Val du Bourget and Haut Gresivaudan

5.2.8.1. Introduction (by Ch. Hannss and P. M. Grootes)

In the Departements of Savoie and Isere in the French inner Alpine region and its foreland, numerous lignite deposits have been found (Bourdier, 1 9 6 1 , 1962).

Samples for the Early Weichselian dating project were obtained f r o m the Val du Bourget and the Haut Gresivaudan (fig. 5.12). These t w o valleys are connected by the Cluse de Chambery. Its entrance f r o m the Isere valley is obstructed by deposits of a medieval landslide, overlying sands, gravels and moraines. This region, the Seuil des Marches, extends approximately 50 m above the valley floor of the Isere situated at 270 m.

Val du Bourget

In the Val du Bourget, between Chambery and the Lac du Bourget several lignite layers have been f o u n d on both sides of the valley at altitudes between 250 and 340 m (fig. 5.12; c.f. Bourdier, 1 9 6 1 , 1962). These lignites occur in late pleistocene deposits viz. the banquette du Tremblay on the west side and the banquette de Sonnaz on the east side of the valley. In the former we f i n d lignites in the gravel pit of the airport of Bourget, north of Montarlet (265 m ) , near Servolex (275 m ) , in the valley of the Ruisseau de Chapitre (280—290 m) and near the Cote Chevrier (285 m). In the banquette de Sonnaz we f i n d lignites in the gravel pit of Voglans (250 and 280 m), in Bouvard (260 m), near Le Fromaget (250 and 278 m), in the gravel pit of the Combe Noire (280 m), in Pessey—Sonnaz (between312 and 322 m

Fig. 5.12. Val du Bourget and Gresivaudan area.

I : banquette de Sonnaz; 1 — Voglans, 2 — La Croix Rouge, 3 — Pessey—Sonnaz.

II : banquette du Tremblay; 4 — Servolex.

I l l : banquette de Barraux; 5 — La Flachere.

and at 341 m) and north of La Croix Rouge (315 m). Most of these lignites have

£een mined.

Up till now age determinations using isotopic enrichment have been made for Voglans (250 and 280 m), La Croix Rouge (315 m), Pessey-Sonnaz (341 m) and Servolex (275 m) (fig. 5.12).

Haut Gresivaudan: La Flachere

La Flachere (45° 23' N Lat, 5° 57' E Long) is situated in the Haut Gresivaudan approximately 20 km south of Chambery (fig. 5.12). The alluvial valley floor is situated at an altitude of approximately 250 m. Late pleistocene deposits reaching up to 400 m have been preserved on both sides of the valley. The deposits mainly consist of a succession of clays, sands and gravels in which in some places organic layers occur. At an altitude of 333 to 336 m lignites have been found on both sides of the valley (Bourdier, 1961, 1962; Hannss et al., 1976).

5.2.8.2. Stratigraphy (by Ch. Hannss and P. M. Grootes)

Val du Bourget Voglans

In the gravel pit of Voglans (45° 3 7 ' N Lat, 5° 55' E Long) approximately 7 km north of Chambery t w o seams of lignite are exposed (fig. 5.13). The lower one at an altitude of 250 m is 2.30 m thick and partly forms the floor of the gravel pit.

The altitude of this seam changes much in horizontal direction. The western part of the seam is somewhat disturbed and t i l t e d , probably due t o glacial tectonics. The upper lignite seam is visible in the wall of the pit between 280 and 282 m altitude.

T o the westside of the gravel pit both lignite beds have been cut by glacial erosion.

The stratigraphy that can be observed in the exposure makes it improbable that the lower lignites of Voglans have been derived f r o m the lignite seam at 280 m. Be-tween the t w o layers fluviatile sands overlie gravels w i t h some sand lenses. The lower part of the wall could not directly be observed, because it was covered by material that had fallen d o w n .

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Fig. 5.13. Stratigraphy of the gravel pit of Voglans.

1 — coarse gravel with some sand lenses, 2 — gravel with sand lenses, number of sand lenses decreases in the higher part, the gravel becomes coarser, 3 — 280 m lignite, showing an erosion surface, 4 — dark coloured clay containing remnants of plants, 5 — stratified grey sands, 6 — medium coarse fluviatile gravel with sand lenses, 7 — cross bedded sand with gravel layers, 8 — gravel with many sand lenses, 9 — 250 m lignite, layer is disturbed and tilted, 10 — light grey unstratified clay, 11 — blue unstratified clay containing lignite lenses, 12 — grey unstratified clay.

