湖南邵阳地区茅口期晚期重力流沉积的发现及意义*

湖南邵阳市邵阳大道两侧出露的龙潭组下段(中二叠世茅口期晚期)发育一套地质特征典型、沉积序列明显的重力流沉积。重力流沉积以块状砂岩、粉砂岩及泥岩互层为主,砂岩底面发育有重荷模、槽模、工具痕及冲刷充填构造,砂岩内部有块状层理、平行层理、包卷层理及粒序层理,互层的泥岩中见类似于古网迹的痕迹化石。可识别出砂质碎屑流、浊流与滑塌沉积。剖面下部含泥砾块状砂岩发育,剖面中上部以发育薄到中厚层砂岩、粉砂岩与泥岩互层为特点。根据重力流沉积物特征及其垂向序列特征,建立了重力流沉积模式,将海底扇划分出内扇、中扇与外扇。通过与附近的短陂桥矿区的龙潭组下段沉积特征进行对比研究,结合华南地区岩相古地理特征,认为位于华南盆地范围的邵阳地区,在中二叠世茅口期晚期(龙潭组下段)发育的重力流沉积,可能意味着华南盆地在扬子陆块和华夏陆块之间的深大断裂在中二叠世末期曾经发生拉张,形成裂谷盆地。

Discovery and significance of gravity flow deposits of the late Maokouan in Shaoyang area of Hunan Province

The gravity flow deposits with typical geological characteristics and depositional sequence are developed in the Lower Longtan Formation(late Maokouan in Middle Permian),which are well exposed in a road cut cliff along the Shaoyang aveneue in the suburb of Shaoyang City,central Hunan Province. The gravity flow deposits are mainly composed of massive sandstones, interbedded siltstones and mudstones. Sedimentary structures including load cast,flute cast,tool mark and scouring-filling structures are developed at the bottom of the sandstones,whereas massive bedding,parallel bedding,convolute bedding and graded bedding are developed in the internal part of sandstones. Trace fossils similar to Paleodictyon are found in interbedded mudstones. The deposits of sandy debris flow and turbidity current,and slumps are recognized based on the sedimentary structures seen in the outcrops. The lower part of the section is developed with muddy-gravel-bearing massive sandstone and the upper part was dominated by interbedded sandstones with thin to middle-thick beds,slitstones and mudstones. The gravity flow sedimentary model is established based on the characteristics of gravity flow deposits and their vertical sequences,and the inner fan,middle fan and outer fan facies belts are subdivided. Based on the comparison with sedimentary characteristics of the lower part of the Longtan Formation in the adjacent Duanpiqiao exploration area,and referred to the overall Middle Permian palaeogeography of southern China,we believe that the gravity flow deposits developed in the Late Middle Permian(lower Longtan Formation)in the Shaoyang area imply the deep fault between the Cathaysia landmass and the Yangtze landmass had stretched and formed a extensional trough basin during the late Maokouan.