The Lithostratigraphic Characters Of The Mesoproterozoic Dahongyu Formation In Xingcheng Area, Liaoning Province

This paper is based on the author's works when join the project of"Research on Field Study Base Construction"and"1:250000 Scale Geological Mapping Project of Jinzhou City"carried out by Jilin University. The paper chose the Mesoproterozoic Dahongyu Formation at Xingcheng area as the object, and studied the lithostratigraphic divisions, correlations, and paleogeography.The Dahongyu Formation was established by Gao Zhen-xi in 1934, the type section is at the Jixian County, Tianjing City. The lower boundary of the unit is at the base of so said"small white quartz stone"which is over the Tuanshanzi Formation (some researchers suggested that the boundary should at the base of the upper layer of"big white quartz stone"), The lowest grayish white quartz sandstone bed of the Gaoyuzhuang Formation parallel disconformable contact with the underlying Dahongyu Formation. At the Jixian Section, the lower member of Dahongyu Formation mainly consists of quartz sandstones and feldspartic quartz sandstone, with K enrich volcanics, purple tuff and tuffenous and dolomitic quartz sandstones, the sediments with various kinds of beddings, and with ripple marks and mud crack on the surface. The Upper member of Dahongyu Formation consists of flint banding dolomites, with a few volcanics, contains stromatolites.In the geologic map and report (1:200000 scale, Jinxi-Xingcheng Breadth) made by the Team of Geologic Survey of Liaoning Province (1983), they clarified two types of rock assembles from the Dahongyu Formation, one is terrigenous clastic rocks in domain with some carboniferous rocks; another type is also terrigenous clastic rocks in domain but with clay stones. After observing and measuring 4 sections at Xingcheng area, the author suggested that the Dahongyu Formation at study area is composed of marine terrigenous clastic rocks. According to Its distribution, association, and characters, the unit at current area could be subdivided into three members, from the bottom up: Member 1 is composed composite breccia and conglomerate of quartz sandstone component; Member 2 is mainly composed of quartz sandstone, interbeded with a few silt sandstone and shale; Member 3 is composed of silt sandstone and shale, interbeded with a few quartz sandstone.Based on stratigraphic measuring, geological mapping, and stratigraphic correlation, we found that the division of Changcheng Series at Tiemashan area in the former Geologic Map of Xingcheng (scale 1:200000, 1983) is not right, the so said"Changzhougou Formation"at the are are actually the Member 1 and 2 of Dahongyu Formation, and the"Chuanglinggou Formation"should belong to Member 3 of Dahongyu Formation.Having correlated the Dahongyu Formation with other places of Xingcheng area on the rock associations and characters, this paper concluded that at Tiemashan and Jiashan area the Dahongyu formation developed more completely; at other places the beds conglomerates are broadly developed, but the finer beds of sandstones are not developed.Since Chang Shao-quan (1976) named the"Xingcheng Movement", there is still no consensus view towards it. It is widely accepted that there is a unconformity boundary between the base of Dahongyu Formation and the under strata and ancient basement at Xingcheng area and some other places, but most of the researchers don't think it belongs to a crust up movement. At Xingcheng area, several sections show that the Dahongyu Formation onlapped over different part of Tuanshanzi Formation, Chuanlinggou Formation, Changzhougou Formation or even directly over the Achaean granites. The breccias from the lowest part of Dahongyu Formation is quite complex, but could be observed that the gravels are mainly form the underlying units, this account for the existence of"Xingcheng Movement"do occurred after the deposition of Tuanshanzi Formation.A further correlation with other sections within Yanshan Alacogen area especially the Jixian section showed that the Dahongyu Formation at Xingcheng area is characted of only contains terrigenous clastic rocks but with no carboniferous rocks, and partly interbeded with sedimentary tuff. And this paper concluded that those views of making the"small white quartz stone"should be reasonable, the K enrich volcanics should belong to Dahonghu Formation other than Tuanshanzi Formation. The Mesoproterozoic volcanism mainly occurred after the"Xingcheng Movement"that is charactered of the unconformity boundary over the Turanshanzi Formation.This paper analyzed and summarized the Dahongyu Formation period paleogeography combined with the paleogeographic characters of the period at the whole Yanshan Alacogen. The Xingcheng area located at the eastern edge of Yanshan Alacogen, the north is Jianping ancient continent, the south is the Shanhaiguan ancient continent, the east is the Yiwulvshan heave, so the base shows that the south is steep and the north is gently, the sediments is basically the same with that at Jixian section. Owning to the affect of"Xingcheng Movement"that occurred at the end of Tuanshanzi Formation's depositing period, the sea water regressed completely at Xingcheng area. The crust uplifted, the area became the land, those already sedimented Tuanshanzi Formation, Chuanlinggou Formation, Changzhougou Formation and Achaean basement were under erosion. During the period of Dahongyu Formation started deposit, the sedimentary base declined, and transgression area enlarged, even east to Fanhe area of Liaoning Province. Because of the uprising of continent, at the areas of basins'edge, the Dahongyu Formation sedimented unconformity over the underlying units (Changzhougou Formation, Chuanlinggou Formation, and Turanshanzi Formation), and some times even on the Achaean granites (ancient basement). There is only one bed of volcanic tuff had been found from the Dahongyu Formation in Xingcheng area, it implied that the area may at the distant place from the center of volcanic erupted, so the volcanism is not overt.