Monday, April 4, 2011

Symposium Field Trip Bursts With The Unexpected – Day One

The two–day field trip with Southern Utah University’s First Annual Geology Symposium this past weekend was bursting with new and unexpected ways of looking at current ideas. However, what also burst themselves into oblivion in the geologic wildlands of Utah, Nevada, and Arizona were the tires on our rental vehicles.

IMG_6156Beaver DamMountains Utah

I have been on quite a few field trips over the years, and each adventure generally included maybe a single flat tire. It was unusual if each vehicle had a flat but not at all out of the realm of possibilities. Considering the rocky back roads we travel and the ginormous amount of gear we pack, flat tires are pretty much a given. Aggravating, but a given.

But five flats with three vehicles? We would be out of spares in a bad way, bouncing along on borrowed air in the middle of nowhere USA.
 
The first day started out innocently enough as we drove in blissful ignorance of the cruel fate that awaited us. We listened attentively at evidence for strike–slip movement on the Gunlock Fault, mapped as a down-to-the-west normal fault at the eastern edge of Great Basin…
 
IMG_6121Gunlock FaultTrace
 …picked up Pentacrinus fossils of the early Upper Jurassic Carmel Formation (170 million years old)…
 
IMG_6119Early UpperJurassic PentacrinusFossils
Pentacrinus fossils

 …and examined stripes of pumice within the ≈22.8 million year old Bauers Tuff.
 
IMG_6126Pumice StripesInBauers Tuff
Pumice in BauersTuff

As cars whizzed around a bend on old highway 91, we took our lives in our hands to stop and look at another road cut displaying the approximate location where the Gunlock and Red Reefs Faults come together.
 
IMG_6127Gunlock Fault OldHighway91
Gunlock/Red Reef Fault on old highway 91

At the next stop, we found slickensides that might indicate left-lateral strike-slip movement, or may indicate a local “flexural” slip where the rocks flexed during a folding event.
 
IMG_6141 Slickensides
Slickenside



These rocks are both “arms” of the same synform or downward fold. A small wash runs between them.     



IMG_6142ArmOf Synform
Leaning on one arm of the fold

IMG_6145ArmOf Synform
The other arm of the fold

A short drive south brought us to the awesomely awesome ≈1.74 billion year old metamorphic rocks of the Beaver Dam Mountains. This is my home sweet home, my undergraduate research and senior project study area.
 
IMG_6148 Metamorphic RocksBeaverDam MountainsUtah
Metamorphics in road cut, old highway 91, Beaver Dam Mountains

Next is a gravity slide block of Mississippian Redwall Limestone, ≈ 340 million years old.  These rocks were once continuous across the crest of the Beaver Dam range but slid off from the east by catastrophic landslides around 5-10 million years ago in the Late Miocene. Until this trip, I had never been able to wrap my mind around what happened to these blocks.

IMG_6154Gravity SlideBlock Mississippian RedwallLimestone
Gravity slide block


Gravity slide block with Joshua Tree

The blocks were caught up in the upper plate of the Castle Cliffs shear zone and slid down onto Mesquite (NV) Basin deposits, sheared and attenuated to a fraction of their original thickness.
 
IMG_6160Gravity SlideBlockOnBasinFill
Gravity slide block atop basin deposits


A well–graded back road led from old highway 91 to Cedar Pockets campground on I–15, our rest stop for the night. Along the way, we learned that the vertical relief from the peak of nearby Mt. Bangs to the bottom of the Mesquite Basin is ≈30,000 feet, the largest relief in the Basin and Range.  

There was much geological arm–waving as we discussed mountains parallel to the extensional direction of the Basin and Range, the dip of Permian rocks coming off the Colorado Plateau, anticlines, and the orientation of other Paleozoic rocks.





IMG_6174PermianRocksBeaverDamMountains
Permian rocks
IMG_6175PaleozoicRocks VirginMountains
 Paleozoic rocks of the Beaver Dam/Virgin Mountains surrounding the Mesquite Basin

It was right about now when we had our first flat tire.  I will not even mention where we located a jack.
 
IMG_6180 ChangingTheFlat_Beaver DamMountains

Several hardy chaps replaced the blowout with the spare while others stood by giving advice. I walked around and took pictures of a bunny.
 
IMG_6186
 

IMG_6185 jackrabbit

The next morning dawned warm and clear. Surely, that had been our first and only flat. Little did we suspect what fate had in store for us!
 
IMG_6192Cedar Pockets Campground VirginRiverGorge AZ
Cedar Pockets campground, Virgin River Gorge, AZ

IMG_6189Cedar Pockets campgroundVirginRiverGorgeAZ
Virgin River Gorge, AZ

To be continued…


References:
Biek, R.F., 2009, Ancient Landslides of the Beaver Dam Mountains, Washington County, Utah, http://geology.utah.gov/surveynotes/snt41-2.pdf  p.4-6, accessed 4/4/2011

2 comments:

  1. The more vehicles the more chances.

    Great geology!

    So Pentacrinus fossils have the star shape? I've only seen a few mixed in with the crinoids at the north rim.

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  2. Isn't the star shape beautiful? We didn't spend much time looking for them since this was a structural stop - Bob just picked that piece up as we walked up the hill.

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