Guidebook 6. Field Trip Guide Book for Precambrian Geology of Northwestern Cook County, Minnesota

Abstract

An exceptionally complete record of Precambrian history is recorded in the rocks exposed in Cook County, Minnesota. In northwestern Cook County, in the vicinity of the Gunflint Trail the Lower Precambrian is represented by a metavolcanic succession, which was intruded by the somewhat younger Saganaga Tonalite. These rocks are unconformably overlain by the Middle Precambrian Animikie Group, consisting of the Gunflint Iron Formation and the Rove Formation. In northeastern Cook County, a gently dipping angular unconformity separates Middle Precambrian and Upper Precambrian strata. There, a thin basal sandstone, the Puckwunge Formation, is overlain by volcanic rocks of the North Shore Group. The Logan intrusions and the Duluth Complex intrude and truncate Middle and Upper Precambrian rocks and comprise the major part of the Upper Precambrian section in northwestern Cook County. Although the geology of Cook County was summarized by Grout and others (1959), geologic mapping since 1962 has considerably revised the earlier geologic interpretation. Because much of this work is unpublished as yet, a comprehensive summary is presented here. The discussion is meant to provide a framework for the specific aspects of the geology which the chosen stops illustrate.Mileages for this trip are listed by stop as distances in miles along Minnesota 12 (The Gunflint Trail) going both northwest from Grand Marais and southeast from Trails End Campground, a round-trip distance of about 120 miles. Figure 1 indicates the location of the Gunflint Trail as well as the general geology of the area. A larger scale geologic map of the field trip area together with all the field trip stops is shown in Figures 2 and 3, while the cross section on Figure 2 and the block diagrams of Figure 4 represent the gross structural relationships between the units encountered on the field trip.