松辽盆地东部宾县凹陷青山口组介形类生物地层与生态环境

Ostracode Stratigraphy and Palaeoenvironment of the Cretaceous Qingshankou Formation in East Songliao Basin

实测松辽盆地东部宾县地区鸟河畔的两条剖面。鸟河剖面上下岩性变化较小,以灰绿色泥岩、泥质粉砂岩为主,含数层黄铁矿薄层及介形虫灰岩薄层。红石山剖面岩性岩相变化较大,下部为灰绿色,黄绿色泥岩,上部出现大套红层。通过介形类组合特征,判断出这两个剖面属于青山口期沉积。其中,鸟河剖面对应青山口组一段上部,含介形类Triangulicypris torsuosus-Triangulicypris torsuosusvar.nota组合带。红石山剖面属于青山口组二段下部,含介形类Cypridea dekhoinensis-Li mnocyrpridea copiosa组合带;及叶肢介Nenestheria sp.、Cratostracus sp.、Dictyestheia sp.,化石富集成层。通过介形类古生态分析,松辽盆地宾东地区在青一段沉积时期为浅湖环境,湖水约为半咸水至微咸水。介形类的属种单调,但优势种Triangulicypris torsuosus极为繁盛。青二段沉积时期为滨浅湖环境,湖水盐度较青一段沉积时期降低,为淡水或微咸水。生物组成以腹足、轮藻、叶肢介以及带瘤介形类的出现为特征。宾县地区在青山口组沉积时期生物具有与松辽盆地其他地区相同的化石面貌和古生态特征。其一致性表明该时期宾县地区同属于松辽盆地。青一段至青二、三段之间微体化石古生态特征发生转变,暗示松辽盆地水体条件在该沉积阶段的演变。

Non-marine ostracod,which is very sensitive to variations of the water body in which they live,is of importance to be used as an indicator to reconstruct the lacustrine environment. By analyzing the ostracodes properties such as shell morphology,ornamentation,disarticulated/articulated ratio,and degree of shell calcification,we try to reconstruct the paleoenvironment of the Songliao basin,including the palaeogeography,palaeohydrology,palaosalinity and palaeoclimate.The two sections were measured in the east periphery of the Songliao basin. According to the ostracods assemblages,we refer the strata to the Qingshankou Formation. The Niaohe section is composed of the 1st Member of the Qingshankou Formation,and the Hongshishan of the 2nd Member of the Qingshankou Formantion. Biostratigraphically,the Triangulicypris torsuosus-Triangulicypris torsuosus var. nota assemblage zone has been recognized in the Niaohe section,which can be correlated with other sections in the central Songliao basin,yet the biostratigraphy of the Hongshishan section can be correlated with the Cypridea dekhoinensis-Limnocyrpridea copiosa assemblage zone of the basin.The strata mentioned above were deposited in two distinctive environments. The Niaohe section is of the feature of sub-shallow water environment,which is characterized by the low biodiversity and extremely high abundance. The salinity is about 5 （brackish water）. The Hongshishan section shows a shallow and fresh water （fresh to oligohaline water） environment characterized by the presence of gastropods,stoneworts and ostracodes with nodes. Generally speaking,the palaeoenvironmental change of Binxian depression during this period was identical with the Songliao basin,which implies that the Binxian depression sub-basin might be a part of an integrated Songliao basin at that time. Furthermore,the changes in ostracode palaeoecology between 1st Member and 2nd Member of the Qingshankou Formantion suggest the changes in palaeohydrology of the period.