FRACTURE APERTURE MODERATES THERMAL WEATHERING PROCESSES IN FRACTURED SEDIMENTARY ROCKWALLS

The Niagara Escarpment, situated in the Great Lakes region, is a highly fractured sedimentary cuesta integrated into urban infrastructure along its extent. Recent work suggests that pre-existing fractures contribute significantly to the disaggregation of exposed rockwalls, and that thermal conditions play a predominant role in this process. However, minimal work has been conducted to examine the effect of fracture aperture on temperature variability on the rockwall and its thermal regime, particularly in this temperate region. To investigate this, we instrumented six pre-existing fractures of varying aperture with thermistors at 12 cm depth and their corresponding proximal rock surfaces on a southeast-facing rockwall in Hamilton, Ontario. Rock surface and fracture temperatures were measured in one-minute increments to evaluate the potential for thermal weathering processes, and to examine surface-fracture dynamics. The temperature range and maximum temperatures recorded in the narrowest (6 mm) fracture were notably larger than those recorded in fractures with wider apertures (10-32 mm). This suggests that, beyond a threshold aperture, small fractures can experience larger temperature changes that make them particularly susceptible to weathering. In addition, while temperatures recorded within fractures and on the rock surface showed diurnal cyclical oscillations, with fractures being warmer than the rock surface in the evening and cooler during the day, the temperatures recorded in narrow fractures were more closely correlated with surface temperature than in larger fractures. Our data suggests that all fractures on the exposed rockwall of the escarpment are subject to surface-fracture temperature dynamics which shift with aperture size and may exacerbate or moderate thermal weathering processes.