Oxygen and carbon isotope analysis of the Mooreville Chalk and late Santonian-early Campanian sea level and sea surface temperature changes, northeastern Gulf of Mexico, U.S.A.

Oxygen and carbon isotope profiles for strata of the Mooreville Chalk (upper Santonian-lower Campanian) of the eastern Gulf Coastal Plain, U.S.A., show correlations with published curves for these isotopes. The δ18O curve exhibits a strong similarity to the Exmouth Plateau δ18O curve from ODP drilling sites offshore northwestern Australia, and the δ13C curve can be correlated with the δ13C curve from the English Chalk Trunch section.A thermal maximum probably occurred in the northeastern Gulf of Mexico during the latest Santonian (∼83.8 Ma), as evidenced by the minimum δ18O values in the lower Mooreville beds. A δ13C positive excursion occurs at the same stratigraphic level, which has been recognized as the “Santonian/Campanian boundary event” worldwide. After this event, ocean surface water temperature decreased throughout the early Campanian. This carbon isotope excursion is followed by a plateau in δ13C values with a peak value occurring in a condensed section (∼80 Ma), which has been correlated to a downlap or maximum flooding surface on seismic data from offshore Alabama. The section characterized by increasing δ13C values corresponds to a marine transgression. The interval characterized by decreasing δ13C values corresponds to regression and progradation. The maximum flooding event occurred ∼0.8 Ma later than the thermal maximum event.The Mooreville chalk/marl cycles are most likely a product of fluctuations in siliciclastic sediment influx into the northeastern Gulf of Mexico modulated by the precession band of the Earth's orbital cycles. Higher carbon isotope values occur in the marl beds indicating that these beds were formed in a more anoxic/dysoxic environment characterized by higher clay, silt input and higher organic carbon accumulation.