CHARACTERIZATION OF TUSCAN FORMATION LAHAR FLOW UNITS, CHICO AREA, NORTHERN CALIFORNIA

The 2-4 Ma Tuscan Formation in Northern California is a primarily volcaniclastic deposit consisting of multiple breccia and conglomerate units interbedded with pyroclastic deposits, and braided stream sediments. The breccia and conglomerate units are interpreted as lahar deposits by their lack of bedding, greater than 20% ash content in the matrix, and exclusively volcanic clast content. Some units exhibit reverse grading. Pyroclastic deposits include the Nomlaki and Ishi Tuffs that are interbedded with the lahar deposits. Previously, a primarily basalt-rich breccia unit was analyzed and found to correlate to lava flows from Mt. Yana, a stratovolcano that was part of the ancient Cascades (1). To accurately characterize the compositions of the lahar deposits from the Tuscan Formation, clast samples from an andesitic lahar deposit in the Big Chico Creek Ecological Reserve, northeast of Chico, California, were analyzed in hand sample, petrographic thin section, and by x-ray fluorescence (XRF) for major element analyses. The 4 m thick flow unit is a clast-supported breccia that is greater than 90% andesitic clasts, and less than 10% dacitic clasts. The angular matrix is clay to fine sand in size, and is resistant to weathering. In hand-sample, phenocrysts of olivine, pyroxene, hornblende, biotite, and plagioclase are observed in many clasts. The composition of clasts in lahar deposits of the Tuscan Formation can be used to help correlate the debris flows with the lavas of Mt. Yana and to reconstruct the origins of Tuscan Formation lahar flow units. (1) Lindberg, et al., 2005.