Monday 7 December 2020

Modelling turbidites off the East Coast of Africa.

As the terminal part of sedimentary source-to-sink systems, deep-sea deposits have been used to reconstruct past climates, sediment and carbon budgets, and the distribution of anthropogenic pollution. Bottom currents, i.e. density-driven circulation in the deep ocean, and sediment gravity flows are the main processes that form and modify these deposits. While the influence of bottom currents on submarine channel architecture has been recognised, interpretation has been hampered by a lack of direct monitoring data. Modern, integrated data sets, including direct measurements and monitoring of bottom currents, are of great importance in understanding the complexity of oceanographic processes, and the preservation of strata within submarine channel complexes.

In a paper published in the journal Geology on 18 March 2020, Arne Furhmann and Ian Kane of the Department of Earth and Environmental Sciences at the University of Manchester, Mike Clare of the National Oceanography Centre, Ross Ferguson, also of the Department of Earth and Environmental Sciences at the University of Manchester, Edwin Schomacker of Equinor, Enrico Bonamini of Eni Upstream and Technical Services, and Fabio Contreras of Eni Rovuma Basin, present the results of a study which combines an integrated subsurface study (three-dimensional seismic, core, and well-log data) with modern seafloor geomorphology and near-bed bottom-current measurements to develop a process-product–based sedimentological model for bottom current–influenced submarine channel complexes.

The Jurassic to Palaeogene basins offshore of East Africa formed during the breakup of Gondwana. Following Pliensbachian to Aalenian northwest-southeast rifting, about 2000 km of continental drift took place along north-south–striking lineaments, such as the Davie Ridge fracture zone and the Sea Gap Fault, between the Kimmeridgian and Barremian. Cessation of rifting was marked by active seafloor spreading between Madagascar and India, leading to the development of the present-day East African passive continental margin. Albian transgression resulted in the development of the extensive deep-marine deposits that are the focus of this study. Major river systems drained the African continent and supplied fine-grained sediment, while additional sediment was shed from uplifted rift shoulders along the palaeocoastline.

(A) Location of the subsurface data (red outline I) and modern analogue (red outline II), the Sea Gap Fault (SGF) and the Davie Ridge Fracture Zone (DFZ) along the East African Margin. Dotted lines refer to regional oil/gas license boundaries. Wells were used for biotratigraphic correlation; Well A is marked by red dot. (B) Lithology and tectonic history of the deep-water basins offshore of Tanzania. Colours of seismic horizons correlate to the interpreted seismic cross sections in panels (E)–(I). Quat, Quaternary; EARS, East African Rift System. (C), (D) Contour map of mid–Campanian (C) seismic horizon and root mean square (RMS) amplitude extraction of the base Turonian to mid-Campanian (D). Coarse-grained sediment (high amplitudes) is influenced by drift-related topography (low amplitudes); white arrows mark direction of sediment gravity flows. TWT, two-way traveltime. (E) Seismic cross section showing Well A (blue line) and lateral seismic facies variation. (F) Seismic cross section showing drift-confined slope channels and sediment waves east of Sea Gap fault. (G) Seismic cross section displaying spatial distribution of drift deposits and Sea Gap fault. (H), (I) Zoom of hybrid levee-drift deposits in Upper Cretaceous and Paleogene (top) with interpretation (bottom). Note overall similar trend of coeval channel and drift migration. Furhmann et al. (2020).

Furhmann et al.'s study used high-resolution 3-D seismic reflection data (covering 4885 km²) and 14 exploration wells provided by Equinor ASA and ExxonMobil. Seismic data were tied to the biostratigraphically calibrated wells (and core data of Well A) to map seismic and stratigraphic geometries offshore Tanzania. The average vertical resolution in the Upper Cretaceous of our study area is about 20–30 m (average velocities of about 2.9–3.3 km per second, and average frequency of 35 Hz). The data have a bin spacing of 12.5 × 12.5 m and a 4 ms sampling rate, and are processed in the Society of Exploration Geophysicists normal polarity to zero phase, where a peak represents a downward increase in acoustic impedance. High-resolution modern seafloor data covering the modern analog, offshore northern Mozambique, comprises extensive (65 × 50 km) multibeam bathymetric data acquired by our study using an autonomous underwater vehicle (5 m bin size), and a focused (190-m-wide) bathymetric survey using a remotely operated vehicle (0.6 m bin size). Down-looking 1200 kHz acoustic Doppler current profilers deployed by Eni energy company on three single-point deep-water moorings measured near-bed (5.5 m above bed) current direction and velocity every 10 minutes from March 2013 to September 2014.

