Conodont biostratigraphy of the middle pennsylvanian Sandia Formation and lower gray mesa formation in north-central New Mexico



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Middle Pennsylvanian strata in central New Mexico are assigned to the clastic-dominated Sandia Formation, and the overlying carbonate-dominated Gray Mesa Formation. In northern and central New Mexico the Sandia Formation represents the base of the Pennsylvanian and is recognized by synorogenic deposits associated with the Ancestral Rocky Mountain Orogeny. The Sandia Formation is generally composed of a mixed siliciclastic and carbonate unit, with thick quartz rich sandstones, conglomerates, and conglomeratic sandstones that rest on Proterozoic igneous and metamorphic rocks. The overlying lower Gray Mesa Formation is a cherty, cliff-forming limestone unit that includes mudstones, fossiliferous wackestones and packstones. Occasional beds of sandstone, siltstone and shale occur. Based on previous sparse fusulinid data, the age of the Sandia was interpreted to range from the late Morrowan through the Atokan, while the Gray Mesa was early Desmoinesian in age. Five sections containing the Sandia and lower Gray Mesa (Elephant Butte and Whiskey Canyon members) formations were sampled to derive a feasible conodont-based biostratigraphic framework, from north to south: the Type Sandia section and Tejano Highway section in the Sandia Mountains, the Cedro Peak Z section in the Manzanita Mountains, the Sepultura Canyon section in the Los Pinos Mountains, and two Presilla sections in the Cerros de Amado, east of Socorro. Three evolutionary lineages of Neognathodus species were recognized that form the major basis for a series of six conodont faunal intervals: The Neognathodus atokaensis-N. colombiensis II lineage, the N. bassleri-N. asymmetricus lineage, and the N. uralicus-N. caudatus lineage. Faunal Interval 1 is defined by the first occurrence of N. atokaensis. Faunal interval 2 is split into subintervals, 2a, defined by the first occurrence of N. "pre-colombiensis," and 2b, defined by the first occurrence of Idiognathodus gibbus. Faunal Interval 3 is split into two subintervals, 3a, defined by the first occurrence of N. colombiensis, and 3b, defined by the first occurrence of N. bothrops. Faunal Interval 4 is defined by the first occurrence of N. asymmetricus and Faunal Interval 5 is defined by the first appearance of I. robustus. Faunal Interval 6 is defined by the first appearance of N. intrala. These faunal intervals improve on the work by Saelens (2014) and Treat (2014) in New Mexico and can be correlated to the Midcontinent conodont zonation of Barrick et al. (2013). Fauna Intervals 1, 2 and 3a are middle to late Atokan in age. The poorly defined Atokan-Desmoinesian boundary appears to lie between 3a and 3b, near the first occurrence of N. bothrops. Faunal Intervals 3b, 4, 5, and 6 extend through most, if not all of the lower Desmoinesian (Cherokee Group equivalent). The oldest conodont faunas (FI 1 and 2) were obtained from the base of the Sandia Formation at the type Sandia section, east of Albuquerque, and the middle of the formation in the Presilla A section in the Cerros de Amado, east of Socorro. The age of the upper beds of the Sandia and the transition to the carbonate-dominated overlying Gray Mesa Formation varies from section to section. Commonly, the uppermost beds of the Sandia and basal beds of the Gray Mesa contain a FI 3 fauna, which is like the Red House/Gray Mesa transition to the south (Saelens, 2014). However, in the Los Pinos Mountains (Sepultura Canyon section) FI 3 appears in the middle of the Sandia Formation.



Geology, Conodonts