广西合山晚二叠世碳酸盐岩型煤系层序地层分析

A Sequence Stratigraphic Interpretation on Late Permian Carbonate Coal Measures in the Heshan Coal Field, Southern China

广西合山煤田晚二叠世合山组是在浅水碳酸盐台地背景下形成的典型的碳酸盐岩型含煤岩系 ,其沉积环境有滨外陆棚、生物礁、开阔台地、潮坪和泥炭沼泽等。在包括合山组和大隆组在内的整个晚二叠世地层中 ,可以识别出5个层序界面 ,并可根据这些层序界面将区内上二叠统划分为 4个层序。层序 从合山组底面到合山组下段顶部的四 下 煤层底板硅质岩层之底面 ;层序 包括从四 下 煤底板到四 上 煤底板的一套地层 ;层序 为从四 上 煤层底板到二煤层之下铝土质泥岩底板的序列 ;层序 包括从合山组二煤层之下铝土质泥岩底板到大隆组之顶。其中层序 、层序 和层序 厚度在合山煤田范围内变化较大 ,说明受盆地基底沉降作用控制强烈 ,层序 则表现为多次的煤层—石灰岩旋回性 ,并且在合山煤田甚至桂中地区稳定分布 ,说明可能主要受全球海平面变化控制。与滨海平原靠陆一侧的冲积体系的陆源碎屑岩含煤岩系不同 ,陆表海碳酸盐岩型煤系三级层序中的最大海泛带底部以该层序中向上变薄至最薄的一层煤的底面为代表 ,如研究区三 中 煤层底面即为层序 中的最大海泛带底部 ,其下为三级层序中的海侵体系域 ,其上为高位体系域。三级层序中有包含四 上 、三 下 、三 中 、三 上 等煤层为界的 4个四级层序 。

The Late Permian Heshan Formation in the Heshan coal field is a typical carbonate coal measures formed in a shallow water carbonate platform setting. Up to seven coal seams have been preserved with numbers 2, 3A, 3B, 3C, 4A, 4B and 5, in descending order. Four third-order unconformity-bounded sequences have been identified. Sequence Ⅰ is from the bottom of the Heshan Fm to the base of the cherts beneath coal Seam 4B. Sequence Ⅱ is from the cherts beneath Seam 4B to the base of Seam 4A; Sequence Ⅲ is from base of Seam 4A to the base of the bauxitic mudstone beneath Seam 2; Sequence Ⅳ is from the base of the bauxitic mudstone beneath Seam 2 to the top of the Heshan Formation. Sequences Ⅰ, Ⅱ, and Ⅳ are strongly controlled by regional differential subsidence. Sequence Ⅲ shows cyclicity of the coal and carbonate rocks, with extensive distribution in the central Guangxi area, reflecting strong control by sea level changes. Within the Sequence Ⅲ, four fourth-order sequences are recognised, each consisting of a basal transgressive coal seam and overlying highstand tidal flat and open platform limestones. In the Heshan coalfield, fourth-order sequences and coals within the Sequence Ⅲ thin first and then thicken upwards, with the thinnest being the fourth-order sequence with coal Seam 3B. As a result, the thickest coal in the carbonate platform setting is formed at the transgressive surface and the thinnest coal is formed at the bottom of the maximum flooding zone.