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 km 2 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 flooding surfaces are overlain by retrogradational barrier or Gilbert-delta deposits, inferred to represent transgressive systems tracts. These, in turn, are overlain by fossiliferous marine limestones inferred to represent condensed sections. Condensed sections are overlain by progradational Gilbert-delta deposits inferred to be highstand deposits. Locations of incised valleys, coarse fluvial deposits, Gilbert deltas, and small phylloid algal buildups were fault controlled. Biostratigraphic zonation indicates that the Minturn sequences have durations of hundreds of thousands of years (fourth-order sequences) and are correlative with major marine transgressions identified in the midcontinent region. Local tectonic movements are interpreted to have exerted the primary control on the thicknesses of sequences and the lateral distribution of deposits within them, whereas eustatic fluctuations are interpreted to have exerted the primary control on the vertical distribution of deposits within each sequence.

You do not currently have access to this article.