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Kansas Geological Survey Open-file Report 2003-82 |
4.3 Discussion
Sequences of the Cherokee Group record abrupt changes from marine to non-marine conditions, followed by periods of rapid relative seal-level rise. Each successive sequence begins with a relative drop in sea level resulting in incision into the lower sequence. Following each incision, relative sea level rises, filling paleovalleys with estuarine deposits as part of the transgressive systems tract. During the early transgressive systems tract interfluves remain subaerially exposed. Since fluvial deposits filling incised valleys were not observed, a lowstand systems tract was not identified in any of the sequences. Within the estuarine deposits of the upper portions of paleovalleys, rooting and development of thin and laterally discontinuous coal were observed.
The preservation and continued growth of peat is dependent on a rising water table and high accommodation achieved by a relative base level rise (McCabe and Shanely, 1992). The movement of destructive fluvial processes landward enhances preservation of peat during the late transgressive systems tract, while peat accumulation attempts to keep up with an accelerating relative rise in sea level (Aitken, 1994). During the late transgressive periods of the Cherokee Group sequences, sea level rise resulted in flooding of interfluves, increased accommodation, and a rise in water tables. Widespread coastal successions capped by progressively thicker and more regionally extensive coals were developed (i.e. Riverton, Weir-Pittsburg, and Mineral coals). In addition to being thicker and more laterally continuous, late transgressive systems tract coals are of higher quality (i.e., lower ash, and higher gas content). The end of each Cherokee Group transgressive systems tract is characterized by regionally extensive phosphatic black shales interpreted as the maximum flooding surface. This shale also commonly overlies the thickest and most extensive coal of a sequence.
The close association with marine shales and coals resulted in relatively higher sulfur content of Cherokee Group coal. Coal gas content may also be augmented by gas generated from the shales and migrating into the immediately underlying high porosity coals. Following each major transgression, the highstand systems tract is reflected by relatively rapid progradation of offshore transitional environments over marine environments followed by progradation of marginal marine and then non-marine environments.
The most extensive, thickest, and highest quality coals (i.e. low ash) within each Cherokee Group sequence are interpreted to occur at the end of the deposition of the transgressive systems tract near the maximum flooding surface (i.e. Riverton, Weir-Pittsburg, and Mineral coals). Generally, coals tend to thicken and become more laterally extensive up through the transgressive systems tract and thin upward through the highstand systems tract (Aitken, 1994). However, Cherokee Group coals are not restricted to the upper part of the transgressive systems tract and can occur in any systems tract. Coals that form during the LST, HST or lower TST are typically thinner, laterally discontinuous and have higher ash contents.
4.4 Mechanisms for Sequence Development
The highest resolution of changes in relative sea level is on the order of about 240 Ka, although changes in relative sea level may have varied substantially during these periods (Walton, 1995). Mechanisms such as tectonics, and climatic changes are not resolvable on this time scale. The high frequency, and widespread nature of Middle Pennsylvanian sequences over eastern Kansas and into adjacent states suggests an allogenic mechanism such as glacio-eustacy. Previous studies established that the upper Paleozoic was a time of large-scale continental glaciation resulting in glacio-eustasy (Heckel, 1977).
The Cherokee Group sequences are interpreted as the product of high-frequency progradational pulses of sedimentation during a major relative sea level rise throughout the Desmoinesian. Perhaps the best explanation for controls on the observed sequence development in the Cherokee Group of southeastern Kansas is glacio-eustasy. Tectonic changes in elevation and subtle paleotopographic features may however, have locally influenced the nature and development of Cherokee Group sequences.
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