The Hunt for Jurassic Bivalves: A Short Trip

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Here is a small trip only minutes away from downtown Salt Lake City. If you would like the exact location, PM me. I hadn’t been to this site in a couple years so it took @Earth Chemistry and I a couple hours to re-discover it so here’s a little history about the area. 

 

Ok first, the structural geology in this area is quite famous as it is a large pair of synclines.

From https://geology.utah.gov/wp-content/uploads/synclines.gif

They were made in the Sevier Orogeny about 120 Ma to 50 Ma ago.

 

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The layer we want is the Twin Creeks Limestone as it has fossils in several different layers. Here’s the top of the bed I visited a couple years ago.

 

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On the lower end is a large amount of bivalves. This was the layer we were after.

Here’s a look back at downtown SLC from the trail. 

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Here is the Hogle Zoo and This is the Place Heritage Park at the opposite direction

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Now the way we took was a longer route because I wanted to check the underlying Triassic layers for fossils.

No such luck much of it is buried under Lake Bonneville gravel deposits such as this one from a geologic blink of an eye at 15,000 years old.

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Here’s the Nugget Sandstone on the Triassic-Jurassic Boundary from when Utah looked like the Sahara desert with aeolian sand dunes. A quiet windswept desert that probably resembled Mars for a time but is now part of a green mountain front uplifted by the Wasatch fault in the last 26 million years. The same layer is called the Navajo Sandstone in Southern Utah and can be seen in Zion NP.

 

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But alas, there are no fossils for us to collect as it was during one of the largest mass extinctions in the history of Earth, inside the rocks is just raw geology not even an echo of life.

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But things would be changing soon.

The layer we wanted outcrops just downstream in geologic time. Sea levels started to climb and fast.

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A change from most likely one of the hottest deserts at the time to a limestone producing marine shelf occurred in only a few million years. Now this site was visited by me last year (

) but it was very brief so here’s a few better pictures of the site and some of the fossils that can be collected there.

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First, the limestone wall with fossils from Jurassic once life made a comeback from the end-Triassic mass extinction.

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The rock slab while has the occasional fossil embedded inside is not the primary layer for collecting as it is as hard as limestone can get. It does preserve some of the mountain building in the form of evidence called en echelon fractures. The layer we want is just above and the fossils weather out of it to the slope down below.

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Now the fossils:

The biggest intact with a bit of original shell material left.

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A yellow sandy layer has some too with some tectonic fractures shown as well.

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A few odd cross sections of some seen above.

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If you’ve read this far, thanks for reading!

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For those keeping track:

Red: no fossils found

Green: fossils found

Source: Geologic History of Utah by L.F. Hintze & B.J. Kowallis

[fossilsonwheels]

Very informative post. One of the things I have found most interesting about getting into fossils has been learning about how the earth has changed and being able to understand those dramatic changes through the fossils. My background is in extant wildlife research so it is really quite interesting to relate that information to the current conditions on our planet. Thank you for sharing.

[grandpa]

Great trip report.  I appreciate the science supporting your report.  Understanding the stratigraphy of an area certainly aids in finding the evidence of past life.  Neat.  Thanks for taking us with you.