Temporal and spatial evolution of Mesozoic drainage systems feeding the deepwater Atlantic passive margin of Morocco: Tarfaya Basin

During the Mesozoic, the hinterland of the Atlantic passive margin of Morocco suffered km-scale post-rift exhumation of Precambrian and Palaeozoic terrains, accompanied with enhanced subsidence offshore. The continental to... [ view full abstract ]

During the Mesozoic, the hinterland of the Atlantic passive margin of Morocco suffered km-scale post-rift exhumation of Precambrian and Palaeozoic terrains, accompanied with enhanced subsidence offshore. The continental to shallow marine record of this Mesozoic succession is exposed onshore in the Tarfaya Basin and comprises extensive coarse clastics. We present new detailed depositional environment models and renewed biostratigraphy that refines the existing palaeogeography of the Tarfaya Basin. An integrated multi-proxy study involving extensive sedimentary logging, subsurface data, provenance and structural modelling, is allowing a better understanding of the entire margin. The results illustrate the scale and evolution of drainage systems feeding the deepwater basins, and offer enhanced temporal and spatial resolution.
On the northern margin of the basin, red beds are exposed along the coast. Previously dated as Early Cretaceous, extensive fieldwork and new biostratigraphy suggests a date no younger than Early Jurassic. This is more consistent with the facies style and stratigraphic location, and is also supported from mapping and seismic interpretation trends offshore. The coarse clastics were deposited as alluvial fans and proximal fluvial facies very likely during the syn-rift and early post-rift phases of the Atlantic opening. To the south, new biostratigraphic data suggests a conformable Middle Jurassic alluvial plain sandstones and siltstones with common interbedded caliches and fluvio-lacustrine carbonates, overlain by cyclic, mixed siliciclastic/carbonate of Middle to Upper Jurassic age. The carbonates were deposited on a shallow carbonate shelf as oolitic shoals and lagoonal facies, with variable terrigenous input from nearby local small- to medium-sized fluvial systems.
Further south, the Early Cretaceous is characterised by the Tan Tan fluvio-deltaic system, which can be correlated with age-equivalent deepwater turbidites on the Canary Islands. These deepwater sediments are being targeted as possible hydrocarbon reservoirs offshore Morocco. Onshore, a thick sand-rich succession (~2000 m thick) records two major regressive/transgressive cycles, with clastic pulses during the Valanginian and Late Barremian. In this succession, prevailing fluvial facies comprising stacked channel fills interbedded with fine-grained overbank deposits, are punctuated by marine shoreface sandstones and overlain by thick Aptian-Albian nearshore to offshore sandstones, siltstones and clays.
Bedform dimensions, the areal extent of the depositional fairway, sediment volume calculations and preliminary 2D seismic interpretation offshore record a large fluvial-dominated delta system comparable in scale to the modern Mississippi River. Ongoing low-temperature geochronology and palaeotopography modeling supports both local sediment source from the Anti-Atlas and a large continental drainage basin extending hundreds of km inland.