New Mexico Geological Society Annual Spring Meeting — Abstracts

The Nacimiento Formation (TN) in the San Juan Basin of northwestern New Mexico is a mud-dominated, fluvial and lacustrine deposit which conformably overlies the Paleocene Ojo Alamo Sandstone and is unconformably overlain by the late paleocene?-early Eocene San Jose Formation. The TN attains a maximum thickness of about 700 m.

The type section of TN is located on the southern end of Mesa de cuba, about 6.5 km southwest of Cuba, NM. A composite section was measured in sections 11 and 14, T20N, R2W, Sandoval County. The dominant lithologies are: yellowish gray (5Y 7/2), light gray (N7) and brownish black (5YR 2/1) claystones (about 70%), yellowish gray (5Y 7/2) and grayish orange (10YR 7/4), very fine to mediumgrained, poorly sorted, askosic, immature sandstones (about 20%) and minor amounts of siltstone, silcrete and lignite. Thickness of the TN at its type section is 215 m.

The surface section measured at the type area can be correlated with an induction laterolog from the Benson Mineral Group Federal 15-21-2 no. 1. (SW1/4 SE1/4 sec. 2, T20N, is located about 11 km to the north. This log could subsurface reference section for the TN type section. the base of the TN is at 1787.7 m and the top is at thickness of 253 m. Total sand thickness is 37 m or is considerable contrast between clay and sand lithologies in log signatures and correlations of relatively thick (2-14 m) , laterally extensive sandstones between the surface and the subsurface suggest at the type area of the TN that long distance subsurface correlation within this unit is possible

Fossils collected from the TN at the type locality indicate an early Paleocene (Torrejonian) age for the upper 165 m of the section. The age of the base of the TN at the type locality is not known as precisely, but presumably 8 R2W) which serve as a In the log, 2040.6 m, a 15%. There
is also early Paleocene.