more advanced analyses are undertaken.
Article
A comparative study of the sequence of sedimentary structures in ancient and modern fine-grained turbidites is made in three contrasting areas. They are (1) Holocene and Pleistocene deep-sea muds of the Nova Scotian Slope and Rise, (2) Middle Ordovician Sevier Shale of the Valley and Ridge Province of the Southern Appalachians, and (3) Cambro-Ordovician Halifax Slate of the Meguma Group in Nova Scotia.A standard sequence of structures is proposed for fine-grained turbidites. The complete sequence has nine sub-divisions that are here termed T0 to T8. “The lower subdivision (T0) comprises a silt lamina which has a sharp, scoured and load-cast base, internal parallel-lamination and cross-lamination, and a sharp current-lineated or wavy surface with ‘fading-ripples’ (= Type C etc. …).” (= Type C ripple-drift cross-lamination, Jopling and Walker, 1968). The overlying sequence shows textural and compositional grading through alternating silt and mud laminae. A convolute-laminated sub-division (T1) is overlain by low-amplitude climbing ripples (T2), thin regular laminae (T3), thin indistinct laminae (T4), and thin wipsy or convolute laminae (T5). The topmost three divisions, graded mud (T6), ungraded mud (T7) and bioturbated mud (T8), do not have silt laminae but rare patchy silt lenses and silt pseudonodules and a thin zone of micro-burrowing near the upper surface.The proposed sequence is analogous to the Bouma (1962) structural scheme for sandy turbidites and is approximately equivalent to Bouma's (C)DE divisions. The repetition of partial sequences characterizes different parts of the slope/base-of-slope/basin plain environment, and represents deposition from different stages of evolution of a large, muddy, turbidity flow. Microstructural detail and sequence are well preserved in ancient and even slightly metamorphosed sediments. Their recognition is important for determining depositional processes and for palaeoenvironmental interpretation.
Article
The East China Sea (ECS) is a typical marginal sea located between the Eurasian continentand west Pacific Ocean. In this study, we review state-of-the-art research progress on the possiblesinks of the Changjiang-derived sediments in the ECS during the late Quaternary. The majorsinks of these sediments in the ECS are on the outer shelf and the Okinawa Trough during the lastglacial maximum corresponding to a lowstand of sea level. During the deglacial marine transgression, the gently dipping shelf was rapidly inundated and strong tides prevented fine sedimentfrom deposition on the open shelf, resulting in the development of a unique tidal sand ridge system. With sea level reaching the present situation and the modern marine environment being completedin the early Holocene, the Changjiang sediments mostly accumulated in the river's estuaryto build a large delta, with only a fraction reaching the inner shelf and coastal embayments. Thelate-Quaternary changes in monsoon-climate-induced river flux, sea level and oceanic circulationprimarily controlled the source-to-sink transport of the Changjiang sediments in the ECS, and furtherdetermined the stratigraphic framework and sedimentary facies on the shelf. © 2016 The Author(s). Published by The Geological Society of London. All rights reserved.
Article
Abstract The outer parts of a number of small Late Jurassic sandy deep-water fans in the northern North Sea are dominated by the stacked deposits of co-genetic sandy and muddy gravity flows. Sharp-based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast-rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a post-depositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water-rich sand bed just deposited by a co-genetic concentrated gravity current. As such, they are referred to as ‘linked debrites’ to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low-friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low-concentration gravity currents should be more important here. In fact, evidence for transport by low-concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast-rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast-charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand-rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.
Article
The ultimate provenance of muddy sediment in the southwest of Cheju Island, East China Sea, remains enigmatic thus far. In this study, rare earth elements (REEs) were used to investigate sediment provenances of cores E03-6, E03-10 and E03-11 taken from the mud patch. Discrimination plots based on REE fractionation parameters and trace elements suggest that the sediments deposited during the last deglacial period (> 15 ka) were derived predominantly from the paleo-Huanghe (Yellow River) which might have delivered sediments directly into the northeastern East China Sea during the lowstand of sea level. The coarse-grained sediments deposited at transgressive stage (15–6 ka) were primarily sourced from the Changjiang (Yangtze River) and partly from Korean Peninsula, probably transported by tidal currents. In comparison, the clayey sediments deposited at highstand stage (< 6 ka) were mostly derived from the modern and old Huanghe. In particular, the fine-grained sediments eroded from the old Huanghe Delta in the southwestern Yellow Sea can be transported to the northeast of the East China Sea by the coastal current and the Changjiang Freshwater Plume as well, and finally trapped within a cyclonic upwelling gyre. The dispersal and deposition of terrigenous sediments in the northeast of the East China Sea are remarkably controlled by the oceanic circulation related to sea level variability. The variable depositional rates and drastic river-sea interaction during the late Quaternary make it difficult to reliably reconstruct a high-resolution paleoenvironmental change in the river-dominated shelf sea. Nevertheless, geochemical approach can provide important constraints on sediment source-to-sink transport patterns in this typical pericontinental sea.
