大民屯凹陷古近系沙河街组三段扇三角洲前缘沉积生物扰动特征

Bioturbation in fan delta front deposits in the 3rd Member of the Paleogene Shahejie Formation in the Damintun Sag,Liaohe Depression

为探究陆相扇三角洲前缘沉积生物扰动特征,以辽河拗陷大民屯凹陷沈检5井古近系沙河街组三段扇三角洲前缘沉积岩芯为研究对象,对生物遗迹化石进行了鉴定,统计了垂向剖面上生物扰动层厚度和扰动指数的分布特征。依据主微量元素参数反映的沉积条件,探讨了生物扰动层厚度和扰动指数与沉积环境的响应关系。研究结果表明,沈检5井S34Ⅱ和S34Ⅰ油层组发育生物扰动构造,生物遗迹化石以古藻迹(Palaeophycus)和漫游迹(Planolites)占优势为特征。生物扰动构造主要发育于水下分流间湾微相沉积物中,扰动层厚度在5~51 cm,扰动指数以3~5级为主。主微量元素参数分析表明S34Ⅱ和S34Ⅰ油层组沉积期的古气候、古生产力、古盐度和古水深条件相近,即为相对温湿的古气候、低古生产力、淡水—微咸水盐度条件和浅水沉积环境。沉积环境变化幅度小是造成S34Ⅱ油层组与S34Ⅰ油层组发育的生物扰动层厚度和扰动指数差异不明显的主要原因。但当前研究还难以明确单个油层组内生物扰动层厚度和扰动指数变化的主控因素。该项研究加深了对陆相浅水扇三角洲前缘沉积环境遗迹化石组成和生物扰动指数变化特征的认识。

Objectives:To investigate the features of bioturbation in shallow-water fan delta front deposits.The core from the 3rd Member of the Paleogene Shahejie Formation from the Well Shenjian-5 was studied,which was deposited in the shallow-water fan delta front in the Damintun Sag,Liaohe Depression.Methods:We have identified the trace fossils,counted the bioturbated layers thickness and bioturbation index along with the vertical core profile.Based on the elemental composition analysis of rock samples,we discussed the relationships between the depositional environment and the bioturbated layer thickness and bioturbation index.Results:The results show that bioturbation structures were observed on the core of S34Ⅱand S34Ⅰoil layer groups,characterized by the dominance of the Palaeophycus and Planolites trace fossils.Bioturbation structures were found mainly developed in the inter-distributary bay deposits.The thickness of bioturbated layers is from 5 to51 cm,and the bioturbation index is from 3 to 5.The thickness of bioturbated layers and bioturbation index vary with no clear evolutionary trend along the vertical profile.Conclusion:The analysis of elemental parameters demonstrate the paleoclimate,paleoproductivity,paleosalinity,and paleowater depth conditions of S34Ⅱand S34Ⅰoil layer groups are similar,they were deposited in relatively warm and humid paleoclimate,low paleoproductivity,freshwater to brackish water salinity,and shallow water environments.The similar depositional environment is the main reason causing the small differences in the thickness and bioturbation index of bioturbated layers between S34Ⅱand S34Ⅰoil layer groups,as well as for the non-significant evolutionary trend of bioturbated layer thickness and bioturbation index along the vertical profile.However,it is still difficult to determine the main factors influencing the bioturbated layer thickness and bioturbation index in a single oil layer group.This study provides further evidence for identifying the terrestrial shallow-water fan delta front environment and improves understanding of the features of the trace fossils and bioturbation intensity in the environment.