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

By
Sarah P. Slotznick
Sarah P. Slotznick
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA
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Don Winston
Don Winston
Department of Geosciences, University of Montana, Missoula, Montana 59812, USA
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Samuel M. Webb
Samuel M. Webb
Stanford Synchrotron Radiation Lightsource, Menlo Park, California 94025, USA
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Joseph L. Kirschvink
Joseph L. Kirschvink
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA, and Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Toyko 152-8550, Japan
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Woodward W. Fischer
Woodward W. Fischer
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA
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Published:
September 01, 2016

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.

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GSA Special Papers

Belt Basin: Window to Mesoproterozoic Earth

John S. MacLean
John S. MacLean
Department of Physical Science, Southern Utah University, 351 W. University Boulevard, Cedar City, Utah 84720, USA
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James W. Sears
James W. Sears
Department of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, Montana 59812-1296, USA
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Geological Society of America
Volume
522
ISBN print:
9780813725222
Publication date:
September 01, 2016

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