New Mexico Geological Society Annual Spring Meeting — Abstracts

Triassic strata exposed along the flanks of the Nacimiento and Jemez uplifts (Sandoval County, northern New Mexico) represent two tectonosequences. The lower tectonosequence is a relatively thin (up to 39 m) package of red-bed I sandstones assigned to the Middle Triassic Anton Chico Member of the Moenkopi Formation. The upper tectonosequence disconformably overlies the Moenkopi Formation or older, Permian strata, and is assigned to the Chinle Group (Agua Zarca, Salitral, Poleo, and Petrified Forest Formations).

The Moenkopi Formation includes strata previously assigned to the Permian Bernal Formation by most workers, and mostly consists of grayish red sandstone, siltstone, and intraformational conglomerate, with minor mudstones as much as 39 m thick. The Anton Chico Member of the Moenkopi Formation disconformably overlies the Permian Glorieta Sandstone and is disconformably overlain by the Agua Zarca Formation.

Chinle Group strata around the Nacimiento and Jemez uplifts are a thick (up to 400 m) sequence of red-bed mudstones with a relatively thick basal unit of sandstone and extrabasinal conglomerate. The Agua Zarca is as much as 61 m thick and mostly white-to-brown, trough crossbedded quartzose sandstone and siliceous, extraformational conglomerate. The overlying Salitral Formation is as much as 102 m thick and mostly purplish, smectitic mudstone. The Poleo Formation disconformably overlies the Salitral and is as much as 41 m thick and mostly grayish yellow, trough-crossbedded litharenite, subarkose and intrabasinal and siliceous conglomerate. Above the Poleo, the Petrified Forest Formation is as much as 340 m thick and dominated by reddish brown, smectitic mudstone. South of San Miguel Canyon (T19, R1 W), the Poleo Formation essentially pinches out, though thin (<20m), lenticular equivalents of the Poleo are locally present as far southeast as Vallecito Creek (T16N, R2E). Furthermore, throughout Sandoval County, the lower portion of the mudstone dominated interval above the Agua Zarca Formation, even where the Poleo Formation is absent, is dominated by purplish mudstone characteristic of the Salitral Formation. Therefore, to the south of San Miguel Canyon, the Salitral and Poleo Formations can be recognized locally, but the thick, mudstone dominated section above the Agua Zarca is assigned to the Petrified Forest Formation. The Correo Sandstone Bed crops out as a bench-forming sandstone up to 15 m thick at the top of the Petrified Forest Formation along the southern flank of the Nacimiento uplift. Fossil vertebrates indicate the Petrified Forest Formation ranges in age from Adamanian to Revueltian (latest CarnianNorian) and support lithostratigraphic correlation of the Triassic strata exposed in Sandoval County to nearby Triassic outcrops in the Chama basin, Lucero uplift, and Hagan basin.