--> Abstract: Sealevel Change and the Depositional History of a Shallow, Coastal Aquifer System, Northern Monterey, CA, based on Indicator Geostatistics, by J. A. Erskine and A. T. Fisher; #90920 (1999).

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ERSKINE, JON A., and ANDREW T. FISHER
Earth Sciences Department, University of California, Santa Cruz, CA

Abstract: Sealevel Change and the Depositional History of a Shallow, Coastal Aquifer System, Northern Monterey, CA, based on Indicator Geostatistics

We conducted a multidisciplinary investigation of shallow aquifers and confining layers within a coastal groundwater basin in northern Monterey County, CA. Shallow sediments within this basin comprise a Holocene, unconfined, aeolian and valley-fill aquifer, and an underlying, Pleistocene aquifer of fluvial and alluvial silt, sand, and gravel. These two aquifers are separated by a confining unit of clay, silt, and sand, previously interpreted to be a lagoonal and shallow marine deposit of latest Pleistocene to Holocene age. The 40 km2 study area contains 231 wells and borings, from which cuttings and core description data were digitized and compiled at 0.6-m depth intervals. Additional geophysical logging (natural gamma-ray and induction resistivity) data was collected in 55 wells. This information was used for proxy lithological identification based on calibration from core descriptions, and combined with the core and cuttings data to generate a database of lithological interpretations at 22,000 X-Y-Z locations. These data were used to generate geostatistical realizations, using a binary (clay versus not clay) indicator scheme for the confining unit and shallow, valley-fill aquifers. The realizations indicate a complicated, three-dimensional geometry that is not apparent in earlier cross-sections of the confining unit, and appears to record multiple sealevel changes, most likely associated with isotope Stage 5. This interpretation has important implications for the hydrogeology of this site and the Salinas Valley in general, as well as other coastal California basins.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California