Sedimentation And Its Motivation Of The Paleogene Fine-grained Sedimentary Rocks In Dongying Sag

Fine-grained sedimentary rocks are sedimentary rocks composed of clay or silty sediments with particle size less than 62.5?m.Fine-grained sedimentary rocks are widely developed in Dongying Sag,with rich and varied lithofacies types.According to core observation,thin slice data,analysis of laboratory test data and logging data,this paper carried out the lithofacies division and characterization,analysis of sedimentary processes and study of sedimentary cycle driving factors of the Paleogene fine-grained sedimentary rocks in Dongying Sag.In this paper,according to the difference of mineral composition of lamina,7 kinds of lamina combination are founded,they are calcite lamina—clay-silty mixed lamina(type I),calcite lamina—organic-rich clay lamina(type ?),clay-silty mixed lamina—organic-rich clay layer(type ?),calcite lamina—organic lamina(type ?),calcite lamina—clay-silty mixed lamina—organic lamina(type ?),dolomite lamina—organic-rich clay lamina(type ?)and dolomite lamina—gypsum lamina(type ?).Combining the mineral composition,lamina assemblage,lamina pattern and organic matter content,we can divide the rocks into 36 kinds.According to the vertical characteristics of the elements,the evolution of sedimentary environment in the study area can be roughly divided into five stages,and the climate has experienced the transformation from dry and cold to the warm and humid,and finally to the dry and cold.The characteristics of lithofacies is different under vary environmental conditions.The mixed fine-grained sedimentary rocks are developed in many kinds of environment.Under the condition of arid climate and strong evaporation,dolomite and type VI and type VII lamina combination are easy to deposit.Silty limestone and limestone lithofacies,and the lamina combination of type I and type II are generally formed in the relatively warm climate.The deposition of the clay siltstone and the silty clay rocks as well as the type III lamina usually requires a certain amount of terrigenous clasts input.According to the analysis of the deposition characteristics and the sedimentary environment,we sum up the sedimentary model that "climate-terrestrial input" dynamically controlled the deposition of lamina.The deposition of lamina is generally because of suspension deposition,the biochemical action.The strong hydrodynamic condition and the effect of the biological disturbance can lead to the discontinuity of the lamina,while the massive fine-grained sedimentary rock is mainly deposited under the action of the turbidity current or debris flow.It is found that the direct driving factors are different by analyzing the characteristics of sedimentary cycles at different scales.The sedimentary cycles of several meters to dozens of meters are mainly driven by the Milankovitch Cycle,and the long eccentricity cycle is the main factor leading to the existence of the periodic climate.Under the modulation of long eccentricity,precession plays an important role in the development of sedimentary cycle of lithofacies scale.In addition to being influenced by climate change and terrestrial detritus input,sedimentation events,such as gravity flow,are also important factors driving the sedimentary cycles of several centimeters to dozens of centimeters.The formation of micrometer-millimeter scale sedimentary cycle is directly controlled by seasonal climate.