松辽盆地晚白垩世“海侵”事件争论及其解决建议

Arguments and solutions on the transgression event in the Late Cretaceous of Songliao Basin

松辽盆地北部已进入非常规页岩油勘探阶段,为了阐明坳陷层页岩形成的古环境特征,针对半个世纪以来松辽盆地晚白垩世“海侵”事件的争论,从古生物、地球化学及地质研究等10个方面进行了梳理和分析,指出有关“海侵”事件的争论主要存在赞成和否定2种观点。地球化学专家和学者主要是持赞成观点,他们甚至通过生物标志化合物与有机质丰度分布特征,推测出东部亚细亚海通过依—通地堑侵入松辽盆地的“海侵通道”;古生物专家和学者主要是持否定观点,他们认为古生物化石以淡水和微咸水为主,少量的高盐度古生物化石并不具有“海侵”的唯一指向性。研究认为:争论焦点集中在水体咸化是湖泊盐度自然分层还是海水侵入所致,但存在3个方面的问题,一是古生态环境对比等时性问题,二是地质研究对古生态环境研究的约束性问题,三是不同领域研究的系统性与同步性问题;强调要充分利用丰富的连续取心井资料及盆地东部边缘完整的地质剖面,在5—6级高频层序地层格架内,同步进行系统的层序地层、沉积充填演化、古生物及地球化学方面分析,才能对坳陷层页岩是否属于“海侵”成因的问题得出可靠的认识。

At present,North Songliao Basin has entered the stage of unconventional shale oil exploration.In order to clarify the characteristics of formed paleoenvironment of depression-layer shale,focused on the debate of the transgression event in the Late Cretaceous of Songliao Basin for half a century,the detailed combing and in-depth analysis were carried out from 10 aspects such as paleo-biology,geochemistry and geology,and it was pointed out there are two main opinions in the debate on the transgression event,namely,yes and no.Most of geochemical experts and scholars agree with the positive view,and even infer that the East Asia Sea invades into Songliao Basin by means of Yi-Tong Graben-the transgression channel with the help of distribution characteristics of bio-marker compounds and organic matter abundance;while paleontologists and scholars mainly hold the negative view,believing that the paleontological fossils are mostly freshwater and brackish water,and a small amount of high-salinity paleontological fossils do not have the only directivity of transgression.The study shows that the focus of debate is whether the salinization of water body is caused by the natural stratification of lake salinity or seawater intrusion.Now,there are three aspects of the problem:first,the isochronism of paleoecological environment comparison;second,the constraint of geological research on the study of paleoecological environment;third,the systematization and synchronicity of research in different fields.It is emphasized that rich continuous coring well data and complete geological section on the eastern edge of the basin should be fully used,and the analyses on the sequence stratigraphic formation,sedimentary filling evolution,paleontology and geochemistry in the 5-6 high-frequency sequence stratigraphic framework should be systematically conducted so as to get a reliable understanding that whether depression-layer shale is the cause of the transgression or not.