A composite reference section for terminal proterozoic strata of southern Namibia.
A composite reference section for terminal proterozoic strata of southern Namibia.
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DOI：
10.2110/jsr.68.1223
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发表时间：
1998-11
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影响因子：
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通讯作者：
B. Saylor;A. Kaufman;J. Grotzinger;F. Urban
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文献类型：
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作者：
B. Saylor;A. Kaufman;J. Grotzinger;F. Urban
综合层序地层学和化学地层学数据得出了晚元古代地层单位与纳米比亚寒武纪Witvlei和Nama群对比的框架。再加上精确的锆石U-Pb年龄约束，这些对比使得构建用于标定元古界末期年代地层学的复合参考剖面成为可能。纳米比亚的参考剖面从两个明显的冰川地平线开始，一直延伸到元古界-寒武纪界线的100万年内。这两个冰川地平线可能代表了北大西洋地区更为人所知的两个截然不同的Varanger时代冰川。从两个冰川层位中较高的一个向上，复合地层学保存了后Varanger末期元古代海水中最厚和最完整的碳同位素变异性记录之一。识别出四个碳同位素化学地层学区间：(1)冰后负三角洲13C漂移(NPG区间)；(2)正三角洲13C值增加的上升区间(Pr区间)；(3)以正三角洲13C下降为特征的下降区间(Pf区间)，最终接近零或负值；以及(4)中等正的、相对不变的三角洲13C值区间(I区间)，一直延伸到包含元古代-寒武系界线的不整合。这些化学地层段中的每一个都可以在世界各地相距甚远的相关剖面中识别出来。通过将放射性校正的纳米比亚地层中的沉积速率与北极加拿大和阿曼相关剖面的沉积速率进行比较，估计年轻的瓦朗热冰川结束的最大年龄为564 Ma，25m.y。比之前估计的要年轻。
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.