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通过对鄂尔多斯盆地陇东地区三叠系延长组孢粉型有机壁微体化石的调查,在长73―长72段发现了浮游植物Sigmopollis cf.laevigatoides-Botryococcus braunii组合。该组合以极其丰富的蓝藻占优势,疑源类和绿藻类也较丰富,组合优势度高,而分异度低,常常显示单种优势。大量蓝藻类的勃发指示温暖和富营养的水体环境,与该盆地当时所处的低纬度亚热带气候区相一致,而延长组沉积时期大范围的火山喷发可能为蓝藻的发育提供了丰富的氮、磷等营养物。绿藻类Botryococcus braunii的大量出现表明本地区晚三叠世为淡水—微咸水内陆湖泊。保存完好的蓝藻化石密集成层出现于纹层状富有机质沉积中,显示可能为季节性分层的深水缺氧沉积环境。蓝藻化石分布的高丰度段长73―长72段恰好是本地区的主要烃源岩,并且烃源岩有机碳含量与蓝藻化石丰度密切相关,表明蓝藻类可能是延长组优质烃源岩有机质的重要生物来源。
Based on the investigation of sporopollen-type organic wall microfossils in the Triassic Yanchang Formation in the Longdong area, Ordos Basin, the phytoplankton Sigmopollis cf.laevigatoides-Botryococcus braunii assemblage was found in the Chang 73-Chang 72 stage. The combination is dominated by a very rich pool of cyanobacteria, with a greater abundance of suspect species and green algae, a higher combination of superiority, and a lower degree of differentiation, often showing a single superiority. A large number of blue-green algae blooms indicate a warm and eutrophic water environment consistent with the low-latitude subtropical climatic zone at that time in the basin, while the extensive volcanic eruptions during the Yanchang Formation deposition period may provide abundant nitrogen for the development of cyanobacteria, Phosphorus and other nutrients. The numerous occurrences of the green algae Botryococcus braunii indicate that the Triassic in the area is freshwater-brackish water inland lakes. Well-preserved cyanobacterial fossil assemblages occur in laminar organic-rich sediments and may indicate seasonal stratification of deep-water anoxic depositional environments. The high-abundance segment length 73-72 in the distribution of cyanobacteria fossils is the main source rock in this area, and the organic carbon content of the source rock is closely related to the abundance of cyanobacteria, indicating that the cyanobacteria may be the source rocks of the Yanchang Formation An important biological source of organic matter.