PALAEOLAKE SHORELINE SEQUENCING USING GROUND-PENETRATING RADAR: PALAEOLAKE ALVORD, OREGON AND NEVADA

Field, map, and aerial photo reconnaissance in Alvord Basin have focused on identifying late Pleistocene and Holocene depositional shoreline features (e.g., tombolos, spits, barriers) in the basin. Features in the different areas of the basin are well-defined and their spatial extents easily mapped; however, absolute – or even relative – ages of shoreline features are not clear. Subsequent activity along several faults in the basin has resulted in differences of up to several meters between elevations of the shorelines across the basin, hampering correlations.

Ground-penetrating radar (GPR) was chosen as a tool in constructing the palaeolake shoreline hydrograph by matching up depositional sequences at the different locations. Three sites were selected to represent different circulation nodes in the basin – Miranda Barrier Spit (north), Bone Creek Littoral Spit (west), and Calderwood Embayment Mid-bay Barrier (east). All three sites consist of depositional features with different drift directions. Transects ranged from 280 to 600 meters in length and were aligned normal to the strike of each depositional feature. Data were collected using 100 MHz antennae with a separation of 1-meter and a 0.25-meter step.

Signal penetration was shallow (~ 4 meters) at all three sites, but sufficient to locate and identify near surface morphostratigraphic features. The Miranda and Calderwood sites provided the best images. We hypothesize that the subsurface environment at the Bone Creek site consisted of poorly sorted and much larger clasts than the other two sites because of differences in fetch and length of drift from source material.

Radar imagery from Miranda and Calderwood shows the relationship of the highest elevation feature as a later transgressive stage over the lowest shoreline. In these images, the surface of the lower elevation barrier can be traced underneath the upper shorelines. In the Miranda image, a topographically-defined shoreline intermediate between the upper and lower stages does not continue as a reflector under the highest shoreline, suggesting this was formed during a recessional stillstand. These results demonstrate the utility of GPR in constructing a palaeolake hydrograph.