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Postcards from the Field

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Nels Iverson
Sills near and sills far! Cornudas Mountains, Otero County, NM
January 24, 2022

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.

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Dan Koning
Bone from a huge ancient camel found in the San Marcial basin San Marcial basin
December 20, 2021

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.

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Jacob Thacker
Hoodoo in Late Cretaceous sandstone
November 29, 2021

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.

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Snir Attia
Folded Paleozoic limestones at the edge of the Rio Grande
November 29, 2021

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.

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Dan Koning
Audio-magnetotelluric survey East of San Marcial, NM
October 20, 2021

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.

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Dan Koning
Interesting features in the Sandia Granite Sandia Mountains, Albuquerque, NM
October 8, 2021

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.

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Kevin Hobbs
Eruptive history in Rio Grande del Norte National Monument Taos County, NM
September 23, 2021

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.

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Gravity Survey of the Marcial Basin near Fort Craig, NM
August 20, 2021

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).

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Matt Zimmerer
Sunrise in the San Juan Basin
August 20, 2021

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.

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Silicified volcanic ash deposits in the Nacimiento Formation at Ceja Pelon Mesa Sandoval County, New Mexico
August 3, 2021

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.

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Vertical Late Cretaceous strata Zuni Mountains, New Mexico
August 3, 2021

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.

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Dan Koning
Desert Pavement in the Ft. Craig Area Socorro County, NM
May 17, 2021

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.

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Kevin Hobbs
El Cerro de Tomé Tome, NM
April 14, 2021

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.

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Dan Koning
Quartz veins and faults Death Valley CA & Magdalena, NM
April 14, 2021

Many gold and silver mines follow quartz veins that were precipitated from hydrothermal groundwater. The first photo [from Death Valley] shows such a quartz vein (just below the hand), which is about 20 cm wide. In the lower-right of the photo is a smooth rock face (covered in dust) that corresponds to a fault plane (dipping to the left). Just above this fault plane is another quartz vein that is 3-4 cm-wide. Many quartz veins form parallel to faults, like what is shown in the first photo.

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Jacob Thacker
Conjugate Faults in the Zuni Mountains Zuni Mountains, NM
April 14, 2021

Geology is truly a multidisciplinary science. On any given day we might work at the intersections of anthropology to zoology. Rock deformation (how rocks break and bend) is one example where these intersections help us to better understand our Earth. Engineering tests conducted to determine the strength of materials have shown us that applying a compressional stress to both sides of a rock will cause it to break in characteristic patterns. One pattern is known as conjugate faults, where failure of the rock results in a pair of faults that form a 60° wedge, of which the maximum compression direction (i.e., the squeezing direction) bisects the middle of the wedge. Armed with this experimental information, detailed field measurements of conjugate faults and other deformation features can tell us the different states of stress that Earth’s crust has enjoyed.

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