Development of Late Paleoproterozoic Aragonitic Seafloor Cements in the McArthur Group, Northern Australia
Peter R. Winefield, 2000. "Development of Late Paleoproterozoic Aragonitic Seafloor Cements in the McArthur Group, Northern Australia", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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Laterally extensive beds of acicular, radiating carbonate fans, locally known as “Coxco needles”, are particularly common within a distinct stratigraphic interval (—1640 Ma) in the Proterozoic of northern Australia. In the southern McArthur Basin, they are the distinctive feature of the Coxco Dolomite Member and occur throughout a number of lithofacies across the platform. Mounding and onlapping of sediment laminae, their upwardly divergent aspect, brecciated Coxco needle clasts infilling synsedimentary fractures, and their intimate association with stromatolites supports the precipitation of Coxco fans from ambient seawater directly onto the seafloor. Individually, they consist of acicular crystal casts up to 10 cm long, which form radiating, bottom-nucleated fans. Needle terminations are commonly blocky or square and in cross section appear pseudohexagonal with crystal casts generally having six-sided forms. Needles consist internally of an irregular mosaic of dolospar cement easily distinguished from the more finely crystalline dolomicrite matrix. These features are entirely consistent with criteria for the recognition of aragonite in ancient carbonate sequences and imply an original aragonitic mineralogy for the Coxco fans.
The sequence through the middle McArthur Group (Emmerugga Dolomite, Teena Dolomite, Coxco Dolomite Member, and Barney Creek Formation) is broadly transgressive, and precipitation of Coxco needles occurred during the onset of a period of tectonically induced subsidence. The mechanism for the widespread chronostratigraphic precipitation of CaCO3 is thought to be upwelling of highly alkaline, HCO3 −-rich anoxic bottom water onto the carbonate platform coeval with changes in the bathymetry of the basin. Mixing with relatively Ca2+-rich surface waters resulted in widespread precipitation of carbonate seafloor cement (i.e., Coxco fans) across the platform in several distinct lithofacies. The reason that macroscopic carbonate cement formed in preference to widespread precipitation of finely crystalline micrite remains unclear, although it is suggested that elevated concentrations of Fe2+ and Mn2+ in the basin waters may have inhibited micrite precipitation and thus favored development of macroscopic seafloor Coxco fans.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.