I'm actually going to visit these rocks early next week for a couple days. This is north of the Bay Area near the charming little coastal town of Gualala (which is fun to say out loud).
In this photo (note coin for scale) we see a complex mixture of pebbly sandstone (near bottom), coarse sandstone, and siltstone and mudstone (darker-colored rock). Note how the coarse sandstone seems to surround the large area of dark mudstone. The mudstone areas are actually clasts within a thick (>5 m) sandy turbidite bed. We call these "rip-up clasts" as they were deposits on the sea floor that were ripped up by a subsequent turbidity current, incorporated into the flow as a clast, and eventually deposited. Muddy material can be quite cohesive and can stay together as a coherent clast like this. Think about making a mudball vs. a sandball with your hands. The sand disaggregates much easier.
As mudstone rip-up clasts go, these are probably slightly above average in size. We commonly see smaller (centimeter scale) and once in a while you'll see a deposit with larger rip-up clasts.
Friday, July 06, 2007