Facies-Specific, Calcitic and Bimineralic Ooids from Middle and Upper Cambrian Platform Carbonates, Western Newfoundland, Canada

Abstract

ABSTRACT Oolites in the Port au Port Group consist of intercalated gray oolite, deposited on the shallow-subtidal mobile fringe of ooid sand shoals, and brown oolite, which accumulated on intertidal sand flats along the shoal crests. Gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport. Brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe. The present fabric of the ooids is the result of protracted burial diagenesis. Cortices of finely and coarsely preserved radial ooids, excluding radial-concentric ones, and superficial radial ooids were Mg calcite originally; they changed to calcite, still retaining fine fabric, and subsequently to a variety of cortical fabrics during shallow burial, and altered to blocky fabrics during deep burial. Concentric ooids and radial-concentric ooids were bimineralic originally, composed of aragonite (now micrite and blocky calcite) and Mg calcite (now radial-fibrous calcite). Primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial; resulting voids were filled with blocky calcite during deep burial. Correlation of original ooid mineralogy, fabric, and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in Cambrian time. This correlation is absent in modern deposits, suggesting that more widespread chemical and organic controls now prevail.