Genesis of the Pyroclastic Rocks of the Lower Cretaceous Yingcheng Formation in the Jiutai area, Southeast Uplift of the Songliao Basin

There are four volcanic edifices in the study area, containing five facies and fourteen sub-facies typical rock types of the Classification Scheme of Volcanic Rocks in the Songliao Basin. Each volcanic facies or sub-facies is... [ view full abstract ]

There are four volcanic edifices in the study area, containing five facies and fourteen sub-facies typical rock types of the Classification Scheme of Volcanic Rocks in the Songliao Basin. Each volcanic facies or sub-facies is located in different parts of the volcanic edifice, with different characteristics of occurrence, following certain rules on planar distribution and vertical superposition. The main sequence of the acidic eruption cycles in this area consists of explosive stage, effusive stage and extrusive stage, among which the type of volcanic explosion belongs to Plinian, the type of volcanic effusion is similar to Strombolian, and the type of volcanic extrusion is Pelean-like. Pyroclastic rocks are widely distributed in the study area, and can be classified into welded pyroclastic rocks and common pyroclastic rocks. The former, generated from hot pyroclastic flow, have significant structures of welding. The typical lithologies are rhyolitic welded rocks composed of various granularities of pyroclasts from block to tuff, consolidated by cooling. The latter, generated from base surge, are well-layered, and some of them have various kinds of bedding types. The typical lithologies are rhyolitic (block-bearing) breccia-bearing tuff, rhyolitic detritus and crystal fragments tuff, consolidated by compaction. Granularity analysis results indicate that the main transport process of the base surge deposits is predominant by suspension, but also accompanied by saltation and traction. The diversifications of grain size distribution and depositional beddings in different layers reveal the changing dynamics of base surges.