Variation Of Depositional Environments And Its Biogical Responses Of Early Triassic (Helongshan Formation) In Chaohu Area, Anhui Province, China

Located in Anhui Province, South China, the Northern margin of the Yangtze Block, Chaohu area is one classic area for Triassic researches because there developed unabridged Lower Triassic. The Helongshan Formation in this area, corresponding to mid-upper Smithian, is about 17.19-30.10 meters thick, can be subdivided into 5 parts according to lithologic features. Part 1 is mid-thick mud limestones intercalated with thin-layered calcareous mudstones; part 2 is banding limestones interbeded with calcareous mudstones, in which microbial mats developed; part 3 is mud limestones and banding limestones interbeded with calcareous mudstones; part 4 is realy like part 3 in lithology, but pyrite is rich in this part; part 5 is dominated by calcareous mudstones, microbial mats and stromatolite lenticles can be found in the lower part and mud limestones gradually increased in the upper part.The microbialites, which were found in the Helongshan Formation, are important materials for reconstructing paleoenvironments, analysising the relationship between environment changes and biological response, discovering the timelimit, process and mechanism of the early Triassic biotic recovery. Dominated by fine-grained sediments, current structures such as wavy beddings, lenticular beddings, flaser beddings, bimodal cross beddings and gutter-filling structures, organic structures such as microbialites, microbially induced sedimentary structures(MISS) and trace fossils, and special minerals such as framboidal pyrites together with minor halite crystals, authigenic quartz are preserved in the Helongshan Formation. Carbonate microfacies indicate that the Helongshan Formation was deposited in open-restricted platform with low energy. Chaohu area experienced intertidal, lagoon, intertidal and shallow subtidal environments successively during the depositional period of the Helongshan Formation. Sedimentary characteristics and microfacies characteristics also indicate that there was an abnormal change at the part 4 of Helongshan Formation.In this paper, many microfossils such as cyanobacteria, green-algae, coccolith and sulfar bacteria have been indentified in microbialites of the Helongshan Formation. The distribution of macro and micro fossils indicates that abrupt biotic variation events happened at the early and late Helongshan Formation depositional stages. On a regional scale, the earlier event might be related to volcanic eruption and/or marine regression, ammonites and conodonts were mauled heavily, but bivalves were not affected significantly, and microbialites bloomed after the event. On the contrary, the later event was corresponding to global change, which had serious effects on bivalves, but minor effects on ammonoids and conodonts, accompanied by Microbialites development. The microbialite development model, which constructed on the basis of paleontologic characteristics and "excessive productivity hypothesis", suggests that the microbialites developed in the late depositional stage of the Helongshan Formation might promote the biotic recovery and can be considered as a sign of the biotic recovery.Isotope geochemistry and element geochemistry analyses manifest that the Helongshan Formation was deposited in restricated shallow ocean near land, normoxic and hypoxic conditions changed rhythmically during the early Helongshan depositional period; it was hypoxic to anoxic during the late Helongshan depositional period till to the end. Salinity of seawater was lower in the. early period and back to normal level in the late period of the Helongshan Formation. The excursion ofδ13C near the top of Helongshan Formation (Smithian\Spathian) shows that a rapid increase of the marine productivity and abundant biotic deposits. This deduction supports the microbialite development model strongly.