Origin and Tectonic Significance of High Fluid Pressures in the California Coast Ranges
Authors: Berry Frederick A.F. 
Published: December 2007

Anomalous, high fluid potentials exist within the miogeosynclinal Great Valley and eugeosynclinal Franciscan sequences of Jurassic-Cretaceous age within the Coast Ranges and at depth on the west side of the Central Valley, Calif. These sediments are dominantly mudstones with low fluid transmissibilities.

Certain problems exist as to the probable regional distribution of these high fluid potentials. Low-fluid-potential areas such as The Geysers geothermal occurrence are present in the Franciscan of Northern California within a region that generally seems to be characterized by high fluid potentials. These low potential areas are attributed to fracture zones with potential areas are attributed to fracture zones with channel-type flow whose transmissive characteristics greatly exceed those of inter-granular flow. Where the transmissive characteristics of such a fractured zone decrease downwards from the surface, the probable fluid pressures at depth within that fracture are interpreted as being at near-hydrostatic values. Other fractures whose transmissive characteristics decrease upwards from zones with anomalously high fluid pressures are interpreted to have extraordinarily high pressures are interpreted to have extraordinarily high fluid potentials at relatively shallow depths. In both cases, a near static fluid pressure would exist along the fracture. Thus, we conclude that the Franciscan of Northern California probably is characterized regionally by near-lithostatic fluid pressures at depth but that extensive fracture systems probably cause a very erratic distribution of fluid potential zones near the surface. Fracture zones with both low (i. e., near-hydrostatic) and high (i. e., near-lithostatic) fluid potentials probably exist at various depths from the surface. probably exist at various depths from the surface. The Geysers dry steam occurrence is envisioned as a fracture zone that has low fluid potentials by virtue of a decrease of the transmissive characteristics of a fracture system with depth but that happens to be located within a local region of high heat flow possibly caused by the existence at depth of a magma chamber.

An abundance of direct fluid pressure measurements within the Great Valley section of the Sacramento Valley demonstrates the existence of such high fluid potentials, but only one such measurement exists within the San Joaquin Valley. The regional chemistry of the waters (membrane-effluent type) within the Tertiary of the San Joaquin, however, suggests that this water has been extruded from a widely distributed series of mudstones and other rocks that are undergoing compaction. The presumed source for this widespread compacting sequence is the underlying Great Valley sediments with their postulated high fluid potentials. potentials. We conclude that the anomalous, high fluid potentials of Tertiary sediments within folds on the west side of the San Joaquin Valley reflect indirectly the presence at depth of high fluid potentials in the underlying Great Valley section. The origin of the folds is attributed to dynamic tectonic compression caused by current deep-seated Bear diapirism of Great Valley mudstones and related rocks that possess near-perfect plastic properties by virtue of their possess near-perfect plastic properties by virtue of their near-lithostatic fluid pressures. The closed gravity minimum over the south end of South Dome-Lost Hills Anticline is postulated as being the result of a diapir of serpentine or similar material.

It is postulated that a fault zone, named herein the West Side Fault, probably exists at depth along the west side of the Central Valley. This buried West Side Fault is envisioned as having an intermittent near-surface expression in the form of faults such as the Midland Fault or long linear folds such as the Kettleman folds. The diapirism presumed to be responsible for these folds is postulated as occurring along this fault. postulated as occurring along this fault. JPT

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