Jurassic hardgrounds and Holocene lava flows in southwestern Utah

Santa Clara, Utah — Team Jurassic Utah spent the day on Galen Schwartzberg’s Independent Study project, the sclerobionts of the Carmel Formation. We began with the above carbonate hardground at the appropriately named Hardground West (HW) locality. (Just a few meters south of C/W-157 and Ethan Killian’s oyster balls; 14.9 meters stratigraphically below them.) A carbonate hardground is a synsedimentarily-cemented seafloor. Above the ruler can be seen the top layer of this hardground with many round holes. They are bivalve borings called Gastrochaenolites. They are what tell us that this was a rocky surface exposed to marine life. Below the ruler the top layer has been removed, revealing that the limestone beneath is full of internal molds of aragonite-shelled bivalves. The aragonite likely dissolved early in the diagenetic process. Did this aragonite then provide the calcium carbonate to cement the layer above? Maybe, but we haven’t yet come up with a test of this hypothesis.

Here Galen is inspecting a hardground. Note that she knows how to protect herself from the relentless sunlight!

One of our hardgrounds with truncated borings. There is an interesting cluster pattern here.

Encrusting organisms that require hard substrates are another identifying feature of hardgrounds. Here our friends the oysters Liostrea strigilecula are attached to a hardground. These are left valves with the right valves removed.

At the end of the day we stopped to walk around the remarkably fresh lava flows near where we’re staying in Santa Clara. These are part of the large Santa Clara Lava Field, which extends from the north rim of the Grand Canyon in Arizona northwards to Fillmore in central Utah. The flows in this area are as young as 32,000 years old. Above Nick and Galen are standing on a pressure ridge.

There are numerous large lava tubes in this volcanic field. Nick and Ethan stand near a large one with a collapsed roof.

Nick made this image. The flows are very difficult to navigate, so I gained new respect for lava hoppers.

This Google Earth image shows the black lava flows near the Airbnb we are inhabiting (the red pin). Urban growth of Santa Clara is eating away at the basalt. Our home here is called, in fact, “Lava Falls”.

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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