@inproceedings{fdi:010012912,
  title = {{C}aract{\'e}risation et formation de la cangahua en {E}quateur : analyse pr{\'e}liminaire (chimie, min{\'e}ralogie, p{\'e}trographie) de quelques types de cangahua},
  author = {{Q}uantin, {P}aul and {Z}ebrowski, {C}laude},
  editor = {},
  language = {{FRE}},
  abstract = {{T}his study concerns the indurated horizons of some volcanic ash soils in the {I}nterandine {V}alley of the {E}cuador. {T}hese indurated horizons were cropped out by erosion on the piedmont of the sierras, under a subarid to subhumid climate, and then they have been called cangahua. {T}he soils within they are intercalated are composed of several various pyroclastic deposits. {T}he soil profiles show one, two or several indurated horizons. {T}he present observations concern a soil climo-toposequence near {B}olivar, viz: downward a soil profile with 2 cangahua horizons; at the midslope a profile with only one horizon of weathered tuff; a little above, another soil with a weakly weathered consolidated ash fall layer; and finally, at the summit on the paramo, an andosol profile with two horizons of weathered tuff; as well as three cangahua samples from other parts of {E}cuador, but located downward of the piedmont. {T}he aim of this study is to compare the properties of these various indurated soil materials. {W}e will show hereafter the first data about some chemical, mineralogical and microscopic features of these soils and especially their indurated horizons. {W}e determined the composition in major elements of the fine earth and of the less than 2 um fraction, at first from a "triacid" extract which dissolved mainly the weathering products, and further on the residue of unweathered volcanic glasses and minerals. {T}his later one can give us good information on the parent pyroclastic material, which is varying from an alkaline trachyte to a dacite and a rhyodacite within the same soil profile or from a site to another one. {T}he chemical composition of the weathering product varies according to ther ainfall regime and its progressive increase from down to upward (climotoposequence). {T}he molar silica/alumina ratio of this product is decreasing from the values 5-4 downward, to 3 at mid-slope, and 2-1.5 upward. {I}ndeed the silica content decreases according to the increase in rainfall intensity. {T}his fact is also parallelly enlighthened by the clay mineral composition, made downward of smectites and badly crystallized 2:1 clays, the a mid-slope of 1 nm hallosyte, and upward only of allophane. {D}ownward of the piedmont the outcropping cangahua is made of silica-rich 2:1 clay minerals; it doesn't contain any allophane but often a little of calcite and probably os opal. {H}owever, as well as upward as downward, all soils and indurated horizons keep a high amount of almost unweathered volcanic glasses and minerals.},
  keywords = {{CENDRE} {VOLCANIQUE} ; {PYROCLASTITE} ; {INDURATION} ; {TOPOSEQUENCE} ; {CHIMIE} {DU} {SOL} ; {MINERALOGIE} ; {PETROGRAPHIE} ; {CANGAHUA} ; {EQUATEUR} ; {ANDES}},
  numero = {},
  pages = {29--47},
  booktitle = {{S}uelos volcanicos endurecidos},
  year = {1997},
  URL = {https://www.documentation.ird.fr/hor/fdi:010012912},
}