On the Essence of Stratigraphic Diachronism from the Basic Sequence Stratigraphic Model—An example from the Devonian sequence stratigraphic framework of the Dianqiangui basin

The principle of ubiquity of diachronism proposed by Shaw in 1964 is now one of the mainstays of modern stratigraphy.Since the end of 1970s,sequence stratigraphy which was based on seismic stratigraphy has been an effective methodology for studying facies transplant within the framework of chronostratig raphy and sea-level changes.It changed the principle of subdivision of sedimentary rocks and initiated one crucial revolution of sedimentary geology and stratigraphy.In the basic sequence stratigraphic model,stratigraphic overlap and offlap induced by relative sea-level fluctuations clearly suggest the principle of ubiquity of diachronism,and imply the essence of diachroneity.The two kinds of facies-change surfaces (dynamic and static facies-change surfaces) produced by two kinds of sedimentation (lateral and verticalag gradation) in stratigarphic record represent two types of diachroneity:the diachronism of facies-change and diachr onism of punctuated sur face.As a result,they together represent two basic forms of diachronism in stratigraphic record.The Devonian in the Dianqiangui Basin takes the shape of alarge wedge,thinning laterally from south to north,which was resulted from the earlier transgressive pinch-out event and later regressive pinch-out event during the Devonian.The sequence stratigraphic framework of the studied area indicates imper fection and ubiquity of diachronism of stratigraphic records,and that recognition and study of the two kinds of facies-change surfaces and two types of diachronism are keys to the construction of sequence stratigraphic ramework as well.