--> ABSTRACT: Stratigraphic Architecture of the Dakota Aquifer (Cretaceous) in Kansas, by Howard R. Feldman, John Hopkins, David R. Collins, John H. Doveton, P. Allen MacFarlane; #91020 (1995).

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Stratigraphic Architecture of the Dakota Aquifer (Cretaceous) in Kansas

Howard R. Feldman, John Hopkins, David R. Collins, John H. Doveton, P. Allen MacFarlane

Exploitation of the Dakota Aquifer, which underlies the western two-thirds of Kansas, is expected to increase dramatically as water supplies from the High Plains and alluvial valley aquifers diminish in the future. Stratigraphic models of the Dakota Aquifer are important for the development of hydrogeologic models. Well log correlation has been a primary tool for development of stratigraphic models of the aquifer, however delineation of sequence boundaries has been difficult for two reasons: 1) in some areas the lithologies above and below the sequence boundaries are very similar and show no change in log response, and 2) the mid-Dakota Formation sequence boundary is imbedded within a dominantly non-marine mosaic of facies that lacks any regional markers. These problems h ve been largely overcome by displaying gamma-ray well-log cross sections on a workstation designed for seismic applications. Cross sections range up to 200 miles long and display 150 logs simultaneously. These seismic-style displays reveal previously obscure stratigraphic relationships. The well-log cross sections are supplemented by cores and outcrops.

The basal Cretaceous unconformity is now readily recognizable because of the slight angular discontinuity between Cretaceous and pre-Cretaceous units. Shale on shale contacts are discernable based on truncation of subtle Permian markers. The basal Cretaceous contact is mostly flat with a few paleovalleys. The largest are up to 200 ft deep and 10 miles wide. The relief is partly erosional, but may have partly resulted from dissolution and collapse of underlying Permian evaporites. The basal Cretaceous sequence in Kansas is the Cheyenne/Kiowa Sequence that typically grades upward from fluvial through deltaic sandy facies at the base to storm-influenced, upper offshore shales. The base of the overlying basal Dakota or J sequence is marked by discontinuous sandstone and mudstone overlying marine shale with laterally continuous thin sandstone beds. The basal contact is of low relief (generally under 50 ft), and paleovalley drainage systems are not well defined. The contact is difficult to identify in areas where the Dakota contains little sandstone. In central Kansas the basal Dakota unconformity is overlain by 200 to 300 ft of continental facies including fluvial sandstones, thick paleosols, overbank deposits, and lignites. The uppermost Dakota Formation in central Kansas is marine influenced nearshore or deltaic sandstones that are abruptly overlain by the marine Graneros Shale. In western Kansas the basal Dakota Formation consists of thin fluvial sandstone (generally under 50 ft thick) that is overlain by successively offshore, parasequences. The Huntsman Shale is a mid Dakota marine transgressive shale recognized in eastern Colorado and western Kansas. In this area the base of the upper Dakota or D Sequence is marked by a minor basinward shift in facies. In central Kansas the sequence boundary is embedded in a mosaic of continental facies bounded by minor erosional surfaces. No single surface can be reliably traced more than a few miles. The seismic-style display of gamma-ray cross sections, however, reveal a tendency for sandstone bodies to occur within a zone in the upper third of the Dakota Formation that may be the updip expression of the basal D Sequence boundary. The transgressive surface of erosion above the Dakota Formation is overlain by the Graneros Shale with parasequences that onlap landward.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995