西准噶尔乌尔禾地区哈拉阿拉特组火山碎屑复理石的发现及地质意义

The Discovery and Geological Significance of Pyroclastic Flysch of Hala'alate Formation in Urho Area of West Junggar

火山碎屑复理石是组成以火山碎屑为主且具有鲍马层序的海相特殊沉积组合,是火山碎屑物经地震诱发和推动形成浊流后再堆积的产物。在新疆西准噶尔乌尔禾地区上石炭统哈拉阿拉特组新发现一套厚约638m的火山碎屑复理石建造,因火山喷发作用类型和成岩作用类型的多样性以及碎屑物经火山机构叠积的复杂性,其组成中常常近源、远源成分混生,火山熔岩与火山碎屑岩类混生,火山凝灰岩等细碎屑与火山角砾混生;其粒级悬殊,磨圆度较差,分选程度不一。较之经典陆源碎屑复理石,火山碎屑复理石总体厚度较小,分布有限,成分以火山碎屑为主,“三屑”齐全;胶结物多凝灰质,连续的多个序列间忽厚忽薄、渐变性差且多缺失E段;层理、层面构造不发育;砾石多继承和保留了原火山角砾岩外形,因而是其独有特征和有别于陆缘碎屑复理石的主要鉴别标志。这些重要地质特征为甄别火山碎屑复理石丰富了岩相学标志,也为盆山地层划分和储层精准对比提供了新的制约。

The pyroclastic flysch,a special marine sedimentary assemblage with Bouma sequence,is compositionally dominated by pyroclasts,and is the product of re-accumulation of pyroclastic turbidity flows triggered and driven by earthquake.A new set of pyroclastic flysch formation with thickness of about 638m was discovered in the upper Carboniferous Hala'alate Formation in Urho area of West Junggar(Xinjiang),which is characterized by mixed constitutions between near-source and far-source pyroclastics,volcanic lavas and pyroclastic rocks,fine-grained pyroclastic rocks(such as volcanic tuff)and volcanic breccias with a great difference in granularity,poor roundness and varying degrees of sorting due to the divers types of volcanisms and diageneses as well as the complexity of pyroclastics affected by superpositions of different volcanic apparatus.Compared with the typical terrigenous clastic flysch,the pyroclastic flysch generally possesses smaller thickness and limited distributions,and is dominated by pyroclasts containing crystal and vitric fragments and detritus in composition,and the cements are mostly tuffaceous.In addition,the thickness varies greatly and irregularly between some successive sedimentary sequences,showing poor gradual changes,and the upper-most components of Bouma sequence(E segment)are generally missing in the field.Moreover,the stratification and stratigraphic structures are not well developed within the pyroclastic flysch,and gravels mostly inherit and retain the shapes of the primitive volcanic breccias.The above are the peculiarities and the main distinguishing marks of the pyroclastic flysch that differ significantly from the continental clastic flysch.These important geological features not only enrich the identification characters of pyroclastic flysch,but also provide basin analysis in the region,and also provide new constraints on the stratigraphic division and accurate reservoir comparison between mountain and basin in the West Junggar.