EFFECTS OF SEDIMENTATION, TECTONICS, AND GLACIO-EUSTASY ON DEPOSITIONAL SEQUENCES, PENNSYLVANIAN MINTURN FORMATION, NORTH-CENTRAL COLORADO

The Pennsylvanian Minturn Formation was deposited adjacent to the Ance stral Front Range on the eastern margin of the actively subsiding Cent ral Colorado Basin during a time of high-frequency eustatic sea level changes attributed to glaciations in the Southern Hemisphere, In the 1 00 km(2) study area near McCoy, Colorado, up to nine depositional cycl es composed of marine and nonmarine deposits are recognized, Three tec tonic blocks were subsiding at different rates in the study area. Deta iled correlation of measured sections along well-exposed outcrop belts has shown that seven of these cycles extend continuously across the f ield area, which includes two delta complexes, Two cycles are absent o n the central block, The study area is well suited for examining the r ole of local tectonic vs. eustatic processes in controlling the distri bution of sedimentary deposits. In the study area, cycles are bounded by erosional surfaces with as much as 43 m of relief, These surfaces a re inferred to be sequence boundaries and are overlain by braided fluv ial 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, infer red to represent transgressive systems tracts. These, in turn, are ove rlain by fossiliferous marine limestones inferred to represent condens ed sections, Condensed sections are overlain by progradational Gilbert -delta deposits inferred to be highstand deposits, Locations of incise d valleys, coarse fluvial deposits, Gilbert deltas, and small phylloid algal buildups were fault controlled, Biostratigraphic zonation indic ates that the Minturn sequences have durations of hundreds of thousand s of years (fourth-order sequences) and are correlative with major mar ine transgressions identified in the midcontinent region, Local tecton ic movements are interpreted to have exerted the primary control on th e thicknesses of sequences and the lateral distribution of deposits wi thin them, whereas eustatic fluctuations are interpreted to have exert ed the primary control on the vertical distribution of deposits within each sequence.