【作者】 赵国伟；

【导师】 王成文； 孙跃武；

【作者基本信息】 吉林大学 ， 古生物学与地层学， 2013， 博士

【摘要】 奥陶纪经历了诸如生物大辐射事件、奥陶纪末生物灭绝事件等一系列的变化。气候与环境的变迁无疑是推动这些变化的因素之一。海水是气候和环境变化的重要载体，海平面的变化能够在一定程度上反映了气候和环境变化的性质和范围，并能够直接与浅水陆表海海底演化、海底生态及生物多样性联系起来，在塑造海洋沉积体系过程中发挥着重要作用。在研究海平面变化的众多技术手段中，保存于海相碳酸盐岩中的碳、氧、锶同位素已经证明是重建奥陶纪环境变化、对不同沉积条件下或者不同古大陆沉积层序进行对比的一种有效工具。塔里木作为地质历史中一个主要地块，在奥陶纪位于北半球的中低纬度地带。巴楚地区位于塔里木西北部，在中—晚奥陶世经历了台地、台地边缘和斜坡的演化过程，广泛发育的碳酸盐岩地层，为研究碳、氧、锶同位素的组成与演化提供了很好的素材。针对巴楚地区碳酸盐岩露头的地层、古生物、岩相、沉积环境等研究也为在该地区开展碳、氧、锶同位素地层学和海平面变化研究提供了坚实的基础。为了重建塔里木巴楚地区中—晚奥陶世海平面变化，探讨巴楚地区碳、氧、锶同位素演化规律，本文在对塔里木盆地巴楚地区中—上奥陶统南一沟剖面和良里塔格组建组剖面实测的基础上，结合牙形刺等化石资料，建立了具有精确生物（年代）地层格架的碳、氧、锶同位素变化曲线，并依据碳、氧、锶同位素对海平面变化的响应机理，结合层序地层学，重建了该地区海平面变化曲线。最终获得以下成果：1.选作进行碳、氧、锶同位素测试的碳酸盐岩样品中未见或者极少见矿物、微晶结构的改变，赋存于岩石中的古生物体在结构上亦未发生改变或极少改变。δ18O值除一个样品为-11.7‰（鹰山组顶部）以外，均位于-10‰~-4‰之间。δ13C与δ18O值得分布是相当离散的，无明显的正向线性关系。表明塔里木盆地巴楚地区中—上奥陶统碳酸盐岩样品受到成岩蚀变的影响较小，碳、氧、锶同位素组成基本能够代表原始沉积时期海水的同位素组成。2.在塔里木盆地巴楚地区中—上奥陶统首次识别出4个δ13C和δ18O正向旋回及与其相应的4个87Sr/86Sr的反向旋回，分别命名为IC-1、IC-2、IC-3和IC-4。IC-1旋回出现在一间房组，IC-2旋回出现在恰尔巴克组和良里塔格组底部，IC-3和IC-4旋回出现在良里塔格组。3.以塔里木盆地巴楚地区中—晚奥陶世碳同位素记录为基础，通过与柯坪地区和塔中地区的对比，建立了塔里木板块中—晚奥陶世碳同位素演化谱。达瑞威尔中期碳同位素正向漂移（MDICE)在巴楚地区一间房组首次识别出来。桑比期末或凯迪期初的古腾伯格碳同位素正向漂移（GICE）在巴楚地区、柯坪地区和塔中地区均能识别。这些具有全球对比意义的碳同位素正向漂移事件在塔里木板块的出现，说明塔里木板块碳同位素变化同全球碳同位素演化有着良好的可对比性。4.塔里木板块的δ18O值自达瑞威尔期到桑比期呈现逐渐上升的趋势，在凯迪初期有一个下降以后，随后重新开始上升，并在赫南特期达到最大。这与全球通过腕足壳体和牙形刺体获得的氧同位素变化具有较好的一致性。这表明巴楚地区氧同位素虽然会受到成岩作用的改变而造成数值的减小，但是仍可保存部分原始沉积时期海水的同位素信息，在一些大的事件上可以进行全球对比。5.塔里木板块巴楚地区87Sr/86Sr值从一间房组的0.7090左右经过恰尔巴克组沉积时期快速降低到良里塔格组的0.7080左右。这与全球锶同位素从达瑞威尔期到桑比期87Sr/86Sr的快速降低趋势相一致。表明塔里木盆地巴楚地区中—晚奥陶世的锶同位素演化具有全球性的特点。6.通过对碳、氧、锶同位素与层序地层的综合研究发现，碳、氧、锶同位素与层序地层的短期旋回有着良好的吻合性。一间房组下部延续了鹰山组开始时期的海侵，中上部海平面有所下降，但在恰尔巴克组重新开始快速海侵，海平面达到最大，恰尔巴克组是海侵最大时期形成的凝缩层。良里塔格组底部有短期的海退，但随后重新开始海侵。通过与全球海平面变化的对比，结合塔里木板块构造运动及沉积速率等影响，表明巴楚地区中—奥陶世海平面主要受到全球海平面变化的影响，但是区域性构造活动和沉积速率对海平面的变化也发挥了很大的作用。7.通过对塔里木盆地巴楚地区中—晚奥陶世礁滩体发育层段的碳、氧、锶同位素的研究，发现生物礁体发育层段普遍具有比同时期地层要高的δ18C、δ18O、87Sr/86Sr值。碳同位素数值的增加无疑是因为礁滩体发育期生物生产率和埋藏作用加强引起的。氧同位素的增加可能是生物礁造成局部地势升高，露出地表遭受风化，引起周边海水中盐度增加造成的。锶同位素数值的增加，则可能与生物体对具有较高87Sr/86Sr的陆源物质的吸附有关。8.通过对塔里木盆地巴楚地区中—晚奥陶世礁滩体发育层位与相对海平面变化的研究，提出在海平面较高时期不利于礁滩体的发育，礁滩体主要发育于海平面上升初期。9.通过对不同微相单元礁滩体储层物性的分析显示，礁基、礁翼和礁内滩具有更高的孔隙度和渗透率，礁核、礁间和礁盖的孔隙度和渗透率则较小。这表明礁基、礁翼和礁内滩是较有利的储层。礁滩体储层随着孔隙度的线性升高，渗透率呈现对数级别的升高，表明礁滩体中孔隙度与渗透率有较好的相关性。这为在礁滩体发育层段寻找有利储层提供了帮助。总之，上述工作的深入开展，对提高塔里木盆地中—上奥陶统研究程度，进一步与全球典型剖面对比，扩大塔里木盆地中—上奥陶统礁滩相地层油气资源前景等方面，具有重要的科学意义和实际应用意义。更多还原

【Abstract】 The Ordovician underwent a series of manifold changes, e.g. the GreatOrdovician Biodiversification Event (GOBE) and the end-Ordovician extinction.Climatic and environmental changes are undoubtedly major drivers of these changes.Sea water is an important carrier of climate and environment; sea level, which canreflect the possible nature and range of climatic and environmental changes, exerts afirst order control on the architecture of marine sedments and is intimately related tosea-floor and substrate evolution, benthic ecology, and biodiversity in shallow-watercratonic seas. Among a wide array pf tecjmoqies developed for interpreting sea-levelchange, carbon, oxygen