Depositional history of the Tuscan Formation (Pliocene) in the Richardson Springs Quadrangle, Chico, California

Abstract

The Pliocene-aged Tuscan Formation is a well-exposed, but poorly understood unit that outcrops along the eastern portion of the northern Sacramento Valley and continues into the subsurface to serve as the region’s primary aquifer for agriculture, municipal, and domestic stakeholders. A better understanding of the depositional history in outcrop can improve groundwater models that have not adequately represented the heterogeneity of facies which can then lead to better management practices of the Tuscan aquifer. Previous studies used subtle compositional changes to divide the Tuscan into four homogeneously distributed units with no regard for lateral facies changes. This study utilizes new techniques to map volcaniclastic facies using Time-Correlative Surfaces (TCS) to delineate hydrogeologic units within a 23 square km area in Mud Creek Canyon (Richardson Spring Quadrangle), north of Chico, California. We present an updated geologic map with a newly acquired Digital Elevation Model (DEM) using drone-imagery as well as stratigraphic panels consisting of 11 measured sections to represent the complex interactions of 3 main facies associations: Fluvial Deposits (FD), Hyperconcentrated Flood Flow (HFF), and Debris Flow Deposits (DFD). The TSC were then used to develop successive generalized cross-sections through time depicting the depositional evolution of the Tuscan Formation in the study area. Deposition began with HFF deposited into a pre-existing channel. The HFF deposits were heavily eroded creating a large channelized system filled with FD, with the thalweg of the channel located to the north of the field area. DFD was then deposited into the channel thinning to the south. HFF was then emplaced on top of the DFD filling the channel to capacity. A new channel formed to the north and began to fill with more volcaniclastics. DFD then overflowed the banks of the second channel and flowed to the south creating a wedge-shaped deposit. HFF was then deposited over the entire field area. A large channel was then eroded in the southern portion of the field area that was then filled with alternating HFF and DFD ending in a very thick DFD. In summary, our study supports a depositional model that envisions a more confined system than previously imagined consisting of a cyclic pattern of increased debris flow activity followed by a return to normal streamflow erosion and deposition.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)