2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 4
Presentation Time: 8:00 AM-12:00 PM


GINN, Fawn M., LAST, William M. and HALDEN, Norman M., Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, MB R3T 2N2, Canada, fm_ginn@umanitoba.ca

Manito Lake is a large, perennial, Na-SO4 dominated saline to hypersaline lake located in the northern Great Plains of western Canada. Significant water level decrease over the past several decades has led to reduction in volume and surface area, as well as an increase in salinity. Today, the lake is 15% of its mid-20th century volume and 46% of its former area. The salinity has increased from 10 ppt to about 50 ppt TDS.

This decrease in water level has exposed large areas of nearshore microbialites. These organogenic structures range in size from several cm to over a meter and often form large bioherms several meters high. They have various external morphologies, vary in mineralogical composition, and show a variety of internal fabrics from finely laminated to massive and clotted. Dolomite and aragonite are the most common minerals found in these structures, however, monohydrocalcite, magnesian calcite, and hydromagnesite are also present. The dolomite is nonstoichiometric and calcium rich; the magnesian calcite is about 6 mol% MgCO3. AMS and bulk radiocarbon dating has indicated an age of some of these structures to be as much as 920 14C yr, but there is abundant evidence for modern carbonate mineral precipitation.

Carbonate mineral formation /diagenesis associated with microbialites is a topic that has captivated the attention of Earth scientists for decades. There has been considerable debate about the fundamental mechanisms by which the carbonate structures are forming and even whether or not there is biogenic mediation involved. In Manito Lake, petrographic evidence confirms a strong organic involvement in the formation of these structures. Nonetheless, inorganic and trapping mechanisms also appear to play a role.

Variations in the morphology, mineralogy and elemental composition with respect to the fabric of the microbialites give us valuable insight into the recent environmental history of the lake.