Microfossils, Macrofossils, and Stromatolites from Middle Proterozoic Belt Supergroup, Montana: ABSTRACT

Abstract

Essentially unmetamorphosed strata exposed in the eastern part of the Beltian basin contain a wide variety of middle Proterozoic fossils and stromatolites. Organically preserved microfossils are abundant in shales of the Chamberlain Shale and Newland Limestone (ca. 1,400 m.y.) in the Little Belt Mountains. This assemblage includes tubular filaments which appear to represent mainly the preserved sheaths of Lyngbya-like oscillatoriacean cyanophytes and sphaeromorphs which might in turn represent the preserved outer sheaths of colonial coccoid cyanophytes or possibly the encystment stages of eukaryotic algae. The sphaeromorphs include forms (e.g., Kildinella sp.) which are potentially useful for intercontinental biostratigraphic correlation. Tabular microstructure defined by hematite particles and probably representing outlines of sheaths of oscillatoriacean cyanophytes in calcareous stromatolites of the Snowslip Formation (ca. 1,100 m.y.) in Glacier National Park represent one of the few occurrences of Proterozoic microfossils preserved within wholly calcareous rocks. Microfossils also occur in association with syngenetic sulfides in shales of the Appekunny Argillite (ca. 1,300 m.y.) in Glacier National Park. Macroscopic carbonaceous compressions occur in shales of the Newland Limestone (ca. 1,400 m.y.) in the Little Belt Mountains and probably represent macroscopic, and possibly eukaryotic, algae. Stromatolites are particularly abundant, diverse, and well exposed in strata ranging from 1,400 to 1,100 m.y. old in Glacier National Park. In addition to having paleoenvironmental significance, inclined conical stromatolites provide paleocurrent information useful in basin analysis, and branched columnar forms are potentially useful time-stratigraphic indicators. End_of_Article - Last_Page 468------------