REEFS IN A DETERIORATING EPEIRIC SEA: UPPER ORDOVICIAN MICROBIAL BIOLITHITES, RED RIVER GROUP, SUBSURFACE SASKATCHEWAN, CANADA

Patch reefs <3 m thick occur in Upper Ordovician Red River strata of southern Saskatchewan, part of the Tippecanoe Sequence that covered much of central North America. Reefs and intercalated grainstones are present in transition beds between the Yeoman Formation and the Lake Alma Member of the Herald Formation. The Yeoman consists of Thalassinoides burrow-mottled dolomitic limestones containing a normal-marine biota of nautiloids, brachiopods, gastropods, corals and calcareous algae, whereas the Lake Alma is composed of laminated and wave-rippled mudstone and peloidal grainstone, overlain by nodular and bedded anhydrite. The layer-cake aspect of these units suggests a basin-wide shallowing-upward setting.

Biolithites are thrombolitic in that their microbial framework consists of mm- to cm-sized knobs and crusts of clotted micrite locally with stromatolitic lamination and hemispheroids of laterally juxtaposed, radiating micrite-walled tubes (Ortonella or Cayeuxia). Sporadically incorporated are also: laminar stromatoporoid-like objects <0.5 cm thick; sinuous to loosely coiled tubular skeletons 0.5-1 mm wide (gastropods?); micrite-walled, Terebella-like tubes (worms?); Wetheredella (foraminifers?); and ramose bryozoans. In the matrix are ostracodes plus scattered brachiopods, gastropods and crinoid ossicles. Cavities contain peloids and micrite with multiple generations of Planolites burrows suggestive of progressive induration. Biolithites exhibit fibrous calcite cementation, synsedimentary recrystallization, mimetic dolomitization, and anhydrite cementation and replacement.

The microbially dominated Red River patch reefs defy the current paradigm of reef evolution. The biota contrasts markedly with that of other known Middle and Upper Ordovician and Silurian shallow-water biolithites, in which stromatoporoids, corals and other representatives of the mid-Paleozoic diversity maximum are conspicuous. Thus, although ostensibly still marine, the environmental setting must have been ecologically stressed. Possible factors include temperature increase and short-lived salinity fluctuations, phenomena more likely to occur in epeiric seas than in marginal shelves in global greenhouse conditions.