A marine origin for the late Mesoproterozoic Copper Harbor and Nonesuch Formations of the Midcontinent Rift of Laurentia

Abstract

The c. 1.1 Ga Copper Harbor and Nonesuch formations of the Keweenawan Supergroup exposed along the Canadian-United States shorelines of Lake Superior are part of the surface exposures of the Laurentian Midcontinent Rift. These units have long been considered non-marine in origin and have figured prominently in ideas regarding the evolution of microbial life and the redox conditions of Earth's ocean-atmosphere system at the close of Mesoproterozoic time. However, these rocks also host hydrothermal metal deposits, the emplacement of which may have compromised primary geochemical signals that are used to underpin those ideas. Here we highlight new sedimentological observations to provide an independent framework for assessing the depositional setting and geochemistry of those strata. We show that the totality of sedimentological features leads to the conclusion that parts of the upper Copper Harbor Formation and the entirety of the Nonesuch Formation were deposited along tide-and waveinfluenced shorelines and in shallow-marine settings under evaporitic conditions. 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 cross-lamination showing bimodal (herring-bone) sediment transport directions, desiccation cracks and metre-scale hummocky cross-stratification. Further, evaporite fabrics and pseudomorphs after gypsum in the upper 200 m of the Copper Harbor Formation and in numerous stratigraphic positions within the Nonesuch Formation indicate that the water body was saline, not fresh. The emerging palaeogeographic image is one of a large, shallow-marine embayment with fringing sabkha-like shorelines. Ideas about late Mesoproterozoic biospheric evolution and Earth's surface redox and oxygenation that rely on the Nonesuch Formation and Copper Harbor stromatolites having been deposited within a lacustrine setting require reassessment.

The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago

“…Volcanism continued in the rift basin until at least 1803.52 Ma ± 0.23 myr to the east of the Keweenaw Peninsula on Michipecoten Island. An undated ash bed was observed in the Copper Harbor Conglomerate c. 20 m below the base of the Nonesuch Formation in a core from well JBY-002 in the northeastern Keweenaw Peninsula (Jones et al 2020). The end of volcanic activity therefore overlapped the start of a period of clastic sedimentation that includes the Nonesuch Formation, but sedimentation of the shale itself appears to have started shortly after the end of surficial igneous activity.…”

“…The Nonesuch Formation contains numerous sedimentary indicators of shallow water deposition, including extensive microbially induced sedimentary structures (Sheldon 2011), desiccation polygons ( Fig. 1d), mud cracks (Jones et al 2020) and raindrop impressions ( Fig. 1e).…”

“…These and other sedimentary fabrics may occur in a range of shallow water habitats and are not necessarily diagnostic of lacustrine settings. This, along with purported geochemical indicators of saline conditions, have led to proposals that the Nonesuch formed in shelfal (Jones et al 2020)or perhaps estuarine (Stüeken et al 2020) settings. However, new mineralogical studies focused on the redox chemistry of the Nonesuch environment continue to support a lacustrine interpretation (Slotznick et al 2018), whereas the geochemical evidence suggestive of marine conditions may actually be an artefact of the associated hydrothermal activity and copper mineralization.…”

“…Recent questioning of the lacustrine nature of the Nonesuch Formation (Jones et al 2020;Stüeken et al 2020) is a reminder of the difficulty in proving a freshwater origin for rocks that lack fossils of an unambiguous terrestrial provenance. However, biological clues can help to distinguish between marine and non-marine depositional settings.…”

Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide

“…While these arguments are indeed consistent with a lacustrine interpretation, they cannot rule out a marine influence. Jones et al (2020) recently documented sedimentary features, including hummocky cross-stratification and tidal indicators, that are most parsimoniously explained by an open connection to the global ocean. The modern geographic setting of the basin does not prohibit a marine connection during the time of basinal deposition, perhaps akin to large modern bays and seaways.…”