【作者】 张华；

【导师】 时志强；

【作者基本信息】 成都理工大学 ， 地质学， 2011， 硕士

【摘要】 上扬子地区P-T转折时期广泛发育各种类型的错时相碳酸盐岩,主要包括微生物岩、蠕虫状灰岩、条带状灰岩、纹层灰岩以及扁平砾石砾岩。微生物岩整体呈丘状产出,内部纹层发育；其主要岩性为粒泥灰岩和泥粒灰岩；岩石内部生物较少,种类单一,主要为一些介形虫、小型腹足和小型腕足等生物化石。P-T过渡时期的蠕虫状灰岩发育于下二叠统长兴组生物碎屑灰岩之上,其所属时代为晚二叠世。蠕虫状灰岩主要由浅色“基质”和深色“蠕体”两部分组成,蠕体方解石含量较高,泥质含量较低；而基质方解石含量较低,泥质含量较高。根据蠕体分布形态将蠕虫状灰岩分为三类,即蠕体呈顺层连续的线纹状分布的蠕虫状灰岩、蠕体呈顺层断续的点线纹状分布的蠕虫状灰岩以及蠕体呈异形杂乱状分布的蠕虫状灰岩。各类蠕虫状灰岩在形成演化上具有一定的先后顺序,蠕体呈顺层连续状分布的蠕虫状灰岩最先形成,顺层断续状次之,蠕体呈异形杂乱排列的蠕虫状灰岩最后形成。各类蠕虫状灰岩在成因上相互联系,其形成与沉积分异作用、成岩作用以及后期水动力改造作用有关。条带状灰岩常与泥晶灰岩、粉砂质泥岩等呈互层产出。镜下显示,条带常为规则的泥质、泥灰质薄层或不规则的泥质团块。泥质纹层被压实后对碳酸盐团块的干扰现象比较普遍,显示不规则的灰质团块是后期压实作用的结果。扁平砾石砾岩见于广元上寺和青川大沟里P—T界线地层剖面中,其常与薄层状、纹层状泥质灰岩、灰质泥岩共生,并伴随有丘状交错层理、菊花状构造、底面侵蚀构造等反映风暴作用的标志性沉积构造。研究表明,扁平砾石砾岩的形成大致经历了以下四个阶段：(1)薄层状灰岩—灰泥岩快速沉积阶段；(2)灰岩等富碳酸盐层变形、破碎；(3)富泥层对富碳酸盐层的进一步破坏；(4)风暴等对前期砾屑的侵蚀、搬运阶段。P-T生物大灭绝后,全球环境剧变。错时相沉积在这一时期广泛出现,指示了当时紊乱、异常的大气条件和海洋环境。微生物岩在大灭绝后迅速出现,表现出微生物等对当时恶劣的环境具有较强的适应能力,并经历了存活—稳定—繁盛的自身演化过程；条带状灰岩、纹层状灰岩、蠕虫状灰岩以及扁平砾石砾岩的岩石学特征及成因机制表明,这些错时相沉积的形成具有相似的海洋化学条件和沉积背景。原始沉积的成层特征保存完好是这些错时相共有的特征,其可能反映了生物大灭绝后底栖后生动物的减小和生物垂向扰动能力的降低；而可能由深部碳酸盐过饱和的贫氧海水上翻以及微生物新陈代谢引起的同沉积海底胶结作用,则是这一时期这些错时相沉积体或沉积构造得以形成的又一关键因素。薄层状灰岩、纹层状灰岩等静水沉积物之上,普遍发育与强水动力条件有关的扁平砾石砾岩、角砾状灰岩、丘状交错层理粉砂岩,反映了特提斯洋在这一时期曾经历过从停滞到动荡的海洋环境变化。大规模的风暴沉积物在全球下三叠统地层中广泛出现,表明了当时的气候处于极端异常的状态,气候的剧烈变化可能与当时盛行的巨型季风有关。停滞的海洋循环、底层海水普遍缺氧,碳、氧、硫同位素负偏、H2S和CO2的增多,海水碱性增强、盐度升高、同沉积海底胶结作用的广泛发生等构成的早三叠世海洋环境,是错时相沉积得以发育并作为P—T过渡时期重要沉积类型的基础。强烈的巨型季风、频繁的风暴活动等外界因素对沉积物的侵蚀、再改造,最终导致类型丰富的错时相碳酸盐岩在全球这一时期广泛再现。灭绝事件后,贫瘠的生物圈对地球系统的反馈作用随之减弱,原本混乱的地球系统在得不到生物圈有效制约的状况下,进一步恶化直至走向极端,而以错时相为代表的异常沉积以及风暴岩的大规模出现正是始于对环境走向极端的自然响应。我们将生物圈对地球系统的这一调节过程称之为Gaia效应。更多还原