and strontium isotopes in marine carbonates have beenproved to be an effective tool to track Ordovician environmental change and correlatesedimentary sequences from different areas or even different continents. Tarim, as oneof the independent blocks, was situated in the middle and low latitudes in theNorthern Hemisphere. Continuous carbonate rocks, deposited in the sedimentaryenvironments including platform, margin and slope during Middle–Late Ordovician inthe Bachu area of Tarim Basin, provide material for studying the composition andevolution of carbon, oxygen and strontium isotopes. Detailed studies on stratigraphy,paleontology and lithofacies in this area provide the substantial basis for the presentresearch on the isotopic evolution and sea-level changes.Based on lithostratigraphy and conodont biostratigraphy from the Nanyigousection and the stratotype section of the Lianglitag Formation in the Bachu area,Tarim Basin, the Middle–Upper Ordovician biostratigraphically calibrated δ13C,δ18O and87Sr/86Sr curves and sea-level curves are reconstructed. The followingachievements are obtained: 1. The carbonate samples selected for carbon, oxygen and strontium isotopeanalysis show few crystal structure changes and paleobiological structure alterations.The values of δ18O are mostly between-10‰~-4‰(except a value of-11.7‰in theYingshan Formation). Meanwhile, the values of δ13C and δ18O without distinctivelinear relationship are quite discrete. All these indicate that the carbon, oxygen andstrontium isotope compositions involved in this study were less altered by thediagenesis, and can reflect the original isotope compositions of sea water.2. Four δ13C, δ18O positive cycles and four87Sr/86Sr negative cycles arerecognized for the first time in Middle–Upper Ordovician in the Bachu area, TarimBasin, and are named as IC-1, IC-2, IC-3and IC-4respectively, of which, IC-1cycleoccurred in the Yijianfang Formation, IC-2occurred in the Qiaerbake Formation andthe lowermost part of the Lianglitag Formation, IC-3and IC-4are mainly occurred inthe Lianglitag Formation.3. Based on the records of carbon isotope from Middle-Late Ordovician in theBachu area and the comparisons with the Kalpin and Tazhong areas, the carbonisotope evolution spectra in Middle-Late Ordovician is reconstructed for the wholeTarim Plate. The mid-Darriwilian δ13C excursion (MDICE) is recognized for the firsttime in the Yijianfang Formation in the Bachu area. The Guttenberg δ13C excursion(GICE) is recognized in the uppermost-Sandbian or lowermost-Katian in the Bachu,Kalpin and Tazhong areas. This indicates that the evolution of carbon isotope in Tarimshows a good consistency with the global carbon isotope evolution.4. The δ18O values in the Bachu area showed an increasing trend during theDarriwilian and Katian, a slight decrease in the early Katian, and another increase inthe Katian, which are correlated with the global oxygen isotope curves based onbrachiopods and conodonts. Although δ18O