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Pleistocene stratigraphy and morphogenesis of La Dombes an alternative hypothesis [Hypothèse pour la stratigraphie et la morphogenèse pléistocène de La Dombes.] - article ; n°2 ; vol.22, pg 85-96

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Bulletin de l'Association française pour l'étude du quaternaire - Année 1985 - Volume 22 - Numéro 2 - Pages 85-96
Deux groupes principaux de formes et dépôts d'origine glaciaire se différencient à la surface du plateau de la Dombes. Le premier recouvre la partie centrale et septentrionale du plateau et présente des paléosols atteignant 5 m d'épaisseur. Le second occupe la bordure du plateau, au-dessus de la partie aval de la vallée de la Saône ainsi que des vallées du Rhône et de l'Ain. Les paléosols ont une épaisseur environ de moitié inférieur aux précédents, sur les dépôts formant les lobes d'Ars-Les Echets qui recoupent le modelé glaciaire de la Dombes centrale. Ces deux groupes se rapportent au minimum à deux stades glaciaires différents. Mais ils représentent vraisemblablement une série stratigraphique beaucoup plus complexe se situant à la partie inférieure et moyenne du Pléistocène Moyen. Les sédiments glaciaires de la Dombes sont réinterprétés comme des moraines de fonte sous pression, des faciès de glissement, quelques moraines d'ablation et des alluvions d'eau de fonte sous-glaciaire abondantes. Une telle association est interprétée comme le produit de la fusion sur place d'une série de lobes glaciaires de piedmont, présentant de vastes étendues de glace morte et un mode de dépôt à dominante supraglaciaire et proglaciaire. L'écoulement supraglaciaire et l'encaissement des eaux de fonte se sont effectués sur un terrain à culots de glace, selon un système de crevasses radiales qui a contrôlé la sédimentation et la mise en place d'un relief de bosses et creux alignés, disposés en éventail, datant de la fonte des glaces et non de leur avance. Les moraines terminales ne forment pas autour de la Dombes une ceinture continue. Mais elles soulignent l'emplacement d'une série de lobes qui correspondent, du Nord au Sud, à des extensions glaciaires d'âge différent.
Two principal sets of forms and deposits are distinguished on the plateau of La Dombes. The first occurs on the central and northern part of the plateau and displays palaeosols at least 5 m thick. The second group occupies the border of the plateau in the S part of the Saône valley and overlooks the valleys of the Rhône and Ain. The palaeosols are only about half as thick on the deposits forming the Ars-Les Echets lobes which cut across the glacial morphology of the central part of La Dombes. These two principal groups of deposits were derived from at least two glacial stages and probably represent a much more complex stratigraphic series located in the lower and middle part of the Middle Pleistocene. The near-surface glacial sediments of the central part of La Dombes have been reinterpreted as meltout tills, slide faciès, occasional flowed tills, and abundant meltwater sands and gravels. Such a sediment association is interpreted as the downwasting of a series of piedmont ice lobes with widespread dead ice, and predominantly supraglacial and proglacial deposition. Radial crevasse systems, supraglacial meltout, and meltwater trenching in an ice-cored terrain controlled sedimentation and the development of the linear, hummocky morphology. The radiating drift ridges of La Dombes were laid down mainly during ice dissolution rather than advance. There is no evidence for a continuous contemporaneous belt of end moraines around La Dombes. However, the moraines are disposed in a series of arcs which correspond from N to S to glacial extensions of different ages.
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Armelle Billard
Edward Derbyshire
Pleistocene stratigraphy and morphogenesis of La Dombes an
alternative hypothesis [Hypothèse pour la stratigraphie et la
morphogenèse pléistocène de La Dombes.]
In: Bulletin de l'Association française pour l'étude du quaternaire - Volume 22 - Numéro 2-3 - 1985. pp. 85-96.
