Fluvial Architecture of a Cascades-Sourced “Paleo-Sacramento River” System Within the Lower Tuscan Formation in the Northern Sacramento Valley, CA

Greene, Todd J.
[email protected]

Although very little is known about deeper sources of groundwater (~500-1500 feet below ground surface) in the northern Sacramento Valley, interest in a potential “Lower Tuscan Aquifer” is growing stronger as greater stresses are imposed on surface and shallower groundwater systems. This study represents the first step to better understand the fluvial architecture of the Pliocene-aged Lower Tuscan by developing a regionally consistent hydrostratigraphy that can serve as the foundation for groundwater models used to predict well water drawdrown effects from pumping within the Lower Tuscan.

Although there are no known regional time-stratigraphic surfaces, exploiting provenance differences in Coast Ranges-sourced sediment (metamorphic) from Cascades-sourced sediment (volcanic) can help delineate time-equivalent packages within the fluvial-dominated Tuscan and Tehama formations. Using geophysical logs as well as point-count data from sandy portions of well cuttings samples from 4 different wells, regionally mappable zones spanning an area approximately 130 square miles identify key areas where high percentages of porous sand (i.e. water-rich zones) exist within the Lower Tuscan.

At the base of the Lower Tuscan, a sharp change occurs based on sandstone composition and well-log character. The change separates an underlying mixed source (Coast Ranges and Cascades-sourced) sand-poor system from an overlying Cascades-sourced sand-rich interval approximately 250-300 feet thick interpreted to be a previously unrecognized north-south trending "paleo-Sacramento River" deposited within a sandy braided depositional environment. This thick sandy system has a sharp western boundary based on good well control and an apparent diffuse eastern boundary where well control is sparse. The sand-rich system is overlain by a gradual return to a mixed sourced system that persists to the present day.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013