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Objective The Wufeng Formation-Longmaxi Formation black rock series in eastern Sichuan and western Hubei is a focus for marine shale gas exploration in South China. These show great differences in sedimentary characteristics， which record major geological events during the Ordovician–Silurian transition. Further knowledge of these differences is of benefit to exploration for shale gas in western Hubei， and helps to understand global paleo-climate and sea-level changes during that time. Methods The sedimentary properties， main influencing factors and sedimentary processes of the black rock series in the Wufeng Formation and Longmaxi Formation were studied in detail by petrological， sedimentological and geochemical analysis. Results The sedimentary attributes of the Wufeng Formation are similar in both study areas， but in the Longmaxi Formation they differ significantly. In eastern Si-chuan， the large thickness of the Longmaxi black shale with completed graptolite zones and thick silty shale in the Rhuddanian-Aeronian transition is overlain by thick， organic-rich shale. Three long-term sedimentary cycles are recognized in the Longmaxi black rock series， but in western Hubei the Longmaxi black shale is very thin and only two long-term sedimentary cycles are identified. Neither silty shale nor Coronograptus cyphus and Demirastrites triangulatus graptolite zones are present in the Rhuddanian-Aeronian transition， but thin lime-bearing dolomitic mudstone covered by thin organic-rich shale are developed. Additionally， the Longmaxi black shale contains less quartz in eastern Sichuan than in western Hubei. Conclusions In this study， tectonic paleogeomorphology and paleoclimate changes are considered to be the main factors influencing the different evolutionary histories of the Wufeng Formation⁃Longmaxi Formation black rock series in the two study areas. During the Upper Ordovician， eastern Sichuan and western Hubei both subsided as a result of the Cathaysia Block⁃Yangtze Block collision. The paleogeomorphology and sedimentary environments in eastern Sichuan and western Hubei are not significantly different， but continuous tectonic compression beginning in the Early Silurian caused non-uniform subsidence. Continuing subsidence in eastern Sichuan and gradual uplift in western Hubei led to significantly different thicknesses of the Rhuddanian black shale in the two regions. Global sea-level fall during the Rhuddanian⁃Aeronian transition related to intensified glaciation further exacerbated the sedimentary differences in the two study areas， resulting in a sedimentary gap in western Hubei and abundant sandy deposits with low TOC in eastern Sichuan. Subsequent significant sea-level rise following global climatic warming promoted the development of black shale in both areas， although thicker in eastern Sichuan than in western Hubei. However， gradually intensifying tectonic compression and gradual sea-level fall， along with sufficient terrigenous sediment supply， sustained a sedimentary environment in both regions that led to the termination of development of the Longmaxi black shale in the two study areas.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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