【作者】 曾德勇；

【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】 可可西里盆地中新世发育一系列古湖泊,而高原内陆湖泊沉积易受气候波动的影响,具有典型明暗叠置纹层的叠层石有效地记录了气候振荡信息,对湖相叠层石的研究意义重大。本文以沉积学、地层学及古生物学相关理论为指导,针对可可西里盆地中新统五道梁组湖相叠层石岩石学、微生物学特征,运用野外和室内、宏观与微观相结合的方法,以常规显微薄片、扫描电镜观察为基础,结合常量/微量元素、稳定C、O同位素等地化资料,详细描述叠层石宏观、微观结构特征,深入研究叠层石发育期沉积环境特征,并获得可可西里地区高分辨率的气候信息。这对丰富叠层石成因理论和探讨青藏高原隆升都有一定的意义。首先利用野外分层描述、室内显微薄片及扫描电镜观察,对可可西里地区1口含叠层石的钻探全取芯井剖面和4条前人实测露头剖面岩石学特征进行详细分析,尤其是各剖面中叠层石灰岩的出露特征;总结了可可西里地区中新世五道梁组主要岩性类型有泥灰岩、泥晶灰岩、叠层石灰岩、颗粒灰岩和膏溶角砾岩。盆地南北向分布的剖面对比显示中新世地层分布厚度极不均匀,呈北厚南薄的横面展布,底部与下伏渐新世雅西措组常呈微角度或平行不整合接触,顶部被剥蚀。其次运用宏观与微观相结合对叠层石进行观察,发现叠层石类型极其丰富,有波状、菜花状、椭球状、锥状、球状、柱状以及它们的组合形态,归纳起来主要有以下4种类型:穹隆状叠层石、丘状叠层石、柱状叠层石和层状（波状）叠层石;叠层石明暗纹层保存完整,形态上主要有层带状、波状和柱状,纵剖面上常组合出现;叠置纹层中富屑层有明显的颗粒结构,颗粒类型主要有内碎屑、球粒及少量陆源碎屑;富藻纹层为致密状的高有机质泥晶方解石构成,表现为泥晶结构。根据叠层石结构的差异性,按成因不同,将叠层石划分为骨架叠层石、凝集叠层石和细粒叠层石三种类型。再次,详细观察钻探取芯发现剖面纵向上可分辨出三个叠层石发育期,自下而上对应厚度分别为72-103m、22-37m和9-13m,且每个阶段对应环境皆主要为浅湖,仅仅第二次早期沉积为滨湖环境。第一次叠层石形成于构造环境稳定,造叠微生物生活于水下45cm水位至近水表层活动,并以较均一的沉积速率形成纹层;第二次叠层石形成于浅湖环境的极浅水区,且可能周期性的暴露更有利于该类叠层石的出现;第三次叠层石中同沉积的粉砂级陆源碎屑偏多,推测离湖岸更近,更趋于滨湖环境。此外,利用微量元素、常量元素,C、O同位素测试,对叠层石样品地球化学特征分析表明:Sr/Ba值从1左右至2.5以上,叠层石湖泊水体盐度可能处于微咸水-咸水条件;Ca/Mg比都在50以上,生物作用干预强烈,浮游生物大量繁殖促使碳酸钙沉淀;Fe/Mn比相对其他岩性较高,Fe²⁺含量偏高,叠层石发育区水体较浅,还原性更强;叠层石纹层C、O同位素值表现趋同,富藻层和富屑层δ（13）^C值皆为正值,δ（18）^O值为负,富藻纹层的同位素值又明显高于富屑层。碳氧同位素值表明藻纹层形成于蒸发量高、湖泊咸化的冬半年干旱期,而较为湿润的夏半年因风暴频发带来大量的陆源碎屑形成富屑层。在此基础上,构建了叠层石纹层生长模式图及本区叠层石横向展布图。最后,联系可可西里地区早中新世干旱的气候背景,分析叠层石沉积期气候曾持续湿润,湖泊降雨量远大于蒸发量,且形成“五道梁古大湖”。推测可可西里地区中新世早-中期海拔高度已超过了2000m,并已超过影响行星风系的临界值,阻挡了来自海洋湿润气流进入内陆,导致气候变干。而叠层石沉积期湖泊气候相对湿润,可能是区域性气候循环的结果。更多还原

【Abstract】 There were a series of Miocene lakes in the Hoh Xil Basin, while lacustrine sediments in the plateau were vulnerable to climate fluctuation. The paleoclimatic informations were recorded effectively in the typical light and shade folds of the Miocene stromatolites. Thus the study of the lacustrine stromatolite layers is of great significance.Based on sedimentological, stratigraphical, palaeontological analysis and related theories, the macroscopic and microcosmic structural characteristics and sedimentary environment of stromatolites were deeply researched. Combining outdoor and indoor, macro and micro investigation, in turn according to the petrological, microbiological characters of Miocene lacustrine stromatolites of Wudaoliang Group in Hoh Xil Basin , high resolution climatic informations in research area were obtained. Thin-section observation, scanning electron microscopy （SEM）, major elements/trace element analyses, table C/O isotopeand,etc. were used to study the genetic theory of stromatolites and to discuss the uplift of Qinghai-tibet Plateau.Firstly, the petrological features of a major drilling-coring profile and four minor profiles previous measured in Hoh Xil Basin were described in detail according to field strata description, thin-section observation and SEM. The outcrops of stromatolites were especially paid attention. The main lithological types with muddy limestone, micritic limestone, stromatolites, grainstone limestone and anointed dissolve breccia in Wudaoliang Group were also summarized. The comparison of south-north strata showed the strata thickness was extremely uneven. There was a higher thickness in the north and a thinner one in the south. The bottom of Wudaoling Group overlied on the Oligocene Yaxicuo Group with a tiny angles or parallel unconformity, which indicated that the toppest layers of Yaxicuo Group was eroded to a certain degree.Secondly, combining the macro and