Palynostratigraphy of Ordovician strata, Canning Basin, Western Australia. Part One: acritarchs and prasinophytes

This paper is the initial part of a comprehensive bipartite monograph of palynomorphs (viz., acritarchs, prasinophyte phycomata, and chitinozoans) that are represented profusely in marine lower Palaeozoic strata of the Canning Basin, Western Australia. The prime aim is to establish a palynologically based zonal scheme for the Ordovician sequence as represented in five cored boreholes drilled through the Lower to Middle Ordovician strata of the central-northeastern Canning Basin. These strata embrace the Oepikodus communis through Phragmodus-Plectodina conodont zonal interval and comprise (in ascending order) the Willara, Goldwyer, and Nita formations, of inferred early Arenig to Llanvirn age. All three formations contain moderately diverse and variably preserved palynomorphs. The palynomorph taxa, detailed systematically in the current Part One of this monograph, comprise 66 species of acritarchs and six of prasinophytes. Of these, two species of prasinophytes and 11 of acritarchs are newly established: Cymatiosphaera meandrica and Pterospermella franciniae; Aremoricanium hyalinum, A. solaris, Baltisphaeridium tenuicomatum, Gorgonisphaeridium crebrum, Lophosphaeridium aequalium, L. aspersum, Micrhystridium infrequens, Pylantios hadrus, Sertulidium amplexum, Striatotheca indistincta, and Tribulidium globosum. Pylantios (typified by P. hadrus), Sertulidium (typified by S. amplexum), and Tribulidium (typified by T globosum); are defined as new acritarch genera. Three new combinations are instituted: Baltisphaeridium pugiatum (PLAYFORD & MARTIN 1984), Polygonium canningianum (COMRAZ & PENIGUEL 1972), and Sacculidium furtivum (PLAYFORD & MARTIN 1984); and Ammonidium macilentum PLAYFORD & MARTIN 1984 and Sacculidium furtivum (PLAYFORD & MARTIN 1984) are emended. An appreciable number of palynomorph species are not formally named owing to lack of sufficient or adequately preserved specimens; others are compared but not positively identified with previously instituted species. The ensuing Part Two of this study will complete the systematic-descriptive documentation, i.e., chitinozoans, and evaluate the Canning Basin palynoflora in terms of its chronological and stratigraphic-correlative significance.

Palynostratigraphy of Ordovician strata, Canning Basin, Western Australia. Part Two: chitinozoans and biostratigraphy

This second and concluding part of a comprehensive palynological study of the Lower to Middle Ordovician succession of the central-northeastern Canning Basin completes the systematic documentation of the palynomorphs, i.e., chitinozoans, and formulates a palynostratigraphic zonation scheme embracing all three constituent formations of this investigation, viz., the Willara, Goldwyer, and Nita formations. A total of 21 species of chitinozoans (five genera), detailed systematically herein, are identified. Although chitinozoan recovery per sample proved variable, the following species occur fairly persistently in the productive samples: Belonechitina micracantha, Conochitina subcylindrica, C. poumoti, C. langei, Calpichitina windjana, and Rhabdochitina magna. Five, stratigraphically successive acritarch/prasinophyte assemblage zones, ranging in age from early Arenig through late Llanvirn, are proposed as follows (ascending order): Athabascaella rossii Assemblage Zone (corresponding to the lower Willara Formation; and dated as early-mid Arenig); Comasphaeridium setaricum Assemblage Zone (upper Willara and lowermost Goldwyer; late Arenig-earliest Llanvirn); Sacculidium aduncum Assemblage Zone (lower Goldwyer; early Llanvirn); Aremorica-nium solaris Assemblage Zone (middle and lower upper Goldwyer; mid Llanvirn); and Dactylofusa striatogranulata Assemblage Zone (upper Goldwyer and lower Nita; late Llanvirn). Four chitinozoan assemblage zones, stratigraphically coinciding (within the limits of sampling) with the acritarch/prasinophyte zones, comprise (in ascending order): Lagenochitina combazi Assemblage Zone (equivalent to the A. rossii and L. heterorhabda Assemblage Zones); Conochitina langei Assemblage Zone; Conocbitina subcylindrica Assemblage Zone; and Belonecbitina micracantha Assemblage Zone. Chronostratigraphic assignments are based principally on associated conodont and graptolite faunas. Whereas the acritarch/prasinophyte zones bear scant similarities to those established globally elsewhere, the chitinozoan zones show significant affiliations with those known from Laurentia.

New Pliocene and Pleistocene acritarchs from the North Atlantic, Arctic Ocean and Bering Sea

Acritarchs have received limited attention in palynological studies of the Cenozoic, although they have much potential both for refining Neogene and Quaternary stratigraphy, especially in mid- and high northern latitudes, and developing palaeoceanographical reconstructions. Here we formally describe and document the stratigraphical and palaeotemperature ranges (from foraminiferal Mg/Ca) of four new acritarch species: Cymatiosphaera? aegirii sp. nov., Cymatiosphaera? fensomei sp. nov., Cymatiosphaera? icenorum sp. nov. and Lavradosphaera canalis sp. nov. In reviewing the stratigraphical distributions of all species of the genus Lavradosphaera De Schepper & Head, 2008, we demonstrate their correlation potential between the North Atlantic and Bering Sea in the Pliocene. Additionally, Lavradosphaera lucifer De Schepper & Head, 2008 and Lavradosphaera canalis sp. nov., while not themselves overlapping stratigraphically, have morphological intermediates that do partially overlap and may represent an evolutionary trend consequent upon climate cooling in the Late Pliocene. Finally, we show that the highest abundances of the acritarchs presented here were living in the eastern North Atlantic, in surface-water temperatures not very different from today.