| The studied area (Hanshan and Chaohu district of Anhui Province) locates in the northeastern part of the Yangtze plate. The Lower Carboniferous Series in this area is widely distributed and rich in marine animal fossils. It is divided into Jinling Formation, Gaolishan Formation and Hezhou Formation from bottom to up, which is one of the most important studying areas in the lower Yangtze area.This study focused on the Gaolishan Formation in Lower Carboniferous Series which is exposed in Xiaoci Mountain profile, Hanshan and Qinlin Mountain profile, Chaohu. The Formation has a thickness between 9.5-16.4m. According to lithology, it can be divided into three sections from bottom to up:the first section consists of variegated mudstone interbedded with thin bedded yellow-brown argillaceous sandstone, at the bottom limonites and a small amount of debris fossil plants are visible; the second section is made up of combination of interbeded thin bedded black mudstone, gray-green thin bedded mudstone and grey bioclastic limestone(sometimes, argillaceous sandstone). Gray-green mudstone at the bottom contains lots of marl calcirudite (clumps), some of which consist of coral and brachiopod fossils; The third section is alternating beds of yellow thin bedded mudstone and thick bedded argillaceous sandstone. There are horizontal bedding, a small amount of biodetritus in mudstone, worm trail structure and brachiopods in argillaceous sandstone. The Gaolishan Formation is both parallel disconformable with overlying Hezhou Formation and underlying Jinling Formation.Corals, brachiopods, bivalves, fusulinid, trace fossils and a small amount of fragment of plant fossils are founded in the studied Gaolishan Formation. Among them, a large number of rugose coral fossils are well-preserved. According to their assemblage, the studied Gaolishan Formation can be divided into two fossil zones:1. Arachnolasma intermedium -Yuanophyllum combination zone; 2. Lonsdaleia combination zone.Through systemic geochemical analysis of rare earth elements of mud rock from Xiaoci Mountain profile and Qilin Mountain profile of Gaolishan Formation in Hanshan and Chaohu Area, the results show that the total content of rare earth elements (REE) of the mud rock from Gaolishan Formation is high. LREE/HREE values, (La/Yb)N, (Ce/Yb) N, (La/Sm) N, (Gd/Yb) N value and REE distribution patterns indicate that there are relative enrichment of LREE and relative loss of HREE in the studied area. value reflects the water medium belongs to reducing condition at that time. The slope of LREE in the distribution curve is large and HREE is relatively small. Eu shows a clear negative trough, which suggests a significant negative Eu anomaly. Ce is normal or slight negative anomaly. (La/Yb) n value that represents the degree of differentiation of REE is regarded as a symbol of the sediment deposition rate. In term of Gaolishan Formation profile from bottom to top, (La/Yb)n value generally shows a gradual upward trend, indicating that deposition rate in the upper part of the studied Formation was generally lower than that in the lower part. According to the characteristics of rare earth elements and their ratios as well as La/Yb-REE diagram combined with paleogeographic research, the sediments provenance of the studied Formation originated from Zhangbaling paleocontinent. The parent rock mainly came from the upper crust composited of a mixture of ancient granite and sedimentary rocks.Through synthetical analysis of the characteristics of the Palaeobiota, rare earth elements and its significances from the studied Gaolishan Formation, it is considered that the Gaolishan Formation was depositied in the shallow muddy reductive water environment with a fluctuations of sea level frequently and rich argillaceous material supply. So the deporsit environment of Gaolishan Formation was not conducive to the living and development of Palaeobiota and the emergence of a large number of coral and brachiopod fossils in the Formation were the results of stong storm action. |
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