Sequence Structure and Depositional Filling of Northern Songliao Basin During Shrinkage Stage

The shrinkage stage is an important part of basin evolution.The research actuality about the sequence stratigraphy of rift basin among faulting stage,depression stage and shrinkage stage shows that there is very little systematic research about the sequence stratigraphy during shrinkage stage,and there is vague understanding of its sequence boundary,structure,and main control factors.In the view of this problem,we carried out a systematic research about the sequence stratigraphy in Sifangtai formation and Mingshui formation which correspond to the shrinkage stage in Songliao basin.One supersequence was defined by two regional unconformities,and it can be divided into six sequences.The comparison and analysis on the geologic feature between depression stage and shrinkage stage in Songliao Basin shows that it is a shallow lake with smooth topography,and its representative characteristics are low-amplitude structure,low accommodation space,frequently up and down of base level,and so on.The representative characteristics of sequence boundary are gentle angular unconformity,low-angle on-lap and transfer surface between ascending and falling.The sedimentary facies of this region contain meandering river,meandering river delta,braid river delta,lakes and so on.In their sequence structure,they were divided into two system tract:transgressive system tract and regressive system tract.There is no obvious incised valley in basin margin and few sub-lacustrine fan was found.Sands are mainly in the transgressive system tract.Because of stable structural setting,sequences were controlled mainly by palaeoclimate and palaeotopography.