Sargent fault - Frank Groffie's miscellany

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I stumbled across this intriguing geologic fault locality while on a leisurely hike among the waterfalls in Uvas Canyon Park in late 2018. Here, the Sargent fault is exposed, and there are numerous neotectonic geomorphic features showing latest Quaternary (including Holocene, likely) dextral surface fault offset totaling about 75 m.

Back at home, I did some online research. I discovered that if anyone anytime earlier had noticed this fault exposure and the associated geomorphic features, then their findings had not made it into any published geologic maps. Apparently, no geologist had appreciated what's on display here: a locality where recent movement along the Sargent fault, one branch of the San Andreas fault system, is on vivid display. The surface expression of neotectonic movement along the Sargent fault over the last few tens of millenia shows up here as a quirky set of tiny hollows, bent streamflows, and warped ground, along with a semispectacular waterfall thrown in for good measure.

I reached out to the U.S. Geological Survey. Two geologists from the USGS's Menlo Park branch, Carol Prentice and Belle Philibosian, were interested in my findings and accompanied me on a field trip to this locality on April 2, 2019. There, in the field, they confirmed my shutter ridge interpretation, although they may have been nearly as perplexed by some of the tiny localized late Pleistocene and/or earliest Holocene deposits as I was. Whether Prentice, Philiosian, or other geologists with the USGS will follow up on these findings and have the resources to perform any paleoseismologic studies at this site remains to be seen.

In the interim, there is this writeup I prepared in spring 2019:


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