19 mars 2019
Auditoire SUD 06 - Place Croix du Sud
Processes of induration in pyroclastic deposits: the case of Cangahua, Ecuador
Indurated volcanic deposits occur worldwide. In Ecuador, they are known as “cangahua”, a Quechua word for “hard, sterile soil”. About 15% of the cultivated area in the Highland region of Ecuador is underlain by cangahua. Improper cultivation practices lead to erosion and exposure of the hard cangahua material. The properties and mode(s) of formation of cangahua are not well understood. This thesis aims at shedding new light on the origin and processes responsible for the formation of cangahua.
Three outcrops located in the Central and Northern Highland were studied. These are comprised of tephra deposits, cangahua layers and soil horizons. Chemical and mineralogical analyses revealed that the parent materials of the cangahua layers were sourced from the closest volcanic centers. Most cangahuas probably relate to individual explosive eruptions although some may have formed on tephra from different volcanoes. Grainsize analyses suggested that the parent materials correspond either to pyroclastic fall or flow deposits.
Both geological (syn-deposition) and pedological (post-deposition) processes contributed to induration of cangahua’s parent material. As a result, the weathering status of cangahua can vary significantly. Thus, cangahua classification should consider the stage at which induration occurs: geogenetic (syn-deposition induration via geological processes), pedogenetic (post-deposition induration via pedological processes) and pedo-geogenetic (syn-deposition induration via geological processes reinforced by post-deposition induration via pedological processes). Mineralogical analyses revealed that pedo-geogenetic cangahua layers dated at 23 and 32 cal ky BP were exposed to alternating wet-dry climatic conditions after and just before/during the Last Glacial Maximum in Ecuador.
Jury members :
- Prof. Pierre Delmelle (UCLouvain), supervisor
- Prof. Sophie Opfergelt (UCLouvain), supervisor
- Prof. Marnik Vanclooster (UCLouvain), chairperson
- Prof. Veerle Vanacker (UCLouvain), secretary
- Prof. Qiuzhen Yin (UCLouvain)
- Prof. Jean-Marc Baele (Université de Mons, Belgium)
- Prof. Alain Bernard (ULB, Belgium)