走滑拉分盆地层序构成特征——以胶莱盆地莱阳群为例

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

胶莱盆地莱阳群有4个层序界面和3个三级层序,其中SB1、SB2和SB4是区域性不整合面,而SB3是平行不整合面。根据水进体系域中泥质含量和水进体系域(TST)与高水位体系域(HST)的厚度比值,将层序类型划分为浅水型和深水型,将冲积型层序直接判定为浅水层序,将TST中含有深湖相沉积的层序直接判定为深水型。层序Ⅰ,莱阳地区为深水型层序,以滨浅湖相沉积为主;诸城地区缺失TST和HST,层序类型不能确定,低水位体系域(LST)以冲积扇相沉积为主。层序Ⅱ,莱阳地区为深水型层序,以湖相沉积为主,发育了厚层暗色半深湖泥岩;诸城、高密和莒县地区为浅水型层序,以冲积扇相、河流相沉积为主。层序Ⅲ,莱阳和诸城两地为深水型层序,以浅湖—半深湖相沉积为主,发育了厚层暗色泥岩;高密和莒县两地为浅水型层序,以冲积扇相—河流相沉积为主。胶莱走滑拉分盆地层序特征表现为:(1)靠近走滑断层的区域发育浅水型层序,而远离走滑断层的区域发育深水型层序;(2)低水位体系域发育叠瓦状扇体;(3)低水位体系域下部单个扇体厚度小,上部单个扇体厚度大。

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.