By Pinkie Young
Hundreds of years ago, a majority of the Bay Area’s shoreline was a mix of tidal marshes and wetlands. By the 1900’s, much of that land was drained and artificially filled to be used for industry, agriculture, and housing. My hometown of Richmond is now mostly fill, but across the street from Miller/Knox Regional Shoreline lies an outcrop of rock that tells a much, much older story.
The “What’s in your Watershed” segment of our newsletter has been a longstanding channel to showcase living creatures in our local watersheds. This month, I’d like to focus on what was in our watershed as our current topography was forming around 70 million years ago. Take a trip through Richmond and you’ll notice it’s mostly flat, except for a hilly section of land in the southwest. Make your way to the Miller/Knox Regional Shoreline parking lot and look across the street to get a closer view. It might not look like much from far away, but upon closer inspection (or if you have a geologically inclined friend) you’ll see some beautifully layered rocks.
According to the US Geological Survey, these rocks are turbidites formed during the Cretaceous period (between 145.5 and 65.5 million years ago). Turbidites are a type of sedimentary rock that form underwater due to ancient ocean turbidity currents. Along the shoreline of every land mass is a continental shelf, which is the edge of a continent that lies under the ocean. The shelf eventually slopes down, connecting the continent to the ocean floor. Over time, the sediment that gathers on the continental shelf builds to a critical mass, and the downhill flow of water caused by increased sediment rapidly deposits the sediments onto the ocean floor. Think avalanche style flow underwater on the ocean shelf, over and over again, for millions of years – now that’s what I call a turbidite! The turbidites at Miller/Knox are one part of the larger Novato Quarry terrane that extends from Bodega Bay to Albany (the Albany Hill landmark is actually the southernmost visible section of rock in the terrane).
The western United States is largely composed of accreted terranes due to tectonic plates crashing into each other. An accreted terrane is one, or many, blocks of subducting oceanic crust that break off and are added to the continental crust. While the turbidites at Miller/Knox originally formed horizontally underwater, millions of years of plate subduction and the subsequent movement of Earth’s crust caused them to slowly be uplifted into what we now know as the Novato Quarry terrane.
Sources:
Blake Jr, M. C., Graymer, R. W., & Jones, D. L. Geologic Map and map database of parts of Marin, San Francisco, Alameda, Contra Costa, and Sonoma Counties, California. U.S. Geological Survey Miscellaneous Field Studies MF 2337, Online Version 1.0. (2000). https://doi.org/10.3133/mf2337
Sigloch, K., Mihalynuk, M. Intra-oceanic subduction shaped the assembly of Cordilleran North America. Nature 496, 50–56 (2013). https://doi.org/10.1038/nature12019
