Shallow faults, Upper Cretaceous clinoforms, and the Colonsay Collapse, Saskatchewan

Abstract

Reflection seismic data in the vicinity of the Colonsay potash mine near Saskatoon, Saskatchewan, reveal numerous shallow normal faults with vertical displacements of as much as 25 m. The faults cut the glacial deposits and Upper Cretaceous rocks to a maximum depth of about 400 m. Horizontal length ranges from a few hundred to a few thousand metres. Similar faults are also exposed in several widely separated open-pit coal mines of southern Saskatchewan and may be common elsewhere in the prairies. Such faulting is considered to be a result of Tertiary to Quaternary extensional tectonics and may have been aided in this region by the melting of gas hydrates. The study area includes the east flank of the Colonsay Collapse, a subsidence structure caused by dissolution and removal of salt in the 200 m thick Devonian Prairie Evaporite Formation. Salt was removed in two stages prior to or during early glacial time and in late-glacial time. Subsidence was gradual, with no observable faulting. A complex of clinoform structures about 120 m thick and prograding east-northeast occurs in the Upper Cretaceous Lea Park Formation in the study area and may correlate to the Alderson Member of southern Saskatchewan. Strong seismic reflections within the complex could be due to gas-filled porosity. Where undermined by salt removal in the Colonsay Collapse, the clinoform structures now form drape antiforms.