Paleoenvironments of the mid-Ordovician (Upper Caradocian) Trenton limestones of southern Ontario, Canada: Storm sedimentation on a shoal-basin shelf model

The mid-Ordovician (Caradocian) limestones of southern Ontario were deposited on a shelf undergoing collision with a magmatic arc. Within the general deepening-upwards sequence, shoals and islands complicate the facies patterns. Around these shoals and islands, carbonate sediments can be divided into nine lithotypes reflecting shallow agitated, to deep, quiet marine environments. Many of these lithotypes show good evidence of storm deposition. The lithotypes can be grouped into natural associations which define shoal, intershoal, slope and basinal facies—though the basins were probably less than 100 m deep. The closest recent analogues of these Ordovician environments occur on the Arabian shelf of the Persian Gulf, and on the Sahul shelf of northern Australia which is undergoing collision with the Banda arc. In both these environments, local shoal-basin shelf topography controls the detailed carbonate shelf sedimentation, which on a large scale is controlled by storm and tsunami effects on a seaward sloping ramp. Such shoal-basin and ramp models now seem more suitable in explaining carbonate facies in epeiric seas, than the flat slope models previously proposed. Glacio-eustatic sea-level changes may have controlled the larger aspects of carbonate sedimentation on the Ordovician shelf, as they did and continue to do now, on the recent shelves. Such changes may explain the localization of the variety of Ordovician hardgrounds which we previously described.