Abstract: Eustatic Versus Tectonic Controls on Depositional Sequences in the Pennsylvanian Minturn Formation of North-Central Colorado

Karen Houck

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 50 mi² study area near McCoy, up to 9 depositional sequences composed of marine and nonmarine deposits are recognized. Three tectonic blocks (two grabens, one horst) were actively subsiding at different rates in the study area. Detailed correlation along well-exposed outcrop belts has shown that 6 of these sequences extend continuously across the field area, including two delta complexes. Three sequences are absent on the central horst block. The study area is ideal for examining the role of local te tonic versus eustatic processes in controlling the distribution of sedimentary deposits.

Depositional sequences are bounded by erosional surfaces with as much as 30 meters of relief. Sequence boundaries are overlain by braided fluvial conglomerates inferred to be transgressive valley-fill deposits. Marine flooding surfaces overlie the conglomerates. 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. Biostratigraphic zonation indicates that the Minturn sequences have durations of 100s of thousands of years (fourth-order sequences) and are correlative with the major marine trans ressions identified by Ross and Ross as well as Heckel in the Midcontinent region. Thus, though local tectonics has exerted the primary control on thicknesses of sequences and lateral distribution of deposits within each sequence, eustatic sea level fluctuations have exerted the primary control on vertical distribution of deposits within each sequence.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994