Hybrid turbidite–contourite systems of the Tanzanian margin

Abstract

Extensive 3D seismic datasets acquired during exploration offshore southern Tanzania have revealed the complex architecture of two contrasting styles of hybrid turbidite–contourite deposits that formed in the Cretaceous (Albian–Early Campanian) and Paleogene (Paleocene–Oligocene). Both sequences are characterized by migrating channel-levee complexes, interpreted to record, and be diagnostic of, the synchronous interaction of eastward, downslope flowing turbidity currents and northerly, along-slope flowing contour currents. Flow stripping of the fine-grained suspended part of the turbulent flow by weak contour currents led to the formation of expanded levee-drifts on the northern (downstream) side of the channels, which prograded southwards (upstream), driving southwards migration of the turbidite channel axis. The difference in the architecture of the two successions is due to the variation in slope topography at the time turbidite activity commenced and the frequency of coarse clastic input into the basin. Cretaceous (Albian–Campanian) turbidite systems were strongly controlled by the position of pre-existing contourite drifts and moats. The contorted geometry of the system provided loci for the deposition of Cretaceous reservoirs comprising thick, amalgamated channel deposits with a high net-to-gross ratio (N:G) and good vertical connectivity, and intra-slope fans with lower N:G and poor vertical connectivity. Paleogene turbidite channels initially evolved on a smooth slope. Sustained southward channel migration produced Paleogene reservoirs comprising complexly laterally-connected sheets of channel and lobe deposits above a southward-younging, diachronous compound unconformity. In both hybrid systems, contour current influence modified the geometry of the turbidite systems, resulting in temporal and spatial partitioning of the depocentres on the slope.

Distinguishing between Deep-Water Sediment Facies: Turbidites, Contourites and Hemipelagites

“…Turbidity current properties are becoming better known from observational and experimental data [41][42][43][44][45][46] coupled with theoretical modelling [47][48][49], although there is still a range of uncertainty over some estimates. Based on a considerable volume of earlier work [16,17,[50][51][52][53][54][55][56], as well as more recent studies, we can summarise the principal properties as follows (see also [1,26].…”

“…Some good modern examples have been described from the Columbia gateway in the SW Atlantic [137][138][139], the Tanzanian margin [140] and the Sicily gateway in the Mediterranean [135]. Some of the sediments described from mixed drift systems, such as those on the Antarctic Peninsula margin [141] and Tanzanian margin [140], have been interpreted as the result of sediment supply from turbidity currents, followed by the capture of the fine suspended cloud by active bottom currents. This material is then deposited at varying distances from the supply channel across the adjacent levees or mixed drift bodies in a down-current direction.…”

Hybrid turbidite-drift channel complexes: An integrated multiscale model

“…4Aii). During this time, the channel and levee deposits are reworked and redistributed, forming one-sided, hybrid levee-drift deposits to the north (sensu Shanmugam et al, 1993;Palermo et al, 2014;Sansom, 2018;Fonnesu et al, 2020). 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 leewave conditions (Flood, 1988).…”

“…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 (e.g., Talling et al, 2012;Rebesco et al, 2014). While the influence of bottom currents on submarine channel architecture has been recognized, interpretation has been hampered by a lack of direct monitoring data (Shanmugam et al, 1993;Gong et al, 2018;Sansom, 2018;Fonnesu et al, 2020). Modern, integrated data sets, including direct measurements and monitoring of bottom currents, are of great importance in understanding the complexity of oceanographic processes (Fierens et al, 2019;Miramontes et al, 2019;Thiéblemont et al, 2019), sediment gravity flows (Clare et al, 2016;Azpiroz-Zabala et al, 2017;Symons et al, 2017), and the preservation of strata within submarine channel complexes (Gamberi et al, 2013;Vendettuoli et al, 2019).…”

The evolution of submarine slope-channel systems: Timing of incision, bypass, and aggradation in Late Cretaceous Nanaimo Group channel-system strata, British Columbia, Canada

“…Our data set sheds light on the fidelity of the stratigraphic record and also provides temporal context for the commonly recognized stratigraphic evolution of submarine channels. Horizontally to vertically aligned channelform stacking patterns are observed globally over a wide range of temporal and spatial scales (spanning distances up to 20 km laterally and >1000 m vertically, over time scales ranging from 10 4 to 10 6 yr; Jobe et al, 2018;Sansom, 2018). Although calculated durations may not be directly applicable to systems of varying longevity, their relative comparison is informative and potentially useful for evaluating the temporal significance of this pattern in other deposits.…”