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Gordian Chuks Obi1,
John Holbrook2
(1) University of Nigeria, Nsukka, Nsukka, Nigeria
(2) Southeast Missouri State University, Cape Girardeau, MO
Abstract: Campanian - Maastrichtian Sequence Stratigraphy of the Anambra Basin, Southern Nigeria
Four stratigraphic sequences (S1, S2, S3, & S4) are identified in the Campanian - Maastrichtian Anambra basin fill in the vicinity of the Enugu cuesta, Enugu State, Nigeria. Sequence S1 is separated from the pre-Campanian, strata by a type 1 sequence-bounding angular unconformity dating 77.5 my. The S1 transgressive and highstand systems tracts comprise open marine shale and biomicrite (Nkporo Shale) deposited during the relative sea level rise that followed the Santonian uplift of the Abakaliki anticline in the Benue trough. Shoreface Nkporo deposits are truncated above by the basal type 1 sequence boundary of S2 (71 my), which is overlain in turn by fluvial valley-fill strata of the Owelli Sandstone. Transgressive and highstand marine deposit of the Enugu Shale overly the Owelli in angular unconformity reflecting possible Campanian reactivation of Abakaliki and/or Benue structures. Maastrichtian Mamu Formation fluvial and shoreface strata overlie the Enugu shelf deposits with an erosional type 1 unconformity (68 my) and a correlative regressive surface of erosion, and comprise sequence 3. Fluvial strata of the Ajali Formation overlie the Mamu unconformably (67 my), and are in turn overlain by shelf and shoreface Nsukka strata, which together comprise sequence 4 and complete the Maastrichtian section.
The above arrangement of systems tracts and lithofacies document pulsating withdrawal of the Atlantic sea from the Benue trough during Campanian - Maastrichtian time. Proximal fluvial and distal marine character of these lithofacies and wedge-shaped geometry of this section are a function of the continued asymmetrical subsidence to the southwest, of the Anambra platform during latest Cretaceous time.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana