The Genoa River basin is underlain largely by granitoid rocks which are of three types-quartz diorite-granodiorite, adamellite, and granite-adamellite-and to a lesser extent by metasediments and coarse elastics. Two types of surface water are present in the drainage basin, an Na-Cl type and a mixed-cation HCO3-Cl type. The genesis of these two water types isrelated primarily to differences in rate of chemical weathering of the three granitoid rock types. Mixed-cation HCO3,-Cl waters drain quartz diorite-granodiorite and adamellite terrain, but not granite-adamellite terrain, whereas the reverse is the case for the Na-CI waters. The quartz diorite-granodiorite and closely associated adamellite rock suites contain more minerals which are more readily weathered than does the granite-adamellite rock suite. These minerals (calcium plagioclase, biotite and hornblende) supply Ca, Mg, Na, and HCO3 to the waters through rapid dissolution. Where the rate of chemical weathering is high, the surface waters are characterized by a mixture of atmospheric salts and soluble products of weathering (mixed-cation HCO3-Cl type). Where the chemical weathering rate is low, the surface waters are dominated by atmospheric salts (Na-Cl type). The chemical weathering rate of the underlying bedrock remains as the controlling factor in the genesis of the two water types, even during low runoff periods when both the groundwater contribution to stream flow and the rate of evaporation are high.
Amazonian chemical weathering rate derived from stony meteorite finds at Meridiani Planum on Mars
