Bedrock Map of the Ester Lake Quadrangle, Lake and Cook County, The Boundary Waters Canoe Area Wilderness, Northeastern Minnesota

Abstract

This map portrays detailed bedrock geology of an 8 mile2 area adjacent to the US-Canadian border in the Ester Lake 7.5 minute quadrangle. The area was incorporated in previous regional scale-mapping by Leith (1910) and Gruner (1941), and parts were mapped in the 1970’s and 1980’s by graduate students of the University of Minnesota Duluth. Historically, mapping was focused on lakeshores and waterways. A wind storm in 1999 and subsequent Cavity Lake forest re of 2006 generated impressive exposure of bedrock and easier access to inland areas. A preliminary geologic map of the fire area was generated by Jirsa and Starns (2008), based on previous work and some localized traverses. Subsequent mapping by Jirsa and students of the Precambrian Research Center during the 2008- 2012 field seasons will allow publication of a final map of the Cavity Lake fire area in 2012. This map depicts diverse geological structure and lithology representing crustal evolution spanning the Archean to Proterozoic Eons. The Archean rocks represent a granite-greenstone terrane within the Wawa subprovince of the Superior Province of the Canadian Shield. The Ester Lake 7.5-minute quadrangle consists of metabasaltic rocks of unknown age, but possibly equivalent to the Ely Greenstone (ca. 2722), the 2690 Ma Saganaga Tonalite (Driese and others, 2011), and arkose, conglomerate, greywacke, slate, and iron-formation of the Knife Lake Group. Field evidence indicates that the Saganaga Tonalite was uplifted and subaerially eroded, generating tonalitic detritus to nearby basins, now manifest as Knife Lake Group strata. All rock types on the map sheet appear to have been metamorphosed to low greenschist facies.