Citer ce document / Cite this document :
Billard Armelle, Derbyshire Edward. Pleistocene stratigraphy and morphogenesis of La Dombes an alternative hypothesis
[Hypothèse pour la stratigraphie et la morphogenèse pléistocène de La Dombes.]. In: Bulletin de l'Association française pour
l'étude du quaternaire - Volume 22 - Numéro 2-3 - 1985. pp. 85-96.
doi : 10.3406/quate.1985.1532
http://www.persee.fr/web/revues/home/prescript/article/quate_0004-5500_1985_num_22_2_1532Résumé
Deux groupes principaux de formes et dépôts d'origine glaciaire se différencient à la surface du plateau
de la Dombes. Le premier recouvre la partie centrale et septentrionale du plateau et présente des
paléosols atteignant 5 m d'épaisseur. Le second occupe la bordure du plateau, au-dessus de la partie
aval de la vallée de la Saône ainsi que des vallées du Rhône et de l'Ain. Les paléosols ont une
épaisseur environ de moitié inférieur aux précédents, sur les dépôts formant les lobes d'Ars-Les Echets
qui recoupent le modelé glaciaire de la Dombes centrale. Ces deux groupes se rapportent au minimum
à deux stades glaciaires différents. Mais ils représentent vraisemblablement une série stratigraphique
beaucoup plus complexe se situant à la partie inférieure et moyenne du Pléistocène Moyen. Les
sédiments glaciaires de la Dombes sont réinterprétés comme des moraines de fonte sous pression, des
faciès de glissement, quelques moraines d'ablation et des alluvions d'eau de fonte sous-glaciaire
abondantes. Une telle association est interprétée comme le produit de la fusion sur place d'une série de
lobes glaciaires de piedmont, présentant de vastes étendues de glace morte et un mode de dépôt à
dominante supraglaciaire et proglaciaire. L'écoulement supraglaciaire et l'encaissement des eaux de
fonte se sont effectués sur un terrain à culots de glace, selon un système de crevasses radiales qui a
contrôlé la sédimentation et la mise en place d'un relief de bosses et creux alignés, disposés en
éventail, datant de la fonte des glaces et non de leur avance. Les moraines terminales ne forment pas
autour de la Dombes une ceinture continue. Mais elles soulignent l'emplacement d'une série de lobes
qui correspondent, du Nord au Sud, à des extensions glaciaires d'âge différent.
Abstract
Two principal sets of forms and deposits are distinguished on the plateau of La Dombes. The first
occurs on the central and northern part of the plateau and displays palaeosols at least 5 m thick. The
second group occupies the border of the plateau in the S part of the Saône valley and overlooks the
valleys of the Rhône and Ain. The palaeosols are only about half as thick on the deposits forming the
Ars-Les Echets lobes which cut across the glacial morphology of the central part of La Dombes. These
two principal groups of deposits were derived from at least two glacial stages and probably represent a
much more complex stratigraphic series located in the lower and middle part of the Middle Pleistocene.
The near-surface glacial sediments of the central part of La Dombes have been reinterpreted as meltout
tills, slide faciès, occasional flowed tills, and abundant meltwater sands and gravels. Such a sediment
association is interpreted as the downwasting of a series of piedmont ice lobes with widespread dead
ice, and predominantly supraglacial and proglacial deposition. Radial crevasse systems, supraglacial
meltout, and meltwater trenching in an ice-cored terrain controlled sedimentation and the development
of the linear, hummocky morphology. The radiating drift ridges of La Dombes were laid down mainly
during ice dissolution rather than advance. There is no evidence for a continuous contemporaneous belt
of end moraines around La Dombes. However, the moraines are disposed in a series of arcs which
correspond from N to S to glacial extensions of different ages.Bulletin de l'Association française 1985/2-3, pages 85-96
pour l'étude du Quaternaire
PLEISTOCENE STRATIGRAPHY
AND MORPHOGENESIS OF LA DOMBES
AN ALTERNATIVE HYPOTHESIS
by Armelle BILLARD* and Edward DERBYSHIRE**
ABSTRACT
Two principal sets of forms and deposits are distinguished on the plateau of La Dombes. The first occurs on the central
and northern part of the plateau and displays palaeosols at least 5 m thick. The second group occupies the border of the
plateau in the S part of the Saône valley and overlooks the valleys of the Rhône and Ain. The palaeosols are only about
half as thick on the deposits forming the Ars-Les Echets lobes which cut across the glacial morphology of the central part
of La Dombes. These two principal groups of deposits were derived from at least two stages and probably represent
a much more complex stratigraphie series located in the lower and middle part of the Middle Pleistocene.
