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Sedimentology of - Géologie de la France

1. D E and in minor proportions of LF F Fig 9 lithofacies column on the left side of the log Individual units of LF D rarely exceed 10 m or so in thickness and only these HCS dominated units may properly adressed as tempestite bars as proposed in Eschards model Since most bars LF D are capped by wave rippled and partly mudcracked intertidal sediments LF E Fig 9 most of their deeper part probably deposited in uppermost subtidal environments less than 15 m allowing for compaction that is above fairweather wave base Therefore the most important factors concerning sand reworking and redepo sition may have been fairweather wave and tidal current activity The sedimen tary structures preserved in any particu lar deposit are not necessarily produced by the most frequent type of redeposi tion however but by the final event reworking this sediment Where the depth of storm erosion exceeded the fair weather accumulation rate only storm generated sedimentary structures are preserved within the redeposited rem nant sediment Thus a storm dominated sedimentary record is indicative for intense storm wave action at the sea floor but does not imply that storms were the main factor for sediment accu mulation 2 The possibility of spontaneous ini tiation and stationary accretion of off shore sand bars by storm wave action may indeed be doubted Where open shelf sediment is redistributed by storm
2. osits Nora NOFFKE Edgar NITSCH Etude sedimentologique des depots clastiques de plate forme de l Ordovicien inf rieur Montagne Noire France G ologie de la France n 4 1994 pp 3 19 13 fig 1 pl photo Key words Shale Sandstone Arenigian Lithofacies Continental margin sedimentation Tempestite Herault Roquebrun area Montagne Noire Mots cl s Shale Gr s Arenig Lithofaci s S dimentation marge continentale Tempestite H rault R gion Roquebrun Montagne Noire Abstract This study investigates the lowermost Arenigian siliciclastic deposits of the Roquebrun area eastern Montagne Noire France The whole sequence consists of cyclically arranged shallow marine to peritidal shales and sand stones Based on lithology and sedimen tary textures six lithofacies types are distinguished in this paper A sandy noduliferous shale B sandy shales with thin crossbedded sandstone beds C hummocky crossbedded sandstones with thin shale interbeds D amalgamated hummocky cross stratified quartzare nites E thin bedded shale sandstone intercalations with wave ripples linsen and flaser bedding and F shales and siltstones with sparse quartzarenite beds Three alternative facies models are discussed 1 offshore bar model open shelf Lithofacies A to C with intercalated sequence of offshore bar deposits D and E surrounded by re stricted marine interbar deposits F 2 tectonic rise mod
3. G rer amp Comprendre 4 n Tarif de Base 3 France 382 FF CO tranger 450 FF Ci joint mon r glement par Ch que bancaire l ordre des Editions Eska I Virement postal aux Editions Eska C C P Paris 1667 494 Z l E i i l l i i l 1 l i l 1 1 t f j l l I i l l 1 I I i l Ln ue soe cum sam vot one mec me re sil ou une non mm mt ue ou 0e mme mm HE ee ER GRR PRE F POS SONO PANNE NON MAN ARR a ZT DE net GS HN OS aae ld RO TAYE ARIE CANON aap pam sete CURE MINE MM Spp apa ana m D UN HAN NE DNS SNS ZEN SR ZOPE PU MN ANNE SANS Sits GE DOM NM OGM MANN RE Dies l
4. E littoral environment of foreshore and lagoonal shoreline Skolithos facies dominated by fairweather rede position by waves and currents wave rippled and partly mudcracked sandy shales and quartzose sandstones Daeda lus Planolites F lagoonal mud with spillover sands shed in from the beach barrier LF D and E by landward directed storm surges sublittoral Cruziana facies but somewhat restricted faunistically from the open sea sandy shales and siltstones with minor quartzose tempestites Cru ziana Possibly part of LF E represents the coastline landward of these lagoons The appearent lack of backshore and dune deposits thought to be characteris tic of beach barrier depositional systems Davidson Arnott and Greenwood 1976 Reinson 1984 Eliott 1986 is probably due to intense storm and wave erosion In Mesozoic to Cenozoic Umes protec tive plant growth represents an important factor in the preservation potential of supralittoral deposits Reineck and Singh 1986 Due to the absence of land G OLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE EE PP nn EEE plants supralittoral erosion may have taken place at higher rates in Ordovician times This problem however is present in any facies model for the Cluse de Orb Fm since tidally exposed sand flats accessible for eolian redeposition are proposed in any instance for lithofa cies
5. contrast with earlier Ordovician deposits of the region Tremadocian c 40 m Ma Tetragraptus zone of lower Arenigian c 100 m Ma thickness and biostratigra phic data from Courtessole et al 1982 The estimate of 2 3 Ma for the duration of the deflexus zone may indeed be wrong but even then the accumulation rates would have been much higher than during Tremadocian times the accumu lation of 1 500 m of sediment at 40 m Ma would take more than 35 Ma which is much more than the duration of the complete Arenigian stage Since the depositional surface did not agerade above sea level this implies accelerated subsidence of the Roquebrun area during the deflexus zone Subsidence rates of more than 200 m Ma strongly imply synsedimentary tectonic activity e g by thrust loading in orogenic fore land basins or extensional rift pull apart basin generation There is however no unequivocal structural or GEOLOGIE DE LA FRANCE N 4 1994 sedimentological evidence of Ordovi cian thrusting in the Montagne Noire or related areas but Upper Ordovician transtensional faulting has been docu mented from the paleogeographically related Cantabrian region of northern Spain Martinez Catalan et al 1992 For the Montagne Noire tectonic activi ty has not yet been documented prior to the Silurian when the Lower Paleozoic rocks were gently tilted and eroded be fore they were unconformably overlain by Upper Silurian and Lower Devonian
