Patterns of Silicification in Permian Pelecypods and Brachiopods from Wyoming

Abstract

ABSTRACT Silicified pelecypods and brachiopods are common in the Ervay, Franson, and Grandeur Members of the Permian Park City Formation of central and western Wyoming. Study of hundreds of these fossils has resulted in recognition of five patterns of silicification. Pattern I is characterized by megaquartz crystals projecting inward from and perpendicular to the skeletal boundary, whereas fossils exemplifying Pattern II consist of concentric laminae of chalcedonic quartz. In Pattern III, an outer lamina of microcrystalline quartz grades abruptly inward to laminated chalcedonic quartz which in turn grades abruptly inward to megaquartz. Shells demonstrating Pattern IV involve megaquartz crystals aligned parallel to the skeletal boundary and lying adjacent to beekite discs. Pattern V consists of megaquartz crystals aligned parallel to the skeletal boundary. In brachiopods, these crystals typically exhibit relict skeletal microstruture. Pelecypod wall layers constructed originally of aragonite are characterized by Patterns I, II, III and IV. Pattern V characterizes all brachiopods and originally calcitic layers of pelecypods. Patterns I, II, III and IV are interpreted to result from filling of voids created by dissolution of skeletal carbonate. Pattern V results from concurrent carbonate dissolution and silica precipitation.