Sequence stratigraphy in a strike-slip pull-apart basin: An example from the Laiyang Group in the Jiaolai Basin

Four sequence boundaries are recognized in the Laiyang Group from the base upwards in the Jiaolai Basin: SB1,SB2 SB4 as regional unconformities and SB3 as a parallel unconformity,according to which the Laiyang Group falls into three third-grade sequences.In the light of the mud contents in the transgressive systems tract TST and the thickness ratios of the transgressive systems tract and highstand systems tract HST,the sequences in the study area may further be subdivided into shallow-water and deep-water types: the former including the alluvial sequences and the latter including the deep-lake sequences within the transgressive systems tract.Sequence I is represented by the deep-water sequences composed of littoral and shallow lake deposits in the Laiyang region.The sequence types remain uncertain in the Zhucheng region due to the absence of the transgressive and highstand systems tracts.The lowstand systems tract LST in this area consists of the alluvial fan deposits.Sequence Ⅱ is represented by the deep-water sequences composed of lake deposits with thick dark semi-deep lake mudstone in the Laiyang region,whereas by the shallow-water sequences composed of alluvial fan and fluvial deposits in the Zhucheng,Gaomi and Juxian regions.Sequence Ⅲ is represented by the deep-water sequences composed of shallow lake and semi-deep lake deposits with thick dark mudstone in the Laiyang and Zhucheng regions,and by the shallow-water sequences composed of alluvial fan and fluvial deposits in the Gaomi and Juxian regions.The Jiaolai strike-slip pull-apart basin is characterized by: 1 shallow-water sequences along the strike-slip faults and deep-water sequences far from the strike-slip faults; 2 imbricated fans within the lowstand systems tract,and 3 smaller thickness of the lower fans and greater thickness of the upper fans within the lowstand systems tract.