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.

SummaryThe Coastal Basin of Tanzania has historically been one of the best-studied areas of the East African margin, with a stratigraphic succession that records the evolution from the Gondwana breakup phase, through transform and passive margin phases to the East African Rift phase. However, published stratigraphic columns based on onshore exposures and wells show considerable variation regarding the recognition and duration of major unconformities, the completeness of the succession and the age and correlation of some lithostratigraphic units. The recent upsurge in deep water exploration on the Tanzanian slope has provided a wealth of 3D seismic and well data. These data have calibrated the more complete offshore succession and revealed in detail the complex depositional architecture resulting from the interaction of turbidite and contourite systems on the Tanzanian slope for the first time. This study presents new information on the offshore stratigraphy, including ages of key unconformities and reservoir units, and integrates the offshore and onshore successions to provide a new stratigraphic scheme for Tanzania, comprising 9 unconformity-bounded tectono-stratigraphic megasequences. This scheme provides the first complete picture of stratigraphic evolution across the margin from the Jurassic to the Neogene, establishing Tanzania as the best-calibrated part of the East African Margin.

Deposition of the Callovian–Ryazanian Humber Group of the UK Central Graben occurred during rifting and long-term relative sea-level rise, which acted to suppress the formation of eustatically forced Exxon-type sequence boundaries. The superposition of highly variable halokinetically controlled subsidence means that classic, passive-margin derived sequence stratigraphic models are not appropriate to describe stratigraphic evolution in this rift setting. The sequence stratigraphy of the Humber Group has been re-evaluated using a transgressive–regressive sequence model, where maximum regressive surfaces are employed as sequence bounding surfaces. The Humber Group comprises two megasequences which reveal distinct phases of evolution of the basin. The latest Callovian–Kimmeridgian megasequence comprises a conformable sequence stack which lacks significant internal unconformities and records progressive marine flooding and overall backstepping onto the basin flanks during a phase of active rifting. The Volgian–Ryazanian megasequence is condensed and highly fragmentary due to punctuation by a number of unconformities which are consistently recognizable throughout the basin. The onset of this change in architectural style corresponds to the oldest unconformity at the base of the Volgian–Ryazanian succession, termed the Base Volgian–Ryazanian Unconformity, of latest Kimmeridgian to earliest Volgian age. The patterns of erosion of the Callovian–Kimmeridgian megasequence and the intra Volgian–Ryazanian unconformities record the effects of dramatic redistribution of underlying salt accompanied by probable uplift of the Forties–Montrose High and J Ridge, resulting in major modification of the basin morphology and the severing of possible earlier links with the Fisher Bank Basin. The kinematics of this event are equivocal, but it is possible that restricted Volgian–Ryazanian depocentres resulted from localized salt collapse rather than basement extension. Widespread erosion of Callovian–Kimmeridgian Humber Group sediments may have occurred in some areas where Volgian–Ryazanian Kimmeridge Clay deposits now overlie pre-Jurassic strata, and exploration models must incorporate the effects of Volgian reconfiguration in order to accurately predict reservoir distribution.

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