湘西北永顺——龙山地区早志留世三角洲沉积

湘西北早志留世龙马溪期至溶溪期为三角洲沉积，可划分出前三角洲、三角洲前缘和三角洲平原三个亚相。前三角角洲亚相以龙马溪组为代表，已鉴别出原地泥页岩、浊积岩和底流沉积三一相，其中后二者主要见于龙马溪组 ；三角洲前缘亚相以小河坝且为代表，已鉴别出试状砂粒滩、礁间泥微相；三角洲平面亚相以溶溪组为代表，已鉴别出分流河道及沼沼泽微相。该三角洲沉积的特征是入海河流流速低而流量大，泥沙含量高而粒级细小，在三角洲前

THE EARLY SILURIAN DELTAIC SEDIMENTS IN YONGSHUN LONGSHAN AREA,NORTHWESTERN HUNAN PROVINCE

The Yongshun-Longshan area in northwestern Hunan Province was an area of deltaic sediments from Longmaxi Formation to Rongxi Formation of Lower silurian.The deltaic sedimentary system can be divided into predelta,deltaic front and deltaic plain facies.The predelta was composed of the Longmaxi Formation,in which three microfacies were identified:in situ shale and mud,turbidites and bottom current deposits.The later two facies mainly developed in the lower part of the Longmaxi Formation.The deltaic front was represented by the Xiaoheba Formation,in which four facies were recognized:front sand sheet,reef,oolite(bioclastic shoal),inter reef(shoal)mud.The Rongxi Formation was deltaic plain which includes branched channels and marsh.The river for building the delta was characterised by low velocity,high discharge,high concentration of fine clay and sand,and associated with reef(shoal) in delta front.