Palaeolake Shoreline Sequencing Using Ground Penetrating Radar: Lake Alvord, Oregon, and Nevada

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Field, map, and aerial photoreconnaissance in the Lake Alvord basin has focused on identifying late Pleistocene depositional shoreline features (e.g., tombolos, spits, barriers). Features in different areas of the basin are well defined, and their spatial extents are easily mapped; however, absolute---or even relative-ages of shoreline features are not clear. Ground penetrating radar (GPR) was used to distinguish between intermediate and highstand stage shorelines during what is thought to have been the latest Pleistocene, threshold-controlled lake cycle. Radar transects of 280 and 600 m imaged a spit and a baymouth barrier at sites in the northeastern quadrant of the basin where transects were aligned normal to the strike of each depositional geomorphic feature. Signal penetration with 100 MHz antennas was shallow (-4 m), but resolution was sufficient to locate and identify gross morphostratigraphic features. Flooding surfaces are shown to correspond to intermediate stage lake surface elevations, and the absence of a flooding surface at the elevation of the highest shoreline indicates this to be the maximum lake illrface elevation during this cycle. Elevations of intermediate lake stage elevations and highstand stage elevations were consistent at the two sites, with the highstand elevations corresponding closely to the basin threshold at Big Sand Gap. These data provide a firstorder approximation of lake stage sequence and the degree of postdepositional neotectonic activity and illustrate the utility of GPR when used in context with field measurements in distinguishing transgressive and highstand features.