摘要:晚三叠世龙门山前陆盆地形成早期的沉积环境长期存在争议。本文通过对须家河组中泥页岩的元素地球化学分析，并结合古生物、矿物组成和沉积特征对须家河组的沉积环境进行分析，探讨有机质富集模式。研究表明，龙门山前陆盆地须家河组沉积时期属于温暖潮湿的热带—亚热带气候。其中，须一段为海相沉积，泥页岩主要形成于还原环境，可见少量黄铁矿，有机质富集受氧化还原条件控制。须二段—须五段的地球化学特征、古生物特征和泥页岩有机质特征上具有相似性，与须一段的海相沉积有明显差别，在与里海进行分析对比后认为，从须二段开始，须家河组逐渐转为陆相沉积。研究表明，须二段—须五段为半咸水—淡水沉积，泥页岩中黄铁矿消失，菱铁矿出现，结合V/Cr、U/Th和dU等地化指标反映，泥页岩应沉积于弱氧化的水体。须二段—须五段泥页岩TOC含量与古生产力和沉积速率相关性好，有机质富集受古生产力和沉积速率共同控制。前陆盆地持续沉降的构造背景下形成的泥页岩具有较快的沉积速率，使得有机质在弱氧化条件下也能得到较好的保存，在沉积速率大于10 cm/kyr，古生产力充足的条件下，有机质富集和保存不再受氧化还原条件控制。
关键词: 晚三叠世/
龙门山/
前陆盆地/
须家河组/
泥页岩/
沉积环境
Abstract:Longmenshan foreland basin formed in the Late Triassic, but the sedimentary environment of that period has long been controversial. This study based on analysis of the geochemistry, and combined with paleontology, petrology and sedimentary characteristics of Xujiahe Formation in Longmenshan foreland basin, discuss its sedimentary environment and the pattern of organic matter enrichment. The study shows that the Xujiahe Formation belongs to warm and humid tropical-subtropical climate during the sedimentary period. Xujiahe 1st member is marine facies, and the shale is mainly formed in the anoxic condition. There is a small amount of pyrite can be seen in the shale, and the enrichment of organic matter is controlled by the REDOX conditions. The study found that the geochemical characteristics, paleontological characteristics and organic characteristics of the shale of Xujiahe 2nd～5th member have similarities, and there is a significant difference with the marine sediments of Xujiahe 1st member. After comparison with the Caspian Sea, it is believed that the Xujiahe Formation gradually turned into continental sedimentation from Xujiahe 2nd member period. And Xujiahe 2nd～5th member should be deposit in the brackish water-fresh water. The pyrite in the shale of Xujiahe 2nd～5th member disappeared, but the siderite appeared, according to the geochemical indicators such as V/Cr, U/Th and dU indicate that the shale should be deposited in weak oxidation condition. There is a good correlation between the TOC and paleoproductivity and sedimentary rate of shale in the Xujiahe 2nd～5th member, paleoproductivity and sedimentary rate are the major controlling factors for the enrichment of organic matter. The organic-rich shale deposit in the foreland basin, which is fast and continuous sinking, has a fast sedimentary rate, and the organic matter could be well preserved in the weak oxidation condition. This study suggests that when the sedimentary rate is faster than 10 cm/kyr, and have sufficient productivity, the enrichment and preservation of organic matter in the shale will not be controlled by REDOX conditions.
Key words:Late Triassic/
Longmenshan/
Foreland basin/
Xujiahe Formation/
Shale/
Sedimentary environment
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