Neoproterozoic carbon isotope stratigraphy of the Amadeus Basin, central Australia
Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central Australia, comprise a geochemical record of Neoproterozoic seawater that facilitates correlation with other basins in Australia and worldwide. This study includes new carbon and oxygen isotope measurements of Cryogenian and Ediacaran carbonate in the northern Amadeus Basin that, in combination with previous data from Tonian strata of the Bitter Springs Formation, form a composite Amadeus Basin Neoproterozoic δC record. Key features of the Cryogenian-Ediacaran portion of this record include (1) negative δC excursions in cap carbonates overlying Cryogenian glacigenic deposits of the Areyonga and Olympic Formations (i.e., the Rasthof and Maieberg excursions, respectively); (2) a mid-Cryogenian negative excursion in the Ringwood Member of the Aralka Formation (seemingly correlative with the Taishir excursion of SW Mongolia); (3) a subsequent mid-Cryogenian positive excursion (the Keele Peak excursion) in the Limbla Member of the Aralka Formation; and (4) relatively low-magnitude excursions in the Ediacaran Julie Formation. In comparison with other Neoproterozoic stratigraphic successions, key portions of the overall Neoproterozoic δC record that appear to be absent from the Amadeus Basin based on existing data include large-magnitude negative excursions (Islay and Trezona excursions) that immediately preceded basaland terminal-Cryogenian (i.e., Sturtian and Marinoan, respectively) glaciations. These "missing" δC excursions are associated with significant unconformities in the Amadeus Basin. δC and δO from carbonate clasts in diamictite of the Areyonga and Olympic Formations suggest that deposition of Islay- and Trezona-equivalent strata was limited or nonexistent in the Amadeus Basin, as opposed to the possibility that these strata were deposited but subsequently removed by glacial erosion. Our new Amadeus Basin results refine Cryogenian and early Ediacaran correlations among the Amadeus Basin, Adelaide rift complex, and Namibia. The lower part of the Ringwood Member, which was deposited during early stages of the Cryogenian nonglacial interlude, includes two negative δC excursions that are also evident in previous results from the Tapley Hill Formation of the Adelaide rift complex. The upper part of the Ringwood Member contains abundant stromatolites and seems to correlate with Cryogenian stromatolite reefs of the Balcanoona Formation in the Adelaide rift complex. The overlying Limbla Member in the Amadeus Basin correlates with the Yankaninna, Etina, Amberoona, and Enorama Formations of the Adelaide rift complex, as well as carbonate from other continents that record the Keele Peak excursion. δC from the early Ediacaran Olympic cap of the Amadeus Basin is (1) comparable to previous results from the Maieberg and Elandshoek Formations in Namibia; and (2) consistent with diachronous deposition of Marinoan cap dolostone in the Amadeus Basin. Additionally, new δC from the Julie Formation of the Amadeus Basin is similar to previous results from the late Ediacaran Kuibis Subgroup in southern Namibia. These correlations underscore that Ediacaran strata of Namibia, as well as the Cryogenian nonglacial interlude strata of the Adelaide rift complex, are roughly an order of magnitude thicker than stratigraphic counterparts in the Amadeus Basin. The wide disparity in sedimentation rates implied by this finding is likely attributable to fundamental differences in the Neoproterozoic tectonic settings of the three locations.