【作者】 张利伟；

【导师】 洪天求；

【作者基本信息】 合肥工业大学 ， 矿物学岩石学矿床学， 2010， 博士

【摘要】 安徽省巢湖地区位于下扬子地块北缘,保存有完整的早三叠世沉积记录,是华南地区下三叠统研究的经典地区之一。该地区下三叠统和龙山组相当于Olenekian阶Smithian亚阶的中上部,厚度在17.19-30.10m之间,根据岩性特征可划分为5段：第1段为中厚层泥质灰岩夹薄层钙质泥岩；第2段为条带灰岩与钙质泥岩互层,条带灰岩中微生物席发育；第三段为泥质灰岩、条带灰岩、钙质泥岩三种岩性互层；第四段与第三段的主要区别在于岩层内含丰富的黄铁矿颗粒;第五段岩性以钙质泥岩为主,下部含微生物席、叠层石薄层或透镜体,向上泥质灰岩层含量增加。其中微生物岩的发现为沉积古环境重建、环境变化与生物响应关系分析和早三叠世生物复苏时限、过程、机制等问题的探讨提供了重要材料。和龙山组以细粒沉积物主,发育波状层理、透镜状层理、脉状层理、侵蚀槽等流水成因沉积构造和微生物岩、微生物沉积构造(MISS)、遗迹化石等生物沉积构造,产出草莓状黄铁矿、石盐晶体和自生石英晶体。碳酸盐岩微相分析表明,和龙山组属浅水低能的开阔—局限台地相沉积,由下向上经历了由潮间—泻湖—潮间—浅水潮下的水体变浅然后又变深的环境变化过程。沉积特征与微相特征还显示,和龙山组第4段沉积期存在海洋水化学条件异常现象。通过光学显微镜与扫描电镜等实验手段,在和龙山组微生物岩中鉴别出蓝藻、绿藻、颗石藻和硫细菌等微体化石,结合和龙山组菊石、双壳、牙形石等生物类型的分布特征判断,巢湖地区在和龙山组沉积早期与晚期分别存在一次生物演化上的突变事件。和龙山组沉积早期的事件具有区域性,可能与火山喷发、沉积环境水体变浅有关,菊石、牙形石受影响较严重,双壳类则未受到太大影响,事件后巢湖地区微生物席发育(和龙山组第2段)。和龙山组沉积晚期的事件是全球性环境变化的结果,造成适合灾变期环境的双壳类生物的绝灭,菊石、牙形石等生物受影响较小,伴随着微生物席、叠层石等微生物岩的普遍发育(和龙山组第5段)。根据古生物发育特征和“生产力过剩假说”建立的微生物岩发育模型表明,和龙山组沉积晚期的生物演化突变事件是环境改善的结果,和龙山组沉积晚期的微生物岩普遍发育对生物复苏起促进作用,是生物开始复苏的标志。同位素地球化学与元素地球化学数据分析表明,和龙山组沉积于近陆局限浅水环境,沉积早期(第一、二、三段)海洋环境在在富氧与缺氧状态之间周期性变化,和龙山组沉积晚期存在一段明显的贫氧—厌氧期,直到和龙山组沉积末期才变为富氧环境。和龙山组沉积期从早到晚存在盐度逐渐升高的过程,早期海水受淡水影响盐度偏低,晚期回归正常水平。和龙山组顶部(Smithian\Spathian界线附近)碳同位素变化特征显示,和龙山组沉积末期存在海洋生产力与生物埋藏量的快速增加,这一结论有力支持了微生物岩发育模型的可靠性。更多还原

【Abstract】 Located in Anhui Province, South China, the Northern margin of the Yangtze Block, Chaohu area is one classic area for Triassic researches because there developed unabridged Lower Triassic. The Helongshan Formation in this area, corresponding to mid-upper Smithian, is about 17.19-30.10 meters thick, can be subdivided into 5 parts according to lithologic features. Part 1 is mid-thick mud limestones intercalated with thin-layered calcareous mudstones; part 2 is banding limestones interbeded with calcareous mudstones, in which microbial mats developed; part 3 is mud limestones and banding limestones interbeded with calcareous mudstones; part 4 is realy like part 3 in lithology, but pyrite is rich in this part; part 5 is dominated by calcareous mudstones, microbial mats and stromatolite lenticles can be found in the lower part and mud limestones gradually increased in the upper part.The microbialites, which were found in the Helongshan Formation, are important materials for reconstructing paleoenvironments, analysising the relationship between environment changes and biological response, discovering the timelimit, process and mechanism of the early Triassic biotic recovery. Dominated by fine-grained sediments, current structures such as wavy beddings, lenticular beddings, flaser beddings, bimodal cross beddings and gutter-filling structures, organic structures such as microbialites, microbially induced sedimentary structures(MISS) and trace fossils, and special minerals such as framboidal pyrites together with minor halite crystals, authigenic quartz are preserved in the Helongshan Formation. Carbonate microfacies indicate that the Helongshan Formation was deposited in open-restricted platform with low energy. Chaohu area experienced intertidal, lagoon, intertidal and shallow subtidal environments successively during the depositional period of the Helongshan Formation. Sedimentary characteristics and microfacies characteristics also indicate that there was an abnormal change at the part 4 of Helongshan Formation.In this paper, many microfossils such as cyanobacteria, green-algae, coccolith and sulfar bacteria have been indentified in microbialites of the Helongshan Formation. The distribution of macro and micro fossils indicates that abrupt biotic variation events happened at the early and late Helongshan Formation depositional stages. On a regional scale, the earlier event might be related to volcanic eruption and/or marine regression, ammonites and conodonts were mauled heavily, but bivalves were not affected significantly, and microbialites bloomed after the event. On the contrary, the later event was corresponding to global change, which had serious effects on bivalves, but minor effects on ammonoids and conodonts, accompanied by Microbialites development. The microbialite development model, which constructed on the basis of paleontologic characteristics and "excessive productivity hypothesis", suggests that the microbialites developed in the late depositional stage of the Helongshan Formation might promote the biotic recovery and can be considered as a sign of the biotic recovery.Isotope geochemistry and element geochemistry analyses manifest that the Helongshan Formation was deposited in restricated shallow ocean near land, normoxic and hypoxic conditions changed rhythmically during the early Helongshan depositional period; it was hypoxic to anoxic during the late Helongshan depositional period till to the end. Salinity of seawater was lower in the. early period and back to normal level in the late period of the Helongshan Formation. The excursion ofδ13C near the top of Helongshan Formation (Smithian\Spathian) shows that a rapid increase of the marine productivity and abundant biotic deposits. This deduction supports the microbialite development model strongly.更多还原