Pecos, NM
February 11, 2022
Dr. Matt Heizler from the NMBGMR, Dr. Karl Karlstrom from UNM, and their students have been collaborating on projects to understand erosion rates in the southwest by dating detrital sanidine (a potassium feldspar: KAlSi3O8). The New Mexico Geochronology Research Laboratory at the NMBGMR has been developing the capability to date minute sanidine grains found in river gravels. Sanidine crystals are spread widely across the landscape when massive volcanoes erupt ashes. If these crystal grains can be recovered from river gravels, the youngest of the grains will provide a maximum age for the gravel deposit. The height of the gravel deposit above the modern river can be divided by the maximum age to calculate an erosion rate. Gravel deposits at multiple heights can be used to refine our understanding about how erosion rates have changed over time. This knowledge can help geologists reconstruct the geologic history of our dynamic landscape.
I saw a student talk about utilizing the detrital sanadine method and remembered a remarkable set of gravel deposits I had mapped about 20 years ago high above the Pecos River near Pecos, NM. The highest of these gravels caps Rowe Peak on Glorieta Mesa, and is about 400 meters above the modern river. Karl and Matt were intrigued, so Karl and I, along with two of his students, Benjamin Rodriguez and Cameron Reed, set out to sample these gravels. After some 4WD adventures and a few missteps on bad roads, we made it to the base of Rowe Peak and hiked to the top. The gravel deposit here is quite thick and contains large cobbles (up to 50cm) of Proterozoic metamorphic and igneous rocks, as well as Paleozoic sedimentary rocks from the Pecos Wilderness area.
We dug two pits to get below the modern soil, and sampled the fine grained sand between the cobbles. Hopefully, there will be sanidine in these samples. If so, we'll sample more sites, and other lower gravel deposits, and will be able to reconstruct the incision history of the Pecos River.
— Adam Read, NMBGMR
