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.