The near-surface glacial sediments of the central part of La Dombes have been reinterpreted as meltout tills, slide
faciès, occasional flowed tills, and abundant meltwater sands and gravels. Such a sediment association is interpreted as the
downwasting of a series of piedmont ice lobes with widespread dead ice, and predominantly supraglacial and proglacial
deposition. Radial crevasse systems, supraglacial meltout, and meltwater trenching in an ice-cored terrain controlled
sedimentation and the development of the linear, hummocky morphology.
The radiating drift ridges of La Dombes were laid down mainly during ice dissolution rather than advance. There is
no evidence for a continuous contemporaneous belt of end moraines around La Dombes. However, the moraines are
disposed in a series of arcs which correspond from N to S to glacial extensions of different ages.
Key-words : Palaeosol thickness, Radial drift ridges, Middle Pleistocene, Piedmont ice downwasting, Arcuate end moraines,
Supraglacial landform-sediment association.
RESUME
UNE AUTRE HYPOTHESE POUR LA STRATIGRAPHIE ET LA MORPHOGENESE PLEISTOCENE DE LA
DOMBES
Deux groupes principaux de formes et dépôts d'origine glaciaire se différencient à la surface du plateau de la Dombes.
Le premier recouvre la partie centrale et septentrionale du plateau et présente des paléosols atteignant 5 m d'épaisseur. Le
second occupe la bordure du plateau, au-dessus de la partie aval de la vallée de la Saône ainsi que des vallées du Rhône
et de l'Ain. Les paléosols ont une épaisseur environ de moitié inférieur aux précédents, sur les dépôts formant les lobes
d' Ars-Les Echets qui recoupent le modelé glaciaire de la Dombes centrale. Ces deux groupes se rapportent au minimum
à deux stades glaciaires différents. Mais il représentent vraisemblablement une série stratigraphique beaucoup plus complexe
se situant à la partie inférieure et moyenne du Pleistocene Moyen. Les sédiments glaciaires de la Dombes sont réinterprétés
*UA 141 CNRS, 1, place Aristide- Briand, 92195 Meudon (France).
** Department of Geography, University of Leicester, Leicester, LEI 7RH (U.K.). 86
comme des moraines de fonte sous pression, des faciès de glissement, quelques moraines d'ablation et des alluvions d'eau
de fonte sous-glaciaire abondantes.
Une telle association est interprétée comme le produit de la fusion sur place d'une série de lobes glaciaires de piedmont,
présentant de vastes étendues de glace morte et un mode de dépôt à dominante supraglaciaire et proglaciaire. L'écoulement
supraglaciaire et l'encaissement des eaux de fonte se sont effectués sur un terrain à culots de glace, selon un système de
crevasses radiales qui a contrôlé la sédimentation et la mise en place d'un relief de bosses et creux alignés, disposés en
éventail, datant de la fonte des glaces et non de leur avance. Les moraines terminales ne forment pas autour de la Dombes
une ceinture continue. Mais elles soulignent l'emplacement d'une série de lobes qui correspondent, du Nord au Sud, à des
extensions glaciaires d'âge différent.
Mots-clefs : Paléosols, rides morainiques radiales, Pleistocene Moyen, Fonte sur place des glaces de piedmont, Moraines
terminales en arcs, Formes et sédiments supraglaciaires.
La Dombes is a large plateau about 1500 km2 in (iii) a complex hummocky zone of glacial deposits
area lying between the valleys of the Rhône and Ain in the S and SW, bounded by a series of morainic
rivers on the S and SE and Sâone valley to the W. Its mounds and ridges running from W of Ars to SE of
mantle of glacial debris records the greatest extension les Echets.
of Pleistocene ice in the French Alpine piedmont.
