THE DEPOSITIONAL ARCHITECTURE OF THE LACUSTRINE DELTAIC SYSTEM WITH SUBAQUEOUS DISTRIBUTARY CHANNEL-MOUTH BAR COUPLETS OF THE YAN’AN FORMATION IN NORTHERN ORDOS BASIN

Based on genetic stratigraphic analysis of the Yan＇an Formation of northern Ordos Basin, a new type of the deltaic system is recognized. Within this system, subaqueous distributary channel-mouth bar couplets are developed beyond the modification of wave processes, without sheet sandstones between the couplets. Morever, a deltaic system unit is divided into three process stages in detail, i.e. constructive, abandoned and destructive. stages, during the second of which the thick coal seams are formed. The recognition of the deltaic system with subaqueous distributary channel-mouth bar couplets can be used as a tool for interpreting the history of basin fill.