Zoophycos Characteristics In Respondance With The Variations Of The Sedimentary Environments Of The Early Permian Taiyuan Formation In Western North China

The Lower Permian Taiyuan Formation in western North China Plate is a set of the coal measure and marine deposits in littoral—neritic,lagoonal,and tidal flat environments on the carbonate rock platform of epicontinental sea.Plenty of trace fossils characterized by Zoophycos are well preserved in limestones.The present work has mainly studied the trace fossils in the 4 chosen outcrop sections of the Lower Permian Taiyuan Formation at Jiaozuo,Yuzhou in Henan province,and Lingchuan,Taiyuan in Shanxi province,highlighted in the analyses of Zoophycos occurences,distributions,sizes,and the sedimentary environments of Zoophycos host rocks,color variations of Zoophycos infillings in the limestone layers of every individual section,and primarily explored the rules of Zoophycos as the regional correlation markers of sedimentary environments,and the evolution laws of transgressions and regressions,in combination with the reported analyses in the related ichnofabrics and geochemical constituents.It has been concluded that:(1)densly distributed small Zoophycos(spreite width=1~3mm)and Zoophycos with black infillings demonstrate obvious relevance with deeper and oxygen-deficient(exaerobic)environments(distal shallow sea);(2)the trace fossil Zoophycos may act as palaeo-bathometric indicators,espcially the small ones with 1~3mm in spreite widths and black infillings can work as the correlation markers of the rock layers deposited in deeper waters(lower shallow sea).The regional correlations using such markers suggest that the Taiyuan Formation underwent 2 larger-scale transgressions in its deposition and 2 major ingression surfaces occurred,which can be treated as regional isochronous surfaces in the study area.The first major transgression appearred in the middle L3 limestone bed at Xishan of Taiyuan city,the middle to upper L1 limestone bed at Fucheng of Lingchuan county,the middle L4 limestone bed at Liuzhuang of Jiaozuo city,and the middle L2 limestone bed at Longhuashan of Yuzhou city.The second major transgression occurred in the middle L5 limestone bed at Xishan of Taiyuan city,the middle to upper L7 limestone bed at Fucheng of Lingchuan county,the middle L8 limestone bed at Liuzhuang of Jiaozuo city,and the upper part of L6 limestone bed at Longhuashan of Yuzhou city;(3)in accordance with the regional temporal and spatial correlation of the sedimentary environments,the change tendency of ingression and degreession in this area is suggested,i.e.,the first transgression was firstly Jiaozuo and then Yuzhou,following Lingchuan,finally Taiyuan,which indicated that the palaeogeographical pattern of the North China Plate has transformed from the uprising south and subsiding north status in the Late Carboniferous to the situation with higher northwest and lower southeast in the Permian,and the transgression direction was from SE to NW.Such seawater ingression order and the palaeogeographical pattern remained the same in the second transgression.The order of regression turned out to be firstly Taiyuan,and then Lingchuan,then Yuzhou,and finally Jiaozuo,which demonstrated that the altitudes at Jiaozuo were lower than both Shanxi and western Henan.In general,the Lower Permian Taiyuan Formation in the study area consists of two time transgression and regression cycles.Each cycle begins with bay and/or lagoon environments,intercalated with tidal flats and sandy barrier islands,and evolved into proximal—distal—proximal shallow seas,intervened by tidal flats and/or swamps,and finally turned into bay-lagoon and tidal flat-swamp environments.