During its maximum advance, the Rhône glacier
extended some 80 km beyond the Alpine front, co Northern and Central La Dombes
vering the southern part of La Bresse. The sediments
underlying the glacial deposits consist of Pliocene
marls (les marnes de Bresse) which have been locally The elongated drift hummocks of the central re
gullied and then infilled and mantled by the strongly gion decline in both relative and absolute altitude
weathered Plio-Pleistocene gravels known as les towards the WNW, and their elongation ratios (a : b
alluvions jaunes (Fleury and Monjuvent, 1984). A axes) increase in the same direction. Between Am-
number of sections show that the sediments visible at bérieux and St. André-de-Corcy, they give way to
the plateau surface bury older glacial deposits, parti some almost flat surfaces which are cut across by a
cularly at Ars where a gravelly drift covers a Plei series of transverse drift ridges, some of which consti
stocene tillite containing weathered crystalline peb tute the outer margins of the SW hummocky zone
bles. (area iii).
The highest altitudes in La Dombes are found in The alluvions jaunes exposed N of Ville-Sollier
the S from where the undulating surface of the glacial appear to underlie the tills and associated glacial
sediments slopes down gently northward to deposits laid down by the Abergement-Clémenciat
250-270 m in the outermost end moraines. A large glacier lobe. Along the Sâone valley, the alluvions
number of artificial ponds fill the smooth depressions jaunes have been reworked into a series of lacustrine
between the low hills, making some of the morpholog deltas with well developed foreset beds, overlain by
ical trends more readily visible. From a morphologic bedded silts, the whole making up the Peyzieux
al point of view, however, the region is more comp terrace as described by Monjuvent et al. (1984). A
lex, and three distinct areas may be recognized as decalcified palaeosol with a banded B horizon (B en
follows (Fig. 1 and 2) : bandes) about 6 m thick can be seen at the top of the
(i) a northern and central part characterized by a gravels. It may be concluded (following Mandier
system of elongated hills converging on Rignieux and 1984 a) that these gravels were laid down in a gl
St. Eloi in the SE (altitude 300-310 m) where they acier-dammed lake during the glacial advance before
the maximum northward extension of the ice in La reach their maximum relative relief of 15-20 m. They
radiate to N and NW and their relief becomes att Dombes. As an alternative to this, it may be suggested
that they date from a glacial stage older than that of enuated as far as the two juxtaposed arcs of the
outermost end moraine series, one between Corgenon the northern end moraines, being laid down in waters
and Chanoz-Chatenay, and the other at l'Abergement ponded by an ice advance which just reached the
— Clémenciat. southern plateau.
Just W of Châtillon-sur-Chalaronne a succession (ii) on the NE side of the region, a lateral de
of over 9 m of Pleistocene sediments is exposed. At pression perched above the Ain River between Pont
the base is at least 1 m of very poorly sorted and d'Ain and Bourg-en-Bresse consisting of a stepped
rounded glaciofluvial gravels in a loose sandy — série of surfaces mantled by glacial sediments and
gravel matrix. This is succeeded by about 30 cm of outwash as far as the outermost (Seillon) end mor
bedded and laminated soft clays, silts and sands aine ; and 87
Corgenon BOURG EN BRESSE
Chanoz-Chatenay
FORET DE
Condeissiat SEILLON L'Abergement
-Clémenciat Tossiat
Certines Ville Jolliers
Lent
Croix de 1 'Orme <0 /Peyzieux Châtillon-sur-
Chalaronne
Rossettes
Dompierre • Hautes •• Basses
La Chapel le-du- Pont d'Ain
Châtelard
iBeauregard Ambérieux-en-Dombes
• Ars
1VÏLLEFRANCHE
Toussieux • St. Jean de Thurigneux Rignieux Trévoux • St.Eloi •
St. André Pouilleux
Le Montellier •deCorcy ►Parcieux
#Genay
Montanay
Les Echets
CaiTloux-sur-
Fontaines
Fig. 1. — La Dombes : places mentioned in the text.
