Weathering and paleosol formation in the 1.1 Ga Keweenawan Rift

At ∼1.1 Ga, the North American Craton began to rift, resulting in flood-style basaltic volcanism. During the phase of active tectonism, some of that basalt was weathered and redistributed by fluvial processes. New observations near the northwestern shore of Lake Superior have also revealed the presence of seven paleosols formed on weathered, immature sediments derived from the basalt that are described here for the first time. Only three other paleosols have previously been described from this part of the geologic record, so these new paleosols represent a significant new discovery. The paleosols are weakly developed and exhibit physical and chemical characteristics similar to Phanerozoic Entisols. In contrast to many Precambrian paleosols, the Keweenawen paleosols have been subject to minimal post-burial alteration. Among the paleosols, three distinct pedofacies are recognized: (1) waterlogged/gleyed fluvial-proximal paleosols, (2) dry fluvial-distal paleosols, and (3) cumulative fluvial-proximal paleosols. Despite these facies differences, using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt, with one of the paleosols showing an additional extra-basinal rhyolitic component. By considering the mass-balance behaviour of a variety of alkali and alkaline earth elements, it is possible to rule out significant potassium metasomatism, suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions, and that they form an important new source of data about Mesoproterozoic weathering.