Wooster’s Team Utah 2020: On a Jurassic Tidal Flat

Hurricane, Utah — Our second day was devoted to measuring, describing and sampling Will’s stromatolite-bearing rocks in the lower half of the Co-op Creek Limestone Member of the Carmel Formation. This locality is only a couple of hundred meters west of Juda’s study location yesterday. The rocks are very different: lime mudstones with beautiful markers for their tidal flat origins. We worked in a deep wadi and thus had cliff sections with some bedding plane exposures. Above the team is describing the top of a depositional cycle. (I don’t know why Nick is giving me the side-eye!)

These are bedding-plane exposures of the top of a laterally-linked hemispheroids stromatolite unit.

Just above the previous stromatolites are these desiccation cracks. The tiny pockmarks may be raindrop imprints. The mudcracked units are thick enough in some places to make unusual sedimentary columnar bedding.

These are casts of evaporative gypsum or anhydrite nodules.

An intraclastic limestone grading into a breccia was one of our marker horizons. These rocks are often referred to as “evaporative breccias” because they are associated with the dissolution of evaporite mineral layers and collapse of the mudstones above.

These are delicious columnar stromatolites that made mounds on the sediment surface. The stromatolites are like thick fingers reaching upwards.

This close view shows the packing of the stromatolites. It is almost hexagonal.

An even closer view shows that the stromatolites were burrowed while still relatively soft. Were the trace-makers feeding on the decaying cyanobacterial mats inside? The interstitial sediment in the burrows and between the columns appears to be dolomitized.

Can’t have tidal sediments without herringbone cross-stratification, can we? These structures indicate bidirectional currents, likely from storms or tides.

Lunch in the shade! We had much more sun than yesterday.

Another successful day of field geology. We celebrated at the Veyo pie shop, now a Wooster Utah tradition.

(Links to the First Day, Second Day, and Third Day.)