Late Quaternary Stratigraphic Evolution of the Southwestern Louisiana Inner Continental Shelf: Paleo-Landscape Preservation Potential and Implications for Offshore Sand Resources

摘要

Understanding how exposed coastal plain shelves respond to sea-level rise is important to determine paleo-landscape and associated intact prehistoric cultural resource preservation potential within the modern shelf. During sea-level rise, wave and tidal ravinement eroded the coastal plain interfluves. Backstepping fluvial and estuarine sedimentation within incised valleys potentially preserved landscape features below the depth of ravinement. Along-strike variable shoreface retreat rates locally favored preservation landward of barrier islands, shoals, and headlands as adjacent landscapes were eroded. Shallow seismic and scant vibracore data lack the resolution to discern the nature of paleovalley fills and determine if features such as paleosols, middens, etc. are present. Moreover, constraining the age of paleovalley fills is difficult because of their time-transgressive nature, limited absolute dates, difficulty correlating onshore-offshore, and stratigraphic incompleteness. To interpret better internal stratigraphy, structure, and relative age of individual paleovalleys, a geographic information system (GIS) compilation of the southwestern Louisiana continental shelf was developed from industry hazards survey maps with interpreted paleovalley deposits for 131 blocks, seismic profiles, and boring data. Five Upper Pleistocene to Holocene stratigraphic units and four associated unconformities are recognized. For a few units, unambiguous correlations can be made with units previously dated, and characterized onshore for their sedimentology and geoarchaeology. Chronostratigraphy is refined based on cross-cutting relationships and identification of bounding surfaces identified in previous studies adjacent to the southwestern Louisiana shelf, and indicates that existing models used for predicting prehistoric cultural resource potential should be revisited to account for multiple sea-level fluctuations and further refined with absolute dating methods.