Diagenetic Mechanism Of Caliches Of Mega-Dunes In Badain Jaran Desert,China

Caliches emerge in the local parts of the mega-dunes,Badain Jaran Desert,NW China.It is not a universal phenomenon in this desert.They were found only in the slopes of the northeast side of Nortu lake and the north side of Miao haizi lake of the desert.They are wind stratifications cementation of calcium carbonate,which are epidiagenisis products in surface sand soil.The formation,evolution and denudation of caliches contain abundant information of climatic-environmental and secondary diagenetic changes.It is of great significance to the source of moisture of mega-dunes,development and evolution of desert.The cement composition,cementation mechanism and formation environment of the caliches of the Badain Jaran Desert are studied systematically and comprehensively for reconstructing paleoenvironmental changes and proving scientific evidences for other desert scientific matters.Through the detailed investigation on the field characteristics of caliches,and via the lab methods of polarized light microscopy,cathodoluminescence,scanning electron microscopy and energy spectrum analysis,and carbon and oxygen isotopes,the main conclusions are following:Caliches occurred only in the windward slopes of some mega-dunes.Some of the caliches are single thin layer,which are far from each other;some are densely multiple parallel layers and some are large scale herringbone or horsetail stratifications,stretching far in long distances.The vertical distances between the tin monolayers are varing.The dip directions of the caliches layers are alwys opposite to those of windward inclined slopes of the mega-dunes.Monolayer thickness of the caliches is about 1-3mm.The thin layers of caliches,near the bottom of a mega-dune and next to Miaohaizi lake shore,are extremely dense.The exposed thicknesses are large on vast areas.Their colors are amaranth.The monolayer thicknesses are about2-5mm.The microscopic characteristics of caliches are those,their main clastic particles are quartzs,feldspars and rock debris,which are medium-coarse in diameter.The degrees of quartzs alteration are little,but feldspar's alternations are deep.It was found that feldspars were substituted by calcite and iron calcite.The degrees of roundness of the particles are sub-aggular to sub-round in psephicity and there are corrosion holes on their surfaces.The main cement composition is calcite,with partial ferric calcite,dolomite and kaolinite.They are granular or micritic.Local voids were not completely filled by the cements.The cementing patern are basal and pore types.The geochemical characteristics of the cementing material are those,the carbon isotope values in the high and middle altitudes of the dunes are mainly positive,but low.Carbon isotopes of caliches at the bottom of the mega-dunes are negative.Oxygen isotopic values of the caliches are all negative.The linear relation between¹³C and ¹⁸O value of caliches at Nortu lake is obvious and the distribution points are discrete,but,that at Miaohaizi lake is poor and the distribution points are concentrated.The diagenetic mechanism of caliches are bellow:(1)the leaching of atmospheric rainfall and the intense evaporation of surface soil water after rain or shower,the rapid dissolution,evaporation and condensation of carbonate made the carbonate cementing sand layers,coupled with the extremely strong porosity and permeability of the sand.Due to the high temperature on the surface soils during summer,the active chemical fluid solution caused the dissolution and recrystallization of some feldspar clastics in the sediments.(2)The enrichment of atmospheric water condensatation in sand layers caused by different diurnal or seasonal temperature also led to the caliches formation.(3)The purple-red caliches at the bottom of the mega-dunes are related to the decrease of lake water table,which might be caused by arid climate.The carbonate concentration in the lake water increased and the consolidation of sand layer near the lakeshore were formed during arid periods.The epidiagenesis of the caliches reflected the cold-warm and arid-wet change of the climate in the desert.However,the lake water table was higher during wet periods than arid periods in the Badain Jaran Desert.