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Hybrid Turbidite-Drift Channel Complexes: an Integrated Multiscale Model A https://doi.org/10.1130/G47179.1 Manuscript received 7 November 2019 Revised manuscript received 27 January 2020 Manuscript accepted 28 January 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Hybrid turbidite-drift channel complexes: An integrated multiscale model A. Fuhrmann1, I.A. Kane1, M.A. Clare2, R.A. Ferguson1, E. Schomacker3, E. Bonamini4 and F.A. Contreras5 1 Department of Earth and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK 2 National Oceanography Centre, European Way, Southampton SO14 3HZ, UK 3 Equinor, Martin Linges vei 33, 1364 Fornebu, Norway 4 Eni Upstream and Technical Services, Via Emilia 1, 20097 San Donato Milanese, Milan, Italy 5 Eni Rovuma Basin, no. 918, Rua dos Desportistas, Maputo, Mozambique ABSTRACT sedimentological model for bottom current– The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity influenced submarine channel complexes. flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past GEOLOGICAL SETTING flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such The Jurassic to Paleogene basins offshore of hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution sea- East Africa formed during the breakup of Gond- floor data, and near-bed current measurements from two sites where eastward-flowing gravity wana (Salman and Abdula, 1995). Following flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data Pliensbachian to Aalenian northwest-southeast from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, rifting, ∼2000 km of continental drift took place deposited over a period of ∼20 m.y. (Upper Cretaceous to Paleocene). High-resolution mod- along north-south–striking lineaments, such as ern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, the Davie Ridge fracture zone (DFZ) and the which are related to northward-flowing near-bed currents with measured velocities of up to Sea Gap fault (SGF) (Fig. 1A), between the 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins Kimmeridgian and Barremian (Reeves, 2018) (with respect to bottom currents), with inhibited deposition or scouring on the upstream (Fig. 1B). Cessation of rifting was marked by flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated active seafloor spreading between Madagascar muddy siltstone, which progressively step back into the channel axis over time, result in an and India, leading to the development of the interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow present-day East African passive continental directions contrast with those of previous models, which lacked direct current measurements margin (Reeves et al., 2016). Albian transgres- or paleoflow indicators. We finally show how large-scale depositional architecture is built sion resulted in the development of the extensive through the temporally variable coupling of these two globally important sediment transport deep-marine deposits that are the focus of this processes. Our findings enable more-robust reconstructions of past oceanic circulation and study. Major river systems drained the African diagnosis of ancient hybrid turbidite-drift systems. continent and supplied fine-grained sediment, while additional sediment was shed from up- INTRODUCTION Gong et al., 2018; Sansom, 2018; Fonnesu et al., lifted rift shoulders along the paleocoastline As the terminal part of sedimentary source- 2020). Modern, integrated data sets, including (Smelror et al., 2008; Fossum et al., 2019). to-sink systems, deep-sea deposits have been direct measurements and monitoring of bottom used to reconstruct past climates, sediment and currents, are of great importance in understand- DATA AND METHODS carbon budgets, and the distribution of anthro- ing the complexity of oceanographic processes This study used high-resolution 3-D seis- pogenic pollution (Sømme et al., 2009; Masson (Fierens et al., 2019; Miramontes et al., 2019; mic reflection data (covering 4885 km2) and et al., 2010; Rebesco et al., 2014; Drinkorn et al., Thiéblemont et al., 2019), sediment gravity 14 exploration wells provided by Equinor ASA 2019; Kane and Clare, 2019). Bottom currents, flows (Clare et al., 2016; Azpiroz-Zabala et al., (Norway) and ExxonMobil (USA). Seismic data i.e., density-driven circulation in the deep ocean, 2017; Symons et al., 2017), and the preservation were tied to the biostratigraphically calibrated and sediment gravity flows are the main pro- of strata within submarine channel complexes wells (and core data of Well A; Figs. 1C and cesses that form and modify these deposits (e.g., (Gamberi et al., 2013; Vendettuoli et al., 2019). 