The effects of sedimentary facies and palaeogeography on the formation and distribution of the deep groundwater of the Cretaceous strata in the Ordos Basin

The integration of sedimentary geological and hydrogeological survey and sample analysis of the representative Cretaceous hydrological boreholes and outcrops has disclosed that during the formation of the Cretaceous aquifers,the fluvial deposits were wide-spread in the Huanhe and Luohe Formations in the northern part while the lake deposits occupied in the Huanhe Formation,and desert deposits occurred in the Luohe Formation in the southern part of the Ordos Basin. The framework of sedimentary facies and palaeogeography has exercised a major control not only on mineral compositions such as feldspar,clay minerals,calcite and gypsum in the aquifer systems,soluble salt contents and their spatial distribution,but also on the spatial distribution of the aquifers and impermeable beds,regional distribution of the deep groundwater circulation and alternation. The water-rock interactions influencing the formation of the hydrochemical fields and distribution of the groundwater include the dissolution of sulfate minerals,carbonate minerals and silicate minerals, rock salt leaching and cation exchange. Controlled by sedimentary facies and palaeogeography,groundwater circulation and water-rock interaction,the groundwater from the Huanhe and Luohe Formations are characterized by lower TDS,enriched freshwater and dominant HCO₃^- type in the northern part whereas higher TDS,enriched brackish water-saline water,and HCO₃· SO₄^- type in the southern part of the basin.