Gamma-Ray Logs and U–Pb Zircon Geochronology—Essential Tools to Constrain Lithofacies Interpretation of Paleoproterozoic Depositional Systems
Michael J. Jackson, Peter N. Southgate, Rod W. Page, 2000. "Gamma-Ray Logs and U–Pb Zircon Geochronology—Essential Tools to Constrain Lithofacies Interpretation of Paleoproterozoic Depositional Systems", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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Outcrop-derived gamma-ray curves, lithofacies, and U-Pb SHRIMP zircon ages are integrated to provide a better understanding of accommodation history in the Paleo-Mesoproterozoic Nathan Group of northern Australia. This chronostratigraphic analysis significantly revises earlier lithostratigraphic interpretations of a 1200-m-thick succession of sandy carbonates. Rather than a continuous succession deposited in a complex series of lacustrine environments, it consists of three completely separate second-order supersequences, each a few hundred meters thick and deposited over a few million years. These supersequences are separated by major stratigraphic breaks (tectonically enhanced sequence boundaries) each approaching a duration of probably 10 million years. Each supersequence comprises several third-order sequences, which themselves contain many higher-order cycles, deposited in a series of continental, shoreline, and inner-rarnp to outer-ramp environments. Transgressive, high-energy, continental to shallow marine, mixed clastic-carbonate facies dominate most of the sequences. The middle supersequence, however, preserves deeper-water (mostly sub-storm) stromatolitic facies in one sequence, and storm-reworked clastics in another. These are interpreted as condensed intervals deposited around their respective maximum flooding surfaces and are succeeded by regressive facies that probably represent highstand systems tracts.
New correlations between the Paleoproterozoic carbonate successions of the McArthur Basin and approximately time equivalent clastic successions in the Mt. Isa area, some 400 km to the southeast, are proposed.
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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.