Abstract: Several features such as sedimentary system,lithology and lithofacies and sandbody distribution in the Dainan Formation of Subei half-graben fault basin neither follow the general patterns of the half-graben faulted basins in Eastern China,nor follow the common patterns that trench belts are the main locations where fans and sandbodies occur in continental basins.Through a large number of core data observation,it is found that there is a special type of sedimentary rock composed of sand grade coarse debris of mudstone,which shows a certain universality and regularity.The special lithologic section is comprehensively identified on the basis of core observation and description and in combination with the thin section observation,paleontological assemblage and some peculiar phenomena of logging responses.The study shows that this set of strata belongs to fans resulted from mud debris flow,a type of gravity flow.The fan bodies can be divided into 3 belts including root-fan,middle-fan and outer-fan.Laterally,this kind of fan develops in the slope break zone of faulted lake basins which is trench zone of lacustrine basin.Vertically,these fans are mainly found in LST of the Dainan Formation of Subei fault basin.No such fans occur in TST,and some occur in HST.Subei half-graben fault basin was formed in a special geological setting.A large complete depressed lake basin in the lower tectonic zone disintegrated into several mutually separated half graben fault depressions.Multi factors including confining of thick mudstone in the 4^th Member of Funing Formation,old and new dual provenances,tectonic difference,various transportation flow patterns and mutiple types of slope break zones coupled together and controlled the sedimentary characteristics of Dainan Formation.The rules are found that the mud debris flow fans and mud-rich fans are developed in slope break zone on the concave side of fault depression and sand-rich fans occur in slope break zones on the convex side of the fault depression.

Key words: mud debris flow fan, multiple factors coupling, petrologic characteristics, sedimentary genesis, Dainan Formation, Subei Basin