High-Resolution Carbon Isotope Stratigraphy: Separating Global from Local Effects on a Jurassic Carbonate Ramp of the U.S. Gulf Coast

Pigott, Kulwadee L., Michael H. Engel, Richard P. Philp, University of Oklahoma, Norman, OK

The sequence stratigraphy of the Upper Jurassic (Oxfordian) Smackover Formation was investigated by chemostratigraphic analysis in the Conecuh and the Manila Embayments of southwest Alabama, Gulf Coast of United States. &delta¹³C_carb results demonstrate the effectiveness of using stable carbon isotopes in evaluating carbonate ramp stratigraphy.

The &delta¹³C_carb record from cores suggests that isotopic sequence stratigraphic sig­nals can be preserved regardless of the depositional environment and diagenetic differ­ences. Four short-term cycles, indistinguishable because of local effects, could be differen­tiated. Chemostratigraphy indicates the lowest sequence boundary was not concordant with a lithostratigraphic boundary between the Norphlet and the Smackover Formations. During a relative sea level lowstand which was associated with a disconformable boundary, local and global effects caused a negative &delta¹³C_carb excursion. In the ensuing relative trans­gression and sea level highstand, low, or no siliciclastic contamination increased carbonate sedimentation and resulted in heavier &delta¹³C_carb values. While lateral paleoenvironment changes during the Smackover sequence were recorded by lithological and elemental vari­ations, the sequence stratigraphy of the Smackover can be vertically revealed exclusively by applying a &delta¹³ C_carb approach. The interpretation is supported by a global correlation with other Oxfordian sequences and to the third-order eustatic cycles.

Carbon isotope chemostratigraphy is a powerful tool for helping decipher variable effects in carbonate sequence analysis, providing correlative insight into the timing of intra­basinal depositional events. Carbonate ramp platforms such as the Smackover, owing to rapid lateral changes with small changes in vertical sea level, provide ideal proxies for test­ing hypotheses of global versus local sea level effects upon carbonate development.