Particularités de l’évolution du système fluviatile solognot dans le bassin de la Loire au Quaternaire. Corrélations possibles

Abstract

A stratigraphic scale for the Quaternary, based on stepped terraces and weathering facies of the deposits, has been proposed for the middle Loire (“Ligerian system”) in the Cretaceous rim of the southwestern Paris Basin (Macaire, 1981 and 1986). In Sologne, a sub-basin within the Loire basin (“Sologne system”), recent mapping has revealed multiple terraces associated with the “old soil” facies described by Honorato (1971). The present article compares the two stratigraphic scales and suggests equivalences. The pre-Ligerian hydrographic network of the Neogene can be divided into two zones: - one corresponding to Sologne, the hydrographic hub and permanent centre for the flows and deposition of particles from the Massif Central, - the other, with a course fluctuating between N-S and E-W, situated upstream and downstream of Sologne; the associated deposits are very discontinuous. This differentiation is also found in the “Ligerian” hydrography of the Quaternary. Quaternary river deposits of the middle Loire From Nevers to Angers, the Loire flows through Mesozoic and Cenozoic formations of the southern Paris Basin (Fig. 1), its valley ranging from 2 to 20 km wide and 40 to 80 m deep, depending on the section. River alluvia are limited to a maximum lateral extent of 20 km, and cover only a small part of the valley sides that supplied them; they are arranged in a sequence of terraces, the number of which varies from section to section (Table 1): - Between Nevers and Gien, 5 to 6 levels of terraces can be observed: these include the Bourbonnais sands and clays at about +50 m above the Loire, a high terrace (+30 m), a middle terrace (about +10 m), a low terrace (+5-6 m), and the low alluvial plain. - In the Sologne around Orléans, the terraces are laterally extensive, with the “Upper Gravels” or “pre-Ligerian Gravels” (Denizot, 1927) on the plateaus, and levels Fv (+30 m), Fw (+15 m), Fx (+6-8 m) and Fy (“hillocks”, +2-3 m) above the flood plain Fz (Gigout et al., 1972). The Fv and Fw levels can combine to form a polygenic glacis. - In Touraine, the plateaus are covered with “post-Helvetian” sands and gravels (about +40-50 m above the Loire), from which the upper sand/gravel unit has been assigned to the Upper Pliocene (“montdorian” minerals). The terraces comprise Fv (+30-35 m), Fw (+15-20 m), Fx (+8-10 m) and Fy (+5-6 m) above the Loire flood plain Fz (Macaire, 1981). In its middle section, the Loire has cut through plateaus covered with Upper Pliocene deposits (Fig. 2), and 5 or 6 separate morphological flats have been itemized in the valley, including the present-day alluvial plain (Fig. 3b). A similar number of terraces are identified along the river’s major left-bank tributaries, the Cher, the Indre, the Vienne and the Creuse (Macaire, 1981, 1983). The river deposits are dominated by sand/gravel and are rich in minerals (Babonaux, 1970; Gigout et al., 1972; Tourenq, 1972 and 1989; Macaire, 1981; Brossé, 1982). Over 50% of the gravel consists of granite, metamorphic rock, basalt and quartz; limestone and flint make up the local drift. The sand, mainly quartzose, contains 30% feldspars (including 1/3 plagioclases), minor limestone and some mica; the heavy minerals (3-4% of the grains) are varied and include mainly pyroxenes (70-80% of the transparent grains). The older drift is poorer in minerals than the recent drift (Macaire, 1985). Weathering has produced “old soils” with differing facies and mineral paragen-eses that have been used as the basis of a lithostratigraphic scale (Horemans, 1961; Macaire, 1981 and 1986) (Fig. 3a). Thus, the sub-present Fz unit contains drifted soils that are poorly to non-evolved; the Fy unit is lightly weathered and displays a yellowish-brown argilloferric vermiculite matrix (Bt horizon); the Fx unit is moderately weathered and has a brown smectite and kaolinite matrix (Bt); the Fw unit is strongly weathered, reddish brown (Bt), rich in kaolinite and displays a bleached degraded A2 horizon; and the Fv unit is very strongly weathered, variegated (Btg), rich in smectite and kaolinite and degraded in its upper part (A2). The Fv facies represents the final degree of evolution. River deposits in Sologne The Miocene sand/clay deposits of Sologne are drained by the Cosson, the Beuvron and the Sauldre which make up a sub-basin (5000 km2) within the Loire Basin (Figs. 1 and 4). Quaternary river alluvia cover approximately 90% of the area. The valleys are less than 40 m deep, but up to 8 terraces are observed (Fig. 5b). The “Upper Gravels” (“pre-Ligerian Gravels” of Denizot [1927], and annotated Fv or Fwa on different maps), including Upper Pliocene “montdorian” minerals, partly cover the plateaus (Fig. 2). To the south and southeast, these gravels grade into alluvial fans (Fv), below which the Fwb (+15-30 m), Fx (subdivided into Fxa, Fxb and Fxc; +10 to +30 m), Fy (subdivided into Fya and Fyb; +2 to +15 m) and Fz or Fy-z (flood plain) levels are identified (Table 1 and Fig. 6). The Quaternary alluvia of the Sologne sub-basin are more clayey and poorer in minerals than those of the Loire. The sands are mainly quartzose with exclusively potassium feldspars, ubiquitous heavy minerals and scarce metamorphic minerals, as in the Sologne formations from which they derive (Rasplus, 1982). They also include quartz and flint gravel and pebbles. The weathering facies and pedogenesis show similarities with those of the “Ligerian” system, and also some specific features described by Honorato (1971) (Fig. 5a and b). The most recent soils (units 1 and 2) are mostly influenced by hydromorphic processes. Units 3 and 4 are characterized by an argilloferric Bt horizon which is brown (unit 3) to reddish (unit 4), respectively similar to the Fx and Fw units of the “Ligerian” system. The bleached A2 horizon is also present though, unlike in the “Ligerian” system, it affects the Bt horizons at an early stage of their development (unit 3). The clay minerals (prevalence of montmorillonite over kaolinite and illite) mostly reflect a legacy of the Sologne formations (Honorato, 1971). The most evolved facies (unit 5), not described by Honorato, was observed on the plateau gravels. Comparison of the “Ligerian” and “Sologne” systems The number of terraces and the amount of river-deposit cover in the “Ligerian” and “Sologne” basins reflect different sedimentary balances and river dynamics (Meade, 1988). The Loire has a fairly steep longitudinal gradient (5 to 0.5‰) and thus only a small proportion of the sedimentary materials from the Massif Central has been stored due to the river’s rather substantial hydraulic energy. In Sologne, where the unconsolidated Neogene substratum was easily eroded, the low gradients of the Cosson, the Beuvron and the Sauldre (0.2 to 0.5‰) and the valley sides have limited the removal of sedimentary matter by running water; this accounts for the high amount of alluvial sediment cover (around 90%) in this sub-basin. The morphological evolution of Sologne during the Quaternary can be seen on the DEM image (Fig. 7): erosion has tended to remove the Upper Gravels. Also noted are the recent reactivation of the main regional faults, the N 80° E Bourbonnais palaeochannel (Upper Gravels), and the dissymmetric character of the present valley sides, which is probably of tectonic origin. The river terraces of the middle Loire basin are of climatic origin (Gras, 1963; Gigout et al., 1972; Macaire, 1981). The Loire’s hydrodynamics and the production of sediments were strongly influenced by the snow cover and the glaciers of the Massif Central during the glacial stages (Goer de Herve, 1972; Veyret, 1978; Church and Ryder, 1972). The cold and wet periods were periods of high sediment storage in the middle Loire, in sharp contrast with the river’s typical, mainly erosive activity. In the Sologne sub-basin, the Quaternary climatic fluctuations were less marked than in the Massif Central. However, the high number of terraces shows that the hydrodynamics and the particle flows were sensitive to these fluctuations. The Loire’s highly sandy alluvia were unfavourable to the development of hydromorphic processes, since the fragile minerals supplied an abundant argilloferric matrix which gradually plugged the Bt pedological horizon (Fv facies). In Sologne, however, the more clayey recent alluvia (units 1 and 2) favoured hydromorphic processes; the mineralogically poor sands produced only a sparse argilloferric matrix (units 3 and 4), except in the Pliocene plateau deposits (facies 5). In both systems, the degraded A2 horizons point to a relatively long period of pedological development —subsequent to the deposition of unit 3 and associated with podzolisation in Sologne (Honorato, 1971), and prior to the deposition of unit Fx and of palaeoclimatic origin in the Loire (Macaire, 1981). Degradation was furthered by a mineralogically poor material, which may account for its prompt occurrence in the sandy Sologne alluvia (3 to 5). Possible equivalences between the terraces and the alluvial lithofacies of the “Ligerian” and “Sologne” river systems are suggested in Table 1. Conclusion Despite their geographical proximity, the hydrodynamics and the sedimentary balance of the middle Loire and the Sologne differed during the Quaternary. This can be explained by the fact that each of these river basins had its own tectonic, morphological, lithological and climatic characteristics.