| Yanshan area is the most developed areas of Proterozoic strata in North China,which is renown with its continuous strata,stable sedimentation and little late deformation.For the above factors,this section is one of the typical representative section in our country,even in all the world.Changzhougou Formation,Chuanlinggou Formation,Tuanshanzi Formation and Dahongyu Formation compose Changcheng Group,which is a first order long-period transgression rock series,with a total thickness up to 2610 meters.Tianjin Jixian section,is mainly composed of terrigenous clastic rock,with some carbonate rocks and small amount of mixed sedimentary rocks.The long time span of Precambrian resulting to difficult to preserve fossils,together with the low isotopic data accuracy,give rise to huge trouble and challenge to study the stratigraphic division,correlation and paleogeographic evolution.Based on sequence stratigraphy theory,the Changcheng Group can divide into 14 third order sequences,which can further combine to 4 second order sequences.This paper studies the sedimentary trend of Changcheng Group and can provide basic factual material for future study.The bottom of Changzhougou Formation develops a set of nearly 400 meters thick non-mature pebbly sandstone,which is interpreted as fluvial deposition;however,from the study of Jixian section,it can be found that the meter-scale circle and its superimposed pattern have the characters of tidal flat sedimentation.Together the above evidence with the sedimentary environment,it can be inferred that the late Neoproterozoic sandstone should interpreted as transgression tidal flat sedimentation.The silty sandstone rock wall,developed in Jixian section,is the typical structure of tidal flat,the formation mechanism has confused geologists since long time.Based on field observation,this paper studies the silty sandstone rock wall and MISS,like wrinkle structure,multidirection,and all the formation progress of these structures are concerned with microbial mat. |
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