Article
The Lower Mekong River has been an important research topic for at least 15 years, notably in the fields of hydrology, fluvial geomorphology, and the impact of dams. Recent papers refer mostly to the impact of the Lancang chain of hydroelectric dams constructed on the Chinese section of the river. Among the pending scientific questions are (1) the upstream–downstream variations in the concentration and yield of suspended sediment and (2) the relative importance of sand in the total yield. The general consensus among the scientific community is that the relative importance of sand in suspended load is not the main scientific concern despite its extensive presence in the Mekong River channel, as noted by geomorphologists, and despite its extreme importance for the stability of the delta shoreline in Viet Nam.
Article
Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well-mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face-face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre-scale, and four microfacies are recognized: Well-sorted siltstone forms millimetre-scale combined-flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt-streaked claystone comprises parallel, sub-millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave-supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay-rich mudstone has little silt and may represent late-stage settling of fluid mud, or settling from wave-dissipated fluid mud. It is difficult or impossible to correlate millimetre-scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined-flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope-flowing, wave-supported fluid mud. In the upper part of the studied section, centimetre-scale interbeds of very fine to fine-grained sandstone show wave ripple crests trending shore normal, whereas combined-flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north-east drove shore-oblique geostrophic sand transport whereas simultaneously, wave-supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small-scale bioturbation of mud beds co-occurs with interbedded sandstone but stratigraphically lower, sand-free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom-water oxygen content collectively inhibited benthic fauna in the distal prodelta.
Article
Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run‐out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain‐back for long distances into basinal lows. Deposition of ungraded mud (TE‐3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE‐2) and finely laminated mud (TE‐1) most probably result from floc settling at lower mud concentrations. Grain‐size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar‐laminated (TD) and ripple cross‐laminated (TC) non‐cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain‐size break beneath the ripple cross‐laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar‐laminated sand can be deposited by low‐amplitude bed waves in dilute flow (TB‐1), but it is most likely to be deposited mainly by high‐concentration near‐bed layers beneath high‐density flows (TB‐2). More widely spaced planar lamination (TB‐3) occurs beneath massive clean sand (TA), and is also formed by high‐density turbidity currents. High‐density turbidite deposits (TA, TB‐2 and TB‐3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low‐density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain‐size texture. Clean‐sand debrites can extend for several tens of kilometres before pinching out abruptly. Up‐current transitions suggest that clean‐sand debris flows sometimes form via transformation from high‐density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low‐strength cohesive debris flows produce extensive deposits restricted to distal areas. These low‐strength debris flows may contain clasts and travel long distances (DM‐2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM‐1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.
Article
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
Article
Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one-dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow.
Article
Turbidity currents may be generated in the oceans as part of the sequence from landsliding through debris flow to turbidity current flow. Three aspects of this sequence examined here are 1) the transition from landsliding to debris flow, 2) the mechanics of subaqueous debris flow, and 3) the transition from subaqueous debris flow to turbidity-current flow. The transition from landsliding to debris flow, as observed in the subaerial environment, occurs readily if water is incorporated into the landslide debris as it is jostled and remoulded during downslope movement. Remoulding and incorporation of water reduce the strength and increase the fluid behavior of the debris, thereby causing it to flow rather than slide. Incorporation of only a few percent water typically decreases the strength of landslide debris by a factor of two or more; therefore, landslide debris commonly becomes very fluid with incorporation of a small amount of water. The ready availability of water in the marine environment suggests that conditions are favorable for the development of subaqueous debris flows from subaqueous landslides. Debris flow has been modeled as flow of a plastico-viscous substance, which has a yield strength and deforms viscously at stresses greater than the yield strength. The conditions required for movement of a subaqueous debris flow are described in terms of a critical thickness of debris, which varies directly with strength and inversely with submerged trait weight and slope angle. Within a debris flow, viscous shear occurs where shear stress exceeds the shear strength of the debris, but where shear stress is less than shear strength the material is rafted along as a nondeforming plug. Distinct zones of viscous shear and nondeformation exist in a subaqueous debris flow. Transition from subaqueous debris flow to turbidity-current flow involves extensive dilution of debris-flow material, reducing the density from about 2.0 gm/cm3 to about 1.1 gm/cm3. In experiments, subaqueous debris-flow material was mixed with the surrounding water by erosion of material from the front of the flow and ejection of the material into the overlying water to form a dilute turbulent cloud (turbidity current). The amount of mixing, and hence the size of the turbidity current, varied inversely with the strength of the debris. Conditions that cause mixing at the front of a subaqueous debris flow are illustrated by analyzing flow around a half-body, with boundary-layer separation. Turbidity, currents also may be generated from subaqueous debris flows by mixing water directly into the body of the flow, behind the front, although this type of mixing was not observed in experiments. Mixing into the body of the flow can result from flow instability, either by breaking interface waves or by momentum transfer associated with turbulence, but available information suggests that mixing due to instability is inhibited by the presence of clay and coarse granular solids in debris. Mixing by erosion from the front of a debris flow is favored as being a more typical process of generating turbidity currents because this mixing is a natural consequence of debris flowing through water; it requires no special conditions to operate.
