Carbonate Rock Microfacies And Ichnocoenosis In The Ordovician Majiagou Formation, Yuntai World Geopark

Based on the field observation including lithologies, sedimentary structures, diagenesis, composition and distribution characteristics of body fossils and ichnofossils developed in the Ordovician Majiagou Formation carbonate rocks of the Yuntai World Geopark, through the experimental analysis and well-done material, the following conclusions can be drawn: Eight kinds of rock microfacies have been identified in study area, including dolomicrite microfacies, gypsum dissolved breccia dolostone microfacies, dolomitic micritic limestone microfacies, gravel crumbs limy dolostone microfacies, micritic limestone microfacies, (with) biological clastic micritic limestone microfacies, sparry pellet limestone microfacies, sparry sand crumbs/gravel crumbs limestone microfacies, which controlled by identified five microfacies environments; The sedimentary evolution of the Majiagou Formation in the study area experienced seven sedimentary cycles, each of the cycle begins with the deposit of intertidal zone (or supratidal zone), change into the deposit of subtidal open platform while the sea water gradually becomes deeper, and then turn into the deposit of intertidal zone when the sea water continuously becomes shallower, seven times of sea level change process in the sedimentary period of the Majiaogou Formation can be recognized.9 ichnogenera and 14 ichnospecies of trace fossils in the Majiagou Formation of the Study area can be identified, including Thalassinoides, Chondrites, Phycodes, Helminthopsis, Scolicia, Gordia, Planolites, Palaeophycus, and Rhizocorallium, which can be divided into domichnia, Fodinichnia, Pascichnia and agrichnia in the light of ecological type.According to the composition and distribution characteristics of the ichnofossils and the carbonate rock microfaices features of surrounding rocks, three ichnocoenosis can be divided into: (1) Thalassinoides -Rhizocorallium ichnocoenosis, remains in low dynamic carbonate platform shallow water environment near the side of the intertidal zone; (2) Helminthopsis - Planolites ichnocoenosis, this community remains nearby the normal weather wave base surface while affected by storm deposit; (3) Chondrites - Helminthopsis ichnocoenosis, this one developed in the environment of lean oxygen, carbonate platform under the storm wave base surface, which represented the highest sea level.