QUANTITATIVE COMPARISON OF THE FOSSIL ASSEMBLAGES BETWEEN THE EVENT AND BACKGROUND MUDSTONES FROM THE EARLY CAMBRIAN CHENGJIANG LAGERST(A)TTE, YUNNAN
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Abstract:
Qualitative and quantitative variations in preservation (completeness, degree of decay) and fossil composition between event and background layers demonstrate that: (1) for event deposits, fossils are unevenly distributed in different layers, and the number of fossils shows no relation to the thickness of each layer, and the fossil compositions suggest conspecific and polytaxic concentration in some layers. But for background layers, the number of fossils related to the thickness of the layer. Usually the thicker background layer is the more fossil individuals it contains. (2) Comparison of fossil compositions between event layers and background layers indicate dramatic differences. Event layers show more complexity than background layers in composition of fossils and the structure of paleocommunity. Fossil assemblages of event layers may reflect variation of palaeocommunity from different horizons. (3) Decay processes of most organisms in event layers took place after they were buried, or the organisms were buried when even they were still alive. But the organisms of background layers had suffered long decay processes before they were finally buried. As well the fossil assemblages were significantly affected by time-averaging. (4) The fossil assemblages of event layers were autochthonous or parautochthonous, and show no bioturbation after the burial. The presence of abundant articulated shells and sponge fossils in the background layer indicates that they deposited in a stagnant and anoxic bottom water environment with rare scavengers and limited bioturbation.
