| Deep water deposits developed in Ordovician Pingliang Formation in Pingliang of Gansu Province. According to the field measurement of Yindongguanzhuang Section with thickness of 146 meters, it can be divided into 30 small layers, and 3 segments on larger scale. There are mainly calcarenite, micrite, calcisiltite, and shale, with calcirudite and mudstone interbeds, and many types of sedimentary structure such as horizontal bedding, parallel bedding, cross bedding, and wavy bedding developed in the formation. On the basis of comprehensive analysis including field measure, thin section, and geochemical data, it is concluded that four types of deep water sedimentation developed in Pingliang Formation at Yindongguanzhuang Section, i.e., deep water autochthonous deposit, contour current deposit, turbidite deposit, and debris flow deposit. Besides the autochthonous deposit, the turbidite deposit and debris flow deposit mainly developed in lower part, in upper part the mixed sedimentation of turbidite and contour current developed, and the contour current deposit increased on scale. According to the vertical variation of some geochemical indicators including V, Sr, 87Sr/86Sr, Sr/Ba, Sr/Cu, Rb/Sr, B/Ga, U/Th, V/Cr, Ni/Co, V/(V+Ni), Ceanom, and Ce/La, it is shown that the sedimentary evolution in Plingliang Formation is sea level relatively increasing, climate from humid to drought, salinity gradually increasing, and reducing environment gradually strengthening. In addition, many indicators such as V, 87Sr/86Sr, and Rb/Sr can reflect different types of deep water sediments based on typical samples mainly in medium-up part of the section. It is shown that deeper water, drier climate, larger salinity, and stronger reducing environment were favorable for the contour current deposits; shallower water were favorable for gravity flow deposits; deeper water, warmer and wetter climate, and smaller salinity were favorable for autochthonous deposits. |
