ABSTRACT

Silcrete is widespread in the Thelon Basin and lower Coppermine Homocline, two Early to Middle Proterozoic cratonic cover sequences in the northwest Canadian Shield. In the Thelon Basin, silcrete occurs as silicified lateritic weathering profiles, weathered tops of rhyolite flows, and silicified terrestrial volcaniclastic sediments. In Coppermine Homocline, silcrete comprises silica-cemented rock fans at the basal unconformity, silicified evaporitic fracture-fills in basement, and silicified ephemeral lake sediments. Petrographic studies of the silcretes indicate that they are strikingly similar to Cenozoic silcretes from southern Africa and Australia formed under arid climates. The silcretes in both Proterozoic sequences are associated with climate-sensitive sediments, such as eolian tes and evaporites, that also suggest an arid environment of deposition. Geochronologic data indicate that the silcretes represent part of an arid paleogeographic province that existed in the northwest Canadian Shield at about 1.85-1.7 Ga. Although the Proterozoic silcretes are petrographically similar to Cenozoic arid climate analogs, the lack of land plants in the Precambrian suggests that geochemical criteria used to distinguish Cenozoic arid and humid climate silcretes may not be applicable to Precambrian examples.