Article
Four principal mechanisms of deposition are effective in the formation of sediment gravity flow deposits. Grains deposited by traction sedimentation and suspension sedimentation respond invidually and accumulate directly from bed and suspended loads, respectively. Those deposited by frictional freezing and cohesive freezing interact through either frictional contact or cohesive forces, respectively, and are deposited collectively, usually by plug formation. Sediment deposition from individual sediment flows commonly involves more than one of these mechanisms acting either serially as the flow evolves or simultaneously on different grain populations. -from Author
Article
The characteristics of a depositional agent are reflected in the texture of the sediment. This relationship is particularly evident if the texture is represented by two parameters of the grain size distribution: C the one percentile and M the median diameter. CM patterns formed by sample points of a deposit are characteristic of the agent of deposition. Since the first discussion of CM patterns (Passega 1957) more than ten thousand analyses of sediments ranging in age from Paleozoic to Recent, sampled in basins of all types and sizes, including the Rhine graben, mobile basins of the Apennines and southeast Iran, and Cratonic Sahara basins, confirmed the conclusions of the earlier paper. They also disclosed new relationships between CM patterns and sedimentation, valid under greatly different conditions. The most notable progress of recent research was the construction of paleobathymetric maps and logs showing the depth of ancient seas and the variations of this depth in a stratigraphic section. Paleobathymetry is geologically important as it seems related to such characteristics of the sediments as facies, permeability and possibly also petroleum productivity. CM patterns are a means for analyzing transportation mechanisms and determining the mechanisms that form peculiar deposits, such as stratigraphic traps. Turbidites and certain tractive current deposits have several common characteristics. Their texture shows that they may have a common origin as wave suspensions. The many applications of grain size analyses indicate that they are a versatile means of investigation that discloses the orderly arrangement of a number of geological factors. This order is a fertile field for geological research.
Article
Muds and mudstones are the prime control on fluid flow in sedimentary basins and near-surface environments. As the world's commonest sediment type, they act as aquitards in sedimentary basins, restricting water flow and influencing the development of overpressure. In petroleum systems they act as source rocks for nearly all oil and much gas, determine migration directions between source and trap in most settings, and act as seals to many reservoirs. In near surface environments they not only control natural flow, but have been commonly used to restrict leakage from waste disposal sites. This volume focuses on the previously poorly described physical properties of muds and mudstones in both near-surface and deep basinal settings. Amongst its contents are reviews of the compaction, permeability, thermal conductivity and mechanical properties of mudstones, experimental studies of transport properties, and case studies of the importance of fluid flow in both hydrogeological and petroleum systems.
Article
Flume experiments have shown that muds can be transported in bedload as floccule ripples and deposited at current velocities that would suffice to transport and deposit sand. A new set of experiments provides firsthand observations of the processes that shape and propagate mud ripples. Sediment is transported over the stoss side in the form of diverging boundary-layer streaks, the carriers of the bulk of the bedload floccule freight. At the brinkline these streaks become point sources of sediment that feeds avalanches of floccule-rich sediment lobes. These propagate down the slip face like classic mudflows on a hillside. Geometries of ripples are very similar to those produced in sandy sediments, even though the floccule ripples contain as much as 90 vol% water.
