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Abstract

The c. 1.1 Ga Copper Harbor and Nonesuch Formations of the Keweenawan Supergroup exposed along theCanadian-United States shorelines of Lake Superior are part of the surface exposures of the LaurentianMidcontinent Rift. These units have long been considered non-marine in origin and have figured prominently inideas regarding the evolution of microbial life and the redox conditions of Earth’s ocean-atmosphere system atthe close of Mesoproterozoic time. However, these rocks also host hydrothermal metal deposits, the emplacementof which may have compromised primary geochemical signals that are used to underpin those ideas. Herewe highlight new sedimentological observations to provide an independent framework for assessing the depositionalsetting and geochemistry of those strata. We show that the totality of sedimentological features leadsto the conclusion that parts of the upper Copper Harbor Formation and the entirety of the Nonesuch Formationwere deposited along tide- and wave-influenced shorelines and in shallow-marine settings under evaporiticconditions. Evidence for this interpretation includes the abundance of flaser, wavy, linsen and pinstripe bedding,ubiquity of reactivation surfaces and mud drapes associated with all ripple forms, superposed sets of ripple crosslaminationshowing bimodal (herring-bone) sediment transport directions, desiccation cracks and metre-scalehummocky cross-stratification. Further, evaporite fabrics and pseudomorphs after gypsum in the upper 200m ofthe Copper Harbor Formation and in numerous stratigraphic positions within the Nonesuch Formation indicatethat the water body was saline, not fresh. The emerging palaeogeographic image is one of a large, shallowmarineembayment with fringing sabkha-like shorelines. Ideas about late Mesoproterozoic biospheric evolutionand Earth’s surface redox and oxygenation that rely on the Nonesuch Formation and Copper Harbor stromatoliteshaving been deposited within a lacustrine setting require reassessment.

Item Type:	Article
Authors/Creators:	Jones, SM and Prave, AR and Raub, TD and Cloutier, J and Stueken, EE and Rose, CV and Linnekogel, S and Nazarov, K
Keywords:	Midcontinent Rift, Keweenawan Supergroup, Copper Harbor Formation, lacustrine, Nonesuch Formation, Mesoproterozoic
Journal or Publication Title:	Precambrian Research
Publisher:	Elsevier BV
ISSN:	0301-9268
DOI / ID Number:	https://doi.org/10.1016/j.precamres.2019.105510
Copyright Information:	© 2019 Elsevier B.V. All rights reserved.
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