Occurrence characteristics and depositional model of graptolites from the black shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation

Based on the pervious researches, the occurrence characteristics and depositional environment of graptolites from the black shale of the upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Yichang area are analyzed in detail through integrating mineral and geochemical data. The results show that the occurrence characteristics of graptolites from the black shale in the Wufeng and Longamxi Formations are different, with graptolites abundance decreasing gradually from the lower part to the upper part. Carbonaceous shale always has a lot of graptolites, and most of graptolites are disordered except some of them are stored regularly. The graptolites abundance in greyish-green shale and black siliceous shale is low and most of graptolites are composed of fragments smaller in the black siliceous shale. Graptolite diversification in the black shale also shows regular variations, and the graptolite diversification in the Hirnantian is much lower than that in the Aeronian and Katian. Comprehensive analysis indicates different graptolite occurrence characteristics usually have various TOC, surrounding rock properties, oxidation-deoxidation conditions, palaeoenvironment and hydrodynamic conditions. The shale with rich graptolites abundance often has high value of TOC, with their quartz content usually ranging from 40% to 60%. Graptolites can be deposited both in weak oxidation and deoxidation conditions, but the deoxidation environment is much more favorable for graptolites sedimentation and preservation. The vertical variations in graptolite diversification and graptolites abundance are obviously influenced by palaeoenvironment, and the lower graptolite diversification and massive disappearance of graptolite in the Hirnantian are caused by glaciation and sea-level changes. Disordered graptolites are the results of suspended sedimentation, while the graptolites with directional alignment indicate reformation by ocean current. Finally, based on the different graptolite occurrence characteristics and their sedimentary geological significance, their sedimentary environment is discussed in detail and the depositional model of graptolites in the Wufeng and Longamxi Formations is proposed.