Postcards from the Field

Limestone and dolomite are carbonate sedimentary rocks generally formed in marine settings from the precipitation of calcium in seawater, often via biological processes. Limestones are rich in the calcium carbonate minerals aragonite and calcite, while dolomite contains the mineral dolomite (CaMg(CO3)2). These rocks may preserve fossils, helping geologists to determine the age of carbonate-containing formations. The porosity of some carbonate rocks can make them efficient petroleum reservoirs. Carbonate rocks are also excellent hosts for some types of ore deposits, such as Mississippi Valley Type lead-zinc deposits.

WDI is currently collaborating with the Pecos Valley Artesian Conservancy District (PVACD) to build web applications to help manage and share data in the Pecos Valley region of southern New Mexico. This work is part of a U.S. Bureau of Reclamation WaterSMART applied science cooperative grant, which began in 2020.

In January 2023, The WDI team went to Roswell, New Mexico to visit the PVACD to learn in-person about details of their data workflow challenges and day-to-day operations related to water data. This visit also included getting feedback on some new applications WDI has in development.

Just before the winter break, Bureau of Geology and NM Tech staff and students hosted the US Geological Survey (USGS) during field work related to the critical minerals in mine wastes project. Co-Principal Investigators Dr. Virginia McLemore, Senior Economic Geologist and Bonnie Frey, Chemistry Lab Manager, along with other Bureau staff joined by Dr. Dan Jones, Geobiologist with NMT’s EES department and numerous students collected rock, water, and bio samples at the Copper Flat Mine near Hillsboro NM. USGS geologists Robert Seal and Kate Campbell-Hay observed our field procedures as part of future work. The NM Bureau of Geology

Not only do geologists collect waters from the surface and wells, but we occasionally collect water samples from underground adits. An adit is a flat or slightly declining underground tunnel with only one entrance to the surface that miners use to access the ore body, generally to remove ore. When mining stops, the adits locally flood. In the Critical Minerals in Mine Wastes project funded by the U.S. Geological Survey, we are examining mine wastes for their critical mineral concentrations.

Over several weeks in November, I have been visiting a set of dikes, sills, and plugs that outcrop across eastern New Mexico. These igneous rocks represent the easternmost magmatism related to the past 350 million years of tectonic activity responsible for shaping what is now western North America!

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.