--> Recently Revealed Hybrid Turbidite-Contourite Systems of the Tanzanian East Africa Margin

AAPG ACE 2018

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Recently Revealed Hybrid Turbidite-Contourite Systems of the Tanzanian East Africa Margin

Abstract

The Tanzanian offshore area is emerging as the best calibrated part of the East African Margin, following successful deep-water exploration since 2010. New 3D seismic and well data has revealed for the first time the stratigraphy and complex architecture of the slope and offers the opportunity to study hybrid turbidite-contourite systems in detail, for several hundred km along strike and over 100 km in a proximal-distal direction. The Tanzanian slope is characterised by two end-member geometries of hybrid turbidite-contourite systems, which reflect differences in the way the systems interact and directly influence reservoir type, architecture and connectivity. The Albian-Campanian systems are characterised by the basinward coalescence of individual canyons into fewer trunk systems with convoluted pathways across the slope, controlled by the position of pre-existing contourite drifts. The turbidite channel systems show a generally southward migration direction and become increasingly entrenched as the intervening drifts grow. The contorted geometry of the system provides loci for the deposition of thick, amalgamated intra-slope fans. The Paleocene to Oligocene hybrid systems exhibit a similar cycle of growth and entrenchment, but have a markedly different geometry and associated reservoir architectures. The supply channels prograded into the basin together with the development of broad levees, which grew progressively into large, asymmetric, hybrid levee-drifts, separating relatively straight channel fairways with a direct west-east pathway down the slope. The channel fairways show a very consistent southward migration direction, which is particularly marked and regular in the Oligocene. This complexity, coupled with the stepwise southerly migration, produces a diachronous, climbing sheet of complexly connected sandbodies with greater reservoir heterogeneity than the ponded slope fans seen in the Cretaceous. The Oligocene channel systems increase their angle of climb to become vertically stacked by the Early Miocene, after which they become disrupted by widespread, large-scale mass transport complexes which completely re-model the slope architecture. The development of models for the hybrid turbidite-contourite systems of Tanzania is at an early stage, and debate remains about the direction and nature of the contourite currents, the mechanisms of interaction of turbidity and contour current processes and the relationship to sequence stratigraphy.