Postcards from the Field

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).

On a recent trip to the San Juan Basin to study erosional processes, bureau field geologists woke to beautiful views of the basin stratigraphy basking in the morning light. The San Juan Basin, located in the four corners region, formed approximately 75 million years ago during mountain building activity.

These silicified beds are common in the upper Nacimiento Formation, where they often form resistant bluffs or the caps of hoodoos. Previously interpreted as pedogenic silcretes or the result of cementation by groundwater, we now think that these are volcanic ash deposits for reasons highlighted in the accompanying photos. The field photograph shows two such beds: a lower one marked by a white arrow is truncated by an upper bed marked by a blue arrow. Just below the red arrow, the lower bed is truncated and the upper bed cuts across it. The "draping" of the upper bed over pre-exisiting topography is a hallmark of volcanic ash deposits. Some erosion of the lower bed must have occurred prior to deposition of the upper bed.

These vertical fins of rock create a stunning landscape at the western edge of the Zuni Mountains in McKinley County (west-central New Mexico). The first photo shows a close up of the sandstones that make the erosionally resistant fins here, which are part of the Dilco Coal Member of the Crevasse Canyon Formation. The little valley to the left (east) is a shale/mudstone/coal layer that easily erodes away, leaving the resistant sandstones as sentinels that bear witness to New Mexico’s past tectonic activity. In the background on the photo’s left side you can see three distinct stratigraphic beds of near-vertical Late Cretaceous Gallup Sandstone with intervening valleys of shale and mudstone layers.

Desert pavements have developed on a terrace surface in the Fort Craig area. They consist of a surface-armor of gravel clasts that overlie a certain type of soil structure called Av peds.

El Cerro de Tomé comprises the remnants of a Pliocene volcano that likely was once much larger than the present-day 383-feet-high hill climbed by so many Good Friday pilgrims. Along the trails to the summit, one can view xenoliths of "country rock" (the pre-existing rock through which the volcano's lava was erupted) within the andesite that erupted approximately 3.5 million years ago.