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Primary, Non-Milankovitch Marl-Limestone Parasequences, Upper Cretaceous, Alabama
David T. King, Jr.
Primary marl-limestone parasequences that lack orbital periodicity occur in late Campanian (81-80 Ma) highstand systems tract deposits within the upper part of the Mooreville Chalk, Alabama Coastal Plain. The marl-limestone parasequences, ranging from 1.1- to 5.2-m thick, were deposited in a regime of episodically fluctuating relative sea-level controlled by buildup and relaxation of intraplate stresses. In these parasequences, the coccolith-rhabdolith-foraminiferal marl beds are flooding-phase hemipelagic sediments and the limestone (calcisphere packstone) beds are benthic sediments that represent stillstand and subsequent, relatively rapid-shallowing conditions.
The marl beds have a constant clay content of approximately 30 percent, however among their framework silicate grains a fining-upward texture and an upward-increasing relative abundance occurs. The limestone beds are rich in benthic-algal calcispheres and contain other benthic fauna including echinoderms, bivalves, and benthic foraminifera. The average carbonate content in the limestone beds is approximately 90 percent. The marl beds contain a simple ichnologic assemblage of dense Thalassinoides, whereas the limestone beds have a complex ichnofabric dominated by Thalassinoides but also including Chondrites, Teichichnus, and Zoophycos. Differences in mean whole-rock 18O ( PDB) isotopic ratio values between marl (-3.1) and limestone (-1.9) are attributed to temperature and salinity differences between benthic and pelagic waters. Higher mean 13C ( PDB) isotopic ratio values in marl (-0.3) versus those in limestone (-2.8) reflect enhanced preservation of organic matter during relatively high rates of shelf-marl deposition. These high rates are confirmed by higher total organic carbon values for marl (average = 0.3 %) than for limestone (average = 0.1 %).
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