The oldest recognized proxies for low atmospheric oxygen are massive iron-rich deposits. Following the rise of oxygen ∼2.4 billion years ago (Ga), massive iron formations (IFs) largely disappear from the geologic record, only to reappear in a pulse ∼1.88 Ga, which has been attributed to sea-level transgressions, changing ocean chemistry triggered by intense volcanism, or lowered atmospheric oxygen levels. The North American Gogebic Range is one of the rare records of this pulse and even more uniquely has exposures of both volcanics and IF, providing an ideal field locality to investigate triggers for this pulse of IF. To determine the environmental context and key factors driving IF deposition after the initial rise in oxygen, we made detailed observations of the stratigraphy and facies relationships and present updated mapping relationships of the Gogebic Range Ironwood Iron Formation and the Emperor Volcanics. This work expands existing mine datasets and logs to constrain variations in stratigraphy. Our results are the first to quantitatively constrain thickness variations along the entire Gogebic Range and tie them to syn-sedimentary faulting along listric normal faults and half grabens. Furthermore, our dataset suggests that initiation of intense syn-basinal volcanism linked to a large igneous province does not coincide with initial iron deposition, thus cannot be invoked as a causal trigger. Finally, the possibility of iron deposition in a shallow-water environment suggests that the post-GOE IF pulse may not reflect global marine transgressions, but instead a chemocline shallowing due to changing global atmospheric oxygen.
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We thank the Bergmann group for early comments and C. Condit and D. Ojakangas for helpful discussions. This manuscript benefited from insightful and detailed reviews from Peir Pufahl, Philip Fralick, and an anonymous reviewer. We also appreciate the professional and comprehensive editorial handling by Josh Feinberg. Financial support for this research was provided by the MIT EAPS W. O. Crosby Postdoctoral Fellowship and the Johns Hopkins EPS Morton K. Blaustein Postdoctoral Fellowship to A. Eyster and the Packard Foundation to K. Bergmann. There are no real or perceived financial conflicts of interests for any of the authors.
© 2021. The Authors.