Article
Results from a small set of laboratory experiments are presented here that help further constrain the processes governing the production of turbidity currents from impulsive failures of continental shelf and slope deposits. Three mechanisms by which sediment can be transferred from a parent debris flow to a less-dense turbidity current were observed and quantified. These mechanisms are grain-by-grain erosion of sediment from the leading edge of the parent flow, detachment of thin layers of shearing material from the head of the parent flow, and turbulent mixing at the head of the parent flow. Which transfer process dominates an experimental run depends on whether the large dynamic stresses focused on the head of the debris flow are sufficient to overcome a effective yield strength for the parent sediment+water mixture and on whether the dynamic stresses are sufficient to induce the turbulent flow of the parent mixture. Analysis of data from Marr et al. [Geol. Soc. Am. Bull. 113 (2001) 1377] and Mohrig et al. [Geol. Soc. Am. Bull. 110 (1998) 387] support the use of a shear strength to dynamic stress ratio in constraining necessary critical values for occurrence of the different production mechanisms. Direct sampling of turbidity currents using racks of vertically stacked siphons was used to measure both the quantity of sediment eroded from the heads of non-mixing parent flows and the distribution of particle sizes transported by the developing turbidity currents. Acoustic backscatter imaging was used to better resolve the internal boundary separating any turbulent mixing zone near the front of a flow from unmodified parent material.
Article
Progressive change of microstructure in pelagic clays of different three orders of burial depths, several centimeter, several meters and several hundred meter depths, was observed by measurement of anisotropy of magnetic susceptibility (AMS) and observation by scanning electron microscope (SEM) and thin section. At depths of several centimeters (void ratio >3.0), peds of several 10 μm in diameter are linked by long and narrow connectors in edge-face (EF) contact and pore sizes between the peds are large. At depths of several meters (void ratio 3.0–2.0), the shape of the connectors changes from narrow to broad. The pore size between the peds decreases with burial depth. At depths of several hundred meters (void ratio
Article
CM grain-size diagrams of clastic deposits were introduced in 1957. Since that time a large number of diagrams representing most environments were constructed. Discussions of parameters C and M by the writer and by others showed that these parameters are indicators of hydraulic conditions under which sediments were deposited. In a 1975 paper Vandenberghe discussed the CM diagram of the Boom Clay, Belgium, and excluded that CM diagrams gave indications on the mechanism of sediment deposition. In order to answer Vandenberghe's assertions, the writer systematically compared the numerous CM diagrams representing suspension deposits with the characteristics of the environments. Result of this comparison is that CM diagrams give coherent indications on deposition which agree with the known environment characteristics. The Boom Clay diagram is no exception. Another result of the present study is that, in environments as a whole, two types of currents can be recognized, distinguished by the relation between bottom turbulence and grain-size of sediment in suspension. Deposits of these two current types and distribution of bottom turbulence are indicated by CM diagrams. Characteristics of these currents vary with the environment and help the reconstruction of ancient environments. Information given by CM diagrams has limits and should be integrated with other evidence on the environment. However, contrarily to the information given by ecology or sedimentary structures, the evidence provided by grain-size has the advantage of being universal, as it can be obtained in all clastic deposits. This evidence therefore is a means for comparing and classifying clastic deposits as a whole.
Article
The clay fabric of fluid-mud deposits was investigated with the aim of identifying characteristic features that potentially apply to the recognition of fluid-mud deposits in the stratigraphic record. We examined both experimentally formed and natural fluid-mud deposits. The clay fabric of experimentally formed fluid-mud deposits is characterized by aggregates of clay particles in face-to-face contact with each other (herein termed ‘FF-aggregates’), with long-axis lengths of up to 20 μm. Some flocs that formed in fluid with a high initial suspended sediment concentration (ISSC), such as fluid mud, would have a high preservation potential as FF-aggregates; once they have settled to the sea floor, to distinguish them from flocs that form in fluids with a lower ISSC. Aggregates that are similar to FF-aggregates are also observed in natural fluid-mud deposits that formed in a modern tide-dominated estuary at the mouth of the Rokkaku River in Kyushu, Japan. Thus, in conjunction with formally proposed lithofacies and ichnofacies features of fluid-mud deposits, the observation of FF-aggregates is potentially useful in identifying fluid-mud deposits when examining limited volumes of muddy samples or thin- to very thin-bedded muddy deposits in the stratigraphic record.
