Palaeo-Environmental Study Area P14 Oleron Island, Near La Rochelle, West Coast, France
Total Page：16
File Type：pdf, Size：1020Kb
Palaeo-environmental Study Area P14 Oleron Island, near La Rochelle, west coast, France PALAEO-ENVIRONMENTAL STUDY AREA P14 OLERON ISLAND, NEAR LA ROCHELLE, WEST COAST, FRANCE Plate P14 View of the beach at ‘La Perroche’ marsh on Oléron Island, Atlantic coast, France, showing the peat layer exposed on the beach, evidence of changing coastal environments 1. LOCATION 1.1_ Geographical Description Oleron Island, 175Km2, is located on the Atlantic coastline and belongs to the Département de Charente Maritime (Plate P14a and Figure P14.1). Situated North of the Gironde estuary, Oléron Island isolates the silty bay of Marennes-Oléron from the Atlantic Ocean. The Charente and the Seudre rivers, flowing into this bay, are bordered by vast saltmarshes. Marennes- Oléron bay is largely open to the ocean at its northern extremity. At the southern end Pertuis de Maumusson is a narrow inlet with strong tidal currents. It is a low,flat, spindle-shaped island experiencing constant transformation due to erosion or sedimentation. Belonging to the Aquitaine Basin, Oleron Island is composed of sedimentary rocks. The island is orientated NW-SE following the axis of an anticline (Anticline of Gémozac). On the south-western part of the island, stretches of sands, sandstones, clays and Cenomamian limestone develop parallel to the coast (Figure P14.2). 1.2 Coastal description The western rocky coast of the island is punctuated by a succesion of small littoral marshes separated from the ocean by sandy dunes. Most of the marshes have become pasture but some of them are still swamps. Peat formation is found in most of these coastal marshes. Theses peat layers can extend under the dunes and appear at low tide on the beach. 1 Palaeo-environmental Study Area P14 Oleron Island, near La Rochelle, west coast, France A coastal dune system surrounds the whole island. The dunes were formed at the end of the Holocene transgression due to the erosion of the calcareous substrate or to the destruction of ancient sandy dunes. These sandy formations are more developed to the south (Massif de St Trojan), and to the east (Massif des Saumonards). Marennes-Oléron bay, isolated from the open ocean by the island, is subject to a constant supply of sediment from the Charente and the Seudre rivers. 1.3 Regional Authority Conseil Régional de Poitou-Charentes Conseil Général de Charente Maritime. Town councils of St Pierre d'Oléron, Dolus d'Oléron, St Georges d'Oléron, St Denis d'Oléron, La Brée-les- Bains, Le Château d'Oléron, Le Grand-Village-Plage, ST Trojan-les-Bains. UNIMA ( Union des marais de La Charente Maritime) group of cities (Syndicats Intercommunaux) or associations for the management of marshes. 1.4 Designation (protected areas) IGN map, 25 000e, 1330 OT 2. MODERN HUMAN GEOGRAPHY The main activity of the island, together with fishing and oyster-farming, is tourism. Its population is 19 000 during the winter. 3 CONTEMPORARY COASTAL PROBLEMS: (DUNE MOVEMENT, EROSION, ACCRETION; I.E COASTAL CONTEXT) The north of the island is a hard rocky coast of jurassic limestone. To the west the coast is composed of a narrow sandy foredune lying on a rocky cenomanian substrate which isolates the littoral marshes from the ocean. The southern oceanic side and the north-eastern bay side are characteristic dune accreting coasts (massif de saint trojan and massif des saumonards). Soft muddy coasts with wide intertidal mud flats are located on the south-eastern side of the island facing the Marennes Oléron bay. The coastal cliffs of the northern part of the island experience a maximum erosion of 20 to 30cm/yr. The dune accreting coasts to the south-east and the north-west are generally eroding except the Massif des Saumonards and the Pointe de Gasteau which are in accretion. To the east the dune coast ,which isolates littoral swamps, has a rate of erosion of 60 to 70cm/yr. The soft muddy coast in the south-western part of the island is slowly accreting (a few centimeters per year). Only the sandy ridge of the Pointe de Bellevue is accreting south-eastwards involving erosion to the north and an infilling of the internal part of the bay. 2 Palaeo-environmental Study Area P14 Oleron Island, near La Rochelle, west coast, France 4. HISTORICAL, ARCHAEOLOGICAL AND PALAEO-ENVIRONMENTAL SETTING OF THE COASTAL COMMUNITY 4.1 Sedimentological and palaeo-environmental data concerning the study area At the begining of the Holocene the sea invaded the river valleys formed by erosion during the last glaciation. A deep ria system developed. During the last stages of sea-level rise, the Pertuis de Maumusson channel (20m deep), separated the island from the continent. Since 6000 years B.P., constant coastal changes have occurred: the western rocky, wave dominated coast underwent intensive erosion . The Atlantic south-western coast, supplied by littoral drift, accreted to constitute the Saint-Trojan dune system. On the less dynamic bay coasts, the wide mudflats were supplied with fluvio-marine sediments from the Charente and the Seudre rivers. Another important accreting dune system lies on the eastern coast (massif des Saumonards). 