Study on the Paleo-sedimentary Environment by the Occurrences of Concentrated Molluskan Fossils in Liu-Chung-Chi Formation, Southern Part from Pa-Chang-Chi, South-West Area of Taiwan

• Main Authors: Chung, Wen-Shang, 鍾文憲
• Other Authors: Chung Kuang-Chi
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/70976352438711606792

碩士 === 國立成功大學 === 地球科學系 === 84 === ABSTRACT This study intended to infer the reasons of the concentrated fossil bed to be formed by the occurrences of concentrated Molluska fossilsin Liu-Chung-Chi Formation and based on the inferences of these causesto reconstruct the paleo-sedimentary environment. In the field work of Liu-Chung-Chi Formation, 11 sections and 50 outcrops with fossil have been completed. In order to have precise basis for describing the fossil concentrated bed in the field, this work makes some measurement of the fossil from outcrops extensively and defines the range og high, middle and low fragmental degree whichcan indicate separately the size of fossils or fossil fragments showed in outcrops in the range below 0.5cm, 0.5cm to 1cm and above 1cm. The density is the ratio in the unit stretched line. The ratio of above 0.2 means high density, that of 0.2 to 0.1 means middle density and that of below 0.1 means low density. The results of fragmental degreeand fossil density may use for the reference of fossil occurrence.(Johnson,1960) There were 76 genus including 115 species having been examined fromfossil samples. The paleoecological analysis in this work was aimed onthe fossil assemblages. By the result of the similarity ratio analysis, there are 24 fossil ecological areas. The differences of organism habits between the south part and the north part may indicate that the sedimentary environment were different. By the analysis of livingenvironments, 14 assemlages were defined and they belong to the sandy bottom of 50 meters depth. The trophic structure analysis (Scott,1978)shows multi-environments of Liu-Chung-Chi Formation and could be predicted the general framework of ecological environment. According to these analysis and combined with the occurrences offossil distributed in the field (Kidwell,1985 ; Norris,1986 et al) andthe possible lithological signlificance on the organism (Purdy,1964 ; Scott,1978 et al), this work can conclude eight reasons of the formation of concentrated fossil bed. According to the evolution of the sedimentary environment of thesesections accomplished by the paleo-environment reconstruction from eachoutcrop and the relations among each section, this work can find fourcycles of transgression and regression. The shallowest environment should be around the shoreface and the deepest one should be to thebottom of middle shelf. This result could be correlated with the universal paleo-sea-level evolution which can correlate with the secondregression of this study. The environmental evolution of each section in the Liu-Chung- ChiFormation was not in uniform conditions. The south part Tzeng-Wen-Chisection have special organism assemblage. The result of analysis showedthat the section should belong to a special bay sedimentary environmentand it did not reflect the conditions of sections of north part. According to these data, this study can infer that the frequent changein the north part of Liu-Chung-Chi Formation should locate in the foreland basin of tectonic structure and that condition may due to somecomplex environment factors. However the sedimentary environment of thesouth part would be major of bay sedimentary thick-beded muddy rock under the balance of basin deepening and sedimentation.Key words : Liu-Chung-Chi Formation, Molluska, Fossil Occurrences.