Fig. 1. — La : localités mentionnées dans le texte.
which are overlain, in turn, by a lensoid unit some 2.5 across the top of this mélange is 1.5 m of very poorly
— 3 m thick of subangular pebbles and cobbles in a sorted glaciofluvial gravels. The whole succession is
compact sandy — silt matrix. Both matrix strength draped by a variable cover of up to 2 m of loessic silts.
and dissolution of the limestone clasts varies late In the upper part of the Châtillon section there is
rally. This material is interpreted as a till which has a palaeosol with a reddish-brown truncated Bt hori
suffered some subglacial loading. It is overlain by zon about 2.5 m thick above decalcified B and BC
1-2 m of glaciofluvial gravels in a rather poorly sorted horizons, the total thickness being 5-7 m, reaching
sandy matrix, above which lies about 3 m of steeply down to the till unit. The former limestone clasts
dipping and laterally variable gravels, sands, silts and within the till have been dissolved, leaving cavities
clays making up a channel fill which contains some with residual powder. A similar palaeosol was des
faults. Also present is a gravel dike produced by the cribed from Chanoz-Châtenay by Bourdier (1963). At
filling of a crack from above with slide debris. Small Le Moléron, 2 km W of Dompierre, at least 2 m of
normal faults run parallel to the sub-vertical margins ripple cross-bedded sand and fine gravel grades
but there is no large-scale deformation. Cutting upwards into over 1 m of till flow, stony in parts, with 88
1
0 2 4
F/^. 2. — />r//f morphology of La Dombes interpreted from topographical maps and field mapping.
Fig. 2. — Morphologie glaciaire de la d'après les cartes topographiques et les relevés de terrain. 89
well-developed flow structures, lying below a 1 m surface (2 11 00 ±500 and 25700 ± 2800/2400 : Evin
cover of yellow-grey mottled stony silt. At Lent, 6 km and Billard, unpublished).
to the N, a small exposure of sub glacial till is
overlain by meltout till and bedded sands and gravels
locally faulted owing to collapse during glacier The S and SW hummocky zone
wastage. Similar sedimentary associations occur at
other sites in the central region including the 4.5 m
sequence at Beaujeu (4.5 km E of Châtillon-sur-
The S and S W areas of La Dombes may be divided Chalaronne) and at La Chapelle-du-Châtelard. into two parts. The first series of arcuate ridges and Throughout this area the palaeosol developed on the mounds of glacial deposits if located between the glacial sediments reaches up to 7 m in thickness. environs of Ars southwards to a point beyond St.
Jean-de-Thurigneux. The second centres around the
Les Echets closed depression and extends from
The Pont d'Ain — Bourg-en-Bresse depression Montanay in the W almost to Montluel in the E. The
Les Echets area contains the boldest morainic relief
(average 20 m) and the highest drift altitudes in the
whole of the La Dombes plateau. This area gives way This depression is separated from the central part to the E to a more gently rolling landscape bounded of La Dombes by a higher belt running N-S which is by the arcuate ridges to the S of Le Montellier then asymmetrical, its steeper slopes facing the depres continuing to the S of Rignieux as also noted by sion. At its S end, this higher surface at 340 m is cut Monjuvent et al. (1984) and Mandier (1984 b). in two, Bois de Priay and Mont Margueron, with
The alluvions jaunes outcrop on the left (E) flank alluvions jaunes at the top with no record of a glacial
of the Sâone between Beauregard in the N to Trévoux sediment cover.
in the S and form an extensive terrace surface. Below This perched depression is made up of several this surface the St. Euphemie terrace, SW of Ars, and elements. The highest is the morainic surface of the Parcieux, Genay and Cailloux-sur-Fonlaines fans Tossiat. The large section associated with the A 40 which descend westward towards the Sâone were autoroute at la Croix de l'Orme consists of a thick produced by meltwater deposition during the down- gravel with many sandy layers in the lower part, the wasting of the ice in these two areas. The Ars and Les units increasing in number and coarseness Echets arcuate moraine systems cut across the westupwards and passing at the top into thin discont erly trending drift morphology of the central part of inuous till. This gravely till is extremely poorly sorted La Dombes. (sorting coefficent 5.2) with a compact, hard matrix
Above the tillite at Ars, the succession consists of strengthened by post-depositional cementation. A
gravelly ice-contact stratified drift overlain by two reddish brown palaeosol has developed near the top
loessic silt units with a reddish palaeosol at the top of this sequence. Its thickness varies from 2.5 m
of the lower silt. At St. Jean-de-Thurigneux the where truncated, to 5-7 m where preserved in situ, and
ice-contact stratified drift interdigitates with meltout even more along weathering pipes. This morainic
till. At Les Fromentales, 0.5 km NW of Toussieux, surface runs northward to the Seillon moraine.
