CONODONT BIOSTRATIGRAPHY OF THE MIDDLE PENNSYLVANIAN (ATOKAN-DESMOINESIAN) SUCCESSION IN CENTRAL NEW MEXICO

The Pennsylvanian Ancestral Rocky Mountain Orogeny created a series of fault-bounded uplifts that lay adjacent to elongated marine basins in which thick sections of sediments accumulated. The oldest Pennsylvanian strata in central New Mexico are dominated by siliciclastics assigned to the Sandia Formation in the north. Southward, the Sandia grades into the Red House Formation, in which carbonate beds are common. A thick, shallow water carbonate succession overlies the Sandia and Red House formations. Across much of central New Mexico, the lowermost carbonates are assigned to the Gray Mesa Formation, but in the north they are called the Porvenir Formation. Previous biostratigraphic work, based on fusulinids and macrofossils, suggested that the Sandia and Red House are late Morrowan to Atokan (Derryan) in age and that the transition to the overlying carbonate section occurred near the beginning of the Desmoinesian.

Numerous conodont samples were collected as part of a series of stratigraphic studies on Pennsylvanian strata in New Mexico. Sampling concentrated on the uncommon, thin limestones in the Sandia and Red House formations. In the Gray Mesa and Porvenir formations, packstones and grainstones were sampled in preference to the more common wackestones. Recovery of conodonts was variable, but a composite succession of conodont faunas was constructed. Species of Neognathodus provide the best biostratigraphic resolution, but Idiognathodus species provide some additional datums. Comparison with the Midcontinent conodont zonation shows that the New Mexico succession contains a more complete record of conodonts across the Atokan-Desmoinesian boundary.

The oldest conodont faunas recovered from the Sandia and Red House formations are of middle Atokan age (N. atokaensis Zone), and the upper parts of these units range into the late Atokan (N. colombiensis Zone). The Atokan-Desmoinesian boundary is best approximated by the appearance of N. bothrops and an unnamed Idiognathodus species. The transition to the Gray Mesa and Porvenir formations varies in age, ranging from late Atokan to early Desmoinesian at different locales. Subdivision of the Gray Mesa/Porvenir can be made using Neognathodus species (N. asymmetricus, N. intrala) with Idiognathodus species (I. obliquus, I. robustus).