The Silurian-Devonian disconformity in southern Ontario

Abstract

ABSTRACT The sub-Devonian (Gedinnian) disconformity in southern Ontario is developed upon the Upper Silurian Bertie Formation. Two erosion surfaces are recognizable: pre- and post-Oriskany sandstone. An extensive system of Silurian joints and fractures is developed within the Bertie beds. A few of these are filled with Oriskany-type orthoquartzitic sand and conglomerate, and represent subaerial solution prior to Oriskany deposition. The others are filled with Springvale sand and conglomerate and represent solution and widening during subaerial exposure following Oriskany deposition. Below the disconformity surface, a system of solution vugs and extensive leached patches stained with hydrocarbons is present in the uppermost Bertie dolomite. The vugs and leached patches are vadose and represent extensive freshwater solution during subaerial exposure. The mottling is not developed below the disconformity beneath the Oriskany, which suggests that leaching and vug porosity developed only during the second erosional period. The disconformity surface consists of a solution topography of rounded ridges, solution pits and other small-scale karren forms. Straight to gently curved borings of the morphological ichnogenus Trypanites are present on the disconformity surface and were established during the marine transgressions. The walls of the Trypanites borings are penetrated by silica-infilled algal microborings. No evidence for the presence of an encrusting epizoan fauna was found. Rounded solution pitting developed on the surface may be the result of an algal or moss cover. Dendritic, branching and single linear etchings developed on some parts of the disconformity greatly resemble the etchings produced by land-plant root or rhizome systems. Analysis of sedimentological and paleontological data leads to the recognition of a marine transgressive-regressive sequence in which two distinct subaerial disconformities are present.