Research On The Early Cretaceous Soft Sedimentary Deformation In Lingshan Island,Shandong Province

Soft-sediment deformation develops at deposition or shortly after,during the first stages of the sediment's consolidation.Own to the lacking of obvious characters of soft-sediment deformation,it is difficult to identify whether the folds or strata are formed by soft-sediment deformation or formed after the consolidation of the rocks.Due to the anisotropy of magnetic susceptibility(AMS)is sensitive to rock deformation,with the deformation increasing,the AMS of the rocks will change in progress.Therefore,the AMS is widely used to study the rock deformation.In addition,the remanent magnetization obtained in different stages of the folds of the rocks also is different.Therefore,in this paper,rock magnetic and paleomagnetic study was conducted on the soft-sediment deformation of the Early Cretaceous Penglai Group in Lingshan Island,Shandong Province.It will be helpful to identify the soft-sediment deformation by the AMS and remanent magnetization characteristics.Lingshan Island is located in the Sulu orogenic belt,where the early Cretaceous Laiyang Group clastic rocks and Qingshan Group volcanic rocks were developed.Among them,Penglai Group developed many soft-sediment structures formed by slump,including curly bedding,flaming structure,water discharge structure,pillow structure,etc.It is a natural laboratory for studying soft-sediment deformation.In this study,192 paleomagnetic samples were collected from 4 sections of the Laiyang Group,Qiancengya,Chuanchang,Laohuzui and Beilaishi sections.Rock magnetic and stepwise thermal demagnetization experiments were conducted on all samples.The AMS results show that the horizontal stratum in the Qiancengya section,the Laohuzui section and the Beilaishi section hold the normal deposition fabric whose smallest axis of susceptibility is perpendicular to the strata.While the vertical stratum in Qiancengya section and the Chuanchang section have relative dispersed fabric,which could be the result of soft-sediment deformation.Stepwise thermal demagnetization results show that,in the geographic coordinates,the high temperature components(HTCs)of the Qiancengya,Laohuzui and Beilaishi sections direct NNW and downward with a moderate inclination;and the HTCs of the Chuanchang section direct SSE and upward with a moderate inclination.The negative fold test suggests that the HTCs could be obtained after folding.However,the HTCs hold dual polarities,suggesting that the HTCs are not the result of remagnetization.In this case,the HTCs could be obtained after the soft-sediment deformation but before the deposition of the overlying stratum Qingshan Group.By applying the tilt correction according to the attitude of the Qingshan Group,the paleomagnetic pole corresponding to the HTC is located at 80.8°N,17.3°E,A95=1.57°.Important developments include:(1)revealing a large number of sedimentary fabric and disturbance fabric features;(2)obtaining important post-deposition remanence in clastic rocks and possible primary remanence in volcanic rocks;(3)Based on paleomagnetic comparison,it is believed that there is no obvious difference in paleolatitude between the Lingshan Island and North China plates in the early Cretaceous period,and they should be part of the North China block.