The Pennsylvanian System in the Sacramento Mountains, New Mexico, USA

Abstract

Pennsylvanian sedimentary rocks in the Sacramento Mountains, New Mexico, comprise an ~1 km thick stratigraphic section. The Morrowan-Desmoinesian Gobbler Formation was deposited by shallow marine processes in and near the Alamo clastic trough. In this trough, the Desmoinesian-Missourian Gray Mesa Formation (Bug Scuffle Member, Gobbler Formation) is a relatively thin unit (Space History Member) representing the glacioeustatic Amado event. The Missourian-Virgilian Beeman Formation includes the lower, siliciclastic Indian Wells Canyon Member and overlying, carbonate-rich Horse Ridge Member. The Virgilian Holder Formation consists of algal bioherms (Little Dry Canyon Member) overlain by the mixed carbonate-siliciclastic Mill Ridge Member. The Virgilian-Wolfcampian Bursum Formation is mixed siliciclastic-carbonate strata that represent shallow marine and nonmarine paleoenvironments. Animal and plant remains occur throughout the section. Unit age determinations are primarily based on conodont faunas recovered from the Gobbler, Gray Mesa, and Beeman Formations. Many conodont faunas correlate with Midcontinent cyclothems. Extensive algal and foraminiferal fossils also were identified in limestones from the section and contributed to age determinations. The Beeman Formation in particular contains an extensive Missourian macroflora. The macroflora is of “mixed” composition, containing typical wetland elements intimately intermixed with taxa indicative of seasonally dry habitats. A seasonally wet-dry background climate is indicated. It is unlikely that droughttolerant plants were transported exclusively from “uplands.” Some plant remains have arthropod-feeding evidence. Previous analyses identified late Paleozoic ice-age glacioeustasy as the primary depositional driver of Pennsylvanian sedimentation in the Sacramento Mountains. We question this because of problems with those analyses and because of ample evidence of local tectonics and microclimate changes as important drivers of sedimentation in this area. Three Pennsylvanian Ancestral Rocky Mountain orogeny tectonic pulses can be identified in the Sacramento Mountains: Morrowan-Atokan, Missourian, and late Virgilian-Wolfcampian.