Journal of the Geological Society, London, Vol. 147, 1990, pp. 687-692, 4 figs. Printed in Northern Ireland The Silurian of the Pyrenees J. M. DEGARDIN Universite‘ de Lille 1, U.F.R. des Sciences de la Terre, Laboratoire de Ge‘ologie Stratigraphique, URA 1365 du CNRS 59655 Villeneuve d ’Ascq Cedex, France Abstract: A detailed stratigraphical study of the Silurian rocks of the Pyrenees and analysis of their graptolite and conodont faunas has allowed the construction of composite lithostratigraphical succes- sionsand accurate correlations between them. Thelithostratigraphy reflects generally continuous deposition of pelitic and shale sediments across the whole of the Pyrenean range from Rhuddanian to Homerian times. There is, however, some variation eastwards where sandier intercalations appearin the Rhuddanian. Also, carbonate facies were deposited in the Mouthoumet Massif during the latest Telychian and then from Sheinwoodian times spread across the major part of the central and eastern Pyrenees.Pelitic sedimentation persisted during the Gorstian and Ludfordian but withsome differentiation into more clastic facies westwards and more carbonate-rich facies to the east. These features of sedimentation became more pronounced during Pridoli times and continued until the Early Devonian. The Silurian lithofacies of the Pyrenees are interpreted as epicontinental deposits formed in low energy, reducing environments on the northern marginof the Gondwanan continent. The Pyreneesform a mountain chain over 450km long importantareas where the variousformations are well between France and Spain, reaching a maximum width of represented and where they are best dated by graptolites some 200 km. Situated between the Aquitaine Basin to the andconodonts (Fig.2). Inthe late Ordovician(Ashgill), north and the Ebro Basin to the south, this region comprises sedimentation was characterized by arenaceous deposits in anumber of structuralbelts that strike parallel to the the Pays Basque and in the eastern Pyrenees, particularly in general trend of the range. There is an almost symmetrical the region of Camprodon and the Agly Massif. Finer detrital arrangement of structures about an axial region know as the deposits are also recorded in most parts of the Pyrenees, High Palaeozoic Range, which was long known as the Axial except forthecentral Pyrenees where interbedded PrimaryBelt (Fig.1). The main structuralelements from limestones occur. The variety of facies at this time and the north to south are: the northern marginal zone, the north abundant, varied fauna reflect theirdeposition as shelf Pyreneanzone, the HighPalaeozoic Range, the south sediments. Throughoutthe whole of the Pyrenees atthe Pyreneanzone, thearea of theAragon syncline and base of the Silurian, in Rhuddanianand Aeronian marginal Sierras. The Palaeozoic formations crop out mainly correlatives, the facies are very homogeneous, comprising inthe High PalaeozoicRange and also in thenorth black pelites which are richer in organic matter than thoseof Pyrenean massifs (Figs 1 and 2). the Ashgill deposits.Some interbedded limestones are The first observations on fossiliferous Silurian formations present only in thecentral Pyrenees (the Bosost and in the Pyrenees date back to the nineteenth century. These Bagnhes de Luchon areas). However, a schistose sequence involved the first discoveries of graptolites byBoubCe with sandstonelenses is presentin theCamprodon area (1845),which weredescribed later by Barrois (1892). theeast.to At the Ordovician-Silurian transition Bresson (1903) then gave a review of the Palaeozoic strata sedimentation was continuous;homogenization of facies within the ‘Axial Range’ of the Pyrenees, in which he took place rapidly, with fine, very dark detrital sedimenta- pointed out the importance of Silurian rocks (then termed tion occurring on a shelf, accompanied by rapid shallowing Gothlandian).These latter strata, represented by the leading to a reducing environment. During Telychian times ‘Carburized Schists’ and which form a characteristic facies carbonates were deposited in the Mouthoumet Massif. throughout the Pyrenean belt, are a valuable datum for the DuringSheinwoodian and Homerian times, pelitic study of Palaeozoicformations in the High Range. It is sedimentation was ubiquitous across the Pyrenees, except in notable otherwise for the lack of, or the poor preservation theeast where carbonate sedimentation also took place. of, fossils. From the beginning of thiscentury, numerous Carbonate beds are also known from the central Pyrenees. regional studieshave further improved our knowledge of In the Mouthoumet Massif, gypsiferous lenses are related to lithological distributions and stratigraphy. The occurrence of lagoonal sedimentation, reflecting trends