The Ancient Tsunami Event And Sedimentary Environment Of The Mesoproterozoic Dahongyu Formation In The Xingcheng Area, Liaoning

The strata of Mesoproterozoic Changcheng System are well developed in the Xingcheng area, Liaoning Province, China. The successions are composed of the Changzhougou Formation (Fm.), the Chuanlinggou Fm., the Tuanshanzi Fm., and the Dahongyu Fm., and they are sedimentary contact over the basement of Archean’Suizhong Granites’. Recently, under the support of the China Geological Survey (1:250000Scale Regional Geological Mapping Project of Jinxi City, Liaoning Province), the features of the Changcheng System in study area are detailed revised. The rare ancient tsunamite are found in the Dahongyu Fm., which is probably the oldest tsunami event in the North China Craton. The identification of tsunamite, triggering mechanism and magnitude of tsunami event are studied systematically by analysis and comparison in sedimentology. The meaning of the tsunami event in the global tectonic setting and palaeogeography is discussed.The feature of the Changcheng System in the Xingcheng area is summarized and revised, and the section at Jiashan was here proposed as a typical section in this area. The feature of the Dahongyu Fm. is detailed studied, and it could be correlated with that in strata type of the Jixian County, Tianjin. However, it also has some special Characteristics, since the area is tectonically located in the eastern margin of the Yanshan Taphrogenic Trough. The whole of the formation could be subdivided into4members:the1st Member is composed of thick bedded quartz conglomerates, and pebble contained arkose conglomerates, and a suit of specially formed breccia (tsunamite) in some places; the2nd Member is composed of grayish-white thick bedded quartz sandstones and thick to huge thick bedded feldsparthic quartz sandstones, and interbedded with some grayish-white thin bedded fine-silt sandstones and tuff; the3rd Member is grayish-black silt shales and interbedded with grayish-white feldsparthic quartz sandstones; the4th Member is composed of lithic quartz sandstones and fine-silt sandstones. The Dahongyu Fm. unconformably overlays the Changzhougou Fm., the Chuanlinggou Fm. or the Tuanshanzi Fm., and even the Archean granites, it is explained as a crust movement’Xingcheng Movement’, and there is a debate on it. In current thesis, this angular unconformity relationship is re-observed and confirmed.The Dahongyu Fm. preserves the ancient tsunami event in the Xingcheng area. This paper detailed describes the special breccia from the bottom of the Dahongyu Fm. at Chapen’an, Yueliangshan, and Jiashan. These breccia are markedly different from normal sediments, and are analyzed in terms of sedimentary petrology, sedimentary texture and sedimentary sequence, and sedimentary environment. Based on the analyses of sedimentary facies, the special breccia is identified as tsunamite. They were deposited in abnormal high-energy sedimentary conditions, and belonged to parautochthonous and allochthonous deposits. After the comparison with the ancient and modern examples of tsunamite, and based on the lithofacies palaeogeography in the Xingcheng area, there are some features to identify the tsunamite:rip-up clasts; boulders exclusively appear in the deposits; poorly sorted gravels; fining-upward sequences; complex sediment sources of the underlying strata; erosional bases. These combined features also widespread apply to identification of other ancient tsunamite.The tectonic setting of the Xingcheng area is detailedly discussed, and it is close to the eastern edge of the ancient Yanshan-Fanhe Seismic Belt (1700-1200Ma). The Luanxian-Jianchang Fault lies at the southeastern edge of the Taihang-Yanshan Taphrogenic Trough, and occurred at the early stage of the Dahongyu Fm., it is characterized by abrupt and pulsing activities. The study area is about50km from this fault, and is affected by the fault. The tsunami was probably triggered by the earthquake, which resulted from the activities of the Luanxian-Jianchang Fault in the early Mesoproterozoic. Based on the analyses of lithofacies palaeogeography and sedimentary facies, the depositional process was discussed in the following:(1) Generation and propagation;(2) Inundation and deposition;(3) Backwash flow;(4) Normal background deposits. Acorrding to international magnitude of tsunami, the magnitude of the acient tsunami is discussed.The sedimentary environment of the later Tuanshanzi Stage-the later Dahongyu Stage is analysed in the paper. Stromatolites occur in the top of the Tuanshanzi Fm., they belong to the Changcheng System type stromatolite zone(Gruneria-Eucapsiphord) in Yanshan and Taihang Ranges. According to the lithostratigraphic characteristics, and the relationship between the vertical distribution rule of stromatolites’ morphology and the paleoenvironment, the sedimentary environment history of the Tuanshanzi Stage in the Xingcheng area should be from regression (shallows to intertidal zone and supratidal zone) to transgression (intertidal zone and supratidal zone to intertidal zone and subtidal zone). The sedimentary facies was a tidal flat environment in the later Tuanshanzi Stage, and the depth of water cannot exceed50m. The sedimentary facies of the Dahongyu Stage are littoral-shallow marine. Therefore, the studied area was in an open coastal environment, and the terrain was flat in the Mesoproterozoic. The direction of tsunami waves should be from SW to NE, it is the similar direction as the later marine transgression.Based on the analyses of the regional and global tectonic backgrounds, the Yanshan Taphrogenic Trough, the North China Craton and the Columbia Supercontinent were in the same global extensional tectonic setting. Having considered the simultaneity and the consistent geodynamic pattern of regional and global tectonic backgrounds, some events, such as tsunamis and volcanic eruptions should be recognized as effects of the breakup of the Columbia Supercontinent in the North China Craton.