Berger, Ingeborg Agnete; (1993) Salts and surface weathering features on alluvial fans in northern Chile. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The spatial association between salts and surface weathering features is examined on three similar alluvial fans in salar basins across a transect of different environments in Antofagasta Province, North Chile. The sites represent the Central Valley (Fan A), the Pre-Cordillera (Fan B) and the High Andes (Fan C). Fan surfaces of different ages were identified by desert pavement, rock varnish and soil development. Combined ionic and XRD analysis shows that calcium sulphate is prevalent, and its deposition onto the fans is primarily aeolian. This salt is found inside split clasts, and on fans A and C it is strongly related to cracking and clast splitting (disintegration into two or more pieces). There is no such link with calcite, the predominant salt on Fan B, but weathering here may be due to small quantities of leonardite and halite, and to agencies other than salt; if CaS04 was present, it has been removed. There is no specific, statistical link between the presence of salts and the proportion and the percentage of clast surface area affected by flaking, granular disintegration, pits, hollows and cavernous weathering. Lithology is apparently not an important control on clast splitting and cracking, except on the younger segment of Fan B, where the dominance of ignimbrite, possibly in combination with more aggressive microenvironments, helps to account for more pronounced weathering. Granular disintegration is statistically linked to ignimbrite, and hollows and cavernous weathering may be related to the prevalence of porphyritic andesite. Portland tablet weathering during one year seems to reflect moisture availability: the most aggressive environment is in the High Andes, followed by the Central Valley where the camanchaca (sea fog) is important, and the even drier Pre-Cordillera. Rates comparable to those in polluted London, U.K., point to considerable potential for weathering despite the aridity. The evidence suggests that calcium sulphate hydration and thermal expansion may be dominant throughout the year on fans A and C. Crystallization should occur when enough moisture is available and thus several times a year on Fan C. The extreme aridity of Fan A suggests that long term stability is required for such weathering, and indeed an optically stimulated thermoluminescence date suggests the fan surface to be at least 230,000 years old.

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