INTEGRATED SEQUENCE STRATIGRAPHY, ST. STEPHENS QUARRY, ALABAMA: IMPLICATIONS TO EOCENE-OLIGOCENE GLOBAL CHANGES

The Eocene/Oligocene transition was the most profound change in global climate of the past 50 m.y. St. Stephens Quarry, Alabama (SSQ) has provided one of the global reference sections for this event, yet the basic relationships among sequences, sea level, temperature, and biotic events at this site have been shrouded in controversy. We integrate magnetostratigraphic, biostratigraphic, benthic foraminiferal stable isotopic, benthic foraminiferal faunal, lithostratigraphic, gamma log, and sequence stratigraphic studies from a corehole drilled at SSQ by ARCO. Late Eocene to mid-Oligocene Chrons C16-C10 are well represented and hiatuses are relatively short (<0.5 m.y.) across sequence boundaries at SSQ. We interpret sequence boundaries at the North Twistwood Creek/Cocoa Sand contact (Chron C15n), the mid-Pachuta Marl (mid-C13n), the Shubuta/Bumpnose contact (latest C13r; the earliest Oligocene event), the Mint Spring/Red Bluff contact (C13n/12r boundary), the Byram/Glendon contact (C12n/C11r), and the Bucatunna/Chickasawhay contact (late C11r = the “mid” Oligocene fall). A major oxygen isotopic increase of 1.0 ‰ is associated with the Chron C13r/13n boundary (ca. 33.6 Ma), Shubuta/Bumpnose contact, an erosional surface, and a biofacies shift in the corehole; using the magnetobiostratigraphy on the corehole, this event can be confidently identified as the global Oi1 isotopic increase and maximum. This provides a first-order correlation between the Oi1 increase and a controversial sequence boundary, linking glacioeustatic lowering (estimated as ~55 m from backstripping in NJ) and margin erosion. A precursor oxygen isotopic increase of 0.5 ‰ in mid-Chron C13r at SSQ correlates with a similar increase at Pacific Site 1218; the lack of any significant sequence stratigraphic change at the level of the precursor shift in the SSQ corehole suggests that this was primarily a cooling, not an ice-volume, event. The ca. 33.6 Ma sequence boundary is associated with a very short hiatus (<<100 k.y.) and can be directly linked to the greatest rate of eustatic fall inferred from oxygen isotopes; in contrast, maximum oxygen isotopic values of Oi1 occur above the sequence boundary, suggesting that the lowest eustatic lowstand was associated with resumption of deposition in the coastal plain.