川西前陆盆地上三叠统沉积特征

Sedimentary characteristics of the Upper Triassic in western Sichuan foreland basin

应用层序地层学方法将川西前陆盆地上三叠统划分为4个三级层序和12个体系域。在层序格架内,研究了不同时期的古地理背景、物源、沉积类型和沉积相展布,明确了上三叠统存在龙门山古陆、米仓山—大巴山古陆、康滇古陆、江南古陆4个物源,识别出上三叠统海湾、冲积扇、曲流河、辫状河、扇三角洲、辫状河三角洲、曲流河三角洲和湖泊8种沉积相类型。层序Ⅰ沉积时期,盆地处于海陆交互相向陆相转变的时期,整体具有填平补齐的沉积特征,发育海湾等沉积;层序Ⅱ沉积时期,川西前陆盆地成为统一的内陆湖盆,其湖侵期为须家河期最大的湖侵期,主要发育三角洲沉积和湖相沉积;层序Ⅲ沉积时期,米仓山—大巴山古陆提供充足的物源,在米仓山—大巴山两逆冲推覆带前缘广泛发育大型冲积扇、辫状河、辫状河三角洲沉积体系;层序Ⅳ沉积时期,主要受江南古陆的影响,发育三角洲沉积和湖泊沉积。整体上,川西前陆盆地沉积相带的展布格局受构造控制,具有平行构造带展布的特点。盆地西北部陡坡带发育粗粒的冲积扇、扇三角洲—湖底扇,西南部陡坡带发育辫状河三角洲、北部—东北部陡坡带发育辫状河三角洲,而在东南部缓坡带则发育远源的曲流河、曲流河三角洲。

The Upper Triassic in the western Sichuan foreland basin can be divided into 4 third order sequences and 12 system tracts. The palaeogeographical setting, provenance, sedimentary rock types and sedimentary facies distribution in different periods have been studied on the basis of the sequence stratigraphic framework. Four areas including Longmenshan oldland, Micangshan-Dabashan oldland, Kangdian oldland, Jiangnan oldland comprise the detrital provenance areas for the Upper Triassic in the western Sichuan foreland basin. Eight sedimentary facies are developed, i.e., estuarine, alluvial fan, meandering fluvial, braided fluvial, fan delta, braided fluvial delta, meandering fluvial delta, and lacustrine facies. During the deposition of sequence I, the basin was transferred from marine-continental alternating facies to continental facies with a filling and reconstruction character. Estuarine facies and tidal flat facies were developed in this period. The unified inland basin was formed during the deposition of sequence Ⅱ, and its transgression was the most extensive during the deposition of Xujiahe Formation. Deltaic and lacustrine facies were developed during this period. During the deposition of sequence Ⅲ, abundant sediments were provided by the Micangshan-Dabashan mountains. Alluvial fan, braided fluvial, and braided fluvial delta were well developed along the Micangshan-Dabashan mountains thrust frontal zones. During the deposition of sequence Ⅳ, influenced by the Jiangnan oldland, deltaic and lacustrine facies were developed. The distribution of sedimentary facies in the western Sichuan foreland basin is mainly controlled by tectonics, and the facies are distributed paralleling to the structural belt. The coarse-grained alluvial fan, fan delta and lake floor fan were developed along the northwestern steep slope belt and the braided fluvial delta was developed along the southwestern steep slope belt, the northern-northeastern steep slope belt, while the meandering fluvial and meandering fluvial delta were developed in the southeastern ramp slope belt.