贺兰山-桌子山区太原组和山西组三角洲沉积体系的沉积构成和聚煤作用

贺兰山-桌子山区太原组和山西组可划分为四个成因地层单元,总体是一套受波浪和潮汐改造的河流三角洲体系。其相构成主要包括水道充填(冲积分流水道、潮河混合水道、潮道、决口小型水道)、进积砂坝(指状河口砂坝、受波浪改造的河口砂坝、受潮汐改造的河口砂脊、边缘席状砂坝、决口充填)、细粒越岸泛滥和间水道细粒潮坪的砂泥互层、间湾和前三角洲泥岩、滨浅海碳酸盐岩以及泥炭沼泽沉积等。从晚石炭世到早二叠世,三角洲体系的沉积构成样式发生了明显变化,反映出早期受波浪作用较强、中期潮汐作用变得明显、晚期河流作用迅速增强的总体演化。同时指出,富煤部位主要分布于次级坳陷内,而且富煤带常发育于冲积三角洲平原和过渡带(太原组下部)或潮河混成三角洲平原沉积组合中(太原组上部和山西组底部)。

DELTAIC SEDIMENTARY ARCHITECTURE AND ACCUMULATION OF TAIYUAN AND SHANXI FORMATIONS IN HELAN-ZHUOZI RANGE,NORTHWEST CHINA

Taiyuan and Shanxi Formations,which can be divided into four stratigraphicunits,have been interpreted as fluvial deltaic systems modified by wave and tid-al energy.The systems are mainly composed of:(1)fining-upward channelfills including fluvial distributary channels,tide-influenced distributary channels,crevasse channels and tidal channels;(2)coarsening upward progradational bars,comprising finger-like river mouth bars,wave-and tide-modified delta front sandridges,marginal or distal bars and crevasse delta fills;(3)overflood and tidalflat interbedded fines;(4)interdistributary bay and prodelta mudstones and(5)peatswamp deposits.The depositional style of the delta system changed significantlyfrom Late Carboniferous to Early Permian.The delta system was mainly modifiedby wave energy during early stage(Lower Taiyuan Formation),and later tidalcurrent reworking became relatively strong with regional transgression(Upper Tai-yuan Formation),and finally(Shanxi Formation)the fluvial process increasedrapidly to bc predominant.It is suggested that the evolution of the depositionalstyle is attributed to the change in sea level,basin morphology and sediment input.The study also shows that the coal-rich zones are coincided with the subdepressionand associated with the fluvial-dominated delta plain and transitional zone(LowerTaiyuan Formation)or tide-fluvial dominated delta plain(Upper Taiyuan Forma-tion) deposits.