Miocene igneous activity in the Northern Subbasin, offshore Senegal, NW Africa

More than 70 enigmatic discordant seismic anomalies of limited lateral extent, associated with eye-shaped structures and folds, have been imaged on 2D seismic data from the offshore Northern Subbasin of the Senegal Basin, NW Africa. In this paper we define their distribution, characterise their morphology and compare them to similar features imaged by seismic data along the NE Atlantic Margin that have been demonstrated to be igneous sills, hydrothermal vents and intrusion-related forced folds. We propose that the Cretaceous and Paleogene strata in the northernmost part of the offshore Northern Subbasin were heavily intruded by igneous sills during the Miocene. The proposed timing of intrusion is coincident with that previously suggested for the development of the Cayar Seamount located 100 km NNW of Dakar and onshore volcanism in the Cap-Vert area near Dakar and in the Cape-Verde Islands. This stage of Miocene igneous activity is interpreted to be closely associated with the coeval collision between Africa and Europe. The interpreted igneous sills give rise to an unexpected “soft” seismic response. The preferred explanation for the “soft” appearance of the sills is that the majority of sills are thin (<90 m) and hence their seismic appearance is affected by interference between the acoustic response from the top and base of the sills. The identification of shallow-level igneous intrusives in the offshore Northern Subbasin has important implications for hydrocarbon exploration in the Senegal Basin and possibly more broadly for hydrocarbon exploration in Atlantic basins offshore NW Africa.