摘要
松辽盆地嫩江组二段下部沉积期发生了大规模的湖侵事件,在整个盆地范围形成一套黑色泥页岩夹油页岩。然而,前人对嫩江组二段的地层及相关地质事件的研究主要基于井下岩心,地表露头不多见。通过调查,在松辽盆地东缘农安县青山口第二松花江沿岸发现一新的剖面——岳王城剖面,出露连续的嫩江组一段顶部—二段下部地层。通过对该套地层进行厘米级的描述和详细的分层,将该剖面地层划分为24层,累计厚度31.54m。第1层为嫩江组一段顶部,为青灰色砂质泥岩,含少量介形类化石;第2~24层为嫩江组二段下部,主体为暗色泥页岩夹油页岩、泥灰岩等,含丰富的介形类、叶肢介、鱼类,少量有孔虫及其他生物。此外,在嫩江组底部还发现一层火山灰和一层磷块岩。对其中的介形类化石进行初步研究,识别出11属18种。通过介形虫古生态和沉积学研究,并结合其他手段,初步探讨了该沉积期的湖泊环境,认为嫩二段下部沉积期经历了一次大规模湖侵,从一段顶部的滨浅湖过渡为二段下部的深湖环境,之后湖平面在波动中逐渐下降。
Large-scale lake transgressions occurred in the lower part of the 2~(nd) Member of the Nenjiang Formation in the Songliao Basin, which deposited a set of dark mudstone and black shale with intercalation of oil shale throughout the basin. However, previous researches on strata and related geological events in the 2~(nd) Member of the Nenjiang Formation were mainly based on the cores, with very insufficient attention paid to outcrops. A new section called Yuewangcheng section was found along the Di'er Songhua River in Qingshankou, Nong'an County, eastern margin of Songliao Basin, which successively exposes the top of the 1 stMember to the lower parts of the 2~(nd) Member of the Nenjiang Formation. In this study, the strata were described in centimeter-level and divided in detail into 24 layers with a cumulative thickness of 31.54 m. The first layer is the top of the 1 stMember of the Nenjiang Formation,which deposited bluish gray sandy mudstone with a small quantity of ostracods. 2~(nd) -24 thlayers constitute the lower part of the 2~(nd) Member of Nenjiang Formation, which is composed of dark shale with oil shale and marl, characterized by rich ostracods, phyllodes and fish,with a small amount of foraminifera and other organisms. In addition, a layer of volcanic ash and phosphorite was found at the bottom of Nenjiang Formation. 18 species of 11 genera of ostracods were preliminarily identified. On the basis of sedimentology and ostracod palaeoecology combined with other evidence, the lake environment is preliminarily discussed. It is suggested that a large-scale lake transgression occurred in the lower part of the 2~(nd) Member of Nenjiang Formation. The shore-shallow lake environment at the top turned to a deep lake environment at the bottom, and then the lake level was gradually declined in fluctuation.