Abstract

In the eastern Gulf Coastal Plain (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 containing more abundant clastics; both are characterized by their overall fine-grained nature. Some previous studies have interpreted the limestone beds to represent accumulations of calcispheres produced by attached, benthonic algae. An alternative interpretation for the Arcola invokes a pelagic origin for these beds, in which the calcispheres derive from a planktonic algal source. Data and observations from five localities have been used to evaluate these models and determine the most likely genesis of the Arcola Limestone Member. On the basis of this evaluation, a pelagic depositional setting is inferred for the limestone beds of the Arcola. 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. High calcisphere and low terrigenous detritus contents in the limestone beds and near total absence of calcispheres in the marls indicate periodic reductions in turbidity, leading to favorable paleoceanographic conditions to support "blooms" of the calcisphere producing dinoflagellates.