Abstract:
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.
