Postcards from the Field

Sills near and sills far!

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This photo looking south depicts several different perspectives of sills! In the foreground, Snir Attia (right) and Mason Woodard (left) are standing on the exposed margin (orange line) of a bedding-parallel igneous sill. Erosion has scoured out the sill to the right of the orange line, exposing the underlying limestone beds. In the midground, the top and bottom contacts of the same sill are exposed in the hillslope on the opposite side of a canyon (dashed lines)! The intersection of geologic layers and the earth’s surface can create intricate and surprising patterns. Flat Top (left) and Alamo Mountain (right) are visible along the skyline in the background.
(click for a larger version)

Nels Iverson

Looking north from the flanks of San Antonio, the sills that make up the top of Flat Top (right) and Alamo Mountain (left) can be seen in a different light. In the foreground, Snir Attia and Mason Woodard are seen making observations and discussing the geology of the pluton-shaped intrusion that forms San Antonio Mountain itself.
(click for a larger version)

Nels Iverson

Simplified map showing the general layout and location of the Cornudas Mountains along the NM-TX state line.
Dr. Virginia McLemore

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.

The first photo shows one sill intruding into Paleozoic limestones. The sill sits on top of one of these limestone beds that has been scoured out by water, with the orange line in this photo marking the sill’s margin. The sill is ~10 m thick and dips into the adjacent hill, where both the top and bottom intrusive contacts are preserved (dashed orange lines). This site is a fantastic example of these types of intrusions!

The mountains making up the skyline to the south in the background of the photo, Flat Top (Left) and Alamo Mountain (Right), are themselves larger sills! The view from the flanks of San Antonio Mountain, provides a complementary perspective of the same two sills that make up Alamo Mountain and Flat Top, as shown the in second photo.

— Snir Attia, Field Geologist, Nels Iverson, Geochemist, and Mason Woodard NMT EES Graduate student