Small-scale basin-slope carbonate cycles in Trenton limestones (Middle Ordovician), southern Ontario

The Middle Ordovician marine transgression is marked by a simple stratigraphic sequence from supratidal and tidal-flat carbonates, through lagoonal and shoal carbonates, into offshore and finally deep-shelf carbonates. Within the offshore carbonate succession, contemporary peninsulas, islands, and shoals complicate the detailed facies distributions, but show features similar to the basin-slope carbonate models proposed for other carbonate deposits, albeit on a smaller scale. Two end-member cycles are proposed. The proximal cycle consists of coarse pebbly biosparites, commonly showing the characteristics of grain flows, passing up into cross-bedded, fine-grained biosparites, which then are overlain by micrite and calcareous shales. Calcareous shales occur interbedded throughout the cycle. This cycle is interpreted as having been deposited in bypass channels on slopes around the Middle Ordovician islands or shoals. The distal cycle consists of poorly washed biosparite passing up into interbedded micrite and calcareous clay, and is interpreted as being deposited near the base of the slope or in the adjacent basin. Both types of cycles have hardgrounds on their coarser units, indicating long periods of nondeposition and/or erosion after deposition. A close analogy can be made, both in microfacies and depositional environment, with the Holocene Arabian shelf of the Persian Gulf. Furthermore, both have similar tectonic situations-carbonate shelves on ancient shields undergoing collision with a magmatic arc.