Below the lignite at 250 m bluish grey clay was observed in many places. The thickness of this clay bed is u n k n o w n . Apparently the 250 m lignite is replaced by clay beds on the west side. Above the 280 m lignite again fluviatile sands and gravels occur, having a thickness of 50 m.

La Croix Rouge

The lignite seam of La Croix Rouge (45° 3 6 ' N Lat, 5° 5 5 ' E Long) approximately 3.5 km north of Chambery lies at an altitude of 315 m. It is exposed in the en-trance of a former, now f l o o d e d , mine. A t the base of the lignite a bluish grey clay is overlain by a silty clay containing little branches. Higher up the bluish grey clay gradually changes into dark organic clay which finally grades into lignite w i t h some embedded clay layers. The total thickness of the organic clay and lignite is approxi-mately 2 m. Below we f i n d sands and gravels of u n k n o w n thickness. This material is obviously of moraihic origin (c.f. Vivien, 1896, p. 20).

Fig. 5.14. Stratigraphy at Pessey—Sonnaz.

1 — fine sand, orange, well stratified, 2 — silty clay, light grey, 3 — lignite, rich in clay, containing molluscs, 4 — clay, locally exposed, containing sand lenses, 5 — bluish grey clay, increasingly sandy near the top, 6 — thin lignite layer, rich in clay, containing molluscs, 7 — dark blue com-pacted clay, 8 — sand, 9 — lignite, very rich in clay, 10 — clay containing plant remnants, 11 — stratified, bluish grey clay containing a few rem-nants of branches, 12 — unstratified gravel in sand bed, about 50% limestone pebbles, 13 — clay con-taining a 1 cm thick lignite layer, 14 — main lignite bed, 15 — dark silty clay containing wood remnants, 16 — silty, bluish grey clay.

Pessey—Sonnaz

In Pessey—Sonnaz (45° 37' N Lat, 5° 55' E Long) approximately 5 k m north of Chambery lignite seams have been found between 312 and 322 m and at 341 m (fig. 5.14). The lowest bed of lignite between 312 and 314 m has a considerable thickness. It has extensively been mined. It is probably the same seam as found near

La Croix Rouge about 1 k m t o the south. I t was not exposed when the samples were taken. Under the principal lignite seam clays containing molluscs, thin lenses of lignite and bones of fishes and mammalia have been f o u n d (c.f. Mortillet, 1850a, p. 116; 1850b, p. 2 0 6 - 2 0 7 ) . Between 314 and 318 m t h i n lignite layers alternate with beds of clay. These accumulations are covered by a sand layer of 3 m thickness and 2 m of clay. In the clay a t h i n band of lignite has been observed at 322 m.

A t 341 m t w o t h i n ( ^ 3 cm) layers of lignite, separated by 10 cm of clay rich in molluscs, are found embedded in a bluish grey, locally dark organic clay layer. The highest parts of the banquette de Sonnaz consist of sands and gravels. On t o p of these layers we f i n d morainic material of the Last Glacial. Therefore the lignites most probably have been formed prior to the last large glacial extension.

Servo/ex

Near Servolex (45° 3 6 ' N Lat, 5° 52' E Long) approximately 6 km north-west of Chambery lignites have been exposed at an altitude of 275 m in several places along the slopes of a highway and other roads under construction. They were also found in t w o borings. The lignite of Servolex is generally rich in wood and frequently contains pine cones. A m o n g the fossil woods spruce (Picea) is most common. The lithology of the lignite changes rapidly in horizontal direction.

Below we f i n d lacustrine clays containing pieces of wood and lenses of lignite.

Generally, the lignite of Servolex is covered by gravels. The banquette du Tremblay, in which these lignites are f o u n d , is locally Covered by t i l l of the Last Glacial. This can especially be observed in the gravel pit of the Ruisseau des Combes. Near Servolex the till even contains small parts of lignite.

v Haut Gresivaudan: La Flachere

Along the road between la Buissiere and la Flachere a number of lignite seams are exposed at an altitude of 333 to 336 m intercalated in unstratified bluish grey clay containing molluscs (c.f. Depape and Bourdier, 1952, p. 83). Because the under-and overlying deposits are not exposed at this place the stratigraphy has t o be deduced f r o m observations at other localities (fig. 5.15).

