川西坳陷上三叠统须家河组米兰科维奇旋回的识别及天文年代标尺的建立

Cyclostratigraphy is the subdiscipline of stratigraphy that deals with the identification, characterization, correlation, and interpretation of cyclic variations in the stratigraphic record and, in particular, with their application in geochronology by improving the accuracy and resolution of time-stratigraphic frameworks. Since the recognition of 'Milankovitch' orbital forcing as the 'pacemaker of the ice ages', cyclostratigraphy has attracted more and more attentions from geoscientists and stratigraphers because it can considerably enhance both relative and absolute age controls at a resolution of 10-100 Ka, better understanding of the sedimentology, quantification of rates and duration of sedimentary, climatic, and paleontological processes, as well as possibilities for correlation within and beyond the basin. Due to the lack of lithologic association and biologic criteria which can be used to correlate across the whole basin, the stratigraphic division and correlation based on lithostratigraphy, seismic stratigraphy, sequence stratigraphy, event stratigraphy, and tectonic movements in the Upper Triassic Xujiahe Formation in Sichuan Basin have been debate during the past several decades. In order to resolve this problem and improve the stratigraphic resolutions for hydrocarbon prospecting and exploration in the basin, this study undertakes spectral analysis of high-resolution total gamma-ray well-logging record, Th/K, and magnetic susceptibility to determine the dominant frequency components and test whether Milankovitch orbital signals are registered in the rhythmic successions. A high-resolution astronomical time scale can be constructed through astronomical tuning of the numerical proxies to the long-period eccentricity curve in the most recent calculated variations of the earth's orbit. After detecting stratigraphic discontinuities and evaluating its absent duration by moving window spectral analysis and wavelet analysis methods, we can construct the isochronous stratigraphic framework in Sichuan Basin by correlating the 405-kyr eccentricity signals among all the field sections and drilling borehole sections.

四川盆地上三叠统陆相地层缺乏可进行区域对比的岩性和生物标志层，盆内基于岩石地层、层序地层、事件地层、构造运动等的地层划分对比方案长期以来存在争议，严重制约了对地层分布规律的深入认识。米兰科维奇理论中天文旋回可进行全球对比的原理，可以为等时地层划分对比提供时间标尺，有望解决标准不统一的现状。本课题拟以川西坳陷上三叠统须家河组为例，选择典型野外露头剖面和沉积中心区钻孔，以自然伽马总量、钍钾比和磁化率作为反映古气候变化的替代指标，通过频谱分析法判断地层旋回的驱动机制及不同峰值频率对应的地层厚度。在确定为天文轨道驱动后，将替代指标与天文周期解决方案中长周期偏心率曲线进行调谐对比，通过露头与钻孔剖面的整合性对比，最终建立纵向分辨率高达405kyr的天文年代标尺。结合滑动窗口频谱变换和小波分析得到的沉积间断及其缺失时间，对比其它钻孔剖面识别出的405kyr偏心率信号，则可建立盆地范围内的等时地层格架。

旋回地层学是近年来国际上兴起的一个地层学分支学科，用于识别、描述、对比以及解释地层中周期性或准周期性的旋回变化，尤其可应用于地质年代学中，用来提高时间地层格架的精确性和分辨率。自从Hays于1976年在《Science》上发表文章报道他们在第四纪深海钻探岩芯中发现了米兰科维奇旋回以来，国际地学界对旋回地层学的研究如火如荼。在中国，虽然一些学者自上世纪九十年代以来对该学科进行了有益的尝试性研究，但却一直没有得到发展，甚至没有中文资料系统的介绍旋回地层学。因此，课题首先详细归纳、总结旋回地层学的相关概念、发展历程、研究方法和研究现状，并通过三个不同时代的应用实例，引申出旋回地层学的研究意义。然后，对四川盆地上三叠统须家河组和鄂尔多斯盆地中、上三叠统延长组这两个实例进行详细解剖和系统研究，利用地层中识别出的405 kyr长周期偏心率旋回，估算两个岩性地层单位的沉积持续时间，并建立盆地范围内的等时地层格架，力争将级次结构及持续时间已知的米兰科维奇旋回引入油气地质勘探，满足其对研究精度日趋提高的要求。. 课题的主要结论是：. 1.根据四川盆地上三叠统须家河组两条野外剖面及三口钻孔自然伽马测井曲线的滑动窗口频谱分析发现，五条研究剖面均不同程度存在沉积间断或剥蚀，但以剖面中识别出的405 kyr长偏心率信号为基础，并假设每个沉积间断持续时间不超过一个405 kyr长偏心率周期，晚三叠世须家河期大致相当于Rhaetian阶（209.5-201.3Ma）。. 2.假设晚三叠世须家河期相当于Rhaetian阶，根据四川盆地几口钻孔自然伽马测井曲线中识别出的405 kyr长偏心率信号，估算出每口钻井大致的剥蚀或无沉积的持续时间。. 3.根据鄂尔多斯盆地部分钻孔自然伽马测井曲线中识别出的405 kyr长偏心率信号，初步计算出鄂尔多斯盆地延长组的沉积持续时间至少为约30 Myr，比晚三叠纪Rhaetian阶（201.3-~208.5 Ma）和Norian阶（~208.5-~227 Ma）的持续时间略长。