La "Longue séquence" pliocène de Marchésieux (Manche). Résultats analytiques et premiers éléments d'interprétation

Abstract

The palaeoclimatic records of the past five million years, reconstructed from oceanic sediments, illustrate the control of orbital parameters (precession, obliquity and excentricity) on global climate. The cooling trend that accompanied the end of the Neogene was also marked, towards -2.4 Ma, by the rapid development of ice sheets in the northern hemisphere. Although the history of the world’s climate reconstructed from oceanic records is becoming increasingly detailed, data concerning the evolution of coastal and continental environments during the course of these changes remains mainly fragmentary. The objective of Project AR42, financed by the Scientific Division of BRGM, was to locate and analyse the beginning of the major Plio-Quaternary climatic change (Pretiglian estimated at -2.4 million years) and to highlight the quantification factors and their impact on the environment (Courbouleix et al., 1992; Garcin et al., 1993). Considering the aims of the Marchésieux “Long Sequence ” study, it was necessary to use a multidisciplinary approach incorporating naturalistic as well as physical methods. The drilled target is at Champeaux in the Marchésieux district of the Manche Department (x = 335.84; y = 1172.29; z = + 9 m NGF(1)). The continuous borehole core provided 159.3 m of sediments covering a period of about 1 million years and dated as Reuverian and Pretiglian. It enabled the first episode of glaciation in the northern hemisphere (-2.4 Ma) to be analysed and probably the first period of cooling (about - 3.1 Ma) in the basal part of the borehole. The analyses have provided a great deal of data that now enable a fine reconstruction of the responses to climatic change in both the marine environment (sedimentology, evolution and modification of faunal populations, etc.) and continental environment (evolution of the vegetation, resumption of erosion, formation of acolian dunes,etc.). The marine palaeoenvironment. Four units have been distinguished from bottom to top of the borehole: - First unit (160 to 100 m): from the contact with the Permian basement at 160 m, homogeneous bioclast-poor clayey Sand. The mineralogical composition of the clays in the sediment close to the basement/fill contact shows a clear predominance of illite and a high percentage of chlorite. The probable clastic origin of these clays indicates the nearby presence of a crystalline basement that was undergoing weathering, with the weathered products being transported over a short distance before being redeposited when the basin filled with water. The depositional environment of this unit corresponds first to the lower infralittoral and then to the upper circalittoral, i.e. to depths of about 30 to 40 m evolving to 40 to 80 m. The palaeobiocenosis is typical of a ‘Coastal Detrital’ or ‘Silted Heterogeneous Sand’ environment with mainly weak unidirectional current hydrodynamism and relatively little detrital deposition. The aquatic vegetation is poorly developed, although the ostracods are represented mainly by forms associated with phytal environments. The sea bottom with Spongiae, the scleractinians, the stylasterids, and the bioturbations all indicate a well-oxygenated environment with normal salinity. The upper part of this unit (140 m to about 100 m) yielded the only calcareous nannofossils of the series, albeit not very abundant, indicating the most open conditions to the ocean for the whole of the “Long Sequence”, which is only weakly oceanic. The organisms in this interval for the most part indicate fresh water. Note in particular the development of bottoms with siliceous Spongiae reminiscent of present day Nordic bottoms. This observation is confirmed by the 16O/18O isotopic curve which shows, over this interval, stable values with a low variability and a mainly cold trend. Second unit (100 to 65 m) constituted of highly bioclastic heterogeneous clayey sand with many basement lithoclasts; tracks of organisms on the bottom beds are to be noted. Siliceous sponge spicules have disappeared, glauconite grains become increasingly scarce and driftwood debris originating from the continent are reported (microfacies and palynofacies). This unit corresponds to an upper circalittoral environment where the depositional energy was on the whole clearly greater than that of the first unit. The hydrodynamism corresponds to slightly bioclastic silty sand in movement over a gravelly bottom; it is accompanied by an increase in detrital deposits from the continent. The bottoms seem richer in aquatic vegetation, as indicated by the presence of grazing organisms. They were well oxygenated and some currents rich in nutritive deposits were probably present. The water temperature seems to increase from the bottom to the top of the unit: this observation is confirmed by the 16O/18O isotopic curve which shows a general warming trend in spite of much greater variability than previously. Two peaks are