Modern bottom-current and seafloor data, offshore northern Mozambique. (A) Bathymetric map of the seafloor including location of moorings and cross sections. AUV, autonomous underwater vehicle. (B) Furrows and scours at the seafloor indicate the dominance of north-flowing bottom currents. ROV, remotely operated vehicle. (C) Cross sections X, Y, and Z showing less-steep canyon walls (drift) at the northern flank of canyons. (D) Bottom-current velocity measurements for each mooring indicate strong seasonal variability. Gray bars on time axis represent times in which data were recorded. (E) Cumulative vector plot shows dominant trend of currents flowing to north. Furhmann et al. (2020).

The deep-marine sedimentary systems of the Upper Cretaceous offshore Tanzania were strongly influenced by topographic relief associated with the Sea Gap fault and large drift deposits. These drifts are characterized by low-amplitude, parallel to wavy reflectors, which show a lateral, upslope accretion toward the southwest. A 1650-m-thick drift (central drift) with horizontal dimensions that exceed the data coverage of the 3-D seismic survey toward the north (maximum west-east width of 50 km) dominates the stratigraphy. Smaller drifts as much as 12 km wide and 30 km long, to the west of the central drift, migrated obliquely along the northwest-southeast–oriented upper slope. Located in their topographic lows are concave-up, roughly 3500-m-wide high-amplitude reflectors, interpreted as coarse-grained submarine slope channel fills. These drifts show characteristics of levees, developed on the northeastern side of the channel complexes, and migrated up dip (southwest), confining and stepping into the channels. One-sided hybrid levee-drift deposits on the northeastern sides of channel systems are recognized from the Upper Cretaceous to modern systems. East of the Sea Gap fault, large drifts, low-amplitude sediment waves (wavelengths approximately 1.5–2.5 km, about 100 m thick), and distal slope channel complexes indicate similar channel and drift interaction along the lower slope. The time-continuous migration toward the southwest, the low seismic amplitude of the drifts, and the dimming of high-amplitude channel fills to the north suggest relatively persistent, long-lived bottom-current flow.

Acoustic Doppler current profiler measurements offshore of Mozambique show a dominance of northward-directed bottom currents, which typically attain near-bed velocities between 0.2 and 0.4 metres per second, with a maximum of 1.4 metres per second. Cumulative vector plots of each deep-water mooring calculated from the current speed and duration show the northward transport of the near-bed water masses along the entire slope. These currents are likely related to the East African Coastal Current (moorings B and D) and deeper Antarctic Intermediate Water. Annual variation in current strength may relate to the seasonal occurrence of eddies along the Mozambique Channel. Bathymetric observations of north- to north-northeast–oriented obstacle-scour features and linear furrows match the measured current direction from acoustic Doppler current profilers, and indicate bottom-current velocities of about 0.2–1.0 metres per second, The rounded, less-steep northern channel flanks are interpreted as drifts stepping into the channels.

Well A intersects high-amplitude, sandstone-dominated channel fills. These high amplitudes decay toward the northeast, where they interfinger with low amplitudes of muddier drift deposits. The gamma-ray log shows repetitive low-intensity intervals that grade into spiky well-log responses, interpreted as coarse-grained sandstone packages that fine upward into mud-prone heterolithic sediments. Core facies are dominated by (1) turbidites, with lesser debrites and hybrid event beds (facies Fa1–Fa4); 2) muddy siltstones interbedded with 5–10-cm-thick, sharp-based sandstones grading into mottled siltstone with starved ripples and laminae, interpreted as reworked low-density turbidites (facies Fa5); and (3) muddy siltstones, 0.5–2 m thick, with parallel lamination, rare cross-cutting laminae, and ripples, interpreted as bottom-current deposits (facies Fa6). The well-sorted, narrow grain-size range of facies Fa6 suggests relatively weak, but long-lasting, flow. Individual cross-cutting laminae that are overlain by ripples indicate short-term increases in bottom-current strength and erosion. The limited occurrence of normal and inverse grading, well-preserved primary sedimentary structures, and relatively uniform grain size distinguish these deposits from those of other contourite facies models. The removal of organic matter in combination with high silt sedimentation rates could account for the suppression of bioturbation and the preservation of primary sedimentary structures.

(A) Sedimentological log of Well A, offshore Tanzania. Vfs, very fine sand; Fs, fine sand; Ms, medium sand; Cs, coarse sand. (B) Well A core photographs: (i) drift deposits interbedded with turbidites; (ii) reworked turbidites interbedded with toes of drift; (iii) drift facies transitioning into muddy turbidites. Zoom on individual facies associations (Fa): Fa6, drift deposit with parallel, cross-cutting and ripple-laminated muddy siltstone (I); Fa5, coarsening up into sharp-based thin low density turbidites (LDTs) with starved ripple lamination and bioturbated bed tops (II); Fa1, LDT, including muddy cross-lamination (III) and muddy climbing ripple lamination (II); Fa2, transitional flow deposits; Fa3, debrite; Fa4, slumped deposits. (C) Well A to seismic tie shows seismic amplitudes in the background with the correlated wiggles along the well path (red line). Seismic pick D represents a tie point between seismic and well; dotted lines are seismic horizons; blue bar represents the cored interval (A). GR, gamma ray; API, standardized unit of the American Petroleum Institute; TWT, two-way traveltime. Furhmann et al. (2020).

