Evidence of a large upper-Cretaceous depocentre across the Continent-Ocean boundary of the Congo-Angola basin. Implications for palaeo-drainage and potential ultra-deep source rocks

The analysis of 2D deep-seismic-reflection profiles across the slope and abyssal plain of the Angola oceanic basin reveals the existence of a significant and formerly unknown depocentre beneath the giant Cenozoic Congo deep-sea fan, between 7000 m and 9000 m depth, deposited directly onto the Aptian oceanic crust. The unit, which is up to 2.5 km thick and extends for more than 200 km basinwards of the Continent-Ocean boundary, is probably aged Albian-Turonian. Its radial fan-shaped depocentre is centred on the present-day Congo River outlet and contains at least 0.2 Mio km(3) of sediments. These observations and the results from flexural modelling indicate that (1) the location of the Congo River's outlet has remained fairly stable since the Late Cretaceous, and (2) the basal unit was indeed sourced by a palaeo-Congo River probably located nearby the present-day one. Thus, the Atlantic sedimentary system related to the exoreism of the Congo River is much older than previously thought. Thermal modelling indicates that the maturation history of this upper-Cretaceous deposits is highly influenced by the interaction between the initial high heat flow of the young oceanic crust and further increase in sediment supply due to the progradation of the overlying Tertiary deep-sea fan during the Miocene. Hence, despite low present-day heat-flow values, should the unit have source rock potential, its basal section may be currently generating hydrocarbons. All in all, the results from our models also suggest that the interplay between an initially high heat flow and the further high sediment supply in areas of major river input, may be a key factor for the thermal maturation of potential source rocks deposited onto a present-day cold oceanic crust. (C) 2009 Elsevier Ltd. All rights reserved.