CONODONT BIOSTRATIGRAPHY OF THE PORVENIR FORMATION (DESMOINESIAN, PENNSYLVANIAN), SOUTHEASTERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO, USA

The Porvenir Formation consists of marine limestones, gray shales, and sandstones that conformably overlie the Sandia Formation and are overlain with local angular unconformity by the Alamitos Formation in the southeastern part of the Sangre de Cristo Mountains. We collected conodonts from two sections representing two of the three described facies, the type locality representing the southern, dominantly carbonate facies, and a reference section representing the northern sandstone-shale-limestone facies. Our goals include describing the conodonts, clarifying local age relationships, and utilizing biofacies and magnetic susceptibility to interpret depositional environments.

Conodont faunas from these sections are dominated by Idiognathodus. Hindeodus and Neognathodus are locally common; Adetognathus is uncommon; and Diplognathodus, Idioprioniodus, and Ubinates are rare. Conodont biofacies indicate that the paleoenvironment varied from open marine conditions to nearshore, shallow-water euryhaline environments of varying energy levels.

At the Porvenir type locality the Neognathodus Index (NI), a primary biostratigraphic indicator for the Desmoinesian Series, shows a general increase from the bottom where it is 2.65 to the upper part where it is 4.29. The similarity in NI between the upper part of the Sandia, 2.30, and the lower 50 feet of the Porvenir, 2.33, supports their conformable relationship. At the northern reference locality the Porvenir NI is 4.57. Thus, the Porvenir there is younger than the top of the Porvenir at the type locality necessitating a revised correlation between these sections.

Magnetic susceptibility was measured on samples collected at one foot intervals in the carbonate lithologies of the lower 36 feet of the Porvenir type section. The SI mass normalized magnetic susceptibility of the majority of the samples is ±0.002 m3/kg. Variations in magnetic susceptibility results in several horizons between 0 and 7 feet where susceptibility increases up to 0.007 m3/kg and a single larger increase to 0.026 m3/kg at 34 feet. The increases in susceptibility are likely due to changes in the primary minerals for which there are a variety of potential causes such as climate change.