The Gray Mesa Formation (Middle Pennsylvanian) in New Mexico

The Gray Mesa Formation is a distinctive interval of ledge- and cliff-forming, cherty limestone in the middle of the Pennsylvanian succession of New Mexico. This unit contrasts with generally slope-forming shale, sandstone, and limestone above and below. Outcrops extend southward from the Nacimiento and southern Sangre de Cristo Mountains, near the latitude of Santa Fe, to the Robledo and northern Organ Mountains near Las Cruces. The Gray Mesa Formation is largely of Desmoinesian age, but both its upper and lower contacts cross time lines and vary from locally unconformable to gradational and arbitrary. Limestone textures are diverse, but lime mudstone and fossiliferous wackestone and packstone are most prevalent. Interbeds of shale, siltstone, and sandstone were sourced from nearby Ancestral Rocky Mountain uplifts. Depositional cycles on the scale of 2 to 10 m record repeated eustatic fluctuations of water depth from below storm-wave base to peritidal, frequently capped by subaerial exposure. Except on the eastern margin of the Uncompahgre uplift and along the southern Pedemal uplift, Ancestral Rocky Mountain uplifts were relatively inactive during Gray Mesa deposition. Reduced tectonic activity meant reduced influx of terrestrial clastics, favoring limestone deposition. Paleoclimate was seasonal wet-dry, fluctuating from subhumid to semi-arid. New Mexico during the late Middle Pennsylvanian was markedly less arid than Colorado and Utah, where thick successions of evaporite rocks accumulated. Wind-blown dust from deserts to the north may be the source of the silica in the prolific chert characteristic of many limestone beds in the Gray Mesa Formation.