towards emersion numerousgraptolites remains the key to stratigraphical (Ovtracht 1964). During Gorstian and Ludfordian times, the subdivision andcorrelation between the differentsections facies were again varied. In the west, in the Pays Basque, which have varied tectonic settings. The Silurian formations intercalations of sandstones mayoccur within the pelitic comprise mostly monotonous sequences of black, graphitic sediments. In theeastern Pyrenees, carbonate sedimenta- shales, but there is some lithological variation at the base tion took place with, locally, thinly beddedsiltstone of the system wherecoarser detrital deposits occur oc- deposits. Inother regions, pelitic sedimentation was casionally, and at the top where carbonateintercalations are predominant,except in thesouthern area (Sallent de present along almost the whole of the range. Gallegoregion, Andorre, Bar-Toloriuand Camprodon) where carbonate nodules are scattered in the pelites. Lithostratigraphy and environments During Pridoli times the diversity of the facies was an Laterallithostratigraphical variation in the Silurian of the important feature. In the west, rhythmic sedimentation of Pyrenees can bedemonstrated by reference to some pelites and sandstones was continuous from the Pays Basque 687 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/687/4890535/gsjgs.147.4.0687.pdf by guest on 28 September 2021 688 J. M. DEGARDIN Palaeozoic rm] JurassicCretaceousand Eocene 0 post-Eocene Fig. 1. Location and major structural divisionsof the Pyrenees. tothe Argeles-Gazost area.In the central Pyrenees, levels. These enable identification of various biozones and sedimentation wasessentially pelitic though this was some correlation between the different successions through- sometimesinterrupted by the formation of carbonate out the region. deposits which are either thickly bedded or nodular in form. Initial studies of graptolites, made mostly in the central In theeastern Pyrenees, carbonate facies are always Pyrenees (Garonne Valley, SenteinandCamprodon followed by detrital facies. The occurrence of numerous regions), indicatecorrelations with the upper part of the carbonate nodules in the Andorre, Cerdagne and Roussillon Llandovery and with the Wenlock Series. The absence of areas isalso worthy of note.Around the Mouthoumet any fauna at the base of the Llandovery Series has led some Massif, fine detrital facies are present within the pelitic authors to suggest that there is a stratigraphic break at this suites. point in the Silurian of many areasin the Pyrenees. From this brief survey of Silurian facies in the Pyrenees, Recently obtained data fromthe Llandovery Series, one can conclude that the deposits present are essentially however, have led to arejection of this hypothesis, the the product of epicontinental sedimentation. Secondly, the absencebeing explainedinstead by the discovery of a sedimentaryenvironments represented display characteris- decollement at the base of the Silurian. tics of a reducing environment. This is reflected in the high Among the 86 graptolite species assigned to 13 genera level of pyritein the sediments and in the nature of the collected from the Silurian of the Pyrenees, only a few derivedorganic matter. Certain studies (Graciansky et al. ‘index’ graptolites are present. Dating is based essentially on 1987) have shownthat marine-derived organic matter is associations of species. Figure 3 shows a group of species more sensitive to oxidation than continental-derived organic characterizing zones 19 to 21 (AeronianStage) of the matter. It follows that marine-derived organic matter would Llandovery Series.It comprises Monograptus triangulatus have to beburied rapidly in order to bepreserved. This (Harkness), M. delicatulus Elles & Wood, M. intermedius wouldnot thebe case for sedimentswhere the (Carruthers), M. distaw (Portlock), M. involutus Lapworth, sedimentation rate was low, such as those in the Silurian of M. sedgwickii Portlock, Pristiograptus regularis (Tornquist) the Pyrenees. Thus only a reducing environment can explain and M. communis Lapworth. This group is present in the the good preservation of the organic remains. Pays Basque,the central Pyrenees and the Camprodon region. A latergroup of specieswhich includes Monograptus Biostratigraphy proteus (Barrande), M. plunus (Barrande), M. undulatus The biostratigraphy of the Silurian of the Pyrenees is based Elles & Wood, M. turriculatus (Barrande), M. spiralis mainly onthe rich graptolitefauna collected from the Geinitz and M. halli (Barrande) indicates the base of the pelites,and on conodonts extracted from thecarbonate Telychian Stage. This group is very well represented in the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/687/4890535/gsjgs.147.4.0687.pdf