湘西北龙山、永顺地区龙马溪组潮控三角洲沉积的发现--志留纪“雪峰隆起”形成的新证据

通过露头沉积学与岩石学分析，在湘西北龙山、永顺地区下志留统龙马溪组中首次发现完整的潮控型三角洲沉积，也是华南地区龙马溪组中存在三角洲相的首次报道。识别出前三角洲、三角洲前缘和三角洲平原3个亚相及10个微相。前三角洲亚相以滨外泥页岩为主，并发育低密度浊积岩，主要见于龙马溪组底部和下部；三角洲前缘亚相包括席状砂、分流间湾、河口砂坝和远砂坝等；三角洲平原亚相包括分流河道、分流间湾、天然堤和决口扇等，主要见于龙马溪组中部、上部。通过分析剖面垂向序列及其指示的沉积环境变化，提出本区龙马溪组三角洲相碎屑岩是构造挤压背景下“雪峰隆起”开始形成的重要沉积响应。该三角洲沉积的发现，对于理解扬子东南缘早志留世构造-古地理转型的时限与过程具有特殊意义。

Tide-dominated Deltaic Deposits in Lungamachi Formation,Longshan-Yongshun Regions,Northwestern Hunan: The initial sedimentary responses to outset of “Xuefeng uplift”

The detailed sedimentological and petrological studies reveal that, for the first time, tide-dominated deltaic deposits are well-developed in Lungamachi Formation, Longshan-Yongshun regions, northwestern Hunan. Three subfacies and ten microfacies were identified by outcrops observation and petrological analysis. The prodelta subfacies consists of shelf mudstones and low density turbidite. Microfacies of the delta front include frontal sheet sand, interdistributary bay, channel mouth bar and distal sand bar. The delta plain subfacies consists of distributary channel, interdistributary bay, natural levee and marsh. The cycles of sedimentary sequences and relative sea level change are furthermore discussed. The deltaic sequence from the prodelta deposits to the delta plain deposits showed three upward-coarsening successions that, respectively, consisted of five to six sub-successions and overlain by an upward-fining succession of tidal flat. Paleocurrent data further indicate that the source of Lungamachi Formation should be located in the SE of the study area(“Xuefeng uplift”). The results show the significance of the deltaic deposits in Lungamachi Formation, which are probably initial sedimentary responses to outset of “Xuefeng uplift” during Rhuddanian-Aeronian and recordings for transition of tectonic-basin pattern on the margin of Upper-Middle Yangtze.