A New Depositional Framework for Massive Iron Formations after The Great
Oxidation Event
Abstract
The oldest recognized proxies for low atmospheric oxygen are massive
iron-rich deposits. Following the rise of oxygen ~2.4
billion years ago, massive iron formations largely disappear from the
geologic record, only to reappear in a pulse ~1.88 Ga,
which has been attributed to passive margin transgressions, changing
ocean chemistry triggered by intense volcanism, or lowered atmospheric
oxygen levels. The North American Gogebic Range has exposures of both
volcanics and iron formation, providing an ideal field locality to
interrogate the relationship between the lithologies and investigate
triggers for this pulse of iron formation. To determine the
environmental context and key factors driving post-GOE iron formation
deposition, we made detailed observations of the stratigraphy and facies
relationships and present updated mapping relationshipsof 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 datasets suggest that initiation of major local
volcanism does not coincide with iron formation deposition, thus, local
intense volcanism cannot be invoked as a causal trigger. Finally, the
possibility of iron formation deposition in a shallow water environment
suggests that the post-GOE iron formation pulse may not reflect global
marine transgressions, but instead a chemocline shallowing due to
decreased atmospheric oxygen.