To understand the interaction of sediment gravity flows and bottom currents, it is important to consider these processes in terms of their orientation, thickness, velocity, steadiness, and persistence over time. Furhmann et al.'s model is based on geometries and facies stacking patterns considered to be representative of strong (i.e. velocities of meters per second), short-duration (minutes to days), unsteady, episodic turbidity currents, and relatively weak (i.e., velocities of centimeters per second) but quasi-steady bottom currents (over geological time scales). For turbidity flows, velocity values were collated from recent monitoring studies, while a compilation of published bottom-current velocity data augments our own measurements from offshore Mozambique. Bottom-current velocity typically fluctuates annually, with average recorded velocities in the region of 0.4 m/s and short-lived peaks of up to 1.4 m/s; flows of these velocities are able to entrain and redistribute the silt- and sand-grade sediment deposited by turbidity currents.

(A) Sedimentological model of hybrid levee-drift systems: (i) with concurrent gravity-driven turbidity currents; (ii) during dominance of bottom currents. Red arrow represents turbidity current; blue arrows represent bottom current. (B) Laterally offset channel complexes after repeated intervals of bottom current– dominated and turbidity current–dominated deposition. (C) Graphic visualization of the spatial variation of turbidity currents and bottom-current flows over time. Velocity (U) values are taken from published work and bottom-current measurements offshore of Mozambique. Furhmann et al. (2020).

When sediment gravity flow systems are active, the channels undergo cycles of erosion, bypass, and deposition under unsteady flow conditions. Thick, amalgamated, high-density turbidites are deposited in the channel axis, laterally changing into individual turbidite beds with preserved bed tops in channel off-axis positions (facies Fa1 and Fa2). Direct interaction of turbidity currents and northward-flowing bottom currents is short lived, but is anticipated to result in partial flow-stripping of the super-elevated fraction of turbidity currents. For the majority of the time, sediment gravity flows are subordinate to the north-flowing bottom currents. During this time, the channel and levee deposits are reworked and redistributed, forming one-sided, hybrid levee-drift deposits to the north. Deceleration and partial deflection of bottom currents interacting with the topography of the channel cause high accretion rates on the upstream-facing channel flank (relative to the bottom current) under lee-wave conditions. Thick homogenous muddy siltstones (facies Fa6) deposited by bottom currents step into the channel and ultimately interfinger with the channel margin (facies Fa1 and Fa2) and reworked overbank and/or levee facies (facies Fa5). For this reason, there is minimal levee development on the bottom current–upstream side of the channel. Sedimentary facies and architecture of the hybrid turbidite-drift channel systems are therefore controlled by the frequency of sediment gravity flow activity and the relative persistence and strength of bottom currents. Deposit modification would mostly occur during periods when bottom currents dominate; during this time, the strength and character or direction of the bottom current system would be variable due to, for example, seasonal eddies and benthic storms. However, changes in sediment flux (i.e., frequency of sediment gravity flows) and fluctuation of bottom currents over geological time scales (10 million years) govern the large-scale architecture and stacking pattern of hybrid turbidite-drift channel complexes along the East African margin. 

Modern and ancient submarine slope channels offshore of Tanzania and Mozambique are, and were, formed by episodic, unsteady, high-energy but short-duration, east-flowing turbidity current events superimposed on long-lived, quasi-steady, northward-flowing bottom currents. The channels are bordered by hybrid levee-drift deposits on their bottom current–downstream (northern) sides, which step progressively southward. Channels have steep eroded margins on their bottom current–upstream (southern) side, and gently dipping downstream flanks where the drift-levees step into the channel. We relate the upstream-migrating levee-drifts to lee-wave conditions as bottom currents traverse the channel. The continued development of the drift-levee pins the channels to the slope for protracted time periods. Well core data indicate that the 'toes' of the drift stepping into the channel are dominantly finely laminated siltstones, and that the internal channel architecture and facies distributions are strongly controlled by turbidity- and bottom-current interaction. Furhmann et al.'s integrated study is likely applicable to many other drift systems globally, and provides new quantitative data to enable the inference of bottom-current direction from ancient sedimentary sequences, which can be applied to existing and future studies.

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