beneath 1-1.5 m of loessic silt is found a rounded Below the Tossiat surface lies the La Rippe-Certi- cobble gravel in a gravelly sand matrix with lenses of nes terrace at the base of which is found the Croix de mottled yellow fine sand containing abundant pebl'Orme sand and gravel overlain by coarse outwash. bles many of which are sub vertical (? dropstones). On this surface a reddish brown soil has developed This unit is interpreted as ice — contact stratified with a thickness of about 5 m, or more in weathering drift and pond sediments, but the fabric is too diffuse pipes. This terrace appears to relate upstream to the to give clear evidence of any palaeoflow direction. Rossettes-Hautes morainic mounds (on which the Some 1.5 km NE of Toussieux over 3 m of oxidized depth of weathering exceeds 3.5 m : Mandier 1984 b). but unweathered till is exposed with striated cobbles
The SE part of the Pont d'Ain depression is and a compact sandy silt matrix with occasional small
covered at Rossettes- Basses by ice-contact stratified gravel lenses. The till fabric in this series of ovoid
drift. The reddish palaeosol on this surface is rather mounds running WNW to ESE is somewhat diffuse
less than half the thickness of the soil described from and this, together with the particle size curve indi
La Rippe and La Croix de l'Orme, i.e. 2.5 m or less. cating some eluviation of fines by meltwater, suggests
Two more recent terraces are located within the a meltout origin. At Pouilleux (2.5 km SW of St. Jean
abandoned valley which lies along the axis of the de Thurigneux) there is a large exposure in an arcuate
depression. Downstream the lower valley floor is series of mounds running roughly E-W. It has a core
occupied by alluvium of the last glacial stage, as of compact sandy-silty lodgement till with a clear
14 C dating of wood 8-10 m below the orientation evident in both the sandy-silt matrix (Fig. shown by 90
3) sand and pebbles indicating ice movement from mentology and pedological data, Bourdier (1963)
between SW, and SE. In detail, however, the orien distinguished three groups of deposits at the top of
tation of shear zones and pebbles is complex, a the plateau, as follows.
lthough all show a S component. Evidence of thru- (i) In the S and SW, including Ars, he found two
shing and shearing from the S is evident throughout loessic silts and one interglacial buried soil above the
this site. The sequence shows progressive eluviation glacial sediments.
of fines from south to north across the site to the (ii) In central La Dombes, he observed three north end where the material is a coarse glaciofluvial loessic silts and two interglacial buried soils above sandy gravel. The steep morainic hummocks at Le the glacial sediments. Michon, near Les Echets consist of crudely bedded
(iii) In the N he distinguished the sediments as coarse glaciofluvial gravels and coarse eluviated
being older because of the great thickness of the (meltout) tills. Here the pebbles are imbricated with
« red » palaeosol. a mean orientation to the SE (150°) in contrast to the
SSE, S and SSW orientations further N. The first two groups were assigned to the so-called
Riss, and the third to the so-called Mindel. It must In this part of La Dombes, the palaeosols are often
be emphasized that the term Mindel does not imply truncated on the moraine crests as can be seen at
that a sediment belongs to the antepenultimate or, Pouilleux and Le Michon, reducing the thickness of
indeed, to any specific glaciation, but it does indicate the palaesol profile to 1-1.5 m. At Ars, the palaeosol
a relatively old deposit. Preliminary palaeo-magnetic is about 2.5 m thick but it is 2.5-3 m on the gravel fans
measurements of sediments from Châtillon-sur-Cha- close to Parcieuxand Cailloux-sur- Fontaines.
laronne, La Chapelle du Châtelard, Tossiat and La
Rippe-Certines gave normal polarities (Geophysics
Institute, Academy of Sciences, Prague, unpublished
results). They may- thus be tentatively placed in the Interpretations
lower part of the Brunhes period.
Attribution of the sediments of N La Dombes to
The stratigraphy of the glacial sediments of La the Mindel is still accepted by Bornand et al (1976).
