deposits appear to have be reworked by water, exhibiting successive winnowing of finer material
and resulting in the deposition of cobbles and sand; these cobble and sand deposits are conducive
to groundwater flow. Successive reworking of mudflow deposits by streams meandering across
the fan during non-eruption periods resulted in erosion and re-deposition of the sediments.
Ultimately, all the sediments were compacted and cemented into rocks. The result is that in some
areas the water-bearing units are confined by less permeable units, such as mudflow and channel
overbank deposits, while in other areas the more permeable units are amalgamated into thick
sequences, as channels carrying sand sequentially cut down into one another. The water-bearing
units shift in location and thickness across the fan due to the distributary nature of the streams
reworking the deposits. None of this is described nor depicted in Technical Memorandum 3.
The stratigraphic interpretation depicted in Figure 3 (Technical Memorandum 3), as
interpreted by the DWR from electric resistivity logs, well logs, and well cuttings, is open to
interpretation. Electric resistivity logs do not constitute primary data when determining lithology.
Instead, rock types are inferred from the shapes of the curves generated as the signal passes
through the rock materials in a process known as inverse modeling. This requires electric logs to
be correlated with actual rock chip samples, such as those generated from drilling wells, or other
types of information. The problem with using well cuttings to correlate with kicks in the electric
resistivity logs is that the exact depth from which the sample comes cannot be known precisely
due to the time it takes for the cuttings to travel to the surface in the drilling mud. A better way to
correlate resistivity logs with actual rock types is to take solid cores of the material at the same
time the well is drilled, but coring is expensive.
A comparison of recent work by Spangler (2002) and Skartvedt-Forte (2006) provides an
example of how two very different interpretations can be generated from similar data. Electric
resistivity logs across the Sacramento Valley were correlated by Spangler (2002), in conjunction
with the DWR, on the basis of a distinctive, widespread, high-resistivity kick. The kick was
inferred to represent a permeable, water-bearing unit within the Tuscan Formation consisting of
interbedded coarse volcanic sand, fine gravel, and tuffaceous siltstone. The high resistivity in the
kick indicated the presence of water. The Tuscan confining layer, consisting of fine-grained
massive volcaniclastic breccia (mudflow-type deposits) and reworked fine volcanic tuff, was
assigned extremely low resistivity, probably due to the assumed absence of water. Skartvedt-
Forte (2006) interpreted electric resistivity logs for the Tuscan Formation within the vicinity of
Chico based on detailed stratigraphic measurements and drilling core. In her interpretation,
volcaniclastic breccia displayed a very prominent blocky resistivity signature with an abrupt base
and top. Tuff intervals were distinguished on the basis of an extremely high sharp peak. Water-
saturated conglomerates and volcaniclastic conglomerates displayed the third highest resistivity,
with a signature that extended outward at the base and receded at the top. Water-saturated
sandstone and volcaniclastic sandstone exhibited a signature that was not as strong as
conglomerate but stronger than siltstone. The lowest, flattest signature on the log represented
siltstone or mudstone. If Skartvedt-Forte’s (2006) interpretation is correct, then the Tuscan sand
and gravel water-bearing unit does not thin to the south and west, nor does the fine-grained
Tuscan confining layer become thicker to the west to interfinger with the Tehama Formation, as
suggested by Spangler (2002). Instead, it is the fine-grained, impermeable volcanic breccia unit
that thins to the south and west, resulting in less confinement, and the water-saturated
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