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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 53 (1983)No. 4. (December), Pages 1051-1096

Petrogenesis of Early Cambrian Reef Limestones, Labrador, Canada

Noel P. James, Colin F. Klappa (2)

ABSTRACT

Bioherms and biostromes in the Lower Cambrian Forteau Formation of southern Labrador and western Newfoundland are rich in skeletal and nonskeletal components and display a wide spectrum of synsedimentary and postdepositional cements. 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 omponents 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.

Comparison with petrographic, trace element, and stable isotopic characteristics of later Phanerozoic andmodern carbonates indicates that 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 is interpreted to have taken place as the sediments reacted first with mixed meteoric and marine groundwaters in a phreatic lens and then with percolating meteoric waters in a vadose diagenetic environment. Deep burial triggered further alteration in the form of fracturing, fabric-specific dissolution, and precipitation of blocky calcite spar or dolomite or both.

The styles of particle and cement preservation, together with a consistent geochemical signature, imply that organisms in warm seas during early Cambrian time were secreting skeletons containing the same spectrum of carbonate mineralogies as tropical marine invertebrates do today, and that metastable carbonates (aragonite and Mg calcite) were being precipitated in buildups on the shallow sea floor as they are in the modern ocean.

These findings suggest that 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.

"Plus ca change, plus c'est la meme chose."


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