Sedimentary Facies And Ichnocoenose From The Lower Permian Taiyuan Formation, Western Henan Province

On the basis of description and observation of outcrops and cores, and the detailed analysis on petrology, sedimentology and ichnology,the Early Permian Taiyuan Formation of Western Henan can be divided into five sedimentary facies in this paper: (1)distal shallow marine facies, (2)proximal shallow marine facies, (3)bay and lagoon facies, (4)tidal flat and peat swamp facies, and (5)barrier island (sandy shoal and bar)facies. The distal shallow marine facies is dominated by dark muddy biolastic limestone, biolastic micrite and marl and is formed in deep carbonate platform environment under storm wave base. The proximal shallow marine facies includes five different types of carbonate storm deposits and represents high energy carbonate platform environment under fair weather wave base. The bay and lagoon facies consists of barrier island, lagoon, bay, mixed tidal flat, peat flat and muddy flat deposits and is preserved in low energy restricted sedimentary environment at high reducibility condition. The tidal flat and peat swamp facies comprises tidal channel, tidal creek, sandflat, mixed flat,mudflat and peat flat deposits and occurs in periodically exposed shallow tidal environment. High maturity medium and fine quartzose sandstone is dominant in the barrier island (sandy shoal and bar)facies and is deposited in high energy coastal environment.The trace fossils in the Early Permian Taiyuan Formation of Western Henan Province are mainly from the Upper Limestome Member and the Lower Limestome Member, only a little in the Middle Clastic Member. They display a higher ichnoabundance and ichnodiversity and include 10 ichnogenera and 11 ichnospecies such as Zoophycos brianeus,Zoophycos villae, Teichichnus rectus,Nereites isp., Chondrites isp., Rhizocorallium isp.,Thalassinoides suevicus,Ophiomorpha nodosa,Gordia marina,Taenidium satanassi, Planolites isp. and so on. Three ichnocoenose can be recognized from the Taiyuan Formation according to the composition and distribution of trace fossils and sedimentology of the host rock. Zoophycos—Nereites ichnocoenosis is formed in distal shallow marine environment below storm wave base and indicates low oxygen level in the interstitial waters within the sediments at the site of trace emplacement. It only occurs in L5 of the Lower limestone Member and represents the sedimentary record when marine transgression reached its maximum within depositional period of the Taiyuan Formation. Zoophycos—Taenidium ichnocoenosis is preserved in proximal shallow marine environment above storm wave base and is commonly made in high energy storm deposits. Ophiomorpha—Thalassinoides ichnocoenosis is formed in the marlite or calcareous siltstone and thin-bedded fine sandstone in tidal flat and lagoon sedimentary environments. The last two ichnocoenose are widely distributed in Lower and Upper Limestone Members, which indicating the dominating depositional framework of alternating occurrence between high energy shallow marine storm deposits and low energy lagoon and tidal flat deposits above storm wave base in the Taiyuan Formation of Western Henan Province.