ABSTRACT: Depositional Setting of the Arcola Limestone Member (Campanian) of the Mooreville Chalk, Eastern Gulf Coastal Plain: Assessment of the Pelagic and Benthonic Depositional Models

TEW, BERRY H. , Geological Survey of Alabama, Tuscaloosa, AL

In the eastern Gulf Coastal Plain area of Alabama and Mississippi, the Arcola Limestone Member (Upper Cretaceous-Campanian Stage) of the Mooreville Chalk consists of indurated limestone that is rhythmically interbedded with chalky marl. The limestone beds are calcisphere wackestones and packstones that contain very little terrigenous clastic detritus, whereas the chalky marls are foraminiferal and nannofossil wackestones in which clastics are more abundant. Both the limestone and marl units are characterized by their overall fine-grained nature.

Previous studies have interpreted the limestone beds of the Arcola to represent accumulations of calcispheres produced by attached, benthonic algae similar to modern calcareous dasycladaceans. An alternative interpretation for the depositional setting of the Arcola invokes a pelagic origin for the Arcola limestone beds. In both models, the marl units are interpreted as hemipelagic deposits.

Physical and geochemical data collected from five localities have been used to assess the relative merits of these depositional models and to determine the most likely genesis of the Arcola Limestone Member. Data from thin-section petrography, carbonate and total organic carbon content determination, and stable isotope composition analysis (oxygen and carbon) were included in this study. These data indicate a pelagic depositional setting for the limestone beds of the Arcola. Further, cyclical alternation of pelagic and hemipelagic conditions associated with fluctuating input of terrigenous clastic detritus into the depositional basin best explains the rhythmically interbedded nature of the limestone and marl units.

AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas