SYNSEDIMENTARY CEMENTS AS PALEOPROTEROZOIC PLATFORM BUILDING-BLOCKS, PETHEI GROUP, NORTHWESTERN CANADA

Peritidal and subtidal carbonates of the Pethei Group are a complex ar rangement of fibrous and micritic synsedimentary precipitates, carbona te grains, and detrital micrite, Synsedimentary precipitates, present largely as stromatolitic laminae, constitute the bulk of the Pethei pl atform, providing about 75% of the total carbonate, They occur as stro matolitic precipitates (precipitates closely associated with stromatol itic laminae) which constitute about 60% of the rock and encrusting pr ecipitates (precipitates outside stromatolites and between grains, i.e . cement) which constitute about 15% of the rock. The most abundant pr ecipitates are fibrous, in the form of stromatolitic microdigitate sta lks (tufa) and masses of radial fans, as well as encrusting pore-filli ng fans, isopachous rinds, and laminar crusts, Micrite precipitates oc cur as stromatolitic clotted micrite and encrusting isopachous rinds, The ratio of fibrous to micritic precipitates is 2:1. Stromatolitic pr ecipitates are arranged in three distinct microfabrics: (1) ubiquitous , spar-micrite couplets of fibrous and clotted micrite laminae; (2) de ep-water vertical fibrous masses that precipitated over micritic preci pitate cores; and (3) supratidal microdigitate crusts of branching fib rous stalks, Micritic cements are interpreted to be of biogenic origin , having precipitated somehow in association with microorganisms. Fibr ous precipitates are interpreted to be largely abiotic Since stromatol itic and encrusting precipitates are petrographically similar, they pr obably share similar origins. There is little evidence of sediment tra pping and binding as a mechanism for stromatolite formation. The abund ance of synsedimentary precipitates varies with platform geometry: sub tidal ramps > subtidal rimmed shelves > subtidal open shelves > periti dal rimmed shelves. While the basic building blocks of the Pethei plat form, synsedimentary precipitates, carbonate grains, and detrital micr ite, are the same as those in Phanerozoic platforms, their distributio n and relative proportions are different, These differences reflect a Proterozoic carbonate factory which extended across the entire platfor m and which produced carbonate largely by in situ precipitation.