6. de puissance Des traces de Daedalus et des coquilles de lingules repr sentent les fossiles les plus fr quents Le lithofaci s E est form par des s ries finement stratifi es de schistes et de gr s quartzeux ripple marks et stratifications obliques Daedalus est galement pr sent Enfin le lithofaci s F regroupe des schistes et des siltites au sein desquels sont parfois intercal s des bancs de gr s quartzeux En ce qui concerne les traces fos siles on rencontre Cruziana mais pas de Phycodes Ces six lithofaci s forment des s ries cycliques et r guli res La partie inf rieure de la s rie est constitu e de cycles du type B C A B C Au dessus vien nent des cycles DEDE F DEDE auxquels succ dent des cycles BC puis l ensemble se termine par des schistes monotones d pourvus de termes ar nac s Le passage brutal des cycles B C aux cycles D E F indique une c sure dans l histoire s dimentaire L absence de termes ar nac s dans la partie sup rieure est un autre fait marquant dans la s dimentation Plusieurs indices montrent l existen ce d une activit tectonique syns dimen taire dans les zones proches du milieu de d p t Le taux d accumulation est beaucoup plus lev pendant l Ar nigien inf rieur qu au Cambro Tr madocien Ceci s explique par une acc l ration de la subsidence mettre en relation avec une tectonique en expansion De m me la disparition brutale du mat rie
7. marine deposits Feist and Sch nlaub 1974 If this unconformity stems from compressive or extensional tectonic acti vity is still open to debate The same is true for a local upper Cambrian discon formity that may indeed be non tectonic in origin Boyer and Guiraud 1964 For the Lower Ordovician however we deduce from the high rates of subsidence cited above that some kind of tectonic activity has occurred Facies model A Open shelf and offshore bar deposits In his study of the Cluse de l Orb Fm Eschard in Courtessole et al 1985 interpreted the muddy sequences of this formation as sublittoral open shelf deposit In his model the mica ceous quartzarenite units are thought to result from storm induced accumulation of sand to lens shaped offshore tempes tite bars accreting vertically up to fair weather wave base and capped by wave rippled littoral deposits In this case the Cluse de l Orb Fm may have been formed on the open shelf far from any exposed land surface There are however some hydrogra phic and paleoecological arguments against this model 1 Eschard in Courtessole et al 1985 considered the quartzitic bars of the Cluse de l Orb Fm to consist of rela tively homogeneous hummocky cross bedded deposits 20 40 m in thickness Detailed facies logging of these bars however revealed complex interstratifi cation of hummocky crossbedded wave rippled and muddy deposits 1 e of LF
8. depo sits due to storm generated suspension clouds It is indeed this minor proportion of sandy material that makes the diffe rence between LF A proper and the facies of the Landeyran shales There is indeed no clear evidence of deep water conditions from benthic fossil assem blages though a shift to more pelagic environments may be suggested On a paleoecological basis Dabard and Chau vel 1991 concluded that the depositio 11 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE nal depth of the Landeyran Fm has been less than 200 m The only distinctive property of the Landeyran shales compared to LF A of the Maurerie and Foulon formations seems to be the lack of sand prolifera tion from the former source area Sandy material is transported subtidally even to pelagic shelf environments due to tidal and oceanic currents and storm surges Sharma 1975 Nelson and Creager 1977 Walker 1984 Sand free environ ments seem only to develop where sand laden bottom surges are cut off from a given area either by divertion to subma rine channels or entrapment within intra shelf subbasins or by the definite cease of sand proliferation to the shelf From a theoretical point of view three alternati ve causes can be proposed 1 a very rapid and large relative rise in sea level Dabard and Chauvel 1991 Chauvel written comm 1994 that drowned the former source area from our point of view the
9. microprobe dating of up to 3 84 b y old detrital zircons Tectonophysics 157 pp 81 96 GEZE B 1949 Etude r gionale de la Montagne Noire et des C vennes m ridionales M m Soc G ologie de la France N S r 29 1 62 215 p GU RANGE LOZES J BURG J P 1990 Les nappes varisques du sud ouest du Massif central G ologie de la France n 3 4 pp 71 106 HARLAND W B Cox A V LLEWELLY P G PICKTON C A G SMITH A G WALTERS R 1982 A Geologic Time Scale 131p Cambridge Univ Press Cambridge HARLAND W B ARMSTRONG R L Cox A V CRAIG L E SMITH A G SMITH D G 1989 A Geologic Time Scale 2nd ed 263p Cambridge Univ Press Cambridge HUP P 1959 Quelques faits s dimentologiques dans F Ordovicien de Saint Chinian H rault Ann H bert et Haug 9 pp 79 84 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE CRE CB ROT EPO TCP et ESSA REE SET a ca A NL MARTINEZ CATALAN J R M HACAR RODRIGUEZ M P VILLAR ALLONSO P PEREZ ESTAUN A GONZALES LODEIRO F 1992 Lower Paleozoic exten sional tectonics n the limit between the West Asturia Leonese and Central iberian zones of the Variscan Fold Belt in NW Spain Geol Rdsch 81 2 pp 545 560 MATTAUER M PROUST F 1963 Sur le d versement vers le Sud et le style tectonique en t tes plongeantes des structures hercyniennes du versant sud de la Montagne Noire CR Acad Sci Fr D 257
10. pp 3967 3970 MATTE P 1986 Tectonics and plate tectonics mode for the Variscan Belt in Europe Tectonophysics 126 pp 329 374 MIALL A D 1991 Stratigraphic sequences and their chronostratigraphic correlation J Sedim Petrol 61 pp 497 505 NELSON C H 1982 Modern shallow water graded sand layers from storm surges Bering Shelf a mimic of Bouma sequences and turbidite systems J Sedim Petrol 52 pp 537 545 NELSON C H CREAGER J S 1977 Displacement of Yukon derived sediment from Bering Sea to Chukchi Sea during the Holocene Geology 5 pp 141 146 NELSON C H DUPRE W FIELD M E HOWARD J D 1982 Variation in sand body types of the eastern Bering Sea epicontinental shelf Geol Mijnb 61 pp 37 48 NOFFKE N 1992 Das untere Arenig Ordovizium der Synform von Roquebrun Montagne Noire Frankreich Diplom Arb Inst Geol Paldont Univ Tiibingen 55 p Unpubl OCZLON M S 1992 Gondwana and Laurussia before and during the Variscan Orogeny in Europe and related areas Heidelb Geowiss Abh 53 pp 1 56 Paris F ROBARDET M 1990 Early Paleozoic paleogeography of the variscan regions Tectonophysics 177 pp 193 213 PETTUOHN F J POTTER P E SIEVER R 1987 Sand and sandstone 2nd ed 553 p Springer Berlin Punt A G 1988 Sharp based shoreface sequences and offshore bars in the Cardium Formation of Alberta their relationship to relative c
