Log Springs Formation
Log Springs Formation
The Log Springs Formation is a rock layer in the Jemez, Nacimiento, and Sandia Mountains of New Mexico, United States. It is thought to be Namurian in age, dating from late Mississippian to early Pennsylvanian. The formation consists of red continental beds that grew from reworked terra rossa soils, filling karst valleys in the underlying Arroyo Penasco Group. Outcrops are limited, but near the type section in the southern Jemez Mountains the formation is about 25 meters thick.
The lower 3 meters are hematitic shales with small 1–5 mm ooids or pisolites. The upper part is an upward-coarsening sequence of cross-bedded, reddish sandstones that contain clasts of Precambrian rocks such as gneiss, greenstone, and quartz. No fossils are known, but the age is inferred from surrounding beds. The logs record the onset of tectonic uplift in the Ancestral Rocky Mountains and probably correlate with the Molas Formation in the Animas Valley.
Historically these beds were included in the Sandia Formation, but geologist A. K. Armstrong showed they are separated from Mississippian beds by an unconformity and are overlain by beds older than most of the Sandia, so he named them the Log Springs Formation in 1955. It underlies the Osha Canyon Formation and overlies the Arroyo Penasco Group, with a thickness of about 25 meters. The primary rock type is ferruginous shale, with sandstone also present. The type section is named for Log Springs, near 35.6469°N, 106.8582°W in New Mexico.
This page was last edited on 3 February 2026, at 06:50 (CET).