New Mexico Geological Society Annual Spring Meeting — Abstracts

Pennsylvanian lithostratigraphic nomenclature in NM has developed in piecemeal fashion over the past century, and in some cases is inconsistent, redundant, or otherwise inappropriate. The interplay between paleogeography, tectonics, eustasy and climate has produced depositional sequences of significant variability from place to place, but the occurrence of most Pennsylvanian sequences in isolated fault-block rages has emphasized apparent differences between local sequences rather than the broader depositional patterns that tie them together. Review of Pennsylvanian stratigraphy throughout NM leads to the following recommendations: 1) use of the term Magdalena Group should be discontinued, as it has been applied in so many contradictory ways as to be meaningless; 2) the Madera Formation is best regarded as a Group wherever it occurs. It consists of two or three major subdivisions (formations), including a lower massive cherty limestone unit of Desmoinesian age, reflecting widespread carbonate shelf environments, followed by marine environments that were increasingly, though not everywhere evenly, affected by influx of siliciclastic seiments derived from increased tectonic activity in several mahor local uplifts. This upper, Missourian-Virgilian interval is still dominated by limestones (mainly thinner and noncherty), but with clastic beds approaching or locally surpassing the limestones in total thickness. Locally, at the top of the Madera Group, a third formation, in which red, purple and green nonmarine clastics predominate over thin marine limestones, can be recognized (e.g., Red Tanks and Bursum Formations); 3) formal names, where needed, should be applied to these Madera Group formations, to replace such informal terms as "lower gray limestone" and "upper arkosic limestone" members. The earliest valid formal names for these units are Gray Mesa and Atrasado, first applied in the Lucero uplift area; 4) the Madera Group can be recognized from the southeastern Sangre de Cristo Mountains southward along the north and west margins of the Orogrande Basin, to the Caballo and possibly Robledo Mountains; 5) the Gray Mesa and Atrasado Formations can be recognized from the Sierra Nacimiento to the Caballo Mountains; in some areas these names should replace local nomenclatures 6) current formation nomenclature in the San Andres, Sacramento and Franklin Mountains is appropriate 7) some portions of the largely ignored but validly establisged lithostratigraphic nomenclature of Thompson (1942) could be usefully employed as local member names within the broadly conceived formations noted above; 8) names for subsurface Pennsylvanian lithostratigraphic units in NM should be the same as those applied to surface exposures, rather than (current practice) Group names derived from unrelated sequences in central Texas.