Elsevier

Geobios

Volume 59, April 2020, Pages 29-46
Geobios

Original article
The Zanclean palaeofloras around the Mont-Dore strato-volcano: A window into upper Neogene vegetation and environments in the Massif Central (Puy de Dome, France)

https://doi.org/10.1016/j.geobios.2020.01.001Get rights and content

Abstract

New Pliocene macrofloras and microfloras perfectly preserved from the Mont-Dore (Puy-de-Dôme, Massif central, France) have been reinvestigated. Samples come from different stratigraphical levels collected from three localities, Lac Chambon, La Gratade and Pont de Chocol. The 40Ar/39Ar radiometric datings bracketing the Chambon Lake and La Gratade fossil-bearing horizons give 4.46 ± 0.05 Ma and 3.94 ± 0.04 Ma, respectively, representative of the Zanclean Stage (= Brunssumian B and C), a period of the early Pliocene (5.32 to 3.6 Ma), much older than previously thought (i.e., Piacenzian). Pont de Chocol is considered to be close biostratigraphically, or even identical in age to La Gratade. Detailed morphological evaluation of leaf morphotypes completed with pollen analysis contributed to the better taxonomic knowledge of these palaeofloras. The overview focuses on floristic and phytostratigraphical characteristics of the defined stratigraphical units and their dating and correlation with previously defined palaeofloristic units of the Massif central. The study provides detailed identifications of plants representing 11 gymnosperms among which Pinaceae (pollen and winged seeds), Cupressaceae including “Taxodioid” pollen grains and Sciadopitaceae families (only pollen). Besides, different angiosperm trees and shrubs have been determined from both micro- and macro-remains. Fagaceae is the most diversified with several foliage of beech and deciduous oaks, while diversified Juglandaceae contribute mainly as abundant leaf remains of Carya, Pterocarya and Juglans; Ulmaceae with numerous leave of two Zelkova species, and also Ulmus. Various other deciduous dicotyledonous such as Alnus, Betula, Carpinus, Populus, Acer laetum and A. interrigenum are well documented. Leguminosae are recorded by a small number of leaflet imprints. Evergreen shrubs of Buxaceae (Buxus pliocenica) are scarce. All these taxa contribute to a rich biodiversity of these Pliocene assemblages. All three sites point towards riverine forest habitats dominated by hygrophilic diversified woody plants while in the surrounding plains and slopes the thermophilic elements were scarce and mesophilic taxa abundant as temperate elements. This vegetation can be compared with mixed mesophytic forests depicting a climate cooling during the two considered periods (ca. 4.46 and 3.95 Ma).

Introduction

Tertiary volcanogenic deposits of the Puy de Dôme (Massif central, France) have been explored for a long time and are a place of interest to palaeobotanists because of the abundance and well-preserved micro- and macrofossil plants. The earliest written record of Mont-Dore region fossil flora dates back to the end of the 19th century (Boulay, 1892). This work summarized all the discoveries that pointed vegetated Tertiary landscapes from the French Massif central (Varennes, Perrier, Niac, Pas de la Mougudo, Ceyssac, Saint-Vincent, Joursac, La Gratade and Pont de Chocol).

During the first half of the 20th century, Grangeon and Rudel (1959) reported some fossil plants in Chambon Lake outcrops and detailed the taxonomic studies; the flora which should belong to this locality contains a small assemblage of 9 species: Bambusa cambonensis N. Boul. (Arundinaria metake Sieb.), Alnus glutinosa Gaertn., Fagus silvatica L., Fagus pliocenica Sap., Populus tremula L., Ulmus effusa Willd., Acer pseudocampestre Ung., Acer Nicolai N. Boul. (affinity A. opulifolium Willd.), Carpinus betulus L., and one family, Poaceae. Later, this study was completed by Elhai and Rudel (1961) with a microfossil analysis. La Gratade and Pont de Chocol were studied by Grangeon and Rudel (1962); most of the fossil flora samples come from La Gratade outcrop, with 23 taxa identified as Torreya nucifera Sieb. et Zucc., Potamogeton quinquenervis N. Boul., Salix sp., Populus tremula L., Populus alba L., Betula macrophylla Heer, Alnus glutinosa Gaertn., Carpinus orientalis Mill. (fruits and leave), Carpinus, Corylus avellana L., Fagus sylvatica Sap., Castanea vesca Gaertn., Quercus hispanica Rer., Quercus sp., Zelkova ungeri Kovats, (crenata and acuminata), Ulmus sp., Juglans regia L., Carya sp., Acer laetum C.A. Mey, Acer sp., and Tilia sp. Then a detailed microfloral study was produced by Elhai and Rudel (1965) from the lignite clay from Pont de Chocol. The microflora of La Gratade has not been previously studied. Today, all these sites are widely scattered in the literature, most of them are not updated and not well constrained by radiometric dates. The more recent and detailed updates concern only the Miocene plant assemblage of Murat in the French Massif Central (Roiron, 1991, Legrand, 2003, Legrand, 2010).

