AN OUTCROP SEQUENCE DIVISION MODEL: EXAMPLES FROM THE DEVONIAN BASIN IN NORTHERN AND CENTRAL GUANGXI

In terms of the falling stage systems tract (FSST) which developed between the regressive surface of marine erosion and the ravinement surface or regressive surface of subaerial erosion,the present paper defines a ravinement surface above the FSST as the type Ⅲ sequence boundary.This new type of sequence boundary is different from both the type Ⅰ and type Ⅱ sequence boundaries,and can be more gearned to actural circumstances of the Devonican outcrop sequence stratigraphic features in northern and central Guangxi. The key to understand the new type of sequence model,on one hand,is to distinguish some special genetic boundary surfaces,which include regressive surface of subaerial erosion,regressive surface of marine erosion,ravinement surface,exposure surface and palaeokarsti-fication surface and so on.On the other hand,the facies sequence analysis of the outcrop sections is much more important.Different sedimentary systems tracts may display distinctive aggradational,progradational and/or retrogradational association patterns.At last,the basic work of outcrop sequence stratigraphic research is to recognize sedimentary textures and structures.According to the model and method mentioned above,the Devonian strata in northern and central Guangxi can be divided into seven Vail-scale sequences,of which two are type Ⅲ sequences,with a time duration of about one to tens of Ma.