11. quartzitique en lits pais litho faci s D intens ment bioturb par des traces verticales de Daedalus oblit rant presque le litage primaire Cluse de la Fm de l Orb Val l e de l Orb pr s de Maynard meter scale beds represent the most spectacular facies of the Cluse de l Orb Formation detailed description given by Eschard in Courtessole et al 1985 Parallcl lamination is seen at the bases of some individual beds and truncated by HCS Rounded mudclasts can be found on bedding planes Occasionally small shallow channels intersect the hummocks l Red concretions up to Im in diame ter occur in some beds They are ellip soidal in shape and somewhat flattened parallel to bedding Adjacent bedding textures bend around their margins enclosing those laminae that can be traced right through the nodules in a less compacted state and hence indicate an early diagenetic 1 e pre compactional origin of these concretions Cements within these concretions are partly sili ceous due to pressure solution and reprecipitation processes but mainly composed of Fe bearing carbonate side rite or Fe calcite J J Chauvel written comm 1994 The trace fossil assemblage sharply contrasts with those of the preceding lithofacies Daedalus halli Rouault predominates accompanied by less ubi quitous Skolithos burrows and few Pla nolites The internal texture of beds up to Im thick has been completely reorga nized by bi
12. rocks in other parts of Southern Europe Brenchley ef al 1986 Durand 1985 This type of storm dominated record of depositional processes is in accordance with paleogeographic reconstructions that place Southern Europe in a high latitude position that is in a climatic zone near the subpolar convergence cf Introduction Its situa tion may be compared to modern subarc tic shelves e g the Bering Sea where aside from oceanic and tidal currents storm reworking and redeposition is one of the most important depositional pro cesses Sharia et al 1972 Sharma 1975 Nelson and Creager 1977 Nelson 1982 A clastic ramp barrier shoreline facies model as favored here is consis tent with the lithofacies characteristics and relationships and avoids the difficul ties associated with an open shelf off SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE shore bar model In both models the shale sandstone alternations of the Mau rerie and Foulon formations are interpret ed as shallow marine deposits of a graded shelf ranging from a deep muddy distal facies to shallow sandy storm dominated proximal facies Contrary to the offshore bar model of Eschard in Courtessole et al 1985 however the crossbedded quartzarenites and intercalat ed mudstones of the Cluse de Orb Fm are proposed here to represent barrier beach and lagoonal deposits The deposi tional site of the Roquebrun de
13. waves dispersal and levelling to some equilibrium surface would rather be expected than accumulation of distinctly localized sandy bars surrounded by muddy spacings Though offshore sand bars and ridges are common features on present shelves most of them seem to be remnant beach barrier deposits drowned during the Early Holocene transgression Field 1980 Nelson et al 1982 Storm reworking of these palimpsest sands tends to produce widespread continuous sheet sand bodies Field 1980 Swift er al 1983 rather than isolated patches of amalgamated tempestites Unfortunately the Ordovician deposits of the study area are too heavily tectonized to map the original shape of the sandstone bodies in question If after all an offshore bar geometry is suspected one has to pro 13 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE sea level storm wave base Fig 12 Proposed depositional environments of the Ordovician deposits of the Roquebrun area and distribution of selected trace fossils A outer shelf mud B distal tempestite facies below storm wave base C proximal tempestite facies above storm wave base D shoreface E littoral and lagoonal shoreline deposits on both sides of F F lagoonal sublittoral with spillover lobes Not to scale Fig 12 Milieux de d p ts propos s pour les d p ts ordoviciens de la zone de Roquebrun et dis tribution de certaines traces fossil
14. which predominate in the upper parts of the formation The ded sandstones siltstones and shales alternating with 20 40m thick succes sions of medium grained micaceous quartzarenites represent the Cluse de l Orb Formation Gr s de la Cluse de l Orb Courtessole ef al 1982 The sandy shales and decimeter thick sand SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE Devonian RMXT rn sss Landeyran ene Formation ha d ae 250m Gray Shale Mamber Cluse de l Orb Formation 200m Setso Member Lower Member unnamsd Maur rle Formation 1000m Lower Arenigian pars stones of the succeeding Foulon Forma tion Couche de Foulon Dean 1966 lithologically resemble those of the Maurerie Fm below With a sharp break in lithology the uppermost Foulon sand stones are followed by gray and black sand free shales of the Landeyran For mation Schistes du Landeyran Thoral 1941 Due to pre Devonian erosion this uppermost unit is only preserved in the southern district of the Roquebrun syn form Lithofacies types On the basis of lithological and tex tural characteristics we distinguish six lithofacies types referred to as Lithofa cies LF A to F They differ from each other in sand to mud ratios sedimentary textures and in their trace fossil assem blage Shelly fossils were not subject of this study and paleontologic data cited below are mainly qu
15. 1992 Sedimentary basins 628 p Springer Berlin ELLIOTT T 1986 Siliciclastic shorelines n Reading H G ed Sedimentary environments and facies 2nd ed pp 155 188 Blackwell Oxford ENGEL W FEIST R FRANKE W 1981 Le carbonif re antc st phanien de la Montagne Noire rapports entre mise en place des nappes et s dimenta tion Bull B R G M Fr 2 I n 4 1980 1981 pp 341 389 FEIST R SCHONLAUB H P 1974 Zur Silur Devon Grenze in der stlichen Montagne Noire Siid Frankreichs N Jb Geol Pal ont Mh 4 pp 200 219 FIELD M E 1980 Sand bodies on coastal plain shelves Holocene record of the U S Atlantic inner shelf off Maryland J Sedim Petrol 50 pp 505 528 FREY R W PEMBERTON S G 1984 Trace fossil facies models Jn Walker R G ed Facies models 2nd ed pp 189 207 Geosci Canada Reprint Ser 1 GEBAUER D GRUNENFELDER M 1974 Rb Sr whole rock dating of late diagenetic to anchimetamorphic Paleozoic sediments in southern France Montagne Noire Contr Min Petrol 47 pp 113 130 GEBAUER D GRUNENFELDER M 1977 U Pb systematics of detrital zircons from some unmetamorphosed to slightly metamorphosed sediments of Central Europe Contr Min Petrol 65 pp 29 37 GEBAUER D WILLIAMS I S COMPSTON W GRUNENFELDER M 1989 The development of Central European continental crust since the Early Archaean based on conventional and ion
16. Courtessole et al 1982 1985 who also published a preliminary map of this region based on fossil occu rances A tentative geological map of the study area has been worked out by Noffke 1992 Fig 1 C is a simplified sketch of this map Due to the rarity of fossils this map is based primarily on lithologic and trace fossil evidence The lithostrati graphic units used therein and herein Fig 2 were defined after outcrop occu rances and descriptions given by Cour tessole et al 1982 1985 The whole sequence is intensively folded and fault ed throughout the study area and logs of undisturbed sections of more than a few tens of meters are not available In thicker sections tectonically introduced gaps and repetitions have to be consi dered All stratigraphic logs dealing with more than a few meters in vertical suc cession therefore are derived by compi lation of data from different oufcrops Nevertheless the order of lithologic and hence facies succession seems to be well established Courtessole et al 1981 1982 1985 Dabard and Chauvel GEOLOGIE DE LA FRANCE N 4 1994 etats Seta tet ts 4 Massif Central Mt Peyroux nappe black outcrop area of Ordovician rocks study area C wrtararere SRR eR EHO H ES Seeeewaneee LELELELLLS arante teeee TALE mnm ssammmssaasssas seem anareerer arene Fak ye FS fe sean COLLE LL SE EH Ee SHH ERE EERE sous ss C po
