Abstract

Diverse detrital source areas surrounding the Sacramento Valley of northern California and the abundance of deep water wells from which sand samples can be acquired make this an ideal natural laboratory in which to apply actualistic sand petrofacies methods in solving hydrogeologic subsurface problems. Discriminant analysis identifies four distinct compositional suites (petrofacies) representing four distinct sources: metamorphic (M), volcanic (V), mixed volcanic and metamorphic (VM), and mixed volcanic, metamorphic, and sedimentary (VMS). Proportions of aphanitic metamorphic, volcanic, and sedimentary lithic fragments (using the Gazzi-Dickinson point-counting method) are the primary distinguishing parameters.

The combined surface and subsurface mapping of lithofacies and petrofacies confirms that the Tehama and Tuscan Formations interfinger in the subsurface near the center of the northern Sacramento Valley. The Pliocene–Pleistocene Tehama Formation along the west side predominantly consists of metamorphiclastic detritus derived from the Klamath Mountains and the Coast Ranges, including the Franciscan Complex and the Great Valley Group. The Pliocene–Pleistocene Tuscan Formation along the east side predominantly consists of volcaniclastic detritus derived from the southern end of the Cascade Range and the Modoc Plateau. These petrofacies results complement regionally integrated surface mapping and subsurface data in order to construct regional cross sections showing basin architecture.

The general correspondence between lithofacies and petrofacies provides a powerful tool for further investigation, but the lack of correspondence in a few cases indicates where additional work is warranted. The simultaneous study of sand composition and three-dimensional geologic relations provides additional insights regarding the architecture of aquifers, knowledge of which improves water-resource development and management.

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