Abstract

Niger Delta depositional patterns are complicated by structural collapse of prograding deposits into underlying mobile shale. Delta field, along the northwest margin of Niger Delta, is located on the dominantly extensional proximal margin of a major collapse structure. This structure defines a sub-basin "depobelt" within this prograding clastic wedge. The Miocene deltaic Agbada Formation was deposited on a succession of cuspate normal faults displaced basinward under the weight of accumulating sediments. This nearly 2-km-thick succession contains five erosionally bounded sequences. Where not deeply incised, successive sequences become progressively thinner upsection, and change from dominantly upward-coarsening deltaic successions to dominantly channel sandstones interbedded with paralic mudstones. Older sequences thicken basinward to a greater extent onto downdropped fault blocks and across crests of associated rollover anticlines. Basal erosion of younger sequence boundaries is narrower and deeper along elongate trends extending landward into footwall blocks.

Along the axis of deepest sequence-boundary incision, fills contain a relatively thin layer of basal, high-sinuosity channel sandstones that are abruptly overlain by shale. The rest of the deposits within sequences are separated vertically into two parts: a lower part defined by hundred-meter-thick, basinward-inclined bed sets that coarsen upward, and an upper part with higher-frequency parallel seismic reflections that record vertical aggradation of growth strata after erosional basin-floor topography was buried. Broad constraints on the age of deposits from biostratigraphic studies suggest a tentative link between sequence-boundary development and sea level but do not constrain the timing of sequence incision and filling relative to sea-level stage. Vertical facies changes within individual sequences do not support formation of erosional sequence boundaries by lowstand fluvial incision; rather, sequence boundaries are interpreted to form by faulting of the sea floor, the slumping of rapidly accumulated sediments and erosion by submarine mass-flow channels into footwall blocks. Deeply incised sequence boundaries may be more likely to develop during initiation of a new depobelt, as deltas first prograde to the shelf edge onto undercompacted shales.