Below the lignites probably sandy fluviatile deposits exist resting on t i l l w i t h boul-ders. These layers can be observed in the gravel pit in La Gache. Bourdier (1962, p. 24—25, fig. 24) considers the till t o be of Rissian age. Hannss et al. (1976) favour the opinion that it belongs t o a glacial advance during the beginning of the Wurm.

On t o p of the lignites 40 to 50 m of fluviatile gravel deposits are f o u n d , containing both limestone pebbles and igneous material. A t 348 m these layers are interrupted by 0.5 m of unstratified clay. On t o p of the gravels a bank of pure limestone

380 m

Fig. 5.15. Stratigraphy of the banquette de Barraux near La Flachere.

1 — slope deposit of angular limestone debris, 2 — band of loam and clay, 3 — unstratified gravel, almost without limestone, 4 — slope deposit consisting of reworked gravel, 5 — unstratified coarse gravel, 6 — gravel bed forming a conglomerate, 7 — unstratified clay, 8 — lig-nite, rich in wood, 9 — bluish grey clay, 10 — unstratified bluish grey clay with molluscs.

deposits has been observed. On the surface we find locally some morainic material belonging to the Last Glacial.

5.2.8.3. Pollen analysis

Val du Bourget (by W. H. E. Gremmen)

In this report we present simplified pollen diagrams. Only pollen curves are shown which are of importance for the purpose of this paper, viz. a short characteristic of the past vegetation and its climatic implications. The complete diagrams and a more comprehensive discussion will be published later.

La Croix Rouge

Pollen diagram 1 (fig. 5.16) shows results of the palynological examination of the lignite layer at La Croix Rouge, about 315 m above sea-level. The bottom zone of the diagram (samples 1—8) is characterized by a dominance of Pinus, while

Artemisia pollen values are high in the lowermost samples. In the upper part of the zone the Pinus values decrease. In the succeeding zone (samples 9—14) coniferous pollen values are low, whereas thermophilous trees are fairly well represented.

Particularly Cory/us, Ulmus and Quercus were frequently f o u n d . In the upper zone of the diagram (samples 15—32) coniferous pollen, specifically Picea, again dominates. In the upper part of this zone Artemisia reaches high values. On the basis of the fluctuations in the curves for Pinus and Picea and in the A P / N A P ratio, the upper zone can be subdivided into 4 subzones. In sample 25 the value of Betula pollen is calculated as a percentage of the pollen sum excluding Betula.

The middle zone of the diagram reflects the vegetation of a relatively warm period. In sample 11 the sum of thermophilous tree pollen values (Corylus, Quercus, Ulmus, Fraxinus, Tilia and Acer) amounts to about 80%. This warm period is preceded as well as succeeded by much colder periods. During these, of the thermophilous trees, only Corylus could maintain itself to some extent.

Although the middle part of the diagram points t o climatic conditions which were favourable for the expansion of thermophilous trees, the pollen evidence does not point t o an interglacial vegetation development. Very probably the duration of the warm period was rather short. Consequently, the lignite of La Croix Rouge, or at least its middle part, should have been deposited during an interstadial period, probably of Early-Weichselian age.

Vog/ans II

Near Voglans t w o lignite layers are f o u n d , viz. Voglans I at about 250 m above sea-level, and Voglans II at about 280 m. The layers are separated by about 30 m of coarse sands and gravels.

Some of the results of the palynological examination of the upper lignite (Voglans II) are shown in diagram 2 (figure 5.17.a). It should be mentioned that only the spectra w i t h odd numbers are included in this diagram. A t the b o t t o m of the diagram a conspicuous AInus dominance can be observed, while thermophilous trees, such as Corylus, Carpinus, Ulmus and Acer, show relatively high pollen per-centages. Between spectra 3 and 5 Picea increases markedly, while the rise in the Pinus curve takes place above spectrum 5. From spectrum 5 onwards, up to the t o p of the diagram, coniferous pollen dominates. However, the continuous curves, w i t h values of a few percent, for thermophilous trees indicate that these species could maintain themselves in the area.

The pollen percentages in spectrum 3 are biassed by the anomalously high local AInus values. If AInus is excluded f r o m the pollen sum, thermophilous trees show fairly high percentages, whereas those of Picea and Pinus remain low. In view of the good representation of Carpinus, w i t h a corrected pollen value of circa 33%, we are probably dealing w i t h a section of the Eem Interglacial. In that case the larger part