SEDIMENTOLOGY AND FORAMINIFERA CONTENT OF THE BUCATUNNA CLAY MEMBER (OLIGOCENE) IN SOUTHWESTERN ALABAMA

The Bucatunna Clay Member of the Byram Formation (Oligocene, c. 34-25 million years BP) is sporadically exposed throughout portions of southeastern Mississippi and southwestern Alabama. This project targeted an exposure of Bucatunna Clay that crops out along Randons Creek near Monroeville, Alabama. Exposure is generally excellent and in places is laterally continuous for over 40 m allowing for detailed sedimentological and paleontological analysis. Here, the Bucatunna Clay is predominantly composed of dark clay/silty clay. Thin alternations with silt and well sorted quartz sand are interpreted to reflect rapid, short duration fluctuations in depositional energy, specifically water velocity. These intervals are also enriched in comminuted plant material and display evidence of soft sediment deformation, both of which are consistent with flooding events in quiet probably fluvial environments (e.g., oxbow lake, swamps). Other intervals at the study site contain in addition to dark clay, thin beds of well-sorted, laminated and cross-bedded quartz arenite sand, foraminifera-rich intervals and mollusc-dominated shell beds. Comminuted carbonaceous detritus is still present in these shelly beds, but less abundant, suggesting a transition to brackish water conditions. Foraminifera analysis is ongoing, but preliminary data suggests a limited biota consistent with marginal marine deposition. One explanation is that sea level rise flooded what had previously be a primarily non-marine embayment. With more comprehensive analysis that is being done in a companion project, it may be possible to better resolve the Oligocene history of sea level change during deposition of the Bucatunna Clay. The research produced in this study provides important geological information by which to better understand this interval of Cenozoic sedimentation in southern Alabama and will be pivotal for associating the plant fossil content to the depositional and climate conditions during the Oligocene when they lived.