241 / 2019-06-29 20:53:59

Grain-size characteristics of the coastal deposits of Daihai Lake, northern China

grain-size analysis, grain-size parameter, transport mechanism, the Daihai Lake, modern deposits

"The present is the key to the past" is one of the most important concepts within geology. Through understanding the present, can understand the past and lay the foundation for predicting the future. Compared with other natural records (such as polar ice, tree rings, corals, loess, etc.), lacustrine deposits have a wide range of distribution, cover a large time span, have good continuity, are of high resolution, and are a source of a large amount of information thus being effective indicators for the restoration of the global environment. On the other hand, lacustrine deposits can provide oil and gas reservoirs, especially in China, continental lake basins are one of the most important oil and gas reservoirs. Thus, research on modern lacustrine deposits can provide guidance for oil and gas exploration and development.
The research methods of lacustrine deposits include geological methods (petrology and paleogeography), geophysical methods (especially seismology) and geochemical methods. Grain-size analysis for the texture characteristics of lacustrine deposits is one of the paleogeographic research methods which is simple, rapid, and economical and is not affected by biological effects. Lacustrine deposits often adopt predictable trends showing a ring-like distribution from the shore to the center of the lake. The size of these deposits continuously reduces, and the rhythm of conglomerate, sandstone, siltstone, mudstone appears, which means the sediments gradually become finer. This is due to variation in hydrodynamic conditions which gradually weakened from the shore to the center of the lake. This makes it possible to predict the depocentre of the lacustrine deposits according to the sediments grain size distributions. Generally, the increase of the sediments grain size reflects the decline of the lake level, the shrinkage of the shoreline to the lake center, and a dry climate; the smaller the grain size of the sediment reflects the rise of the lake level, the expansion of the shoreline to the surrounding area, and a humid climate. Further, it can be inferred that the coarse sediments indicate a dry climate at a low water level, and the fine sediments indicate a humid climate at a high water level.
The Daihai Lake which located in Liangcheng County, Inner Mongolia Autonomous Region, China, provides excellent exposure to modern lacustrine deposits. In this study, a total of 34 field samples were systematically collected and were analyzed for grain-size distribution and a possible mechanism for their transport was discussed. The samples were sorted by standard deviation σ1, and the results conclude that 10 samples are well sorted, these include 1-2, 1-3, 1-4, 1-7, 1-10, 1-12, 2-10, 4- 1, 4-2, 4-3; 12 samples are poorly sorted, including 1-8, 1-9, 1-13, 2-1, 2-2, 2-3, 2-4, 2 -5, 2-6, 2-7, 2-8, 3-1 respectively; whereas the remaining samples are moderately sorted. Overall, these samples show nearly a moderate sorting, thus reflecting relatively stable water/wind dynamic conditions. The degree of asymmetry of the grain size distribution was judged by the skewness SK1. The results show that 10 samples are positively skewed, they comprise 1-1, 1-6, 1-7, 1-13, 2-2, 2-4, 2-7, 3- 1, 3-3, 3-5; 6 samples are very positively skewed, comprising 1-5, 1-9, 1-1-1, 2-5, 2-8, 2-11 respectively; whereas, the remaining samples are nearly symmetrical. Overall, the fine grains predominate the grain size distributions. The sharpness of the frequency curve is measured by the kurtosis KG and shows that the frequency curves of 9 samples are sharp, these include 1-1, 1-6, 1-8, 1-9, 2-5, 2-6, 2-7, 3-1, 3-5; the frequency curves of 4 samples are very sharp, including 1-5, 1-1-1, 2-8, 2-11 respectively, and the rest of the samples frequency curves are moderately sharp. Overall, the kurtosis is moderately leptokurtic.
There are obvious differences in the probability cumulative curves of wind transport and hydrodynamic transport. The bar and aeolian dune probability cumulative curves all exhibit a “tri-segment” pattern. The rolling population of the wind-forming sand dunes have similar slopes with the bouncing population, while the bar does not have this feature. The beach sediments are of a “bi-segment” pattern and the wind-forming sand dunes are “one-segment” pattern. According to the features of the probability cumulative curves, different types of microfacies can be determined.