RI-07 Stratigraphy and Petrology of the Type Fond du Lac Formation Duluth, Minnesota

Abstract

The Fond du Lac Formation of Upper Keweenawan (Precambrian) age is exposed along the St. Louis River near Duluth, Minnesota. It is inferred to be more than 800 feet thick in this area, but only 300 feet are exposed. The formation consists predominantly of lenticular beds of red sandstone and siltstone and interbedded shale, but conglomerate beds containing clasts of vein quartz, basalt, felsite, chert, and quartzite are common. Physically isolated exposures of quartz-pebble conglomerate previously assigned to the Lower Keweenawan are reassigned to the Fond du Lac Formation because the conglomerate (1) grades into sandstone and shale that are similar in lithology to definite Fond du Lac strata, and (2) it contains heavy minerals identical to those found in the Fond du Lac sandstones. The sandstone of the Fond du Lac Formation is arkosic or subarkosic, consisting of 36-68 percent quartz, 5-29 percent feldspar, 1-10 percent rock fragments, 1-15 percent matrix material compos ed of quartz, illite, chlorite, and rare kaolinite and biotite, and 1-20 percent cement of hematite, calcite, quartz and dolomite. Heavy minerals of the Fond du Lac Formation are leucoxene aggregates, apatite, tourmaline, zircon, magnetite- ilmenite, and garnet. The siltstone and shale, although finegrained, are mineralogically equivalent to the sandstone. Analysis of cross - bedding and other sedimentary directional indicators imply that the formation was derived from a terrane consisting of igneous, high-grade metamorphic, and sedimentary rocks that probably was situated to the west of the Fond du Lac basin of deposition. Although the basal quartz-pebble conglomerate was deposited on an irregular surface during a transgressive sea in early Fond du Lac time, most of the formation was deposited by fluvial-deltaic processes, as indicated by the presence of filled channels, intraformational fragments, mud cracks, ripple marks, rain imprints, and extensive large - and small-scale cross - bedding.