A Comparative Study On The Early Carboniferous Reef Of Cat Ba Island,Vietnam And Langping Area,Guangxi,China

Organic reef is a very important research topic in geological sciences,which was built of sessile organisms,essentially built in places with carbonate deposition.In the geological history,reef was an extremely complex ecosystem with a significant development and evolution history.In this thesis,a lot of geological field work was conducted using sedimentology,theories and methods of paleontology,stratigraphy,paleoecology and the ancient lithofacies,and more research was carried out on the reef in Cat Ba Island and Guangxi Langping area.Detailed research was focused on the reef stratigraphic features,Early Carboniferous reef types,and reef-building process and the developmental characteristics in two areas;dividing the area reef communities,carrying out a comparative study on palaeoecology,reef communities,facies stratum and palaeoenvironment in two study places.The reefs is located in two different basins.The Cat Ba Island reef belongs to the basin in northeastern Vietnam,and the Guangxi Langping reef belongs to Dianqiangui basin.Although the two are in different basins which in the Early Carboniferous were carbonate platform,they have similar background in the evolution and paleogeography.So most of the Cat Ba Island area and Guangxi Langping area in Early Carboniferous,various benthic organisms were prosperous,with strong biological rock-forming effect.The study areas are covered with massive carbonate rocks,where had been a large number of various types of biological beach which produced different types of reef.Most of the coral reef in Cat Ba Island in Early Carboniferous are patch reef,with small scale about 6 meters high,about 12 meters lateral extension.The main reef-building organisms were rugose corals Donophyllum sp.and Thysanophyllum sp.,with a large number of solitary corals,foraminifera,crinoids,brachiopods and gastropods and so on as the reef-inserted organisms.Langping area in Guangxi,in lower Carboniferous output relatively large-scale reef and the coral reef in Xiadong village is a representative one,about 50m high,and 260m long.The main reef-building organisms were Diphyphyllum,Lithostrotion and Thysanophyllum.Adhering organisms included solitary corals,foraminifera,crinoids,brachiopods,gastropods,algae and some microbes and suspected biological source.The two types of reef species show different characteristics,community characteristics and other aspects.In this thesis,coral reef communities in both study areas were meticulously divided.Two consecutive biological communities in Cat Ba Island coral reef were recognized,representing different development stages of the reef,which was Donophyllum community and Thysanophyllum community;the main reef builders in Guangxi Langping area reef were plexiform groups Diphyphyllum and Siphonodendron and reef development was continuous.Furthermore,three consecutive biomes were identified,which represented different development stages,namely Diphyphyllum community,Siphonodendron-Lithostrotion community and Diphyphyllum-Syringopora community.Apparently,the two reef communities and their ancient ecological characteristics in Cat Ba Island and Langping area were significantly different.In this thesis,comparative study on reef systems in the two areas was carried out,which included sedimentary environment analysis,reef facies depositional process analysis,microfacies analysis and establishment of the sedimentary facies model.Vietnam Cat Ba Island coral reef consists of packstone,grain limestone and coral barrier grid,representing open platform environment,and three sedimentary microfacies were identified:?open platform;? coral reefs phase;? platform shoal facies.Six microfacies phase type were identified in Guangxi Langping,representing six sedimentary microfacies:?deep water open platform;?platform protected phase;? shallow water open platform;? reefs phase;?inner carbonate platform slope facies;?inner carbonate platform shallow slope facies.The research in Cat Ba Island and Guangxi Langping will contribute to the futher research on the Carboniferous reef.