Page 18 - Beaumont Basin Watermaster

Beaumont Basin Watermaster
2013 Reevaluation of the Beaumont Basin Safe Yield 3-Apr-15

3.0 Hydrogeologic Setting

The Safe Yield analysis presented herein utilized a groundwater flow model that has been
updated and refined from a pre-existing USGS groundwater flow model of the Beaumont Basin
area. A detailed description of the geology and hydrogeology of the area, which formed the
conceptual basis for the groundwater model update, is presented in Rewis et al. (2006). This
section provides a general overview of the hydrogeologic setting with an emphasis on areas
where the conceptual model has been updated to reflect data collected since the previous model
was developed (2003).

3.1 Geology

3.1.1 Geologic Units
Geologic units in the Beaumont Basin area can generally be grouped into crystalline basement
complex, Late Cenozoic sedimentary rocks, and Quaternary alluvium (see Figure 11). The
crystalline basement complex consists of Mesozoic age (approximately 245 to 65 million years
before present) granitic rocks and older metamorphic rocks. These rocks are generally
considered to have a very low permeability and, at depth beneath younger sediments, form the
effective base of the aquifers in the area. Late Cenozoic consolidated and semi-consolidated
sedimentary rocks of the San Timoteo Formation occur in the southern portion of the Beaumont
Basin and in the hills surrounding the basin (see Figure 11). At depth, these sediments overlie
the crystalline basement complex. The upper portion of the San Timoteo Formation is relatively
permeable and forms the lowermost portion of the aquifer system in the Beaumont Basin.
Surface sediments in the Beaumont Basin and nearby lowlands consist of unconsolidated to
semi-consolidated Quaternary alluvium consisting of interbedded layers of sand and silt with
localized gravel and cobbles. Where saturated, the Quaternary alluvium forms the primary
aquifer in the Beaumont Basin.

3.1.2 Geologic Structure

The regional geology of the San Gorgonio Pass area is largely a function of faulting associated
with the San Andreas Fault system (see Figure 11). Numerous faults have been identified and
mapped throughout the Study Area (Rewis et al., 2006). In general, two primary fault systems
have been identified: 1) the Banning Fault, and 2) the San Gorgonio Pass Fault Zone.

The Banning Fault is a regional structural feature that trends northwest/southeast across the
northern portion of the Study Area (see Figure 11). This fault is sub-parallel to the San Andreas
Fault System to the north and shows similar movement (right-lateral strike slip) (Rewis et al.,

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