Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Proterozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with precise U-Pb zircon age constraints, these correlations make it possible to construct a composite reference section for use in calibrating terminal Proterozoic chronostratigraphy. The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the Proterozoic-Cambrian boundary. The two glacial horizons may represent each of two distinct Varanger-age glaciations better known from the North Atlantic region. From the higher of the two glacial horizons up, the composite stratigraphy preserves one of the thickest and most complete available records of carbon-isotope variability in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negative delta 13C excursion (Npg interval); (2) a rising interval (Pr interval) of increasing positive delta 13C values; (3) a falling interval (Pf interval) characterized by decreasing positive delta 13C and culminating in near zero or negative values; and (4) an interval of moderately positive, relatively invariant delta 13C values (I interval) that extends up to the unconformity that contains the Proterozoic-Cambrian boundary. Each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world. By comparing sediment accumulation rate in the radiometrically calibrated Namibian stratigraphy with sediment accumulation rates in correlative sections in Arctic Canada and Oman, a maximum age of 564 Ma is estimated for the end of the younger Varanger glaciation, 25 m.y. younger than previous estimates.

中文翻译：

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纳米比亚南部元古界末期地层的复合参考剖面

综合层序地层学和化学地层学数据为纳米比亚末元古代至寒武纪 Witvlei 和 Nama 群中地层单元的相关性提供了框架。再加上精确的 U-Pb 锆石年龄限制，这些相关性使得构建复合参考剖面以用于校准末元古代年代地层学成为可能。纳米比亚参考剖面从两个不同的冰川层开始，一直延伸到元古代-寒武纪边界的 100 万年内。这两个冰川层可能代表了北大西洋地区更为人所知的两个不同的 Varanger 时代冰川中的每一个。从两个冰川地平线中的较高处向上，复合地层保留了瓦朗厄尔末元古代海水中碳同位素变化的最厚和最完整的可用记录之一。识别出四个碳同位素化学地层区间：（1）冰后负 delta 13C 偏移（Npg 区间）；(2) 正增量 13C 值的上升区间（Pr 区间）；(3) 下降区间（Pf 区间），其特征是正 delta 13C 下降并最终接近零或负值；(4) 一个适度正的、相对不变的 delta 13C 值区间（I 区间），一直延伸到包含元古代-寒武纪边界的不整合面。这些化学地层区间中的每一个都可以在世界各地广泛分离的相关剖面中识别出来。