GEOLOGICAL SIGNIFICANCE OF THE FISH-BEARING CONCRETIONS NEAR THE SMITHIAN-SPATHIAN BOUNDARY,OLENEKIAN,EARLY TRIASSIC

Fossil-bearing carbonate concretions are found in Early Triassic marine strata of many regions,including Madagascar in southern Africa,Spitsbergen in Europe and British Columbia in North America.In Chaohu of Anhui Province,Longtan near Nanjing of Jiangsu Province,and Jurong of Jiangsu Province,there are many carbonate concretions in uppermost Smithian mudstones,and some of them are fish-bearing and commonly known as"fish nodule".The"fish nodule layers"are below the Smithian-Spathian Boundary,and they are preserved in dark gray thin-bedded mudstones.Lithological study shows that the mudstones were deposited in anoxic environments.The carbonate carbon isotope values(δ13C)of"fish nodule layers"are positive and show an increasing stratigraphic trend.The "fish nodule layers"is regarded as marker of the Smithian-Spathian boundary.Based on carbon and oxygen isotope data,palynological analysis,global distribution of ammonoids,and biodiversity data,we infer that a significant climatic and environmental change occurred at the end of the Smithian,which caused a biological crisis.The development of fossil-bearing concretions coincides with this environmental change and the Smithian-Spathian crisis.Thus,the"fish nodule layers",the stratigraphic pattern of carbon isotopes,and changes in the latitude distribution of ammonoids are regarded as a manifestation of the Smithian-Spathian global oceanic event.