柴达木盆地西北缘上干柴沟组泥岩地球化学特征与古环境古气候意义

Geochemical Characteristics and Paleoenvironment Paleoclimate Significance of Mudstone in the Shang-Gan-Chai-Gou Formation at the Northwestern Margin of Qaidam Basin

柴达木盆地西北缘新生界古环境对揭示盆内的油气形成与演化有着重要的理论和实践意义。本文以盆地内上干柴沟组冷湖四号井中的26件泥岩样品为研究对象,通过常量、微量元素的变化特征来分析其古环境古气候演化历程。结果表明,所有样品的Sr/Ba<1、Th/U>2,指示为陆相淡水环境;V/Cr、Ni/Co、U/Th和V/Sc值分别为0.07~0.01、3.25~2.18、0.48~0.18和8.82~5.53,均指示氧化环境,与其棕褐色的颜色特征反映的沉积特征相吻合,推测由喜马拉雅中期运动导致的盆地北缘隆起致使湖盆下沉、湖面下降形成氧化环境有关。古气候指标Sr/Cu值为13.35~5.91,表明气候总体偏温湿,温湿气候期大致可分为两期:31.0~28.7、27.7~22.5 Ma,干旱气候也大致分为两期:28.7~27.7 Ma、22.5~21.0 Ma, Rb/Sr、Fe/Mn值指示的气候温湿变化趋势与此基本一致,沉积学和古生物学特征同样表明该沉积期总体处于温湿环境。温湿气候、淡水环境响应了该时段全球升温的气候背景。阶段性干旱气候的出现,与前期可能受青藏高原隆升的影响,后期主要与北特提斯海退有关。

The study of the Cenozoic paleoenvironment in the northwest margin of Qaidam basin has great theoretical and practical significance for revealing the formation and evolution of oil and gas in the basin. In this paper, 26 mudstone samples from the Lenghu No.4 well in the Shang-Gan-Chai-Gou formation were studied, and the paleoenvironmental and paleoclimatic conditions were discussed according to variation characteristics of major and trace elements in these mudstone samples. The results showed that: nearly all Sr/Ba values were less than 1, while all Th/U values were greater than 2, in according with the terrestrial freshwater environment;values of V/Cr, Ni/Co, U/Th, and V/Sc ranged from 0.07 to 0.01, 3.25 to 2.18, 0.48 to 0.18, and 8.82 to 5.53, respectively, indicating the oxidation environment, which is also showed by the brown color sediments. It can be speculated that the uplift of the northern basin margin by the mid Himalayan movement resulted in the subsidence of lake basin and the decline of lake level, and thus formed the oxidation environment. The paleoclimatic index Sr/Cu was 13.35-5.91, indicating that the climate was generally warm and humid, and, approximately, periods of warm and humid climate were 31.0-28.7 and 27.7-22.5 Ma, while dry periods were 28.7-27.7 and 22.5-21.0 Ma. The temperature and humidity trends indicated by Sr/Cu, Rb/Sr, and Fe/Mn were basically the same. The sedimentological and paleontological characteristics also showed that the temperature and humidity environment should have been maintained for the whole sedimentary period. The warm and humid climate and the freshwater environment reflected the climate background of global warming in this period. The appearance of the periodic arid climate might be affected by the uplift of Tibet Plateau in the early stage, and mainly related to the regression of north Tethys in the late stage.