Abstract: Neotectonic Implications - Revision of the Ben Lomond Mountain Marine Terrace Stratigraphy, Santa Cruz County, California

Gerald E. Weber, Jeffrey M. Nolan, Erik N. Zinn

Based on recent geologic mapping we propose the following revisions to the Ben Lomond Mountain marine terrace stratigraphy: (1) The lowest emergent terrace (Santa Cruz Terrace) contains only 1 wave-cut platform and shoreline angle (Highway 1), not 3 as indicated by Bradley and Griggs in their 1976 study. This wave-cut platform probably corresponds with the 103 ka (isotope stage 5c) sea-level high stand. (2) The designation of the Greyhound Rock and Davenport wave-cut platforms within the Santa Cruz Terrace as platforms formed by separate sea-level high stands is not justified by field evidence. (3) Only 1 wave-cut platform and shoreline angle are present north of Scott Creek. The Greyhound Rock wave-cut platform is an intertidal bench, not a marine strandline formed by a separate sea-level high stand. (4) Field evidence for the Davenport wave-cut platform and shoreline angle is very weak. The lone field exposure at Davenport is a fluvial terrace of San Vicente Creek. Five refraction seismic lines showing a 7-9 foot high step are, therefore, the sole evidence of the existence of the Davenport wave-cut platform.

Bradley and Griggs, 1976 study of the Ben Lomond Mountain marine terraces has been the basis for subsequent analysis of Pleistocene uplift rates and neotectonics. The presence of 3 wave-cut platforms in the Santa Cruz Terrace proved a dilemma (they preclude constant uplift), resulting in ambiguity of terrace ages and uplift rates. The recognition of only 1 wave-cut platform and shoreline angle in the Santa Cruz Terrace reduces this ambiguity. It suggests the Santa Cruz Terrace corresponds with the 103 ka sea-level high stand and that the uplift of Ben Lomond Mountain has been constant. Uplift rates vary from about 0.35 m/ka near Santa Cruz to 0.48 m/ka near Greyhound Rock. Work funded by USGS NEHRP award #1434-93-G-2336.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California