THREE-DIMENSIONAL MODEL OF THE VALLEY FILL DEPOSITS OF THE CENTRAL VALLEY, CALIFORNIA

A three-dimensional (3D) texture model was developed to help characterize the aquifer system of the Central Valley, California, for a ground-water flow model. The Central Valley aquifer system is comprised of heterogeneous valley-fill deposits that underlie 50,000 km2 in the Sacramento and San Joaquin Valleys. The model was developed by compiling and analyzing approximately 8,500 drillers' logs, ranging from 4 to 950 meters below land surface, that were selected to define the 3D lithology of the valley. The lithologic descriptions on the logs were simplified into a binary classification of coarse- or fine-grained. The percentage of coarse-grained sediment, or texture, was then computed from this classification for each 15-meter depth interval of the driller's logs. A 3D texture model was developed for the entire valley by three-dimensional kriging the percentage of coarse-grained deposits on to a 1.6-kilometer spatial grid at 15-m depth intervals from land surface down to 800 m below land surface.

The 3D texture model results correlate to source areas, independently mapped geomorphic provinces, and to factors affecting the development of fluvial fans. Fine-grained deposits in the Sacramento Valley are partially derived from volcanic rocks, and, in the western San Joaquin Valley, from Coast Range shales and other marine rocks. The eastern San Joaquin Valley is predominantly coarse-grained alluvial and fluvial fill derived from Sierra Nevada granite. The texture distribution also correlates to differences in the basin, channel, fan, and interfan geomorphic provinces. Factors affecting the development of fluvial fans include subsidence rates, valley width, and the spacing, alignment, and tributary source area of the main tributaries to the Sacramento and San Joaquin Rivers. Because hydraulic conductivity is usually correlated to the percentage of coarse-grained deposits, the texture model results explain regional, spatial, and vertical heterogeneity in the aquifer system.