U-Pb dating of pedogenic calcite near the Permian−Triassic boundary, Karoo Basin, South Africa

作者：Heriberto Rochín-Bañaga, Robert A. Gastaldo, Donald W. Davis, Johann Neveling, Sandra L. Kamo, Cindy V. Looy, J. W. Geissman · 发表于：Geological Society of America Bulletin · 年份：2023 · DOI：10.1130/b36968.1 · 被引用次数：8 · 研究领域：Paleontology and Stratigraphy of Fossils、Geochemistry and Elemental Analysis、Geological formations and processes

We report U-Pb age determinations of carbonate nodules from an in situ paleosol horizon in the Upper Permian Balfour Formation and from several horizons of pedogenic nodule conglomerate (PNC) in the Triassic Katberg Formation, Karoo Basin, South Africa, using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). The paleosol sample yields an age of 252 ± 3 Ma, which overlaps with a previous high-precision U-Pb zircon date from a volcanic ash deposit 2 m above the paleosol. This relationship demonstrates the reliability of using LA-ICP-MS dating techniques on terrestrial pedogenic calcite. Two PNC samples collected at the base of the Katberg Formation within the same sandstone unit yield ages of 255 ± 3 Ma and 251 ± 3 Ma. The age of 251 ± 3 Ma overlaps with the high-precision U-Pb zircon date below the PNC and is a maximum age estimate of deposition for the base of the Katberg Formation. Our results show that reworked nodules in the same concentrated conglomerate lag can be of different ages, but that similarly aged nodules are spatially associated. In addition, two PNC samples collected higher in the section yield ages of 249 ± 3 Ma and 241 ± 3 Ma, providing maximum depositional ages for the lower to middle Katberg Formation for the first time. We demonstrate that pedogenic carbonate nodules can be dated with meaningful precision, providing another mechanism for constraining the age of sedimentary sequences and studying events associated with the Permian−Tr...