Postcards from the Field

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.

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.

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.

New Mexico is a great place to study extensional tectonics, or what happens to Earth’s crust as it is stretched and thinned. The Reserve graben, a small rift basin in western Catron County, is part of a longer fault system, including the Plains of San Agustin, at the southeastern margin of the Colorado Plateau. It also sits within the massive Mogollon-Datil volcanic field, where some of the largest explosive eruptions in North America’s history have occurred. The San Francisco River and its tributaries have cut down through the sedimentary rock that filled in the basin, exposing many of the basin’s internal faults.

"I have been mapping the Santa Fe Group and post-Santa Fe Group deposits near San Marcial over the past year. In December, I came across this neat exposure of axial sand laid down by the Rio Grande about 2-3 million years ago. The first photo shows very thin-thin beds of volcanic pebbles interbedded in the white, medium-grained river sand. Above the blue pen are a couple of spheroidal clay balls. These balls formed when the river ripped up a pre-existing, hardened clay bed (presumably on a floodplain). The ripped-up clay rolled downstream like a piece of gravel, becoming rounded in the process. If the clay ball rolls across pebbles on the bed of the river (like the pebble beds seen below the pen), then the pebbles can stick to the clay -- like rolling a ball of cookie dough over chocolate chips and having the chips stick to the outside of the ball-of-dough. An example of such an "armored clay ball" is seen in the second photo, with a blue pen for scale. The clay ball in the second photo may actually be comprised of smaller clay balls that have been compressed together to form one larger clay ball, which was later armored."

New Mexico offers a much wider variety of mineral collecting opportunities than my home state of Alabama. This is certainly the case with fluorescent minerals, a major focus of my mineral collection. One mine that offers attractive fluorescent material is the Dictator Mine, located near Winston in Sierra County. The Dictator Mine is a small, former producer of zinc, lead, copper, and silver. According to the 1934 NM Bureau of Mines and Mineral Resources Bulletin 10, which details the ore deposits of Sierra County, the deposit was known to prospectors as early as 1880.