Samples from three well cores in the Pipe Creek Junior Reef (Niagaran-Cayugan) in the' Wabash Formation of northeastern Indiana revealed an indirect and imperfect relationship between dolomite and clay content. A microscopic examination of the larger than two micron fractions of residues that were insoluble in acetic acid, and semi-quantitative X-ray analyses of the smaller than two micron fractions revealed illite to be the only clay mineral present in 91 samples. Illite comprised approximately 50 percent of the insoluble residue and more than 90 percent of the aluminum oxide-containing minerals. No magnesium silicates were found.Illite and dolomite contents were calculated from spectrochemical analyses of Al203 and MgCO3 respectively in 94 reef-influenced samples from three Indiana Geological Survey well cores. The regression and linearity of the dolomite/illite relationship revealed an imperfect correlation in two of the three cores.Nineteen thin sections of textural' varieties, from the core that did not reveal a strong illite/dolomite correlation, and eight thin sections from a portion of interbedded limestones and dolostones in the pipe Creek Junior quarry reef flank revealed the authigenetic origin of dolomite and the probable detrital origin of illite. The concentration of illite and other insolubles by post-micritization microstylolitization occurred along seams of carbonate solution and high permeability which developed selectively within the micritic matrices. Dolomite selectively crystallized in those more permeable sediments following the complete or nearly complete lithification of the reef.Ball State UniversityMuncie, IN 47306