FORMATION, COMPOSITION AND STRUCTURE OF THE TUSCAN FORMATION IN NORTHERN CALIFORNIA

The Tuscan Formation (northeast of Chico, CA) consists of conglomerate, breccia and sandstone created from a series of debris flows and lahars initiated from volcanoes south of Lassen Peak. The debris flows and lahars responsible for the deposition of the Tuscan Formation ranged in strength and sediment load, from very fine hyperconcentrated sandy flows to massive deposits of clast supported breccia. High-energy stream flows contain many bedding features, including lenses of cobble size clasts and stream channels cutting through flow units. Clast imbrication suggests flow direction to be primarily towards the southwest. The volcanoes responsible for the Tuscan were Mt Maidu and Mt Yana, which are extinct now but were active during the Pliocene. Evidence of types of lavas that built Mt Yana and Mt Maidu are now eroded remnants of lava flows that make up the clasts of the Tuscan Formation. The primary compositions of the clasts are basaltic andesite, pyroxene andesite and dacite. Previous research in the area found that the dip of the beds is between 3 and 30 degrees but current work suggests that the area is more complex. It was not possible to correlate flow units across Big Chico Creek Canyon (a distance of approximately 1.5 km), suggesting that structural complexities such as faulting folding or larger dip angles are present. The south side of the canyon exposes units higher in the stratigraphic sequence than the north side, consistent with the idea that a fault is located in the canyon, perhaps related to the structures associated with the Chico monocline. A more comprehensive understanding of the structural geology of the Tuscan Formation is important because it comprises a large aquifer in northern California.