GEOLOGY OF THE McCOY AREA, EAGLE AND ROUTT COUNTIES, COLORADO

Abstract

The McCoy area lies in the transition zone between the ancestral Front Range Highland and the Central Colorado Basin. The Precambrian crystallines, here mostly granite, are overlain by sedimentary rocks of Cambrian age. Limestones overlying the Cambrian rocks and previously mapped as Ordovician are assigned a Mississippian age. The Pennsylvanian McCoy formation is re-defined to include over 3500 feet of coarse arkosic sandstones and grits with interbedded shales and limestones. A Walchia bed is recognized as a member of the formation. Red siltstones overlying the McCoy formation conformably are defined as the State Bridge siltstone and are either Pennsylvanian or Permian. Above the siltstone are thin representatives of some of the Mesozoic formations to the west: the Shinarump, Chinle, Entrada, Morrison, Dakota, Benton, and Niobrara formations. The sedimentary formations were deformed during the Laramide orogeny into open asymmetrical folds and then faulted. The mosaic pattern of the faults is interpreted to mean deformation due to vertical forces rather than horizontal. A monzonite stock was intruded in Laramide times, after the faulting. Following prolonged erosion, extensive basalt and andesite flows were poured out in miocene time. After another period of erosion volcanic activity broke forth again. River terraces and three erosion surfaces are recognized. The Black Mountain surface upon which the Miocene lavas were spread was warped before the Sheephorn surface developed. The Blacktail surface was formed some 500 feet below and into this the Colorado River has cut another 500 feet.