Cyclicity in Upper Mississippian Bangor Limestone, Blount County, Alabama

Raymond L. Bronner

The Upper Mississippian (Chesterian) Bangor Limestone in Alabama consists of a thick, complex sequence of carbonate platform deposits. A continuous core through the Bangor on Blount Mountain in north-central Alabama provides the opportunity to analyze the unit for cyclicity and to identify controls on vertical facies sequence.

Lithologies from the core represent four general environments of deposition: (1) subwave-base, open marine, (2) shoal, (3) lagoon, and (4) peritidal. Analysis of the vertical sequence of lithologies in the core indicates the presence of eight large-scale cycles dominated by subtidal deposits, but defined on the basis of peritidal caps. These large-scale cycles can be subdivided into 16 small-scale cycles that may be entirely subtidal but illustrate upward shallowing followed by rapid deepening. Large-scale cycles range from 33 to 136 ft thick, averaging 68 ft; small-scale cycles range from 5 to 80 ft thick and average 34 ft. Small-scale cycles have an average duration of approximately 125,000 years, which is compatible with Milankovitch periodicity. The large-scale cycles have an aver ge duration of approximately 250,000 years, which may simply reflect variations in amplitude of sea level fluctuation or the influence of tectonic subsidence along the southeastern margin of the North American craton.

The dominance of subtidal deposits in the core suggests a location near the seaward margin of the Bangor platform. Analysis of the Bangor in more cratonward and more basinward locations will test the feasibility of using cyclicity for stratigraphic correlation and hopefully indicate the overall influence of tectonic subsidence on cyclicity.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.