Abstract: Geochemical indication of late Holocene Evolution of Little Manitou Lake, Saskatchewan, Canada

Lisa Sack, W. M. Last

Little Manitou Lake is a relatively large, perennial, hypersaline lake that occupies a long meltwater channel in the northern Great Plains of western Canada about 100 km southeast of Saskatoon, Saskatchewan. Most of the modern and Holocene sediment in the lake has been generated within the basin itself, either by endogenic inorganic precipitation or by authigenic processes. These endogenic and authigenic precipitates, composed of mainly very soluble sulphate salts and sparingly soluble carbonates, provide an explicit record of chemical and hydrological fluctuations which have occurred over the past 2000 years in the basin. These fluctuations, in turn, may be interpreted with respect to brine evolution and are ultimately related to climatic fluctuations in the region. Brine composition in Little Manitou has fluctuated between Na-SO₄ and Mg-Na-SO₄-Cl types during the past 1000 years, while water levels and overall salinities have remained relatively constant Over the past 70 years, however, Little Manitou has experienced significant changes in water level and brine composition, with salinities ranging from less than 70 ppt to greater than 200 ppt.

Variations in mineral composition and chemistry of the sediment indicate that Little Manitou Lake was a relatively deep, freshwater lake about 2000 years ago. However, by about 1900 yr B.P. a shallower and more saline water body existed in the basin. Water levels gradually increased after about 1500 years ago resulting in the development of a meromicitic, saline to hypersaline lake characterized by periodic carbonate (aragonite) whitings. Water levels

again decreased about 1000 years ago, resulting in the breakdown of meromixis and the initiation of subaqueous evaporitic salt precipitation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994