四川省乐威煤田晚三叠世层序古地理及聚煤特征

Late Triassic Sequential Paleogeography and Coal-Accumulation Characteristics in Lewei Coalfield,Sichuan Province

通过对四川乐威煤田晚三叠世含煤地层野外露头、钻孔岩心、测井、古生物等资料分析,剖析了该区含煤地层的沉积环境和层序地层特征,在此基础上恢复了各层序的岩相古地理格局并分析了聚煤特征。乐威煤田晚三叠世含煤地层主要发育河流、三角洲和湖泊相;共识别出5个典型的层序界面,其中包括下伏雷口坡组与小塘子组之间的侵蚀不整合面,以及上覆珍珠冲组与须家河组之间的地层颜色突变面,在含煤地层内部的层序界面主要为下切谷砂体冲刷面;将晚三叠世含煤地层划分为四个三级层序,大致与小塘子组,须家河组一、二段,须家河组三、四段,须家河组五、六段相对应;以三级层序为作图单元,恢复了各层序的岩相古地理格局,主要的古地理单元为河流、曲流河三角洲和滨湖,从层序Ⅰ到层序Ⅳ滨湖范围明显缩小,河流及三角洲范围扩大;根据古地理格局可推测晚三叠世研究区主要物源为四川盆地西南的康滇古陆及东南的江南古陆,西北部龙门山及北部米仓山-大巴山对研究区影响不大;各层序煤层发育都较好,但分布有差异,层序Ⅰ和层序Ⅲ煤层较厚,煤层主要发育于分流间湾沉积中,其次为河流冲积平原沼泽和滨湖沼泽,聚煤中心主要在煤田的中部及东部,西部聚煤不好。

Through the analyses of late Triassic coal-bearing strata field outcrop, drill core, well logging and paleontologic data in the Lewei coalfield, Sichuan, analyzed sedimentary environment and sequence stratigraphic characteristics of coal-beating strata in the area; on this basis have restored lithofacies-paleogeographic pattern of each sequence and analyzed coal-accumulation characteristics. The coal-bearing strata in the area have mainly developed fluvial, deltaic and lacustrian facies. 5 typical sequence boundaries have been identified, including erosional unconformity between underlying Leikoupo Formation and Xiaotangzi Formation, surface of strata color discontinuity between overlying Zhenzhuchong Formation and Xujiahe Formation, and sequence boundaries within coal-beating strata are mainly incised valley sand scouring surfaces. The late Triassic coal-bearing strata can be partitioned into 4 third order sequences, roughly corresponding to the Xiaotangzi Formation, first and second members, third and fourth members, fifth and sixth members of the Xujiahe Formation. Taking the third order sequences as the mapping unit, have restored the lithofacies- paleogeographical pattern of each sequence, main paleogeographical units have fluvial, meandering fiver delta and lakeshore facies, the range of lakeshore facies has been considerably narrowed from Sequence I through to Sequence IV, while fluvial and delta facies extended. Based on paleogeographical pattern can speculate the Kangdian old land to the southwest and Jiangnan old land to the southeast of the Siehuan Basin are the main provenances of the late Triassic study area, while the Longmenshan to the northwest and Micangshan-Dabashan to the north have less contribution to the study area. The coal seams in each sequence have been well developed, but their distribution is different; coal seams in Sequence I and Sequence III are rather thick, mainly developed in the interdistributory bays, and then in alluvial plain bogs and lakeshore bogs, coal-accumulation center is in the central and eastern part of the coalfield, while the westeru part is not in good condition.