GEOLOGIC MAPPING AND FLOW IDENTIFICATION OF THE TUSCAN FORMATION, NORTHERN CALIFORNIA

The need to understand the stratigraphy and depositional history of the Pliocene-aged Tuscan Formation has accelerated recently due to its role as a potential regional aquifer in the northern Sacramento Valley. It covers an area of over 5180km2 near the southwest side of the Cascade Range Province to the Chico Monocline in northern California. Previous works identify Tuscan rocks broadly as conglomerates, breccias and sandstones that were created from a series of debris flows and lahars initiated from volcanoes of the Ancient Cascade Range south of Mt. Lassen. Age constraints on the Tuscan Formation are from the Ishi Tuff (1.8 Ma) and the Nomlaki Tuff (3.27 Ma). Based on clast types within the Tuscan flow deposits, some of the southern flows have been correlated to Mt Yana, a coeval volcanic center of the Pliocene Cascade Range.

However, detailed correlation within the Tuscan Formation is very difficult due to the lack of datable primary volcanic deposits. In addition, previous correlation schemes fail to account for multiple volcanic centers as well as the variety of depositional facies. Current research of the Tuscan Formation uses mapping and detailed lithologic description of individual flow units. The identification criterion includes clast populations in terms of textural descriptions and lithologic characterization, as well as matrix lithology from thin-section description. Since there has been no formal, detailed geologic mapping done in the area on a flow-by-flow basis, this work attempts to correlate flow units along and across Big Chico Creek to determine the lateral continuity of individual flows, as well as to determine the depositional facies and flow types that deposited the Tuscan Formation. We will present results of our work in a relatively small area (15.5km2) in an effort to contribute to larger scale correlations of Tuscan flow units. Outcrop stratigraphy and sedimentology of the Tuscan Formation will enable the creation of a geologic framework for the area that can potentially be continued into the subsurface as well. This research is being done concurrently with others that are studying detailed textural features in both outcrop and microscopic scales.