Biostratigraphy of the Ordovician to Silurian Renheqiao Formation in the Baoshan Region, Southern Yunnan, China
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摘要:
滇西保山地区广泛发育奥陶系至志留系仁和桥组黑色岩系。为了建立其笔石地层时空对比框架,以云南保山市施甸县半坡剖面和茶地剖面为研究对象,开展了生物地层学和岩石学研究。研究表明,研究区仁和桥组总厚度约71.6 m,底部发育一层厚约25 cm的古风化壳,与下伏蒲缥组呈平行不整合接触,证明研究区晚奥陶世曾暴露于地表。基于高分辨率笔石化石调查,识别出了上奥陶统赫南特阶至志留系兰多维列统特列奇阶共9个笔石带,自下而上依次为
Metabolograptus persculptus 带(仁和桥组第1笔石带,简称R1)、Akidograptus ascensus 带(R2)、Parakidograptus acuminatus 带(R3)、Cystograptus vesiculosus 带(R4)、Coronograptus cyphus 带(R5)、Demirastrites triangulatus 带(R6),Lituigraptus convolutus 带(R7),Stimulograptus sedgwickii 带(R8)和Spirograptus guerichi 带(R9)。与扬子地区龙马溪组笔石生物地层对比发现,保山地区仁和桥组9个笔石带能够与龙马溪组LM1~LM9笔石带相当。此研究成果为保山地区仁和桥组页岩气调查评价建立了良好笔石生物地层学基础。Abstract: The Ordovician to Silurian Renheqiao Formation, characterized by a sequence of well-preserved black shales, is extensively developed in the Baoshan Block, located in the southwestern part of Yunnan Province, China.Objective The primary goal of this study is to establish a graptolite biostratigraphic framework for the Renheqiao Formation. To achieve this, the research focuses on the Banpo and Chadi sections in Shidian County, Baoshan City, Yunnan, which are critical outcrops for biostratigraphic analysis in the area.
Methods A combination of high-resolution biostratigraphic and lithologic investigations was conducted to precisely define the stratigraphy and identify key biozones within the formation.
Results The Renheqiao Formation in the study area is approximately 71.6 meters thick, with a basal bed of about 25 cm of paleo-weathering crust. It unconformably overlies the Pupiao Formation, indicating that the study area was subaerially exposed during the Late Ordovician. Based on careful fieldwork and high-resolution surveys, a total of nine graptolite zones were identified, spanning from the upper Hirnantian in the Ordovician to the Silurian Llandovery Telychian stage. These zones are identified in ascending order as follows: the Metabolograptus persculptus Zone (R1), Akidograptus ascensus Zone (R2), Parakidograptus acuminatus Zone (R3), Cystograptus vesiculosus Zone (R4), Coronograptus cyphus Zone (R5), Demirastrites triangulatus Zone (R6), Lituigraptus convolutus Zone (R7), Stimulograptus sedgwickii Zone (R8), and Spirograptus guerichi Zone (R9).
Conclusion Comparisons with the Longmaxi Formation (LM1–LM9) in the Yangtze region reveal that the nine graptolite zones of the Renheqiao Formation in the Baoshan Block correlate well with those of the Longmaxi Formation. This correlation not only strengthens the regional stratigraphic framework but also provides a solid basis for future shale gas exploration and assessment in the Baoshan Block.
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图 5 茶地剖面部分笔石照片
a. Metalograptus ojsuensis Koren’ and Mikhaylova in Apollonov et al. 1980,样品点CD-3; b. Metabolograptus persculptus Elles and Wood 1907,样品点CD-3; c. Akidograptus ascensus Davies 1929,样品点CD-4; d. Parakidograptus acuminatus Nicholson 1867,样品点CD-4; e. Cystograptus vesiculosus Nicholson 1868,样品点CD-4; f. Cystograptus ancestralis Štorch 1972,样品点CD-4; g. Demirastrites triangulatus Harkness 1851,样品点CD-7; h. Rastrites guizhouensis Mu 2002,样品点CD-10; i. Coronograptus cyphus Lapworth 1876, 样品点CD-10
Figure 5. Partial graptolites photos form the Chadi Section
图 6 半坡剖面部分笔石照片
a. Euclimacograptus Riva 1989, 样品点BP-3; b.Normalograptus sp., 样品点BP-6; c. Glyptograptus sp., 样品点BP-7-0; d.Demirastrites magnificus Pribyl and Munch 1942, 样品点BP-0 ; e. Rastrites peregrinus Barrande 1850, 样品点BP-9-1; f, n, o. Stimulograptus sedgwickii Portlock 1843, 样品点BP-9-1; BP-11; BP-12; g. Rastrites distans Lapworth 1876; h. Spirograptus guerichi Loydell et al. 1993, 样品点BP-9-2; i. Monograptus yichangensis Ni 1987, 样品点BP-0-7; j. Demirastrites triangulatus Harkness 1851, 样品点BP-2-1; k. Spirograptus guerichi Loydell et al. 1993, 样品点BP-12; l. Rastrites distans abbreviatus Lapworth 1876, 样品点BP-2-1; m. Oktavites gracilis Wang 1978, 样品点BP-6-1; p. Petalolithus sp., 样品点BP-7-1; q. Pribylograptus argutus Lapworth 1876, 样品点BP-7-3; r. Pernerograptus Pribyl 1941, 样品点BP-9-2; s. Demirastrites triangulatus, 样品点BP-0-0; t. Spirograptus guerichi Loydell et al. 1993, 样品点BP-9-2; u. Lituigraptus convolutus Hisinger 1837, 样品点BP-7-3; v. Pernerograptus qijiangensis Ye 1978, 样品点BP-8-1; w. Monograptus gemmatus Barrande 1850, 样品点BP-9; x. Pristiograptus sp., 样品点BP-1; y. Pristiograptus sichuanensis Ye 1978, 样品点BP-0-4; z. Monograptus xizangensis Mu and Ni 1975, 样品点BP-6
Figure 6. Partial Graptolites photos form the Banpo Section
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