SPELEOTHEM DEPOSITS IN A PROTEROZOIC PALEOKARST, MESOPROTEROZOIC DISMAL LAKES GROUP, ARCTIC CANADA

A well-preserved paleokarst within the Mesoproterozoic Dismal Lakes Group, Arctic Canada, displays extensive fracturing of the host dolostone, grikes that extend up to 12 m into underlying bedrock, and spectacular cave-collapse breccias up to 35 m thick. Karst development is interpreted to have occurred during an abrupt fall in relative sea level resulting from localized tectonic uplift during emplacement of the ~1.27 Ga Muskox igneous intrusion. Sedimentary deposits within the grike network include flowstone, pisoids, carbonate microspar, and detrital quartz sand/silt, and represent one of the oldest known speleothem occurrences. Here we use petrographic and cathodoluminesence analysis to determine the depositional history of these speleothem deposits.

Primary speleothem deposition consists of a thin layer of detrital quartz cemented with micrite. Quartz grains aligned with their long axes parallel to both vertical and horizontal surfaces and the dull luminescence of micrite suggests subaerial transport of silt, grain adhesion to bedrock via a thin fluid film, and carbonate formation during evaporation of marine-derived pore fluids. Secondary speleothem deposition consists of 1-5 mm thick flowstone. Pendant nature of cements and bright luminescence suggests vadose deposition from non-marine fluids. Tertiary and later speleothem generations are marked by a complex assemblage of pisoids, carbonate microspar, and at least two generations of detrital quartz. Angular quartz silt occurs as discrete, subhorizontal layers and is interpreted as having been deposited from episodic flow of surface waters through the grike system. The final stage of speleothem formation consists of unsorted, well-rounded quartz sand and carbonate pisoids cemented by dully luminescent carbonate microspar. Adhesion of some quartz grains along pisoid margins suggests a combination of carbonate precipitation in standing pools, transport of both carbonate and siliciclastic elements, and ultimate deposition in fluid-filled grikes. Together, these observations suggest speleothem deposition in a variety of marine, phreatic (?), and vadose environments. Isotopic and elemental analysis of carbonate phases is currently underway to further characterize depositional environments.