A New Location of Modern and Holocene Lacustrine Dolomite Formation: Manito Lake, Saskatchewan, Canada

Major advances have occurred in our understanding of modern dolomite formation and penecontemporaneous dolomitization over the past several decades. The northern Great Plains region of western Canada contains a wide range of carbonate-precipitating lacustrine settings, ranging from deep, meromictic perennial lakes to shallow, hypersaline playas. Although the NGP limnological system is sulfate dominated, nearly every saline lake in the region is supersaturated with respect to common carbonate minerals like calcite and aragonite. Non-detrital Holocene dolomite occurs in only ten basins throughout this large region. Foremost among these dolomite-precipitating lakes is Manito Lake, located in west-central Saskatchewan. This is a large (65 km²), deep (z_max: 22 m) perennial saline (~45 ppt TDS) lake in which modern and late Holocene dolomite coexists with other endogenic and authigenic carbonate precipitates, including aragonite, monohydrocalcite, calcite, and Mg-calcite. Like many other lacustrine dolomites, Manito Lake dolomite is microcrystalline (less than 1 μm to 5 μm), Ca-rich and poor to well ordered. It occurs as relatively pure hardgrounds and as a component of nearshore microbialites. It also forms isopachous cements in consolidated siliciclastic shoreline sediments. It has been shown that the ¹⁴C reservoir effect is negligible in Manito Lake, thus radiocarbon dating provides an accurate estimate of carbonate mineral genesis.

Although we have not yet demonstrated inorganic or biomediated precipitation of dolomite in the open offshore areas of the lake, there is no petrographic or geochemical evidence to suggest the mineral formed by diagenetic replacement of a precursor carbonate. Manito Lake dolomite is most likely forming by primary inorganic and biomediated precipitation at or near the sediment-water interface (i) in pore spaces of coarse siliciclastic sediments (i.e., beachrock), (ii) as fine laminae associated with microbialites, and (iii) as a major component of mudstone hardgrounds and pavements.