Résumé
Au Pliocène supérieur, sur la bordure subalpine du bassin de Valensole, les apports caillouteux deviennent très abondants et la fin du remblaiement est marquée par la mise en place de grands cônes torrentiels dont la brèche de Balène est l’un des principaux témoins. L’augmentation en volume et en gabarit des cailloutis est corrélée avec la tectonique de l’arc de Castellane. Vers l’aval, les cônes convergent en direction de la cuvette lacustre de Puimoisson où les apports caillouteux s’interstratifient localement avec des dépôts lacustres ou palustres. Plusieurs gisements de mammifères permettent une chronologie des dépôts lacustres. Le gisement de Cornillet (zone de Saint-Vallier, vers 2 Ma) situé entre les deux premiers bancs de la brèche de Balène, date en ce point le début de la progradation des cônes torrentiels. Le nouveau gisement de Valensolette, situé pratiquement au toit des cailloutis, indique un âge compris entre 1 et 0,7 Ma. En raison de ce contexte favorable, une étude magnétostratigraphique a pu être entreprise. Le corps de la brèche est entièrement de polarité négative à l’exception de deux niveaux de polarité directe, l’un situé vers sa base, un peu au-dessus du gisement de Cornillet, l’autre vers son sommet, approximativement au niveau du gisement de Valensolette. Ainsi, globalement la brèche prendrait place dans la période inverse de Matuyama, débutant dans la première partie de cette période (2,6-1,77 Ma), un peu avant l’événement d’Olduvai (1,95-1,77 Ma) et prenant fin dans la deuxième partie de la période de Matuyama, après l’événement de Jaramillo (1,05-1 Ma). La continuation des dépôts caillouteux dans ce secteur du bassin, comme leur persistance bien après la fin du chevauchement, impliquent deux conditions : 1) un nourrissage important et permanent en matériel détritique, vraisemblablement sous une double influence, tectonique et climatique ; 2) la stabilité du niveau hydrographique local.
Mots-clés : Faune mammifères, Biostratigraphie, Paléomagnétisme, Pliocène supérieur, Pléistocène inférieur, Morphodynamique, Alpes de Haute-Provence, Bassin Valensole
Abstract
The wide distribution of end-Pliocene torrential gravelly deposits along the Subalpine border of the Valensole Basin, of which the Balène breccia is the most important, indicates drastic erosional conditions. The causes were either tectonic, associated with the Subalpine thrust, or climatic, but both may be involved. The stratigraphic record is apparently continuous and sedimentology reveals a succession of environmental events. The aim of the present paper is to put these events into a chronological frame and assess their importance and their relationships. Topmost facies The different topmost facies and their lateral variations (Fig. 1) illustrate the diversity of environments encountered in the Subalpine piedmont. The fluvial conglomerates of the Asse and Verdon paleo-rivers are a common type of deposit in the area, whereas the Puimoisson basin was a site of lacustrine or palustrine marly and limy sedimentation. Local fans deposited at the foot of the reliefs during the Late Pliocene, of which the most important is the Balène breccia (Dubar, 1984), converge towards the Puimoisson basin and provide evidence of an important increase in coarse deposits. The gravelly layers contain interbeds of lacustrine or palustrine origin (Fig. 2), the oldest of which (interbedded between the first two gravelly layers) is remarkable because of its extension and of the fact that it includes the Upper Pliocene fossil mammal-bearing locality of Cornillet. Near the paleo-valleys of the Asse and Verdon rivers, gravelly deposits are so important that they constitute a well individualized formation that was deposited just before the cutting of present valleys. Deformation at the margin At the foot of the Subalpine thrust on the northeastern border of the basin, the Pliocene formation is tilted at the contact of the calcareous reliefs. In the Ponteilhas thalweg (Fig. 1), the tectonized formation dips steeply and is exposed over a 70-m-thick section; Lower Pliocene conglomerates and red paleosoils are observed below the whitish marly deposits of the Puimoisson Formation (Upper Pliocene), and the Balène breccia (10 m thick) ends the sequence at the top. This section represents an interval of ca 1 million years. The lower levels dip more steeply (ca 75°) than the upper ones (ca 10°), with the dip decreasing upward – a pattern that indicates synsedimentary deformation. It can be inferred that the Subalpine thrust was due to continuous tectonism and that the gravelly deposits of the upper part of the sequence testify to the progression of the Subalpine thrust front. Paleontologic and paleomagnetic data Mammals Several faunas are recognized from the Puimoisson, Grenouillet and Les Deffends localities in the Puimoisson area and the Ségriès, Cornillet, Plaine de Balène and Valensolette localities in the Ségriès area (Figs. 1 and 2). The paleontological data are: Puimoisson N.D: Large mammal remains found more than 20 years ago indicate the Montpellier - Roussillon fauna (Guérin et al., 1970), and recently collected fossils of Hipparion crassum and Anancus arvernensis (discovered 30 m below the top of the Puimoisson Formation and very similar to specimens from Montpellier) give strength to the former dating. Grenouillet: The fauna found in a bed 15 m below the top of the Puimoisson Formation includes the following rodents (Bachelet, 1990): Mimomys cf. gracilis, Mimomys cappettai, Stephanomys cf. donnezani and Apodemus dominans. It is roughly contemporaneous with the fauna of Sète, a reference fauna of the biochronological scale