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Doushantuo Formation
The Doushantuo Formation () is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran geological period and the more familiar fauna of the Cambrian explosion of lifeforms whose descendents are recognizable. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma (million years) ago at its base to about 551 Ma at its top, predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon peninsula of Newfoundland, and recording conditions a good forty to fifty million years before the Cambrian explosion.
==Sedimentology==
The whole sequence sits on an unconformity with the underlying Liantuo Formation, which is free of fossils, an unconformity usually being interpreted as a period of erosion. On that unconformity lie tillites of the Nantuo Formation - cemented glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Marinoan / Varangian glaciation (the third and last of a series of very extensive glaciations during a period called the Cryogenian -- ('Snowball Earth'). This latest Cryogenian glacial level is tentatively dated ''ca'' 654 (660 ± 5) — 635 Ma (million years ago).
The Doushantuo formation itself has three layers representing aquatic sediments that formed as sea levels rose with the melting of worldwide glaciation. Biomarkers indicate highly saline conditions, such as might be found in a lagoon, low oxygen levels, and very little sediment that had been washed off land surfaces.
The richest finds (the ''Lagerstätte'' itself) lie at the bottom of the middle stratum, with a date about 570 Ma, thus from some time after the great Gaskiers glaciation of (± 1 - 582.1 ± 0.4 Ma ).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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