Evidence for Paleoproterozoic cap carbonates in North America
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Abstract The long-conceived idea of the glacial origin of Blaini diamictite of Lesser Himalayan Neoproterozoic succession reached its maxima when the diamictites and capping pink limestone were attributed to the Neoproterozoic Snowball Earth event and its aftermath, respectively. Occurrences of diamictite-limestone association in two different levels have also been correlated with the Sturtian and Marinoan glaciations. Critical review, however, reveals that the interpretations of the glacial origin of diamictites are not well founded. The diamictite-limestone association, which occurs at the lower part of a thick, light brown shale unit and laterally grades into light brown shale, primarily indicates episodic surge events in an otherwise tranquil condition favorable for hemipelagic sedimentation. The lithology, bed geometry, internal organization, and disposition of the diamictite bodies suggest deposition of debris flow fan lobes along fault scarps in a rift setting. Emplacement of subaqueous debris flows is indicated by the associated deposits of entrained turbidity currents. The limestone also bears the signature of claciturbidites. The appearance of diamictite bodies and associated limestone in two distinct levels is not a stratigraphic disposition; on the contrary, the deposits were dislocated and repeated by two successive regional thrust faults. The Chemical Index of Alteration (CIA) values of the light brown shale and the matrix of the diamictites indicate that these sediments formed through prolonged subaerial weathering. The events leading up to development of the rift system and evidence of prolonged weathering within the basin-fill sediments are consistent with supercontinental break up, the prologue of Snowball Earth.
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1. Introduction Since over a decade, numerous studies have postulated that extremely low global temperatures (-50 °C) existed during successive separate glaciations in the Cryogenian period (770–580 Ma). This would explain not only the presence of ice at sea level near the equator, but also an icy cover on all oceans (Snowball Earth Hypothesis; Kirschvink, 1992; Hoffman et al., 1998; Hoffman & Schrag, 2002). The original suggestion of a “global” glaciation in the Neoproterozoic by Harland (1964) was partly based on paleomagnetic data (Harland & Bidgood, 1959) pointing to low paleolatitudes for these glacial deposits. The latter, widely distributed on all continents, are sharply overlain by a cap carbonate unit, interpreted as the result of a sudden switch back to a greenhouse climate related to the increase of atmospheric carbon dioxide due to volcanic degassing (Hoffman & Schrag, 2002). Despite the absence of many typical “glacial” features (e.g., faceted and striated clasts, dropstones, etc.), most Neoproterozoic diamictites were considered as glacial or periglacial deposits. However, not all reported Neoproterozoic diamictites were interpreted in this way, but also as the result ofsyntectonic sedimentgravityflows (Eyles & Januszczak, 2004, 2007) associated with widespread rifting of the Rodinia Supercontinent. In this paper, we present a new macro- and microscale structural analysis of the Upper Diamictite Formation (UDF) in the West Congo Supergroup (WCS) of the Democrati
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Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of δ13C and δ18O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ13C chemostratigraphy possesses some unique characteristics: (1) stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, (2) increase of the δ13C values from −3 to +5‰ across the C2/C3 boundary, (3) no δ13C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high δ13C value (+2.3‰) and a faint positive correlation between δ13C and δ18O values in the C1 unit. Evidence of quite low δ13C anomalies (with a nadir of −41‰), ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high δ13C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.
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The Neoproterozoic Glaciation (known as “Snowball Earth”) is one of the remarkable events in Earth history. In this paper, the late Neoproterozoic tillites at Lantian, Xiuning, South Anhui have been studied in detail by means of stratigraphy and chemostratigraphy. Lithologically, the Leigongwu Formation can be divided into 3 units: the upper diamictite, the middle dolomite and black shales and the lower diamictite. Comparison of C-isotope results of the cap dolomites, which are about -5‰ (PDB), with δ 13C profiles of contemporaneous carbonates around the world suggests that the two tillites of the Leigongwu Formation may represent two glaciations and correspond to the Sturtian (710—730Ma) and Marinoan (590—600Ma) tillites, respectively.
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An enduring enigma of Neoproterozoic Earth history is the intimate association of glacial diamictites with typical warm-water carbonates. Among the many hypothesized explanations for this paleoclimatic dichotomy are high orbital obliquity, true polar wander, reduced solar luminosity, snowball albedo, CO2 drawdown, stagnant ocean overturn, and reinterpretation of diamictites as mega-impact ejecta. The Otavi carbonate platform on the Congo craton in Namibia contains two discrete intervals of diamictite and associated glaciomarine deposits, sandwiched by thick carbonates from which we have obtained detailed carbon-isotopic records. From subsidence analysis, we estimate maximum rates of shallowwater sediment accumulation. The magnitude and duration of isotopic variations permit critical assessment of the existing hypotheses.
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