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Sedimentary Model for Mixed Depositional Systems: Conceptual Implications

Authors:

Abstract

Along- and down-slope processes are fairly common processes along continental margins. The aforementioned processes and their interaction can build large mixed / hybrid (turbiditic-contouritic) depositional systems. These systems are characterized by a variability and complexity of features. Globally, several mixed systems have been identified on the Cenozoic, however their representation in the Mesozoic remains severely understated. This issue is aggravated if we consider that the diagnostic criteria for mixed systems is not fully defined and, therefore, not used to improve the examples already described in the literature. This work aims to contribute to a better understanding of the dynamics between along- and down-slope processes, by identifying significant modifications in physiographic features and stacking architectures, and by discriminating the mechanisms responsible for the formation of each feature and how they operate through time. To approach these objectives, we are studying two key areas: 1) a modern example on the Pacific Margin of the Antarctic Peninsula, characterized by a remarkable depositional system with several contourite mounded drifts and turbidite channels in the continental lower slope and continental rise; and 2) the Cretaceous record on the Argentine Margin, which comprises an extensive mixed system on the continental slope. This study is based on a compilation of swath multibeam bathymetry, high- and low-resolution seismic reflection datasets, well borehole data and sediment cores. The two examples share similar downslope elongated mounds along the lower slope and rise with asymmetric morphologies, marked by a smooth, aggradational side and a steep, eroded side with signs of mass movements. However, while the Antarctic Peninsula has a dendritic network of gullies and channels on the upper continental slope that converge into single turbidite channels on the lower slope and rise, the Argentine Margin is characterized by individual large channels that start at the continental shelf / upper slope and cut through the topography. The interpretation and integration of these results aims to propose a conceptual model for depositional mixed systems that will clarify the role and influence of bottom currents versus turbidity currents.
Sedimentary Model for Mixed Depositional Systems: Conceptual Implications
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Sara Rodrigues1*, F. Javier Hernández-Molina1, Renata Giulia Lucchi2, Robert Larter3, Claus-
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Dieter Hillenbrand3, Michele Rebesco2, Karyna Rodriguez4, Neil Hodgson4
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1 Department of Earth Sciences, Royal Holloway University of London, Egham Surrey, TW20 0EX, UK,
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*Sara.Rodrigues.2017@live.rhul.ac.uk
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2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante, Trieste, 42/c
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34010, Italy.
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3 British Antarctic Survey, High Cross Madingley Road, Cambridge, CB3 0ET, UK.
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4 Spectrum Geo Ltd, Dukes Court, Duke Street, Woking Surrey, GU21 5BH, UK.
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Along- and down-slope processes are fairly common processes along continental margins. The
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aforementioned processes and their interaction can build large mixed / hybrid (turbiditic-contouritic)
12
depositional systems. These systems are characterized by a variability and complexity of features.
13
Globally, several mixed systems have been identified on the Cenozoic, however their representation in
14
the Mesozoic remains severely understated. This issue is aggravated if we consider that the diagnostic
15
criteria for mixed systems is not fully defined and, therefore, not used to improve the examples already
16
described in the literature.
17
This work aims to contribute to a better understanding of the dynamics between along- and down-
18
slope processes, by identifying significant modifications in physiographic features and stacking
19
architectures, and by discriminating the mechanisms responsible for the formation of each feature and
20
how they operate through time. To approach these objectives, we are studying two key areas: 1) a
21
modern example on the Pacific Margin of the Antarctic Peninsula, characterized by a remarkable
22
depositional system with several contourite mounded drifts and turbidite channels in the continental
23
lower slope and continental rise; and 2) the Cretaceous record on the Argentine Margin, which
24
comprises an extensive mixed system on the continental slope. This study is based on a compilation of
25
swath multibeam bathymetry, high- and low-resolution seismic reflection datasets, well borehole data
26
and sediment cores.
27
The two examples share similar downslope elongated mounds along the lower slope and rise with
28
asymmetric morphologies, marked by a smooth, aggradational side and a steep, eroded side with signs
29
of mass movements. However, while the Antarctic Peninsula has a dendritic network of gullies and
30
channels on the upper continental slope that converge into single turbidite channels on the lower slope
31
and rise, the Argentine Margin is characterized by individual large channels that start at the continental
