--> ABSTRACT: Solution-Subsidence Origin, Architecture, and Hydrocarbon-Trapping Mechanisms of Basal-Pennsylvanian Morrow Fluvial Valleys, Southwest Kansas, by Charles E. Bartberger, Thaddeus S. Dyman, and S.A. Cook; #90906(2001)

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Charles E. Bartberger1, Thaddeus S. Dyman2, S.A. Cook1

(1) Consultant, Denver, CO
(2) U.S. Geological Survey, Denver, CO

ABSTRACT: Solution-Subsidence Origin, Architecture, and Hydrocarbon-Trapping Mechanisms of Basal-Pennsylvanian Morrow Fluvial Valleys, Southwest Kansas

Basal-Pennsylvanian fluvial valleys of the Morrow Formation in southwest Kansas formed through a combination of subsidence and incision. Regional isopach maps integrating 3-D and 2-D seismic data with subsurface data clearly define valley segments formed primarily by subsidence with minor amounts of incision. Subsidence occurred in earliest Pennsylvanian time and was caused by dissolution of anhydrite in the Mississippian St. Louis Formation, about 300-feet below the basal-Pennsylvanian depositional surface. Beneath downwarped valley segments, seismic data show thinning of a stratigraphic interval that includes a 150-gross-foot anhydrite zone in the lower St. Louis Formation. Core data show presence of collapse breccia in this interval within the valley. Basal-Pennsylvanian fluvial sandstones generally rest unconformably on Mississippian Chester strata, and wireline-log data show that the Chester section commonly is thicker in downwarped portions of valleys than outside valleys, reflecting preservation from erosion through subsidence. Valley segments connecting downwarped stretches formed by fluvial incision, reflected by lack of seismic thinning in the underlying anhydrite-bearing interval, and often by complete erosion of Chester strata. No controlling mechanism or geographic pattern of anhydrite dissolution has been discerned, but internal architecture of the valleys reveals a complex history of valley filling and valley switching, governed in part by changing patterns of solution-controlled subsidence. Hydrocarbon traps often have stratigraphic components associated with type and geometry of valley-fill deposits. Sequence-stratigraphic analysis integrating data from cores and wireline logs in one downwarped valley segment shows five episodes of fluvial incision and deposition followed by relative sea-level rise and backfilling of the valley with estuarine sediments. The deepest, youngest channel, filled with estuarine sandstone and shale, curves through the valley and provides a significant component of the trap for gas in older fluvial sandstones structurally downdip.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado