Abstract

Siliciclastics dominate the Canadian Mesozoic and carbonates are rare. Recent Canadian carbonates are rarer yet, but they do exist. In a review for the Canadian Reef Inventory Project, significant reefal carbonates were recognized only in the Triassic of the Canadian Cordillera and in the Jurassic - earliest Cretaceous of offshore Nova Scotia. But what were termed ‘atypical reefs’ in the clastic-rich central cratonic area were found to be not so acceptable in the carbonate realm by many students of reefs. ‘Atypical reefs’ and carbonate accumulations included the following: tufas/travertines, lacustrine stromatolites and oncolites, marine peritidal stromatolites, oyster banks, shell bank/channel coquinas, sponge mounds, deep-water coral banks/thickets, and serpulid mounds. Other carbonates which were not considered are oolites, marls and chalks. Recent Canadian deposits that could serve as examples and analogues may include temperate climate carbonates of the “foramol” association consisting of foraminifera, bryozoans, barnacles and mollusks (especially bivalves); oyster banks; red coralline algal microbiostromes and rhodoliths; deep sponge mounds (under ice!); deep coral banks; hydrothermal vent or seep-associated tube worm-clam communities; and nonmarine groundwater-associated tufa deposits (including petrified beaver dam) and lacustrine stromatolites in both arid and humid climates.

Of what significance and interest to students of siliciclastics are these rare and typically thin carbonate deposits that might make them worthy of study? (1) Though rare, they are highly facies-specific for certain tidal or shelf settings, for example, estuarine or lagoonal oyster reefs. (2) They are indicative of slow or nondeposition and thus record low sediment supply and/or interdeltaic and/or bypass situations which in turn may indicate early structure such as might occur over a salt dome. (3) Alternatively, they may be indicative of sequence breaks in marine settings and associated (or subsequent) high sea-level stands with low sediment input. Tufas, like caliche carbonate soil profiles, indicate subaerial sequence breaks. (4) They, as coquinas, may provide evidence of temperate/cold or arid climates, both by resulting from low sediment supply and by not being leached subsequently. (5) Surprisingly, they may also be direct indicators of hydrocarbon accumulations. Methane in seeps forms the food base of certain Recent worm tube-clam communities and apparently was also the basis of a serpulid mound in the Cretaceous of the Canadian Arctic Islands.

In discussing these ‘atypical reefs’ and deposits at this conference, I wish to foster an appreciation for such carbonates in those who will be examining the surrounding sediments. Your help in reporting any examples of such deposits would be welcomed. Eventually we may all get a better understanding of these potentially highly useful and interesting but rare carbonates that occur in default of high or even moderate siliciclastic sedimentation.