The Terrace Development And Environmental Change In The Jinhua-Quzhou Basin

The river terrace is one of the most important materials for environmental change information in the record basin.Jinhua and quzhou basin located in the upper reaches of the Qiantang River,Jinhua river and Qujiang segment of cross-strait flows through the basin can be seen the multi-level terraces.the formation is closely related to the Qiantang River basin since the Quaternary environmental change.Based on six typical sections from all levels terraces located in in Jinhua and Quzhou Basin.this paper studied types and distribution of all levels of terraces as well as sediments of the physical and chemical characteristics by means of sedimentology, environmental geochemistry in order to extract information of environmental changes in Jinhua and Quzhou Basin with combing with regional strati graphic correlation.The main findings are as follows:(1) The type characteristics of the river terraces in Jinhua and Quzhou Basin.Three terrace levels are well developed in Jinhua-Quzhou basin, the terrace level T1,T2and T3.From the old to the new mark as T3, T2, T1,They are mainly base terraces and alluvial terraces, among which T3is mostly base terraces with purplish sandstone as lower bedrock formed in the cretaceous period and poorly sorted gravel layer as the upper part. Due to intense corrosion of the wind,its top is usually Vermicular and the upper part is nothing but powdery clay and clay. And unconformable contact with the bedrock There is vermiculated red earth and homogeneous red earth,above the gravel layer and the bottom of vermiculated red earth is pebbly silty clay in part of the sections.There is sediment texture with windy rich in aeolian grain size and homogeneous in the upper vermiculated red earth and homogeneous red earth.T2and T1terraces is most of the accumulation of terraces,the gravel layer is in the terrace underground for the degree of weathering The gravel layer above or eolian sediments or floodplain sediments.Jinhua river-Qujiang segment of cross-strait terrace retains differently.(2) The distribution characteristics of the river terraces in Jinhua and Quzhou Basin.Qujiang river terraces in Jinhua section are well preserved that three terrace levels are widely spread, as well as Jinhua River’s south side terraces;the distribution of the terraces in Jinhua section and the north side of Jinhua river is less than that of the south side of Jinhua river, only the first terrace and the third terrace have been preserved.Tl terraces are widely distributed in Jinhua and Qujiang modern valley on both sides of,and constitute a relatively wide floodplain plain peaceful farm plains;T2terrace can be finded in the south bank of Jinhua river and qujiang river intermittent scattered, and now remain in the vicinity of the Tangxi;T3terraces distribute and retent both north and south sides in the Qujiang Jinhua section and Jinhua river. Distribution into pieces is in the south bank of Jiangtang Tangxi Yafan, the North area of Luodian Xianqiao TongYa was scattered,however, the distribution height of the same terrace is variable and depends on the region in the basin. the distribution of the terraces present a basic pattern of two high intermediate low.(3) The sediment characteristics of river terraces in Jinhua and Quzhou Basin.Silt and clay are dominant components in homogeneous red earth and vermiculated red earth layers in T3terraces,and the content of dust particle(10μm-50μm) is high;The frequency curves in a higher content of the entrained component and secondary components,mobile components is lack.Poor sorting,skewness is nearly symmetric and positively skewed distribution,kurtosis is medium and the broad peak state;Two layers experienced a strong chemical weathering from the constant element composition and weathering indicators.the mean of SiO2, Al2O3,Fe2O3of T3homogeneous red earth were67.22%～71.98%,12.6%-13.47%,5.2%-5.85%;the mean of SiO2, Al2O3,Fe2O3of T3vermiculated red earth were62.45%-68.93%,13.23%-15.11%,5.43%-8.2%;Most of the element content of less than1%.the characteristics of high CIA,low ba and low SiO2/A1203,Si02/R203.The average of CIA, ba value, SiO2/Al2O3and SiO2/R2O3of homogeneous red soil in T3were respectively87.56%～88.81%,0.19～0.23,8.86～9.76,6.9～7.73;The average of CIA, ba value, SiO2/Al2O3and SiO2/R2O3of vermiculated red soil in T3were respectively87.71%～90.5%,0.19～0.22,7.02～7.77,5.22～6.3; They indicate that vermiculated red earth layer in the ground uplift outcrop has experienced strong weathering process.Silt(4-63μm) and sand(>63μm) are dominant components, and clay(<4μm)content is small in floodplain layers from