云南兰坪-思茅盆地白垩纪中期海侵事件及其与钾盐成矿的关系研究

The rapid sea level rising and resulting paleoceanographic change in the Middle to Late Cretaceous has receiving much attension. The evaporates-bearing Mengyejing Formation in the Lanping-Simao Basin, southern part of the East-Tethys domain, might be affected by sea water. However, for the discrepancy of the age and sedimentary environment of the Mengyejing Formation, these potash-bearing deposits are never been linked to sea level rising in the mid-Cretaceous. Otherwise, evaporates of the Mengyejing Formation may be sourced from sea water, but when and where are the sea water from has been a subject of debate and controversy. This research mainly focus on the mid-Cretaceous evaporates-bearing Mengyejing Formation. The major composition of fluid inclusion in evaporates, organic geochemistry, and stable isotopes are mainly performed combined with sedimentology and palaeontology to bulid the chain of evidences of marine incursion in the mid-Cretaceous in the Lanping-Simao Basin, aiming to ascertain the compositive identifying features for the marine incursion, clear the periods and direction, and reveal the relationship between marine incursion and potash mineralization. The results are not only significant to reveal the change of palaeo-environment and global correlation in the mid-Cretaceous time, but also of importance to understand the origin of potash salts and guide for further potash exploration in the Lanping-Simao Basin.

白垩纪中晚期全球海平面的快速上升及其古海洋变化一直是国际研究热点。东特提斯域南段兰坪-思茅盆地含蒸发岩的勐野井组可能受到了海水的影响，但由于对其时代和沉积环境的认识差异，一直以来并未与白垩纪中期的海平面上升事件相联系。另外，尽管勐野井组蒸发岩可能来源于海水，但对海水来源还存有分歧。本项研究以白垩纪中期勐野井组含盐地层为研究对象，以蒸发岩流体包裹体成分分析、有机地球化学和稳定同位素地球化学等为主要手段，结合沉积学和古生物学方法，建立兰坪-思茅盆地白垩纪中期海侵事件的证据链，确定海侵存在的综合识别标志，明确海侵的期次和方向，进而查明海侵与钾盐成矿的关系。研究成果不仅对揭示东特提斯域南段白垩纪中期古环境变化和全球对比具有重要的科学意义，而且对于探讨该区钾盐成因并指导进一步的找矿具有重要的实践意义。

白垩纪时期是地球著名的温室气候和高海平面时期。东特提斯域思茅盆地发育白垩纪中期勐野井组含钾盐的蒸发岩沉积，但其物质来源一直存有争议。据此，本项目开展的主要研究内容包括以下三大方面：1）白垩纪中期勐野井组海侵事件证据链研究；2）海侵期次与方向研究；3）白垩纪中期海侵与钾盐成矿关系研究。四年来，项目组通过连续的野外科学考察和室内分析，获得了以下重要成果。第一，系统建立了思茅盆地白垩纪海侵的多学科证据链。发现了反映潮坪沉积环境的多种沉积构造，揭示了勐野井组原生石盐包裹体成分与白垩纪中期古海水成分相一致，获得了反映古盐度增加的元素和同位素指标，首次发现了特征性的海相生物标志物24-正丙基胆甾烷和翼囊藻，从而从地质-地球化学-古生物-包裹体等多维度建立了勐野井组沉积时期海侵的证据链；第二，通过系统的物源分析，揭示了在自早白垩世晚期以来至晚白垩世，存在着一条自北而南流向的大流域的古河流——古湄公河，沟通了北部的思茅盆地和南部的呵叻盆地；通过古地理重建，提出副特提斯海在早白垩世的海侵首先到达塔里木盆地东部，进而在白垩纪中期，进一步到达四川、思茅和呵叻盆地，形成广域的统一的陆表海盆地；而由于副特提斯海的海退，导致大量水气被带走，加剧了区域的干旱化，形成了白垩纪中期北至兰坪-四川，南到呵叻高原的蒸发岩盆地；第三，通过钾盐锶同位素和石膏层自生黄铁矿的研究，认为思茅盆地晚白垩世以来热液活动频繁，多期次的热液作用对盆地金属成矿有积极作用，但由于对勐野井组钾盐沉积的改造作用，使得原生钾盐发生蚀变，普遍发生重结晶作用。研究结果对揭示思茅盆地钾盐成矿规律有重要的科学意义。