NIX, MICHAEL A., Employable Geologist, Cottondale, AL

ABSTRACT: Use of Well-Log Signatures to Demonstrate Time Equivalency of Deltaic and Carbonate-Platform Deposits in the Black Warrior Foreland Basin

Foreland basins may be sites of transition from clastic to carbonate sedimentation. Clastic-wedge sediments originate at active plate margins, whereas carbonate production occurs along the craton margin in the absence of clastic sedimentation. Clastic and carbonate deposits interfinger within a mixing zone of the two end members.

The Bangor Limestone was deposited on the northeast flank of the Black Warrior foreland basin, representing a carbonate buildup on the East Warrior platform. Upward-shoaling carbonate depositional cycles are separated by southwest-thickening mudstone beds which are distal deltaic deposits of the Parkwood Formation. The clastic to carbonate mixing zone produces a lithostratigraphic unit which generates a characteristic resistivity log signature. Distribution of distinct log signatures demonstrates the relationship of clastic to carbonate sedimentation.

The Bangor/Parkwood interval is divided laterally into four different vertical sequence types across the basin; each of the sequence types is defined by a distinct pattern in the log signature. The lower part of the Bangor/Parkwood interval is more carbonate-rich toward the platform. The middle part consists of mudstone which grades to limestone bordering the platform. The upper part represents the mixing zone that forms the lithostratigraphic unit which migrates northeastward with increased influx of clastic sediments. Each sequence type corresponds to successive limits of clastic sediments, forms a linear band parallel to the platform edge, and illustrates northwest-southeast continuity of stratigraphy. Because of this relationship, the time-rock correlations separating Parkwood del aic progradations can be extended through the mixing zone into the carbonate platform, indicating time equivalency of the Parkwood Formation with the Bangor Limestone.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.