Taphonomie et minéralisation de bois carbonatisés du Purbeckien de l'Usine, à Cherves-Richemont (Charente)

Abstract

RÉSUMÉ Les faciès purbeckiens de Cherves-Richemont, en Charente (SO France), comprennent au-dessus de la série évaporitique des gypses de Chamblanc une série marno-calcaire fossilifère. Dans le secteur de l'Usine, cette série carbonatée présente deux faciès remarquables par leur contenu en bois fossiles carbonatisés. En surface persistent des blocs relictes d'un conglomérat calcaire, disloqué par les labours, riche en encroûtements tubulaires, carbonatés et fibroradiés, d'axes végétaux, associés à des fragments de fusain. Ce faciès conglomératique comporte également des galets d'os de grands reptiles et des écailles de poissons Lepidotes Agassiz, 1832. À environ 40 cm de profondeur, une fouille sur le site a permis de mettre au jour, dans une couche marneuse, des tronçons métriques de bois carbonatisé, encroûtés par des sphérules fibreuses carbonatées. L'analyse géochimique des bois confirme une minéralisation exclusivement carbonatée (ni gypse, ni silice). Les lames minces réalisées dans les bois révèlent une fine carbonatisation à l'échelle cellulaire, surtout hyperblastique ou polyblastique, vraisemblablement d'origine microbienne. La détermination des bois au microscope électronique à balayage permet de les attribuer au genre Agathoxylon Hartig, 1848, témoignage fréquent des gymnospermes mésozoïques. Le milieu de dépôt est interprété comme continental ou margino-littoral. La position stratigraphique de ces faciès à bois carbonatisés, un peu au-dessus des gypses berriasiens de Champblanc, en fait un équivalent latéral probable du site berriasien final à dinosaures et bois fossiles d'Angeac-Charente. ABSTRACT Taphonomy and mineralization of carbonated wood from the Purbeckian facies of l'Usine, Cherves-Richemont (Charente). Lower Cretaceous deposits from the Charentes region, in southwestern France, correspond to Purbeckian facies of continental to brackish environments, locally rich in evaporitic formations (Manès 1850; Pouech et al. 2006, 2014, 2015; Schnyder et al. 2012; Vullo et al. 2014; Moreau et al. 2017b). Three fossiliferous localities have been previously identified: the Berriasian gypsum of Champblanc, near Cherves, with both teeth accumulations and bone beds of fishes, crocodiles and dinosaurs, but relatively poor in fossil wood (Buffetaut et al. 1989; Le Loeuff et al. 1996; Schnyder 2003; Colin et al. 2004; El Albani et al. 2004; Mazin et al. 2006; Rees et al. 2013); the Berriasian marls and calcareous conglomerate of Angeac-Charente, particularly rich in dinosaur megaremains and lignitic accumulations (Néraudeau et al. 2012; Allain et al. 2014; Benoît et al. 2017; Polette et al. 2018; Polette 2019); the Purbeckian to Wealden clay and conglomerate of the Rochefort area, providing large bones of dinosaurs in surface blocks, but never observed in stratigraphical position (Vullo et al. 2012). Another fossiliferous Purbeckian locality from the Charentes region, the Chassiron coast of Oleron Island, is rich in vertebrate and plant remains, but mainly Tithonian in age (Vullo et al. 2014; Moreau et al. 2017b). This paper presents a fourth Charentese Purbeckian locality with the first case of calcite mineralized wood known in Mesozoic deposits from France. The Purbeckian facies is located at Cherves-Richemont, in Charente (SW France), at 3 km east of the Champblanc gypsum quarry, in the L'Usine locality (Figs 1; 2). The stratigraphical position of the marls and the conglomerate with calcite mineralized wood is at mid-distance from the lower to mid-Berriasian gypsum series of the Champblanc quarry, to the east, and the unconformity with the lower Cenomanian sands that erode the Purbeckian series, to the west (Fig. 3) (Coquand 1858, 1862; Platel 1980; Bourgueil et al. 1986). Consequently, the fossil wood locality of L'Usine is a probable lateral equivalent of the uppermost Berriasian bone and wood beds of Angeac-Charente, located at about 25 km only (Gônet et al. 2018; Polette et al. 2018; Rozada 2019). There is no real outcrop at L'Usine, and the limestones with fossil wood can be mainly observed on surface blocks in fields and vineyards. Two outstanding facies, rich in calcite mineralized wood, can be observed. First, the surface of the fields shows abundant relic blocks of a calcareous conglomerate, rich in tubular, fibrous and carbonated incrusting of plant axes, associated to fragments of fusain. The conglomerate contains reptile bone pebbles and Lepidotes Agassiz, 1832 fish scales too. At a depth of 40 cm, an excavation has shown marls containing large pieces of calcite mineralized wood (about 1 m), incrusted by fibrous carbonated spherules (Figs 2; 4). The geochemical analyses of the marls and the fossil wood by X-ray diffraction, realized according to the method of Moore & Reynolds (1997), confirm the mineralization by carbonate only. Dolomite and silica are very scarce and gypsum is lacking (Fig. 5). Polished sections (Fig. 2) and thin sections (Figs 6; 7) of calcite mineralized wood show processes of centripetal mineralization (Higgins 1960; Buurman 1972; Marynowski et al. 2007; Mustoe 2018; Mustoe & Beard 2021). The microscopic observations reveal a fine mineralization at the cell scale, mainly hyperblastic or polyblastic (sometimes oligoblastic, likely of microbial origin. Cell lights are filled by sparry calcite. Some “tuff structures” of wood mineralization are characteristic of certain tuff of filamentous cyanobacteries (e.g. Rivularia C.Agardh ex Bornet & Flahault, 1886, ou Shizothrix Kützing ex Gomont, 1892) common in the geological record (Freytet & Plaziat 1965; Freytet & Plet 1991; Caudwell et al. 1997; Freytet & Verrecchia 1998; Freytet 2000; Pentecost 2003; Hägele et al. 2006; Sanders et al. 2006; Arenas & Jones 2007). A zonation of the mineralization can often been observed on thin sections of wood but corresponds more likely to successive mineralization steps by cyanobacterial mats than to steps of wood growth. The observations of wood fragments with Scanning Electron Microscopy allow to identify a tracheidoxyl (Creber 1972) with araucarioid structures of the genus Agathoxylon Hartig, 1848, a common Mesozoic fossil gymnosperm wood, abundant in the Charentese region of SW France (Fig. 8) (Néraudeau et al. 2002, 2003; Philippe et al. 2008, 2010, 2018; Philippe 2011; Vullo et al. 2014). The pitting is mainly uniseriate, when the punctuation is mainly biseriate for Agathoxylon gardoniense Crié, typical of the mid-Cretaceous deposits of Charentes. Consequently, the Agathoxylon from L'Usine could belong to another palaeoecological morph of A. gardoniense, possibly to another species. The deposit environment that has gathered gymnosperm trunks and branches, eroded reptile bones (e.g. sauropods) and freshwater to brackish fish remains (e.g. Lepidotes) is interpreted as continental to coastal, in freshwater or brackish conditions.