Postcards from the Field

I am drawing on my experience in mapping the badlands northwest and west of Rio Rancho to make a geologic model of hydrostratigraphic units (HSUs) under the northwestern Albuquerque Basin. Hydrostratigraphic units are differentiated based on inferred groundwater-related properties of an aquifer, like permeability or porosity, as opposed to strictly outcrop-related criteria used to differentiate lithostratigraphic units.

The Funzie conglomerate started off as a thick package of pebbles, cobbles, and boulders within a sandy matrix that were eroded off of an immense mountain range formed during the Caledonian orogeny, a relatively early phase of the larger collision between what are now the eastern and southern edges of North America and parts of Europe and South America. This broader event shaped the geology of much of North America, including the Appalachians and the Ancestral Rocky Mountains uplifts! As the Caledonian orogeny progressed, the sediments of the Funzie were buried and deformed. However, the total penetrative strain that these rocks accumulated varied in space due to heterogeneous deformation (see figure). These differences are recorded in the shape of clasts, that is the clasts act as strain markers.

Close to the earth’s surface, rocks undergo brittle deformation by breaking and sliding along faults when subjected to external forces like when continents collide or break apart. However, just ~15 kilometers beneath the surface, higher temperature and pressure cause rocks to flow in response to these same forces and undergo ductile deformation. At these depths, the mineral grains and clasts that make up rocks accumulate penetrative strain and change shape, allowing the entire rock to also change its shape! The depth where the change in deformation style takes place is called the brittle-ductile transition.

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.

Bureau geologists have been mapping the San Marcial basin for the past 5 years. This is the sparsely populated basin near Fort Craig, about 30 miles south of Socorro. Relatively little attention has been paid to the geology here until this mapping effort. Not unexpectedly, after tromping around and making many outcrop observations, we have learned a lot more about the geology in this part of New Mexico.

An ancillary "fruit of our labor" is the discovery of fossils in a few places in the basin. We took Gary Morgan, from the New Mexico Museum of Natural History and Science and an expert on ancient mammal fossils, out to these sites in early November. At the last site, as we were hiking back to the cars and dusk was deepening, Gary spotted a bone the size of a small tree trunk! The first photo shows Gary holding what he later determined to be a radius-ulna (forearm) bone of a humongous ancient camel.

Hoodoos are gravity defying testaments of weathering and erosion. This small hoodoo (note the 14 inch long mapboard for scale) in sandstones of the Crevasse Canyon Formation near Gallup showcases how these features form. The upper dark brown sandstone is much harder and more resistant to erosion than the "friable" (easily eroded and crumbling) light tan sandstone beneath it. Being more resistant to the elements, the dark brown rock erodes much more slowly, while the light tan sandstone beneath it erodes much more quickly. This contrast in erodibility leaves a large cap rock over a small pinnacle.

Bureau field geologists Dan Koning, Jacob Thacker, and Snir Attia joined John Nelson and Scott Elrick of the Illinois State Geological Survey to map folded strata in the Little San Pascual Mountains that sit just east of the Bosque Del Apache. This small mountain range was uplifted during formation of the Rio Grande Rift, but the complexly folded limestones exposed here were deposited over 300 million years ago in the Pennsylvanian! The complex folding and more cryptic, associated faulting likely resulted from regional contraction during the Late Cretaceous Laramide Orogeny.

We are doing an audio-magnetotelluric survey today east of San Marcial. Geologically, we are in the structural accommodation zone between the northern Marcial Basin and the southern Socorro Basin. Hopefully we will get useful results that can tell us something about the variations of depth to bedrock and groundwater salinity changes in this enigmatic area.

Yesterday I saw some neat bedrock features along the northern Tramway trail in the Sandia Mountains, between La Cueva Canyon and the La Luz Trial. The trail is mostly on weathered Sandia Granite, which is actually a biotite monzogranite and granodiorite that is between 1455 +/12 Ma and 1446 +/-26 million years old.

In 2013, President Barack Obama invoked the Antiquities Act of 1906 to create the Rio Grande del Norte National Monument in Taos County. Part of the wording of this 115-year-old act allows for the protection of features of "scientific interest that are situated upon the lands owned or controlled by the Government of the United States", of which there are many in the Rio Grande gorge and surrounding canyons and volcanoes.

This photograph shows Kyle Gallant, a student from NM Tech, taking a reading from a gravimeter during a gravity survey last month. The gravimeter is the tan, box-like object on the paver. Kyle was the primary worker in this gravity survey, which focused on the Marcial basin (rift basin near Fort Craig, between Elephant Butte Lake and Socorro).