17. E A rapid fall in sea level can be in ferred from the sharp and perhaps erosive base of the Cluse de l Orb Fm which can be interpreted in terms of a rapid seaward shift of shoreline sedimentation onto former shelf environments Plint 1988 Yet even this interpretation can not be followed without at least one spe culative preassumption because it involves shoreline sedimentation one has to imply some nearby topographic high to act as a depositional nucleus to prograde from or transgress onto Note that this uplift is not implied to be loca lized at but adjacent to the depositional site There is no petrographic evidence for a new sediment source at the base of the Cluse de l Orb Fm Dabard and Chauvel 1991 If a tectonic rise is in voked to explain the depositional pattern of this formation this inferred rise has obiously not been eroded to its basement but only in its upper sedimentary cover The resedimented material of this cover was in its upper portions the late ral equivalent of the Maurerie Fm and thus of the same Gondwanan prove nance The lack of petrographically dis tinguishable sedimentary material therefore is not necessarily at variance with a local rise of significant topogra phy The need for some local rise becomes even more pronounced when the paleogeographic situation of the Early Ordovician Gondwana shelf is considered e g Paris and Robardet 1990 Cocks and Fortey 1990 the
18. Fr 11 pp 147 148 THORAL M 1935a Contribution l tude g ologique des monts de Lacaune et des terrains Cambriens et Ordoviciens de la Montagne Noire Bull Serv Carte G ol Fr 129 38 320 p THORAL M 1935b Contribution l tude pal ontologique de Ordovicien inf rieur de la Montagne Noire et r vision sommaire de la faune Cam brienne de la Montagne Noire 2 Th se tat Paris 363 p THORAL M 1941 Stratigraphie et faci s de l Ar nig languedocien Ann Sci Nat Univ Lyon C2 pp 99 150 THORAL M 1946 Cycles g ologiques et formations nodulif res de la Montagne Noire N Arch Mus Hist Nat Lyon 1 103 p VarL P R AUDEMART F BOWMAN S A EISNER P N PEREZ CRUZ C 1991 The stratigraphic signatures of tectonics eustasy and sedimentology an overview In Einsele G Ricken W and Seilacher A eds Cycles and events in stratigraphy pp 617 659 Springer Berlin VAN DER VOO R 1988 Paleozoic paleogeography of northern America Gondwana and intervening displaced terranes Comparison of paleomagne tism with paleoclimatology and biogeographical patterns Geol Soc Am Bull 100 pp 311 324 VAN WAGONER J C POSAMENTIER H W MircHimM R M mR Van P R SARG J F LOUTIT T S HARDENBOL J 1988 An overview of the fundamen tals of sequence stratigraphy and key definitions SEPM Spec Publ 42 pp 39 45 VERNAY P 1983 Tectonique et microtectonique
19. Gondwanaland shoreline was located well upon the Sahara Platform far from the South European depositional realm near the shelf edge Lower Ordovician tectonic activity involving local uplift and neighbouring subsidence has been reported from northern Spain Martinez Catalan et al 1992 and highly evolved pre Caradocian palcosols developed at the famous Sardic Unconformity in Sardinia Coccozza and Jacobacci 1975 Sardinia is placed offshore relati ve to southern France in pre Alpine tec GEOLOGIE DE LA FRANCE N 4 1994 B Tectonic rise model C Sequence stratigraphic model nonmarine inferred Fig 13 Diagrammatic comparison of the lithostraligiaphic models for the Roquebrun series as discussed in the text Note that the original geometry of the Cluse de l Orb Fm LF D F is recons tructed different in either model SB sequence boundary ts transgressive surface In the sequence stratigraphic model C nonmarine deposits or paleosols are inferred as a working hypothesis to the south of the Montagne Noire e g on the Saharan platform Fig 13 Sch ma comparatif des mod les lithostratigraphiques de la s rie de Roquebrun examin s dans le texte On notera que la g om trie originale de la formation de la Cluse de l Orb lithofaci s D a F est reconstitu e diff remment dans chaque mod le SB limite de la s quence ts surface de transgression Dans le mod le de s quence stratigrap
20. OGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE gt mudclasts Lingulids Q Dabuulus GP Cruziana Lithofacies Lithology 2 DEF 9 3 d oO 9 m ras T A 3 Fig 9 Measured section of the uppermost 40 m of the Cluse de l Orb Fm Rieuberliou section The columns represent lithofacies succession left shale to sand ratio middle and sedimentary structure right Only those sedimentary structures visible at the outcrop wall have been included in the drawing Mudcracks are more common than depicted in this figure as indicated by their presence in the footwall rubble but hard to identify in cross section at the outcrop wall Fig 9 Section mesur e des 40 m sup rieurs de la formation de la Cluse de l Orb Les colonnes repr sentent une s rie de lithofaci s gauche le rapport gr s sable au milieu et la structure s dimentaire droite Seules les structures s dimentaires visibles l affleurement sont repr sent es sur la figure Les fissures de retrait sont plus fr quentes qu il n appara t sur cette figure comme l indique leur pr sence dans l boulis au pied de la paroi mais sont difficiles identifier sur la coupe transversale de l affleurement posed more specifically A next to B and B next to C as would be expected from Walther s law a geological bias not unequivocally verified in all pa
21. all and gap geometry Of some load casted sandstone beds plate 1 f An about 15 m thick pile of chaotic ball and pillow sandstones in a sandy shale matrix crop out at the Orb river 3 km southeast of Roquebrun detailed des cription in Andrieux and Matte 1967 Some of the sandstone beds reveal Planolites and Cruziana traces at their bases Fig 7 Of the latter three types have been identified Cruziana rugosa d Orbigny C rouaulti Lebesconte and C imbricata Seilacher Contrary to LF C Phycodes is virtually absent from this lithofacies Body fossils are dominated by trilobites accompanied by inarticul ate Brachiopods molluscs and rare echi noderms Courtessole et al 1982 1985 Referring to these authors howe ver the shelly fossil assemblage is less diverse than in the upper Maurerie Fm and therefore supposingly in LF C Interpretation Compared to LF C absence of Phycodes burrows and pro bably diminished diversity of marine body fossils indicate a somewhat restric ted environment for LF F Its stratigra phic position surrounded by shoreface and littoral sediments implies a lagoonal setting Subtidal muddy background sedimentation prevailed during fair wea ther conditions whereas sandy material may have been swept in by tidal currents or storm waves Interbedded thick quart zose tempestites may represent washo ver fans spilled in from the barrier shoreline during major storms Storm erosion and depo
