Numerical Analysis on Soil and Rock Formation of Ancient Gravel Strata Around Nanjing

Research samples were taken from an ancient gravel stratum which is not only a representative soil layer along the middle-lower reaches of the Yangtze River in East China, but also one of the primary Neozoic strata in Naming district. Located mostly on the second and third terraces, the ancient gravel strata formed the geomorphic landscapes of terrace and step. They were complex in constitution, varied widely in stability, of multiple sources, locally derived, and associated with braided streams in the deposition environment. A CIPW (Cross, Iddings, Pirsson and Washington) method modified by the author was used to analyze the soil-rock-forming materials of finer grains (less than 2 mm in size) of the ancient gravel stratum. Results of the analysis showed that the sandy grains were composed of apatite, ilmenite, potassium feldspar, plagioclase, enstatite and free quartz, the clay mainly of kaolinite, and the cement of a combination of silicon, aluminum and iron at a ratio of 46:44:10. In the soil-rock-forming processes, including compactional solidification, water-stable illuviation-cementation t homogeneous overgrowth and so on, the loose soil-rock-forming components gradually changed into consolidated soil and further to the early stage of lithification. Meanwhile, from the analysis, we found that the ancient gravel stratum had been protected by the overlying Xiashu loess or basalt and the overloading resulted in overconsolidated strata. The modified CIPW method was applicable and effective for semi-quantitative analysis.