兴蒙地区晚二叠世海相沉积的古环境重建

In Hinggan-Inner Mongolia region, bryozoan, calcareous alga and other typical marine organic fossils have recently been discovered in many sections in the upper part of late Permian Linxi Formation, e.g. the Guandi section in Linxi county. This evidence indicates massive marine deposits occur in upper part of late Permian system in this area, which has distinct difference from that reported by others in the past, i.e., continental deposits. This project will first set up the precise stratigraphic isochronous framework, and then carry out systematic studies in petrology, mineralogy and inorganic or organic geochemistry of limestone, clay/shale and sandstone, e.g. the B and Sr isotopes values from limestone, the acritarch and organic molecular markers from mud/shale, and the fabric, structure and authigenic mineral from sandstone, focusing on the late Permian marine deposits in Linxi Formation, to get ancient biotypes and ecological characteristics, and to extract geochemical indicators of palaeoenvironmental conditions from marine and continental sediments. We will recognize the formation environments for all kinds of rocks on the basis of a number of geological and geochemical markers, and then depicting the phase sequence superposition and the distribution of facies belt to reveal the space-time distribution of marine deposits of linxi formation, and reconstruct the palaeogeographic pattern and its evolution for the Norhteast China and Hinggan-Inner Mongolia region in late Permian to provide the basis for to select oil and gas favorable area in upper Permian series in Songliao and peripheral region.

在兴蒙地区官地等多个剖面的上二叠统林西组上部发现大量苔藓虫、钙藻等典型海相生物化石，认为在晚二叠世晚期本区可能存在较大范围的海相沉积，这与前人的陆相沉积认识明显不同。本项目以精确地层等时格架为基础，系统研究林西组灰岩、泥/页岩和砂岩的岩石学、矿物学及无机或有机地化特征，聚焦晚二叠世海相沉积，重点解析生物建造类型及古生态特征，提取海、陆相沉积的地球化学环境指标，灰岩侧重B、Sr同位素，泥/页岩突出疑源类孢子和有机分子标志物，砂岩强调结构构造和自生矿物等，综合多项地质和地球化学标志，判别各类岩石成因环境，刻画相序叠置和相带分布，揭示林西组海相沉积的时空分布，重塑兴蒙地区晚二叠世林西期古地理环境及演化，为东北-兴蒙地区上二叠统新层系油气战略选区提供科学依据。

在兴蒙地区官地等多个剖面的晚二叠世林西组上部林四、林五段中发现了大量典型海相化石，认为该区晚二叠世晚期可能发育较大规模的海相沉积。为了揭示林西组海相沉积的时空分布，重塑兴蒙地区晚二叠世林西期古地理环境，本项目系统研究林西组灰岩、泥/页岩等岩性的岩石学、矿物学及无机或有机地化特征，聚焦晚二叠世海相沉积，分析岩性-岩相-古生物特征，应用地球化学环境指标，判别各类岩石成因与沉积环境。建立了林西组标杆地层柱和晚二叠世地层等时格架；取得了兴蒙地区上二叠统林西组上部大量典型海相化石与生物礁的重要发现；确定了林西组地层时代，林四、林五段砂岩样品碎屑锆石U-Pb年龄主要分布在254～267和250～269Ma区间内，对应峰值分别为256Ma和257Ma，而林五段样品中最年轻碎屑锆石年龄（250Ma）及林五段层内安山岩结晶年龄（248Ma）共同限定了林西组地层的下限年龄为早三叠世末期，表明林西组沉积期主体为晚二叠世，并持续至早三叠世末期；林西组岩性-岩相-古生物特征及其对沉积环境的指示表明晚二叠世林西组上部地层的沉积环境以海相为主；林西组灰岩锶同位素87/86Sr分布范围为0.7073～0.7077，平均值0.7076，总体与贵州罗店长兴组海相灰岩的87/86Sr 值基本相当。根据碳、氧同位素分析数据计算的Z值范围在110～123之间，平均为116，很接近海陆环境分界值120。灰岩的常微量元素特征表明林西组灰岩Th/U比值在0.33-2.55之间，平均1.21，V/Ni比值分布范围为0.6-2.2，平均1.5，表明林西组灰岩主要为海相沉积。暗色泥页岩的生物标志化合物和微量元素特征指示泥页岩母质来源于低等水生生物或藻菌类，形成于强还原高盐度沉积环境；恢复了兴蒙地区晚二叠世构造沉积环境，晚二叠世出现兴-蒙张裂海槽，海相沉积环境的发现为东北-兴蒙地区上二叠统新层系油气战略选区提供科学依据。