New Mexico Geological Society Annual Spring Meeting — Abstracts

Kottlowski et al. (1956, NMBMMR Memoir 1, p. 62, fig. 11) identified the southernmost Triassic outcrops in central New Mexico as Dockum Formation strata exposed on the western flank of Big Gyp Mountain in T11S, R2E, Sierra County. Bachman and Harbour (1970, USGS Map 1-600), however, mapped these strata as Permian Artesia Formation. Kottlowski and LeMone (1994, PBS-SEPM Pub. 94-35, p. 41) suggested these strata may be Triassic Moenkopi Formation.

Kottlowski et al. described 27 ft of pale red silty shale and claystone overlain by 25 ft of interbedded pale reddish brown, light gray, greenish gray, grayish red and light yellowish gray calcareous claystone and micaceous siltstone with laminae of silty, feldspathic sandstone. These strata are between the Permian San Andres Formation and Cretaceous Dakota Sandstone. The outcrop they measured is at the confluence of two creeks due north of East Tank in the NW1/4 SW1/4 NW1/4 sec. 12, T11 S, R2E (UTM zone 13, 341912E, 3693026N), where the following strata (in ascending order) immediately underlie the Dakota Sandstone: (1) 0.5 m of pale reddish brown calcareous siltstone and yellowish gray silty, gypsiferous, very fine-grained sandstone; (2) 2.5 m of moderate reddish orange and moderate reddish brown calcareous, gypsiferous ripple-laminated, sandy siltstone; and (3) 0.8 m of pale red and pale reddish brown trough crossbedded and ripple laminated, very fine-grained sandstone. The overlying Dakota Sandstone consists of very pale orange and grayish orange, hematitic quartzarenite and siliceous, clast-supported conglomerate mostly composed of gray and black quartzite and chert pebbles up to 3 cm in diameter.

Contrary to their original description, the red beds exposed on the western flank of Big Gyp Mountain are neither micaceous nor feldspathic. These strata are very fine grained, gypsiferous and are pale red to orange in overall color. No conglomerates are present. In these features, they do not resemble Moenkopi Formation or Chinle Group (includes Dockum) strata. The supposed Triassic strata in the northern San Andres Mountains are more similar to Artesia Formation strata, to which they are assigned. Thanks to Bob Myers and the U.S. Army, WSMR, who made access possible.