Cherty facies of the Late Ordovician Montoya Group, southern New Mexico and western Texas: implications for Laurentia oceanography and duration of Gondwana glaciation
— Michael C. Pope

The Late Ordovician Montoya Group in southern New Mexico and western Texas records predominantly subtidal carbonate deposition on a gently dipping ramp. Rocks of the Montoya Group are almost entirely dolomitized. The medial unit, the Aleman Formation is unique because it contains abundant chart (locally up to 70% by volume). The chart occurs as thin continuous beds of sponge spicules within calcisiltite or nodules within skeletal wackestone and packstone. Skeletal grainstone and muddy peritidal facies contain Iittle chart. The abundance of chert and phosphate in the subtidal facies indicates the Montoya Group formed within a region of strong upwelling, possibly initiated by onset of glaciation during the Late Middle Ordovician.