Iron mineralogy and redox chemistry of the mesoproterozoic Newland formation in the Helena embayment, belt supergroup, Montana
Slotznick SP, Zieg J, Webb SM, Kirschvink JL, Fischer WW. 2015. Iron mineralogy and redox chemistry of the mesoproterozoic Newland formation in the Helena embayment, belt supergroup, Montana. Northwest Geology. 44:55-72
The Mesoproterozoic era falls in the period between 1.85 to 0.85 Ga called “the boring billion”, sandwiched between the rise of multi-cellularity and animals in the Neoproterozoic and the Great Oxygenation Event and rise of oxygen in the Paleoproterozoic (Holland, 2006). However, important evolutionary developments occurred over the period with the rise of eukaryotes and macroscopic organisms (Knoll and others, 2006). The Belt Supergroup contains some of the early micro- and macrofossils that have been attributed to eukaryotes (Horodyski and others, 1989). One particularly well-studied example is found in the Greyson Shale of the Helena Embayment region in southwestern Montana (fi g. 1a). First identifi ed by Charles Doolittle Walcott (1899) and later renamed (Walter and others, 1976), Grypania spiralis is a curved or coiled ribbon, 0.2 to 1.7 mm wide, and 6 to 14 mm in coiled diameter, and has been interpreted as eukaryotic algae as well consortia of several microscopic organisms (Horodyski and others, 1989; Knoll and others, 2006; Walter and others, 1976).