Numerous vertical to subvertical, cylindrical shafts occur in the rugged exposures of the Middle Silurian Amabel Formation bedrock along the Eramosa River, approximately 10 km northeast of Guelph. These shafts vary from < 1 m to 10 m in diameter, with depths as great as 12 m. They can occur as isolated depressions but more commonly are clustered closely together, and many coalesce to form sinuous bedrock walls. These depressions may contain soil, fine sediment, rubble from the surrounding dolomite bedrock, and rounded clasts of Precambrian basement lithologies. Till is conspicuously absent. These shafts occur in close proximity to cavernous pores that were formed during pre-Wisconsinan to Early Wisconsinan time, suggesting a possible karstic origin. The cylindrical shafts, however, with their greater midpoint diameters, sculpted interiors, and inclined axes, and their lack of vertical flutes and till, are more likely to have originated from glaciofluvial discharge at the end of the Late Wisconsinan. The development of these shafts may have been assisted by cavitation, which enhanced the erosional capacity of sediment-laden meltwaters.Résumé : Plusieurs puits cylindriques, verticaux à sub-verticaux, se retrouvent dans les affleurements à relief accidenté de la Formation Amabel (Silurien moyen) le long de la rivière Eramosa, à environ 10 km au nord-est de Guelph. Le diamètre de ces puits varie de < 1 m à 10 m et les profondeurs atteignent 12 m. Ils se présentent sous forme de dé-pressions isolées mais sont généralement regroupés ensemble et forment des murs de roc sinueux. Ces dépressions peuvent contenir du sol, des sédiments fins, des débris de la roche dolomitique environnante et des roches clastiques arrondies de lithologies du socle précambrien. Le till est remarquablement absent. Ces puits se retrouvent tout près d'interstices caverneux qui ont été formés à l'époque du Pré-Wisconsinien au Wisconsinien précoce, suggérant une possible origine karstique. Cependant, les puits cylindriques, avec leurs plus grands diamètres au point milieu, des inté-rieurs sculptés et des axes inclinés ainsi que l'absence de flûtes verticales et de till, tirent probablement leur origine d'une décharge fluvio-glaciaire à la fin du Wisconsinien tardif. La cavitation a peut-être aidé au développement de ces puits, rehaussant la puissance d'érosion des eaux de fonte chargées de sédiments.[Traduit par la Rédaction] Kunert and Coniglio 52
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.
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