New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1946, Wood and Northrop, in their map of the Sierra Nacimiento-Jemez Mountains-San Pedro Mountains (NJS), identified a late Paleozoic uplift in that area, and it came to be called the Peñasco uplift. This uplift has long been identified as a north-south elongate and east-west narrow (~80 km north-south and ~16 km east-west), basement-cored highland. However, detailed stratigraphic study reveals a series of fault blocks that moved intermittently during the late Paleozoic, not a single Peñasco uplift.

In the NJS, Mississippian marine deposits of the Arroyo Peñasco Formation covered a large area of a peneplained Proterozoic basement. The unconformity between the Arroyo Peñasco Formation and overlying nonmarine Log Springs Formation reflects a tectonic pulse near the Viséan-Namurian boundary that caused moderate local uplift and erosion. The Lower Pennsylvanian (Morrowan) Osha Canyon Formation in the NJS rests unconformably on the Log Springs Formation and documents a marine transgression. Coarse-grained sediments are rare to absent in the Osha Canyon Formation, indicating that a Peñasco uplift was not active during the Morrowan.

A tectonic event close to the Morrowan-Atokan boundary was the local onset of the Ancestral Rocky Mountain (ARM) orogeny, and drove deposition of the Sandia Formation on Morrowan and Mississippian strata, and Proterozoic basement rocks across the NJS. The complex facies and thickness distribution of the Atokan Sandia Formation identifies a source area for coarse siliciclastic sediments to the northwest. In the NJS, small local uplifts existed for a short time and were rapidly eroded and covered with sediments during the Atokan. The overlying early Desmoinesian Gray Mesa Formation is a thin but widespread limestone and mudstone unit that indicates the NJS was covered by a shallow sea during a period of little tectonic activity: the Peñasco uplift was not active during the early Desmoinesian.

The overlying late Desmoinesian-Virgilian Guadalupe Box Formation has high amounts of coarse-grained siliciclastic sediments and varies laterally in facies and thickness. The sudden onset of coarse-grained sediment derived from granitoid Proterozoic basement rocks marked a second tectonic pulse of the ARM. Locally, the Guadalupe Box Formation rests on Proterozoic basement (small local uplifts), and facies and thickness variation indicate strong tectonic activity. A third ARM tectonic event near the Virgilian-Wolfcampian boundary drove a change in deposition from mixed marine-nonmarine sediments of the Guadalupe Box Formation to nonmarine redbeds of the Abo Formation. Locally, in the NJS, the Abo Formation rests on Proterozoic basement rocks deposited across small uplifts (Jack Rabbit Flats, Camp Zia area).

Thus, a single Peñasco uplift in the NJS that was active during the entire Pennsylvanian did not exist; only local small uplifts existed for relatively short time intervals during three tectonic phases (Morrowan-Atokan, late Desmoinesian and Virgilian-Wolfcampian). These uplifts provided only small amounts of siliciclastic sediments and were rapidly eroded and buried. The main sources for siliciclastic sediments in the NJS were to the north: the San Luis uplift during the Morrowan-early Desmoinesian and the southern Uncompahgre uplift during the late Desmoinesian-Wolfcampian.