Stratigraphy and Structural Features of the Continental Margin Basin of Senegal and Mauritania

Abstract

Published Géological and geophysical information was compiled and considered with some gravimetric and seismic data of the BGR to obtain a synthesis of the stratigraphy and structure of the Senegal-Mauritania Basin:

Refraction and gravimetric data show the deepest part (ca. 14 km) of the basin to lie under the lower continental slope. It is flanked in the west by anoceanic hinge line in Jurassic basalts with hundred-meter fault throws. In the east faults of acontinental hinge line offset continental blocks up to a kilometer. This structural boundary of the continent had been active until the Tertiary and formed listric growth faults in the Cretaceous section north of Dakar.

Doleritic rocks occurring in Senegal are not associated with rift tectonics of the Atlantic ocean formation.Triassic rift sediments have not been found onshore south of the Precambrian Reguibat High, however during this time and until the Middle Jurassic a peneplanation surface was eroded in eastern Senegal (? rift onset unconformity).

The base of theJurassic section and the time of the first marine transgression are not known, but regional subsidence of the continental margin is assumed to have started during the Liassic. An almost continuous succession ofJurassic to Cenomanian platform carbonates forms a regressive Mesozoic shelf edge with a 4 km high escarpment under the southern continental slope. This most prominent structural feature deepens to the north and is less pronounced off Mauritania. Paleozoic? to Triassic diapirs, originating from an evaporite layer underneath the carbonates at 8 to 10 km depth off Casamance, there occur landward of the Mesozoic shelf edge, while off Mauritania? Liassic diapirs grew from < 6 km depth on the seaward side.

Athick Cretaceous-Tertiary basin offshore Mauritania, with a regional northward deepening of sediment layers, thins in a southern and western direction. Its eastern border is drawn by a group of N-S striking faults along the 200 m-isobath.

Extensive deltaic wedges developed during theCretaceous off Mauritania and possibly drowned the Mesozoic carbonate platform. AnotherCretaceous to Paleogene carbonate platform exists further north off Cape Blanc in the form of a capital letter L. Its southern limb possibly marks the eastern extension of the Kane Fracture Zone, which is tentatively inter–preted to extend into the onshore region of the Arguin Platform.

After the Early Jurassic extensional tectonic phase asecond major tectonic phase during the LateCretaceous established fault controlled anticlines and voluminous igneous intrusions, documented by semi-circular gravity anomalies, in the coastal region of the basin.

During theearly Tertiary carbonate deposition reappeared on the present shelf, while the coast line regressed, as an epicontinental sea flooded almost the whole basin.

In theNeogene a prograding sedimentation pattern dominated again. Renewed activity on deep coastal faults especially in the Miocene and Early Quaternary uplifted deep water sediments and culminated in an intensive alcalinevolcanism on the Cape Vert peninsula, while sea mounts were constructed on the continental slope.Early Miocene slumps and turbidite deposits under the lower continental slope point to the activity of mass wasting processes. The Middle Miocene shelf edge lies under the upper continental slope. During thePliocene/Pleistocene numerous canyons on the continental margin, which coalesce into two major channel systems on the continental rise, cut into the sediments that had outlasted several paleogene and neogene erosional cycles.