noted which tend to indicate a cooling at borehole depths 97/98 m and, more particularly, around 80 m; these two peaks, however, fall within the overall warming trend. Third unit (65 to 32.4 m): the upper limit of the unit corresponds to the gullying contact of the “Faluns de I’Abbaye des Bohons” over the Bosq d’Aubigny Formation. The sediments of this unit comprise heterogeneous bioclastic clayey Sand; the bio/litho ratio shows a clear downward trend in the upper part, corresponding to the appearance then increase in the number of Spongiae spicules. The clayey intervals are characterised, from 62 m up, by the appearance of irregularly interbedded non-swelling minerals which correspond mainly to an inheritance of detrital minerals accompanied by their weathering alteaution products. The depositional environment is on the whole charactorised by the circalittoral infralittoral passage zone (40/50 m). The type of sediment characterises either “Coastal Detrital” or “Silted Heterogeneous Sand”. Signs of euxinic bottoms from 65 to 44 m are to be noted; above, the bottom sediments do not seem to reflect any oxygen deficiency and one notes the establishment of bottom beds rich in Spongiae, indicating the existence of nutritive currents. Signs of transport and of faunal mixing reflect the presence of active bottom currents. The submarine vegetation is rarer than in Unit 2, as indicated by the scarcity in grazing organisms. From the climatic point of view, many signs of a degradation in a cold environment are indicated by the organisms (foraminifera, ostracods, cirripeds, bryozoans, etc.) and this was very likely accompanied by a drop in sea level foreshadowing the stop of sedimentation at 32.4 m. The oxygen isotopic curve envelope confirms the signs of climatic cooling and, at the same time, shows great variability. Fourth unit (32.4 to 0 m): constituted of heterogeneous sandy calcarenite. These facies characterise bioclastic sand in movement over the bottom; only a few horizons reflect silting up. The depth of deposition of this unit is compatible with that of the upper circalittoral stage, the environment reflecting high-energy sedimentation with signs of reducing bottom conditions. The malacofauna constituting a large proportion of rock is not very diversified, essentially represented by Pectinidae; the absence of grazing organisms leads one to presume a fairly poor submarine vegetation. From the climatic point of view, this assemblage can be split, according to the isotopic curves, into two parts: - from 32.4 m to about 18 m one notes a clear warming trend that is suddenly interrupted at about 18 m; - from 18 m to the top marks a new warming trend, although the vartabiltiy of the isotopic values is high. The continental palaeoenvironments. On the continent, other than the hinterland with continuous vegetation represented mainly by Pinus silvestris, the evolution of the landscape from the bottom to the top of the sequence is generally as follows: from 158.50 m to 142 m, firmly established trees of dry and temperate forest environments then, from 142 m to I39 m, one notes an extension of marshy forest conditions with Taxodiaceae and also several signs of aeolian sand transport. From 130 to 110 m, the Taxodiaceae regress to be replaced by Pinus haploxylon. The following period sees the extension of the salt marsh near the shoreline and the development of fresh water aquatic plants (from 104 to 99 m). From 87 m, both the Taxodiaceae and Pinus haploxylon regress, exposing the hinterland to more active erosion which was responsible for the large influx of basement lithoclasts in the basin. At 68 m the arboreal vegetation diminishes on the continent: progression of Ericaceae moors, development of alders, Cyperaceae, and spores of bryophytes (sphagnum) and pteridophytes (ferns). Broad-leaved trees such as Quercus regress. Between 44 and 35.8 m the Ericaceae moor is at its maximum and the climate becomes colder. From 32.4 m to 20 m, one notes the strong resumption of wind-transported sand characterising the remobilisation of a large quantity of aeolian grains (dunes) accumulated during the immediately preceding glacial episode. This had led to a drop in sea level giving rise to emergence of the region and exposing large areas that were less protected on account of the regression of the arboreal vegetation with aeolian deflation. The shell sands and green clay at the top of the sequence reveal an arboreal vegetation dominated by gymnosperms (Pinus silvestris, Sciadopytis and Cupressaceae). Stratigraphy. Due to its favourable geographic position and its coastal environment, the Marchésieux “Long Sequence” has made it possible, in agreement with the physical and biostratigraphic data, to clarify the stratigraphic correlation and appraise the detail of the first stages of climatic degradation of the first Plio-Quaternary glacial episode (Pretiglian: -2.4 Ma). This study has, furthermore, demonstrated the complementarity of naturalistic, physical and geochemical methods in a project of this type.