新疆尼勒克地区下侏罗统三工河组辫状河三角洲沉积

Braided-river Delta of the Lower Jurassic Sangonghe Formation in the Nileke Region,Xinjiang

新疆伊宁盆地早侏罗世为一内陆坳陷盆地 ,沉积了一套冲积—湖泊环境下的碎屑岩沉积体 ,其中三工河期盆地周缘广泛发育辫状河进积到湖泊中而形成的辫状河三角洲。三角洲三层结构清楚 ,由下而上出现前辫状河三角洲亚相、辫状河三角洲前缘亚相及辫状河三角洲平原亚相 ,呈明显的进积层序。辫状河三角洲特征显著 ,明显有别于其它类型的三角洲 ,可见到九种特征各异的微相。辫状河三角洲的主体是由含砾砂岩及中、粗粒砂岩组成的辫状河道砂坝及水下分流砂坝 ,单一砂坝呈下粗上细的透镜状 ,透镜体最大厚度在 0 .5～ 3m不等。垂向上许多砂坝透镜体相互叠置而成巨厚砂体。三角洲中交错层理丰富 ,其中尤以水道砂坝侧向迁移加积而形成的侧积交错层异常发育为标志。

Braided river delta that braided river systems prograde into an inland lake is very abundant in the Lower Jurassic Sangonghe Formation in the Yining Basin,Xinjiang. Their characters are distinct and significantly different from the other types of deltas. The braided river deltas are deposited by river tractive current sediments, controlled by torrential flood in general, and are seasonal sediments. There three sedimentary subfacies from bottom to top: probraided river delta subfacies, braided river delta front subfacies and braided river delta plain subfacies, in the braided river deltas. Nine microfacies are recognised. The deposits of braided river deltas are mainly composed of pebbled sandstone and medium coarse sandstone. The bars of the braided channel and subaqueous distributary channel are the main bodies of deltas, and they take the form of lenticular bodies. A single lenticular channel sand body has a thickness of 0.5m～3m. Many, finger upward lenticular channel sand bodies pile vertically up each other. Cross stratifications are abundant in the braided river deltas, but lateral accretion cross beddings resulting from lateral accretion of channel sand bodies are the most principal sedimentary structures and one of the most distinct character of braided river deltas.