PALEOGEOGRAPHIC ANALYSIS OF THE TUSCAN FORMATION, CHICO, CALIFORNIA

The upper Pliocene age Tuscan Formation crops out on the east side of the northern Sacramento Valley and is part of the Cascade Range volcanic province. It is over 380 meters thick and is comprised of a mixture of volcaniclastic sediments including mud-rich debrites (lahars), sand-rich pyroclastic deposits, braided stream conglomerates and bioturbated siltstones. In order to better understand the distribution of these different lithofacies and the paleogeography of the Tuscan Formation in the Chico area a detailed sedimentologic and stratigraphic study was begun. This initial study is of the lower Tuscan Formation between its base and the intra-formational Nomlaki tuff which occurs approximately 70 meters above the base. In the Chico area, the Tuscan Formation is derived from volcanic vents located near Lake Almanor. The eastern outcrops of the Tuscan Formation are the most proximal deposits and are extremely poorly sorted, matrix supported, mud-rich debrites (lahars) with angular clasts meters in size. These proximal deposits dominate the Tuscan outcrops and can be traced for 20 to 30 kilometers westward into the basin where they become thinner, finer grained and interfinger with channelized fluvial deposits. The fluvial deposits are comprised of coarse-grained, multi-story deposits infilling shallow braided channels and associated reworked, sand-rich pyroclastic deposits. The most distal facies recognized in outcrop are bioturbated siltstones which contain Steinichnus crayfish burrows which suggests a wet flood plain or lake margin environment.