Dombes has been interpreted in several ways since However, more recently all the N, centre and most of
Penck and Bruckner (1909) attributed them to the the S La Dombes has been attributed to the so-called
so-called Riss Glaciation and placed the alluvions « Riss ancien » (Monjuvent et al 1984) or the
jaunes in the Deckenschotter. On the basis of sedi- «complexe Riss ancien» (Mandier 1984a), with
is. ,'K ''*-.-.■■-'■;■
15KU 8.45KX 22. 2 M 9553
Fig. 3. — Scanning electron photomicrograph showing angular, crushed sand and coarse silt grains in a fine silt matrix with both
imbrication and micro-shearing dipping to the S (left). Pouilleux.
Fig. 3. — Photographie en microscopie électronique à balayage montrant des grains de sables anguleux, broyés et de silts grossiers,
inclus dans une matrice finement silteuse avec imbrication et plans de cisaillement plongeant vers le Sud (gauche de la photo).
Pouilleux. 91
M N M N
Fig. 4. — Stereograms showing pebble orientations and dips
at La Piemonte. (a) east end, (b) west end, and (c) composite.
Fig. 4. — Stéréogrammes des orientations et inclinaisons M N
des galets à la Piemonte. (a) extrémité Est de la coupe ;
(b) . extrémité Ouest ; (c) (a) et (b) confondus , MN . Nord
magnétique.
some differentiation (« Riss recent ») along the S
border of the plateau. All the moraines close to the
NE, N and W margins of La Dombes, and the
outermost ridges in the SW have been interpreted as
a continuous contemporaneous end moraine series.
In order to explain the strong differences in the
thickness of the palaeosols, a severe climatic gradient
with the climate becoming more humid toward the N
has been advocated. However, this interpretation
fails to explain the strongly contrasting characterist
ics of sections in close proximity such as those at Ars
and Châtillon-sur-Chalaronne.
The form of the arcuate ridges and mounds around
the Ars area strongly suggests an advance of the ice
The outstanding problem inhibiting differentianorthward up the Sâone valley. The site at Pouilleux
tion of the Ars lobe from the northern lobes is the is of particular interest in this respect, its core of
occurrence along the Sâone valley of the alluvions compact, imbricated lodgement till with abundant
jaunes at the top of the Trévoux plateau lacking a edge-crushed grains (Fig. 3) having a pebble fabric
drift cover or any apparent evidence of glacial eroindicating ice movement from a southerly quarter. At
La Piemonte, the orientation and dip of pebbles in sion or deformation. However, the weakness noted in
the compact but sandy subglacial till shows that ice some locations by Mandier (1984 a e.g. p. 254, 298)
movement directions varied between SW and SE ; the or absence of glacial erosion of, or glacial deposition
upon, the alluvions jaunes may be a general phenomean being slightly W of S (Fig. 4). It is likely that
menon in La Dombes. Attention has already been the same glacier which occupied the Rhône and Ain
valleys also covered the SE margin of La Dombes to drawn to this in the case of the alluvions jaunes in the
the south of Le Montellier and Rignieux, and reached Mt. Margueron and Bois de Priay areas. It is import
Rossettes-Basses. ant to note that the glacier which advanced to the 92
Seillon moraine eroded its own outwash only very (Chasse I and II) and « outer moraines » studied by
slightly at la Croix de l'Orme, and left only a very thin Mandier (1984 a, 1984 b) in the middle Rhône valley.
and discontinuous till at the surface. La Croix de These are likely to date, by comparison with Bièvre-
l'Orme is at an altitude of 260 m and only 14 km S of Valloire, from the stages GL5 to GL3 of Billard (1980,
Seillon : it was clearly overrun by the Seillon ice. The 1984).
plateau surfaces of Mt. Margueron and Bois de Priay The landform-sediment associations of La Domb
lie 70 m above 16-18 km S of the huge Seillon es have previously been interpreted either as a
moraine. The ice that built this «end moraine was subglacially-drumlinized surface or as an « ablation
clearly quite vigorous with a relatively steep gradient. moraine » deposit with subjacent « moraine de
If, as seems probable from modern analogues, the fond », bounded by an outer end moraine extending
gradient was steeper than about 1 : 240 in order to as a continuous belt all the way from NE to SW.
