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The Plattsburg limestone is anomalously thick in the Neodesha-Fredonia area, swelling from less than 10 feet to maximum thickness of 115 feet. Thickening is due to large increases in thickness of two of the three members into which the Plattsburg has been divided. The Merriam limestone (lower member) varies only slightly in thickness, ranging from 1 to 3 feet, but the Hickory Creek shale (middle member) ranges from 1 to 45 feet, and the Spring Hill limestone (upper member) ranges from 3 to 88 feet in thickness.
The principal cause of thickening of the Plattsburg limestone is interpreted to be due to deposition of an extensive, lens-shaped shallow marine bank which rose above the general level of the surrounding sea floor. The shape of the bank is thought to be partly reflected by present thickness variations in the Plattsburg limestone. The bank was at least 14 miles long northwest and southeast, and about 12 miles wide northeast and southwest.
Two smaller, detached thickened parts of the Plattsburg limestone are also present in the area and probably represent small banks. A second cause of thickness variations in the Plattsburg limestone is local structural warping during deposition, which permitted greater thicknesses to accumulate over downwarps and less thicknesses over upwarps. Thickness of the Vilas shale also has been affected by this cause.
Thickness of the Vilas shale has been observed to be inversely related to thickness of the Plattsburg limestone at most localities where the two formations are exposed. Where the Plattsburg limestone is thick, the Vilas shale tends to be thin, and vice versa. The Vilas shale has been interpreted to be an off-bank lateral time equivalent of the thickened Spring Hill member.
Deposition of the bank is interpreted to have been strongly influenced by carbonate-secreting organisms, including crinoids, bryozoans, brachiopods, mollusks, and algae, which flourished over the bank. The organisms may have influenced deposition of silt and clay (Hickory Creek shale member) by exerting a sediment-binding effect, and probably helped stabilize slopes at least as great as 7° on the sides of the bank. In addition, the carbonate-secreting organism contributed large quantities of calcareous material to form the upper part (Spring Hill limestone member) of the bank.
Where thick, the Spring Hill limestone member of the Plattsburg may be divided into three tabular lithologic subdivisions which occur in regular vertical sequence. The lower subdivision has been termed the fragment-pellet subdivision because of the abundance of irregular-shaped fragments and pellets. Much of the fragmental and pelletal material appears to be of algal origin. The middle subdivision is termed the crystalline subdivision because of the abundance of sparry calcite which is closely associated with fragments of carbonate encrusted blades of various forms of calcareous algae. Some of the algal forms resemble the alga Anchicodium, although positive identifications have not been made because of the lack of preservation of essential details. The upper subdivision is termed the alcarenite subdivision because of the abundance of calcarenite composed of grains with varying degrees of rounding and sorting.
During deposition of the crystalline limestone subdivision of the Spring Hill member, carbonate-secreting algae may have imparted sufficient rigidity to the bank to cause it to be wave-resistant and thus allowing it to be classed as a reef. However, during deposition of most of the other parts of the Plattsburg bank, the deposits probably did not possess sufficient coherence to be wave-resistant.
Porosity in the Spring Hill member in outcrops is closely related to limestone lithology. Highest porosity occurs in the crystalline limestone subdivision, where conspicuous pores and vugs occur in visibly crystalline calcite. Some of the crystalline calcite is interpreted to have formed through precipitation in open spaces which were not filled with calcareous mud during deposition because of the sheltering effect of individual fragments of carbonate-encrusted calcareous algae. Other crystalline calcite appears to have been produced through recrystallization of calcareous algae. It seems possible that favorable oil reservoir conditions in certain Pennsylvanian limestone in central and western Kansas may be provided by porous zones in thickened limestones lenses similar in origin to t e thickened Plattsburg in the Neodesha-Fredonia area.
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