Spatial Changes in Tectonic and Stratigraphic Style across a Transform Fault, Offshore Sierra Leone Basin (West Africa): Implications for Potential Reservoir and Trap Architecture

Elenwa, Chinwendu A.; Watkinson, Matthew P.; Anderson, Mark W.

The Sierra Leone basin is the westernmost of the West African transform margin basins, which have been the focus of increasing exploration interest following a series of recent discoveries. Unlike most of the West African passive margin, these basins are dissected by extensions of oceanic transform faults which juxtapose transitional or oceanic crust against rifted continental crust. They frequently contain evidence of transtensional and transpressional tectonics during, or shortly after, the syn-rift phase. We demonstrate how the tectono-stratigraphic evolution of the Sierra Leone basin and spatial changes in transform to passive margin tectonics, control potential reservoir and trap distribution.

The Sierra Leone margin can be split into two sections which are separated by the extension of the Sierra Leone transform fault (SLTF). To the west of the SLTF, the deep-water parts of the basin are underlain by thinned continental crust showing an array of half-graben basins with well-defined syn- and post-rift sequences. Adjacent to the SLTF, these syn-rift basins were inverted early in the post-rift phase through uplift in the hanging-walls of reverse faults. During the post-rift phase (late Early Cretaceous) this region became the continental slope, but inversion lifted the slope above grade. Synclinal basins, in which mid-slope stepped accommodation space was partly controlled by differential compaction, developed above the inverted half-grabens. Mid-slope submarine fan channel system geometry and stacking patterns were controlled by a combination of this stepped accommodation space, eustacy and the effects of confined slumps.

To the east of the SLTF is a conventional passive margin and transform tectonics are absent. Syn-rift half-grabens are not recognized in the deep-water basin because of the presence of through-going seaward dipping reflectors. This suggests that this part of the margin was affected by previously unreported extensive volcanism. The post-rift slope megasequence is unaffected by inversion tectonics. From mid-Cretaceous times, unconfined graded slope and basin sequences show onlapping and offlapping toe-of-slope geometries. These sequences formed during phases when the slope was either below grade, during which time the basin floor facies on-lapped the slope (the slope was largely bypassed), or graded. When at grade, the slope prograded offshore and offlapping shingled turbidite systems developed at the base of the slope.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013