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Abstract:

Despite more than a century of multidisciplinary study, our understanding of the genesis of evaporite minerals in deep-water sedimentary environments remains poor. This is most likely due to a paucity of modern settings in which deep-water evaporites are forming. Several of the present-day deep, saline to hypersaline lakes in the northern Great Plains of western Canada provide excellent opportunities to examine subaqueous salt mineral formation, and to document the processes by which these salts are deposited, preserved, and altered.

Deadmoose, Little Manitou, and Freefight lakes are lacustrine basins in which soluble and sparingly soluble evaporite minerals are accumulating today in the relatively deep offshore areas of the lakes. Both Deadmoose and Freefight lakes are hypersaline and meromictic, with strongly anoxic bottom waters. Little Manitou is also hypersaline but seasonal density and chemical stratification usually breaks down by early to midsummer. The salts in these basins, which are comprised of a complex assemblage of hydrated sodium and magnesium sulfates and carbonates, reflect, in general terms, the high alkalinity of the brines and the overall dominance of Na+, Mg2+, and SO4 2– ions. Modern sediment–trap data show that salt precipitation and sedimentation are extremely rapid, with the precipitates being derived from a variety of positions in the water columns. Salt accumulation is most rapid in the deepest parts of the lakes. The evaporites are generally coarsely crystalline, massive to thickly bedded, and interfinger laterally with laminated organic-rich clays and poorly sorted carbonate-clastic debris derived from nearshore areas. Even in the shallowest basin, the salts show no indication of dissolution or recycling. Indeed, preliminary augering and coring of the late Holocene sediment reveals that relatively thick sections of evaporites have accumulated and are being preserved in the basins.

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