maintain such an active ice front, then these alluvions It has been shown above that the near-surface jaunes surfaces must have been overridden. This glacial sediments of La Dombes are dominated by phenomenon of alluvions jaunes surfaces lacking a rather coarse glaciofluvial gravels (with sorting coefdrift cover is thus found in both the E and W parts ficients in the 3.5-2.0 range) often grading into eluvia- of the plateau in relation to glacial deposits of two ted tills of meltout origin (s.c. ranges 3.0-4.0). Some different ages. Although the problem of the alluvions steeply-dipping gravels are associated with post-de- jaunes at Trévoux is not yet resolved, it is clearly not positional faulting of the water-lain units and are an isolated case in La Dombes. interpreted as a slide faciès. Subglacial lodgement
In the E, it is not known if the Seillon end moraine, tills are also present (all with sorting coefficients
related to the Tossiat surface, and the La Rippe- greater than 4.0), usually below the meltout units, at
Certines terrace belong to one or to two different Lent, Châtillon-sur-Chalaronne and Pouilleux. The
glacial stages. They both belong to the same group of particle size charateristics of the La Dombes deposits
old deposits which includes the hills of the central repeatedly show a gradational series of tills, eluviated
area and the N end moraines. In the W and SW it has tills and meltwater sandy gravels (Figs. 4, 5, 6), with
not been possible to differentiate stratigraphically occasional flow and slide units. Such relationships
between the Ars and Les Echets glacial sediments. have been recognised in a number of ice wastage
However, the evidence appears unambiguous that the areas around modern glaciers and the hummocky
SW and S parts of La Dombes, as well as the terrain associated with them has been shown to be the
Rossettes-Basses area to the N of Pont d'Ain, are product of downmelting of stagnant ice with supra-
covered by glacial sediments belonging to one or glacial and proglacial deposition predominating (e.g.
several glacial stages younger than the period or Boulton 1972 ; Clayton and Moran 1974 ; Eyles and
periods during which deposits of the central and N Slatt 1977). Thin laminated to massive clay silts are
parts of La Dombes were laid down. common in the swales between the hummocks as a
From S to N along the Rhône and Saône basin, result of deposition in temporary shallow ponds. An
there is a strong gradient in the colour of the pa- equivalent of this kind of deposit in La Dombes can
laeosols which pass from red fersiallitics in Bièvre- be seen at Le Perrochet (3.5 km NW of Ambérieux).
Valloire (W of Grenoble to the Rhône valley) to An alternative hypothesis to drumlinization is worthy
reddish lessivé soils in the area of Vienne, then of consideration, therefore, the linear mounds ex
reddish-brown to brown lessivé to the N of Lyon. It pressing the control on sedimentation during ice
is thus impossible to correlate the sediments on the wastage by radial crevasse systems in this low altitude
basis of the colour of the palaeosols developed in the piedmont lobe, with meltwater trenching an ice-cored
upper part of these deposits : they are better corre terrain of a type known as the supraglacial land
lated on the basis of the thickness of the palaeosols. form-sediment association (Paul 1983 : Fig. 7). This
Consequently, it appears reasonable to correlate the process appears to have occurred on at least two
older set of La Dombes (which includes Peyzieux, La scales and on at least two occasions in the past.
Lineaments have been objectively analysed by densi- Chapelle-du-Châtelard, Châtillon-sur-Chalaronne,
and La Rippe-Certines) with the group of high terra tometry and laser scanning of Landsat imagery. This
ces (Tourdan, Anneyron, Ternay, Malaguet) found reveals, in good agreement with the map of line
further S in Bièvre-Valloire and in the Rhône valley aments drawn from topographic sheets and field
S of Lyon. That is to say, this group of deposits is mapping (Fig. 2), two strongly linear sets (to N and
likely to date from glacial stages GL7 and GL6 to NW) in the northern and central La Dombes, and
(Billard 1980, 1984). This accords with a tentative a restricted radial set in the S. The central and N sets
sequence, counting from the top, in the clay pit at les have a well developed mode which is statistically
Carronnières, 4.5 km SW of Pont d'Ain (Billard, significant at the 99.99 and 99.0 % level respectively.
unpublished results). The younger set may therefore The southern group is not statistically significant
be better correlated with the group of middle terraces (even at the 90 % level) because of the multimodal