UPWELLING IN THE MONTOYA GROUP: EVIDENCE FOR PROLONGED LATE ORDOVICIAN GLACIATION

The Upper Ordovician Montoya Group in New Mexico and western Texas is a 2nd-order (10-12 Ma duration) supersequence deposited on a gently sloping ramp. During the Late Ordovician this ramp was situated 10-20° south of the equator and faced the Panthalassic ocean to the west. The Montoya Group contains four stratigraphic units (in ascending order): the Cable Canyon Sandstone, the Upham Dolomite, the Aleman Formation and the Cutter Formation. Phosphate is locally up to 5% by volume in the Upham and Aleman units most commonly occurring as a replacement for shells, as pellets or along hardgrounds. Chert occurs throughout the Montoya Group, though it is particularly abundant, up to 70% by volume, in the Aleman Formation. The chert in the Aleman Formation is comprised primarily of sponge spicules and occurs in two modes: 1) thin, even beds of chert interbedded with calci- or dolosiltite with rare hummocky cross-stratification; and 2) nodular chert in skeletal wacketone/packstone. The abundance of spiculitic chert in the Montoya Group is unusual because it occurs in shallow water (<100’s m water depth) hundreds of kilometers from deep basinal settings. We interpret the co-occurrence of the phosphate and spiculitic chert to indicate that this area was an extensive upwelling zone during the Late Ordovician (Cincinnatian). This interpretation is further supported by the Montoya Group paleogeography along an open marine, west facing continental margin south of the equator. We suggest the upwelling was driven by the initiation of continental glaciers on Gondwana during the Late Ordovician and that glaciation lasted up to 15 Ma.