Euroclay 1999 : conference of the European clay groups association : program with abstracts
- Authors
- Publication Date
- Jan 01, 1999
- Source
- Horizon / Pleins textes
- Keywords
- Language
- English
- License
- Unknown
- External links
Abstract
The Ecuadorian Cangahua is an indurated volcanic formation. For geologists it consists of tuff resulting from induration of ash deposits. For pedologists, this induration could be due to weathering products under a subarid climate. The aim of this paper is to confirm the former tuff formation as well as the play of pedogenesis in the clay formation. Our previous studies (chemistry, petrography, mineralogy, sedimentology) have suggested that the Cangahua originated from pyroclastic flows of dacitic ashes. The deconvolution of grain-size frequency curves (according to Liere et al., 1996) demonstrates the depositional process. Every pyroclastic event has left a composite deposit including flow and surge deposits, ash falls and hydrovolcanic products. Induration of some layers, early after this deposition, could be due to former hydrothermal alteration of minute glass shards, producing amorphous matrix in the incipient Cangahua. Now, at the bottom of paleosols the Cangahua has a matrix made of various clay minerals, which result from subsequent weathering. Indeed, there is a good relationship between the nature of clay minerals and the present climatic conditions. These minerals are varying with the moisture regime from semiarid to perhumid, according to a toposequence. We observe successively : at semi-arid foot-slope, predominant silica-rich clays like beidellite or poorly crystalline 2:1 clays and small amounts of opal and calcite - at semi-arid mid-slope, abundant hallysite and segregation of iron oxides - at perhumid summit, predominance of allophane and occasionally small amount of halloy site. (Résumé d'auteur)