22. base of few sandstone beds interpretation Predepositional ero sion and HCS indicate that storm waves affected the sea floor and deposited the dm scale sandstones in their waning stage Post depositional current rework ing probably occurred asymmetric ripples but there is no evidence of fair weather wave activity Muddy back ground sedimentation prevailed between individual storms as documented by the shaley interbeds The depositional envi ronment of LF C can be regarded as sublittoral open shelf but above storm wave base This view is supported by the presence of Cruziana traces which are thought to represent shallow marine sub tidal environments Seilacher 1964 1967 Frey and Pemberton 1984 The rarity of records of these traces can be attributed to the difficulties of iden tifying them where no change in sediment material has occurred and their low pre servation potential on pre storm sedi mentary surfaces Lithofacies D Amalgamated micaceous quartzarenites PI 1d Description Hummocky crossbed ded quartzitic sandstones arranged in 3 to 5 m thick units of amalgamated deci SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE EE A OS Fig 5 Thick bedded quartzitic sandstone Lithofacies D intensively bioturbated by ver tical Daedalus traces which almost obliterate primary bedding textures Cluse de l Orb Fm POrb valley near Maynard Fig 5 Gr s
23. cassante tardi hercynienne dans la partie centrale du versant sud de la Montagne Noire G ologie de la France n 1 2 pp 31 54 WALKER R G 1984 Shelf and shallow marine sands Jn Walker R G ed Facies models 2nd ed pp 141 170 Geosci Canada Reprint Ser 1 18 GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE RE RER EEE SSSA E Plate 1 Lithofacies types of the Ordovician rocks near Roquebrun a Unstratified marine mudstone with limonitic nodule Lithofacies A b Thin bedded shale sandstone alternations Lithofacies B scale bar c 50 cm c Decimeter thick sandstone heds with shale intercalations Lithofacies C d Thick bedded hummocky cross stratified quartzitic sandstone Lithofacies D e Thin bedded quartzitic sandstones displaying wavy and flaser bedding Lithofacies E f Sandy shales with intercalated thin quartzitic sandstone beds Lithofacies F Large load cast to the left is about Im thick GEOLOGIE DE LA FRANCE N 4 1994 Planche 1 Types de lithofaci s des roches de l Ordovicien pr s de Roquebrun a boues marines non stratifi es avec nodule limonitique Lithofaci s A b alternances gr s shales en couches minces Lithofaci s B chelle c 50 cm c couches de gr s d paisseur d cim trique avec intercalations de shales Lithofaci s C d gr s quartzitique stratifications entrecrois es en
24. cknowledgments This study arose from a Diplomarbeit Noffke 1992 carried out at the Univer sity of T bingen under the supervision of A Seilacher and H HiiBner who are gratefully acknowledged for their steady interest and valuable advice We further benefited from discussions with R Feist Montpellier We thank R Riding Car diff who revised the manuscript and improved our English We further ack nowledge J J Chauvel Rennes and J Rey Toulouse for their critical review of the manuscript and their sug gestious to Clarify some passages B Ligouis T bingen translated the french summary The study was support ed by the T binger Stipendienstiftung ANDRIEUX J MATTE P 1963 Quelques traits s dimentologiques des flyschs cambro ordoviciens et vis ens de la Montagne Noire Feuille de Saint Chinian An 50 000 Bull Cart G ol Fr 59 273 pp 155 178 16 GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE ARTHAUD F 1970 Etude tectonique et microtectonique compar e de deux domaines hercyniens les nappes de la Montagne Noire et l anticlinal de l Iglesiente Th se Sci Univ Montpellier 175 p ARTHAUD F MATTAUER M PROUST F 1966 La structure et la microtectonique des nappes hercyniennes de la Montagne Noire Colloq Et Tect Neuch tel pp 229 241 BECQ GIRAUDON J F 1990 La structure conc in cone e
25. cles of the Foulon Fm The uppermost Foulon prograding sandstones and Landeyran Shales repre sent the succeeding highstand deposits As outlined above Facies succes sions d the Foulon Landeyran bound ary probably developed due to a change in paleogeography and sediment dispers al patterns rather than water depth 15 A major advantage of this model is that there is no need to imply some hypothetical tectonic rise or anactualistic hydrodynamic conditions on the outer shelf The main disadvantage however is that it is based only on a very small outcrop area and a theoretical stratigra phic model instead of a well established two dimensional stratigraphic profile Further the compatibility of sequence stratigraphic models derived from seis mic lines with observable outcrop strati graphy has yet to be improved Miall 1991 Schlager 1991 The validity of facies model C therefore strongly depends on the validity of its main assumption that is how far the stratigra phic model used here for the Gondwana continental margin represents a plausible approximation to its former stratigraphic architecture The Lower Arenigian siliciclastic deposits of the Roquebrun area were laid down in a shallow marine shelf environ ment The most characteristic feature of this sequence are cross stratified sand stones dominated by storm generated internal textures In this respect the Roguebrun deposits compare to Ordovi cian
26. d Chauvel 1991 It seems more likely that the advanced maturity of the micaceous quartzarenites may have been achieved by wave and storm reworking at or near their site of deposition a view supported by the pre sence of wave and storm wave related sedimentary structures Apart from petrologic maturity the step from uppermost Cluse de l Orb hummocky crossbedded amalgamated sandstones LF D to lowermost Foulon hummocky crossbedded sandstones with shale interbeds LF C seems rather slight and compares to shoreface to near shore shelf facies changes frequently described in the geologic literature Reviews given by Elliott 1986 Petti john et al 1987 Einsele 1992 Little can be said about the significance of the lower boundary of the Cluse de l Orb Fm since the uppermost portion of the Setso Member is commonly disturbed by Hercynian faulting and part of the succession may have been lost in some outcrops Immediate erosive contact of LF D on thickening upward LF B seems probable however d Landeyran Formation One of the most striking features of the Roquebrun series is the abrupt cessa tion of sand deposition at the base of the Landeyran Fm Superficially the shales of the Landeyran Fm resemble lithofa cies A in diagenetic facies sideritic nodules and faunal composition Dean GEOLOGIE DE LA FRANCE N 4 1994 Fig 10 Alternations of Lithofacies D E and F in the Cluse de L Orb Formation a Quartzi
