Biological processes for the formation of carbonate concretions in the Upper Triassic Wayao Formation of Guanling, Guizhou

The Guanling biota,which has important scientific significance,has been found at the bottom of the early Late Triassic Wayao Formation near Xinpu Township,Guanling County,Guizhou Province.There are abundant carbonate concretions in the strata in which the biota occurs.There are plant stems in the middle of the concretions.The contents of fossils such as crinoids,bivalves,ammonoids and conodonts in the concretions are notably higher than those in rocks around the concretions.Many fossils are directly attached to the plant stems.The formation of these concretions was affected by biological processes besides chemical processes,when they were being formed.Crinoids,ammonoids and bivalves dwelt on the stems and lived on pseudoplanktons.They were gradually deposited on the sea bottom with increasing biomass of the dwelling organisms and lime content ,and at last concretions were formed.