Seismites within Ordovician–Silurian carbonates and clastics of Southern Ontario, Canada and implications for intraplate seismicity

A regional survey of Lower Paleozoic sedimentary strata (Ordovician–Silurian) exposed across some 7000 km2 of Southern Ontario, Canada has identified the sedimentary record of paleo-seismicity at a time of known faulting and rapid subsidence of Appalachian foreland basins. Shallow marine shoreface sandstones and shales of the Lower Silurian Thorold Formation show laterally extensive beds of large (< 2 m) synsedimentary ball-and-pillow structures outcropping on the face of the Niagara Escarpment near Hamilton. It is argued that these are seismites produced by ground shaking caused by a moderate intracratonic earthquake. Other comparable deformation structures that can be attributed to seismic activity also occur within sandstone–shale shoreface successions of the Upper Ordovician Georgian Bay, Lower Silurian Grimsby and the dolostone DeCew Formations indicating recurring activity. Later seismic activity is recorded in carbonate strata by deformed rhythmically laminated lagoonal dolostones of the mid-Silurian Eramosa Member of the Guelph Formation consisting of intraformational syndepositional reverse faults and horizons of brecciated laminites indicating compressional stresses and mass flow. Deformation structures are exposed in several quarries and can be correlated in drill cores 15 km distant indicating widespread extent. Deformed dolostones are attributed to co-seismic slumping of lagoonal facies from bioherms and are similar to seismites of the same age in closely adjacent parts of the US. These were attributed to reactivation of underlying Precambrian basement structures such as the Grenville Front Tectonic Zone at times of rapid basin subsidence. Paleozoic seismites in Ontario occur close to basement structures that are seismically active at the present day supporting the emerging model for mid-continental seismicity involving reactivation of Precambrian structures.