Tectono-stratigraphic evolution of the Mesozoic continental margins of the Central Atlantic

Abstract

The geology of the continental margins of northwest Africa and northeast South America, surrounding the southern Central Atlantic have been investigated through a fully integrated, multi-disciplinary study to document the tectono-stratigraphic evolution of the Mesozoic post-rift sequence. These sedimentary basins are some of the most prolific underexplored hydrocarbon provinces worldwide. Rejuvenated tectonism associated with the opening of the Equatorial Atlantic in the Aptian interrupted the relatively ‘passive’ post- rift subsidence and was a significant control on sedimentation. Across the southern Central Atlantic, this impacted the source-to-sink system through hinterland uplift (source), tectonic deformation of the basin (sink), and associated effects on sedimentary systems, routing and depositional style. A high-resolution stratigraphic framework has been constructed integrating existing data with new results from biostratigraphic and sedimentological analyses of outcrop localities on the island of Maio, Cape Verde and the re-sampling of seven key scientific boreholes and three exploration wells. Margin-scale chronostratigraphic charts have been constructed to investigate facies distribution and highlight significant regional hiati to enhance understanding of temporal stratigraphic evolution. Major unconformities are recognised extending thousands of kilometres across the Central Atlantic such as the base Albian unconformity (BAU). Local stratigraphic breaks are also identified, typically restricted to one structural domain, i.e. the Late Cretaceous regional composite unconformity (RCU) located on the distal escarpment margin offshore Senegal and The Gambia. The mechanisms generating these regional unconformities have been considered and data suggest plate- scale tectonic events are the major drivers. The middle Berriasian unconformity (MBU) recognised for the first time at DSDP Site 367 has an associated time gap of ca. 5 Myr, linked to far-field effects of North Atlantic breakup within the Iberia-Newfoundland segment. The Aptian-aged transpressional rifting in the Equatorial Atlantic heavily deformed the pre-Aptian carbonate platform stratigraphy on the Demerara Rise creating a suite of compressional structures resulting in major margin collapse. Further observations along the northwest African Atlantic margin suggest the effects of this compression extend ca. 500 km north to Dakar, forming broad intra-oceanic crust folding (DSDP Site 367) and thrusting of the distal escarpment margin. On the Demerara Rise, the deformed pre-Aptian stratigraphy is peneplained by a striking angular unconformity termed the base Albian unconformity (BAU); up to 1 km of sediment has been removed. In subsequent Albian times, shelf edge delta systems prograde to the edge of the escarpment margin depositing siliciclastic sediment across the shelf and to a base-of-slope apron. Quantitative seismic geomorphology offshore The Gambia reveals the submarine canyonisation of the distal escarpment margin forming kilometre-scale canyons on the RCU surface. Submarine lobe deposits at the terminus of these systems are generally debris-rich, containing eroded blocks up to 1 km3 of the underlying carbonate platform. Further unconformities are recognised on the Demerara Rise related to the creation of oceanic crust (intra Late Albian) and continental breakup between S. America (Demerara Rise) and Africa (Guinea Plateau) in the earliest Cenomanian. To aid source rock characterisation, geochemical analysis of 110 samples reveals organic-enrichment in a well-constrained time interval from the Late Albian (NC10a upper nannofossil zone) through the peak (>30% TOC) during the global oceanic anoxic event OAE-2 (Late Cenomanian) into the earliest Turonian (northwest Africa), and Coniacian (northeast South America). This organic-rich interval is the major source rock candidate for generating hydrocarbons along these prospective continental margins.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor) & Mads Huuse (Supervisor)