柴北缘上石炭统混积层系对古海平面及古气候变化的响应

Clastic and carbonate rocks are produced from "exogenous, muddy water" and "endogenous, clean water" environment, respectively. The frequent alternations of clastic and carbonate facies reflect the repeated, major changes in multiple environmental factors, such as the nature of palaeowater, sediment supply and palaeoclimate. During the late-Carboniferous, the northern Qaidam Basin is tectonically stable, and thus providing conditions for development of extremely thick mixed clastic-carbonate deposits, which consist of multistage cycles. It records “icehouse” information such as the glacier activities and their influence to the palaeo sea level fluctuations as well as the frequent palaeoclimate and palaeoenvironment changes, and therefore can be used as tools and bridges for palaeo sea level and palaeoclimate changes study. In this study, we highlight the response of the sedimentary cycles at different scales to the palaeoclimate change. The typical outcrops and drilling cores from the northeastern margin of the basin are used to analyze the palaeo sea level change driven by the palaeoclimate variations. Based on the synergetic evolution of palaeoclimate-environmental events and their related sedimentary records, we analyze the sedimentary characteristics and their major controlling factors of all sequence levels. By the methods of multi-disciplinary, involving sedimentology, sequence stratigraphy and geochemistry, we will reconstruct the regional, short-term and long-term palaeo sea level change history, build the short-term paleoclimatic model, and compare with the global climate glacier climate to understand the relationships between the late-Carboniferous climate-environment and their coupling succession in the Qaidam Basin, and thus making further comparison with the regional or global palaeoclimate and sea level change history for future studies.

碎屑岩和碳酸盐岩分别产自“外源、浑水”与“内源、清水”环境，二者交替互层沉积反映古水体性质、物源供给及气候等环境要素的频繁改变。晚石炭世柴北缘构造稳定，发育了巨厚、多级次嵌套的“碎屑岩-碳酸盐岩混积层系”，记录了“冰室地球”冰川活动下的古海平面大幅度进退和古气候、古环境频繁交替信息，是研究该时期古气候-古海平面变化过程和机制的窗口。本课题以气候-环境事件与沉积记录的协同演化为切入点，针对不同尺度沉积-气候响应关系，围绕气候转换驱动的古海平面变化过程，以柴北缘石灰沟连续完整露头剖面和钻井取芯剖面为研究对象，聚焦不同级次混积层系沉积特征和主控因素，通过系统沉积学、层序地层学和地球化学等多学科交叉技术，恢复区域短期、长期古海平面变化历史，建立短期古气候模型，并对比全球冰期长期古气候模型，揭示柴北缘晚石炭世不同级次混积层系的成因，探讨柴北缘晚石炭世不同尺度古气候-环境耦合演替规律及区域或全球对比。

柴北缘石炭系是潜在的重点油气勘探接替领域，其中，上石炭统广泛发育的“碎屑岩-碳酸盐岩混积层系”记录了区域构造相对稳定背景下，“冰室地球”冰川活动下的古海平面大幅度进退和古气候、古环境频繁交替信息，是研究该时期古气候-古海平面变化过程和机制的窗口。通过对柴北缘上石炭统碎屑岩-碳酸盐岩混积层系开展高精度的沉积旋回记录及其古构造、古海平面变化及其古气候意义的研究，建立了晚石炭世高精度的沉积旋回和层序地层结构，揭示了不同级别层序的沉积结构及其形成过程和控制因素，提出高级别旋回主要与构造沉降演化有关，而高频旋回主要受控于气候变化引起的海平面变化，揭示了不同沉积体系域的沉积构成和成因。主要的进展包括：（1）建立了重点层段柴北缘上石炭统的高精度层序地层序列，划分出4个三级层序，8个四级层序和17个五级层序；2）识别出主要的沉积体系类型及体系域的沉积相构成，包括三角洲、滨岸、河口湾、碳酸盐岩缓坡等沉积体系，其中低位体系域以发育河口湾底部河道沉积为主，海侵体系域以发育河口湾顶部的潮汐砂坝或者碳酸盐岩缓坡沉积为主，高位体系域以发育浪控滨岸和三角洲沉积为主；（3）初步重建了柴东地区晚石炭世相对海平面升降曲线，发现罗苏阶-逍遥阶,该地区相对海平面在四级旋回多次(4次明显)波动叠加的基础上，在三级旋回尺度上呈现逐渐上升的趋势；（4）揭示了层序及沉积体系形成演化的控制因素，认为在构造运动控制了研究区晚石炭世长期海侵旋回变化，构造作用不是控制层序发育及沉积体系演化的主要因素，古气候和古物源补给控制了区内三级层序的发育；海平面变化控制了沉积体系的纵向演化，古地貌和物源共同控制了各层序内沉积体系的展布；（5）混积层系内发育多种生储盖组合类型，其中，下切谷充填型沉积可行形成潜在的有效圈闭。