Recognition and Responses of Milankovitch Cycles in Eocene Shahejie Formation, Dongying Depression

Several studies have revealed that the Eocene fine-grained sedimentary rocks in Bohai Bay Basin is driven by astronomical forces. However it is still not clarified how orbital cycles specifically control the deposition of fine-grained sedimentary rocks and the coupling relationship of paleoclimate- paleolake- sedimentation. In this study, a combination of core and thin section observation, X-ray diffraction, ICP-MS analysis, total organic carbon (TOC) content analysis and cyclostratigraphy analysis were conducted on the lacustrine shale of the Eocene Shahejie Formation (Es4u to Es3l) in the Dongying Depression of Bohai Bay Basin, in order to recognize the Milankovitch cycles and explore the controlling effect of eccentricity, obliquity and precession on the deposition of the lacustrine shale as well as the paleoclimate evolution. The related achievements are as follows: 1. Through pre-processing, sliding window spectrum analysis and multi-taper method (MTM) of the natural gamma ray data of the well NY1, good orbital signals were recognized. And through correlation coefficient method (COCO), the study interval is divided into two sections according to the change of accumulating rates. Filtered eccentricity, obliquity and precession signal curves were obtained for both sections. 2. For the lower Section, obliquity is the main controlling factor for shale deposition and precession is the secondary controlling factor. When obliquity reaches its maximum, the content of dolomite and clay in Shahejie Formation increases, while the content of calcite decreases and TOC also increases. At this point the laminar combinations are mainly dolomite/calcite lamina and organic-rich clay lamina. When obliquity reaches its minimum, the content of calcite increases, the content of dolomite and clay decreases and TOC decreases. At this point the laminar combinations are mainly calcite lamina and clay silt mixed lamina. 3. For the upper Section, eccentricity is the main controlling factor for shale deposition and precession is the secondary controlling factor. When eccentricity reaches its maximum, the content of clay and quartz in Shahejie Formation increases, while the content of carbonate minerals decreases and TOC also increases. At this point the laminar combinations are mainly calcite lamina and organic-rich clay silt lamina. When eccentricity reaches its minimum, the content of clay and quartz decreases, the content of carbonate minerals increases and TOC decreases. At this point the laminar interface is usually blurry. 4. According to a series of geochemistry data analysis, the chemical weathering index (CIA) for the lower section is relatively low, the Mg/Ca ratio and Ni/Co ratio are relatively high, which indicates that the paleo-environment was arid and reductive. For the upper section, CIA is relatively high and the Mg/Ca ratio and Ni/Co ratio are relatively low, indicating a humid and less reductive paleo-environment. The coupling relationship of paleoclimate- paleolake- sedimentation experienced a transition from the lower section to the upper section of the study interval, which is consistent with the shift from obliquity driven to eccentricity driven. The intensification of East Asia monsoons might be responsible for this transition.