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

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.