Geographic Assessment

Preliminary Geologic Assessment of Helen Putnam Regional Park Hazards and Opportunities

Regional Setting

The park is in the volcanic part of the North Coast Range of California. The North American Continent is moving southeast with respect to the crust below the Pacific Ocean. The continent and the ocean “plates” move along a set of northwest trending faults (Alt, 1975, p.8). Most of the motion is along the San Andreas Fault. No faults run through the park site. Several million years ago the oceanic crust was moving northeast. It was pushed under the continent causing folding and thrust faulting (op.cit., p. 2-9) and probably contributing to the volcanic activity in the area.


The park does not have a volcanic form and it is not likely to be a volcanic hazard. Mount Konocti and other Clear Lake Volcanoes are capable of erupting. They will emit ash which has the capability of reaching Helen Putnam Regional Park.


The Burdell Mountain fault is south of the park. The Tolay fault is to the east and the Bloomfield fault is to the northwest of the park. All three of those faults are quaternary-active faults (R.W. Graymer, 2006).   The park is only 20 miles east of the San Andreas fault which caused major destruction in the Santa Rosa area in 1906. The nearby faults and the San Andreas will cause extreme shaking in the park in the future (Miller, 1972).


The park consists primarily of Sedimentary rocks and Quaternary hillslope desposits. The Sedimentary rocks have been dated at around 65 million years old. This is during the Paleocene or Late Cretaceous epoch. There are also outcroppings of Sedimentary rocks from the Pliocene epoch.   The rocks in this area were originally part of the Franciscan Complex, a mix of ocean crust rocks which formed on the ocean floor about 140-142 million years ago as the oceanic plate slid under the continental plate. As a result of the plates sliding, uplift, erosion, and sedimentation...