Study Of Sedimentary Environment Of Dark Sedimentary Rocks In Lower Cretaceous, Zhejiang And Fujian Provinces

A series of terrestrial volcanic-sedimentary rocks of lower Cretaceous widely distribute in Zhejiang and Fujian Provinces, Southeast China, and it has been proved that this dark sedimentary rocks possess hydrocarbon potential. However, it is still being debated that whether the basin underwent a transgression during the lower Cretaceous period. Therefore, it is of great significance to study on the favorable depositional environment conditions for the dark sedimentary rocks.Based on characteristics of field sections, it is found that the dark sedimentary rocks, including Bantou formation in Fujian Province and Guantou Formation in Zhejiang Province, extensively developed in Mesozoic volcanic-sedimentary clastic basins in the above areas. Bounded by Zhenghe-Dapu Fault and Jiangshan-Shaoxing Fault, the dark sedimentary rocks develop well in the region west of the fault while poorly developed in the region east of the fault for the reason of constant volcanic activities.The petrologic features of dark sedimentary rocks in study area indicate the sedimentary environment of lacustrine facies. Meanwhile, marine algal fossils in Shipu, Zhejiang Province, and benthic algae "blade" fossils in Yong’an Basin and Chong’an Basin, Fujiang Province, suggest the influence of transgression.Quantitative analysis of fossil spores and pollen shows that there has been a humid subtropical climate with less climatic fluctuation in the east of the studying area, a humid-half humid subtropical climate with a short term of hot and dry climate in the middle, and half dry subtropical climate with a few times of undulations between half-arid and half-humid climate in the west. Paleoclimate characteristics of the study area coincided with the climate significance instructed by the continental special deposits.In conclusion, with weak volcanic activities and high altitude, water gather in the low-lying land among the mountains in the west area, doing good for the deposition of the dark sedimentary rocks. While volcanic activities are constant in the east area, bringing adversity for the deposition of the dark sedimentary rocks. The deposition of the middle area is between the west and the east. Besides, the marine influence, which is conducive to life, promotes the deposition of the dark sedimengtary rocks.