The Characteristics and Significance of varve in Dakangpu layer of Lower Cretaceous Yixian Basin in Western Liaoning

Laminated structure developed widespread in lacustrine deposits of Lower Cretaceous Yixian Basin in Western Liaoning in Northeast China, and a number of well-preserved lamina developed in Dakangpu bed in the middle of the... [ view full abstract ]

Laminated structure developed widespread in lacustrine deposits of Lower Cretaceous Yixian Basin in Western Liaoning in Northeast China, and a number of well-preserved lamina developed in Dakangpu bed in the middle of the Yixian Formation. In Wangjiagou section, the lacustrine annual lamina was identified in Dakangpu bed, according to the lacustrine structure we can learn to the paleoclimate and paleoenvironment characteristics.
1.The Characteristics of varve
Same the Sihetun beds in Jin—yang basin, there are a lot of varve in the Dakangpu beds in yixian basin. The varve in Dakangpu bed is mainly made up of : (1)grey black mudstone and light yellow green bentonite; (2). grey black mudstone and grey white marl; (3). grey-greyblack shale, flaky after weathering.
2. Deposition rate
By the thickness of lamina couplet and statistics of the data, a high precision varve bed chronology was established, it is beneficial for learning the high resolution paleoclimatic. However, the lake deposition rate can be precisely calculated by measuring the lamina couplet thickness . The first kind lamina couplet thickness is 0.3-0.5mm, the third lamina couplet thickness is 0.05-0.1mm. The second kind of lamina couplet is 0.10-0.20mm measuring under microscope. The different thinckness of the three types of lamina couplet indicated that the deposition rate of lacustrine deposits is not same (respectively in 0.3-0.5mm/a, 0.1-0.2mm/a, 0.05-0.1mm/a) in different periods. Combined with three kinds of varve position in the vertical sequence, the factor that thickness decreases gradually of the lamina couplet in Dakangpu beds indicates that the deposition rate decreases gradually.
3. Paleoclimate
According to the sedimentary sequence, the thickness of varve bed is between 0.3mm to 0.5mm approximately in the initial stage of Dakangbao beds. It indicates that a mass of water poured into basin and climate was humid. In the middle stage of Dakangbao beds, the varve was dominated by endogenic varve deposition. The thickness of varve bed is between 0.1mm to 0.2mm, and the thickness of light-colored varve layer was bigger than initial stage one obviously. It indicates that the temperature has a increase tendency. In the late stage of Dakangbao beds, the fine grained sandstone deposition replaces the varve deposition. It indicates that a mass of water with strong power poured into basin. At the end of Dakangbao beds, the varve is debris varve, and the thickness of layer is between 0.05mm to 0.1mm, it indicates that the amount of water pouring to basin was decrease