Abstract

Devonian and Mississippian rocks cover most of the central part of the Colorado Plateau and range in thickness from a few feet in the eastern part to a combined total thickness of approximately 2000 feet along the western part. Mississippian rocks are composed predominantly of pure carbonates whereas Devonian rocks are composed of carbonates, sandstones and varicolored shales.

Over most of the area the Devonian rests on rocks of Cambrian age and the contact, at least as observed in the San Juan Mountains outcrops, is one of paraconformity. The three major units of the Devonian in ascending order are the Aneth, Elbert and Ouray Formations. There is an intergrading facies relationship between the Aneth Formation and the basal carbonates of the Elbert Formation. The Elbert Formation is divided into the McCracken Sandstone Member and the Upper Elbert Member. There are numerous individual sandstones within the Elbert Formation, so that identification of the McCracken Sandstone Member, as differentiated from other Elbert sandstone units, is a major problem in mapping the Devonian in this area. The Upper Elbert Member represents continued transgressions of the Late Devonian sea. The Ouray Formation is composed of carbonate rocks that are both Devonian and Mississippian in age with the rocks of the two periods being debatably either (1) intergraded or (2) having a significant hiatus between them.

The Mississippian Redwall Formation is considered to be correlated into the subsurface of the Four Corners area well enough to justify the use of the term Redwall Formation throughout much of the area of this report. The Leadville Formation of the San Juan Mountains area is considered equivalent to the Redwall Formation. Member names for the Redwall are given by McKee in ascending order as the Whitmore Wash. Thunder Springs, Mooney Falls, and Horseshoe Mesa Members, all named from Grand Canyon localities. The Thunder Springs Member contains a distinctive chert upon which widespread subsurface correlations are postulated. It is suggested that subdivisions of the Mississippian rocks of this area based on changes in types of carbonate lithology, be used only in local areas and with great caution.