Controls and age of cavernous porosity in Middle Silurian dolomite, southern Ontario

Abstract

Porosity in bioclastic dolomites of the Amabel Formation in the Eramosa River area, northeast of Guelph, Ontario, is characterized by solution-enlarged joints in outcrop (kluftkarren), and vuggy and cavernous intervals in the subsurface. In addition, a network of accessible caves has been dissected by Recent fluvial incision by the present Eramosa River valley. Porosity development was controlled locally by lithostratigraphy and regionally by the progressive downcutting of a deeply incised, buried bedrock valley west of Rockwood. The Amabel Formation is capped by the Eramosa Member of the Guelph Formation, which consists of jointed but relatively nonporous, laminated to medium-bedded, bituminous dolomitized mudstones. The vuggy and cavernous porosity in the Amabel Formation is probably pre-Wisconsinan to Early Wisconsinan in age, beginning more than 60-75 ka, and developed in response to the lowering of hydraulic gradients as the channel of the Rockwood buried valley was progressively incised. Porous intervals at higher elevations were abandoned in favour of deeper flow paths as incision continued, creating a complex gallery of interconnected subsurface conduits. Subsequent ice advances and ablation events eventually filled the valley with tills and interglacial sands and gravels, ending the valley's influence on the development of vuggy and cavernous porosity.