50% of the island is situated below the altitude of 5m NGF. 4.2 Archaeological data concerning the study area (see Appendix P14.1 and Figure P14.3) The island was continuously occupied from the Palaeolithic to the Modern ages. 33 sites are known on the island but 18 of them are particularly representative. The Neolithic period is the most well representated. The Gallo-Roman, Medieval and Modern sites are found everywhere except in the south probably due to the presence of sandy dune systems. The study of 4 Neolithic sites (La Perroche , Ponthezière L'Ecuissière and Ors) located on Oléron Island were used to specify micro-scale coastal changes. 5. THE PALAEO-ENVIRONMENTAL AND ARCHAEOLOGICAL POTENTIAL 5.1 Interpretation of the sedimentary archives of the La Perroche marsh We focused our investigations on a small marsh called La Perroche (Plate P14 and P14a) located on the south-western coast of Oleron Island to try to reconstruct coastal changes. This marsh is separated from the ocean by a narrow foredune and a dam and is surrounded by outcrops of Cenomanian limestone, forming the substrate of the Holocene deposits.The marsh has a flat surface lying around 1m NGF. In order to determine the succession of Holocene palaeo-environments, more than 10 percussion corings were made. The study of the sedimentary sequences allowed a reconstruction of a succession of palaeo-environments which were characterised by their faunal (ostracods) assemblages. Using 14C dating a determination was made of the chronology of this evolution and the infilling of the marsh since the sea reached its present level around 6000 years ago. Data and methods Cores were collected over the whole marsh area (Figure P14.4, Table 1) and on the La Perroche beach from peat formations outcropping at low tide. 8 cores particularly representative of the sedimentary sequences were selected for this study : H 9602, H 9603, L 9818, L 9908 in the western part, L 9909 to the east, L 9817 to the south, L 9906 to the north and L 9910 on the beach. The sedimentological study, based on lithological and grain-size analysis, allowed the definition of palaeo-environments in terms of dynamic processes and marine influences. Ostracods were counted and classified according to species. They were gathered in 4 associations belonging to 2 different environments (Yassini, 1969): 3 Palaeo-environmental Study Area P14 Oleron Island, near La Rochelle, west coast, France · Continental with fresh water and brackish water ostracods (Figure P14.5). · Marine s.l. composed of coastal and marine s.c.species (Figure P14.6). · Benthic foraminifera were mentionned in terms of presence/absence. · Charophytes, mainly consisting of gyrogonites, were semi-quantitatively evaluated. These cores will be successively analysed, explaining the sedimentological and the microfauna data as well as the chronological information obtained by 14C dating. Each study will end with a brief summary of their characteristics in terms of palaeo-environments. Macrofauna analysis was made on the core L 9906 giving more information on palaeo- environments. The variations in shell density and in the quantity of species observed gives us some understanding of the coastal changes which have occurred in the La Perroche marsh (Dupont 2000). Geophysical analysis took place in the marsh to localise the sedimentary units observed in the cores and to measure the depth and the morphology of the calcareous substrate. Two methods were used in this study. The method of "pseudo-sections de résistivité" was used along 3 transects through the marsh and the method of "sondage électromagnétique en domaine temporel" (TDEM method) at 7 points in La Perroche marsh (Figure P14.7). Results The core H 9602 (Figure P14.8) is composed, from 4.5 to 3.6m depth, by coarse sands with pebbles, overlain from 3.6 to 2.2m by clays containing plant debris and silty laminations. The fauna appears at 3.1m in a layer of organic matter.. It is a very poor and varied fauna, composed successively by polyhaline waters species (Loxoconcha rhomboïdea) brackish and marine waters species (Cyprideis, Callistocythere), all calibrated and then probably displaced. From 2.3 to 1.9m, silty sands with shell laminations appear containing predominantly brackish (Cyprideis torosa and Loxoconcha elliptica) and coastal water species while some marine species are also able to subsist.The brackish water fauna seem complete and are not displaced. Lots of foraminifera were observed in these sands showing a marine influence. From 1.9 to 1.7m, clays with plant and shell debris develop. In this sequence, ostracods reach their maximum level (more than 1000 individuals), 50 % of them become freshwater species (Limnocythere inopinata, Candona sp.).