Postcards from the Field

"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.

They might seem timeless, but desert badlands are dynamic landscapes that owe their existence to the changes induced by rapid erosion. These badlands in the San Juan Basin of northwest New Mexico are no different. The sediment eroded from steep badlands slopes seen in the background of this photograph is transported downslope in arroyos and in sheetwash floods.

The Jornada volcano, one of the largest shield volcanoes in NM, is located about 35 miles south of Socorro. This volcano was once thought to be ~760,000 years old, but new dating indicates it is only 79,000 years old, making it one of the younger volcanoes in the region!

In the foreground of this photo, at The Box rock climbing area near Socorro, is a basalt lava flow that has a brown varnish. A fresh piece of the rock is shown on the smaller photo below, with a penny for scale. This is the basalt of Broken Tank, which flowed into the area from the south about 8.5 million years ago. This lava has interested Richard Chamberlin, who mapped it out, and myself for many years because one can use it to infer paleo-slopes and paleo-geography at this time period.

But recently I found out another intriguing fact about this basalt: a piece of this same outcrop is now on Mars!

The Collaborative Groundwater Monitoring Network tracks groundwater levels across the state as part of the Aquifer Mapping Program for the Bureau of Geology. This program is actively seeking wells to fill in gaps around the state where groundwater information and trends are lacking. Working in collaboration with well owners and operators, our team uses tools for groundwater level tracking that include manual measurements with steel tapes, acoustic devices, and pressure transducers.

Studying carbonatites in the Lemitar Mountains, north of Socorro, New Mexico during a pandemic! Carbonatites are carbonate-rich rocks containing more than 50% magmatic carbonate minerals, less than 20% SiO₂, are of magmatic derivation, and typically found in zoned complexes consisting of alkaline igneous and/or carbonatite stocks, ring dikes, and cone sheets.

New Mexicans are no stranger to flash floods. The arroyos around the state often flow during our annual summer monsoon; water quickly collects in dry channels, bringing rock and sediment along with it. Predicting floods in arroyos is still a difficult task: at the Arroyo de los Pinos sediment monitoring station, we are measuring these flows just west of Socorro, NM.

Faults are fractures in Earth’s crust where blocks of rock move in opposite directions relative to one another. The Jornada fault, pictured here near Radium Springs (north of Las Cruces, New Mexico), beautifully displays the juxtaposition of units from fault slip.

Well-developed growth rings in fossil trees in the Ojo Alamo Sandstone, like those seen to the right of the orange notebook in this photograph, likely indicate pronounced annual seasonality of temperature or precipitation when the tree was growing.

"I have been working along Milligan Gulch with Kevin Hobbs and Kristin Pearthree (both of the NM Bureau of Geology) to make a geologic map of the Fort Craig quadrangle. Milligan Gulch is the canyon immediately south of the dirt road to Fort Craig, located about 30 miles south of Socorro. This canyon has a fantastic set of terraces, which can be correlated to terraces flanking the west side of the Rio Grande..."