Facies anatomy of Lower Cambrian archaeocyathid biostrome complex, southern Labrador

Hughes, Steven (1979) Facies anatomy of Lower Cambrian archaeocyathid biostrome complex, southern Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

An archaeocyathid-rich biostrome complex in the middle of the Forteau Formation (Lower Cambrian) of southern Labrador, is divisible into two facies: (1) Oolite Sand Facies, and (2) Biostrome Facies. Stratigraphic and sedimentologic analysis of the eight selected areas indicates that the Biostrome Facies was deposited seaward of the Oolite Sand Facies and that the palcoslope sloped towards the south or south-east. The highly irregular nature of the contact between the two facies suggests that salients and recesses occurred within the Oolite Sand Facies and that the Biostrome Facies was deposited on shallow swales within recesses. -- Seven lithologies make up the Oolite Sand Facies: skeletal grainstone, oolitic grainstone, mixed oolitic grainstone, mixed peloid packestone, mixed oncolitic grainstone, intraclast grainstone, dolostone and dolomitic siltstone. These lithologies are considered to form an oolite shoal complex consisting of an active shoal, stable sand channels, storm-affected supra-tidal islands, and leeward oolite shoal spillover lobes. -- The Biostrome Facies was deposited seaward of, and on shallow swales between, the oolite shoals. This facies consists of five distinct units : Dolomite Mound Unit (BD), Red Mound Unit (BR), Nodular Mound Unit (BN), Oolite Bioherm Unit (BB), and Grey Mound Unit (BG). Each unit consists of two lithofacies : (1) skeletal calcarenite, which does not vary between units, and (2) moundstone, a new nongenetic rock-type applied to mounds or bioherms of uncertain origin. Five different moundstone lithofacies occur, all of which contain archaeocyathids and sometimes calcareous algae, in a mudstone matrix. The moundstones form mounds, biostromes, and rarely bioherms, all flanked by skeletal calcarenites. Dolomitic Moundstones (BD-1) and Nodular Moundstones (BN-1) formed in relatively deep water, seaward of Red Moundstone (BR-1) which accumulated in lateral contact with the action oolite shoal. Grey Moundstone (BG-1) and Archaeocyathid Bioherms (BB-2) accumulated on shallow, protected swales between oolite shoals. The Archaeocyathid Bioherm lithofacies is flanked by 'quiet-water' oolitic calcarenites (BB-1). -- Diagenetic processes which modified the original depositional texture of the moundstones and calcarenites include: bioerosion, dissolution and cementation, neomorphism, dolomitization, silicification, pressure solution (stylolitization), and fracturing. Diagenetic fabrics were formed by these processes operating in a number of diagenetic environments. Micritization, macroborings and facies specific cementation were formed by processes operating in the submarine diagenetic environment. Selective dolomitization occurred in a zone of mixing between meteoric phreatic and marine phreatic waters. Minor dissolution and widespread cementation occurred in the vadose zone. Fabrics formed in the phreatic environment include extensive stylolitization, dissolution, fracturing, pore-fill cementation, and aggrading neomorphism which includes a transitional micrite-microspar-pseudospar fabric affecting archaeocyathid skeletons. Widespread dolomitization and rare silicification and stylolitization occurred in a late, unspecified diagenetic environment. -- The autecology of archaeocyathids, calcareous algae, stromatolites, a new, primitive 'coelenterate', brachiopids, trilobites, hyolithids, echinoderms, and agmata (Salterella), is reviewed and three faunal/floral assemblages are recognised: (1) mound-flanking Skeletal Calcarenite faunal assemblage, consisting predominately of echinoderms with brachiopods, trilobites, hyolithids and Salterella, (2) mound-dwelling assemblage, consisting predominantly of archaeocyathids and calcareous algae with brachiopods, trilobites, coelenterate, sponges, hyolithids and the ichnogenus Trypanites, (3) coelobitic (cavity dwelling) assemblage, found mainly beneath retilaminiform archaeocyathids and consisting of pendant calcareous algae, foraminifera, encrusting archaeocyathids, Archaeotrypa, sponges, trilobites, and worms.

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