DEFINING THE SUBSURFACE EXPRESSION 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 resistant breccias 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.

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. The California Department of Water Resources (DWR) has installed wells throughout valley, tapping into the aquifer of the Tuscan Formation. The e-log data gathered with the development of each well has shown a distinctive division in the units of the Tuscan. Two distinct units are defined in the subsurface based on the well log character. The Upper Tuscan is defined as the shallowest zone dominated by volcanic breccias; the Lower Tuscan is designated by an increase in the resistivity and water bearing potential. The high resistivity zone of the Lower Tuscan is a highly productive unit for agricultural and municipal wells. Until recently there was no core or outcrop that showed the lithology that causes this jump in resistivity.

The Neal Road Landfill site, south of Chico, CA., administered by Butte County Department of Public Works, has revealed a section of these water bearing units. Data from measuring sections in the exposed in the landfill pit has been correlated with e-log data from an adjacent monitoring well. The units that are exposed in the landfill pit are also revealed in 250 feet of two inch core that was recovered in the production of the monitoring well. Study of the three different resources for data correlation has expanded the knowledge of the subsurface character revealing sedimentary packages and more clearly defining the aquifer units of the Tuscan formation.