Abstract

Depositional units preserved on coastal plains worldwide are an important repository of information about large-scale climate change that has occurred during more than 20 Quaternary glacial-interglacial cycles. In general, the lateral and vertical complexity of these depositional units and their response to climatic and sea-level change are poorly understood, making it difficult to place historical and anticipated future climate and sea-level change in a natural geologic context. Mapping Quaternary siliciclastic depositional units on low-relief coastal plains historically has been based on aerial photographs. Accuracy and detail have been hindered, however, by lack of exposure and low relief. High-resolution airborne lidar surveys, along with surface and borehole geophysical measurements, are being used to identify lateral and vertical boundaries of stratigraphic units on the Texas Coastal Plain within upper Quaternary strata. Ground and borehole conductivity measurements discriminate sandy barrier island and fluvial and deltaic channel deposits from muddy floodplain, delta-plain, and estuarine deposits. Borehole conductivity and natural gamma logs similarly distinguish distinct depositional units in the subsurface and identify erosional unconformities that likely separate units deposited during different glacial-interglacial stages. High-resolution digital elevation models obtained from airborne lidar surveys reveal previously unrecognized topographic detail that greatly aids mapping of subtle surface features such as sandy channels, interchannel deposits, and accretionary features on barrier islands that formed during the last interglacial period. An optimal mapping approach for coastal-plain environments employs (1) an initial lidar survey to produce a detailed elevation model; (2) selective surface sampling and geophysical measurements based on preliminary mapping derived from lidar data and aerial imagery; and (3) borehole sampling, logging, and analysis at key sites selected after lidar and surface measurements are complete.