Imaging Sedimentary Structures of the Monterey Dunes Using Ground Penetrating Radar

The Quaternary dunes at Monterey Bay are one of the largest coastal dune\ud complexes in California and contain a valuable record of climate history. In this\ud study, ground penetrating radar (GPR) was used to image sedimentary\ud structures of the dunes at Monterey Bay in order to determine subsurface dune\ud morphology, and to provide insight to paleowind direction and the dunes???\ud response to changes in climate.\ud 32 GPR profiles were recorded at four coastal sites: Marina Dunes\ud Preserve, Lake Court, Stilwell Hall, and Fort Ord Dunes State Park. Profiles\ud ranged in length from 56 to 523 m, and data were acquired with a pair of\ud transmitting and receiving antennas mounted on either a wheeled cart or on a\ud hand-carried stretcher. Signal penetrated to a maximum depth of ~ 12 m with 50\ud MHz antennas, and to ~ 10 m with 100 MHz antennas. The theoretical vertical\ud resolution is 0.6 m for 50 MHz antennas, and 0.3 m for 100 MHz antennas based\ud on velocity at ~ 0.12 m/ns obtained from common midpoint velocity analysis. Up\ud to four radar surfaces and six radar packages were imaged in a Marina Dunes\ud profile, while other profiles typically showed three radar surfaces and packages.\ud Radar surfaces ranged in length from 30 to 300 m, and radar packages ranged in thickness from 2 to 8 m. Several profiles imaged the water table, and its presence\ud attenuated the signal and prevented deeper imaging.\ud All profiles exhibit foreset laminations that are interpreted to be relict\ud slipfaces. Based on the orientation of slipfaces within both relict and active\ud dunes, the dominant wind direction during the Holocene has consistently been\ud from the northwest. Southeast-dipping relict slipfaces are interpreted to indicate\ud grainfall and landward dune migration, as observed in modern active dunes.\ud Trough structures are interpreted to be scour marks created by changing wind\ud directions. Overall, imaged dune sedimentary structures indicate paleowind\ud direction and strength behave similarly to modern climatic conditions of\ud prevailing northwesterly wind during much of the year