Stratigraphy, palaeontology and geochemistry of the late Neoproterozoic Aar Member, southwest Namibia: Reflecting environmental controls on Ediacara fossil preservation during the terminal Proterozoic in African Gondwana
Hall M.,Monash University |
Kaufman A.J.,The Interdisciplinary Center |
Vickers-Rich P.,Monash University |
Ivantsov A.,Monash University |
And 12 more authors.
Precambrian Research | Year: 2013
Common, Ediacaran fossils are well preserved in a Late Neoproterozoic (ca. 545Ma) shallow marine sequence, described here as the Aar Member of the Dabis Formation (Kuibis Subgroup, Nama Group), near Aus in southwest Namibia. This 31-38m thick, shale-dominant unit records the transition from fluvial-shallow marine Kliphoek Sandstone to open marine limestone of the Mooifontein Member of the Zaris Formation, deposited on a subsiding continental margin during a major, regional transgression. Thin sandstone beds contain fossils at a number of levels throughout the Aar Member. Concentrations of Pteridinium were mostly transported in flood-derived sheets, while some Ernietta assemblages are preserved close to in situ. Rangea has also been transported, and is mostly confined to thin sandstone lenses incised into mudstone. Limestone beds, common throughout, include at least two marker horizons that can be followed regionally and show local evidence of storm reworking. Systematic sampling and analyses of limestone reveals enrichment in both 13C and 18O higher in the section, with negative δ13C near the base rising to moderate positive values near the top. The negative-to-positive transition in δ13C values is more pronounced in the east, with all of the lower Aar Member samples consistently depleted in 13C. While this may reflect greater degrees of alteration by meteoric or dewatering fluids, the same carbonates are notably enriched in 18O relative to those at the same stratigraphic position to the west. The overall rise in 13C is attributed to greater proportional burial of organic matter and release of oxygen to surface environments, while the spatial variability is likely the result of a strong surface-to-deep carbon isotopic gradient in seawater. A number of the fossils, especially Rangea, are encrusted with jarosite, an iron-bearing sulphate mineral and common weathering product of pyrite. This observation suggests that preservation of the fossils may have resulted from the rapid encrustation of pyrite on the surface of the organisms as they decomposed and were consumed by sulphate-reducing bacteria within the sandy, near shore sediments. Insofar as pyrite formation requires iron, which is soluble and reactive in anoxic solutions, it is likely that the deeper subtidal environments lacked oxygen. In situ pyritized forms like Ernietta may have developed the capacity to survive under episodically anoxic or sub-oxic environmental conditions, while Pteridinium and Rangea lived within an oxygenated estuarine or fluvial setting and were transported during storms to anoxic, ferruginous environments where they were exquisitely preserved. © 2013 The Authors.
Vickers-Rich P.,Monash University |
Ivantsov A.Y.,Russian Academy of Sciences |
Trusler P.W.,Monash University |
Narbonne G.M.,Queen's University |
And 7 more authors.
Journal of Paleontology | Year: 2013
Rangea is the type genus of the Rangeomorpha, an extinct clade near the base of the evolutionary tree of large, complex organisms which prospered during the late Neoproterozoic. It represents an iconic Ediacaran taxon, but the relatively few specimens previously known significantly hindered an accurate reconstruction. Discovery of more than 100 specimens of Rangea in two gutter casts recovered from Farm Aar in southern Namibia significantly expands this data set, and the well preserved internal and external features on these specimens permit new interpretations of Rangea morphology and lifestyle. Internal structures of Rangea consist of a hexaradial axial bulb that passes into an axial stalk extending the length of the fossil. The axial bulb is typically filled with sediment, which becomes increasingly loosely packed and porous distally, with the end of the stalk typically preserved as an empty, cylindrical cone. This length of the axial structure forms the structural foundation for six vanes arranged radially around the axis, with each vane consisting of a bilaminar sheet composed of a repetitive pattern of elements exhibiting at least three orders of self-similar branching. Rangea was probably an epibenthic frond that rested upright on the sea bottom, and all known fossil specimens were transported prior to their final burial in storm deposits. © 2013 The Paleontological Society.
Wallace M.W.,University of Melbourne |
Hood A.V.S.,University of Melbourne |
Woon E.M.S.,University of Melbourne |
Hoffmann K.-H.,Namibian Geological Survey |
Reed C.P.,Teck Namibia Ltd
Precambrian Research | Year: 2014
Previously undescribed chambered structures are common and widespread in the Cryogenian (post-Sturtian glacial) carbonates of the Oodnaminta Reef Complex (Adelaide Geosyncline, South Australia), the Rasthof-Berg Aukas Formation (Namibia) and the Gauss Formation (Namibia). These carbonate structures have millimetre to centimetre-scale chambers separated by well-defined and generally thin micritic walls. Chamber walls now consist of dolomite, but were probably originally aragonitic. The chambers may have a lobate, polygonal or dendritic morphology and are often further divided into smaller chambers. Chambered structures occur as reefal growth frameworks; as cavity-fillings in neptunean dykes and growth cavities; and as intercolumnar material within stromatolite frameworks. In the Oodnaminta Reef Complex, these structures are only present in the sub-photic deep water framework.These structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the Sturtian glaciation. No precisely analogous structures have been previously described from modern or ancient settings, but the complexity and degree of organization suggests a significant evolutionary advance over older Proterozoic fossils. The closest morphological analogues for the structures are: (a) some types of reef-dwelling sponges; and (b) some complex microbialites from Archaean and Paleoproterozoic carbonates. The structures lack spicules and ostia found in sponges, ruling out a true Poriferan origin. However, it is plausible that they are proto-sponges, sponge-grade organisms, or complex microbial precursors to sponge-grade organisms. Whatever their affinity, we suggest these structures record a significant evolutionary event on the path towards organic complexity. © 2014 Elsevier B.V.