DEPOSITIONAL CHARACTERISTICS OF TUSCAN FORMATION LAHAR FLOW UNITS

The Tuscan Formation of Northern California is located on the western flanks of the foothills of the Sierra Nevada Mountains. The Tuscan Formation consists of a series of well indurated sandstones and volcanic breccias that are interpreted as multiple lahar deposits. Lahar flow units extend from Redding to Chico, once covering an area of 5180 square kilometers, with a maximum thickness of 518 meters (1). Lahar flow units of the Tuscan Formation originated from volcanoes of the ancient Cascades. The southern flow units of the Tuscan Formation have been associated with Mt. Yana, of the Ancient Cascades (2). While detailed mapping of the Tuscan Formation has not occurred throughout the extent of flows, we estimate that in approximately 500 ft of exposure, there are tens of exposed lahar flow units. There are several cases in which rounded cobbles indicated stream flows have eroded through the flow units, suggesting some time elapsed between debris flows. In the absence of large scale features that can be used to correlate individual flow units, this work has produced detailed descriptions that may prove useful for making correlations of flow units within the Tuscan Formation. Two flow units have been studied in detail, which are located in the Big Chico Creek Ecological Reserve, upstream and adjacent to Upper Bidwell Park, in Chico, California. The flow units are dominated by poorly sorted, sub angular to angular, mafic and intermediate composition volcanic clasts, ranging from 0.5 millimeters to 34 mm in diameter. Analyses of clast distributions within the two flow units indicate neither normal nor inverse grading occurred. These depositional characteristics are consistent with cohesive, volcanic debris flows. (1) Brunkal, 2004; (2) Lindberg et al., 2005.