SEDIMENTOLOGY AND STRATIGRAPHY OF THE OLIGOCENE BUCATUNNA CLAY MEMBER (BYRAM FORMATION, VICKSBURG GROUP) , SOUTHWESTERN ALABAMA, USA

The Bucatunna Clay is a relatively thin member of the Byram Formation that is exposed along creeks in portions of southeastern Mississippi and southwestern Alabama. It thickens toward the south and east into Florida where it represents an important subsurface aquiclude. As with much of the Tertiary in the southeastern United States, surface exposure of the Bucatunna Clay is sporadic; however, near Monroeville AL, laterally continuous exposures can be found. Although dominated by dark silty-clay, the Bucatunna Clay Member also contains thin beds of well-sorted, laminated and cross-bedded quartz arenite sand, foraminifera-rich intervals and mollusc-dominated shell beds. Comminuted carbonaceous detritus is common throughout the member, but the outcrop that we are studying is noteworthy for containing several intervals of well preserved and abundant fossil leaves, seeds, and fruits. Identification of specific flora is underway, however, at this point in our study it appears that faunal assemblage is dominated by at least five angiosperm morphotypes. From a stratigraphic perspective, the near continuous exposure of the Bucatunna Clay from the underlying Byram Marl Member to the overlying Chickasawhay Formation in combination with the foraminifera and flora suggest that this outcrop will provide much needed age control for Oligocene strata in the southeastern United States. We also feel that the outcrop will reveal new insight concerning the region’s Tertiary paleoenvironments of deposition. The Bucatunna Clay was likely deposited in a marginal-marine/estuarine/coastal wetland environment, but detailed sedimentological examination including high resolution grain size analysis is needed to confirm this. This presentation will present the preliminary results of our study.