The Knoxville Formation

knoxville fm

As you go east along the south shore of Lake Chabot, the churned, oatmeal-colored volcanic rocks of the Leona Rhyolite give way to shale of the Knoxville Formation. The crust-building activity that created the Coast Range Ophiolite, of which the volcanic rocks of the Leona Quarry are the uppermost member, finally ended in the Late Jurassic some 150 million years ago, and ordinary seafloor mud and occasional sand began to cover the remains to form the Great Valley sequence. This long series of sedimentary rocks includes, bottom to top, the Knoxville Formation, Joaquin Miller Formation, Oakland Conglomerate, Shepard Creek Formation, Redwood Canyon Formation and Pinehurst Shale—a nearly uninterrupted sequence of rocks whose definitive exposures are in Oakland. The Knoxville is said to contain fossils of ammonites and Buchia, a mussel-like mollusk (here are some from farther north).

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4 Responses to “The Knoxville Formation”

  1. jarichmond Says:

    Do you have any idea where the name Knoxville Formation originates? I grew up in the Knoxville area in Tennessee.

    Also, though I’m not sure I’ve ever commented here, I love reading your posts; always very fascinating!

  2. Andrew Says:

    Knoxville is a locality north of Lake Berryessa in northernmost Napa County.

  3. Sheldon White Says:

    There’s some really cool rocks up around Knoxville. I remember driving around up there long ago and finding some big chunks of chromite and some truly amazing banded agate by the side of the road (for those interested in such things). Big bodies of gnarly serpentine.

  4. Andrew Says:

    It’s on my big list of day trips. Naturally, the depositional contact between the Knoxville and the underlying ophiolite (hence the serpentine, chromite etc.) is well exposed there. There’s a big BLM recreation area there as well as the defunct Homestake gold mine, now the McLaughlin Natural Reserve.

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