贵州纳雍地区含煤地层龙潭组中上段三角洲沉积体系

DELTAIC DEPOSITIONAL SYSTEMS IN THE MIDDLE AND UPPER MEMBERS OF LONGTAN FORMATION OF COAL-BEARING STRATA, NAYONG DISTRICT, GUIZHOU

贵州纳雍地区含煤地层龙潭组中上段形成于三角洲沉积体系,其中中段形成于潮汐影响的河控三角州沉积体系;上段形成于坝后河控三角洲沉积体系。每一种沉积体系的持续发展形成了一个沉积幕,分别包括4个成因单元。每个成因单元代表了该沉积体系在某一阶段内受海平面升降影响而沉积的一套完整的进积至后期废弃沉积。这些沉积由5种沉积相,即上三角洲平原;下三角洲平原;水下三角洲平原;三角洲间湾(或边缘)以及三角洲废弃相组合构成。两种不同类型的三角洲沉积体系聚煤作用有着根本的差别:受潮汐影响的河控三角洲沉积体系代表了龙潭组最活动的碎屑沉积部分,聚煤差,煤层普遍不可采;坝后河控三角洲沉积体系相对较稳定,聚煤好,煤层普遍可采。不同环境和相组合中聚煤作用也不同,本区以上三角洲平原和三角洲间湾(或边缘)环境聚煤最好。总之该三角洲沉积体系是我国海陆交替相含煤地层中沉积厚度大、聚煤最丰富的三角洲沉积体系之一。

Recent sedimentologic studies of the coal-bearing Longtan Formation(Upper Permian) in Nayong district of Guizhou reveal the facts that whole middle and upper members of the formation were deposited in deltaic depositional systems, with a thickness of approximately 280 meter, 30 coal seams and rich in coal reserves, which is a large deltaic depositional system to be found and studied in coal-bearing strata of China. The middle member of Longtan Formation was deposited in fluvial-dominated deltaic depositional system with tidal influence. The upper member of Longtan Formation was deposited in fluvial-dominated deltaic depositional system in back-barrier. Detailed facies analysis of two members show the existence of five facies associations such as upper delta plain, lower delta plain, subaqueous delta plain, interdeltaic bay (or deltaic margin), deltaic abandoned facies. Deltaic depositional system of upper or middle member is equal to a depo sitional episode (Galloway et. al., 1983) and corrsponds respectively to upper and lower large cycles. From configuration of the facies associations and evolutionary analysis in each depositional system, eight middle cycles which are attributed to allocyclicity and can make direct regional correlation of coal-bearing strata are found out. Each of them with same attributive part is well known as a genetic unit which represents a complete deposition from the progradationary term of delta system to later abandoned term in a period of relative change of sea level. Individual genetic unit which is attributed to autocyclicity generally contains one more facies associations sequences in vertical profile. In lateral, different facies associations appear facies change. It is reliable to correlate individual coal seam in a facies association range. Over stepping this range, it is usually unreliable. The coal-accumulating features in two types of deltaic depositional systems appear obviously distinction. The fluvial-dominated deltaic depositional system with tidal influence represent the most active debris depo sition of Longtan Formation, so its coal-bearing property is bad. Most of coal seams are unminable. But the fluvial-dominated depositional system in back-barrier is relative stable, with a good coal-bearing property and most of them are minable coal seams. The coal-accumulating features in different environments and facies asso-ciatioas of delta systems are marke'dly different. Upper delta plain and interdeltaic bay (or deltaic margin) are best sites for coal accumulation, usually with thick and laterally continuous seams. Especially, higher quality coals were usually formed in upper delta plain that is a most prospective site for high quality coals. Coals formed in lower delta plain are with moderate thickness, usually local minable, but laterally discontinuous, higher in sulfur and ash, commonly with numerous splits. Coals formed in tidal flat and tidal-dominated delta during the abandoned term of delta system are bad, generally unminable and with high sulfur and ash.