A Wooster Geologist returns to the Jurassic of southwestern Utah

April 16th, 2018

St. George, Utah — This week I’m exploring the wonderful Middle Jurassic Carmel Formation exposed in southwestern Utah. It is a rare bit of solo fieldwork I’m doing to prepare for a Wooster Independent Study expedition here with students and Nick Wiesenberg in about a month. My colleagues, students and I last did research here almost two decades ago, so I wanted to make sure I knew how access and exposures have changed. Better to explore early than be surprised while leading a team! (One new thing in the above image: a lake where I used to wade across the Santa Clara River to get to the Carmel outcrops.)

The Carmel Formation is 100 to 300 meters thick, more or less, through parts of southern Utah. It is relatively thin by Utah Jurassic standards. It is a mostly marine unit, having been deposited in a narrow restricted seaway. I’ve long been fascinated by its fossils, which are almost entirely mollusks and traces of arthropods. It is time to revisit these exposures with new eyes and ideas.

This is a view looking north across typical Carmel Formation outcrops about a half-hour northwest of St. George. The rocks are exposed in strike valleys, which are great for finding bedding plane slabs with fossils and sedimentary features, but miserable for constructing stratigraphic columns. Luckily most of that tedious work is done. In the background are two iconic mountains for the area: Square Top and Jackson Peak. In April 1983, a B-52 bomber tragically crashed into Square Top.

The Carmel Formation is capped unconformably in this region by the Upper Cretaceous Iron Springs Formation, a conglomeratic sanadstone here. It is cemented well, capping the less resistant Carmel limestones and claystones below. This is a typical strike valley exposure of the top of the Carmel.

This is a Google Maps view of today’s field area, which is in the middle of the image stretching diagonally from the reservoir in the southeast (right) to where the dirt road bifurcates in the northwest.

One of the interesting features of the Carmel is a widespread carbonate hardground. Tim Palmer and I published on it a long time ago, but there are still many questions about its formation and the variety of borings on its surface. Above are two fragments found loose in a wadi.

The Carmel has beautiful sedimentary structures, like the ripples on the left, and cool trace fossils like the arthropod trackway (Gyrochorte) on the right.

I’m intrigued with the dominant oysters in the Carmel: Liostrea strigilecula. They formed “oyster balls” (ostreoliths) almost unique to the Carmel, and these large masses of unknown origin and significance. They seem to be small reef-like forms, but also show signs of occasional overturning. They hosted encrusting bryozoans and boring bivalves.

An early morning view of my field area. The reddish unit below is the Temple Cap Formation, with the base of the Carmel white turning into, shall we say, carmel-colored. The water is part of the Gunlock Reservoir.

Today I sorted out access to a few old localities, and found some new ones. Much has changed here in the last 20 years. There are new roads in and around the growing cities of St. George and Santa Clara, the Santa Clara river has been further managed with massive earthen works (in response to frequent flash floods, no doubt), and there seem to be new barbed-wire fences across some Carmel exposures. Nevertheless it felt like old times as I tramped across the gravelly hillsides scanning the ground for geological treasures.

I must add as a footnote an image of this ugly-but-simple bridge over the Santa Clara River at Miner’s Canyon. For years my students, colleagues and I waded through the river here because the water was too deep for our rental vehicles. This meant we had lots of walking to do once we were on the other side, including walking back loaded with rock samples. Now we can just drive across. I have a feeling, though, that this bridge will not survive the next flash flood!

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