CORRELATION OF THE UPPER MINTURN AND LOWER MAROON FORMATIONS FROM MINTURN TO SOUTH PARK, COLORADO: IMPLICATIONS FOR PENNSYLVANIAN FAULT MOVEMENT AND EVAPORITE DEPOSITION ON THE WEST SIDE OF THE ANCESTRAL FRONT RANGE

In the Central Colorado Basin, Pennsylvanian rocks in the Minturn and South Park areas have received intensive study, but stratigraphic relationships between these areas have not been well understood. As part of Colorado’s geological mapping program, a correlation project was undertaken in which the upper part of the Minturn Fm (from the Robinson Member upward; about 2000 feet) and the lower part of the Maroon Fm (about 2500 feet) were measured and described in a series of closely spaced sections extending from Pando (NNW) to Hartsel (SSE). Fusulinids date the Robinson Member as middle Desmoinesian; megafossils date the White Quail Member as Desmoinesian; the ages of the Jacque Mountain Member and the lower Maroon Fm are unknown.

The project provided opportunities to document facies changes, measure paleocurrent indicators, and test hypotheses about the contemporaneous movement of faults in the area. Sections contain alternating marine and nonmarine rocks. Marine rocks are mostly shallow marine and marginal marine deposits of shale and limestone. Nonmarine rocks are mostly braidplain deposits of conglomerate, sandstone, and siltstone. Between Hoosier Pass and Fairplay, fluvial flow was toward the south. Between Fairplay and Hartsel, fluvial flow between the Robinson and White Quail members was toward the south, but above the White Quail Member, flow was toward the northwest. It is inferred that the Gore, Boreas Pass, and Williams Range faults underwent up-to-east movement, and unnamed east-west faults north of Breckenridge underwent up-to-north movement, during deposition of the upper Minturn Formation. This movement decreased gradually during deposition of the Maroon Formation. The Agate Creek Fault underwent up-to-east movement following deposition of the White Quail Member. This movement also decreased gradually during deposition of the Maroon Formation. No evidence was found for movement on the Mosquito Fault.

Contrary to previous correlations, no evaporites were found in the study interval; the evaporites in South Park occur below the Robinson Member. This agrees with what is known about the regional paleogeography. When the Robinson Member was being deposited, an extensive marine inundation was beginning in the Midcontinent, and evaporite deposition was coming to an end in the Paradox Basin.