Today I accompanied a new friend and colleague to REI to try out and purchase some climbing gear. His name is Laurence Tognetti (@ET_Exists) and he’s a fellow graduate student at the School of Earth and Space Exploration at Arizona State University (ASU).
Just having started at ASU myself, I’ve been eager to get out and see some of the local geology but haven’t had a lot of opportunities yet. I’m still settling into my new research group and trying to keep up with a heavy course load while also trying to prepare my M.S. defense, so I’ve been pretty busy.
We decided to head north of Tempe into Scottsdale, first enjoying lunch at Rehab Burger Therapy. I got a veggie burger, which was quite good. Then we headed to REI and Laurence got outfitted for rock climbing, accomplishing our primary goal. On the way back to the Phoenix metro area, we passed a striking outcrop of red rock with large clasts contained within it. I felt the urge to stop and investigate so we made a U-turn and set off geologizing.
The first thing we noticed was different-sized boulders of granite and quartzite. Farther along the path up the butte, we began to see the bedrock. This butte (named Papago Butte) appeared to be composed of a sedimentary breccia, with large clasts (some bigger than us!) of granite and quartzite. The matrix material was a striking-red fine-grained sandstone.
Breccias are defined by highly angular clasts that exist in a finer-grained matrix material. Consequently, the sediments making up the rock are extremely variable in terms of their size. Geologists say that they are “poorly-sorted”. Conglomerates share this characteristic with breccias; however, the larger clasts in conglomerates are rounded instead of angular. The poorly-sorted character and angularity of the clasts in breccias tell geologists that those sediments didn’t travel very far prior to deposition and didn’t spend much time being reworked by surface processes prior to being lithified into rock. So basically, something pretty drastic (and fast) happened to put these sediments in place.
The current hypothesis is that about 17 million years ago, there were mountains composed of granite and quartzite. One day, a large landslide brought pieces of these mountains into a nearby riverbed. The breccia (along with other river sediments) was trapped between down-dropped blocks of bedrock that resulted from faulting associated with stretching of the crust (structures called half-grabens). The events resulting in the formation of these structures is known in central Arizona as the Mid-Tertiary Orogenies (because they occurred between 42 and 15 million years ago). The source of this orogenesis was the steepening of the angle of a subducting slab of crust that had broken off from what is now the western coast of North America.
Since that time, surface processes have eroded the landscape. The most obvious erosional feature we observed was tafoni. These are essentially large, cave-like pockets that form in different types of rocks. I had become accustomed to seeing lots of tafoni in the Red River Gorge in Kentucky so this was a welcome sight for me.
We spent about 15 or 20 minutes in the middle of the afternoon out there. Since it’s August, the sun was REALLY intense. I haven’t done much mid-day outdoor activity since moving to AZ but the elements here are more extreme than I anticipated. By the time we were back in the car, I had finished my water and noticed I was getting a headache. Dehydration happens quickly in the desert, so I know to be better prepared next time!