Coastal paleokarst near the Ordovician-Silurian boundary, Manitoulin Island

Exposures of the Ordovician-Silurian systemic boundary occur at two places on Manitoulin Island, Lake Huron, Ontario. One is in a roadcut and old quarry (Kagawong West Quarry) near Kagawong, and the other is a roadcut on highway 6 south, north of Manitowaning. At both localities the exact position of the boundary is controversial. Within the 0.5-m-thick interval in which most workers place the systemic boundary at these localities, there are two paleokarst surfaces. The paleokarsts mark erosional disconformities above and below an 18-cm-thick argillaceous unit overlying Upper Ordovician limestones (Ashgillian) of the Kagawong beds and underlying the basal beds of the Lower Silurian (Llandovery) Manitoulin Formation. The paleokarst surfaces are characterized by small-scale pitting (Karren), an irregular profile displaying topography of a few centimetres, and an iron oxide crust. The highest parts of the Karren are covered by a thin, epilithic, microphyte carpet (algal and fungal?) within which most of the iron oxide occurs. The Karren is interpreted as having been produced in a coastal environment within the spray or splash zones of the supratidal areas; the microphyte carpets also grew near shore and probably represent coastal intertidal or supratidal epilithic floras that grew at the advancing edge of the early Silurian transgression of the area. These carpets may represent the oldest recognized filament carpets developed on coastal karst of the rocky supratidal or intertidal zones.