Carbonate-Sulfate Mineral Replacements in Diagenesis of Peritidal Limestone: ABSTRACT

Abstract

Petrographic study of crystal textures and fabrics, both original and replacement, has revealed the postdepositional history of carbonate and sulfate minerals in the Silurian Tonolway Limestone. Crystal forms, assumed to have been gypsum originally, are locally common in supratidal facies; they occur as coarse euhedral needles, randomly oriented to bedding, and commonly in clusters. When this gypsum precipitated interstitially in the lime sediment, micrite was often incorporated into the crystals; organic matter, concentrated on side faces, may have blocked nucleation, causing the fibrous crystallization texture. As a consequence of the changing chemistry of pore fluids, a series of mineral replacements began within these crystal forms. Gypsum was first replaced by celest te, as evidenced by its pseudomorphism. The enclosing aragonite sediment inverted to calcite, and strontium was thus freed for the making of celestite. Afterwards, calcitization of the sulfates took place, i.e., each crystal was replaced by a mosaic of very finely crystalline calcite. Calcitization began around inclusions of micrite within the crystal forms. Presumably, decomposition of algal material produced carbon dioxide that was used for the calcitization reaction. Sulfate ions released by this last replacement inhibited the growth of calcite, resulting in its very fine crystallization fabric. Lastly, these calcite pseudomorphs were themselves partly replaced by limpid dolomite. End_of_Article - Last_Page 787------------