A HYDROSTRATIGRAPHIC STUDY OF THE TUSCAN FORMATION

The Plio-Pleistocene age Tuscan Formation crops out on the northwest side of the Sacramento Basin in northern California and extends for approximately 4500 km2 with a maximum thickness of 380 meters. The Tuscan Formation is composed of a mixture of fluvial deposits, pyroclastic flows and thick muddy debris deposits that were deposited as a progradational volcanic fan apron from east to west from a source area near the ancient Lassen volcanic center. The fluvial deposits comprise the aquifer for much of the northern Sacramento Valley.

Despite the importance of the Tuscan Formation as a water-bearing unit, many hydrostratigraphic questions remain unanswered. Questions regarding connectivity, flow paths, and degree of confinement stem from the fact that the fluvial deposits were deposited in a braided stream environment. Channelized beds, in general, cut into previously deposited channels, thus increasing connectivity and establishing preferential flow paths. Conversely, where fine-grained fill material is preserved connectivity decreases and the degree of confinement increases. The channelized beds tend to grade and fine upwards, thus increasing vertical anisotropy. More regionally, muddy debris deposits that extend westward act as laterally continuous aquitards and reduce aquifer connectivity.

I characterized the hydrostratigraphy of a portion of the Tuscan Formation using measured sections, drilling core and subsurface geophysical data. Lithofacies in the measured section and drilling core were defined on the basis of clast or matrix support, clast angularity, grain size and bed thickness. Lithofacies in the geophysical data were defined by fluctuations in recorded downhole resistivity values. Lithofacies were correlated and analyzed geostatistically within the context of the volcanic fan apron model using the Groundwater Modeling System (GMS) software. The results of the analysis indicate that the aquifer system is more confined toward the east due to the predominance of muddy debris deposits and unconfined toward the west with observable flow paths within the amalgamated channelized deposits.