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

Porosity in bioclastic dolomites of the Amabel Formation in the Eramosa Riv er area, northeast of Guelph, Ontario, is characterized by solution-enlarge d joints in outcrop (kluftkarren), and vuggy and cavernous intervals in the subsurface. Ln addition, a network of accessible caves has been dissected by Recent fluvial incision by the present Eramosa River valley. Porosity de velopment 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 Guel ph Formation, which consists of jointed but relatively nonporous, laminated to medium-bedded, bituminous dolomitized mudstones. The vug,oy and caverno us porosity in the Amabel Formation is probably pre-Wisconsinan to Early Wi sconsinan in age, beginning more than 60-75 ka, and developed in response t o the lowering of hydraulic gradients as the channel of the Rockwood buried valley was progressively incised. Porous intervals at higher elevations we re abandoned in favour of deeper now paths as incision continued, creating a complex gallery of interconnected subsurface conduits. Subsequent ice adv ances and ablation events eventually filled the valley with tills and inter glacial sands and gravels, ending the valley's influence on the development of vuggy and cavernous porosity.