Ordovician reef systems and settings in South China before the Late Ordovician mass extinction
Ordovician reef systems and settings in South China before the Late Ordovician mass extinction
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DOI：
10.1016/j.palaeo.2003.12.010
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发表时间：
2004-03
期刊：
影响因子：
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通讯作者：
Yue Li;S. Kershaw;Xinan Mu
中科院分区：
文献类型：
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作者：
Yue Li;S. Kershaw;Xinan Mu
华南陆块奥陶系生物礁主要分布在三个地层单元中：（1）特里马多克组中的粉香组和仑山组上部;（2）特里马多克组晚期的红花园组;（3）阿什吉尔组中的下镇组、三巨山组和大渡河组。因此，该地区记录了部分早古生代礁扩张插曲，并允许在奥陶纪礁相的变化进行更广泛的评估。中Tremadoc期，在扬子地台中部高能带发育以海绵苔藓虫-Calathium-钙质微生物为主的粉香组片礁。柱状非骨架叠层石的伦山组（与Fenhsiang组同时代）发生在东南缘的平台，在那里的生物多样性低的珊瑚礁附着，和一般细粒的沉积物的字符，是符合深度低于正常波的基础。石笔海绵和石海绵是晚期Tremadoc礁的主要造礁者，并广泛分布于宜昌（湖北省）和东至（安徽省）地区，从台地中心到边缘设置。苔藓虫造礁者只出现在平台中心，具有高度多样性的珊瑚礁居民，如腕足动物，三叶虫和鹦鹉螺。然而，在同一时期，在台地边缘，微生物在珊瑚礁的建造中发挥了重要作用，与Calathium和lithistid海绵一起。中晚期Tremadoc lithistid海绵苔藓虫Calathium微生物群落被中期Ashgill珊瑚层孔虫群落所取代，表明奥陶纪期间群落更替持续了相对较长的时间。受扬子区主体黑色页岩的限制，中阿什吉尔礁群仅见于华夏陆台东北部，介于江西玉山和浙江常山之间。扬子地台西缘发育碳酸盐泥丘。华夏陆东北台地下镇组的礁片（部分高浮雕）由珊瑚、层孔类、钙微生物、腕足类和腹足类等生物组成。三巨山组与下镇组的时代相当，含有由丰富的钙质藻类和钙质微生物组成的碳酸盐泥丘。在晚奥陶世的第一次灭绝事件之前，古陆的扩张迫使华夏陆向北延伸，并导致区域相对海平面下降，消除了这一礁复合体。大渡河组碳酸盐岩泥丘产于康滇陆（扬子地台西缘）近岸带，上覆于赫南田南城组之上。总体而言，中国南方的奥陶纪生物礁显示了一系列的栖息地和控制的增长和消亡。
Ordovician reefs of the South China Block occur chiefly in three stratigraphic units: (1) the middle Tremadoc Fenhsiang Formation and the upper part of the Lunshan Formation; (2) the late Tremadoc Hunghuayuan Formation; and (3) the middle Ashgill Xiazhen, Sanjushan and Daduhe Formations. The region therefore records part of the Early Palaeozoic reef expansion episode and permits a broader assessment of change in Ordovician reef facies. During middle Tremadoc time, lithistid sponge–bryozoan–Calathium–calcimicrobially dominated patch reefs of the Fenhsiang Formation occur in the high-energy belt of the central Yangtze Platform. Columnar nonskeletal stromatolites of the Lunshan Formation (coeval with Fenhsiang Formation) occur on the southeastern margin of the platform, where low diversity reef-attached organisms, and the generally fine-grained character of the sediment, are consistent with a depth below normal wave base. Calathium and lithistid sponges were the principal reef builders of late Tremadoc reefs and were widespread in the areas of Yichang (Hubei Province) and Dongzhi (Anhui Province), from the platform centre to its margin settings. Bryozoan reef builders occur only in the platform centre with a high diversity of reef dwellers such as brachiopods, trilobites and nautiloids. However, during the same time, at the platform margins, microbes played an important part in reef building together with Calathium and lithistid sponges. The middle-late Tremadoc lithistid sponge–bryozoan–Calathium–microbial community was replaced by a middle Ashgill coral–stromatoporoid community and shows that the succession of community replacement during the Ordovician took a relatively long time. Restricted by the black shales of the main part of the Yangtze region, middle Ashgill reef complexes can be found only on the northeast platform of Cathaysian Land, between the Yushan (Jiangxi) and Changshan (Zhejiang) regions. Carbonate mudmounds are present on the western margin of the Yangtze Platform. On the northeast platform of Cathaysian Land, patch reefs (some higher relief) with talus and biostromes of the Xiazhen Formation consist of high-diversity biotas of corals, stromatoporoids, calcimicrobes, brachiopods and gastropods. The Sanjushan Formation is age-equivalent to the Xiazhen Formation and contains carbonate mudmounds that are composed of abundant calcareous algae and calcimicrobia. Uplift forced a northward extension of Cathaysian Land and caused a regional relative sea-level fall, eliminating this reef complex prior to the first extinction event of the Late Ordovician. The Daduhe Formation carbonate mudmounds occur in the nearshore belt of Kangdian Land (western margin of the Yangtze Platform) and are paraconformably overlain by the Hirnantian Nancheng Formation. In total, the Ordovician reefs of south China show a range of habitats and controls on growth and demise.