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Accueil du site → Master → Etats Unis → 2004 → La Posta series soils : a petrological, chemical, and statistical analysis of their development in arid and Mediterranean microclimates

San Diego State University (2004)

La Posta series soils : a petrological, chemical, and statistical analysis of their development in arid and Mediterranean microclimates

Carrick, Colleen Marie

Titre : La Posta series soils : a petrological, chemical, and statistical analysis of their development in arid and Mediterranean microclimates

Auteur : Carrick, Colleen Marie

Université de soutenance : San Diego State University

Grade : M.S. San Diego State University 2004

Résumé
In order to investigate the role of climate on the weathering of a common parent rock, this study examines two weathering profiles developed within an arid microclimatic zone and one weathering profile developed within a Mediterranean microclimatic zone. All three profiles that were studied are located on the small biotite facies of the Cretaceous La Posta pluton, Peninsular Ranges, southern California. Petrologic observations, chemical and mass balance data derived from samples studied from the two arid sites suggest that little chemical alteration attended their development from the underlying granodiorite. Molecular A-CN-K and A-CNK-FM diagrams support this interpretation as do mass balance calculations that show that bulk mass change for site 1 is -2.9 +/- 5.1 grams/100 grams of rock and for site 2 -1.3 +/- 5.9 grams/ 100 grams of rock. Both of these results are not statistically significant at the 95% confidence level. In contrast to the data from the two arid sites, petrological, chemical, and statistical data consistently put to significant chemical alteration in the development of horizons C and A. For example, on A-CN-K and A-CNK-FM diagrams samples analyzed from the R horizon consistently cluster beneath those analyzed form the Cr, C, and A horizons. Data derived from the Cr horizon tends to plot within the cluster defined by samples from the C horizon. Specimens analyzed from the C horizon form a crude linear trend parallel to the A-CN or A-CNK joins extending toward the A apex. Unexpectedly, data from the A horizon plot along this trend but approximately midway between the cluster of C and R + Cr samples, a relationship that implies that samples from the A horizon are less weathered than those from the C. Mass balance arguments show that the bulk mass change associated with the development of the C from the R horizon is -14.3 ± 2.5 grams/ 100 grams of rock while the bulk mass change associated with the development of the A from the R horizon is -13.0 ± 2.9 grams/ 100 grams of rock. Hence, petrological, chemical, and statistical data consistently indicate that material in the A horizon is less weathered than material in the C. I speculate that this unusual result stems from the A horizon being constantly resupplied with relatively fresh plutoniclastic debris from surrounding and/or nearby highly jointed outcrops of granodiorite. Such outcrops commonly contain piles of grus that could be easily spread and mixed with the more weathered debris of the C horizon by surface water runoff and gravity.

Présentation

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