VOLCANICLASTIC DEBRIS FLOW PROCESSES INFERRED FROM SEDIMENTARY TEXTURES OF THE SOUTHERN TUSCAN FORMATION, CALIFORNIA

The Tuscan Formation is located along the western flanks of Northern California's Sierra Nevada Mountains. This formation makes a large wedge shaped series of sandstones and volcanic breccias extending approximately 124 km (N-S) from Redding to Chico and 30 km (E-W) from the Sierran Foothills into the Central Valley of Northern California. Volcanic breccias of the Tuscan Formation are commonly interpreted as lahar deposits sourced from volcanoes of the ancient Cascades Arc. Clasts in deposits of the southern Tuscan Formation flow units have been correlated to Mt. Yana, an ancient Cascades volcano located at present day Butt Mountain. However, fundamental questions remain regarding the types of flows that deposited Tuscan rocks (rivers vs. volcanic flows vs. mudflows), their aerial extent in outcrop, and a workable correlation scheme between outcrop exposures. To partially address the first of these unknowns, this study examined sedimentary textures of Tuscan Formation deposits in outcrop, thin-section, and back-scatter electron (BSE) microscopy with the overall goal of documenting the range of depositional facies and flow types that deposited Tuscan rocks. Three main flows were studied in detail. Two flow units are extremely poorly sorted and ungraded volcanic breccias with andesitic to basaltic andesite boulders (up to 1 meter in diameter), cobbles, and gravels in a grain-rich matrix composed of very angular elongate grains that vary from sand-sized to 1-2 microns in diameter. At the micron scale, grains appear to be in a chemically and texturally homogenous matrix of volcanic ash. Extremely sharp angles on clasts attest to the lack of grain-to-grain contact during transport, suggesting a more cohesive flow with minimal grain-to-grain collisions. Volcaniclastic debris flows with high clay content have been attributed to the failure of hydrothermally altered rocks involved in flank collapses. Such flank collapse debris avalanches can produce voluminous lahar flows, as multiple cubic kilometers of volcanic debris are instantly mobilized. If a flank collapse debris avalanche caused the debris flows of the Tuscan Formation, there should be correlative block-an-ash flows more proximal to the source and possibly more dilute flow deposits down depositional dip.