THREE-DIMENSIONAL GEOLOGIC AND HYDROGEOLOGIC SUBSURFACE MAPPING OF THE CUYAMA VALLEY, CA

The Cuyama basin underlies the east-west trending Cuyama Valley in the southern California Coast Ranges. The basin is bounded on the north and south by Pliocene to Pleistocene-age thrust faults forming a structural trough filled with up to 5,000 feet of Pliocene to Holocene -age terrestrial deposits that unconformably overlie Tertiary sedimentary rocks. Groundwater is essentially the sole source of water supply for the growing agricultural community in the Cuyama Valley. Pumping for irrigation has resulted in water-level declines of as much as 300 feet since the 1940s, which has dewatered much of the Holocene alluvium.

Because of continued groundwater extraction from the basin-fill aquifer, the US Geological Survey is developing a three-dimensional (3D) hydrogeologic framework model (HFM) to evaluate the long-term availability of groundwater in the valley. The 3D HFM is built using 3D subsurface mapping techniques to define the geometry and subsurface extent of the structures and main hydrostratigraphic units (HSUs) and textural parameters of the basin-fill deposits. Structures of various orientations offset the basin-fill HSUs and are associated with water-level changes in the basin. Three principal HSUs are defined within the basin: younger and older Quaternary alluvium and the mostly Plio-Pleistocene Morales Formation. HSUs are recognized by their electric log character, degree of cementation, and, where logged, by clast composition. The Morales Formation is characterized by sheet-like deposition and crystalline clasts from sources outside the basin. Post-Morales alluvium is more channel-dominated with lens-like bodies of coarse-grained deposits containing locally-derived clasts.

Within each of these HSUs, data from shallow lithologic logs and deeper electric logs are used to define the percentage of coarse-grained deposits and sorting variations within basin-fill deposits. The spatial distribution of these properties is interpolated for each HSU using basis functions. Well pump tests and specific capacity data are used to semi-quantitatively link these properties to hydraulic properties. The 3D HFM will be used to spatially distribute hydraulic properties for a numerical groundwater flow model of the basin.