DISCRIMINATING QUATERNARY COASTAL-PLAIN STRATA USING AIRBORNE LIDAR AND NEAR-SURFACE GEOPHYSICS: A HELPFUL APPROACH TO LOW-RELIEF GEOLOGIC MAPPING

Depositional units preserved on coastal plains worldwide control lithologic distribution in the shallow subsurface, affect infrastructure design and construction, and are an important repository of information about large-scale climate change that has occurred during Quaternary glacial-interglacial cycles. The lateral and vertical lithologic and stratigraphic complexity of these depositional units and their response to climatic and sea-level change are poorly understood, making it difficult to predict lithologic distribution and 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 traditionally has been based on their expression on aerial photographs and low-resolution topographic maps. Accuracy and detail have been hindered by low relief and lack of exposure. High-resolution airborne lidar surveys, along with surface and borehole geophysical measurements, are being used to identify subtle lateral and vertical boundaries of lithologic units on the Texas Coastal Plain within 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 lithologic 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 aids identification of surface features such as sandy channels, clay-rich interchannel deposits, and accretionary features on Pleistocene barrier islands. An optimal approach to identify lithologic and stratigraphic distribution in low relief 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.