Columnar stromatolites from the Early Proterozoic Schmidtsdrift Formation, Northern Cape province, South Africa - Part I: systematic and diagnostic features

Main articleThe Schmidtsdrift Formation (Transvaal Dolomite) is a Lower Proterozoic stromatolite-bearing carbonate unit. A good succession of columnar stromatolites occurs along the Boetsap River (Northern Cape Province). Using the method of serial sections to draw the gross morphology of columns and following the actual classification based on a succession of different characters, many new groups and forms have been found : Topinamboura insulata is a bulbous column with wart-like projections spreading out everywhere on the smooth surface. Radiatina isotropa presents closely packed radiating columns forming compact bioherms. The small rhythmically superposed columns of Tibia cristata, Tibia plumata and Sapinia fucoides offer tiny bushy columns with a constant crestal zone, a very unusual feature, giving an angular ,shape to the laminae. Some of the stromatolites belong to previously described groups, from the same area as Katernia africana or Katernia perlina new form, which present true bushy columns forming thin biostromes; or, from other parts of the world, as Pilbaria boetsapia and Pilbaria inzeriformis, two new forms, attributed to a Lower Proteroroic group described in Australia. Besides description of morphologies and mode of occurrences, detailed studies of lamina microstructures have been carried out. Very interesting fabrics have been described but, until now, no true organic remains (cells) have been found. The conclusions emphasize the importance of detailed systematic description of stromatolites in order to make a biostratigraphical model of the Lower Proterozoic and to clear up confusions with Upper Proterozoic stromatolites which also present varied assemblages of ramified stromatolites.Non

A palaeoecological model for the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

Reconstructing the spatial distribution of early eukaryotes in palaeoenvironments through Proterozoic sedimentary basins provides important information about their palaeocology and taphonomic conditions. Here, we combine the geological context and a reconstruction of palaeoenvironmental redox conditions (using iron speciation) with quantitative analysis of microfossil assemblages (eukaryotes and incertae sedis), to provide the first palaeoecological model for the Atar/El Mreïti Group of the Taoudeni Basin. Our model suggests that in the late Mesoproterozoic – early Neoproterozoic, the availability of both molecular oxygen and nutrients controlled eukaryotic diversity, higher in oxic shallow marginal marine environments, while coccoidal colonies and benthic microbial mats dominated respectively in anoxic iron-rich and euxinic waters during marine highstands or away from shore where eukaryotes are lower or absent

Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a pylome interpreted as a sophisticated circular excystment structure, and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicate that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy