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Postcards from the Field

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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.