Characterisation of Mesozoic Depositional Systems along the Atlantic Passive Margin of Morocco. North Aaiun-Tarfaya Basin

Abstract

Passive margin coastal basins are recipients of thick sedimentary successions with significant economic interest. Natural resources hosted in these sequences are being targeted around the globe. The evolution of passive margins predicted by classic thermal sag models is being challenged by a relatively recent body of studies highlighting “unexpected” vertical movements in the hinterland and basin during late syn- and post-rift stages. This new element of complexity has direct implications in predicting facies distribution and the volume, routing and type of sediment delivered to the basins. The depositional style and evolution of the post-rift sedimentary sequences exposed onshore the Aaiun-Tarfaya Basin (ATB), Morocco, have been studied to characterise the early post rift depositional sequences to assess the relative control of eustacy versus local and regional tectonic movements in the hinterland. The research has resulted in re-dating of a significant part of the northern ATB outcrops using bivalves and gastropods, previously ascribed to the Cretaceous, to a new Jurassic (Bathonian) and older age, which has implications for the prediction of offshore deepwater packages. Three depositional sequences have been described in this interval, with the overall succession exhibiting a deepening-upward transgressive profile from continental conglomerates and sandstones to shallow marine carbonates and fine-grined clastics. Each sequence shows a similar evolution from basal alluvial fan conglomerates to peritidal fine-grained sandstones, silts and interbedded microbial mats and shallow subtidal carbonates. Finally, they are capped by mixed carbonate-clastic shallow marine lagoon to ramp facies. A minimum thickness of 300 m containing age-diagnostic fossil groups was deposited during the Bathonian. This is coeval with a global eustatic sea-level fall evidencing the strong tectonic control on deposition. The lower Cretaceous post-rift successions comprising proximal fluvial to shallow marine extensively exposed onshore Morocco have been logged along two dip profiles. This has been correlated with distal deep water turbiditic equivalent examined in Fuerteventura. The early Cretaceous was the time of onset and evolution of the large Tantan delta. The first outcrop-based sequence stratigraphic framework has been established for the area. The pre-Aptian progradation of the Tantan delta was followed by subsequent regression and reworking. Four depositional sequences, bounded by amalgamated sequence boundary and transgressive surface, have been identified onshore. Offshore, in the inverted sections exposed in Fuerteventura, a Berriasian sand-prone deep-water fan facies is interpreted to record a Falling Stage System Tract, confidently dated with the discovery of ammonite fauna. The proximal onshore equivalent is represented by alluvial fan conglomerates exposed in extensive outcrops onshore and also penetrated by wells. The fans can be associated with faulted contacts with the basement and represent local sediment input on the basin margin. The thick stacked fluvial packages have a mean NNE palaeocurrent, and suggest a long distance regional drainage provenance from the Reguibat Shield. The Pre-Aptian sand-prone fluvial deposits show a significant thickness variation between transects, suggesting a possible structural control on differential subsidence. The third sequence is represented by marine facies interpreted to record a regional transgression (TST) that can be traced across the entire area. This Aptian transgression is followed by aggradational/progradational fluvial (south) and peritidal to shoreface (north) units ascribed to a HST deposited in a homogeneously subsiding basin at this time. In the equivalent deep offshore sediments outcropping on Fuerteventura a large-scale fining-upward cycle of mud-prone turbidites with increasing carbonate and mud content upwards correlates with the sequence onshore. The last sequences studied o

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Hodgetts (Supervisor), Jonathan Redfern (Supervisor) & Stefan Schroeder (Supervisor)