New Mexico Geological Society Annual Spring Meeting — Abstracts

The southeastern edge of the Albuquerque Basin of the Rio Grande rift is delineated where alluvium from the Los Pinos Mountains laps onto faulted Proterozoic-Mesozoic bedrock along the northern edge of Palo Duro Canyon on the eastern side of Sevilleta National Wildlife Refuge. The hanging wall of a north-northeast-trending normal fault on the southern drainage divide of Palo Duro Canyon preserves a block of Jurassic Morrison Formation (uncommon in the area; Lucas et al., 2016) and a small outcrop of Miocene Popotosa Formation filling a paleo-valley incised into the Morrison. The fault cuts a north-striking, west-facing, Laramide monocline exposed along the footwall. The normal fault strikes N 20-60^o E, whereas the trend of the northwest edge of the paleo-valley is S 207^o W. The fault has at least 120 m of vertical separation of Mesozoic bedrock and at least 10-11 m of separation of the paleo-valley fill. Clasts within the bedded Popotosa Formation conglomerates and pebbly sandstones are predominantly rounded pebbles, cobbles, and boulders of Paleozoic formations exposed to the east of the monocline, silicified veins replacing limestones, intermediate igneous rocks from sills, rare Proterozoic quartz and granitic rocks, and rare ash-flow tuffs. Clasts of basaltic andesite were not noted, but may be rare. Rotated Popotosa clasts adjacent to the fault suggest that they were not cemented at the time the fault was active; rather, the carbonate cement may have been deposited later. The compositions of the clasts in the Popotosa suggest that most of the La Jara Peak basaltic andesite and most of the underlying bedrock formations from the Oligocene ash-flow tuffs, Spears Formation, Baca Formation, and all of the Cretaceous formations were stripped down to the Morrison strata before the paleo-valley was cut. The rare ash-flow tuff clasts suggest that a few remnants were still in the paleo-valley drainage at the time.

Whereas AFT (apatite fission-track) ages of uplifts along the Montosa reverse fault are Laramide, an AFT age south of the Los Pinos Mountains on the hanging wall of the Montosa normal fault is 24.8± 4.4 Ma (Behr et al., 2004), suggesting erosional stripping of the area was underway in early Popotosa time. This may imply that the paleo-valley, fault, and Popotosa fill developed relatively late in mid-Miocene time. Erosional planation across the fault, Popotosa Formation, and adjacent Paleozoic and Mesozoic formations was well underway by early Pliocene time as shown by high-level piedmont alluvial deposits, some predating and some post-dating 3.5-Ma basaltic lava flows in the headwaters. Erosion of Palo Duro, Cibola, and other east-side canyons as much as 20 m deep was episodic over the past 3.5 Ma (Treadwell, 1996). Wide-spread erosion south of the Palo-Duro-Cibola divide stripped broad areas of Mesozoic mudstones, shales, and less cemented sandstones. This area is also south of the gravity-defined arch along the southern edge of the Albuquerque Basin. Drainages adjacent to the arch flow west from the eastern mountain/uplift fronts and then veer to the northwest, suggesting ongoing tectonic activity and subsidence of the southern Albuquerque Basin (Love et al., 2013).

This small outcrop of Popotosa Formation contrasts with other, older exposures along the Manzano Mountain western flank (Lozinsky, 1988; Connell et al., 2002) and thick accumulations encountered in wells in the Albuquerque Basin (Brister, 2004; Hudson and Grauch, 2003).