Blackriveran carbonates from the subsurface of the Lake Simcoe area, southern Ontario: stratigraphy and sedimentology of a low-energy carbonate ramp

摘要

The lowest Upper Ordovician (Blackriveran) stratain the Lake Simcoe area represent the base of a major transgressivesequence of peritidal carbonates and siliciclastics. From detailedpetrographic study of 12 cores that penetrate, in ascending order,the Shadow Lake, Gull River, and lower Bobcaygeon formations,seven distinctive lithofacies are recognized: (1) sandstone,siltstone, and shale; (2) dolostone and calcareous dolostone; (3)lime mudstone; (4) peloidal bioclastic wackestone and packstone;(5) peloidal bioclastic grainstone; (6) ooid grainstone; and (7)intraclastic bioclastic wackestone and packstone. Metre-scalecycles, like those reported from numerous other peritidal deposits,are apparently absent in this succession. In addition, these strata arecommonly interrupted by a variety of hardgrounds. The carbonatestrata can be grouped into four lithofacies associations thatrepresent deposition in generally low energy, tropicalpaleoenvironments along a southward-facing, homoclinal rampthat was frequently influenced by storms. Rapid facies changesalong depositional strike likely resulted from a combination of anirregular basement topography and coastline geometry. Thecoexistence of Heterozoan and Photozoan carbonates in the lowerBobcaygeon Formation probably reflects the increasingincorporation of cooler oceanic waters as the ramp graduallydeepened during the Tippecanoe transgression rather than the onsetof more temperate climatic conditions.