Masters Thesis

Stratal geometries of Tuscan deposits in Big Chico Creek Canyon outcrops and in the subsurface underlying Chico, California

The late Pliocene Tuscan Formation, in the northern Sacramento Valley is an established aquifer system. However, little is known about its hydrogeologic units in terms of dimensions, textures (facies), and heterogeneity. Due to a lack of well data, regionally extensive mappable surfaces, and age control, it is difficult to divide the Tuscan into units that can be correlated. In addition, the Tuscan outcrop has been historically divided by gross lithologic differences without attention to depositional processes or time-correlative surfaces. The existing stratigraphic scheme is therefore inadequate to characterize Tuscan stratigraphy in more local areas of investigations. For this study, the complexity of Tuscan deposits warrants field work and subsurface investigation. Fieldwork methodology involved mapping eight stratigraphic sections, (plus five previously mapped southern sections) on Musty Buck Ridge (MBR) in Big Chico Creek Canyon. Mapping of individual flow units on MBR allowed for recognition of major lithofacies in the Tuscan: breccia (matrix and clast-supported), conglomerate (matrix and clast-supported), massive sandstone (cross-bedded and planar laminated), mudstone and re-worked tuff. Environmental interpretations of Tuscan lithofacies were then grouped together and correlated to define facies associations: volcanic debris flow, hyperconcentrated flow, normal stream deposits, distal debris flow, flood deposits, and re-worked tuff. Major changes of facies associations were used to create timelines (MBR 1-11 from oldest to youngest) binding correlative packages on Musty Buck Ridge. The subsurface component of the study focuses on composition, texture and geometry of the Tuscan Formation by using petrographic analysis from three wells to establish a Red Bluff Formation–top Tuscan Formation boundary. A point count method of individual (metamorphic vs. volcanic) sand grains was conducted to delineate the boundary, combined with data such as driller completion reports and resistivity pattern recognition to complete a 3D surface map and three cross-sections. The mapped surface depicts the topography of the paleo-valley prior to the deposition of the Red Bluff Formation. Establishing the Red Bluff Formation–top Tuscan Formation boundary is important because the city of Chico receives its water both above and below this boundary. Results from this study indicate that Tuscan deposits depict cycles of volcanic activity interpreted to trigger large debris flows, followed by restoration to normal stream flow conditions. Along Musty Buck Ridge, older deposits near the base of sections (MBR1-2) reflect a distal volcanic fan environment interbedded with normal stream flow deposits, that progressively become more proximal in younger northeastern units. MBR2- 5 is dominated by normal stream deposits, interpreted to be one large debris flow (MBR3-4). Younger deposits are dominated by massive debris flows interbedded with normal stream flow deposits (MBR5-11). The outcrop and subsurface results in this study provide a depositional history, stratigraphy, and aid existing models of the hydrogeology of the Tuscan Formation. Development of the spatial geometry of additional Tuscan outcrop cliffs will expand the understanding of the Tuscan Formation. Additional well data will further the identification of other key surfaces in the Tuscan Formation, adding to a growing database that could eventually be used to create an accurate subsurface model of the Tuscan aquifer system.

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