Bioherms and biostromes of the Lower Cambrian Forteau Formation in southern Labrador and western Newfoundland are rich in skeletal and nonskeletal components and display a wide sprectrum of synsedimentary and postdepositional cements. Through petrography, cathodoluminescence and microprobe analysis, three types of components can be distinguished: (1) molds filled with iron-rich or iron-poor calcite spar or iron-rich dolomite (archaeogastropods, hyolithids, brachiopods, ?coelenterates, and Chancelloria); (2) particles composed of iron-poor calcite and showing original or relic texture (trilobites, salterellids, echinoderms, and sponge spicules); and (3) components illustrating both the above fabrics (ooids, archaeocyaths, and Renalcis). Synsedimentary cements are (1) rays r botryoids in which each acicular crystal is a spar-filled mold, or (2) rinds of fibrous calcite commonly with fascicular-optic properties.

Comparison with the petrographic characteristics of Phanerozoic and modern carbonates indicates that those Cambrian particles and cements which are now spar-filled molds were originally high porosity aragonite; those without a void stage and retaining original fabric were calcite or Mg-calcite; those with a fibrous habit were Mg-calcite, and those which exhibit two or more of the above characteristics were low porosity aragonite or high porosity Mg-calcite.

These styles of preservation confirm that organisms in Early Cambrian were secreting skeletons with the same spectrum of mineralogies as those to today and that metastable carbonates (aragonite and Mg-calcite) were being precipitated on the shallow sea floor, as they are in the modern ocean. The absence of similar fabrics at other times in the Phanerozoic probably reflects epeiric sea rather than continental margin sedimentation and not a gradual change in seawater chemistry or atmospheric CO₂ through time.

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