| In view of unclear types of geological structures,low level of evolution and inadequate researches on control factors in the different regions of the Upper Ordovician Lianglitage Formation in Tazhong area,guided by principles of carbonate stratigraphy,sedimentology and geophysics,making full use of seismic,core and thin section data,this paper divided the types of geologic structures at the platform margin and analyzed sedimentary and seismic reflections of different types of structures.Also,factors controlling the evolution of the platform margin were analyzed from the aspects of structure and sea level changes,phase development pattern and the distribution map of the Tazhong platforma margain facies belt have been established,and the evolution of the sedimentary facies and the plane distribution rules under different types of platform structures were analyzed.According to the slope angle,depth,relative thickness as well as plane distribution width of the depositional water in the platform margin and its relationship with fault properties,the platform margin of the Upper Ordovician Lianglitage Formation in the Tazhong area was classified as four kinds of margin structure styles,including fault-controlled steep slope,sedimentary steep slope,fault-controlled gentle slope and weak-rimmed gentle slope.The thickness of the fault-controlled steep-sloped platform margin is large,and there is a significant difference from the intra-platform geomorphology.High-angle slopes develop to one side of the basin;this type of platform has a narrow facies belt(averages 4.76 km),characterized by vertical accumulation,inapparent phase band migration,and with abundant large reefs.In terms of seismic reflection,it is characterized by mound-like and cluttered reflection,medium-weak amplitude and continuity,a small number of sheet-like to layered reflection in middle part with medium-weak amplitude and medium-high continuity.The thickness of sedimentary steep-sloped margin is larger and the slope angle is higher,its facies belt is wider than the fault-controlled one(averages 9.49 km),and it mainly developed vertical deposition,with reef hills developing.Seismic reflection features are as follows:sheet-like reflections in the upper-middle part with medium-strong amplitude and strong continuity;cluttered reflection in the lower part with medium-weak amplitudes and continuity.The thickness of the fault-controlled gentle slope margin is relatively thin,the slope of platform margin and slope belt is small,and the average width of the platform marginal facies belt is 14.02km,reef is basically undeveloped,sedimentary facies belt is broad and regresses toward the platform;the seismic reflection characteristics are with sheet-like reflection,strong-medium amplitude,moderately continuity,and with partially cluttered and mound-like reflections,medium-weak amplitude and continuity.The thickness of weak-rimmed gentle slope margin is thin,the slope of platform margin and slope belt is small with facies belt width averaging 14.91 km.Lattice rocks and the horizontal migration rules of the reefs are not obvious,which needs further exploration.In the seismic reflection appears sheet-like reflection,with medium-weak amplitude,moderately continuity.In the mid-lower part develope cluttered and mound-like reflection,with medium-weak amplitude,and continuity.The fault-controlled steep slope type platform margin mainly developed in the Tazhong 26-82 well area,and the sedimentary steep slope in the Zhonggu 2-3 well area,the fault-controlled gentle slope in the Tazhong 86-Shunxi area and the weak-rimmed gentle slope in the A-dong region.The geologic structures of different platforms are controlled by factors including faults,ancient water depths and the construction of reef and beach bodies,among which,faults are important factors affecting the distribution and migration of platform margins,ancient water depth is an important condition affecting the platform margin structure,and the construction of reef and beach bodies is the material basis affecting platform structures.In the planar distribution of the platform marginal facies,the platform margin of the entire Lianglitage Formation gradually narrowed from northwest to southeast.Liang-5 section is dominated by a large area of beach-to-tank sedimentation,with only a few areas developing sand-debris beaches,and the biological reefs are generally not developed.The facies belt of platform margin in Liang-4 section is narrow,and the sandstone beach deposits dominate the eastern and middle margins with the grit domes showing belt-like deposits.The western section is dominated by intertidal sea sediments,the muddy hills are scattered and the mud mound distributes outside the platform margins.The entire marginal zone of Liang-3 Formation is mainly dominated by sand-debris and sand-mud mounds.There is a small amount of developed reefs near the east,sand-dust-bearing beaches on the west and tank sea sediments in the inner belt.Liang 2 Formation is mainly dominated by sand-debris beach.The reefs and limestone mounds distribute over a large area in the east section,and the belt-like stucco facies distribute in the outer belt along the middle-western section.Some sections of the eastern section of Liang-1 section are denuded.The reefs in the east section distribute in the form of strips along the outer belt.The middle section is dominated by sand and shoal beach deposits and reefs are dotted.The western section is dominated by sand debris beach deposits,only in some wells developed stony mounds.On the whole,the marginal belt in Liang 5-Liang 1 gradually shrinks toward the outer margin of the No.1 slope break zone. |
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