The Geology of the prairie evaporite formation of the Yorkton area of Saskatchewan

Abstract

Beds of grey insolubles alternate with units of clear to grey halite in the lower two-thirds (about 330 feet) of the Prairie Evaporite Formation of the Yorkton area. The upper one-third of the formation consists mainly of clear to grey, anhedral halite containing one or two potash zones. In the potash zones, euhedral halite predominates over anhedral halite. Three types of sylvite, two types of halite, and two types of carnallite occur in the potash zones. These mineral types have characteristic associations. Clear to milky-white sylvite, with red rims of included hematite, is the dominant. sylvite of both. potash zones. It is partly replaced by fine-grained, anhedral, pink sylvite, and is always associated with clear halite but never with carnallite. Large-grained, clear to pinkish sylvite, without hematite rims, occurs in seams in the lower potash zone. This sylvite is associated with mllky-white halite and both halite and sylvite are replaced in part by blood-red, anhedral carnallite. Blood-red carnallite is associated with clear and milky-white halite in the upper potansh zone of Alwinsal Brewer 9-22 well. No sylvite is present and the origin of the carnallite is unknown. The clear halite and milky-white halite of the potash zones, and the clear to grey halite of the rest of the formation, have bromine contents lower than, as well as within, the range for primary halite crystallized from present-day sea water. Values lower than the primary range suggest that solution and redeposition of halite has occuried. Red-rimmed sylvite and large-grained, clear to pinkish sylvite have similar bromine and rubidium values. Rubidium values, below 0.0017 wt. per cent suggest these sylvites formed from pre-existing sylvite; values greater than 0.0020 wt. per cent suggest they formed from pre-existing carnallite. In township 23, range l W. 2 Mer., and in townships 28 and 29, ranges 6 W. 2 and 7 W. 2 Mer., solution of the Prairie Evaporite Formation and collapse of formations below the First White Speckled Shale has occurred. These solution areas underlie "lows" on the Paleozoic erosion surface. Locally, the Paleozoic erosion surface has as much as 550 feet of topographic relief. Blairmore and Lower Colorado sediments are 300-400 feet thicker on the sites of erosional "lows". The erosional "lows", with their thick Cretaceous sequences, are more extensive areally than the solution channels. In seismic investigations, these thick sequences of low velocity sediments cause apparent structural lows, also more extensive areally than actual lows caused by Prairie Evaporite solution. These apparent structural lows, however, may be misinterpreted as signifying areas of Prairie Evaporite solution and areas containing normal potash zones may be overlooked.