The Origin of High-Frequency Platform Carbonate Cycles and Third-Order Sequences (Lower Ordovician El Paso Gp, West Texas): Constraints from Outcrop Data and Stratigraphic Modeling
The Origin of High-Frequency Platform Carbonate Cycles and Third-Order Sequences (Lower Ordovician El Paso Gp, West Texas): Constraints from Outcrop Data and Stratigraphic Modeling
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DOI：
10.1306/d4267afa-2b26-11d7-8648000102c1865d
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发表时间：
1993-05
影响因子：
2
通讯作者：
R. K. Goldhammer;P. Lehmann;P. A. Dunn
中科院分区：
文献类型：
--
作者：
R. K. Goldhammer;P. Lehmann;P. A. Dunn
暗黑破坏神平台的被动边缘继承（下奥陶统，西得克萨斯州）是由二级索克C超层序组，包括一个基底海侵碎屑岩单位，（布利斯砂岩）位于破碎不整合面之上，标志着第二级基底低位海侵阶段），被750米的漂流相关的浅海台地碳酸盐岩（埃尔帕索组）记录了二级高位。由于晚古生代构造的冈瓦纳被动边缘，目前暴露在得克萨斯州是在一个向上倾斜的陆棚位置，缺乏内部的地层几何形状跨越沉积走向，所以序列和体系域被识别的垂直叠加模式的沉积亚相和更高的频率，第五级循环。高频周期的适应图（费舍尔图）测量两个完整的三级层序的三级适应的系统性变化，每个大约200万年。长，60-140米厚，在埃尔帕索群内。这表现在旋回类型的垂直演替、旋回厚度的系统变化以及与Fischer图相关的亚相类型直方图所揭示的亚相变化上。一个完整的埃尔帕索陆棚序列包含一个薄的低水位体系域的石英砂，厚的海侵体系域为主的向上增厚，血栓的潮下旋回，和一个高位体系域的标志是白垩纪，减薄向上的潮缘旋回含有石英砂。从富兰克林山东南到Diablo Arch，东北到Ardmore盆地，再到阿巴拉契亚盆地，通过生物层和旋回叠加模式对比了埃尔帕索三级沉积层序。因此，三级层序似乎是海平面升降的基础上，围绕冈瓦纳周边的可容纳地块的生物地层对比。时间序列分析表明，一个严格的allocyclic，Milankovitch驱动的，冰川海平面升降机制本身是不够的，占较高频率的第五阶和较低频率的第三阶调节周期。我们使用了一个二维的前向计算机模拟比较allocyclic和autocyclic模型的高频周期。的allocyclic模拟叠加一个确定性的，Milankovitch冰川海平面升降驱动器上的低频，三阶的住宿周期，并产生一个二维的合成横截面组成的高频围潮和潮下带周期堆叠成三阶序列。自动循环模拟生成的金斯伯格自动循环机制的高频周期性触发的三阶调节变化自动循环模拟定量复制的自动循环机制的第五阶周期的发展与陆上泥沙运输和向海的潮滩。在这两种情况下，一个三级层序的二维内部相结构和高频旋回叠加模式进行了模拟。模拟中的旋回和亚相叠加模式的重要差异有助于解释Diablo平台旋回和层序。作为自旋回性或米兰科维奇强迫的冰川euclide的替代或补充，我们已经研究了驱动相对海平面高频变化的随机模型。这些有趣的结果基本上保留了岩石记录中观察到的所有特征。我们的平滑sto hastic模拟表明，“周期问题”比目前认识到的要复杂得多。
ABSTRACT The passive-margin succession of the Diablo Platform (Lower Ordovician, west Texas) is represented by the second-order Sauk C supersequence set, consisting of a basal transgressive clastic unit (The Bliss Sandstone) above the breakup unconformity, marking the second-order basal lowstand transgressive phase), overlain by 750 m of drift-related, shallow-marine platform carbonates (the El Paso Group) recording the second-order highstand. Due to late Paleozoic structuring of the Gondwanan passive margin, present exposures in Texas are in an updip shelf position and lack internal stratal geometries across depositional strike, so sequences and systems tracts are identified solely by the vertical stacking patterns of depositional subfacies and higher-frequency, fifth-order cycles. Accommodation plots (Fischer plots) of high-frequency cycles gauge systematic shifts in third-order accommodation of two complete third-order sequences, each about 2 m.y. long and 60-140 m thick, within the El Paso Group. This is expressed in the vertical succession of cycle types, systematic changes in cycle thickness, and variations in subfacies as revealed by histograms of subfacies types tied to Fischer plots. A complete El Paso shelf sequence contains a thin lowstand systems tract of quartz arenite, a thick transgressive systems tract dominated by upward-thickening, thrombolitic subtidal cycles, and a highstand systems tract marked by dolomitic, thinning-upward peritidal cycles containing quartz sand. The El Paso third-order depositional sequences have been correlated by biostrat graphy and cycle stacking pattern from the Franklin Mountains southeast to the Diablo Arch, northeast into the Ardmore Basin, and thence to the Appalachian Basin. Hence, the third-order sequences appear to be eustatic, based on biostratigraphic correlation of accommodation plots around the periphery of Gondwana. Time-series analyses suggest that a strict allocyclic, Milankovitch-driven, glacio-eustatic mechanism alone is insufficient to account for the higher-frequency fifth-order and lower-frequency third-order accommodation cycles. We used a two-dimensional forward computer simulation to compare allocyclic and autocyclic models for high-frequency cycles. The allocyclic simulation superimposes a deterministic, Milankovitch glacio-eustatic driver on a lower-frequency, third-order accommodation cycle and generates a two-dimensional synthetic cross section composed of high-frequency peritidal and subtidal cycles stacked into third-order sequences. The autocyclic simulation is generated by a Ginsburg autocyclic mechanism for high-frequency cyclicity triggered by third-order accommodation changes The autocyclic simulation quantitatively replicates an autocyclic mechanism for fifth-order cycle development linked to onshore sediment transport and seaward progradation of tidal flats. The two-dimensional internal facies architecture of a third-order sequence and the high-frequency cycle stacking patterns are simulated in both scenarios. Important differences in patterns of cycles and subfacies stacking in the simulations shed light on the interpretation of the Diablo platform cycles and sequences. As an alternative or a supplement to autocyclicity or Milankovitch-forced glacio-eustasy, we have studied stochastic models for driving high-frequency changes in relative sea level. These intriguing results maintain essentially all the features observed in the rock record. Our smoothed sto hastic simulations suggest that the "cycle problem" is much more complex than presently realized.