Article: Palaeoenvironmental distribution of Proterozoic microfossils, with an example from the Agu Bay Formation, Baffin Island
Nicholas J. Butterfield and Frederick W. Chandler
A shale sample from the Black Shale Member of the c. 1250 Ma Agu Bay Formation, Fury and Hecla Group, north-west Baffin Island contains abundant, well-preserved microfossils. The assemblage is dominated by small leiosphaerid acritarchs of which c. 15 per cent show structures here interpreted as medial split release structures. Colonial unicells and larger spheroidal acritarchs are uncommon, and filamentous microfossils extremely rare. A single specimen of the highly distinctive acritarch Valeria lophostriata extends the geographic range of this taxon and, in concert with geochronological and chemostratigraphical data, constrains the timing of Fury and Hecla deposition to the early Late Riphean. The overall habit of the Black Shale Member assemblage, including the even bedding plane distribution of fossils, supports the sedimentological and stratigraphical evidence for a mid to outer shelf depositional environment. Incorporating these findings into a review of Proterozoic shale-hosted microfossils reveals a distinct depth/diversity trend in assemblage structure such that five broad zones can be recognized extending from restricted nearshore to basinal environments. The depositional environments of the Neoproterozoic Mineral Fork Formation, Utah, and the terminal Proterozoic Pertatataka Formation, Australia, are reconsidered using this palaeoecological measure of depositional environment.