RAPID FLOODING AND DRAMATIC ENVIRONMENTAL CHANGE IN EAST TEXAS/WEST LOUISIANA BAYS DURING THE LATE HOLOCENE

The nature of past coastal evolution provides clues for how coastlines are likely to respond to future natural and anthropogenic sea level, climate and subsidence changes. Sabine Lake and Calcasieu Lake, along the east Texas and west Louisiana coast, are the flooded fluvial valleys of the Sabine and Calcasieu rivers, respectively. The sedimentary deposits in these bays record the history of Holocene marine transgression and modern highstand conditions. The purpose of this study is to document and describe the nature and rate of sedimentation in bays along a sub-humid segment of the northern Gulf of Mexico coast.

A total of 197 km (86 Calcasieu, 111 Sabine) high-resolution seismic data and 7 continuous cores, up to 20 m in length, along the valley axes were collected to examine the response of the bays to eustatic sea level rise and other forcing mechanisms, such as changes in antecedent topography and climate. Seismic and lithofacies changes in the two bays are remarkably similar to each other and to Galveston Bay. Typical valley fill successions consists of, from bottom to top, fluvial sand overlain bay organic-rich bay head delta and delta plain deposits, These upper bay deposits are overlain sharply by laminated fine-grained middle bay mud and the succession is capped by chenier, tidal inlet, barrier deposits of the lower bay. Major flooding surfaces associated with back-stepping facies record radical changes in bay environments and appear to have been bay wide. The timing of these flooding events is poorly constrained but the sub sea depths roughly correspond with well dated flooding surfaces in Galveston Bay, which separate identical facies. The flooding events in Galveston Bay occurred over a few centuries.

The most recent sea level records from the Gulf Coast for this period of time reflects slow continuous rise, which is inconsistent with our inital observations assuming that sea level rise is the cause of flooding events. Ongoing efforts include developing a detailed radiocarbon stratigraphy for Calcasieu Lake. Better chronostratigraphy will shed light on the forcing mechanisms that caused dramatic changes in the bay environment.