Assessing Early Triassic paleoceanographic conditions via unusual sedimentary fabrics and features
Assessing Early Triassic paleoceanographic conditions via unusual sedimentary fabrics and features
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DOI：
10.1016/j.earscirev.2013.08.015
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发表时间：
2014-10
影响因子：
12.1
通讯作者：
A. Woods
中科院分区：
文献类型：
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作者：
A. Woods
对下三叠统沉积岩记录的分析揭示了各种不同寻常的沉积结构和特征，其中许多在元古代比显生宙更常见。这些所谓的“逆时代相”发育于下三叠统碳酸盐岩台地，从浅层潮下环境到深层盆地环境，包括与碳酸钙降水增强（如同沉积海底胶结物）、微生物岩和因生物扰动强度和深度降低而形成的织物（如蛭状灰岩）相关的特征。控制早三叠世错时相发育的相互作用因素有：1)环境条件（即不同寻常的早三叠世海洋化学）；2)生物因素（如生物扰动和放牧压力）；3)实际的沉积过程（如波浪和海流、沉积）。由于这三个因素通常是相互作用的，因此在使用错时相重建早三叠纪古海洋条件时，考虑每个因素的作用是很重要的。总的来说，错时相表明早三叠世碳酸钙沉积增强，但并不一定表明它们沉积的地方以前存在缺氧或缺氧条件。海水化学对错时相沉积的影响在较深的水环境中最为明显，异常的古海洋条件增加了碳酸钙降水，并通过缺氧、碱性水的上涌导致生物扰动和放牧压力的减少。在生物因素和实际过程更为重要的海岸，环境影响程度有所降低，微生物活动、浅埋或二氧化碳脱气对海水的轻微改造是碳酸钙沉淀发生的必要条件。错时相是重建古海洋条件和研究二叠纪-三叠纪大灭绝恢复过程中环境应力分布变化的有用工具，但必须仔细考虑导致其生长的生物、沉积和地球化学因素。
Analysis of the Lower Triassic sedimentary rock record has revealed a variety of unusual sedimentary fabrics and features, many of which were more common in the Proterozoic than the Phanerozoic. These so-called “anachronistic facies” developed across Lower Triassic carbonate platforms from shallow subtidal environments to deep, basinal environments and include features associated with the enhanced precipitation of calcium carbonate (e.g., synsedimentary seafloor cements), microbialites, and fabrics that formed due to a reduction in the intensity and depth of bioturbation (e.g., vermicular limestones). Three interacting factors controlled the development of anachronistic facies during the Early Triassic including: 1) environmental conditions (i.e., the unusual chemistry of Early Triassic oceans); 2) biotic factors (e.g., bioturbation and grazing pressures); and, 3) actualistic sedimentologic processes (e.g., waves and currents, sedimentation). Because these three factors typically acted in concert with each other, it is important to consider the role of each when using anachronistic facies to reconstruct Early Triassic paleoceanographic conditions. Overall, anachronistic facies are indicative of enhanced calcium carbonate precipitation during the Early Triassic, but do not necessarily indicate the former presence of anoxic or suboxic conditions where they were deposited. The influence of seawater chemistry on the deposition of anachronistic facies was most pronounced in deeper water environments, where anomalous paleoceanographic conditions heightened calcium carbonate precipitation and caused a decrease in bioturbation and grazing pressures via the upwelling of anoxic, alkaline waters. The degree of environmental influence lessened shorewards, where biotic factors and actualistic processes were more important, and slight modification of seawater by microbial activity, shallow burial, or CO2-degassing was necessary for calcium carbonate precipitation to occur. Anachronistic facies are a useful tool for reconstructing paleoceanographic conditions and examining shifts in the distribution of environmental stress during the recovery from the Permian–Triassic mass extinction, but only when the biologic, sedimentologic, and geochemical factors that led to their growth are carefully considered.