ABSTRACT

Subsurface Miocene sediments of coastal Alabama and the adjoining state and federal waters area consist of a clastic wedge varying in thickness from about 500 feet in south-central Baldwin County in southern Alabama to a maximum of about 6,000 feet in the northeastern portion of the Main Pass Area. Relatively deep-water and open-marine transgressive basal Miocene clays and shales unconformably overlie a gently southwestward dipping late Oligocene-earliest Miocene carbonate platform. Middle and late Miocene sediments consist of a regressive offlapping sequence of sand and shale deposited in varying neritic paleoenvironments.

Analysis of planktonic and benthonic foraminifera has resulted in a refined biostratigraphic zonation of these sediments, permitting the recognition of several regional time-equivalent datum levels, or biohorizons. These biohorizons are shown on a series of subsurface cross-sections which indicate the dramatic southwestward thickening of middle and late Miocene sediments within the Cibicides carstensi and Discorbis "12" interval-zones.

The paleoenvironmental history of the Miocene has been reconstructed on a series of paleobathymetric maps drawn for selected regional biohorizons. Among other features, these maps have proven the existence and outlined the margins of previously unrecognized shallow-neritic deltaic sediments in southeastern Mobile County and in the Chandeleur and Viosca Knoll (north) Areas. Analysis of sedimentation rates, which range from less than 25 to 1,370 feet per million years, further aids in understanding the coastal shelf, deltaic, and open-marine depositional history of the Miocene of Alabama and the adjoining state and federal waters areas.