Clinton B. Foster

Gloeocapsomorpha prisca Zalessky, 1917: A new study: Part I: Taxonomy, Geochemistry, and paleoecology

Abstract Gloeocapsomorpha prisca Zalessky , 1917 is emended and redescribed and a neotype is designated from Middle Ordovician kukersites of the Baltic Shale Basin, Estonia. Gloeocapsomorpha was a colonial, probably cyanobacterial organism and its fossil remains are represented by at least three morphotypes. These morphotypes result from growth and life cycle stages which are overprinted by post-mortem changes. Gas chromatography of hydrocarbons from pyrolysed kukersite yields a diagnostic low molecular weight, odd-dominated suite of n-alkanes, maximizing at C19. The emended diagnosis of Gloeocapsomorpha incorporates these biogeochemical criteria. Morphological and biogeochemical characteristics of Gloeocapsomorpha show strong similarities with certain species of the modern Entophysalidaceae cyanobacteria. Entophysalis major, a mat-forming, and sometimes stromatolite-forming, cyanobacterium, is suggested as a modern analogue for G. prisca.

Gloeocapsomorpha prisca Zalessky, 1917: A new study part II: Origin of Kukersite, a new interpretation

Abstract Kukersite is the name given to exceptionally carbon-rich Middle Ordovician oil shales found in the Baltic Shale Basin of Estonia. The organic component of these sediments consists mostly of accumulations of the microfossil Gloeocapsomorpha prisca which, we propose, was an intertidal, marine, mat- forming benthonic cyanobacterium similar to, but not identical with, the extant cyanobacterium Entophysalis major. Kukersite beds are formed in situ or occur as allochthonous deposits resulting from transport of G. prisca from large intertidal areas by water or wind currents into anoxic offshore environments. It is proposed that the cyclical nature of sediments containing kukersite and marine carbonate facies in the Baltic Shale Basin results from changes in relative sea level, probably caused by both local tectonism and eustatic sea level changes.

An Early Ordovician organic-walled microphytoplankton assemblage from the Nambeet Formation, Canning Basin, Australia: biostratigraphic and paleogeographic significance

A well-preserved and moderately diverse organic-walled microphytoplankton assemblage was recovered from the subsurface Lower Ordovician Nambeet Formation, Canning Basin, Western Australia. The microphytoplankton assemblage consists of prasinophyte phycomata (Leiosphaeridia spp.), acritarchs, cyanobacteria (Eomerismopedia maureeniae), degraded algae, chitinozoans and graptolite fragments. The acritarch assemblage comprises 13 genera, one of which is new (Aciculasphaera), 13 named species, three of which are new (Aciculasphaera interrupta, Gorgonisphaeridium martiniae and Loeblichia nambeetense), five species left in open nomenclature (sp.), and one cf. designation. The acritarch assemblage indicates an Early Ordovician (late Tremodocian through Floian) age and displays some similarities with comparable-age assemblages reported from North and South China, Australia and Laurentia. The paleogeographic distribution of the acritarch assemblage indicates that this assemblage represents a low- to mid-paleolatitude occurrence. Sedimentologic and palynologic evidence signifies deposition in a normal marine offshore environment.