AN ANALYSIS OF FRACTURE CONTROLLED EROSION AND ROCKFALLS IN THE NIAGARA ESCARPMENT IN SOUTHERN ONTARIO

The Niagara Escarpment is a geological feature composed of heavily fractured Ordovician and Silurian shales and carbonates stretching east-west, predominantly through southern Ontario and various states including New York, Michigan, Wisconsin and Illinois. Differential erosion of the shale and carbonate strata has generated a steep cliff face bisecting the city of Hamilton, Ontario. Here, more resistant fractured dolostone overhangs undercut shale, presenting structural stability issues. This is of particular concern when in proximity to vital city infrastructure such as access roads. The location, orientation, and aperture of fractures in the different geological formations comprising the Niagara Escarpment were systematically quantified and documented at sites around Hamilton. The aim of this was to understand the distribution of fractures throughout the escarpment, their connectivity, fluid flow properties, and relationship to structural stability. These data indicate dominant fracture sets are concentrated in the caprock of the escarpment. The fractures here can be differentiated into 1) near-vertical bedding confined fractures, which are confined by 2) sedimentary bedding planes which have facilitated fracture migration and controlled resultant fracture geometry. This work has shown that the distribution and nature of fractures throughout the escarpment is the predominant control on the locations and sizes of rock fragments generated by erosion resulting in rockfalls. Moreover, fracture controlled porosity has contributed to fluid flow throughout the escarpment. Thus, overall, fractures have been found to present a significant control in generating rock falls sourced from the escarpment. This is particularly true of the uppermost geological unit, where systematic fracture sets segregate rock fragments along vertical fracture sets and horizontal bedding planes of predictable sizes.