Piercement Structures in Canadian Arctic Islands1
The Sverdrup basin in the Queen Elizabeth Islands of northern Canada contains many piercement structures with exposed cores of gypsum and anhydrite. Several cores are more than 10 sq mi in area. Adjacent anticlines may have unexposed evaporite cores. The basin is about 700 mi long and 250 mi wide. It is filled with more than 40,000 ft of Mesozoic clastic strata underlain by possibly 5,000 ft of Pennsylvanian and Permian strata, including reefoid carbonate rock and an evaporite sequence. Salt is not known to be associated with these evaporites, but its presence is suggested by gravity data.
Piercement structures in the western part of the basin are large, domal features which show little or no evidence of tangential compression; they are probably salt domes which resulted from halokinesis or geostatic loading. Ordovician salt is known to be present in the Cornwallis fold belt, which presumably extends under the basin; it may have been involved in the early history of piercement structures in the central part of the basin.
In the eastern part of the basin some piercement structures are large and domal, but most are relatively small and elongate and are associated with major faults. These appear to have resulted from diapirism initiated by tangential forces during the Laramide orogeny.
Figures & Tables
“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.