Sand provenance in the MSGBC Basin—towards an integrated regional approach to constraining sediment routing and tracing

Cretaceous and younger slope and base-of-slope sediment gravity flow systems form major reservoir targets within the MSGBC Basin. Temporal and regional (along margin, north to south) changes in detrital sand composition and provenance are understudied, as are the key linkages between up-dip composition and provenance, and down-dip (basinal) reservoir supply, presence and quality. Furthermore, few studies seek to understand regional drainage patterns both north and south of the exploration divide centred on Dakar, or to gather complementary datasets at this scale. Part of an ongoing CASP research programme is aimed at resolving this by collecting a suite of samples distributed basin-wide for multi-proxy provenance analysis. The samples are split between comprehensive coverage of modern Senegambian river sands, set alongside key ancillary outcrop sandstones, dominantly from the distal offshore. The data acquired on these samples, together with that available from the literature, provide a first-step towards addressing the issues mentioned above.

As the largest active regional sediment conveyors, the Senegambian rivers provide the best available analogue signature for regional drainage off the West African margin in the past. Initial analysis combined a review of the geology and geomorphology of the c. 3,588,489 km² catchment region, GIS quantification of catchment characteristics, qualitative petrological assessments and the quantitative analysis of translucent detrital heavy mineral assemblages. Current models demonstrate that the sectors of the West African Craton flanking the basin formed a relatively stable regional high, and constituted the main sediment source, with similar surface geology and regional drainage pathways extending as far back as the latest Cretaceous. This stability offers opportunities for detailed integration of sediment generation and sediment routing systems.

Ancient offshore sand fairways likely had three primary input pathways. Our modern river-sand dataset provides baseline signatures for the central and southern sediment conveyors, against which sediment contributions to MSGBC Basin reservoirs can be compared and contrasted. The primary signatures of the Senegal and Gambia river systems’ signature are tied to drainage off the higher-elevation southern and central West African Craton (WAC), and are variably mixed and diluted with reworked lowland Cenozoic sedimentary cover downstream, and with coastal sands near the river mouths. Many of the Senegal and Gambia samples comprise a suite of heavy mineral species that are unstable during prolonged weathering, transport and burial diagenesis and are likely first cycle grains. This signature contrasts with more refractory, mature heavy mineral assemblages found in samples from smaller secondary systems with watersheds confined within sedimentary cover sequences, especially those of the expansive Cenozoic coastal plain. The collection of both upstream and downstream samples provides the opportunity to assess the signature of sands generated during higher efficiency (transmitting more freshly eroded cratonic components) and lower efficiency (incorporating more recycled/weathered signatures) routing conditions in the modern, and to extrapolate these for older systems. These insights may be important because Late Cretaceous sands collected by CASP in the distal offshore province are also contain higher proportions of key unstable grains than might be expected, given their transport distances and burial history.

Integrating our modern riverine provenance data with our onshore proximal, and distal offshore sandstone data has already provided better constraints on the provenance character, feeder pathways and timing of the highest-efficiency Late Cretaceous turbidite systems, which delivered up to coarse grade sand to Maio (Cabo Verde) and to DSDP sites 367 and 368. These early results indicate that the combination of unstable, craton-derived tracer heavy minerals could be used to assess possible sediment pathways and infer the relative contributions of cratonic sectors and recycled detrital components to reservoir units in the offshore. Our results, set alongside industry reports and the available literature, suggest that varietal provenance studies find considerable heterogeneity, which characterises sand types along the exploration fairway, both temporally and spatially. As exploration targets focus on deeper point sourced, canyon fed, base-of-slope and basin floor fan reservoirs, understanding the signatures of regional provenance shifts may prove increasingly important, since unstable HM tracers may highlight the conveyors capable of deeper-water/greater volume sand delivery.