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
1997. The American Association of Petroleum Geologists. All rights reserved.
received January 17, 1996; revised manuscript received October 18, 1996;
final acceptance March 20, 1997.
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
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.