ABSTRACT

The Rio Ameca drainage basin is underlain almost entirely by igneous rocks. The principal clay minerals forming in the soils are halloysite, kaolinite, and montmorillonite. The clay mineralogy of the stream sediments generally corresponds to the soil-clay mineralogy in the drainage basin, but montmorillonite appears to be over-represented in sediments compared to the areal abundance of montmorillonitic soils.

Stream water chemistry is strongly influenced by the porosity and reactivity of the rocks in the drainage basin. Waters draining Tertiary andesites of low porosity contain less than 200 ppm total dissolved solids, and the molar ratios of the dissolved constituents are such that the equation [Average andesite] +H₂O+CO₂[Kaolinite] + [Ca-montmorillonite] + [dissolved load of streams] can be balanced.

Streams draining an unconsolidated rhyolite ash, however, have up to 820 ppm total dissolved solids. The Na/K ratio ill these waters is approximately 10, while this ratio is approximately 1 for both the ash and the soil formed on the ash. On a log a_Na/a_H vs log a_SiO2 diagram, all stream water analyses except those from streams draining the rhyolite ash plot close to the montmorillonite-kaolinite phase boundary. Those from the ash plot in the montmorillonite stability field, although halloysite is the principal clay mineral forming in the soils on the ash. It is suggested that reactions below the observed soil zone control the chemistry of the streams draining the ash.

The chemical denudation rate of the low porosity andesites is 28 metric tons/km²/yr, a typical value for silicate weathering in a range of climates. The rate for the ash is 91 tons/km²/yr, showing the influence of highly reactive starting material.