Middle Pennsylvanian Conodonts of the Southern Sangre de Cristo Mountains, New Mexico

Date

2014-12

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Abstract

The transition from the upper Sandia Formation into the lower Porvenir Formation in the southern Sangre de Cristo Mountains, New Mexico, preserves a significant record of the Ancestral Rocky Mountain orogeny as well as conodont evolution across the Atokan-Desmoinesian Stage boundary. The Sandia Formation is composed largely of shale with interbedded sandstones of varied thickness and granule to pebble conglomerate, with some thin limestone beds. The overlying Porvenir Formation comprises three laterally intergrading facies associations, the most significant of which is a lower dominantly carbonate facies association. The age of the upper Sandia and lower Porvenir formations has been previously interpreted to be late Atokan to early Desmoinesian using fusulinid faunas. Previous work has suggested that deposition of the limestones in the basal Porvenir is diachronous across the southern Sangre de Cristo Mountains. The two sections studied here, the Type Porvenir and the Mineral Hill sections, cross the transition from the upper Sandia Formation into the carbonate facies association of the lower Porvenir Formation. Eighteen species of conodont are described in detail. Three successive conodont faunal intervals are identified based on species of Idiognathodus. Faunal Interval 1, which occurs in the upper Sandia, is characterized by Idiognathodus sp. I and is interpreted to be late Atokan in age. Faunal Interval 2, which occurs in the uppermost Sandia and lower Porvenir, is characterized by I. sp. HI. This faunal interval includes the first occurrence of the fusulinid Beedeina, which is the working definition for the base of the Desmoinesian. Faunal Interval 3, which is characterized by I. sp. HA, was recovered from only the Type Porvenir section. All faunal intervals contain species of Neognathodus, Diplognathodus, Adetognathus and Hindeodus. The Idiognathodus succession from I. sp. I to I. sp. HI may provide a means by which the Atokan-Desmoinesian boundary may be identified in sections where fusulinids are absent.

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Middle Pennsylvanian conodonts, Southern Sangre de Cristo Mountains, New Mexico

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