Abstract

The Calumet and Hecla Conglomerate, a Keweenawan interflow sedimentary unit that is ∼1.1 b.y. in age, consists at the Centennial Mine of a felsitic conglomerate upper part and of ∼1–2 m of siltstone and shale along the base. In these fine-grained sediments, there is a complex submature caliche soil profile represented by an upward increase in calcite and corresponding changes in calcite structures. There is also caliche in conglomerate. All of this calcite predates the period of regional epidote alteration. The caliche is interpreted to have developed in slowly aggrading sediments that were deposited on a rock-floored fluvial plain and took place during the initial stages of slope reversal of the lava plain, from southerly to northerly. This kind of carbonate accumulation in sediments requires a highly seasonal, temperate or tropical climate and represents ∼15,000 yr of weathering. The corresponding Keweenawan paleolatitude has been reported to be ∼30°N.

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