Cyclostratigraphy of the Arcola Limestone Member (Campanian) of the Mooreville Chalk, Alabama and Mississippi, Northern Gulf Coastal Plain
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 decimeter- to meter-scale bedding rhythms of alternating calcisphere limestone and chalky marl. The indurated limestone beds of the Arcola Limestone Member contain abundant Pithonella spherica calcispheres (calcareous dinoflagellates). These pelagic organisms normally inhabit deep-water outer shelf and slope environments but their periodic inhabitation of shelf settings, such as the Late Cretaceous eastern Gulf Coastal Plain basin, under conditions conducive to their proliferation has been documented. The marl beds that rhythmically alternate with the Arcola calcisphere limestone beds can be generally characterized as hemipelagic foraminiferal and nannofossil wackestones. Analysis reveals that the carbonate, total organic carbon, and sulfur contents and carbon stable isotope compositions of the indurated calcisphere limestone beds are significantly different than those of the associated marl units. The rhythms in the Arcola Limestone Member interval are interpreted to result from cyclic changes in terrigenous elastic sediment input into the basin, accentuated by intervals of slightly enhanced carbonate productivity. Cyclicity is inferred to be associated with climate variability (wet and dry periods) and short-term relative sea level variations, augmented by small-scale fluctuations in carbonate productivity. High calcisphere and low terrigenous detritus contents in the relatively pure, indurated Arcola Limestone Member limestone beds, in combination with the near-total absence of calcispheres and much higher terrigenous elastic content in the marl units, indicate periodic reductions in turbidity on the shelf, leading to favorable conditions to support "blooms" of the calcisphere-producing dinoflagellates. The Arcola Limestone Member calcisphere beds represent transgressive pulses during which widespread pelagic conditions characterized the shelf. The interbedded marls of the Arcola accumulated under hemipelagic conditions during progradational regressive pulses.