Stratigraphie du Plio-Pléistocène inférieur de Normandie : les séries marines et fluviatiles des bassins du Seuil du Cotentin

Abstract

Thick Plio-Pleistocene deposits are accumulated in small grabens within the older rocks of the Armorican Massif in the Cotentin (Normandy), and more particularly in the Seuil du Contentin basins which have been subject to a host of lithological and faunal studies. To explain the geometry of the different sedimentary units, the most current hypothesis is to evoke a synsedimentary tectonic activity during the Plio-Pleistocene infilling of the basins. Based on a detailed sedimentary revision of the outcrops and borehole data for these basins, we propose a new stratigraphic pattern that simplifies the previously accepted Plio-Pleistocene chronostratigraphic divisions. Six formations are thus described (Grès coquilliers de Marchésieux Fm., Marnes du Bosq d’Aubigny Fm., Falun de Bohon Fm., Marnes de Saint-Nicolas-de-Pierrepont Fm., Sables de Saint-Vigor Fm. and Sables de la Lande de Millières Fm., recording the passage from marine sedimentation to fluviatile deposition. The determination of a bay-type sedimentary environment beneath the Sables de Saint-Vigor Fm. explains the lateral coexistence of several formations in the Seuil du Cotentin basins. Abridged English version The Armorican Massif contains rare outliers of Pliocene to Early Pleistocene sedimentation in Normandy (Fig. 1). In the Cotentin, such deposits are preserved in small kilometre-size Cenozoic grabens, the largest of which is the Sainteny-Marchésieux graben (Fig. 2). It is bounded to the north by the Paleozoic North Contentin block and to the south by the Precambrian Coutançais block, and lies between the Lessay harbour to the SSW and the Baie des Veys to the NNE. The presence of such Plio-Pleistocene deposits in the Seuil du Cotentin basins was long suspected. But it was only in the last 30 years, with the drilling of many hydrogeological and stratigraphic boreholes, that it has been possible to determine the extensive richness, complexity and large thicknesses of such series in this marshy region with almost no outcrop (Fig. 3). A stratigraphic division of the series is difficult when based only on the study of borehole data, and this mainly cuttings with very little core. Lacking any reference sections, the deposits have historically been divided into a multitude of lithological units with no determination of their geometric relationships and often of their depositional environments (Table 1). The geometry of the Seuil du Cotentin basins has been explained as a simple stratigraphic superposition of formations with no sedimentary inter-relationships. In this hypothesis, the basins present a mosaic of tectonic strips at times subsiding and thus trapping deposits, at times uplifted and enabling a stripping of the earlier deposits by erosion. Through successive and repeated movement of the blocks in both time and space, the geometric relationships between two contiguous facies appear anomalous and compartments must therefore be separated by faults (Fig. 4). The reinterpretation of the series based on a systematic revision of the outcrops and a re-examination of the material from the many available boreholes drilled by auger or rotary bits, and at times cored, leads to another sedimentary model for the infilling of the Sainteny-Marchésieux Basin (Fig. 5). The Plio-Pleistocene sedimentary series is here described as thick sandstone at the base, overlain by marl with limestone intercalations and then sandstone again at the top (Fig. 6). It cannot be determined in its totality from a single vertical section (estimated theoretical cumulative thickness of about 250 m maximum) and shows numerous lateral facies variations that obscure the relationships. Nevertheless, the rich fauna and flora in some of the boreholes has made it possible to determine the biostratigraphic position of the subsurface terrigenous formations. The morphology of the Seuil du Cotentin basins, with a gulf opening into the western Channel, was acquired during the first marine onlaps of the Late Pliocene. This basin was structured by the renewed movement of old N 020° and N 110° faults that were active throughout the Pleistocene. The base of the Pliocene series (Grès coquilliers de Marchésieux Fm.) in the Sainteny-Marchésieux Basin is dated through palynology in the cored Marchésieux borehole (Fig. 7). Characterizing a marine shelf environment, this thick homogeneous sandstone series passes laterally, towards the interior of the basin, to the Marnes du Bosq d’Aubigny Fm. pro parte, which is historically famous due to the discovery of Cenozoic gastropods in old Armorican terrane. This terrigenous formation covers a large diversity of deposits that are all marly, but showing in places clastic quartz intervals and in places shelly or shelly limestone intervals of a coastal intertidal zone (Fig. 8). The pollen assemblages indicate an age ranging between the Late Pliocene and Early Pleistocene (in the sense of the geologists of NW Europe who place the Pliocene/Pleistocene boundary at the first Cenozoic glaciation in the northern hemisphere). The re-examination of the available borehole data indicates that these marly facies pass/grade laterally towards the eastern and western borders, into accumulations of shelly limestone and coarse-grained sandstone that have been lumped under the name of the Falun de Bohon Fm. dated as Pliocene mainly through the foraminifera (PI to PIII biozones) and as Pretiglian by the pollen associations in the Marchésieux borehole. Locally very thick (at least 50 m), these shelly marls are interpreted as the progradation of a system of subtidal megaripples under a tidal dynamics. In the centre of the Sainteny-Marchésieux Basin, where the shelly marl series are no longer present, the marl deposition (at least 60 m) continued up to the beginning of the Pleistocene, from the Marnes du Bosq d’Aubigny Fm. to the Marnes de Saint-Nicolas-de-Pierrepont Fm. The latter formation comprises marly quartz-sand facies containing abundant microfauna and that ended by covering all the Seuil du Cotentin basins. The Sables de Saint-Vigor Fm. is a thick marine unit (30 m minimum) of very well sorted azoic and homogeneous sand, with cross bedding (Figs. 9 and 10), indistinctly overlying the different Plio-Pleistocene marl or shelly marl series and overlapping onto the Permo-Triassic and Miocene borders of the Seuil du Cotentin basins (Fig. 11). In outcrop, the Sables de Saint-Vigor Fm. reveals a tidal regime. At the top of this formation the last sandy deposits have been deformed by glacier block falls, reflecting a climatic degradation and confirming the regressive tendency of the Pleistocene sedimentation in the Cotentin. These marine deposits were then locally eroded by fluviatile medium to coarse clayey sands with conglomeratic lenses. This fluviatile system, at first braided and then anastomosing, flowed along the SSW-NNE troughs in the Seuil du Cotentin basins. Peaty intercalations present at the top of these fluviatile sands provide a valuable chronostratigraphic marker for the end of the Early Pleistocene. The proposed sedimentary model depends entirely on the physionomy of the old coastline, the climatic evolution during the Plio-Pleistocene and the regional structural context, but does not confirm the hypothesis of a neotectonic control of the sedimentation in Normandy.

Key words: Pliocene, Lower Pleistocene, Coastal sedimentation, Fluvial sedimentation, The Channel, Seuil du Cotentin