Saylor B Z, Kaufman A J, Grotzinger J P, Urban F
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Sediment Res A Sediment Petrol Process. 1998 Nov;68(6):1223-35. doi: 10.2110/jsr.68.1223.
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.
综合层序地层学和化学地层学数据为纳米比亚元古宙末期至寒武纪维特弗莱群和纳马群地层单元的对比提供了一个框架。结合精确的铀-铅锆石年龄限制，这些对比使得构建一个用于校准元古宙末期年代地层学的复合参考剖面成为可能。纳米比亚参考剖面始于两个不同的冰期地层，一直延伸到元古宙-寒武纪边界前100万年之内。这两个冰期地层可能分别代表了在北大西洋地区更为人所知的两个不同的瓦兰吉尔期冰期。从两个冰期地层中较高的那个向上，复合地层保存了瓦兰吉尔期之后元古宙末期海水中碳同位素变化最厚且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) 冰期后的负δ13C偏移（Npg区间）；(2) δ13C值上升的区间（Pr区间）；(3) 以δ13C正值减小为特征并最终达到接近零或负值的下降区间（Pf区间）；以及(4) 一个δ13C值为中等正值、相对不变的区间（I区间），该区间一直延伸到包含元古宙-寒武纪边界的不整合面。这些化学地层区间中的每一个都能在世界各地广泛分布的对比剖面中识别出来。通过将经过放射性校准的纳米比亚地层中的沉积物堆积速率与加拿大北极地区和阿曼对比剖面中的沉积物堆积速率进行比较，估计较年轻的瓦兰吉尔冰期结束的最大年龄为5.64亿年，比之前的估计年轻2500万年。
