Characterization of preserved remnants of the Pre-Pleistocene Saprolite on the Canadian Shield: Examples from Northeastern Minnesota.

Abstract

Saprolite has been found in drill core across Minnesota and is overlain by Jurassic, Cretaceous, and glacial sediments. However, in the northeastern part of the State, which is characterized largely by glacially scoured bedrock, saprolite exposures can be found all along the North Shore of Lake Superior and inland where they are most well expressed in cliffside exposures and road cuts. This weathering residuum is typically associated with fault and fracture systems, and in some cases are preserved in the lee of competent rock types. Features of this saprolite include core stones, spherical weathering rinds, grus, and authigenic mineral formation along fractures. The goals of this investigation were to inventory saprolite occurrences and characterize the geochemistry and clay mineralogy with emphasis on two sites where the remnant saprolite is well exposed in outcrops with a complete weathering sequence from least altered to highly weathered zones with authigenic mineral formation. Samples were submitted for major oxide and elemental analysis and mineral species were identified using powder X-Ray diffractometry with emphasis on clay mineralogy. The degree of weathering of samples was estimated using the Chemical Index of Alteration (CIA) (Nesbitt and Young, 1982) and the Chemical Index of Weathering (CIW). Isocon analysis is used to evaluate chemical losses and gains during weathering. At the Crow Creek site, thirty meters of saprolite developed in mafic volcanic and intrusive rocks of the Midcontinent Rift are preserved. The unaltered diabase is composed of plagioclase, orthopyroxene, clinopyroxene, olivine, magnetite, and accessory minerals.The altered rock mineralogy is quartz, chlorite, talc, laumontite, prehnite, smectite, kaolinite, and illite. The Silver Bay site exposes a deeply weathered pendant developed in granophyre. Unaltered granophyre is composed of K-feldspar, augite, apatite, plagioclase, and quartz. The highly altered weathering residuum has abundant iron oxide, calcite, smectite and Kaolinite. The thickness, weathering progression, and authigenic mineral production suggest that these scattered exposures throughout NE Minnesota are the remnant of a once continuous pre-Pleistocene saprolite that covered the Canadian Shield. The age of the weathering is difficult to determine, but weathering on the Shield has been the dominant geological environment at least since the Late Proterozoic about 750 Ma ago, and likely much longer. Therefore, caution should be used when interpreting all of the saprolite in Minnesota as Mesozoic in age.