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Published March 16, 2016 | Published
Book Section - Chapter Open

Iron mineralogy and redox conditions during deposition of the mid-Proterozoic Appekunny Formation, Belt Supergroup, Glacier National Park


The redox state of the mid-Proterozoic oceans, lakes, and atmospheres is still debated, but it is vital for understanding the emergence and rise of macroscopic organisms and eukaryotes. The Appekunny Formation, Belt Supergroup, Montana, contains some of these early macrofossils dated between 1.47 Ga and 1.40 Ga and provides a well-preserved record of paleoenvironmental conditions. We analyzed the iron chemistry and mineralogy in samples from Glacier National Park, Montana, by pairing bulk rock magnetic techniques with textural techniques, including light microscopy, scanning electron microscopy, and synchrotron-based X-ray absorption spectroscopy. Field observations of the Appekunny Formation combined with mineralogical information allowed revised correlations of stratigraphic members across the park. However, late diagenetic and/or metasomatic fluids affected primary iron phases, as evidenced by prevalent postdepositional phases including base-metal sulfides. On the west side of the park, pyrrhotite and chlorite rims formed during burial metamorphism in at least two recrystallization events. These complex postdepositional transformations could affect bulk proxies for paleoredox. By pairing bulk and textural techniques, we show primary records of redox chemistry were preserved in early diagenetic and often recrystallized framboidal pyrite, submicron magnetite grains interpreted to be detrital in origin, and red-bed laminae interpreted to record primary detrital oxides. Based on these observations, we hypothesize that the shallow waters of the mid-Proterozoic Belt Basin were similar to those in modern marine and lacustrine waters: fully oxygenated, with detrital reactive iron fluxes that mineralized pyrite during organic diagenesis in suboxic, anoxic, and sulfidic conditions in sedimentary pore waters.

Additional Information

© 2016 The Authors. Gold Open Access: This chapter is published under the terms of the CC-BY license and is available open access on www.gsapubs.org. Manuscript Accepted by the Society 10 December 2015; First published online March 16, 2016. We thank Rob Thomas, Tim Lyons, Austin Chadwick, and Steven Skinner for assistance with stratigraphic measurements and sample collections under National Park Service (NPS) collection permits GLAC-2012-SCI-0195 and GLAC-2014- SCI-0008. Thanks also go to Jim Sears, Johnny MacLean, Erik Sperling, and an anonymous reviewer for helpful comments that greatly improved the manuscript. Support for this work was provided by the Agouron Institute, a Tobacco Root Geological Society scholarship, a Belt Association student research grant, a Geological Society of America student research grant, the National Aeronautics and Space Administration (NASA) Exobiology program (Fischer), the David and Lucile Packard Foundation (Fischer), the National Science Foundation Graduate Research Fellowship program (Slotznick), a NASA Earth and Space Science fellowship (Slotznick), and a Philanthropic Educational Organization (PEO) Scholar Award (Slotznick). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a directorate of the SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University.

Attached Files

Published - Geological_Society_of_America_Special_Papers-2016-Slotznick-2016.2522_09.pdf



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August 20, 2023
January 13, 2024