The Grain Size Distribution Characteristic Of The Weathering Crusts Underlying Carbonatites And Its Indication Meaning On The Provenance And Formation Processes In Karst Terrains, Guizhou Province

In southwestern China, karst region centered on Guizhou Province is the vastest one in the world and is overlain by non-isopachous red earth. Since carbonate rock is easy to solve, acidic insoluble residua is very low (generally less than 5%), pedogenesis process is very slow, weathering crust volume is intensively reduced during weathering, mother rock texture and semi-weathering layer cannot be retained, contact relation of rock-earth interface shows abrupt change, transitional layer is absent, and macroscopically there isn't direct field geological evidence to support that inheritable characteristic exists between basement rock and overlying weathering crust, the provenance of overlying weathering crusts in karst regions has been on the debate for a long time. Because different viewpoints exist about the genesis of weathering crusts in karst regions, which becomes the obstacle to use weathering crusts to undertake a series of scientic researches, to precisely understand the provenance and genesis of overlying weathering crusts in karst regions is very pressing and important.On the basis of results obtained, this paper selects karst terrains of mosaic occurrence among limestone, dolostone and clastic rock in Guizhou Province (including Jishou profile in western Hunan Province) as research area, attempts to use the grain size analysis method to discuss the grain size distribution characteristic of different basement rocks and overlying weathering crusts in detail. Based on the grain size analysis, merging with magnetic susceptibility, pH value, geochemistry and mineralogy data, Author discusses the provenance and profile development of overlying weathering crusts in karst region of Guizhou Province, and obtains several significant innovations as follows:1. The grain size analysis is a valid and intuitionistic method during the research on the genesis of weathering crusts underlying carbonatites. Sedimentary rocks derived from different sedimentary backgrounds possess different grain size distribution features, and weathering crusts developed on the basis of them inherit the grain size distribution characteristic of protolith rocks, their grain size frequency distribution curves show consistency and gradual transition with protolith rocks. Under the condition of moderate weathering intensity, weathering crusts still retain the information of the provenance. Weathering crusts developed on the carbonatites inherit the grain size distribution featuresof acidic insoluble residua of basement rocks. Weathering crusts developed on the sedimentary rock containing different grain size compositions take on obvious discrepancy in this aspect too.2. The grain size analysis results of different basement rocks and overlying weathering crusts show that the grain size distribution feature of every weathering profile obviously inherits that of its underlying basement rock. Among weathering profiles the grain size distribution features show remarkable diversities, which accounts for that there isn't the same provenance about weathering crusts in karst region of Guizhou Province. Weathering crusts underlying carbonatites are the offspring of basement rock weathering action through the dissolution of carbonates and long-term accumulation of acidic insoluble residua.3. During non-isometric weathering, weathering front (i.e. interface between rock and earth) is a important place of geochemical action. On this narrow interface, carbonate is dissolved wealthily, acidic insoluble residua is decomposed as well. Furthermore, weathering gradient on the weathering front obviously excesses that of weathering profile formed during subsequent weathering, i.e. from acidic insoluble residua of basement rock to the bottom of weathering crust, weathering intensity abrupt increases, and subsequent development of weathering crust is a slow process.4. Owing to plentiful water and heat condition, the phenomenon of eluviation and illuviation widely occurs in weathering profiles. Clay content shows gradual reduction fr...