Article
Mudstone layers in the tide-dominated Bluesky Formation (i.e., the "mud drapes") have enormously variable sedimentary characteristics: they range from 0.1to 20 cm thick, can be homogeneous or internally stratified, and can have sharp or gradational upper and lower contacts. Based on recent flume studies, this diversity is interpreted to reflect the wide range of suspended-sediment concentrations (SSC; < 1 g L(-1) to > 100 g L(-1)) and flow velocities (slack water to several meters per second) that are typical in tidal environments. Four recurring mudstone types are present, each of which is interpreted to have formed under distinct depositional conditions: (1) unstratified mudstone Type 1 (UM1): "classic" mud drapes generated by slow settling from suspensions with SSC values less than 1 g (2) stratified mudstone Type 1 (SM1): cross-stratified mudstone layers deposited by turbulent or transitional turbulent flows with appreciable current speeds (> 0.2 ms(-1)) and moderate SSCs (1-10 g L(-1)); (3) stratified mudstone Type 2 (SM2): horizontally laminated mudstone layers formed under conditions of transitional plug flow with moderate to high SSCs (1-100 g L(-1)) and appreciable currents (> 0.2 ms(-1)); and (4) unstratified mudstone Type 2 (UM2): thick mudstone laminae and beds (> 2 mm thick), with no internal lamination and common soft-sediment deformation, deposited by unstable plug flow or quasi-laminar plug flow, with moderate to high SSCs (1-1000 g L(-1)) and current speeds ranging from slack water to 1-2 ms(-1). UM2 and SM2 mudstone types are equivalent to what recent studies have interpreted as the lithified product of fluid mud and may have been deposited dynamically, under current speeds above the threshold of mud erosion in clear-water flows. This indicates that, in high-SSC settings, "mud drapes" could form over significant portions of the tidal cycle (i.e., not only at slack water) and perhaps continuously over multiple tidal cycles if fluid-mud layers persist. Thus, tidal rhythmites might be poorly developed in areas with high SSC values.
Article
New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about 15 x 106 tons) contribute about 7 x 109 tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about 13.5 x 109 tons annually; bedload and flood discharges may account for an additional 1-2 x 109 tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.-Authors
Article
Turbidites are generally deep-sea deposits. By contrast, the turbitdites which form a large part of the nearly 1000m Quaternary section, cored in well CNR Venice 1, do not seem to have been deposited at sea depths much greater than 200m. Sea depth, determined mostly by ecological analyses of faraminifera, gradually decreases as the basin was filled with sediments. In the upper part of the well, above 302 m, turbidite sedimentation was replaced by alternating fluvitile and very shallow marine deposition. In well CNR Venice 1, turbite characteristics vary with the sea depth and the gradient of sea slopes. If near riveer mouths, the slope gradient is sufficient, sediments debouching from rivers readily form turbity currents. The displaced fauna in sand and in shale turbidites is fresh-water. When gradients decrease, sediments are distributed along the coast and form submarine deltas or coastal shelves. If these shelves are limited by a sandy slope, turbidity currents may originated on this slope. Displaced fauna in the sand turbidites is marine littoral. If sediments of the slope limiting the coastal shelf are silt and clay, muddy turbidity currents can originate provided the gradient is 1% or more. A critical factor of sedimentation of Venice neritic turbidites is the very fine grain-size of the sands. Turbidity current suspensions probably were stable as they could be supported by a moderate bottom turbulence. Suspensions therefore were not likely to settle rapidly when the gradient decreased to form fan-like deposits. They flowed to the bottom of the basin where they spread and made their thickest deposits. Turbidites similar to Venice Quaternary turbidites may be common in ancient neitic basins, where gradients were sufficiently steep. These turbidites can form good petroleum reservoirs as well as impervious shale beds and permeability barriers.
Article
Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi-laminar plug flows. The characteristics of these transitional flows are known to be a function of both clay concentration and type, as well as the applied fluid stress, but so far the interaction of these transitional flows with a loose sediment bed has received little attention. Information on this type of interaction is essential for the recognition and prediction of sedimentary structures formed by cohesive transitional flows in, for example, fluvial, estuarine and deep-marine deposits. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow, turbulence-enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and a quasi-laminar plug flow. Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in turbulent flow, turbulence-enhanced transitional flow and lower transitional plug flow. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under turbulence-enhanced transitional flow and lower transitional plug-flow regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater lee side erosion. As clay concentration increases further from a lower transitional plug flow, ripples cease to exist under the upper transitional plug flow and quasi-laminar plug flow conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross-stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not form in cohesive flows with a Reynolds number smaller than ca 12 000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This interpretation includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain-size segregation and longitudinal flow transformation.