vermiculated red soil belong to T3terrace;There are multiple peak grain-size frequency curves and three-stage structure cumulative frequency curves, this showing the two floodplain layers are more than the upper coarse-grained material in the sediment; Poor sorting,skewness is nearly symmetric distribution, kurtosis state wide peak;Chemical elements are SiO2,Al2O3,Fe2O3in the following vermiculated red earth,the mean of SiO2, Al2O3,Fe2O3of the following vermiculated red earth T3were62.45%-66.65%,15.34%-16.08%,4.8%-5.28%; the soluble component such as K2O, MgO, Na2O and CaO were very low.Most of the element content of less than1%.The average of CIA, ba value, SiO2/Al2O3and SiO2/R2O3of the following vermiculated red earth in T3were87.57%-87.48%,0.21-0.22,7.04-7.12,5.83-5.91;Magnetic susceptibility of textured red soil values was significantly lower than the upper homogeneous red soil in T3terrace. The mean of textured red soil in each section ranged from10.56×10-8m3/kg to19.49×10-8m3/kg, and the mean of homogeneous red soil ranged from27.4×10-8m3/kg to42.83×10-8m3/kg.T2terrace sediment’s dominant particle group grain size was silt, and also was rich in a certain proportion of "dust particles group"(10-50μm). Frequency curve was in a single peak; curve hostage components and secondary components were in higher levels. This showed that the yellow-brown soil had aeolian features; poor sorting, skewness closing to symmetric and positively skewed distribution, medium kurtosis characteristics indicated that the brownish yellow soil was mixed with near-source coarse. The main elements were SiO2, Al2O3and Fe2O3, and the content of SiO2was the most, followed by Al2O3, Fe2O3. The average of SiO2, Al2O3, Fe2O3were69.98%,11.77%,3.31%. While the soluble component such as K2O, MgO, Na2O and CaO were very low. The chemical index of alteration (CIA value) basically increased gradually from the surface to the downward; ba value, SiO2/Al2O3and gradually decreased from the surface. And the average of CIA, ba value, SiO2/Al2O3and SiO2/R2O3were76.62%,0.39,10.89,9.2.This showed it had aeolian features. Magnetic susceptibility values ranged from35.16×10-8m3/kg to51.83×10-8m3/kg, and the average was45.29×10-8m3/kg.T1terrace floodplain layer’s dominant fractions was grain size, its range was from70.16%to90.67%, the average content was84.32%, and the silt and clay content were less. Compared to the T2,T3terrace, sediments were significantly coarse. Sorting coefficient was poor, skewness is very positively skewed, and kurtosis belonged to a very narrow scope. The main elements were SiO2, Al2O3, K2O and Fe2O3, and the content of SiO2was the most, followed by Al2O3, K2O, Fe2O3. The average of SiO2, Al2O3, K2O, Fe2O3were69.11%,12.19%,3.84%,2.34%. While the soluble component such as MgO and CaO were very low. The chemical index of alteration (CIA value) basically increased gradually from the surface to the downward; ba value, SiO2/Al2O3and SiO2/R2O3gradually decreased from the surface. Magnetic susceptibility values ranged from34.78×10-8m3/kg to42.27×10-8m3/kg, and the average was37.58×10-8m3/kg.(4) Terraces within the basin and basin tectonic movement closely linked. A good correlation between the early three river terraces and basin north side of the mountains of gold in North China and development of three-cave in the basin. Qujiang of Jinhua section of the north shore of the stone ridge head Yongchang first-line growth there are two structural surface, and its distribution the height Jinqu Basin T2, T3terrace distribution of height is more consistent, show that the Qujiang the north shore of the two structural surface formation age and T2, T3terrace formation within the basin is basically the same.(5) In combination with regional stratigraphic correlation,terrace level T3. T2. T1corresponding to Zhijiang Zu,Lianhua Zu and Yingjiangqiao Zu, constituted Q2-Q3-Q4time sequence.The chemical and physical characteristics of sediments of three terrace levels recorded the envirenment changing characteristics and trend from Pleistocene in Jinhua-Quzhou Basin.There was the most intense weathering in terrace level T3where was under a warm and humid environment.The weathering of gravel and claybank layer from terrace level T2was weaken gradually.There was the lest intense weathering in terrace level T3which indicated that the warm and humid environment had been weaken.The chemical and physical characteristics changes of each terrace level indicated the fluctuation among the different periods in Jinhua-Quzhou Basin.The three terrace levels in Jinhua-Quzhou Basin formed under tectonic movement,displaying a good correlation with karst caves in Beishan Mountain.