华南陆块白垩纪晚期风成沉积记录及其古气候动力学研究

The Cretaceous Period was characterized by deep perturbations in earth surface system, e.g., atmosphere, lithosphere, oceans, and biosphere; accurate understanding the global climate evolution behavior of this interval is thus one of the major issues in paleoclimatology research. Multiple climate proxies show that the equator-polar geothermal gradient during the Cretaceous ‘Green house’ is extremely low compared with the current earth, meaning much heat of atmospheric or oceanic circulation is meridional transported. However, the pattern of the late-Cretaceous atmospheric circulation is still poorly understood due to the absence of direct geological evidence, especially from the terrestrial-atmospheric system. Wide aeolian deposits in the mid-low latitude regions of the northern hemisphere often occurred in form of desert belt during the Cretaceous period, providing unique information of prevailing winds and paleoclimate. In this study we use aeolian deposit records as monitor for the late-Cretaceous greenhouse climate and focus on the aeolian depositional associations, to establish the vertical paleoclimate-sedimentary environment evolution sequence, to reconstruct the accumulation-preservation processes and the spatial configuration of aeolian deposit in the basin group of South China block, to identify the type of prevailing wind and reconstruct the general circulation patterns by combining with the paleo-wind direction, and to verify the synchronization and asynchronization among the mid- to low-latitude desert belt, the regional climate zone, and the paleotopography along East Asia during the late-Cretaceous. Our study will improve our knowledge of climate change in the late-Cretaceous from a new angle of view and provide the reference to human to realize the current environmental problems such as "greenhouse effect".

白垩纪是地球表层系统大气圈、岩石圈、水圈和生物圈之间深度扰动的典范，了解该时期全球气候演变行为是古气候学研究的重大课题之一。众多气候替代指标表明，与现代赤道至极地的温度梯度相比，白垩纪“温室期”赤-极地温梯度极低，这意味着大量的大气或海洋环流的热量沿纬线传输。然而，白垩纪大气环流格局的重建缺乏大气-陆地系统的直接地质证据，针对全球中低纬度广泛发育的白垩纪风成沉积所蕴含的古风向和气候信息研究则显得尤为必要。本研究将华南陆块典型盆地白垩纪晚期风成沉积记录作为温室期气候演变的监测器，以风成沉积组合为切入点，建立古气候-沉积环境演化序列，研究风成沉积堆积、保存过程及时空分布规律，恢复古风向并判断所属盛行风性质，重塑大气环流分配样式。根据风成沉积体系及地表矢量风向组的变化和转型事件，探讨白垩纪晚期中低纬度沙漠带、气候带及东亚古地形之间的成因关联，重新审视我们对白垩纪晚期温室气候变化的认识。

本研究以风成沉积体系理论为指导，以华南陆块白垩系巨厚红色碎屑岩层为重点研究对象，以风成沉积组合为切入点，开展风成砂体构型分析和床砂建模，配合多元敏感类气候指标建立古气候-沉积环境演化序列，研究风成沉积的堆积、保存机制。同时，关注风成沉积的时空分布特征，恢复古风向信息，重塑白垩纪华南大气环流分配样式和迁移规律；最后围绕岩石圈动力学、冰冻圈和古气候等档案探讨了东亚白垩纪沙漠盆地的成因。研究表明：在区域尺度上，风成沉积的堆积、保存过程受到盆地构造属性和区域潜水面的深刻影响；在大陆尺度上，白垩纪晚期板块俯冲方式的转变可能触发了地表隆升，增强了剥蚀作用和沉积物的可利用性，导致了东亚地区干旱带的扩张和砂质沙漠的建造和堆积。其次，随着诸多冰筏碎屑和多年冻土砂楔遗迹的发现，表明白垩纪温室期的冰冻圈过程客观存在，并且比预期更为活跃；根据现代多年冻土层发育的纬度与高程之间的函数关系，认为白垩纪早期鄂尔多斯盆地腹地的古高程可能达到3000-4000米。此外，华南陆块白垩纪山间沙漠盆地中堆积的风成沙丘沉积物经历了周期性的软变形过程，它被认为可能与季风降雨引发的水文-地震活动有关；古沙漠层序中的大量坡移沉积物构成了油气运移的显著非均质性，其架构多变性导致风成储层中有效沉积圈闭的发育。