1D) to map seismic and stratigraphic geometries Talling et al., 2012; Rebesco et al., 2014). While This study combines an integrated subsurface offshore Tanzania (Fig. 1). The average vertical the influence of bottom currents on submarine study (three-dimensional [3-D] seismic, core, resolution in the Upper Cretaceous of our study channel architecture has been recognized, in- and well-log data) with modern seafloor geo- area is ∼20–30 m (average velocities of ∼2.9– terpretation has been hampered by a lack of di- morphology and near-bed bottom-current mea- 3.3 km/s, and average frequency of 35 Hz). The rect monitoring data (Shanmugam et al., 1993; surements to develop a process-product–based data have a bin spacing of 12.5 × 12.5 m and a CITATION: Fuhrmann, A., et al., 2020, Hybrid turbidite-drift channel complexes: An integrated multiscale model: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G47179.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47179.1/4974090/g47179.pdf by University of Southampton/National Oceanography Centre user on 03 April 2020 ABE40 E41 S7 C ? Drift SGF DFZ Well A ectonic Litho- N Block 5 T history time Seismic Geological horizon Central drift stratigraphy Drift Holocene S8 Pleistocene Quat. Block 4 Pliocene EARS Drift Miocene Drift Block 3 Neogene ? Base Neogene (I) TWT [ms] S9 Mandawa Basin Oligocene -2500 -5000 -7500 Base Oligocene ? Block 2 25 km SGF Eocene E ` Paleogene D Well A S10 Central drift F Block 1 Paleocene ` Base Paleogene (high amplitude artifacts) Rovuma Basin E Tanzania Mid- Campanian Upper G ` S11 ectonic reactivation of SG fault Base Turonian (II) T F 50 km Base Cenomanian Mozambique Mid- Albian Cretaceous ? Area of work G RMS amplitude Channel Lower low high License boundaries Lobe Well locations Drift 25 km SGF N-S continent drift SW NE SW NE E Well A Lv/Drift E ` F F ` Sw Ch TWT TWT Lv/Drift 250 ms Sw 1.25 km 400 ms 2.5 km SW NE W G Drift Drift E ` H G TWT I Ch 5000 m 1000 ms Ch Ch Sw SGF Drift Ch Lv/Drift Lb Central drift H Hybrid levee-drift I Hybrid levee-drift developed to the NE Channel migrating SW “Toe” of the drift stepping into the channel complex Inferred bottom-current direction Inferred bottom current direction Lobe complex SW Hybrid levee-drift NE Hybrid levee-drift developed to the NE Channel migrating SW “Toe” of the drift stepping into the channel complex Inferred bottom-current direction SW Inferred bottom-current direction Lobe complex NE Figure 1. (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. Colors of seismic hori- zons 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; well tie is shown in Fig. 3C) and lateral seismic facies 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47179.1/4974090/g47179.pdf by University of Southampton/National Oceanography Centre user on 03 April 2020 o A X ` Y ` 40 50'E Z ` 4 km 5 m bin size N AUV bathymetry Water depth [m] 0 2800 Mooring: D Mooring:B N Mooring:C 10 Mooring: C 0 W E N 8 10 Figure 2. Modern bot- Mooring: B tom-current and seafloor 45'S N Frequency [%] S o 0 W E data, offshore northern 20 10 Mozambique. (A) Bathy- Zoom metric map of the seafloor 0 W E 8 Mooring: D including location of in B Frequency [%] S moorings and cross sec- 20 tions. AUV—autonomous underwater vehicle. (B) Frequency [%] S Furrows and scours at Inferred flow direction the seafloor indicate Turbidity the dominance of north- current flowing bottom currents. ROV—remotely operated Bottom vehicle. (C) Cross sec- current X Y Z tions X, Y, and Z showing B C SE NW less-steep canyon walls 1.0 (drift) at the northern N 1.4 X X flank of canyons. (D) ` Bottom-current velocity 1.2 measurements for each 1.6 Y Y mooring indicate strong ter depth [km] ` seasonal variability.
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