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The Upper Pennsylvanian (Missourian) Lansing-Kansas City Groups in the subsurface of western Kansas consist of a dozen cyclothems of carbonate and terrigenous clastics deposited on a platform that was gently tilted southward toward the more rapidly subsiding Anadarko basin. Maps of the study area covering western Kansas, derived from several thousand well logs and over 30 cores, describe four cyclothems in the Kansas City Group. Regressive carbonates developed in each of these cyclothems are major petroleum reservoirs in this region and are the focus of this examination.
The regressive carbonates thicken southward (basinward) at rates controlled by the tilt of the platform. Local and subregional variations in thickness and facies distribution are affected by local differential subsidence on the shelf, particularly along broad positive areas that closely correspond with previously active uplifts. Relatively thin carbonates having restricted shallow-marine facies are abundant over these positive areas. Subtle flexures along the shelf, especially where the slope increased basinward, were loci for ooid shoal development caused by wave and current action during shallow-water deposition.
Facies patterns interpreted from core and log-derived mapping demonstrate that neither the flexures nor the broader positive areas of the shelf were consistently active throughout the deposition of all four cycles studied. Hence, despite remarkable similarities, there are distinct variations between the cycles. Furthermore, not all cycles cover the study area to the same extent because (1) in some cycles, regressive carbonates pinch out along the northern (landward) shelf; (2) shallow restricted marine facies can be displaced southward; (3) the marine black shale, the deepest water phase of the cycle, can be missing or only poorly developed; and (4) intense local variations can occur in the early meteoric freshwater diagenesis that affects all cycles over much of the shelf.
Terrigenous clastics in the cycles are composed of thin layers of silty shale and claystone that prograded southward over much of the northern half of Kansas during the regressive phase of each cycle. Clastics from the Ouachitas, important components in cycles in eastern Oklahoma and southeast Kansas, never reached western Kansas until the Virgilian because these sediments were trapped by a relatively deep basin in western and central Oklahoma.
Extensive and prolonged subaerial weathering and associated freshwater diagenesis, marked and sharp vertical changes in the lithofacies, and broad repetitive facies patterns in relatively thin carbonates and shales can be explained by glacial eustatic changes in sea level. The Gondwana glacially influenced eustatic sea level changes had a periodicity and magnitude comparable to those of the Pleistocene. Variations in thickness, carbonate facies patterns, and diagenesis were also strongly influenced by second-order intrabasinal processes, including differential subsidence over positive areas and along breaks in slope.
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