Palomas Basin Geomorphology

It is always exciting when a field geologist comes across an outcrop that yields a lot of information, especially when it pertains to the area’s geologic history. Becca Goughnour and I encountered a pair of such outcrops last June while mapping along the western margin of the Palomas Basin, about 15-20 miles southwest of Truth or Consequences. The outcrops are part of a large exposure at the top of a steep slope, as shown in the first image (an annotated jpeg exported from Google Earth). The two outcrops are marked by the yellow rectangles. In this area, two early Pliocene basalts are present at two different geomorphic levels, separated vertically by 40-50 feet. Underlying the basalts, the bulk of the exposure is of the Miocene-age Santa Fe Group (ancient alluvial fan sediment of the Rincon Valley Formation, which Dan Koning wrote about in aprevious postcard).

Of interest here is not so much the Miocene strata but the early Pliocene features that give clues to the geologic history here at 5–4 million years ago (Ma). The lower basalt has been dated at 4.6 Ma (rounding up from an 40 Ar/ 39 Ar age of 4.55 ± 0.03 Ma) and the
upper basalt appears to correlate to 4.9 Ma basalts 0.5 mile to the southeast. The fact that there is Pliocene sediment deposited under, over, and alongside these basalts allows one to construct a story of paleodrainage and paleogeomorphic activity at this location in the early Pliocene.

Photos 2 offers a view of the northwest exposure and Photo 3 is a close-up of this view. Note how the 4.9 Ma basalt fills a 20-25 ft-deep paleovalley on the right. Sandwiched between the gravelly, Miocene-age Rincon Valley Formation and the 4.9 Ma basalt is a thin deposit of Pliocene sediment. This Pliocene sediment consists of ~2 ft of colluvium overlain by 3–5 ft-thick bouldery debris flow sediment that pinches out to the south (photo 3). The restricted lateral extent of this sequence of rocks (i.e. colluvium, bouldery sediment, and basalt flow) suggests that just before 4.9 Ma, erosion of the landscape here created a paleovalley and a thin, pebbly colluvium formed on the south wall of this paleovalley. Then, a brief period of gravelly aggradation (up to ~5 ft thickness) only partly filled this paleovalley. We can tell that this aggradation coincided with the onset of basaltic volcanism in the area because of the abundant basaltic boulders and cobbles found in the deposit (photo 3). Finally, the 4.9 Ma basalt flow filled in the bulk of the paleovalley and spilled out southwards onto a flat geomorphic surface (labele“spillover” in Photo 2).

Erosion occurred between 4.9 and 4.6 Ma, dropping base level by ~45 ft (Photo 1). Photo 4 illustrates the southeast exposure from a distance. Here, you have a younger lower paleovalley cut into the Miocene sediment and backfilled by Pliocene gravel interfingering with the 4.6 Ma basalt. This Pliocene gravel and the basalt is overlain by sandy colluvium and slopewash (about 20–25 ft thick and annotated in the lower-left corner of Photo 1). On top of the sandy sediment and extending up the slope to the 4.9 Ma basalt flow (annotated in Photo 1 and by the blue line in Photo 4), is a strong calcic soil (stage IV carbonate morphology, the stage where you get CaCO 3 laminations at the top of the calcic horizon).

A close-up shot of the southeast outcrop (Photo 5) indicates that the Pliocene gravel (Npg) in the lower paleovalley can be subdivided into three units: Npg1 fills the lower several meters of the paleovalley. Npg2 is about 1 m thick and has a reddish matrix and extends immediately beneath the 4.6 Ma basalt. However, the bouldery Npg3 extends over the basalt. These interfingering relationships show that this lower paleovalley experienced gravelly aggradation just before 4.6 Ma (Ngp1 and Ngp2), the 4.6 Ma basalt flowed down the paleovalley and over these units, and then erosion occurred to produce a third paleovalley that was ~15 ft deep. This third paleovally was then backfilled by unit Npg3. After Npg3 deposition, fluvial activity moved elsewhere (probably south) and enough time passed (during a period of geomorphic stability) to produce a stage III+ carbonate soil horizon in the upper part of unit Ngp3. Then ~20–25 ft of sandy sediment accumulated. Becca and I did not look at this sandy sediment closely but its geomorphic position between the two basalt flows suggests the sediment was deposited by slopewash and colluvium. Then a prolonged period of geomorphic stability occurred (at least locally, elsewhere the larger Black Mountain-derived canyons were probably incising) and a strongly developed calcic soil formed at the top of the sandy sediment.

From these outcrops we can conclude that the western margin of the Palomas Basin was characterized by net erosion and episodic incision of shallow paleovalleys between 5 and 4 million years ago. But this erosion was sometimes interrupted by brief(?) paleovalley sedimentation and basalt-flow emplacement, and there was local geomorphic stability when paleodrainages shifted elsewhere. In contrast, geologic mapping in the middle of the Palomas Basin suggests relatively constant sedimentation of the distal alluvial fans and Rio Grande during 5 to 4 million years ago (e.g., Elephant Butte geologic map by Koning et al, 2026, GM-84). This contrast illustrates how, in a given moment in time, different parts of a basin can behave differently in terms of being in an erosional vs. aggradational state.