A mid-Ordovician temperate carbonate shelf-the black river and trenton limestone groups of southern Ontario, Canada

Abstract

Mid-Ordovician limestones of southern Ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from Recent tropical carbonate environments. Nevertheless, depositional rates, grain types, faunas, erosion surfaces, mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment. In view of the low latitudes (20°S) indicated by paleomagnetism, this suggests deposition in relatively cold seas, deposition below a tropical thermocline, or that the paleomagnetic data are wrong. Similar features in other Caradocian limestones of eastern North America show that these inferred cooler conditions were widespread. If deposition occurred below a tropical thermocline, most shallow-water Caradocian limestones must be re-interpreted as deeper-water deposits. More likely, in view of the evidence for temperate conditions in shallow water, is that the whole eastern seaboard of North America was affected by cold currents arising from southern polar ice caps, and thus was a temperate shelf environment.