松辽盆地白垩纪湖侵沉积层序与湖海沟通事件的地球化学记录

GEOCHEMICAL RECORDS OF THE CRETACEOUS LACUSTRINE TRANSGRESSIVE SEQUENCES AND THE MARINE-LAKE CONNECTING EVENTS IN THE SONGLIAO BASIN,NORTHEAST CHINA

关键词:  近海陆相盆地  湖海沟通事件  沉积地球化学记录

Key words:  nearshore continental basin  marine-lake connecting events  sedimentary geochemical records

摘要:

松辽白垩纪近海陆相盆地最大湖侵期的泉头组一嫩江组时期,可能发生了三次较大规模的海水入侵事件,时间分别为泉三、四段一青一段(阿尔布期),青二、三段顶部(土仑期)和嫩一、二段(晚桑托一早坎佩尼期)。海水注入使湖盆的水文地球化学和环境地球化学条件均发生改变,并在层序中留下沉积记录。海进期主要表现为:①重同位素组份增加,直至与同期海相层序的同位素组成相同;②介质的盐度指标(Sr/Ba)、碱度指标(Ca+Mg)/(Si+Al)、还原性指标(Zn+Ni)/Ga和硫通量指标(S归-化含量)均系统增加。相反,海退期则上述指标显著降低。根据沉积层序的同位素组成、同期海水的同位素组成和同位素分馏的质量平衡关系,可求出研究层段内同位素的海水来源与淡水来源的比例(混合度)为0-5。并由此恢复出海平面升降曲线。

Abstract:

Situated in the northeast of China (42-49°N,120-128°E),the Songliao Basin is a Cretaceous nearshore continental basin which has been affected significantly by the adja-cent marine transgression/regression.The authors have recognized in,this paper three main marine transgressive records on the maximum lacustrine transgressive sequences of the basin,which occurred separately during the deposition of the third and fourth members of the Lower Cretaceous Quantou Formation (Albian),the top of the second and third members of the Upper Cretaceous Qingshankou Formation (Turonian),and the first and second members of the Upper Cretaceous Nengjiang Formation (late Santonian-early Campanian).Besides fossil and mineralogical evidences,the marine transgressive geo-chemical records on the continental basin sequences are;(l)the sharp increase of the heav-ier isotopes of ³⁴S,¹⁸O and ¹³C,and the ratios of ⁸⁷Sr/⁸⁶Sr,even up to those of the syn-chronous normal marine sequences,and (2)the sharp increase of the geochemical indexes for salinity (Sr/Ba),alkalinity (Ca+Mg)/(Si+Al),reducibility (Ni+Zn)/Ga and sulfur flux (S-normalized concentration).