四川盐源晚三叠世煤系沉积环境与层序地层

晚三叠世为中国南方重要的聚煤期，盐源地区晚三叠世含煤地层主要为博大组和冬瓜岭组。根据野外露头、探槽及岩心资料的沉积学研究，博大组沉积期由于北部长枪-琪木林岛链的障壁作用，主要发育滨岸潮坪沉积，以陆源碎屑岩夹泥质、砂质碳酸盐岩为主，物源区为东南部的康滇古陆，成煤环境为潮坪沼泽；冬瓜岭组沉积期由于大规模海退并伴随周围古陆快速抬升，盆地沉积以河流-三角洲相的碎屑岩为主，成煤环境以分流间湾沼泽为主。构造不整合面在盐源地区广泛存在，表现为博大组与舍木笼组之间、冬瓜岭组与博大组之间的平行不整合面；下切谷砂体底部河流侵蚀不整合面在冬瓜岭组广泛发育，三段、六段底部均见河床底部滞留沉积的粗砂、砾岩。本次研究主要根据区域性构造不整合面、河流侵蚀不整合面及河道砂体的发育特征等，建立了盐源地区晚三叠世层序地层格架，在盐源地区晚三叠世煤系中识别出5个层序界面，共划分为4个三级层序(层序I—IV)。其中层序I对应博大组；层序II对应冬瓜岭组一段和二段；层序III对应冬瓜岭组三段、四段和五段；层序IV对应冬瓜岭组六段、七段和八段。这些层序中，聚煤作用以层序II最强，其次为层序III；在层序内部，又以湖(海)侵体系域聚煤最好，高位体系域次之。

Sedimentary environments and sequence stratigraphy of Late Triassic coal-bearing strata in Yanyuan area,Sichuan Province

Late Triassic is one of the most important coal-accumulating periods in South China, and Boda and Donggualing Formations of Late Triassic are coal-bearing strata in Yanyuan area. Boda Formation was developed in the tidal flat environment associated with the barrier of Changqiang-Qimulin Islands, and the Formation mainly consists of clastic rocks interbedded with muddy and sandy limestone, with a provenance from Kangdian Mountains to the southeast. The coal-forming swamp was developed from the coastal tidal flat environments. Donggualing Formation was formed in the fluvial plain to lacustrine delta environments developed as a result of regression and rapid uplift of the surrounding mountains. The coal-forming swamp at this stage was developed from the delta plain inter-distributary bay and fluvial flood basin environments. Tectonic unconformities were widely distributed in Yanyuan area, including the parallel unconformities between Boda and underlying Shemulong Formations, and between Boda and Donggualing Formations. Erosional surfaces at the base of incised valley channel sand bodies were developed in Donggualing Formation, especially in Members 3 and 6. According to these key surfaces, totally 5 sequence boundaries were identified and 4 third-order sequences were subdivided in the Late Triassic coal-bearing strata. Sequence 1 corresponds to Boda Formation, Sequence 2 corresponds to Members 1 and 2 of Donggualing Formation, Sequence 3 corresponds to Members 3, 4 and 5 of Donggualing Formation, and Sequence 4 corresponds to Members 6, 7 and 8 of Donggualing Formation. Coal accumulation was favored in sequence II, followed by sequence III, while within these sequences, coals were preferentially accumulated in the transgressive systems tract, and then the highstand system tract.