Marine Bank Development in Plattsburg Limestone (Pennsylvanian), Neodesha-Fredonia Area, Kansas

Abstract

The Plattsburg Limestone is anomalously thick in the Neodesha-Fredonia area, increasing from an average of about 20 feet to a maximum thickness of 115 feet. Thickening is due to increase in the Hickory Creek Shale (middle member) from 1 to 45 feet, and the Spring Hill Limestone (upper member) from 3 to 88 feet in thickness.

The Plattsburg Limestone and the overlying Vilas Shale have been studied in outcrops in the eastern part of the Neodesha-Fredonia area and have been traced underground in the central and western parts of the area by means of about 200 drillers logs. Nineteen outcrop sections, measured, described, and sampled in detail, form the basis for lithologic and genetic interpretations. About 150 rock samples were studied microscopically by means of acetate peels and enlarged negative photographic prints made from the peels.

The principal cause of thickening of the Plattsburg Limestone is interpreted to be deposition of an extensive, lens-shaped marine bank that rose above the general level of the surrounding sea floor. The bank was at least 14 miles long in a northwest-southeast direction, and about 10 miles wide. Two smaller, detached thickened portions of the Plattsburg Limestone in the area 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 lesser 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. Part of the Vilas Shale is interpreted to be an off-bank time equivalent of part of the thickened Spring Hill member.

Deposition of the bank is interpreted to have been strongly influenced by lime-secreting organisms, including crinoids, bryozoans, brachiopods, mollusks, and algae. The organisms may have influenced deposition of silt and clay (Hickory Creek member) by exerting a sediment-binding effect, and probably helped stabilize slopes at least as great as 7° on the edges of the bank. In addition, they contributed large quantities of calcareous material to the upper part (Spring Hill member) of the bank.

Where thick, the Spring Hill member of the Plattsburg is divided into three tabular lithologic subdivisions in regular vertical sequence. The lower subdivision contains abundant irregular fragments and pellets, much of it seemingly of algal origin. The middle subdivision contains abundant visibly crystalline calcite intimately associated with encrusting calcareous algae. The upper subdivision contains abundant calcarenite composed of grains of various degrees of rounding and sorting. During deposition of the crystalline subdivision, lime-secreting algae may have imparted rigidity to deposits forming on the bank, thus creating a reef if the bank extended into shallow water. During deposition of other parts of the Plattsburg bank, the deposits probably were not wave resistant.

Porosity in the Spring Hill member is related to limestone lithology. The crystalline limestone subdivision, where pores and vugs are conspicuous in visibly crystalline calcite, is most porous. The thickened Spring Hill Limestone of the Neodesha-Fredonia area may provide an example of a porous limestone lens that might serve as an oil reservoir; some oil pools in Pennsylvanian limestones of central and western Kansas may occur in porous lenses of similar origin.