Abstract

Lower and middle Paleozoic units recognized in the San Juan Mountains include the Upper Cambrian Ignacio Formation, the Upper Devonian Elbert Formation and Ouray Limestone, and the Lower Mississippian Leadville Limestone. This study suggests that the Ignacio should be divided into two members; a lower quartzite conglomerate named the Weasel Skin Member that was deposited in braided stream channels and the overlying shallow marine sandstones called the Stag Mesa Member. The Weasel Skin Member is much older than the Stag Mesa, perhaps Neoproterozoic. The Elbert Formation is divided into three units; the lower McCracken and units B and C. The Ouray is divided into the lower fossiliferous Broken Rib Member and the upper stromatolitic Coffee Pot Member. These subdivisions of the Elbert and Ouray are the same as those of the Parting and Dyer Formations of the Chaffee Group in the central part of the state and deposition was in similar environments in transgressing then regressing shallow seas. The Leadville and Ouray are separated by the presence of a thin sandy zone which is probably the equivalent of the Gilman Sandstone.

Rhyolitic gneiss of known ~1800 Ma age in western and northern Needle Mountains has a lower intercept age of U-Pb zircon discordia of ~550-600 Ma hinting at a Neoproterozoic to Early Cambrian thermal event in southwestern Colorado. This implied thermal event may correlate with regional tectonic uplift and consequent unroofing at ~550 Ma that led to the deposition of the Weasel Skin Member. The Stag Mesa Member of the Ignacio and the McCracken Member of the Elbert depositionally thin to zero at Coal Bank Hill indicating a highland to the northeast. The Elbert thins but crosses this highland and is in depositional contact with the underlying Precambrian. The Broken Rib Member of the Ouray extends across this highland without change. Both the Coffee Pot Member of the Ouray and the Leadville Limestone thin across the highland because of Late Mississippian/Early Pennsylvanian erosion. This highland was positive and controlling sedimentation throughout Lower Paleozoic time.