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首页> 外文期刊>Stratigraphy >Integrating magnetic susceptibility data with sequence stratigraphy in the ironstone bearing successions (Lower Silurian) of eastern North America
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Integrating magnetic susceptibility data with sequence stratigraphy in the ironstone bearing successions (Lower Silurian) of eastern North America

机译：在北美洲东部的铁石承继岩层（下志留纪）中将磁化率数据与层序地层相结合

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摘要

A total of 839 samples of marine sedimentary rock were collected from Lower Silurian (Aeronian-Shein- woodian) strata exposed at localities situated along an inferred proximal to distal transect of the Appalachian Foreland Basin. These samples were classified on the basis of lithology, and analyzed for bulk magnetic susceptibility. The application of several statistical tests suggests a significant correlation between lithology and magnetic susceptibility. The results are considered in the context of models explaining patterns of magnetic susceptibility and the origin of ferruginous strata. In shallow depositional environments rimming the Appalachian Basin, peak magnetic susceptibility values were associated with intervals containing abundant ferric iron, notably the classic "Clinton Ironstones" of east central New York. Elevated values are also associated with glauconitic shale horizons and fossiliferous, dolopackstones as well. In these same sections, beds interpreted as tempestites show lower values than those observed in the surrounding units. Magnetic susceptibility curves generated for these sections display sharp offsets in values at the base of ferric ironstones; the highest values occur near the top of these units. These observations are consistent with models that explain ironstones as sediment-starved and highly reworked transgressive units overlying unconformities. According to this framework,non-ferruginous strata overlying the ironstone would be interpreted as the product of increased sedimentation rates associated with highstand and falling stage, which would "dilute" any iron that is present. However, given continuous changes in sedimentation and sea level, one would expect a gradual tapering of magnetic susceptibility values as sedimentation increased upward through the section. This is not reflected in the data described herein, which demonstrate sharp, negative shifts in magnetic susceptibility values near the upper contact of ironstones, suggesting that additional factors may also influence the rate at which iron concentrated on the sea floor. Although sedimentation rates appear to have an important influence on magnetic susceptibility values, the exceptional concentration of authigenic iron-bearing minerals in these facies may have generated a case wherein magnetic susceptibility values and presumed rate of sedimentation are inversely correlated in certain environments. At localities interpreted as the deepest part of the Appalachian Basin, conglomeratic lag beds and black shales lack elevated magnetic susceptibility peaks found in coeval successions in the surrounding regions. Previous workers have argued for a genetic link between ironstones and the conglomeratic lag beds, and both contain abundant iron bearing minerals. However, the sulfide minerals characteristic of the conglomeratic lag beds contain iron primarily in its ferrous state, which produces magnetic susceptibility values that are not significantly different from over- and underlying strata. The high magnetic susceptibility values associated with sediment-starved horizons in shallow, oxic environments and the lack thereof in deeper, dysoxic environments, suggests that the prevailing redox conditions of a depositional regime exert an important control on magnetic susceptibility values. Although this study is narrow in scope, these findings may provide useful insight toward theories of the origin of ironstones and the controls on magnetic susceptibility values in marine sedimentary rock.
机译：从下志留纪（Aeronian-Shein-Woodian）地层中收集了总共839个海洋沉积岩样品，这些地层暴露于阿巴拉契亚前陆盆地的推断近端样貌至近端样貌样。这些样品根据岩性进行分类，并分析其磁化率。几种统计检验的应用表明，岩性和磁化率之间存在显着相关性。在解释磁化率模式和铁质地层起源的模型的背景下考虑了结果。在围绕阿巴拉契亚盆地的浅层沉积环境中，峰值磁化率值与包含丰富三价铁的区间相关，特别是纽约东部中部的经典“克林顿铁矿”。升高的值还与青冈页岩层和化石的白云质块岩有关。在这些相同的部分中，被解释为风暴岩的床显示出的值低于在周围单元中​​观察到的值。这些部分产生的磁化率曲线在铁铁矿的底部显示出明显的偏移值。最高值出现在这些单元的顶部附近。这些观察结果与将铁矿石解释为沉积物匮乏和高度返工的覆盖不整合面的海侵单元的模型一致。根据该框架，覆盖在铁矿石上的非铁质地层将被解释为与高位期和下降期相关的沉积速率增加的产物，这将“稀释”存在的任何铁。但是，考虑到沉积物和海平面的连续变化，人们会期望随着沉积物在整个剖面中的增加，磁化率值会逐渐减小。这没有在本文所述的数据中得到反映，该数据表明铁矿石的上接触面附近的磁化率值发生了急剧的负向偏移，这表明其他因素也可能影响铁在海床上的集中速度。尽管沉积速率似乎对磁化率值有重要影响，但在这些相中自生的含铁矿物的异常浓度可能会导致在某些环境中磁化率值与假定沉积率成反比的情况。在被解释为阿巴拉契亚盆地最深处的地方，砾岩化的滞后层和黑色页岩缺乏在周围地区的近代演替中发现的升高的磁化率峰值。先前的工作人员认为铁矿石和砾岩层之间存在遗传联系，两者都含有丰富的含铁矿物。但是，砾岩砾岩层的硫化物矿物主要含有铁，处于亚铁态，其磁化率值与上覆地层和下伏地层没有明显差异。高磁化率值与浅层缺氧环境中沉积物匮乏的地层有关，而在较深的低氧环境中缺乏磁化率，这表明沉积体系的主要氧化还原条件对磁化率值具有重要的控制作用。尽管这项研究的范围很窄，但这些发现可能为铁矿石起源理论和海洋沉积岩磁化率值控制提供有用的见解。

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