INTEGRATED LATE EOCENE-OLIGOCENE STRATIGRAPHY OF THE ALABAMA COASTAL-PLAIN - CORRELATION OF HIATUSES AND STRATAL SURFACES TO GLACIOEUSTATICLOWERINGS

We integrated strontium and oxygen isotopic, biostratigraphic, and mag netostratigraphic studies of two upper Eocene-Oligocene boreholes dril led near Bay Minette and St. Stephens Quarry (SSQ), Alabama. Continuou s coring provided fresh, unweathered material for magnetostratigraphic studies, minimizing problems reported from nearby outcrops. Difficult ies with each technique were encountered because of diagenesis, absenc e of marker fossils, and the presence of unconformities; however, by i ntegrating results from isotopic stratigraphy, biostratigraphy, and ma gnetostratigraphy, we correlated these relatively shallow-water deposi ts to the geomagnetic polarity time scale (GPTS). At the SSQ borehole, the upper Eocene to lower Oligocene section is apparently complete wi thin our stratigraphic resolution (0.2-0.5 m.y.), allowing us to estim ate the ages of several stratal surfaces. Late Eocene Sr isotope age e stimates are as expected at the SSQ borehole, but Oligocene ages are a pproximately 1 m.y. older than expected due to diagenesis. At the Bay Minette borehole, a latest Eocene-earliest Oligocene and a late early Oligocene hiatus were detected. We correlate these two hiatuses and st ratal surfaces at SSQ with global deltaO-18 increases inferred to repr esent glacioeustatic lowerings and with evidence for hiatuses on other continental margins: (1) a distinct disconformity at the base of the Chickasawhay Limestone at both boreholes and a hiatus at Bay Minette c orrelates with a global deltaO-18 increase; we revise the age of this surface (equivalent to the TB1.1 sequence boundary) making it approxim ately 2 m.y. older than previously reported; and (2) a surface at the top of the Shubuta Member (lowermost Oligocene) has been interpreted b oth as a condensed section and a disconformity; this surface at SSQ an d a hiatus at Bay Minette correlate with a sharp global deltaO-18 incr ease and with hiatuses on the New Jersey and Irish margins. The timing of the hiatuses and stratal surfaces correlates with the inflection o f the deltaO-18 increases and not with the maximum values, supporting models that indicate that unconformities form during the maximum rates of sea level fall.