A new road cut in the Chilean Coastal Range near Valparaiso, Chile has exposed an ~8m deep complex weathering profile comprising a saprolite (weathered Carboniferous granitoid) overlain by a ~1m layer of fluvial gravels and a ~60 cm thick loess. Soil horizons include a well-developed Bt horizon at 20-100 cm depth, and a ~1m thick duripan formed mostly in the fluvial gravel layer. A ¹⁰Be profile age through the saprolite indicates a minimum exposure age of 2.2 Ma.
The chemical weathering observed in the granite generally requires levels of soil moisture typical of a more humid climate than presently exists in the region (semi-arid), while the formation of the duripan in the overlying gravels is compatible with the current climate. Duripan formation likely took place in a dry climate at the maximum depth of penetration of meteoric moisture, and as it proceeded, further limited the downward flux of moisture, requiring that much of the chemical weathering below took place beforehand. The juxtaposition of duripan and saprolite may suggest a change in environmental conditions since the exposure of the granite; however, reconstructing palaeo-environments requires that the effects of the complex geomorphic history on the weathering processes be understood.
To investigate the geomorphic and weathering history at this site, we are undertaking geochemical, soil property (particle size, Fe oxides, pH), and mineralogical (bulk and clay) analysis of 16 samples collected down profile.
Results so far show that chemical weathering indices (CIA and CIW, based on Al, Na, Ca, and K concentrations) of the saprolite increase towards the surface but do not indicate the intensity of weathering often observed at the top of saprolite sections. This may be the result of conditions unfavorable for intense chemical weathering, or due to truncation of the upper weathering zones prior to fluvial gravel deposition. Clay mineralogy, soil analysis, and meteoric ¹⁰Be will help to test these hypotheses.