Genetic Mechanism And Sedimentary Filling Response Of The Eocene "Opposite Detachment Type Composite Sag" In The Western Of Baiyun Sag, The Pearl River Mouth Basin

The detachment faulting is an important part of the study of crustal thinning,geomorphic denudation and the formation and evolution of faulted basins.Two sets of large-scale detachment fault systems developed in the continental direction and the ocean direction during the Eocene rifting period in the Baiyun deep-water area of the northern continental margin of the South China Sea,forming a detachment-forward dustpan-like fault depression on the crust of Eastern subsag of Baiyun sag,a multi-stage fault-step wide-deep fault depression in the Main subsag of Baiyun sag crust-mantle detachment,and a weak detachment transitional double-fault complex fault depression in the Western subsag of Baiyun sag.The western area of Baiyun Sag is located in the convergence zone of the south-dipping single detachment fault and the north-dipping multi-stage detachment fault system in the convergence zone of the Zhu II,Zhu I and Zhu III depressions in the Pearl River Mouth Basin,forming a unique pattern of’opposite detachment type composite sag’.The structure of the depression is complex,and its special genetic mechanism needs to be further studied,which directly restricts the type of sedimentary system and the Characterization of sedimentary coupling response.Based on the latest high-precision full-coverage three-dimensional seismic data and drilling data,this paper takes structural geology,sequence stratigraphy,cyclostratigraphy,sedimentology and logging geology as the theoretical guidance to carry out the research on the genetic mechanism and sedimentary filling response of the Eocene Wenchang period’s’opposite detachment type composite sag’in the western Baiyun area of the Pearl River Mouth Basin in the northern continental margin of the South China Sea.Taking the complex subsag structure developed in the western Baiyun area and the sedimentary coupling response under its control as the starting point,the structural-sedimentary response studies were carried out around the Panyu 25 subsag(southward detachment zone),Baiyun 3 subsag(northward detachment zone)and its opposite detachment convergence zone.Based on the paleogeomorphological reconstruction and BQART sedimentary flux model,the source-sink coupling model under the transformation of tensional-detachment structure is established.Through the above work,this study has achieved the following understanding:(1)The ternary genetic mechanism model of’pre-existing fault-ductile uplift-magma activity’of’antithetic detachment composite sag’in Eocene rifting period in the western area of Baiyun Sag is constructed.1)The western Baiyun area of the early Wenchang period is located in the sub-scene I_a of the fault depression,corresponding to the first act of Zhuqiong movement.The principal stress direction is NW-SE,oblique to the Mesozoic near EW-NEE pre-existing faults in the Yunkai low uplift,and the activation degree of the pre-existing faults is limited.Most of the growth faults only extend to the upper crust in the form of high-angle normal faults,forming a large number of scattered half-grabens or compound half-grabens.The intensity of crustal uplift and denudation is low,and a small amount of scattered magmatic activity is accompanied by rifting.2)In the late Wenchang period,the western Baiyun area was in the sub-stage of fault depression I_b,corresponding to the Huilu movement.The direction of principal stress deflected to the NNW-SSE direction,and the degree of orthogonality with the pre-existing near EW-NEE faults increased significantly.The pre-existing faults were activated in large quantities and extended to the middle crust with plastic rheological properties.With the strong extension and thinning in the direction of the force,the current pattern of’opposite detachment type composite sag’has been formed in the western Baiyun area,and the fault dip angle has been reduced.The differential upwelling of mantle-derived materials in the strong thinning area causes crustal uplift,which has obvious characteristics of rift migration or transformation,and converges in the deep part of the depression with the detachment transformation,which has a significant control and influence on the early rift structure.(2)The space-time evolution sequence from’multi-center narrow-deep lake fan delta-braided river delta system’to’single-center wide-shallow lake braided river delta-beach bar system’was established under the Eocene Wenchang tensional-detachment tectonic transformation in western Baiyun area.1)The early Wenchang period is dominated by brittle tension,and the pre-existing fault block in the basin has a strong separation effect,showing a narrow deep lake sedimentary appearance with multiple sedimentary centers.The Panyu low uplift,the Yunkai low uplift and the fault block in the basin are supplied by multiple sources.The small and medium-sized fan delta sedimentary system is developed along the steep slope of the fault.The short-axis braided river delta is developed in the northeastern slope of Baiyun 3 sag,which is characterized by fan delta-braided river delta composite system.2)In the late Wenchang period,the ductile detachment started,the sedimentary center converged and migrated along the detachment fault system,and the terrain slope in the depression continued to slow down,showing a single-center wide shallow lake sedimentary appearance.The sedimentary system of the margin changed to the braided river delta system.At the same time,the fault-controlled ductile uplift continued to supply the source in the opposite detachment convergence zone,and the axial braided river delta-beach bar composite model under the limit of fault block uplift was developed.(3)Combined with the BQART model,the sand control mode of source area supply efficiency under the control of ductile uplift under the tension-detachment transformation in the western Baiyun area was constructed.1)In the early Wenchang period,the supply flux was relatively small,the sedimentary volume was small,the fault block in the basin was dynamically supplied,and the overall supply efficiency was low.2)During the late Wenchang period,with the uplift of the peripheral uplift,the area of the secondary basin decreased,the supply flux and the distribution scale of the sedimentary body increased,and the input of the large-scale material source on the long axis enhanced the early supply efficiency.The stagnation of the source strength and fan thickness with the ductile uplift and the formation filling weakened synergistically.The denudation-deposition process is controlled by structural transformation and environmental tuning,which provides conditions for facies development and sand body transformation.In the process of detachment,the positive transformation of the reservoir sand body by the coastal flow formed by the convergence of the depression,the migration and change of the sedimentary center,and the matching of the spatial and temporal coupling relationship between the sedimentary sand body and the sedimentary center during the tension-detachment period are helpful to predict the potential exploration target area in the area.