松辽盆地姚家车站剖面嫩江组一段下部湖相有机质组成及其沉积环境特征

Paleoenvironmental changes during the deposition of the lower Member 1 of the Nenjiang Formation in the Songliao Basin, Northeastern China

姚家车站剖面位于松辽盆地东南部,是研究嫩江组一段烃源岩发育时湖泊沉积环境的一个典型露头剖面。通过测定和分析总有机碳(TOC)含量及其稳定碳同位素组成(δ^(13)C_(org))、特征分子标志物(Pr/Ph、伽玛蜡烷、C30甾烷、芳基类异戊二烯、脱羟基维生素E和GDGTs)的剖面变化,讨论了嫩江组一段下部湖泊有机质组成及沉积环境特征。根据剖面各参数的变化特征,可将其沉积划分为2个阶段。剖面TOC含量为0.06%～3.98%,具有阶段Ⅰ高,阶段Ⅱ低的特征,主要受初级生产力水平和保存条件的控制。δ^(13)C_(org)值揭示出沉积有机质主要来源于湖泊内源。特征生物标志物的剖面变化表明:嫩江组一段沉积期湖泊水体分层,下部水体缺氧,且阶段Ⅰ和阶段Ⅱ的上部湖泊水体缺氧已上升至透光带;湖泊下层水体盐度相对较高,存在短期快速变化。24-正丙基胆甾烷和24-异丙基胆甾烷含量的剖面变化揭示出嫩江组一段时期松辽古湖盆发生了海侵,但海侵是间歇性的,而不是持续的。分析认为:松辽海侵的发生与高盐、缺氧、分层的水体环境及油页岩的形成密切相关。尝试利用TEX_(86)指标定量重建了嫩江组一段下部古湖泊水体温度,结果表明松辽盆地在白垩纪基本处在亚热带气候区。

The Yaojia Station section, on the southeastern side of the Songliao Basin, is a typical outcrop to investigate the paleoenvironment of Member 1 of the Cretaceous Nenjiang Formation. Here, we present the total organic carbon (TOC),δ^13Corg, and biomarker data from the 31 outcrop samples collected from this section. According to the variation in these parameters, the whole profile can be divided into two depositional stages. The TOC content is higher during stage I and lower during stage II. These results indicate that the primary productivity and sedimentary environment mainly controlled the enrichment of the deposited organic matter (OM) in the paleo-lake. The depleted δ^13Corg in the whole profile indicates that the OM was mainly derived from lake algae and aquatic macrophytes. The Pr/Ph ratios, as well as the occurrence of aryl-isoprenoids, gammacerance, and methytrime-thyltridecyl chromans indices, indicate that the water column was stratified and the bottom water column was anaerobic with high salinity in the Songliao Basin. Marine transgression, indicated by the presence of 24-n-propyl-cholestanes and 24-iso-propyl-cholestanes, occurred during deposition of the lower Member 1 of the Nenjiang Formation. The seawater incursions caused a stratified water column with high salinity and anoxicity, favoring the formation of the OM-rich source rocks in this basin. The paleo-lake water temperatures were reconstructed based on the TEX86 index. The estimated temperatures suggest that a subtropical climate prevailed during the deposition of Member 1 of the Nenjiang Formation.