HOLOCENE EVOLUTION OF WESTERN LOUISIANA-EAST TEXAS BAYS PUNCTUATED BY RAPID FLOODING EVENTS AND ENVIRONMENTAL CHANGES

Detailed marine geological studies were conducted in Calcasieu Lake, Sabine Lake, Galveston Bay, Matagorda Bay, and Corpus Christi Bay. The objective of this research was to investigate how the bays responded to past changes in the rate of relative sea-level rise and climatically triggered changes in sediment flux. This information can be used to predict how bays will respond to future changes in climate and sea-level rise. The study area is characterized by a strong, potentially unstable climatic gradient and subsidence rates are among the highest in North America. Recent studies have shown that subsidence is increasing to the point where relative sea-level rise is approaching early Holocene rates. The data set used in this study consisted of dense grids of seismic data and long drill cores that sample through the bay fill. A robust radiocarbon stratigraphy was obtained for each bay.

An important factor regulating bay evolution is the level to which the valley was filled by fluvial sediments prior to initial flooding. This varies between -15m and -30 meters from east to west, which reflects the decrease in the long-term sediment flux of rivers in that direction. Once the valley is flooded, bay evolution is strongly regulated by the antecedent topography of the incised fluvial valley and coastal plain. A third factor regulating bay flooding histories is climatically induced changes in sediment flux. Because these variables differ from area to area, the flooding histories of the bays have been diachronous.

The evolution of each bay was punctuated by rapid flooding events that drastically altered their environmental setting. The main flooding events resulted in landward shifts in bay environments of tens of kilometers and increases in bay area up to 30% in a century or two. Most of these flooding events occurred prior to 5,000 years ago, when the rate of sea level was rising at a fairly steady rate.