27. dans les shales Le gr s pr sente un l ger feuille tage horizontal l int rieur du chenal avec stratification entrecrois e d angle faible stratification entrecrois e mamelons au milieu du lit Lithofaci s C Fm du Fou lon Section de Rieuberliou chelle en cm alternate With sandy shales and thin cm scale sandstones or are stacked upon each other with only thin shaley inter beds It 1s well developed in the upper portion of the Maurerie lower Member but it also occurs in the Foulon Forma tion The frequency of the dm scale sandstone beds is 10 50 10 m Erosive bases displaying gutter casts Fig 4 small scours and prod marks and hum mocky cross stratification HCS are the most prominent feature of these sand stones The tops of the hummocky cross stratified beds may be gradational or sharp sometimes capped by asymmetric ripple marks In some instances howe ver internal textures have been obscured by soft sediment deformation processes convolute bedding load casts ball and pillow structures As in LF B Phycodes circinnatum Richter is the most important trace fos sil Moreover casts of Cruziana rugosa d Orbigny can rarely be found at the bases of some beds e g lowermost Setso Member Shelly fossils are rare and seem to be restricted to reworked bioclasts e g echinoderm or mollusc debris In the lowermost Foulon Fm abraded lingulid shell fragments occur as graded shell bed at the
28. due to tectonic effects seems to include some stratigra phic hiatus indicated by the superposi tion of LF D on LF B This may possibly be due to a relative fall in sea GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE LRA eA a LN PS A ER GS e ar a a TES TE O PT EE 2 ean level e g tectonic uplift or eustatic sea level drop but a sudden increase in sediment supply cannot be excluded until the fine stratigraphy of this bounda ry is better confirmed from undisturbed outcrop sections As outlined above the cessation of sandy input at the Foulon Landcyran boundary is probably related to a change in sediment supply rather than to a chan ge in depositional environment This may have occurred either by topographic isolation from or drowning of the for mer source area Accumulation rates According to Courtessole ef al 1982 the sedimentary pile considered here has to be placed completely within the Didymograptus deflexus zone of the lower Arenigian which starts near the base of the Maurerie Fm and still conti nues during deposition of the upper Lan deyran Fm The duration of this graptolite zone may be proposed to be about 2 3Ma depending on the time scale used Harland et al 1982 1989 If the total thickness estimate of c 1 500 m is correct surprisingly high accumulation rates of 200 to 500 m Ma have to be inferred hese rates strongly
29. el clastic ramp bar rier shoreline facies sequence from open Shelf mud A and distal tempestite facies B through proximal tempestite CY shoreface D to shoreline E and lagoonal deposits F This second model implies a nearby emergent area which is assumed to represent a local tec tonic uplift in this part of the lower Ordovician Gondwana shelf 3 sequen ce stratigraphic model continental mar gin shelf break facies sequence with intercalated peritidal lowstand deposits The depositional environments of litho facies A to F respectively are inter preted in the same way as in the previous model but the coastal deposits of litho Jacies D to F are proposed to rim the exposed Gondwana shelf during sea level lowstand The present field data do not allow to make a clear decision be tween these models From a sedimentolo gic point of view the first model seems to be the least probable In any of these models a short term relative sea level fall during the time of deposition is indicated Syndepositional tectonic activity is indi cated by increased accumulation rates of the early Arenigian deposits and can be inferred from the abrupt cessation of sandy input near the top of the sequence R sum Dans les environs de Roquebrun H rault Montagne Noire on observe Manuscrit re u le 17 d cembre 1993 accept d finitivement le 2 septembre 1994 1 Present adress Carl von Ossietzky Universitat Oldenburg Inst
30. es A boues du plateau ext rieur B faci s de tempestite dista le au dessous des vagues de temp te C fuci y de tempestite proximale au dessus de la base d une vague de temp te D zone infratidale E d p ts c tiers littoraux et lagunaires de part et d autre de F F lagunaire sublittoral avec lobes de d versement Echelle non respect e pose some other mechanism for sand accumulation e g tidal or oceanic currents and to claim intense storm wave rework ing of these bars to explain their HCS dominated internal architecture 3 If adjacent Maurerie and Foulon Formations are taken into account their lithological and ichnofaunal differences to LF F of the Cluse de l Orb Fm clear ly indicate a different environment for the latter cf descriptions of LF C and F respectively Compared to the inferred open marine conditions of LF C the Cluse de l Orb deposits of LF F have to be ascribed to somewhat restricted conditions as implied by the absence of Phycodes and diminished tempestite fre quency At the present state of informa tion this seems to be constrained by the restricted faunal diversity of the Cluse de l Orb Fm compared to Maurerie and Foulon formations reported by Courtes sole et al 1982 1985 In an offshore 14 bar model however this may be achiev ed by the assumption of a period of extraordinary hydrodynamic conditions compared to the normal open shelf enviro
31. es Subsequent authors however classified the Lower Ordovician rocks as flyschoid deposi tional facies Hup 1959 Andrieux and Matte 1963 More recent studies e g Eschard in Courtessole et al 1985 Dabard and Chauvel 1991 recognized them as storm dominated shelf deposits thus returning to a shallow marine inter pretation The diverse brachiopod trilo bite assemblages throughout the succession also indicate shallow marine conditions Dean 1966 Courtessole et al 1981 1982 1983 1985 Ordovician stratigraphy and paleo geographic reconstructions of this region suffer from limited outcrop areas and intense tectonic deformation Much pro gress has been made by the recent bios tratisraphic studies of Courtessole ef al 1981 1982 1983 1985 but as in any biostratigraphic scheme the relative importance of facies dependent versus evolutionary control of the observed fau nal changes has to be assessed carefully Sedimentologic interpretation of the depositional facies may help to elucida de the depositional and environmental history recorded by this sedimentary pile The main purpose of this study is to discuss three alternative facies models for the Lower Ordovician deposits of the Roquebrun area eastern Montagne Noirc which have somewhat different consequences for the pre Hercynian paleogeographic and tectonic evolution of this part of southern Europe Geologic setting The study area is located sou
