川西拗陷中段蓬莱镇组物源及沉积相展布特征

Provenance and sedimentary facies evolution of Penglaizhen Formation in the middle part of West Sichuan depression

在野外剖面观测、岩心描述及测井资料分析的基础上,综合前人研究成果,对川西拗陷中段上侏罗统蓬莱镇组物源、沉积相类型、沉积环境的展布特征及演化规律进行研究。结果表明:(1)根据砂岩碎屑成分分析及物源区构造背景的分析,研究区具有自龙门山前缘至盆地中部砂岩的石英含量逐渐增多、沉积岩岩屑含量逐渐减少的特征,物源主要为西部的龙门山,而北部的米仓山—大巴山对研究区的影响较小。(2)川西拗陷中段蓬莱镇组主要发育冲积扇、河流、辫状河三角洲、湖泊四大类沉积相类型。(3)编制了川西拗陷中段蓬莱镇组砂岩质量分数等值线图及沉积相图,据此认为蓬莱镇组各个时期的沉积环境及砂体展布特征主要受到湖平面升降及物源供给的控制。蓬莱镇组第一段沉积时期,主要以发育冲积扇—湖泊沉积为特征;第二段沉积时期,冲积扇沉积及河流沉积范围逐渐扩大,辫状河三角洲沉积逐渐向湖盆推进,湖泊沉积的范围逐渐萎缩;第三段沉积时期,区内冲积扇沉积、河流沉积范围逐渐萎缩,而辫状河三角洲沉积规模较第二段沉积期增大,湖泊沉积变化不大;第四段沉积时期,主要以发育河流—湖泊沉积为特征,不发育冲积扇沉积,随着辫状河三角洲沉积的推进,湖泊沉积范围逐渐减小。

Based on the field profile observation, core description, Logging data analysis, and synthesis of the previous research results, this paper studies the provenance, types of sedimentary facies and the characteristics and evolution rules of the sedimentary environment of the Upper Jurassic Penglaizhen Formation in the middle part of the West Sichuan depression. The results indicate that： （1） According to the analyses of the clastic constituents of sandstone and the tectonic background of provenance, there are the characteristics in study area that the ratio of quartz in sandstones increases gradually while the ratio of debris in sedimentary rocks decreases gradually from the leading edge of Longmenshan to the middle part of the basin and that the provenance is mainly Longmenshan in the west of study area, while the Micang-Daba mountains in the north contribute little. （2） There develop mainly alluvial fans, fluvial, braid river deltas, lakes sedimentary systems in Upper Jurassic Penglaizhen Formation in the middle part of the West Sichuan depression. （3） People have drawn the contour map of ratio of sandstone and the sedimentary facies diagram of Penglaizhen Formation in the middle part of the West Sichuan depression. They indicate that the sedimentary environment, sand bodies and distribution patterns in the different sedimentary periods of Panglaizhen Formation were mainly controlled by the fluctuation of lake level and the supply of provenance. In the deposition period of the first member of Penglaizhen Formation, there mainly developed alluvial fan-lake sediments in the study area. In the deposition period of the second member, the range of the alluvial fan and fluvial sediments gradually broadened, the braid river deltas sediments gradually carried forward to the lake basin, and the range of the lake sediments reduced gradually. In the deposition period of the third member, the range of alluvial fan sediments and fluvial sediments reduced gradually, while the braid river deltas sediments became wider than that in the second member, and the range of lake sediments did not change obviously. In the deposition period of the fourth member, there mainly developed fluvial, braid river deltas and lakes sediments and no alluvial fan sediments. Along with the braid river deltas sediments carrying forward to the lake basin, the range of lacustrine sediments became narrower than that in the third member.