Microbiotas of the Late Precambrian Hunnberg Formation, Nordaustlandet, Svalbard

Abstract

A distinctive carbonate shallowing-upward sequence occurs within the Upper Riphean (750-800 Ma) Hunnberg Formation of Nordaustlandet, Svalbard. Clastic limestones deposited in an open coastal marine environment pass upward into columnar stromatolitic bioherms, and these, in turn, are overlain by finely laminated lagoonal dolomites. Early diagenetic silicification has preserved microbiotas in each of the three major facies. The lagoonal biota includes a low diversity planktonic assemblage in which two species are overwhelmingly dominant. In strong contrast, the plankton assemblage found in open coastal cherts contains more than two dozen taxa. Importance values are far more equably distributed in this assemblage. Few microfossils are preserved in the biohermal stromatolites, but intercolumnar spaces contain abundant fossils whose taxonomic composition and abundance distribution is intermediate between those of the open coastal and lagoonal biotas. Fossil assemblages of the Hunnberg Formation are useful biostratigraphically; the open coastal plankton biota is clearly latest Riphean in aspect. They also contribute to our understanding of the paleoecological distribution of late Precambrian plankton. This distribution is of potential value in paleoenvironmental reconstruction and is equally important in helping to define the ecological context in which evolutionary interpretations of early microbes must be made. The Hunnberg biota is also important in that it combines features of both "chert facies" and "shale facies" Proterozoic microbiotas and, thus, helps to elucidate the similarities and differences between these two types of microfossil assemblages. The Hunnberg microbiota contains 33 taxa, of which two are formally described as new: <i>Cymatiosphaeroides kullingii</i> n. gen. et n. sp. and <i>Trachyhystri-chosphaera vidalii</i> n. sp.