New Mexico Geological Society Annual Spring Meeting — Abstracts

The Pennsylvanian section exposed on the northern end of the Robledo Mountains (secs. 34-35, T21S, R1W) has not been assigned to a lithostratigraphic unit(s), but instead has been identified by the chronostratigraphic terms Atokan, Desmoinesian, Missourian, Virgilian and “Bursumian.” This section rests on a Cenozoic rhyolite sill, is ~ 420 m thick and is interbedded limestone and shale with a few thin beds of conglomerate. The lowermost Pennsylvanian beds have been termed Atokan without age evidence. The lower 82 m is mostly thick-bedded, cherty limestones that yield the Desmoinesian fusulinid Wedekindellina. A change to red shale with nodular limestone and thin conglomerate marks the Missourian base. Overlying Upper Pennsylvanian strata form upward shallowing cycles in which the cycle boundaries are marked by conglomerate beds or by thin, pedogenically-modified, rhizolithic horizons at the tops of limestone beds. Thick-bedded, cherty limestones at the base of the Hueco Group (Shalem Colony Formation) overlie the cyclic Upper Pennsylvanian section. Significantly, the Robledo Pennsylvanian section is very different (especially the postDesmoinesian part) from that exposed in the Caballo Mountains only 60 km to the north. Instead, except for being relatively thin, the Robledo Pennsylvanian section closely resembles the Horquilla Formation in southwestern New Mexico, especially that at New Well Peak (Big Hatchet Mountains). We thus assign the Robledo Pennsylvanian section to the Horquilla Formation and interpret it as shelfal strata in which glacio-eustatic cycles, not local tectonism, significantly drove sedimentation. This casts doubt on the idea that the Robledo Pennsylvanian section, deposited on the western (Robledo) shelf of the Orogrande basin, was significantly separated from the Pedregosa basin to the southwest by an intervening Florida uplift (highland). Instead, the original idea of the Florida “islands” (shelf) - an archipelago and shallow shelf separating the Orogrande and Pedregosa basins - is more compatible with the stratigraphic data. Subsidence in the Pedregosa basin and along the Florida shelf was relatively even, so that glacioeustatic cycles are better recorded in these sections than in the more tectonicallyinfluenced sections of the Orogrande basin to the east.