Abstract: Sediment hydrodynamic conditions are the basis of the study on sedimentary facies and the key to hydrodyna-mic analysis is the distribution of grain size.On the basis of elaborate observation of cores and facies analysis,characteristics and its relationship with sedimentary environment of probability cumulative grain-size distribution curves of coarse clastic rocks of Es³ in Yongbei area of the Dongying Sag,Bohai Bay Basin are systematically studied through mass grain size data.The grain-size distribution curves can be classified into 4 basic types(including debris flow type,high-density turbidity current type,low-density turbidity current type and tractive current type) and 2 transitional types(including debris flow to low-density turbidity current transitional type and high-density turbidity current to low-density turbidity current transitional type).The debris flow type includes broad convex arc pattern and arc-linear composte pattern reflecting debris flow deposition,and mostly develops in main channels of inner fan of the nearshore subaqueous fan and the fan-delta plain.The high-density turbidity current type consists of one suspension segment pattern and low-slope two-segment pattern reflecting high-density turbidity current deposition,and develops in channels of the nearshore subaqueous fan and aquatic or subaqueous distributary channels of the fan-delta.The low-density turbidity current type contains low-slope two-segment pattern and low-slope two bouncing segment-one suspension segment pattern reflecting low-density turbidity current deposition,and mainly develops in space between main channels and space between braided channels of the nearshore subaqueous fan.The tractive current type includes high-slope two-segment pattern,one roll segment-one bouncing segment-one suspension segment pattern,one bouncing segment-one suspension segment-one transitional zone pattern and two bouncing segment-one suspension segment reflecting tractive current deposition,and mainly develops in middle fan of the nearshore subaqueous fan and fan-delta front.The debris flow to low-density turbidity current transitional type shows step-like multi-segment pattern representing transition from debris flow to turbidity current and develops in subaqueous distributary channels of the fan-delta.The high-density turbidity current to low-density turbidity current transitional type is of one bouncing segment-one suspension segment-one transitional zone pattern and only develops in deeply-cutting channels of the outer fan on behalf of the density decline phenomenon of turbidity current.

Key words: probability cumulative grain-size distribution curve, nearshore subaqueous fan, fan-delta, sedimentary environment, Shahejie Formation, Yongbei area, Dongying Sag, Bohai Bay Basin