Reef-capping laminites in the Upper Silurian carbonate- to-evaporite transition, Michigan Basin, south-western Ontario



Silurian pinnacle reefs in the subsurface of the south-western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef-capping facies in the upper part of the Guelph Formation and the overlying A-1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non-isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south-western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.