32. formation de Maurerie au nord de la zone tudi e buted to the lithofacies types by combi nation of these literature data with only limited field data Our remarks on body fossil occurances have to be regarded to as preliminary therefore We wish to make clear that the relationships be tween body fossil assemblage and depo sitional facies as supposed herein are tentative and have to be confirmed or rejected by further studies Lithofacies A Noduliferous shales and sandy shales PI 1a Description Fissile sandy shales of the Maurerie and Foulon Formations commonly exhibit gray to greenish gray colours black bitouminous sandy shales are restricted to the upper Setso Member of the Maurerie Fm Stratification is indistinct to absent Sand size material dominated by detrital mica flakes makes up c 1 5 of the sediment the higher values are achieved near the facies tran sition to LF B Fossiliferous concretions 5 to 20 cm in diameter can be found throughout the lithofacies They are of variable composition siliceous calca reous and sideritic nodules have been reported Sideritic concretions are the most common variety though in outcrop most of them are weathered to red ochre yielding easily accessible but fragile shelly fossils nodules limoniteux Cour tessole et al 1982 Chevron like sili ccous conerctions resembling cone in cone structures can also be found Bonte 1945 Becq Giraudon 1990 Though n
33. hanges in sea level SEPM Spec Publ 42 pp 357 370 REINECK H E SINGH I B 1986 Depositional sedimentary environments 2nd ed 551 p Springer Berlin REINSON G E 1984 Barrier island and associated strand plain systems Jn Walker R G ed Facies models 2nd ed pp 119 140 Geosci Cana da Reprint Ser 1 SCHLAGER W 1991 Depositional bias and environmental change important factors in sequence stratigraphy Sedim Geol 70 pp 109 130 SCOTESE C R McKERROW W S 1990 Revised world maps and introduction In McKerrow W S amp Scotese C R eds Paleozoic paleogeography and biogeography Geol Soc Mem 12 pp 1 21 SEILACHER A 1964 Biogenic sedimentary structures Jn Imbrie J amp Newell N D eds Approaches to paleoecology pp 296 316 New York SEILACHER A 1967 Bathymetry of trace fossils Marine Geol 5 pp 413 428 SHARMA G D 1975 The Alaskan Shelf hydrographic sedimentary and geochemical environment 498 p Springer New York SHARMA G D NAIDU A S Hoop D W 1972 Bristol Bay a model contemporary graded shelf AAPG Bull 56 pp 2000 2012 Swift D J P FIGUEIREDO A G jr FREELAND G L OERTEL G F 1983 Hummocky cross stratification and megaripples a geological double stan dard J Sedim Petrol 53 pp 1295 1317 THORAL M 1933 Stratigraphie de l Ordovicien de la vall e du Landeyran H rault C R Somm Soc G ol
34. hin crossbedded sandstone beds PI 1b Description This facies includes sandy shales with intercalated centime G OLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE GE GPU EN ter scale sandstone beds and is restricted to the Maurerie and Foulon Formations The sandstones are medium to dark gray in colour only within the Setso Member do black crossbedded sandstones occur The frequency of sandstone intercala tions ranges from 1 2 m near the transi tion to LF A to 10 m adjacent to LF C The sandstone beds show diverse inter nal textures plane lamination unidirec tional small scale plane and concave crossbed sets and climbing ripple lami nation Gradation from medium to fine sand is rarely visible In places graded concentrations of abraded shells can be found at the base of some of the sand stones Apart from undetermined simple burrows Planolites bioturbation is dominated by Phycodes circinnatum Richter Some of the sandstone beds are totally homogenized by bioturbation and their bases are densely covered by Phy codes traces Fig 3 In one instance a probable Zoophycos trace could be reco gnized on a weathered sandstone surfa ce Body fossils are less frequent than in LF A though perhaps of comparable diversity From our field observations we suggest that benthic fossils prevail in the sandstones but planktic and benthic forms can be found in
35. hique C on inf re la pr sence de d p ts non marins ou de pal osols titre d hypoth se de travail au sud de la Montagne Noire par ex sur la plate forme saharienne tonic reconstructions Scotese and McKerrow 1990 Oczlon 1992 Hence the possible existence of some Early Ordovician topographic rise near the Montagne Noire though not yet inde pendantly proven may at least be held as a plausible assumption Facies model C Shelf break sequence stratigraphy A third interpretation can be propo sed however on the basis of sequence stratigraphic models of continental mar gins e g Van Wagoner ef al 1988 Vail et al 1991 In this model deposi tional environments of LF A to F are also interpreted in terms of a shelf to shoreline depositional system Fig 12 but it differs from facies model B in its paleogeographic setting In this third model the shales and sandstones of the Maurerie Fm are interpreted to be highstand deposits laid down in an outer shelf or shelf edge environment discon formably capped by a sequence bound ary erosive base of the Cluse de l Orb Fm A relative fall in sea level shifted the bayline from its normal high stand position on the Saharan Platform to the shelf edge at or near the Roque brun depositional site The nearshore deposits of the Cluse de l Orb Fm repre sent the lowstand deposits of the next sequence superseded by transgressive thinning upward cy
36. ieur de la Montagne Noire M m Soc Et Sci Aude Carcassonne 32 p COURTESSOLE R PILLET J Vizcaino D 1982 Brachiopodes Articul s et Mollusques Bivalves Rostroconches Monoplacophores Gastropodes de l Ordovicien inf rieur Tremadocien Arenigien de la Montange Noire France m ridionale M m Soc Et Sci Aude Carcassonne 63 p COURTESSOLE R PILLET J VIZCAINO D ESCHARD R 1985 Etude biostratigraphique et s dimentologique des Formations ar nac es de I Arenigien du Saint Chinianais oriental H rault versant sud de la Montagne Noire France m ridionale M m Soc Et Sci Aude Carcassonne 99 p DarArD M P CHAUVEL J J 1991 Signature p tragraphique et pal obiologique des variations bathym triques pendant l Ar nig inf rieur dans la Montagne Noire versant sud r gion de Saint Chinian G ologie de la France 2 n 4 pp 45 54 DAVIDSON ARNOTT R G D GREENWOOD B 1976 Facies relationships on a barred coast Kouchibouguac Bay New Brunswick Canada SEPM Spec Publ 24 pp 149 168 DEAN W T 1966 Lower Ordovician stratigraphy and trilobites of the Landeyran valley and the neighbouring district of the Montagne Noire south western France Bull Brit Mus Nat Hist Geol 12 6 pp 247 353 DURAND J 1985 Le Gr s Armoricain S dimentologie traces fossiles milieux de d p t Centre Arm Et Str Socles Rennes M m Doc 3 105 p EINSELE G
