Use of Rhythmic Bedding Patterns for Locating Structural Features, Niobrara Formation, United States Western Interior

Abstract

Milankovitch-type bedding cycles are well developed in the Upper Cretaceous Fort Hays Limestone Member, Niobrara Formation. These time-equivalent cycles can be correlated across much of Colorado, Kansas, and northeastern New Mexico by combining subsurface and outcrop data. Documentation of thickness variations within the regionally persistent Fort Hays bedding sequence furnishes a basis for fine-scale analysis of Late Cretaceous crustal movements within the eastern ramp region of the Western Interior foreland basin. Regional thickness changes in groups of shale-limestone couplets were correlated and mapped in outcrop and in the subsurface to locate structural elements that influenced Fort Hays deposition. In the Denver-Julesburg basin of Colorado and western Kansas, up to 6.1 m (20 ft) of thinning of the section occurs dominantly along northeastwardly trending belts formed during Late Cretaceous reactivation of the Transcontinental arch. Mapping of these small-scale thickness changes in the Fort Hays demonstrates that Cretaceous reactivation of the Transcontinental arch was not restricted to the northern part of the Denver-Julesburg basin. Additional structures may occur as far south as the Colorado-New Mexico border. A northwestwardly thinning trend is also apparent and may have resulted from increased compaction and diagenesis, reduction of sediment input during transgression, or from uplift in the vicinity of the Colorado Front Range.