Keywords: 
• Lungamachi Formation /
• tide-dominated delta /
• sedimentary sequences /
• Xuefeng uplift /
• sedimentary responses

Abstract: 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.