Devonian Alberta

Alberta lay close to the equator during the Devonian Epoch and was mostly covered by alternately expanding and receding seas.  The resulting rocks were largely limestones with some shales.  In the Williston Basin (see map below) extensive reefs were developed.  Arid conditions and restricted circulation in reef-enclosed basins resulted in deposition of impressive thicknesses of gypsum and salt.  The reefs provided permeable structures into which petroleum migrated, resulting in the rich oil deposits in what we now call the Western Sedimentary Basin.