M-193 Bedrock Geology of the Cavity Lake Fire Area, Boundary Waters Canoe Area Wilderness, Northeastern Minnesota

Abstract

The bedrock geology in this part of the Boundary Waters Canoe Area Wilderness is extremely diverse and unusually well exposed (Fig. 1). Many parts of this area were mapped to varied levels of detail in the 1930s, and more recently in the 1970s and 1980s (see Index Map), with efforts focused primarily along waterways. A severe windstorm in 1999 blew down trees in much of the region, and a delayed result in 2006 was the Cavity Lake forest fire. The fire exposed bedrock and allowed comparatively unencumbered access to interior parts of the map area, creating a unique and time-sensitive opportunity for mapping. Fieldwork and compilation of prior mapping was conducted in 2007 and 2008, and a preliminary map was produced (Jirsa and Starns, 2008). That map is revised here to incorporate subsequent fieldwork and geochronologic analyses, including one new date acquired by coauthor Schmitz. The map portrays bedrock that represents crustal evolution spanning the Neoarchean to Mesoproterozoic Eras (Fig. 2), with an emphasis on structural and stratigraphic relationships in the Neoarchean portion. Neoarchean greenstone-granite terrane of the Wawa subprovince of the Superior Province is represented by a succession of mostly mafic to ultramafic metavolcanic rocks (circa [ca.] 2,700 Ma), unconformably overlain by calc-alkalic volcanic and volcaniclastic rocks (ca. 2,690 Ma), and intruded by the Saganaga Tonalite (also ca. 2,690 Ma).