Calcium carbonate "pipes" in the upper part of the Santa Cruz mudstone at Swift St. and West Cliff, Santa Cruz. The carbonate precipitated when methane venting from the ancient (late Miocene) sea floor came into contact with sea water. You can see for yourself why this locality is nicknamed "the toilet bowls".

The group gets their first look at paleo- cold seep deposits at Swift St. & West Cliff. In the uppermost layers of the SC mudstone, you can see the highly fractured grayish stringers of porcellainite - a bed in the mudstone which contains a higher proportion of diatom tests to clay particles.

The top of the Santa Cruz Mudstone at Swift & West Cliff is one of the few places locally where you can see the angular (only 3-5 ^o) unconformity with the overlying Purisima Fm. The basal conglomerate of the Purisima, dated elsewhere at ~6.8Ma, contains breccia of the SC mudstone including clasts of seep carbonates. The Santa Cruz mudstone is estimated to be 9 -7 million years old. So the age of the unconformity is pretty well constrained. Over the thin layer of Purisima you can see the Pleistocene terrace deposits (they start where it's kind of grassy.)

The view from the approach to Yellow Bank Beach. The cliff is the Santa Cruz mudstone. Out of the field of view to the left is the sea arch you run under to access Panther Beach - which is a distinct beach system from Yellow Bank (thanks Curt Storlazzi, USGS.) From both beaches you can see the massive sandstone intrusions derived from underlying Santa Margarita Sandstone.

A fault cuts the cliff above the sea arch. This fault was a conduit for warm tar moving up through the sandstone intrusion, which left a gray stain in its pathway. Ian Gradek found this fault without a map.

The sandstone intrusions formed when course wet sands, overpressured by burial under the Santa Cruz Mudstone, squirted up through cracks in the mudstone. Hydrocarbons migrating up through the strata from their source in the deep basin used the porous sands as a pathway to get through the low-permeability mudstone. When the hydrocarbons passed through the sand, they left tar residues behind along their path. Later, meteoric water passed through the sand formation, leaving a fine banding of iron oxide minerals (limonite) as it flowed through.

Field trip leader Catherine Riihimaki, a PhD student studying geomorphology (the evolution of landscapes), explained the uplift of the Santa Cruz-area marine terraces from the vantage point of Yellow Bank Beach. Modern sea level is cutting the beach where she's standing. During the last high stand of sea level (the interglacial period prior to ours), the ocean cut the flat surface atop the cliffs. In the interim, movement on the San Andreas Fault System caused the land of the area to rise.

Our last stop: Wilder Ranch State Park. Hiking up just a little up the hill from Highway 1, you can get a great view across several sets of marine terraces, as well as a great ocean vista. Wilder has lots of local historical interest, as well as some unexpected features: UCSC post-doc Mark Kessler was able to record real-time evidence of bioturbating burrowing rodents. Gophers may play a major role in moving weathered rock from the bottom to the top of the soil layer, where it can be eroded - thereby affecting the shape of hills around Santa Cruz.




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