ABSTRACT: High-Resolution Stratigraphic Analysis of the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems, Western Denver Basin, Central Colorado

Erle G. Kauffman

Cretaceous strata outcrop along the western Denver basin, allowing detailed north-south facies and sequence analyses. Three 2nd-order marine cyclothems are interpreted from 20-30 sections, utilizing high-resolution event chronostratigraphy integrated with molluscan biozonation. (1) Kiowa-Skull Creek cyclothem (middle late Albian): consisting of the (ascending) Plainview Sandstone (shoreface), the Skull Creek Shale (proximal-medial offshore; peak transgression) upper Skull Creek-Muddy Sandstone transition beds (lower shoreface), and the Fort Collins Member (middle shoreface), Muddy Sandstone, marking peak regression. Third-order sequences and parasequences occur with the cyclothem; the upper sequence boundary (SB) is a channeled lowstand disconformity. (2) Greenhorn cyclot em (latest Albian-middle Turonian): a subsymmetrical cyclothem comprising, on transgression, the Muddy Sandstone (aggrading fluvial channel-middle shoreface) Mowry and Graneros shales (proximal-medial offshore) and medial-distal offshore carbonate facies of the Greenhorn Formation (Lincoln, Hartland, Bridge Creek members; maximum transgression). Several third-fourth order sequences occur within the cyclothem. Regressive facies comprise (ascending): Fairport Shale, Blue Hill Shale, and Codell Sandstone members, Carlile Shale (distal offshore-shoreface). A major disconformity cuts the upper Codell reflecting lowstand and subsequent transgression. Southward this disconformity expands into multiple anastomosing disconformities embracing condensed transgressive sequences of (3) the Niobrara c clothem (late Turonian-early Campanian); (ascending) Juana Lopez, upper shale and basal Fort Hays members (late Turonian-earliest Coniacian). The Fort Hays Limestone and three higher chalk units (Smoky Hill Member) represent third-order transgressive peaks. Final Niobrara regression is marked by the "transition zone" (north) and Apache Creek Sandstone Member (south), Pierre Shale, in the Denver basin. Basement blocks moved in and around the Denver basin contemporaneous with sedimentation in all cycles.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990