Fjölrit RALA - 15.06.2004, Side 77

Soils and paleosols on volcanic rocks of Cantal (Massif Central, France);
Example of Puy Courny, Aurillac
P.Quantin1, J. Dejou2 and M. Tejedor3
1IRD, Paris; 2INRA, Clennont-Ferrand; 3University ofLa Laguna, Spain.
Introduction
Red fersialitic soils were studied in the Cantal on late Miocene basalts (Dejou & al.1982,
Chesworth & al. 1983). The present soils, of Holocene time although on Miocene volcanic
rocks, are aluandic Andosols or andic Brown soils ( Hétier 1975, Moinereau 1977, Chesworth
& al. 1983). A new study of the Puy Coumy cut (after the COST 622 Meeting in Auvergne,
30-05 to 3-06 2001) shows the following succession of volcanic products, ‘alterites’,
paleosol, and present soil, from bottom to top : - a/ over a basanite flow, 7.3 M. years old, a
green ‘alterite’ and a red paleosol; - b/ series of 5 to 6 outburst pyroclastic products,
partially altered; -c/ a weakly altered basalt flow, 6.4 M. years old; - d/ the present soil,
deriving from periglacial deposits of weathered trachybasalt material.
Results
The whole elements chemical composition allows us to approximate the magmatic family of
successive original volcanic materials : a/ lst series : an augite-olivine trachybasalt flow,
evolving up to quartziferous basalt in the red paleosol; b/ 2d series : pyroclasts of
quartziferous latite composition, evolving from dark quartziferous latite to rhyodacite; c/ 3d
series : upper-Cantalian basalt flow, near to augite-olivine trachybasalt; d/ 4* series :
weathered products of trachybasalt.
The mineralogy of clay minerals and the chemical composition show different types of
successive alteration or weathering processes.
1. Paleosol and present soil weathering :
1.1 : The red paleosol, of late Miocene formation, almost completely weathered, is fersialitic
from its chemical composition (Ki = 2.7, Kr = 1.8), as well as its clay mineralogy, of
predominant dehydrated halloysite, a little of disordered smectite and of hematite and goethite
iron oxides, without residual organic matter. ( Ki = SÍO2/AI2O3 mol. ratio ; Kr =
Si02/A1203+Fe203 mol. ratio).
1.2 : The present Brown soil, with eutrophic and weakly andic properties, is only partially
weathered, rich in silica ( Ki = 5.0, Kr = 3.4 ) and residual primary minerals. It differs greatly
from paleosol in its clay mineralogy, of predominant disordered smectite, a little of illite and
non-crystalline iron oxi-hydroxide, and traces of allophane.
2. Two types of ‘alterites’ :
2.1 : ‘pre-meteoric’ : probably formed under hydromagmatic conditions during the deposit
process or immediately after. That is characterised by formation of well crystallised
vermiculite, in the basanite alteration products and overlying green alterite, as well as in
pyroclastic flows of quartziferous latite (blast of bright grey sand or pumiceous lahar) and the
altered ‘cortex’ (crust) of bowls of the upper basalt flow.
2.2 : probably ‘meteoric’ : characterised by poorly ordered smectite clay minerals in red
brownish alterite, brown cinerite, purple brownish tuff, as well in the present andic Brown
soil.
The chemical properties, from top to down of the cut, are characterised by : pH (H2O)
slightly acid in present soil, then slightly alkaline in all ‘alterites’ and red fersialitic soil; ApH
(H2O-KCI) >1, from 1.3 to 1.9, in relation with high permanent charges and CEC clay
minerals, even in halloysitic paleosol; high values in exchangeable bases and bases saturation
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