values are often diagenetically altered andtrend to smaller values, they still reflect part of the original signal of sea water, andare correlated to the global oxygen isotopes evolution.5. In the Bachu area, the strontium isotope curves show higher87Sr/86Sr values of0.7090in the Yijianfang Formation, rapidly drop in the Qiaerbake Formation, and getlower values of0.7080in the Lianglitag Formation. The evolution in the studed areais compatible with the global trends, which are characterized by a major drop in theDarriwilian–Sandibian.6. Comprehensive researches on carbon, oxygen and strontium isotopes withsequence stratigraphy show that the cycles of carbon, oxygen and strontium isotopes are correlated to the short-term cycles of sequence stratigraphy. The sea-levelcontinued the increased trend in the Yijianfang Formation, and got the maximum inthe Qiaerbake Formation. The Qiaerbake Formation is considered to be the condensedsection in maximum flooding. After a regression in the lower part of the LianglitagFormation, a transgression occurred to in the upper part of the Lianglitag Formation.The results show that the global sea-level fluctuations is an influencing factor in theBachu area, and the regional tectonic activities and deposition rates are also anotherdriver to the relative sea-level changes in the Bachu area.7. The reef-bank complexes show higher δ13C, δ18O and87Sr/86Sr values than thatof the coeval carbonate strata. The higher δ13C values are explained in terms ofincreased productivity and enhanced burial of organic matter. The higher δ18O valuesare mainly caused by the increasing salinity in sea water due to the enhancedbiological weathering when the reef-bank complexes are exposed out of the seasurface. The higher87Sr/86Sr values are mainly caused by the adsorption of theterrestrial material.8. The research on the relationship between relative sea-level changes and thegrowth of reefs in Middle–Upper Ordovician in the Bachu area, Tarim Basin, showsthat reefs are mainly developed in the sea-level rising periods, but too deep water isnot beneficial to the growth of reefs.9. The analysis on the porosity and permeability in different microfacies ofreef-bank complex in the Bachu area show that the reef bases, reef flanks and bankspossesses higher porosity and permeability than the reef cores, reef inners and reefcaps. The reef bases, reef flanks and banks are the favorable reservoirs. The values ofporosity and permeability have a positive relationship. When the porosity isincreasing linearly, the permeability increases logarithmically. This is helpful toexplore favorable reservoirs in the reef-bank complex.In sum, the above research is significant for the further study in Middle–UpperOrdovician in the Bachu area of Tarim Basin, particularly for promoting the accuracyin stratigraphic correlation internationally. Meanwhile, it would be also beneficial formaximize oil and gas potential in reef-bank complex in the Bachu area of TarimBasin.更多还原