Recent harvests of macroflora were gathered by J.F. Pastre from 3 localities: Chambon Lake (LCh), La Gratade (LG) and Pont de Chocol (PTC), combined with new radiometric datations, bring us to complete and synthesize the results of former inventories. Despite the local character of such fossil records and the inequitable representation of ancient vegetation, they provide invaluable information about the history and distribution of the floristic elements and offer a great potential for identifying past environmental elements, making a contribution to a new picture of the palaeofloristic biodiversity in this area. The results of the palaeoenvironmental investigations are partly comparable but also complete earlier investigations with the description of 28 new taxa in Chambon Lake outcrop, as well as two species in La Gratade and Pont Chocol. The aims of the present study were: (i) to document, describe and determine the three new collections of plant fossils of Chambon Lake, Pont Chocol and La Gratade outcrops which were recently collected by one of us (JFP), (ii) to compare these fossil plant collections to the one recovered and published by Grangeon and Rudel, 1959, Grangeon and Rudel, 1962 from the same area, (iii) to compare the Zanclean flora of these aeras to other selected Pliocene floras in the Massif central region and from Western Europa, and (iv) to infer the palaeoecology of the three area during the early Pliocene, using evidence from macrofossils and published data from dispersed pollen and spores.

Section snippets

Location and geological settings

The three fossiliferous outcrops studied in this article are all located in the French Massif central, in the periphery of the Mont-Dore strato-volcano (Fig. 1). They were deposited in lacustrine environments reworking contemporaneous pyroclastites and giving tuffites deposits.

The fossiliferous outcrop of the Chambon Lake (Altitude 866 m asl) is situated on the right bank of the Couze Chambon river downstreams from the Chambon Lake and to the West of the strombolian cone of the Tartaret

Macroflora

Plant fragments were recovered by splitting clay containing the impression/compression fossils. Minimal preparation with fine needles was performed to fully expose the plant organs when needed. Cuticles are often present but were not studied here. The images were processed and measurements obtained using Nikon binocular loupe and Canon Elements. The taxonomic determinations established in this study are based exclusively on the gross morphological characters of the gathered specimens, whereas

40Ar/39Ar datations

Full analytical details for individual crystal are given in Tables S1 and S2 (Appendix A). Results are presented as probability diagrams in Fig. 3. In this figure, individual crystal age uncertainties are given at 1σ-level and the weighted mean age uncertainties are quoted at 2σ-level, including J uncertainties. In both sample we found only one main population of crystal as shown by the unimodal probability diragrams (Fig. 3). Despite a slight excess scattered, meaningful weighted mean ages of

Discussion

Pliocene plant assemblages in the French Massif central occur at localities variously dated and in various stage of exploration. Based on published fossil floras for most of them, it appears that most micro- and macroflora assemblages may be less stratigraphically controlled than assumed in several former studies, as demonstrated by Bellon (1971) and Gibert et al. (1977) with the first radiometric controls for several Miocene outcrops in Cantal (Massif central, France).

Based on palynological

Conclusions

The plant material recovered from Chambon Lake, La Gratade and Pont Chocol (Puy-de-Dôme) and newly dated to 4.46 ± 0.05 Ma and 3.94 ± 0.04 Ma by 40Ar/39Ar are among the taxonomically richest lower Pliocene floras in the French Massif central. The plant assemblage of micro- and macrofloras added complementary data of both autochthonous and allochthonous origin. It comprises at least 76 different taxa (pollen flora and macroflora), a number considerably increased with the recent collect of macrofossil

Acknowledgements

We thankfully acknowledge Dr. Monique and Jean Dupéron (CR2P, MNHN and Paris 6 University) who spent a great deal of their time to help us with the identification of macroflora remains. We are grateful to Jean-Pierre Suc and Vincent Girard who contributed to the improvement of the manuscript.

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