The Geology & History of Point Reyes, California
Hull Geological Society
Webinar
John Connor
15th March 2023, 7.30 pm
Abstract
To the relief of some of you, there will be very little in this talk about subduction. The Point Reyes “peninsular” has a completely different geological history from the subducted/accretionary-wedged Franciscan Terranes to its east and southeast, having been transported northwards an estimated hundred kilometres or more, in about the past 10 million years, by strike-slip (right-lateral) movement of the San Andreas and related faults. Since the SAF is still very active, is there reason to think that Pt Reyes (on the Pacific Plate) could move another 100 kms or so in the next few million years, continuing to slide north past the North American Plate … as far as the Mendocino Triple Junction ?
As what became this peninsular moved northwards, younger rocks accumulated on its crystalline basement, such that there is now a variety of mainly Tertiary sediments there, relatively undisturbed when compared with the geologic complexities of the Franciscan Terranes. We’ll examine about a dozen locations on Pt Reyes in this talk, illustrating both the basement rocks and the overburden.
Point Reyes also has an interesting more recent history, from Francis Drake’s encounter with the Miwok indigenous people in the 16th century, to the largest observed surface offset of the great 1906 San Francisco earthquake, to being added to the US National Park system in 1962. From the mid-1800’s there has been extensive ranching on Pt Reyes – initially for butter & cheese; nowadays for both these dairy products and for beef.