Effects of Sedimentation, Tectonics, and Glacio-Eustasy on Depositional Sequences, Pennsylvanian Minturn Formation, North-Central Colorado

Abstract

The Pennsylvanian Minturn Formation was deposited adjacent to the Ancestral Front Range on the eastern margin of the actively subsiding Central Colorado Basin during a time of high-frequency eustatic sea level changes attributed to glaciations in the Southern Hemisphere. In the 100 km2 study area near McCoy, Colorado, up to nine depositional cycles composed of marine and nonmarine deposits are recognized. Three tectonic blocks were subsiding at different rates in the study area. Detailed correlation of measured sections along well-exposed outcrop belts has shown that seven of these cycles extend continuously across the field area, which includes two delta complexes. Two cycles are absent on the central block. The study area is well suited for examining the role of local tectonic vs. eustatic processes in controlling the distribution of sedimentary deposits. In the study area, cycles are bounded by erosional surfaces with as much as 43 m of relief. These surfaces are inferred to be sequence boundaries and are overlain by braided fluvial conglomerates inferred to be incised valley-fill deposits. Marine flooding surfaces overlie the conglomerates. The ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received January 17, 1996; revised manuscript received October 18, 1996; final acceptance March 20, 1997. 2Department of Geological Sciences, University of Colorado at Boulder, Campus Box 250, Boulder, Colorado 80309-0250. Present address: Department of Geology, University of Colorado at Denver, Campus Box 172, P.O. Box 173364, Denver, Colorado 80217-3364. I thank the members of my doctoral dissertation committee at the University of Colorado (Ted Walker, Martin Lockley, Alan Scott, David Budd, and John Pitlick) for their advice and guidance during the course of my research. Financial support was provided by the Rocky Mountain Section of SEPM, the Colorado Mountain Club, Union Pacific Resources, and the Department of Geological Sciences at Colorado University-Boulder. Charles Ross provided fusulinid identifications and biostratigraphic zonation. I thank Paul Weimer (Colorado University-Boulder) for advice and encouragement, and Keith Miller (Kansas State University) for carefully reviewing a preliminary draft of this paper. AAPG reviewers William Devlin, Romeo Flores, Denise Stone, and Elected Editor Kevin Biddle provided many suggestions that greatly improved the final manuscript. I also thank the residents of McCoy, Bond, and Copper Spur, Colorado, for granting access to their property, and for all their kindness while I was working in the field.