patpt

GEOLOGY OF PATRICK’S POINT STATE PARK
		(Developed as part of a field studies course at College of the Redwoods, Eureka, CA)
Students enrolled in Field Studies (Geology 9) at College of the Redwoods recently investigated the geology of Patricks’s Point State Park. Much of our knowledge of this region (and the maps below) come from the investigations of K.R. Aalto of Humboldt State University (see reference below).
Patrick’s Point State Park is located on the west coast of North America approximately 300 miles north of San Francisco and 30 miles north of the College of the Redwoods.
Mesozoic (245-65 million years old) Franciscan Complex rocks and overlying Quaternary (1.6 million to present) sedimentary deposits are well exposed at this location. The figures below show the location and the geologic map of K.R. Aalto.

The highly deformed Mesozoic rocks are interpreted as a sequence of oceanic basalts and sedimentary rock that have been folded and broken during plate subduction. The Farallon plate has been converging (subducting) with the western portion of North America near this location for over 200 million years. The boundary between the remaining portion of the Farallon plate (the Gorda plate) and the North American plate is located approximately 25 miles west of Patrick’s Point State Park (beneath the Pacific Ocean). A more immediate consequence of this subduction process are the frequent large earthquakes which occur throughout this region.
	The Mesozoic Franciscan Complex includes greenstone (low-grade metamorphism of iron- and magnessium-rich igneous rock), conglomerate, graywacke (a mud-rich sandstone), chert, and blueschist facies metamorphic rock (exposed to high pressures and low temperatures).
Quaternary (last 1.6 million years) strata unconformably overlie the Franciscan Complex at Patrick’s Point State Park. These strata include approximately 350 meters of tilted, nearshore marine, and nonmarine sediments. These sediments record glacially-controlled sea level fluctuations as well as evidence of progressive uplift and tilting of terraces.	Reference: Aalto, K.R., 1989, Geology of Patrick’s Point State Park, Humboldt County, California Geology, v. 42, p. 125-133.