Foraminiferal Biostratigraphy, Paleoenvironmental History, and Rates of Sedimentation Within Subsurface Miocene of Southern Alabama and Adjoining State and Federal Waters Areas

Charles C. Smith

Miocene sedimentary rocks of the study area consist of a predominantly regressive sequence of clay and quartzose sand deposited on a carbonate platform which dips toward the southwest at 50-100 ft/mi. This clastic wedge ranges in thickness from 1,000 ft in central Mobile and Baldwin Counties to a maximum of about 5,800 ft in the northeastern portion of the Main Pass area. Analysis of planktonic and benthic foraminifera has resulted in a refined biostratigraphic zonation of these rocks, which indicates that basal Miocene transgressive shale assignable to the Amphistegina "B" interval zone immediately overlies the upper Oligocene regional carbonate platform. Thus, both lower and lower middle Miocene sedimentary rocks are absent throughout the area of investigation. Biostrat graphic analysis of the middle and upper Miocene rocks has resulted in a series of cross sections illustrating the dramatic thickening southwestward into the federal offshore continental shelf and showing the relationships of producing intervals in the Cibicides carstensi and Discorbis "12" interval zones. Paleoenvironmental interpretations are illustrated on a series of maps constructed for selected regional biostratigraphic zones. These maps have outlined previously unrecognized late middle and early late Miocene deltaic sedimentation in the southeastern Mobile County and Chandeleur-Viosca Knoll (north) areas. Study of sedimentation rates, which range from less than 25 up to 1,370 ft/m.y., further aids in understanding the deltaic and coastal shelf sedimentation of the Miocene within A abama and adjoining state and federal waters areas.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.