Paleoenvironmental implications of an expanded microfossil 1 assemblage from the Chamberlain Formation, Belt Supergroup, 2 Montana 3 Zachary R. Adam 1,2 , Mark L. Skidmore 1 , David W. Mogk 1 4 1 Department of Earth Sciences, Montana State University, Bozeman, MT, USA 5 2 Blue Marble Space Institute of Science, Seattle, WA, USA 6 Correspondence should be directed to: zach@bmsis.org 7 8 ABSTRACT 9 The Chamberlain Formation has previously yielded a low-diversity assemblage of early 10 Mesoproterozoic microfossils, but the reported fossils were of limited utility for inferring 11 paleoenvironmental conditions. Here we describe substantially more diverse microfossil 12 assemblages from drillcore of the Chamberlain Formation obtained from the Black Butte mine 13 locality near White Sulphur Springs, Montana. The Chamberlain Formation biota contain 14 abundant Valeria, Leiosphaeridia, Synsphaeridium and Lineaforma, with lesser amounts of 15 Satka, Symplassosphaeridium and Coniunctiophycus. The assemblages partially overlap with, 16 but are distinct from, microfossils recently reported from the Greyson Formation, another unit 17 from the Helena Embayment of the Belt Supergroup. Since the overlapping taxa exhibit similar 18 states of preservation but dissimilar relative abundances, we interpret the assemblages as 19 reflective of distinct paleoenvironmental conditions of the sampled sections of the Chamberlain 20 and Greyson Formations. The Chamberlain Formation assemblages are most comparable to 21 microfossil groupings reported from the Bylot Supergroup of Canada and the Roper Group of 22