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…
…picked up Pentacrinus fossils of the early Upper Jurassic Carmel Formation (≈170 million years old)…
…and examined stripes of pumice within the ≈22.8 million year old Bauers 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.
|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.
These rocks are both “arms” of the same synform or downward fold. A small wash runs between them.
|Leaning on one arm of the fold|
|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.
|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.
|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.
|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.
|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.
Several hardy chaps replaced the blowout with the spare while others stood by giving advice. I walked around and took pictures of a bunny.
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!
|Cedar Pockets campground, Virgin River Gorge, AZ|
|Virgin River Gorge, AZ|
To be continued…
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