重庆地区晚二叠世层序-古地理及聚煤特征

摘要

The sequence stratigraphy and palaeogeography of the Late Permian Longtan and Changxing formations in Chongqing area of southwestern China have been studied based on the data of outcrops and borehole sections. A total of two third-order sequences are subdivided in the Late Permian; sequence I is from the bottom of Longtan Formation to the bottom of C16 coal seam in Longtan Formation, representing a transgressive to regressive cycle; sequence II is from the bottom of C16 coal seam in Longtan Formation to the top of Changxing Formation , representing a second regional transgressive to regressive cycle. The palaeogeographic maps of two sequences have been reconstructed based on the contour maps of a variety of palaeogeographic parameters including the total thickness of strata, the thickness of sandstone, the thickness of limestones, the thickness ratio of siliciclastic rocks to carbonates and siliceous rocks, and the thickness ratio of sandstones to mudstones, as well as the thickness of the total coal seams. Both of the two sequences reflect a vertical and lateral pattern of palaeogeography, which shows that, from the bottom to the top and from the southwest to the northeast, there is a variation in facies units from the tidal flat-delta combination, through the bay-lagoon-tidal flat combination, to the shallow marine combination. In the two sequences, depositional environments are all deeper in the northeast, and shallower in the southwest, with the transgressive pulse of advancing from northeast to southwest. The sediment provenances were mainly from the Kangdian oldland to the west. The favorable coal-forming facies units are mainly the bay-tidal flat facies, followed by residual plain facies, and in the two sequences these facies units controlled coal distribution and they moved towards southwest with a continuous sea-level rise. The thick coal zone in sequence I is distributed in the area bordered by the Liangping-Fuling-Wansheng line to the east, and by the Tongliang-Jiangjin-Xishui line to the west, and the thick coal zone in sequence II is distributed in the southwestern area restricted by Guang'an-Changshou-Nanchuan line to the east with the coal-accumulation center located in the Hechuan-Chongqing-Qijiang zone.%采用EXXON公司“Vail”学派的观点,将晚二叠世划分为2个三级层序.层序Ⅰ发育低位体系域、海侵体系域及高位体系域,比较完整.层序Ⅱ只发育海侵体系域及高位体系域.通过绘制各种单因素等值线图,并与单井剖面相和连井剖面相分析相结合,从点到线再到面,展开古地理分析.2个层序均反映出海侵自东向西的脉动推进、东深西浅的格局,从而产生了潮坪三角洲-海湾(泻湖)-潮坪-浅海沉积模式在垂向上和平面上的展布特征,其中海湾-潮坪及碳酸盐台地构成了研究区基本骨架.物源区主要来自西部康滇古陆.总体上晚二叠世研究区遭遇了一个海水自东北向西南不断侵进的过程,因此成煤环境随着海平面的上升而不断向西南迁移.煤层主要发育在海湾—潮坪环境里,其次为低地残积平原.