Architecture of volcanic sequence and its structural control of Yingcheng Formation in Songliao Basin

Abstract

Yingcheng Formation is a set of volcanic strata composed of lava rocks, volcanic clastic rocks and sedimentary rocks, filled in some fault depressions in Songliao Basin, early Cretaceous. The study about litho-facies succession of Yingcheng Formation in the outcropped area of the southeast margin and in Xujiaweizi fault depression and its distribution based on analysis of seismic data, shows that the sequence of volcanic strata is quite different from the clastic sedimentary sequence. To study the architecture of volcanic sequence and its structural control of Yingcheng Formation in Songliao Basin, in this work, dividing of the volcanic sequence and dating of the sequence boundaries were finished firstly, then displacement and displacement rate of faults were calculated. The results show that, sample ages of top of the first member, the seconde member, and the third member are 127 Ma, 115 Ma, 110.7 Ma, respectively and sample age of the bottom of the third member is 114.7 Ma. The maximum displacement and displacement rate of the fault 1 are 3 km and 300 m/Ma, respectively, and those of the fault 2 are 3 km and 1000 m/Ma. Studies suggest that, the cooling unit of lava rock or pyroclastic rock is a basic genetic stratigraphic unit in volcanic sequence stratigraphy. Cooling units can construct a parasequence reflecting a volcanic eruption stage. A sequence was superimposed by some parasequences, responding to a volcanic active cycle. There are three types of volcanic sequences in Yingcheng Formation: type of explosion, type of effusion and type of mixed explosion-effusion. The surface of the volcanic sequence, an unconformity surface widely spread and traced in seismic profiles, is a base for analysis of volcanic sequence. The development of volcanic sequence was controlled by faulting, and the curves of fault displacement (rate) can reflect this control. The preservation of volcanic sequence was controlled by the type of volcanic structure and the regional subsidence, also different from that of the sedimentary. The type of volcanic structure of Xujiaweizi was a volcanic depression during the forming of Yingcheng Formation, and the breakdowns of volcanoes and structural subsidence were key factors in the volcanic sequences preservation.