Abstract

The Yeoman and Herald formations of the Late Ordovician Red River succession of southeastern Saskatchewan comprise two carbonate–evaporite cycles in the epicontinental Williston Basin. Although these units possess an overall ‘layer-cake’ aspect across a broad area, thickness variations in the Herald Formation show that accumulation was affected by syndepositional flexure, differential subsidence, and displacement of basement faults thought to have been oriented orthogonally NE–SW and NW–SE. The dolomudstones and anhydrites of the Lake Alma and Coronach members host a variety of synsedimentary deformation features—seismites—interpreted to have been induced by earthquakes generated by movements of these faults, and the low-energy, subaqueous environmental setting was conducive to their preservation.

Individual deformed intervals are <0.2 m thick. Folds and small-scale pinch-and-swell structures indicate plastic behaviour of both carbonate and evaporite sediment under compression and shear stress. Brittle failure of stiffer material from compression or extension is shown by microfaults, cracks, disrupted layers, pseudo-intraclasts, breccias and cataclastites. Dikelets in interlaminated dolomudstone and anhydrite record cyclically elevated pore pressure that caused dewatering and shrinkage with concomitant fluidization and injection of carbonate mud and primary granular gypsum. Enterolithic anhydrite exhibits compressed vertical dikes that likely represent injection of granular gypsum. Clay-filled shear-vein arrays occur rarely in dolomudstone. Seismites are absent in bioturbated limestones except for one horizon of molar-tooth structure.

The different kinds of seismites are related to the varying rheology caused by the primary nature of the carbonate and evaporite sediment plus modifications due to deformation-induced dewatering, early diagenesis, and confinement under shallow burial, as well as earthquake magnitude. A minimum shaking intensity of MM=VI was probably required, meaning that events of lesser magnitude were not recorded. Some seismite-bearing horizons can be correlated across the study region. Within the limitations of core control, their geographic distribution varies, probably in relation to the basement faults. Seismites thus serve as proxy signals of syndepositional tectonic activity but also point to components of the basement structural fabric that, if reactivated later, might induce fracture porosity or influence subsurface fluid flow. Syndepositional tectonism undoubtedly had a much more profound influence on many successions than is presently thought, and its effects are more widespread than currently appreciated.