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Summary and conclusions
A holistic approach to studying a stratigraphic unit requires using all data available. For this study I used surface outcrops, subsurface cores, geophysical well logs, drillers' logs, and thin sections of rock samples to decipher stratigraphic, petrographic, and sedimentologic characteristics of sandstones in the Rock Lake Shale Member of the Stanton Limestone.
Subsurface isopach and sandstone-isolith trends of the Rock Lake Shale Member in southern Kansas thin toward the northwest, strongly suggesting a southeasterly source of siliciclastics. The Rock Lake Shale Member contains five distinct sandstone units, informally labeled A-E, in the near subsurface of the detrital facies belt of eastern Chautauqua County, Kansas (fig. 14). Unit A cuts through the underlying Eudora Shale Member and Tyro oolite in T. 34 S. just north of the Oklahoma border and is the oldest sandstone in the Rock Lake Shale Member. Unit B developed later, south of unit A. Unit C is a thin sandstone that lies above unit A and slightly higher than unit B; it appears to be the youngest sandstone in the southern part of the detrital facies belt. Unit D is found only in the middle of the detrital facies belt and may have been deposited penecontemporaneously with units A and B. Unit E is the youngest sandstone in the northern part of the detrital facies belt and appears to be the subsurface extension of the Onion Creek sandstone. These sandstones are separated from each other mostly by mudrocks, and the whole interval is capped by the marine transgressive South Bend Limestone Member, which can be traced from the algal-mound facies belt southward across the detrital facies belt into Oklahoma.
Petrographic analyses indicate that the sandstones of the Rock Lake Shale Member were derived from older sedimentary rocks. Monocrystalline quartz is by far the predominant grain type, averaging 95-96% of the grain components. The relatively rare lithic fragments in the quartzarenites are mainly chert and polycrystalline metamorphic quartz. High degrees of textural and mineralogic maturity indicate a long, repeated transportation history, which in some cases included marine reworking. Low grain-to-grain contact densities indicate that these sandstones underwent only shallow burial and minimal compaction before early cementation.
In order of decreasing abundance, cement types consist of calcite (including ferroan calcite), iron oxides, sericite, and silica. An early stage of calcite cementation probably resulted from meteoric solution of carbonate fossil fragments and adjacent emergent carbonate buildups, when the deposits were emergent during later regression before the South Bend transgression. Pore fluids changed during subsequent transgression and again became supersaturated with carbonate, probably because of warming concomitant with deeper burial. Hydrolysis of feldspars, conversion of expandable clay minerals to mixed-layer varieties in adjacent compacting shales, and dissolution of siliceous fossil fragments resulted in quartz overgrowths on partially compacted quartz grains. Sericite cement also resulted from hydrolysis of feldspars. Iron oxide cements, which are common in outcrop samples but relatively rare in subsurface samples, appear to have resulted from oxidation of ferroan carbonate cements and iron-beafing ground waters related to the modern weathering regime. Replacement of silica by calcite and vice versa were probably related to postdepositional changes in pH and temperature of the pore waters within Rock Lake sandstones. Replacement of calcite by pyrite in carbonate shell fragments probably occurred in low Eh environments created by decay of the organic matter of the shell-forming organism.
Sandstone units in the lower part of the Rock Lake Shale Member are for the most part very fine grained to fine-grained and commonly exhibit ripple marks and cross-laminations and carry sparse marine fossils. In contrast, the sandstones in the upper portion of this member are generally nonfossiliferous and fine- to medium-grained and contain both planar and trough crossbedding. They often form fining-upward sequences with erosional bases. These observations suggest that this member was deposited as a regressive sequence. The lower sands formed under relatively low-energy conditions at the margin of a sea, whereas the higher sands formed under higher-energy conditions in channels, which show the characteristic waning-upward energy sequence of point-bar deposits. Transport directions measured from trough crossbedded sets are to the north and northwest, which supports the isopach-sand isolith indications of a southeastern siliciclastic source.
Using the stratigraphic, petrographic, and sedimentologic data and interpretations and relating these to the regional setting in which the sediments represented by the Rock Lake Shale Member were deposited, I developed a process-response model. This model depicts a fluvially dominated deltaic system that was active during the regressive depositional phase of the Stanton cycle within the Upper Pennsylvanian epicontinental sea. The Rock Lake deltaic system consisted of several lobes, which developed initially in the southern part of the detrital facies belt. After deposition of sediments associated with the initial lobes ceased, new lobes formed to the north along the eastern margin of the Missourian seaway in response to the northward shifting of the sediment-laden fluvial systems. The Timber Hill siltstone may represent the marine reworking of a washout from one of the lobes, and the Onion Creek sandstone represents one of the later delta lobes that formed in the detrital belt. The third and fourth oolite quartz sandstone horizons may represent shoal-water deposits on local highs in interlobe positions that received reduced amounts of siliciclastics, possibly during dry periods. The channel sandstones in Wilson and Woodson counties may represent sands from a now-eroded eastern lobe that were washed westward across the carbonate shelf and into preexisting low areas on the carbonate surface. The mechanism for the seaward movement of sediment is unclear, but it probably was related to unusually high discharges from storms and floods.
The most probable source of siliciclastic sediments of the Rock Lake Shale Member of the Stanton Limestone is eroded quartzose sedimentary rock from the uplifted Ouachita Mountains (fig. 30). This stands in contrast to both older Missouri= sandstones in the Kansas City area and to much older Desmoinesian channel sands in southeastern Kansas, which have much higher percentages of unstable minerals and reflect immature, nearby sediment sources. Thus Appalachian and Canadian Shield sources contributed little if any siliciclastic sediment to late Missourian deposits in southeastern Kansas.
Acknowledgments
This report is based on my 1980 doctoral dissertation for the University of Iowa and was revised for publication by R. L. Brenner and P. H. Heckel. I wish to thank my thesis committee, consisting of R. L. Brenner, P. H. Heckel, G. R. Hallberg, K. F. Clark, R. S. Carmichael, and R. Rajagopal; the Kansas Geological Survey for field support; W. J. Ebanks for access to subsurface information; P. H. Heckel for unpublished field information; M. C. Tynan and R. C. Price for suggestions on earlier drafts of the manuscript; and my wife and family for continuing encouragement and support during the entire project.
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Kansas Geological Survey, Geology
Placed on web Nov. 4, 2010; originally published 1990.
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