Molas and Associated Formations in San Juan Basin-Needle Mountains Area, Southwestern Colorado

Abstract

A review of some of the literature dealing with the Molas formation and its equivalents in the Four Corners area reveals differences of opinion about its age and historical significance. The writers studied and sampled exposures along the northern rim of the San Juan basin and in the Needle Mountains, southwestern Colorado, and made petrographic, insoluble-residue, heavy-mineral, and clay-mineral (x-ray diffractometer) analyses of the samples. Units examined include the Upper Devonian Ouray limestone, Lower Mississippian Leadville limestone, and Mississippian-Pennsylvanian Molas formation. Insoluble residues from the Ouray and Leadville are distinctive; boulders from each were identified in the basal Coalbank Hill member (named herein) of the Molas formation. The boulders are packed in a red clay and silt matrix which overlies the limestones and has sifted down into joints, pits, and caverns in them. Overlying the Coalbank Hill is the Middle member of the Molas, a poorly stratified unit of stream-deposited red clastics. The upper Molas member is made up of well stratified clastics, largely red, and contains marine fossils in its upper part. The upper Molas grades into the overlying green and gray clastics and interbedded li estones of the Pinkerton Trail or Hermosa formation. Illite is the chief clay mineral in residues from the Ouray, the Leadville, limestone boulders in the Coalbank Hill member, and in the Upper member of the Molas. Kaolinite is the chief clay mineral in the matrix of the Coalbank Hill and Middle member of the Molas. The interpretation presented is that a residual deposit accumulated in southwestern Colorado as solution of Mississippian and Devonian limestones took place during post-Osage, pre-Pennsylvanian erosion. The upper part of the old regolith was eroded and deposited elsewhere by streams; some of it was finally deposited in and reworked by the advancing early Pennsylvanian sea. In some areas, deposition of the Molas red clastics continued into early Des Moines time.