Depositional Patterns across Syndepositional Normal Faults, Niger Delta, Nigeria
Ajibola O. Owoyemi1 and Brian J. Willis2
1 ChevronTexaco, Lekki, Nigeria
2 Texas A&M University, College Station, TX
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 nearly 2 km thick Miocene deltaic Agbada Formation 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 down-dropped 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 incisions a relatively thin layer of basal, high-sinuosity channel sandstones are abruptly overlain by a up to several hundred-meter-thick, basinward-inclined bed set that coarsens upward. Above intervals that fill incisions higher-frequency parallel seismic reflections record vertical aggradation of growth strata after erosional basin floor topography was buried. 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 due to 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.