Field and lithologic characterization of the Tuscan Formation lahar flows in Big Chico Creek, Northern California

The Tuscan Formation is a highly resistant series of lahar deposits that crop out along the western margin of the northern Sierra Nevada foothills, forming dramatic cliff faces and steep canyons along Big Chico Creek in Northern California. Previous study of the Tuscan Formation associated lahar units with early Cascade volcanism ~ 2.8 - 3.3 Ma (1). Combined, the flows cover 5200 km2 and have a total thickness greater than 150 m (1). Tuscan flow units are overalin in the west by younger the Great Valley sediments and serve as the primary aquifer for the northern Sacramento Valley. Preliminary fieldwork has revealed a diversity of flow units, dominated by clasts of intermediate composition lithologies. One unique flow unit that is dominated by mafic clasts and matrix is the focus of this work because of its relative limited lithologic range. Major element XRF analyses of clasts from the lahar flow unit are dominated by calc-alkaline, medium-K series basaltic andesites, but range from basalt to andesite. Matrix material is dominated by sand-sized mafic lithics and olivine crystals. We envision the source region for the flow unit to have been a cinder cone or series of mafic cones similar to those found in the modern Cascades. Additional fieldwork and lithologic analyses will help characterize the volcanic setting and possible origins of units within the Tuscan Formation. Ongoing and future research builds on this work to better constrain possible source area(s) and eventually reconstruct the ancient volcanic system from which lahar flows of the Tuscan Formation were derived. (1) Lydon, 1968.