Abstract: Primary, Non-Milankovitch Marl-Limestone Parasequences, Upper Cretaceous, Alabama, U.S.A.

David T. King Jr.

Primary marl-limestone parasequences that lack orbital periodicity occur in late Campanian (81-81 Ma) high-stand 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 owing to buildup and relaxation of intraplate stresses. In these primary parasequences, the coccolith-rhabdolith-foraminiferal marl beds are flooding-phase hemipelagic deposits and the limestone (calcisphere packstone) beds are benthic deposits representing still-stand and subsequent, relatively rapid shallowing conditions.

Differences in whole-rock delta Oxygen-18 (per mill v. PDB) isotopic-ratio values between marl samples, averaging -3.1, and limestone samples, averaging -1.9, are attributable to differences between pelagic and benthic water temperature and salinity. Higher delta Carbon-13 (per mill v. PDB) isotopic- ratio values among marl samples (average = -0.3) versus those in the limestone samples (average = -2.8) result from enhanced preservation of organic matter during relatively high rates of shelfal marl accumulation and preservation. These high rates are confirmed by higher total organic-carbon values for marl samples (average = 0.3 percent) than for limestone samples (average = 0.1 percent).

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana