Research About The Early-Cretaceous Soft-sediment Deformation Structures And Discussion Related To The Triggers On The Lingshan Island,Shandong

The Lower Cretaceous Lingshandao Formation on the Lingshan Island,Shandong,contains abundant gravity-flow sediment and various soft-sediment deformation structures.Based on field work of three sections(Chuanchang,Dengta,Qiancengya),the soft-sediment deformation structures and relative triggers are studied.Lithological units of three sections can be divided into two kinds: thick sandstone and thin interbeddings of sandstone and shale.In view of sedimentary structures and the formation mechanism,thick sandstone is recognized to be formed by three different kinds of mechanism: turbidity flow(TSA sandstone),sandy debris flow(TSB sandstone)and remobilization(TSC sandstone).TSA and TSC sandstones distribute mainly in the Chuanchang and Dengta sections and TSB mainly in the Qiancengya section.Thin interbeddings can be divided into: deformation interval and undeformation intervals.In the deformation interval are lots of soft-sediment deformation structures with different morphologies,scales and stages.Soft-sediment deformation structures show changes in type and quantity of different sections.In the Chuanchang section,soft-sediment deformation structures,which are of most abundance in three sections,can be classified into two kinds: ductile deformation and brittle deformation.Ductile deformation includes folds and boudinage.Brittle deformation includes syn-sedimentary faults.Folds are dominantly recumbent and transform from recumbent to sheath folds along slump direction.Besides,boudinage structures change into syn-sedimentary faults along slump.Slide surfaces are also observed.In the Dengta section,main deformation structures are inclined to recumbent folds.In the Qiancengya section,the deformation structures significantly decrease in both types and quantity and are dominated by load structures.Sedimentary structures and thickness distribution of the undeformation intervals are studied.Sandstones herein are formed by turbidity current based on graded bedding,sharp lower contacts and sharp or gradual upper contacts.Besides,small-scaled deformation structures including flame structures,deformation lamination,are shown in the undeformation intervals and don not change the original sedimentary bedding pattern.Thickening-upward cycles are detected in these intervals.Based on thickness and sedimentary features,the deformation intervals can be classified into two kinds: SSD-1(obviously less than 1m),and SSD-2(next to or more than 1m).The statistical relationship between deformation intervals and undeformation intervals shows that: the SSD-1 occurs when the number of sandstone beds of undeformation intervals rises but the thickness of sandstone beds decreases;the SSD-2 occurs when the number of sandstone beds declines but the thickness of sandstone beds increases.Due to formation mechanism,type and scale of soft-sediment deformation structures,developed laminated shale,coal mass and pyrite,a deep-water slope is inferred for the deposition of the Lingshandao Formation.The slope angle should be low enough to accumulate mass gravity-flow sediment.Field observation shows that three studied sections plus Beilaishi section(formed reported)collectively constitute an intact slope profile from upper slope to slope-bottom.Beilaishi section,in the upper slope,displays slump scarp and slide surfaces;Chuanchang section,maybe in the transition from upper to lower slope,contains slump folds and slide surfaces;Dengta section,in the lower slope,is dominated by slump folds;Qiancengya section,in the slope-bottom or basin plain,shows rare deformation structures and increase in shale abundance.These sections are very meaningful for deep-water hydrocarbon exploration.In conclusion,the existence of slope shows controls,to some degree,about the types and evolution of soft-sediment deformation structure.The soft-sediment deformation structures in the deformation intervals have no direct relationship with overloading and support seismic triggers.However,the small-scale deformation structures in the undeformation intervals occur under the graded sandstone or accompanied with thicker sandstone.This may indicate that these small-scaled deformation structures are formed by overloading.