Ooid Mineralogy and Diagenesis of Upper Mississippian Pitkin Formation, Arkansas

E. Heydari, R. D. Snelling, W.C. Dawson, M. L. Machain

The Pitkin formation is a marine oolitic-bioclastic limestone exhibiting shoal, lagoonal, beach, and tidal-channel facies. Ooids formed on the Pitkin carbonate shelf show lateral and vertical variations in original mineralogy. In a section south of Mountain View, Arkansas, the lower marine shoal and shoreface facies are composed of originally aragonitic ooids. These ooids are replaced by neomorphic calcite and exhibit elevated strontium and low magnesium concentrations. The overlying lagoonal facies are composed of originally calcite ooids that have retained their original radial fabrics. These ooids exhibit low strontium and high magnesium concentrations. Ooids in sections to the west of Mountain View are all calcitic.

Early diagenesis of the formation is dominated by marine cementation. Postdepositional diagenesis encompasses extensive dissolution of originally aragonite and magnesian calcite components, precipitation of a late unzoned ferroan calcite, and minor amounts of saddle dolomite. Grainstone units lack any intergranular porosity. Pressure solution contacts among grains in grainstones are generally uncommon. Cementation alone is responsible for destruction of all of the original porosity in these rocks. Intensive pressure solution is observed only in packstone units not affected by early marine cementation.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.