渤海新近纪古湖发育的直接证据

DIRECT EVIDENCE FOR DEVELOPMENT OF BOHAI PALEOLAKE DURING THE NEOGENE

通过渤海5口钻井的沉积有机质分析,以沉积有机质中浮游藻类丰度和类型、无定形有机质含量变化以及地层序列中富藻沉积层发育程度,作为指示湖相沉积环境发育的标志,确定渤海新近纪湖相沉积层的分布特征及渤海古坳陷湖的发育特征。分析表明,浮游藻类和无定形有机质含量高的层段在馆陶组和明下段都有分布,但发育最好的是在明下段下部。这是渤海新近纪湖相沉积地层存在的直接证据。从湖相沉积发育的差异看,馆陶组和明化镇组时期的古湖有着不同的特征,馆陶组早期渤海的湖泊可能很小,馆陶组中、晚期为湖泊扩张期,从含浮游藻类和无定形有机质沉积层纵向变化频繁出现可以看出浅湖沉积环境的不稳定性。新近纪古湖的盛期是在明下段下部沉积时段,当时渤海淡水湖泊普遍存在。

The distribution of the Neogene lacustrine sediment beds and development of the Bohai paleolake are discussed, based on the analysis of the particulate sedimentary matter from 5 wells in the Bohai Bay. The abundance and type of phytoplanktonic algae fossils, content of amorphous organic matter in sediments and the sediment bed rich in phytoplanktonic algae fossils are regarded as the proxy of lacustrine depositional environment. The analysis results show that the sediment beds containing abundant phytoplanktonic algae fossils and amorphous organic matter are well developed in the lower part of lower member of Minghuazhen Formation, although they are also present in the Guantao Formation and the upper part of lower Member of Minghuazhen Formation. These beds are the direct evidence for the occurrence of the Neogene lacustrine depositional environment. The difference in the development of the lacustrine sediment beds between the Guantao Formation and the Minghuazhen Formation suggests the changes in the Bohai paleolake in the period from the Guantao Formation to Minghuazhen Formation. The Bohai paleolake may be small during the early period of Guantao Formation and expanded in middle and late periods of Guantao Formation. The intermittent occurrence of the sediment beds rich in phytoplanktonic algae fossils reflects changeable depositional environments. The maximum of the Bohai paleolake development is in the period of the lower part of lower Member of Minghuazhen Formation, when large freshwater lake occurred in Bohai region.