A STRATIGRAPHIC EVALUATION OF THE TUSCAN FORMATION, NORTHERN CALIFORNIA

The Tuscan Formation is found in outcrop in the northeast portion of the Sacramento Valley in northern California. It is characterized by a series of late Pliocene lahar deposits from Cascadian volcanic sources located near the area that is now Lassen Volcanic National Park. These mud flow derived sediments were laid down over a million year period creating deposits that once covered 2000 square miles and had a maximum thickness of 1700 feet. Interbedded with these resistant mud flow deposits are volcano-sedimentary deposits of sands, gravels and conglomerates. The Tuscan Formation is a prominent geologic feature forming cliffs, canyons and buttes above the valley floor and constitutes the beginning the foothills of the Sierra Nevada to the east.

The Tuscan is described as a large wedge shape mass that dips and thins to the southwest with the strata continuing beneath the Sacramento Valley floor. The sands and gravels of the Tuscan Formation are the principal water bearing units for the northeastern Sacramento Valley. Measurements of multiple sections of the Tuscan in outcrop have allowed for a reinterpretation of the stratigraphy, and a redefined facies model has been developed for the Tuscan. The correlation of the principal water bearing units in outcrop and in the subsurface contributes to the understanding of the hydrologic characteristics of the unit. This makes it possible to define the geographic distribution, chronologic succession, and paleogeography of the area in the late Pliocene, and will provide insights to the flow of groundwater in the Butte Basin.