32
shelf / upper slope and cut through the topography. The interpretation and integration of these results
33
aims to propose a conceptual model for depositional mixed systems that will clarify the role and
34
influence of bottom currents versus turbidity currents.
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Article
Full-text available
The Argentine Basin is a deep-sea basin located in the South Atlantic Ocean that contains sedimentary deposits derived from different provenances. It is characterized by complex ocean dynamics encompassing diverse spatial and temporal dimensions. The northward subantarctic Malvinas Current and southward subtropical Brazil Current converge at the western margin of the Argentine Basin, resulting in the formation of the Brazil-Malvinas Confluence. Bottom currents, particularly currents flowing alongslope and horizontal eddies, are crucial in shaping the seafloor and in the formation of sedimentary features (e.g. contourites). The poorly understood strength and variability of bottom currents leave the processes that control sedimentation in deep environments unclear. High-resolution (1/12°) reanalysis was used to analyze near-bottom flows and bottom dynamics were compared with seafloor sedimentary characteristics obtained from geophysical datasets and sediment cores. High speeds, up to 3.5 m/s at the surfa ce and up to 1.4 m/s at the bottom, reveal the presence of intense flows in this area. The Zapiola Drift, an approx. 1,200 m high sedimentary deposit located in the central part of the Argentine Basin, is bounded by a zone of high bottom eddy kinetic energy (EKE) that resulted in the erosion of the seafloor and in the accumulation of sandy mud. The Malvinas Current is distinguished by strong and constant currents flowing northwards along the continental slope and by minimal EKE at the bottom. The area of the continental slope along which the Malvinas Current flows corresponds to a contourite terrace, a relatively flat surface composed almost entirely of sandy sediments and with abundant erosional features. The regions of highest EKE activity in the bottom layer is the overshoot of the Brazil-Malvinas Confluence and the Abyssal Plain. Our study highlights the impact of bottom current dynamics on contouritic sedimentation. In some regions, sedimentation is influenced by sporadic processes that occur between intense and weak flow events over time, which are considered intermittent processes. While sedimentation in other areas is controlled by constant flows. A better understanding of the strength and variability of bottom currents will improve paleoceanographic reconstructions based on the sedimentary record.
Thesis
Full-text available
The Gulf of Cádiz is home to the well-studied modern contourite depositional system, which was deposited through the influence of bottom currents sourced from the Mediterranean Outflow Water exiting the Straits of Gibraltar since Pliocene to present. However, the Late Miocene sedimentary evolution and Mediterranean-Atlantic water- mass exchange prior and during the Messinian salinity crisis are poorly understood. Some progress on the characterisation of an ancient Late Miocene contourite depositional system were established from field studies onshore in the Betic and Rifian corridors albeit limited due to outcrop availability, but their downstream continuation in the Gulf of Cádiz has yet to be identified. This is partially due to the complexity of the area resulting from tectonic deformation, whose effect on contourite deposition are ambiguous. Consequently, identification and characterisation of the Tortonian to Messinian interval in the Gulf of Cádiz could answer questions related to the relationship between the Mediterranean and Atlantic during that period, as well as increase our knowledge of contourite deposition in tectonically active settings. This thesis presents a regional-scale study on the Late Miocene evolution of the Gulf of Cádiz, focusing on the role of bottom currents in sedimentation. A detailed seismic stratigraphic analysis was carried out for the available seismic and borehole data acquired from scientific and industry sources, to characterise the sedimentary and paleoceanographic evolution during the later parts of the Late Miocene, assisted by chronostratigraphic correlation. An ancient contourite depositional system is identified consisting of three evolutionary stages: initial-drift, growth-drift, and maintenance-drift, prior to its burial in the latest Miocene. The formation of the Late Miocene contourite depositional system occurred following the main emplacement of the regional Gulf of Cádiz allochthonous unit and can be traced towards southern West Iberian margin. The results allowed us to reconstruct the evolution of the paleo-Mediterranean Outflow Water responsible for the bottom current activity depositing the ancient contourite depositional system until its severe weakening or cessation during the latest Miocene, which led to the Messinian salinity crisis, as well as to suggest its impact on North Atlantic paleoceanographic and climate. This is mainly controlled by continuous uplifting and subsequent closure of the Mediterranean-Atlantic paleo-gateways. These findings also allowed us to understand the influence of tectonic and orbital control on gateway evolution and gravitational processes, and thus on contourite deposition. By unravelling these control factors, they enable us to propose diagnostic criteria of contourite depositional system in tectonically active margins.
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