Triassic-Jurassic of Alberta, Saskatchewan, Manitoba, Montana, and North Dakota

Abstract

Triassic sedimentary rocks in southern Saskatchewan, North Dakota, and southern Montana represent only Early Triassic time. Thicknesses in excess of 700 ft in western North Dakota reflect the depocenters of restricted salt basins. On the basis of lithologic correlation, the Lower Watrous Formation in Saskatchewan is equivalent to the upper part of the Spearfish Formation in North Dakota. Lower, Middle, and Upper Triassic rocks in western Alberta are of marine origin, produce oil and gas, and reach thicknesses in excess of 4,000 ft. Jurassic rocks are widespread because Jurassic seas transgressed from the northwest along the Alberta trough into western Montana, then spread east across Montana into the Williston basin. Lower Jurassic strata are restricted to southwestern Alberta. Middle Jurassic rocks are the most widespread, and are thicker than 500 ft in a depocenter in southeast Saskatchewan and northwest North Dakota. Similar thicknesses are present in the eastern trough. Upper Jurassic strata also are widespread, and are 7,000 ft thick in the Alberta trough and more than 700 ft thick in eastern Montana. Coal, gypsum, oil, and gas are present in commercial quantities in Lower, Middle, and Upper Jurassic formations. Oil is the most significant, particularly in southwestern Saskatchewan and north-central Montana, where 20 fields are estimated to have ultimate production of 411 million bbl. These fields produce primarily from stratigraphic traps in sandstone.