Stratigraphic and ecologic implications of late Precambrian microfossils from Utah

Abundant and well preserved microfossils have been discovered in black shales and siltstones of the glaciogenic upper Precambrian Mineral Fork Formation ncar Salt Lake City, Utah.The rocks are interpreted to have been deposited close to an ice margin in a shallow, restricted marine embayment during glacial retreat.Several distinct morphotypes are discernible, but following Moorman (1974), we interpret most of these as stages in the life cycle of a single planktonic, endosporulating alga, Bavlillellll Jllveolllta (Shepeicva) Vidal.The low taxonomic diversity of this assemblage, coupled with the large population size and almost complete dominance by Bavlillella, suggests an ecologically stressed environment.The source of this stress was probably the melting glacier.On the basis of stratigraphic position and lithological correlation with a radiometrically dated sequence in \Vashington, the Mineral Fork Formation has been considered to be about 800 m.y.old; however, most well dated occurrences of Bavlillella are in rocks of Vendian (650-570 m.y.) age.The presence of Bavlillella Javeolata in Mineral }-'ork strata raises significant questions concerning both the stratigraphic range of this presumed Vendian index fossil and the timing of Late Precambrian glaciation in the l\'orth American Conlillera.