东营凹陷中始新统湖相碳酸盐岩沉积演化及其对古东亚季风气候的响应

Lacustrine carbonates have been demonstrated as important sediments carrying information of lake environment and regional paleaoclimate, whose sedimentary evolution and deposition - climate interactive relationship are the key for decoding deep-time paleaoclimate. However, the response relationship of deposition to climate is still poorly understood, even though lacustrine carbonates are commonly deposited in most lake basins. During the deposition period of the upper fourth member of Shahejie Formation (Es4s), the western Dongying Depression was generally situated in a stable tectonic setting, which was favorable for the wide development of lacustrine carbonates throughout the whole basin. The Es4s carbonates detailedly recorded the climate fluctuation process and evolution history, providing an ideal chance to study the Middle Eocene East-Asian paleaomonsoon climate and the climate - deposition responding mechanism in a lacustrine carbonate system. The key pointcut of this project is that the respective evolution of sedimentation and paleaoclimate are joint by the same sequence stratigraphic framework. Based on integrated cores, well-logging, sporopollen and testing data and an interdisciplinary approach of sedimentology, sequence stratigraphy and geochemistry, this project concentrates on: 1) establishing a high-resolution sequence stratigraphic framework that is unified and comparable throughout the whole basin; 2) clarifying the lacustrine carbonate system from temporal and spatial distribution, controlling factors and sedimentary model; 3) studying the coupling relationship between sedimentary evolution and climatic fluctuation, and revealing the responding mechanism of lacustrine carbonates and the East-Asian paleaomonsoon. This study promotes the study of the evolution history of deep-time paleaoclimate, and thus providing theoretical references for the sedimentary system distribution and prediction in lacustrine carbonates.

湖相碳酸盐岩是记录湖泊沉积环境及区域古气候信息的重要载体，其沉积演化及沉积-气候响应机制是解密深时古气候的关键，我国陆相湖盆中广泛发育湖相碳酸盐岩，但一直缺乏对其沉积-气候响应的系统研究。沙四上亚段沉积时期，东营凹陷西部构造相对稳定，广泛发育的湖相碳酸盐岩记录了该时期气候的波动过程及演化历史，是研究中始新世古东亚季风及沉积-气候响应机制的理想窗口。本申请项目拟以精细层序地层格架中沉积与古气候的协同演化为关键切入点，综合利用钻井岩心、测录井、孢粉及分析测试等资料，通过沉积学、层序地层学、地球化学等多学科交叉手段，建立全盆可对比的、统一的高精度层序地层格架，阐明湖相碳酸盐岩沉积体系时空展布规律、主控因素并建立沉积模式，理清沉积演化与古气候变化之间的耦合性，揭示湖相碳酸盐岩对古东亚季风气候的响应机制，有助于加强对深时古气候演化规律的认识，为湖相碳酸盐岩沉积体系的分布及预测提供依据。

湖相碳酸盐岩是湖盆沉积演化过程中古气候、古环境等沉积条件的重要记录，是当今世界碳酸盐岩领域研究的难点和热点之一。东营凹陷西部沙四上亚段广泛发育碳酸盐岩，但由于长期以来受沉积模式的局限，其成因机制一直认识不足。本项目综合岩心、薄片、地球化学、孢粉及地震、测录井等资料，对东营凹陷西部沙四上亚段碳酸盐岩的层序、沉积特征进行了系统的研究，探讨了湖相碳酸盐岩沉积气候响应过程。研究认为，东营凹陷沙四上亚段湖相碳酸盐岩主要发育于层序地层中的高位体系域，研究区常见7种岩石类型及5种岩性组合，可识别出礁滩、礁丘、浅滩、混积坪及风暴-重力流5种沉积微相，不同微相所含生物、颗粒类型及空间分布有明显差异；礁滩和浅滩两个微相共同构成湖相礁-滩复合沉积体系，主要发育于构造高地顶部，相带主体呈NE-SW向展布，从高位域早期至晚期，礁-滩体系发育规模逐渐扩大；风暴-重力流微相主要发育于台地向东和向南倾的地貌斜坡，具有多期叠置、发育频繁的特征，从早期至晚期风暴沉积作用具有减弱趋势；碳氧同位素古温度计算结果表明湖相碳酸盐岩沉积期为相对温暖的古气候，孢粉组合显示RT和RH指数变化趋势较一致，有湿热同期的特征；区域气候、湖平面与全球气温、海平面叠加效应，可能导致海侵为湖相碳酸盐岩的形成和发育提供了盐度保障；区域和全球相对温暖气候背景、古东亚季风气候条件下盛行的东南风、盆地构造及古地貌、湖平面升降、碳酸盐岩母岩风化及海侵作用等均是湖相碳酸盐岩形成及演化的主控因素，提出湖相碳酸盐岩“气候-物源-盆地”系统成因模式，丰富了对湖盆大规模碳酸盐岩发育及礁-滩沉积体系形成机理的认识。