HOLBROOK, JOHN M., Department of Geosciences, Southeast Missouri State University, Cape Girardeau, MO

ABSTRACT: Influence of Early Cretaceous Intraplate Uplift on Distribution of Lower Cretaceous Valley-Fill Sandstone and Regional Sequence-Boundary Morphology in Southeastern Colorado

Regional exposure and incision of marine Skull Creek and Skull Creek equivalent strata during maximum Kiowa-Skull Creek regression prompted late Albian development of a widespread lowstand surface of erosion throughout southeastern Colorado. This lowstand surface of erosion is preserved as a regionally correlative sequence-bounding unconformity (sequence boundary SB3) between Albian (latest Early Cretaceous) valley-fill strata of the Mesa Rica/basal Muddy Sandstone and underlying Albian Skull Creek equivalents.

Sequence boundary SB3 manifests up to seventy feet of local relief in southeastern Colorado, reflecting preferential incision of Skull Creek equivalents coincident with major southeastward-directed late Albian river valleys. Interfluvial areas separating these paleovalleys coincide in position with well-documented late Paleozoic and older basement structures, namely Las Animas Arch and Turkey Creek and Rampart-Security uplifts, suggesting structural control of Albian drainage patterns through uplift on these structures and associated deflection of Albian rivers to tectonic low areas. Back-filling of this drainage system during latest Albian Greenhorn transgression resulted in preferential deposition of thick valley-fill reservoir sandstone in locations of Early Cretaceous relative sub idence and thin intervals of coastal plain strata over local Early Cretaceous tectonic highs.

Southeastward-directed paleovalleys in southeastern Colorado represent down-dip continuation of a well-documented, structurally controlled drainage network which is recorded to the north in the Denver Basin by thickness trends in highly productive valley-fill strata of the Muddy Sandstone. Valley-fill strata in southeastern Colorado are shallow and locally exposed, yielding limited production. These strata, however, are potentially valuable as surface and near-surface analogs for equivalent Muddy strata.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.