6533b852fe1ef96bd12aab2a

RESEARCH PRODUCT

Sedimentation in the Kandi extensional basin (Benin and Niger): fluvial and marine deposits related to the Late Ordovician deglaciation in West Africa

subject

description

Abstract The Lower Paleozoic detrital succession of the half-graben Kandi Basin in West Africa (Niger-Benin) is about 600 m thick and rests unconformably on the Pan-African basement. Along the western edge of the basin, the base of the succession locally features large glacial fault-bounded paleovalleys. These valleys are filled by the lowermost continental deposits of the Were Formation characterized by massive diamictites with dropstones, and coarse to conglomeratic sandstones associated with large-scale channel structures and internal erosional truncations. The uppermost braided-river deposits of the Were Formation deposited across the entire basin are overlain by the Late Ordovician–Early Silurian storm and tidal sediments of the Kandi Formation, made up of hummocky cross-stratified sandstones and siltstones. Computer-aided analysis of the populations of synsedimentary to synlithification microfaults observed in the Late Ordovician to Early Silurian sediments shows evidence of extensional paleostress tensors with a N90°E to N100°E horizontal σ3 stress responsible for normal displacement along the Kandi Fault. The synsedimentary normal activation of this major fault, inherited from the Precambrian, controls the spatial arrangement of the glacial, braided-stream, storm to tidal, and offshore deposits as well as the deformation of the basin-fill into an asymmetric synsedimentary syncline associated with progressive unconformities. The characterization of glacial features and Late Ordovician deposits from the biostratigraphic distribution of traces of trilobites strongly supports the idea that the deposits of the Kandi Basin are contemporaneous with the melting of the wide ice sheet which overlay the Afro-Arabian Shield during Late Ordovician times. The successive deposits of the Were and Kandi Formations reflect a gradual change from tillites, through glaciofluvial outwash conglomerates (Wa Member), braided-stream sediments (Wb Member), and shoreface barrier sands (Ka Member), to offshore clays and sands (Kb Member). They correspond to reworked, glaciofluvial to marine facies laid down by the Late Ordovician glacial retreat. The Kandi Basin is therefore defined as a staging-post between the Late Ordovician––Early Silurian basins of the Sahara and those of South Africa.