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Jurassic limestone-marl sequences in Northern Spain: Detecting diagenetic signals using rare earth and trace elements.

机译：西班牙北部的侏罗纪石灰岩-泥灰岩序列：使用稀土和微量元素检测成岩信号。

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摘要

Mesozoic rhythmically bedded limestone and marl sequences have been traditionally interpreted to be the result of eustatic sea-level changes related to Milankovitch cycles. An alternative interpretation supports the idea that these calcareous alternations are a product of diagenetic bedding. Stratigraphic analysis is typically used to determine the extent of diagenesis and detect any primary depositional signal in these limestone-marl rhythmites. Rare earth and trace element geochemistry is an overlooked method to distinguish the presence of a primary signal from a diagenetic one. The main objective of this study was to test the validity of using rare earth and trace elements as a method for determining diagenesis in calcareous rhythmites that are part of the Rodiles Formation (Pliensbachian, Lower Jurassic) from coastal outcrops in NW Spain. Data from the application of this geochemical approach, in conjunction with stratigraphic analysis, provides insight into the distribution of elements that are facies-controlled, chemically-controlled, and elements that are independent of both. Results presented here for the use of rare earth elements as a test for a primary signal are inconclusive. Problems with using rare earth elements in this study include the possibility of unrealized REE mobility and signal interference from strontium, which many rare earth elements commonly substitute for. Thorium and strontium presented primary signals that are independent of facies and diagenesis. Of the two, thorium is the best element for use in the detection of a primary, externally controlled signal. Using thorium as a geochemical proxy, it may be concluded that despite diagenesis, diagenetic bedding is not the mechanism behind the formation of these carbonate rhythmites. Therefore, it can be concluded that the Rodiles Formation is the result of eustatic sea-level changes. Furthermore, thorium could be an ideal element for chemostratigraphy of Jurassic and Cretaceous deposits within the same paleogeographic setting as the Rodiles Formation.
机译：传统上，中生代有节奏地层状石灰岩和泥灰岩层序被解释为与米兰科维奇周期有关的海平面不断变化的结果。另一种解释支持以下观点，即这些钙质交替是成岩作用的产物。地层学分析通常用于确定成岩作用的程度，并检测这些石灰岩-泥灰质节律中的任何主要沉积信号。稀土和微量元素地球化学是区分原发信号和成岩信号的一种被忽视的方法。这项研究的主要目的是检验使用稀土和微量元素作为测定西北西班牙露头的罗迪尔斯组（普利恩斯巴赫，下侏罗纪）的钙质节律中成岩作用的方法的有效性。这种地球化学方法的应用数据与地层分析相结合，提供了对相控制，化学控制元素和二者独立元素分布的洞察力。这里介绍的使用稀土元素作为主要信号测试的结果尚无定论。本研究中使用稀土元素的问题包括未实现的REE迁移率和锶信号干扰的可能性，而锶是许多稀土元素通常替代的。 ium和锶呈现出与相和成岩作用无关的主要信号。在这两者中，th是用于检测外部主要控制信号的最佳元素。使用th作为地球化学替代物，可以得出结论，尽管有成岩作用，但成岩作用并不是这些碳酸盐节律岩形成的机制。因此，可以得出结论，罗迪尔斯地层是海平面不断变化的结果。此外，or可能是与罗迪尔斯组形成古地理环境中侏罗纪和白垩纪沉积物化学地层学的理想元素。

著录项

  • 作者

    Hollon, Brittany Elise.;

  • 作者单位

    The University of Alabama.;

  • 授予单位 The University of Alabama.;
  • 学科 Geology.;Geochemistry.;Sedimentary Geology.
  • 学位 M.S.
  • 年度 2012
  • 页码 74 p.
  • 总页数 74
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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