Small lithistid sponge bioherms early middle ordovician table head group western newfoundland canada

Klappa, C.F.; James, N.P.

Bulletin of Canadian Petroleum Geology 28(3): 425-451


ISSN/ISBN: 0007-4802
Accession: 006428129

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A series of small sponge-dominated bioherms occurs within the platformal carbonate sequence of the Table Head Group (Whiterock Stage) in western Newfoundland. These bioherms or organic mounds are 0.2-1.0 m high, 0.5-6.0 m in diameter, circular to oval in plan and hemispherical (convex upward) in shape. Texturally, the mounds are mostly floatstones, each grading upward into bindstones. Skeletal metazoan remains, constituting 40-70% of individual mounds, consist of lithistid sponges (Archaeoscyphiidae, Aulocopiidae, Anthaspidellidae, Eospongiidae) and less abundant orthid brachiopods, nileiid and illaenid trilobites, trepostome bryozoans, planispiral gastropods (mostly Maclurites) and rare orthoconic cephalopods. Pelmatozoan debris and skeletal algae are abundant within the mound matrix and in the basal bioclastic grainstone sheets which underlie and flank individual mounds. Gastropods are most common in the uppermost portions of the mounds; pelmatozoan holdfasts are conspicuous on exposed sponge substrates; macroborings are rare or absent. From the base to the top of the mounds, 4 growth stages often can be delineated: pioneer stage, rippled pelmatozoan-rich sands provide foundation for mound growth; colonization stage, single stick and branching archaeoscyphiid sponges initiate mound growth; diversification stage, club-shaped, funnel-shaped, domal and massive anthaspidellid, archaeoscyphiid, eospongiid and aulocopiid sponges, together with orthid brachiopods and branching to encrusting laminate trepostome bryozoans, form the bulk of individual mounds; and domination stage, plate-shaped to cup-shaped anthaspidellid and aulocopiid sponges encrust high-relief areas. These small zoned structures are one of several different types of metazoan bioherms that developed during the Early and Middle Ordovician times, after the long hiatus following extinction of the archaeocyaths at the end of Early Cambrian time. Different from the more familiar Ordovician coral-stomatoporoid bioherms, they are among the earliest examples of reef mounds dominated by sponges which appear sporadically throughout the Phanerozoic.