can acritarchs be used for ediacaran subdivison?
Uppsala University, Department of Earth Sciences, Palaeobiology, Villavägen 16, SE-752 36 Uppsala, Sweden. Email: Sebastian.Willman@geo.uu.se
The fossil record of the three billion years before the Cambrian is patchy, but shows that early organisms had adapted to a variety of environments and that many major evolutionary innovations such as multicellularity, sexual reproduction and tissue formation occurred early in life history. The fossil record of the earliest prokaryotes is debated (e.g., Schopf, 1993; Brasier et al., 2002) but that of eukaryotic protists is better understood, although by no means well established. Protists and problematic biotas of limited diversity, including e.g., Grypania, are present between 2.5 and 1.5 Ga. Younger rock successions between 1.5 Ga and 750 Ma in age include more morphologically complex fossils such as Chuaria and Tawuia as well as the possible red algae Bangiomorpha (Knoll et al., 2006 gives an up to date summary of Proterozoic eukaryotes). However, it is not until the Ediacaran that organisms of undoubtedly metazoan affinities first appear (e.g., Narbonne, 2005). The Ediacaran biota remains largely problematic and even though the fossils are spread globally their distribution is uneven. Conversely, organic-walled microfossils referred to as acritarchs are commonly recorded throughout Meso- and Neoproterozoic rock successions worldwide.
The Neoproterozoic was a time of major environmental change. At least two, possibly three, and even four global glaciations have been suggested to cover the entire Earth with thick ice during the Cryogenian Period (e.g., Hoffman et al., 1998). Following the last glaciation, the Marinoan glaciation, greenhouse conditions prevailed during the Ediacaran and caused major shifts in ocean geochemistry, oceanic stratification and oxygenation, and evolution of the marine biosphere (Kirschvink, 1992; Canfield, 1998; Shields et al., 1998; Grey, 2005; Fike et al., 2006; Canfield et al., 2007). It was during the Ediacaran that the first major diversification of organic-walled acritarchs occurred. This diversification is preserved in rock successions worldwide, including Siberia, China, Baltica, India and, especially, in the Centralian Superbasin in Australia. Grey et al. (2003) noted that the first appearance of acanthomorphic (ornamented) acritarchs in samples from drillcores in the Officer and Amadeus Basins and the Adelaide Rift Complex, occurred stratigraphically above a bolide ejecta layer, the result of a large impact some 580 Ma years ago (the Acraman impact; Walter et al., 2000).
A problem in Neoproterozoic subdivision has been the lack of biostratigraphic control (Knoll & Walter, 1992), and although Neoproterozoic biostratigraphy is increasingly well-understood it is still a highly debated theme. As a result, anything and everything that can aid in the correlation of Ediacaran successions is especially sought after. The tools for correlation of the Neoproterozoic currently at hand include chemostratigraphy, sedimentology, event stratigraphy (impacts, glacial episodes, volcanic eruptions and the alike), magnetostratigraphy, and the subject of this presentation: Ediacaran biostratigraphy.
The Officer Basin in South Australia is an intracratonic basin that extends some 1400 km in an east-west trend across Western and South Australia. Stratigraphic correlation of the Officer Basin has been based mainly on seismic, magnetic, and gravity studies but was recently reviewed by Grey (2005). The succession studied here (and in Grey, 2005) is the largely siliciclastic lower Ungoolya Group which, in ascending stratigraphic order, consists of the Dey Dey Mudstone, the Karlaya Limestone, and the Tanana Formation. The Dey Dey Mudstone is divided into two units separated by a bed of dolomitic intraclasts; the lower unit is mainly red-brown and occasionally green-grey silty mudstone that was deposited in a fluvial environment and the upper unit is a laminated dolomitic or calcareous siltstone and mudstone deposited in slightly deeper, prodelta and shelf environments (Zang, 1995; Morton, 1997). The Karlaya Limestone consists mainly of thin-bedded micritic limestone with dark grey silty mudstone layers and some limestone intraclasts, deposited on a subtidal shelf, probably below fair-weather wave base (Zang, 1995). The Tanana Formation overlies the Karlaya Limestone and consists of micritic limestone with silty mudstone interbeds deposited in a prodelta and distal delta front to shelf settings (Morton, 1997).
Microfossils were collected from unevenly sampled intervals in the Giles 1, Murnaroo 1, Lake Maurice West 1, WWD 1, Observatory Hill 1, and Munta 1 drillcores. Samples were selected from lithologies that are suitable for palynological preservation (i.e., unoxidized mudstones, shales, and carbonaceous rocks) and collected from both sides of the Acraman impact ejecta layer. The microfossils are permanently fixed in strew mounts and were observed under transmitted light, and documented using a digital camera.
The results of micropalaeontological studies provide further evidence for the acritarch diversification that was so typical for parts of the Ediacaran. Many of the acritarch taxa are stratigraphically constrained and the patterns observed in various drillholes match the patterns first reported by Grey (2005). The new palynological record further supports and enhances the subdivision and stratigraphic correlation of the Ediacaran System in Australia and potentially on a more interregional scale. Acritarchs are well-preserved and diverse, change over short stratigraphic intervals, and allow the recognition of the previously established zones by use of certain acanthomorphic species. The presence of common species and taxonomic similarities between entire assemblages from Australia, Siberia, Baltica, South China and India provide a means for global correlation of the Ediacaran System using palynology. Portions of the Ediacaran System can be confidently proved to be coeval by the occurrence of discrete species distributed across various palaeocontinents and ranging stratigraphically no longer than a few million years. This suggests that they can be used for biostratigraphic analysis and correlation because other fossils are too scarce, too geographically restricted, or too difficult to interpret.
Keywords: Ediacaran, Neoproterozoic, Officer Basin, Australia, acritarchs, biostratigraphy
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