准噶尔盆地南缘中新世湖相叠层石特征及其构造—环境意义

The relationship between tectonic uplift and paleoenvironment evolution since the Late Cenozoic is adressed as one critical scientific issue. A significant and continuous transition in sedimentary facies of the southern margin of the Junggar Basin exists from shore-shallow lacustrine, braided river to alluvial fan throughout the Miocene to present-day. An accurate investigation on evolution of sedimentary environment during the Early Miocene in the southern Junggar Basin is thus favored for better understanding the intracontinental orogenesis and its effects on regional environment. The newly discovered lacustrine stromatolites from the bottom of the Taxihe Formation developed exactly in the earliest Miocene, recording the process of environmental evolution during this critical phase in tectonic transformation in the Tian Shan. This project intends to further constraint the initiation time of significant tectonic uplift and exhumation in the North Tian Shan by accurately investigating spatial distribution and minimum depositional age of the Miocene lacustrine stromatolites, and makes a scruting into the vertical accumulated sequence of these lacustrine stromatolites in the Taxihe section by sampling and analyzing continuously. Multiple approaches in terms of petrology and element - stable isotope geochemistry (including carbon, oxygen and magnesium isotopes) are applied to reveal fabric and geochemical features of the studied stromatolites. The changes of lacustrine salinity and terrigenous debris input can thus be emphatically traced to discuss evolution of the lake area and erosion within the source area during initiation of the Taxihe Formation sedimentation. The expected results will provide new evidences for a better understanding of tectonic-environmental evolution throughout this critical phase in tectonic transformation in the North Tian Shan.

晚新生代以来天山构造隆升与环境变化的关系是一个重大科学问题。准噶尔盆地南缘中新统及上覆沉积体系呈现滨浅湖—辫状河—冲积扇相的过渡变化。揭示准噶尔盆地南缘早中新世的沉积环境变化，对深入认识天山陆内造山过程及其区域环境效应十分重要。新发现的塔西河组底部湖相叠层石恰好沉积于早中新世早期，记录着这一天山构造转换关键时期内的环境演化信息。项目拟通过明确准噶尔盆地南缘中新世湖相叠层石的空间分布及最小沉积年龄，约束北天山显著构造隆升—剥蚀的起始时间；依照塔西河剖面叠层石的纵向堆叠层序连续取样分析，通过岩石学、元素和稳定同位素（碳氧、镁同位素）手段查明叠层石的组构和地球化学特征，明确湖泊水体咸化程度、陆源碎屑输入量变化，揭示塔西河组沉积初期湖盆范围与源区风化剥蚀程度的演变。成果将为清晰认识北天山构造转换关键期内准噶尔盆地南缘的构造—环境演化提供新的证据支撑和信息补充。

晚新生代以来天山构造隆升与环境变化的关系是一个重大科学问题。准噶尔盆地南缘中新统及上覆沉积体系呈现滨浅湖—辫状河—冲积扇相的过渡变化。揭示准噶尔盆地南缘早-中新世的沉积环境变化，对深入认识天山陆内造山过程及其区域环境效应十分重要。准噶尔盆地南缘塔西河剖面塔西河组底部湖相叠层石作为独特的高分辨率沉积载体，忠实刻录着晚新生代北天山构造转换关键时期内的环境演化信息。本项目在厘定第三纪湖相叠层石的平面与纵向分布范围基础上，以塔西河剖面塔西河组底部湖相叠层石为重点研究和解剖对象，运用岩石学、元素与稳定同位素地球化学等分析手段，开展叠层石的组构特征、地球化学特征、构造—环境意义三个方面的研究。具体包括：① 准南中新世湖相叠层石的组构特征研究；② 准南中新世湖相叠层石的地球化学特征研究；③ 准南中新世湖相叠层石的构造—环境指示意义研究。结果表明：天山南北缘前陆中—新生界陆相盆地叠层石发育分布局限，准南塔西河剖面中新世湖相叠层石确为区域构造—环境演化的宝贵沉积记录。准南中新世湖相叠层石明（富屑）、暗（富藻）薄层交替叠置而成的韵律纹层发育，沉积不同阶段的微观岩矿组成差异显著，自核部至边缘泥晶组分逐步增大；叠层石堆叠过程中微观组构特征持续演进，叠层石堆叠后期陆源碎屑沉积矿物明显更为富集。传统与Clumped同位素综合分析示踪准南中新世湖相叠层石沉积期气候趋于干冷，湖盆蒸发作用趋于强烈，新近纪以来湖泛面积大规模萎缩、陆源碎屑剧烈注入，反映着新近纪天山构造活化影响下，北天山的开始广泛回春隆升与强烈剥蚀。该区域构造转折事件的发端被巧妙地解码于特殊的湖相叠层石沉积构造。研究区邻区下伏上渐新统、上覆中-上中新统—上新统沉积记录的研究成果在为盆山耦合进程提供进一步的年代学证据的同时，亦有力地佐证了准南造叠期沉积充填加剧、古气候趋向干旱的区域构造机制。