【Abstract】 Various types of anachronistic facies including microbiolites, vermiculate limestone, ribbon limestone, laminated Limestone and flat-pebble conglomerate were developed in Upper Yangtz area during the P/T transforming interval. Microbiolites were found as dome shapes on outcrop and interior lamina occurred in the microbiolites,which were formed by wackestone and packstone, characteristicing for less organisms and monotonous species inside, such as, ostracodes, minitype proleg and tiny brachiopoda etc..Vermiculate limestone was underlied by the bioclastic limestone in the toppest layer of upper Permian Changxing formation, and the geological age of the vermiculate limestones was infered as late Permian. The vermiculate limestone mainly consisted of light-colored ’matrix’ and dark-colored ’worm grains’, the former contained higher component of calsite and lower component of mud than that of the latter. Vermiculate limestone could be divided into following three main types according to its distribution shape:(1) continuous line shaped, (2) intermittent line shaped, (3) strange and littery shape. The continuous line shaped one was formed first, the second was the intermittent line shaped vermiculate limestone and the strange and littery shape was the last one. It’s thought that all the three types of vermiculate limestones were interrelated to their genesis, and the formation of these rocks were associated with sedimentary differentiation, diagenesis and the rework of the hydrodynamic force after thet were deposited.Ribbon limestone often occurred with interbedded micritic limestone or silty mudstones. The microscopic features showed that ribbon limestone was usually comprised by regular thin mud bed and irregular marl crumb. With the influence of compaction, the mud lamina often disrupted the carbonate crumb, which indicated that the formation of irregular calcareous crumb was resulted from the compaction. flat-pebble conglomerate was often found in P/T Stratigraphic profile in Guangyuan Shangsi and Qingchuan Dagouli. It usually co-occured with thin or lamina argillaceous limestone and calcilutite, and was often accompanied by the storm-generated sedimentary structures, such as hummocky cross bedding, chrysanthemum structure, base erosion structure, etc.. The formation of the flat-pebble conglomerate had experienced the following four stages:(1) the quick deposition of thin limestone-calculutite; (2) the deformation or the breaking of the carbonate-rich layer, (3) the further disruption to carbonate-rich layers by mud-rich layers, (4) the erosion and transportation of the former calcrudite by storm.The globle climate sharply changed after the P/T mass extinction. Anachronistic facies widely appeared during this period, which indicated the disorder and unusual atmospheric conditions of the marine environment.Microbiolites growing rapidly after the extinction demonstrated that the microbiolite could adapt to the degradation condition at that time, and had experienced survival-stable-prosperous evolutionary process. According to the petrological features and genetic mechanism of the vermiculate limestone, ribbon limestone, laminated limestone and flat-pebble conglomerate demonstrate, it was concluded that the marine chemical conditions and depositional setting were almost similar when they were deposited. Stratification was in good keeping of the primary deposit, which was the common character of the anachronistic facies, indicating the decrease of metazoon and the receding of its vertical disturbance. The synsedimentary cementation caused by the upturning of poor-oxygen oversaturated-seawater and by metabolism of microbiolite, was the other main factor for the formation and conserve of sedimentary structure of the anachronistic facies. The laminated limestone, thin-bedded pelmicrite, argillaceous limestone were widely covered by storm-generated brecciform limestone, flat-pebble conglomerate calcirudyte, hummocky cross-bedding siltstone, which reflected that the Tethys during this interval might experience the changes from the sluggish to the oscillating sea. The storm-generated sediments widely emerged around the world in the lower Triassic, which indicates that the atmospheric conditions were in an extremely abnormal state. The severe changes in atmospheric conditions might be correlated with the ’megamosoon’. The stagnant ocean circulation, the comprehensive anoxia in bottom water, the increase of atmospheric CO2 and H2S, enhancing of alkaline and salinity, along with the extensive synsedimentary cementation in seafloor, were the major conditions for the development of the anachronistic facies at the P-T boundary. On the other hand, the external factors such as violent megamonsoon and frequent storms result in the erosion and rework to the sediments, which finally resulted in the reoccuring of the abundant types of anachronistic facies at this time.After the mass extinction, feedback effecting to the earth system by barren biosphere got weaker. Due to the lack of effective restricting by biosphere, the chaotic earth system further deteriorated and finally arrived its extreme. As the representative of the abnormal deposits, anachronistic facies were just the response to an extreme natural environment. Therefore, this process that the biosphere adjusts the earth system was named as Gaia effection.更多还原