Petrogenesis of Early Cambrian Reef Limestones, Labrador, Canada

Abstract

Through petrography, cathodoluminescence, and microprobe analysis, three styles of particle preservation can be differentiated: 1) skeletons with excellent fabric retention as nonferroan calcite (trilobites and Salterella) or both ferroan and nonferroan calcite (echinoids and sponge spicules); 2) skeletal molds completely filled by nonferroan calcite spar, or occasionally a second stage of equant ferroan calcite and dolomite as well (archaeogastropods, hyolithids, brachipods, ?coelenterates, and Chancelloria); 3) fibrous to microcrystalline components which are now either well preserved as nonferroan calcite or are molds filled by equant ferroan calcite and dolomite or both (ooids, archaeocyathans, and the alga Renalcis). Synsedimentary cements, localized to reefs, are: 1) rays or botryoids in which each acicular crystal is a spar-filled mold; or 2) rinds of fibrous calcite commonly with fascicular-optic properties or a partially dissolved fabric in which the solution voids are now filled with iron-rich, equant calcite cement. Those Cambrian particles and cements which are now only spar-filled molds were originally aragonite; those without a void stage and retaining original fabric were calcite or Mg calcite; those with a fibrous to microcrystalline habit in some instances and spar-filled molds in other cases were probably Mg calcite. Alteration to calcite occurred as the sediments reacted with meteoric and marine groundwaters in a phreatic lens and then with percolating meteoric waters in a vadose diagenetic environment. Either conditions of carbonate precipitation have been similar since the earliest Paleozoic or that the ocean-atmosphere system in Early Cambrian time was coincidentally similar to that of today, but may have been quite different at other times during the Phanerozoic.--Modified journal abstract.