PADGETT, DEEMS, Jacobs Engineering Group, Pasadena, CA, and ROY J. SHLEMON, Roy J. Shlemon & Associates, Newport Beach, CA
ABSTRACT: Soil-Geomorphic Assessments of Neotectonism at Venus II Antenna Dish Site, Goldstone Deep Space Communications Complex, San Bernardino County, California
Several moderate to strong aerial-photographic lineaments pass within a proposed two arrayed 34-m antenna dish site at the Goldstone Deep Space Communications Complex in San Bernardino County, California. The lineaments trend to the northwest, consistent with the regional neotectonic framework. Backhoe trenches excavated across the lineaments expose a previously undocumented fault that juxtaposes distal segments of alluvial fans against a low discontinuous ridge of granitic rocks.
A representative soil-stratigraphic sequence consists of near-surface unpatinated granitic alluvium bearing an undeveloped soil profile (C and Ckj horizons) underlain by pair of progressively deeper buried paleosols with moderate profile development (upper with argillic Btk and stage II-III calcic horizons; lower with argillic Btk and multiple stage II-III calcic horizons locally grading into stage IV calcrete). Based on relative profile development, the buried paleosols are judged to mark epochs of regional landscape stability and soil formation inferentially taking place about 35,000-40,000 and 100,000 yr ago, respectively (oxygen isotope stage 3 and 5).
The lower paleosol is vertically displaced more than 1 m, and the upper is offset about 0.3 m. Near-surface, unbroken overlying sediments are probably less than a few thousand years old, judging from lack of geomorphic dissection and soil profile development. Accordingly, recurrent movement has taken place in the last 100,000 yr with the last event occurring after about 35,000 yr ago but before an estimated 2000-3000 yr before present. The previously unrecognized fault may therefore be "active" according to present state of California definition. Consequently, appropriate setbacks and seismic design criteria recommended for the antenna dish sites to mitigate potential seismically induced ground rupture and moderate to strong ground shaking.
AAPG Search and
Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach,
California, May 5-7, 1993.