for small mammals. Les Deffends: Equus stenonis has been collected in a bed 5 m below the top of the Puimoisson Formation (Biquand et al., 1990). Ségriès: A small mammal fauna has been found in the upper part of the Puimoisson Formation (Guérin et al., 1970). The presence of a large Stephanomys (S. cf. thaleri) and of Castillomys crusafonti indicates a fauna more or less contemporaneous with the fauna of Seynes, which defines an immediately younger level than that of Sète on the biochronological scale for small mammals. Cornillet: The fauna has been collected 10 m above the level containing the Ségriès fauna. It was extracted from a lacustrine marl which lies above a several-metre-thick gravelly deposit and just below the Balène breccia. Eleven species of large mammal have been recognized, similar to those of the Saint-Vallier fauna dating from ca 2 million years (Debard et al., 1994). Plaine de Balène: One molar of a much evolved Equus stenonis has been found on the surface of the Balène breccia (ref. in Dubar, 1983 b). Valensolette: This recently discovered locality is at the distal edge of the Balène breccia (Fig. 1). Fossils have been found near the top of the breccia in a bed of colluvial origin (Fig. 2 B) located at the base of a gravelly deposit. Two species of rodents have been determined: Microtus pliocaenicus and Mimomys cf. savini (Fig. 3). The stage in evolution of M. pliocaenicus and the presence of Mimomys cf. savini indicate the lower part of the Biharian small mammal stage, which is at the top of the Lower Pleistocene (Fejfar and Heinrich, 1990). In conclusion, the Cornillet and Valensolette localities provide dating for the Balène breccia: the former, which belongs to the upper part of the lacustrine marly and travertinous Puimoisson Formation, gives a lower boundary age, whereas the latter dates the ultimate deposits of the breccia. Paleomagnetism In a previous study of the uppermost 20 m of the Puimoisson Formation (Biquand et al., 1990) a direct polarity was recognized for the first 10 metres and a reverse one for the last 10 metres. The polarity reversal was interpreted as the boundary between the Gauss and Matuyama epochs (ca 2.6 Ma). In the Balène breccia, which overlies the Puimoisson Formation, paleomagnetic study was possible only for the silty or travertinous beds interstratified in the gravelly deposits. Ten levels distributed among eight sections have been analysed (Figs. 1 and 2), which demonstrates the discontinuous character of the study. The analyses were conducted with a high sensibility inductometer (JR4 type, Geofysica, Brno) in the Laboratoire de Géomagnétisme de St Maur. Eight out of the ten levels show a reverse polarity (Fig. 4a) from which it can be interpreted that the Balène breccia belongs to a reverse epoch. This interpretation agrees with that of Biquand et al. (1990) as the upper part of the Puimoisson Formation, which lies immediately below the Balène breccias, belongs to the first part of the reverse Matuyama epoch (1.95 - 2.6 Ma). Two levels in the Balène breccia appear to have a normal polarity (Fig. 4b). The first recognized at two different points of the Balène breccia (profile A, no. 3 and profile B, no. 3’), and the second corresponding to the Rème - Valensolette section (profile E, no. 9) at the top of the gravelly deposits. New paleomagnetic measurements made on further samples from these levels confirm the normal polarity. The loess from the top of the Balène breccia has a reverse polarity. Taking into account the biochron-ological data, the paleomagnetism of the Balène breccia can be interpreted as belonging to the Matuyama reverse epoch. Two levels are of importance; the Cornillet level (ca 2 Ma), which gives a lower boundary age for the base of the breccia, and the Valensolette level (between 1 and 0.7 Ma), which dates the uppermost deposits of the breccia. According to our preliminary interpretation, the two levels of normal polarity could represent the Jaramillo event (1 - 1.05 Ma) for the youngest, and the Olduvai event (1.95 - 1.77 Ma) for the oldest. The time interval during which the Balène breccia was deposited was thus almost 1 Ma, with deposition starting ca 2 Ma ago (Fig. 5). The age and meaning of the Balène breccia. Neogene to Quaternary morphodynamics of the basin Over a period of 1 million years, there was an important increase of gravelly deposition, with the torrential fans of the upper part of the Valensole Basin being deposited during the end Pliocene and Early Pleistocene. This perenniality has two implications: 1 - The gravelly drift was important and constant, resulting from the tectonism that gave rise to high reliefs at the border of the basin. There was also a probable climatic influence at altitude producing cryoclastic debris (Bourdier, 1962; Gigout,1969; Dubar, 1972). 2 - The local hydrographic base level was stable, which can be inferred from two facts: first, the alluvial deposits are everywhere present in an uppermost position, and second, the Puimoisson basin existed over a long period of time. The downcutting of local valleys began very late (i.e. since the Valensolette fauna) in comparison with the downcutting of the Durance river, which represents the major drainage of the area. The lower boundary age (1.7 Ma) for the downcutting of the Durance river was deduced from paleomagnetic data obtained on terraces of this river (Dubar and Sémah, 1986). The discrepancy between these ages is still not understood.
Key words: Mammals, Biostratigraphy, Palaeomagnetism, Upper Pliocene, Lower Pleistocene, Landform evolution, Alpes de HauteProvence, Valensole Basin (France).
Dernière mise à jour le 02.07.2015