micro structural characteristics, it was found that the stromatolite types were extremely abundant, including wave shape, cauliflower shape, directed asphere shape, cone shape, granlar columnar shape as well as their combination form. While the major three types were seen as vault shape stromatolite, dome shape stromatolite and layer （wavy） stromatolite. The stromatolite layers were well-preserved and were mainly trip-shaped, ripple-shaped and column-shaped. Their combination form were commonly showed in the longitudinal profile. The grains were mainly intraclast, spherulite and a few terrigenous elastic minerals in the grain-rich layers （light layers） , while the algae-rich layers were made of high content of organic matters, displaying micrite structure. The stromatolites in Wudaoliang area might be classified as skeleton, agglutinated and fine grained stromatolites which also implied different genesis.Thirdly, based on detailed observation of the drilling core, three zones of stromatolite could be found. Upwards, the corresponding depth of the stromatolites were respectively 72-103m, 22-37m and 9-13m, and the sedimentary facies of each stromatolitelayer was mainly shallow lake or lake shoreline. The first layer of the stromatolites was developed in a stable tectonic setting, and the microbes which made stromatolites lived underwater 45cm to nearly water surface. The stromatolites of second layer grew in the extremely shallow lake and were likely to appear in periodically shallow waters. While there were more terrigenous minerals in the third layer of stromatolites, which was speculated that it developed in the shoreline lake area closer to shore.In addition, accorrding to the geochemical analyses including the major elements/trace element,C/O isotope of stromatolites sample, the following character s were showed: the Sr/Ba ratios ranged from 1 to above 2.5 and stromatolites maybe grew in the brackish water– salt water. The Ca/Mg ratios could reach above 50 due to strong biological action and thriving plankton urge carbonate precipitation, while if the Fe/Mn ratio and Fe²⁺ were higher, the stromatolites developed in the shallow water and reducibility was stronger. The C isotope values of the stromatolites had similar characters to the O isotope values. And theδ（13）^C values of grain-rich and algae-rich layers were positive, but theδ（18）^O were negative. Generally, the values of grain-rich layers were higher than the algae-rich layers. These values of C and O isotope indicated that algae-rich layer was developed in drouthy winter of strong evaporation and high salinity. In relatively wetter summer, algae-rich layer was formed due to the storms which brought a lot of terrestrial detritus. Based on the above research, the mode chart about stromatolite layer was built.Finally, it was concluded that the climate background was generally of drought during the early Miocene interval in the Hoh Xil region, while it was moist vs each period of stromatolite deposited. The rainfall far exceeded the evaporation so as to a "Wudaoliang Ancient Great Lake" formed. The altitude of Hoh Xil region might have exceeded 2000m in early-middle Miocene. Thus the higher altitude influenced the planetary system of winds, which in turn prevented the moist air into the inland from the ocean and caused the dry climate. But it was relative wet when stromatolite deposited, possibly resulted form the regional climatic circulate.更多还原