NONMARINE FOSSILS FROM LATE PALEOZOIC REDBEDS IN NEW MEXICO AND THE TIMING OF THE ANCESTRAL ROCKY MOUNTAIN OROGENY

Tetrapod tracks, plant fossils and vertebrate body-fossils are the most common nonmarine fossils in Late Paleozoic redbeds in New Mexico (NM). These fossils provide constraints of varying utility on the age of the Rocky Mountain (ARM) orogeny. Fossiliferous redbeds of NM include the Penn-Permian Cutler Formation of north-central NM, the Penn-Permian Sangre de Cristo Formation of northeastern NM, the Permian Abo Formation of central and southern NM, the Penn-Permian Bursum Formation of central and southern NM, and portions of the Hueco Group of southern NM. These units broadly represent a depositional apron south of the ARM, so their age is broadly synorogenic.

Tetrapod tracks are the most common fossils and include the amphibian tracks Batrachichnium and Limnopus and the reptile tracks Dimetropus and Dromopus. They can be dated as Wolfcampian based on correlation to the European Rotliegend. Ostracods from marine intertongues of the Hueco Group suggest that the youngest ichnofaunas may be Leonardian in age. There are two distinct redbed paleofloras in NM: (1) a Desmoinesian paleoflora from Cutler Formation (megasequence 1) at El Cobre Canyon includes Alethopteris serlii and Neuropteris scheuchzeri; and (2) a Wolfcampian paleoflora in the upper Cutler, Abo, Hueco and Bursum formations is dominated by Walchia and Callipteris.

The oldest vertebrate fauna is from El Cobre Canyon; this largely endemic fauna is best explained as a rare example of a non-paludal fauna of middle Pennsylvanian age. Other Cutler, Abo, Sangre de Cristo, and Hueco faunas are homogeneous and are early-middle Wolfcampian in age on the basis of the first occurrences of Eryops, Diadectes, and Sphenacodon and the absence of late Wolfcampian or Leonardian index taxa. One Cutler locality yields Seymouria and may be late Wolfcampian in age. These data indicate that the majority of redbed strata shed into New Mexico from the ARM are early-middle Wolfcampian in age, but that that redbed sedimentation extended from the middle Pennsylvanian into at least the early Leonardian.