鄂尔多斯盆地奥陶系锶同位素地球化学特征

对于鄂尔多斯盆地奥陶系碳酸盐岩锶同位素的研究主要集中于成岩作用识别与划分。本次以盆地内中东部陕北坳陷镇钾1井马家沟组五段(以下简称马五段)以及盆地西缘桌子山地区三道坎组—桌子山组为研究对象,首次报道了未经成岩作用影响的碳酸盐岩的锶同位素数据,基于锶同位素地层学原理,试图恢复该沉积期古环境特征并对奥陶纪地层格架进行再讨论,共获得了如下几点认识:①镇钾1井马五段锶同位素自下而上以反复震荡、整体上略微增加为特征,震荡周期与蒸发岩韵律相吻合。桌子山地区三道坎组—桌子山组锶同位素以小幅波动为特征,整体上呈现出单调递减趋势。②马家沟组的地质年代归属于中奥陶世达瑞威尔中—晚期,而三道坎组—桌子山组的地质年代早于马家沟组,为中奥陶世达瑞威尔早—中期。③镇钾1井马五段处于局限台地-蒸发台地的沉积背景下,其锶同位素与全球演化曲线特征存在差异,说明了在该沉积背景下锶同位素对古环境变化反应更为敏感,能够成为区域古环境分析的重要指标。桌子山地区三道坎组—桌子山组由于处于开阔台地-缓坡的沉积背景下,与全球锶同位素演化曲线拟合度较高。为进一步研究鄂尔多斯盆地的沉积及构造差异提供了新依据,本次研究为全球锶同位素曲线提供了新的基础资料。

Geochemical characteristics of Ordovician strontium isotope in the Ordos basin

The research on Sr isotope of carbonate rocks in the Ordos basin mainly focuses on the identification and classification of diagenesis. In this study, the fifth Member of the Majiagou Formation (hereinafter referred to as Ma5) in the well Zhenjia- 1 and Sandaokan Formation- Zhuozishan Formation in the western margin of the Ordos basin was selected as the research object. This study firstly reports Sr isotopic data of carbonate rocks which are not influenced by diagenesis. Based on the principle of Sr isotope stratigraphy, the characteristics of sedimentary environment were restored and the Ordovician stratigraphic framework was addressed. The following understanding has been obtained. (1) Ma5 Sr isotope of the well Zhenjia- 1 is characterized by repeated vibration, but slightly increasing from bottom to top, with oscillation period coincidentally with evaporative rhythm. While Sr isotopes of the Sandaokan and Zhuozishan formations are characterized by moderate fluctuation and decrease generally. (2) Geological age of the Majiagou Formation can be assigned to the middle- late Darriwillian while the counterpart in the Sandaokan and Zhuozishan formations is earlier and is assigned to the early- middle Darriwillian. (3) Ma5 was deposited under the background of restricted platform- evaporation platform and its Sr curve differs from the global curve in Ordovician, suggesting that Sr isotope is more sensitive to the variation of paleoenvironment and could become an important indicator for regional paleoenvironmental comparison. The Sandaokan and Zhuozishan formations in the Zhuozishan area deposited under the background of open platform- carbonate slope and its Sr curve corresponds well with global curve. This study provides not only new fundamental data for global Sr isotope curve but also new evidence for comparison of the sedimentary and tectonic differences of strata in the Ordos basin.