Limestone Diagenesis and Dolomitization in Mississippian Carbonate Banks in Montana

Abstract

ABSTRACT Waulsortian type carbonate banks in central Montana were diagenetically altered in three major stages: before, during, and after subaerial exposure of the tops of the banks. Before subaerial exposure, cloudy radiaxial fibrous drusy mosaic was precipitated as aragonite on all free internal surfaces. Opaline silica was dissolved, and small amounts of chalcedonic chert replaced parts of fossils and the fibrous mosaic. With partial emergence of the banks, the lime mud and fossils were altered, clear equant-grained drusy mosaic calcite and euhedra of dolomite were formed in centers of cavities lined with fibrous crusts, and seepage refluxion of hypersaline water formed an irregular zone of dolostone at the tops of the banks. Other changes involved weathering of the exposed surface, dilatational fracturing, dissolution of cavelike openings within the banks, and filling of the fractures and openings with crinoid debris. After resubmergence, the banks were engulfed by continued Lodgepole limestone deposition. Late in diagenesis a minor amount of dedolomitization occurred, and, perhaps, microspar (Folk, 1965) formed neomorphically.