云南保山地块香山组和丁家寨组沉积序列与碳同位素研究

Depositional Sequence and Carbon Isotope Chemostratigraphy of the Lower Carboniferous Xiangshan and Lower Permian Dingjiazhai Formations in Baoshan Block,Yunnan: Paleogeographic Implications

通过对保山地块北部西邑地区下石炭统香山组和下二叠统丁家寨组详细的野外剖面实测与钻孔编录,并结合层序地层学、沉积相以及碳氧同位素分析结果认为,下石炭统香山组自下向上可分为三个岩性段,分别对应台前斜坡下部、台前斜坡上部、碳酸盐岩台地三种沉积环境。下二叠统丁家寨组为滨岸冰水沉积,下部为冰碛含砾钙质杂砂岩,发育落石构造。下石炭统香山组δ^(13)CPDB值自下而上逐渐升高,环境可能较稳定,有机质埋藏增加。综合前人古地磁、生物区系对比、大地构造学等研究成果认为早石炭世保山地块较稳定,仍属冈瓦纳大陆北缘。丁家寨组冰碛含砾钙质杂砂岩结束后δ^(13)CPDB值出现强烈负偏,与研究区冰期结束、早二叠世初次温度上升、大量玄武岩喷发等时限高度一致,共同说明了保山地块在早二叠世开始从冈瓦纳大陆北缘裂解出来,并开始向北漂移。该项研究的开展为探讨Submasu地块晚古生代的古海洋、古地理和气候演化具有重要意义,为全球同时期碳氧同位素数据库提供了新资料。

Better understanding of the late Paleozoic sedimentary sequences in Baoshan block were obtained by detailed field measurement,borehole logging combined with sequence stratigraphic and facies studies as well as carbon and oxygen isotopic analysis of the lower Carboniferous Xiangshan Formation and lower Permian Dingjiazhai Formation at the Xiyi area,northern Baoshan Block.Three lithologic units were recognized in the Lower Carboniferous Xiangshan Formation,which are in ascending order represent lower slope,upper slope and carbonate platform depositional settings,respectively.The Lower Permian Dingjiazhai Formation was interpreted to be glacial-marine deposits according to distinctive pebbly calcareous moraine diamictite and local development of dropstones.Theδ13 C PDB profile demonstrates a gradual increasing trend up section in the Lower Carboniferous Xiangshan Formation,indicating a relatively stable sedimentary environment possible due to a steady increase of organic carbon burial.Combined with previous paleomagnetic results,biotic correlation and tectonic studies,it is presumed that the Baoshan Block was in a stable setting as a part of the northern Gondwana.However,theδ13 C PDB values drop dramatically on the cessation of the pebbly moraine carbonate which represents the ending of glacial period,and correspond well with the first temperature rise event as well as massive basalt eruption in the early Permian.The above collectively indicate that the Baoshan Block was separated from the northeast Gondwana and started to drift northwards in the early Permian.The present study not only sheds light on the paleoceanic,paleogeographic and paleoclimatic significance of the Paleozoic Submasu Block,but also provides new carbon isotope chemostratigraphy for the contemporary sedimentary sequences.