Paleontology of proterozoic shales and mudstones: examples from the Belt supergroup, Chuar group and Pahrump group, western USA

Abstract

Proterozoic black shales and other fine-grained siliciclastic rocks contain a variety of fossils. These fossils are illustrated by examples from three Proterozoic units in the western USA—the Belt Supergroup (Middle Proterozoic), the Chuar Group (Late Proterozoic), and the upper Pahrump Group (Late Proterozoic). A variety of microfossils and megafossils occur in the Belt Supergroup, western Montana. Acritarchs and filamentous microfossils are moderately well preserved in sub-greenschist facies black shale in the Little Belt Mountains. The biologic affinity of the acritarchs is uncertain; they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria. Most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities; however, rare 20- and 40-μm-wide filaments could be either prokaryotic or eukaryotic. Higher-grade sub-greenschist facies dark-gray to black mudstones in Glacier National Park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils. They are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism. Several authentic megafossils occur in lower Belt shales in the Little Belt and Big Belt Mountains, Montana. One of these is the carbonaceous compression Beltina danai , which could represent either fragments of a microbial mat or fragments of a sheet-like megascopic alga. The megafossil Grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga. Green to purple mudstones and muddy sandstones in the lower Belt Supergroup in Glacier National Park contain markings that resemble a string of beads; they are interpreted as megafossils that may have eukaryotic algal affinities. Black shales in the Late Proterozoic Kwagunt Formation, Chuar Group, Grand Canyon, Arizona, contain a variety of acritarchs and filamentous microfossils. Of special interest is the vase-shaped microfossil Melanocyrillium , which is interpreted as the sporangium of a eukaryotic and probably megascopic alga. Melanocyrillium also occurs in black mudstone in the Late Proterozoic Pahrump Group, southeastern California.