Silurian Tectonics of Western Avalonia: Strain-Corrected Subsidence History of the Arisaig Group, Nova Scotia

Abstract

The Arisaig Group is a Silurian succession of predominantly shallow marine clastic sediments overlying volcanics, exposed in northern mainland Nova Scotia. Sediment accumulation provides a record of the subsidence of western Avalonia during the interval when terranes were being accreted within the Canadian Appalachians. To calculate the amount of subsidence, one must correct the measured thicknesses for the effects of tectonic strain. Deformed fossils on bedding surfaces indicate strain ratios mainly between 1.2 and 1.6. An empirical porosity-depth relationship is used to correct for compaction. The subsidence curves are then adjusted to allow for variations in water depth and eustatic sea level. The resulting curves show significant variations in subsidence rate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian time, with deeper-water sedimentation during an early Ludlovian eustatic high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidence rate occurred during deposition of the Pridolian Stonehouse Formation. This episode was accompanied by a rapid increase in sediment supply, and a change in paleocurrent flow from southwest to northwest. The rapid Pridolian subsidence probably resulted from oblique collision between Avalonia and the Meguma Terrane to the south.