Postcards from the Field

In sandstone-dominated highlands of the San Juan Basin in northwestern New Mexico, many landscapes are in transition from soil-mantled forested slopes to unvegetated slickrock slopes. This process is altering the hydrology and ecology of the region. In the first photographs, a mature Ponderosa pine has been undercut by arroyo incision, showing the depth of soils in the area (about 2 meters) as well as the gradational weathering of the region's sandstone bedrock (in this case, the Ojo Alamo Sandstone). This rock weathering creates a geological product called saprolite that has properties of both rock and soil. As in many environments, there is no distinct dividing line between rock and soil here. Note how the tree's roots preferentially follow weak fractures in the top of the bedrock - an excellent reminder of the important role that plants take in turning rock into sediment! Since this photo was taken in 2014, this tree has toppled into the arroyo.

The final photograph shows a landscape that has lost practically all soil cover. Small trees, grasses, and shrubs still persist in fractures and low spots, but the landscape is largely unvegetated. Summer rainstorms generate intense runoff since the precipitation cannot soak in through the bare rock as it would through a soil. These runoff events lead to further erosion and soil loss, creating a positive feedback loop; i.e., less soil leads to more runoff, which leads to less soil.

— Kevin Hobbs, Field Geologist, NMBGMR

When visiting geologists come to New Mexico, we take them into the field, even in January. Dr. Tapani Rämö is from the Department of Geosciences and Geography, University of Helsinki and spent a day in the field in the Gallinas Mountains, Lincoln County with Dr. McLemore. Rapakivi syenite sills intrude Permian limestone and sandstone along a closed road in the Gallinas Mountains, Lincoln County, New Mexico. The Finnish word “rapakivi” means disintegrated or crumbly rock or stone and denotes the tendency of certain rapakivi granite types to disintegrate more easily than other granitic rocks of southern Finland (type area of rapakivi granite). Rapakivi texture in the Finnish rapakivi granites is characterized by ovoidal alkali feldspar megacrysts mantled by sodic plagioclase and presence of short-prismatic quartz crystals.

Geologists often find themselves hiking up, over, and down hills, peaks, and valleys in straight lines — despite the topography — to get to our area of interest. On this day, however, geologic mapping near Gallup revealed this natural stairwell that made for a leisurely ascent up tilted beds of the Late Cretaceous Twowells Tongue of the Dakota Sandstone. Most likely, dissolution of calcium carbonate in the rock led to the small pits found across the outcrop, which were further accentuated by chemical and physical weathering processes from surface water runoff and wind to carve out the channel and form the steps.

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.

A sill is a tabular igneous intrusion that is parallel to the planar structure of the surrounding rocks, one of the many possible shapes that magmatic intrusions can form! Bureau geologists are working in the Cornudas Mountains (near the Texas state line in southernmost Otero County, NM) to map out and study the 25 to 40-million-year old magma bodies that tower proudly over the Otero Mesa. This work is part of an Earth MRI-funded collaboration between the New Mexico Bureau of Geology (led by Dr. Virginia McLemore), Texas Bureau of Economic Geology, and USGS to investigate the economic potential for critical mineral resources in the area. New Mexico Bureau scientists Snir Attia and Nels Iverson, along with graduate student Mason Woodard spent a recent weekend mapping several of the intrusive plugs, sills, and dikes found in the area.