37. itut fiir Chemie und Biologie des Meeres Meeresstation Schleusenstr 1 26382 Wilhelmsha ven Germany 2 Institut f r Geologie und Pal ontologie der Universitat Sigwartstr 10 72076 T bingen Germany GEOLOGIE DE LA FRANCE N 4 1994 des gr s et des schistes de l Ordovicien inf rieur affect s par une forte d forma tion tectonique Les d p ts les plus jeunes de cette s rie objet de l tude ont une puissance d environ I 500 m Cette s rie appartient l Ar nigien inf rieur et est recouverte en discordance par le Silurien et le D vonien La pr sence de fossiles marins dans la s rie clastique du groupe de Roque brun indique qu il s agit de d p ts de plate forme Six lithofaci s ont t dis tingu s ABCDEetF Le Lithofaci s A comprend des schistes et des shales d pourvus de niveaux gr seux Le lithofaci s B est compos de schistes analogues dans lesquels sont intercal s de minces bancs de gr s pr sentant souvent des traces de Phycodes Le lithofaci s C correspond des bancs de gr s o sont intercal s des schistes en fins niveaux comportant ga lement des Phycodes Le lithofaci s D comprend des gr s quartzeux sans intercalations de schistes Ces gr s sont constitu s en SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE bancs stratification entrecrois e en mamelons formant des barres bien visibles de quartzites de plusieurs m tres
38. l ar na c dans la partie sup rieure de la s rie a probablement une cause tectonique Les s diments clastiques de Roquebrun s in terpr tent comme des d p ts de mer pi continentale ouverte o pr dominent les temp tes Les lithofaci s A C y sont ainsi consid r s comme des s diments de plate forme caract re proximal subtidal croissant et les lithofaci s D F assimil s amp un complexe lagunaire a chenaux Les lithofaci s D a F ont par ailleurs t interpr t s par Eschard in Courtesolle et al 1985 comme des barres de temp te form es a proximit de la c te Trois sc narios sont propos s pour l ensemble de la s rie tudi e 1 D p t de plate forme externe avec formation intermittente de barres de temp te lors d un niveau marin bus 2 D p t de talus clastique avec formation locale d un faci s de plage sur haut fond tectonique lors d un niveau marin bas 3 Mod le identique de talus clastique mais la formation du faci s de plage est alors la cons quence d un niveau marin plus bas et correspond une limite de s quence D apr s les donn es dispo nibles aucun de ces mod les ne peut tre mis en avant En tout cas le carac t re cyclique du Groupe de Roquebrun est le r sultat de variations r gionales du niveau marin Le plus bas niveau marin est indiqu par le d but du cycle DEF base de la cluse de l Orb Biostratigraphiquement ce niveau se situe dans la zone a Defle
39. least probable possibility 2 channeling of the shelf edge and bypass of the sandy terrigeneous sediment into the deep sea during Landeyran time and 3 tectonically caused entrapment of sediment on its way to the Landeyran depositional sites In the latter specula tive model deep troughs were generated by extensional or wrench tectonics to the south of the study area separating the Montagne Noire area from its Gondwa nan hinterland It should be noted that separation only refers to the pattern of sediment distribution and does not necessarily imply significant tectonic or biogeographical segregation In this case a sand free pelagic but shallow water environment may have formed without significant change in relative sea level e Facies sequences In summary the succession studied can be subdivided into three distinct facies sequences characterized by diffe rent internal architecture Fig 11 1 Maurerie Formation built up of several 60 100 m thick depositional cycles consisting of lithofacies A to C Outcrop Facies A Lithostratigraphy interpretation model C Landeyran Formation Cluse de FOrb Formation Maurerie Formation Fig 11 Composite log and facies distribution for the upper 900 m of the studied section Lithofa cies types range from A open shelf most distal to F lagoonal most proximal Quartzitic Lingu la Sandstones wide stipple display complex facies architectu
40. mamelons en bancs pais Litho faci s D e gr s quartzitiques en lits minces stratification lenticulaire et ondul e Lithofaci s F f argilites sableuses avec intercalation de minces couches de gr s quartzitique Litho faci s F la grande figure de charge gauche mesure environ m d paisseur 19 UN MENSUEL DEUX SERIES Les Annales des Mines sont une revue mensuelle compos e de deux s ries compl mentaires R alit s industrielles et G rer et Comprendre Oe re 3 p gt G les an np ro PRE DOME PME PAAA MAA WANA SEI NON JIE VINE AONNE Drei FN SHH A rin a Mak i ut entr eus UC KE ae Ait ON SAIN Sete m DUR I ee ail Cachet Organisme REALITES INDUSTRIELLES So fois par an cette s rie des Annales des Mines fait le point sur un sujet technique un secteur conomique ou un probleme d actualit Chaque num ro en une viugtaine d arlicles propose une s lection d informations concr tes des analyses approfondies des connaissances a jour pour mieux appr cier les r alit s du monde industriel Les r seaux de service public La technologie nouvelles fronti res Le tunnel sous la Manche 1794 1994 M moires de jeunes ing nieurs Minerais et m taux non ferreux L nergie en France et dans le monde e Vers une gestion globale de l eau e La formation en Europe T l communications nouveaux produits nouveaux services gt Design et entrepri
41. n milieu de d p t siliclastique observations nouvelles sui les cune in cone de P Ordovicien inf rieur de la Montagne Noire sud du Massif central G ologie de la France 2 n 2 pp 11 20 BONTE A 1945 Observations sur les nodules structure cone in cone de l Arenig de la Montagne Noire Bull Soc G ol Fr 5 15 pp 453 478 BOYER F GUIRAUD R 1964 Observations nouvelles sur le passage du Cambrien l Ordovicien dans la Montagne Noire Bull Soc G ol Fr 7 6 pp 515 522 BERRY W B N WILDE P 1990 Graptolite biogeography implications for palaeogeography and palaeoceanography Geol Soc Mem 12 pp 129 137 BRENCHLEY P J ROMANO M GUTIERRES Makco J C 1986 Proximal and distal hummocky cross stratified facies on a wide Ordovician shelf in Ibe ria Can Soc Petr Geol Mem 2 pp 241 255 CoccozzA T JACOBACCI A 1975 Geological outline of Sardinia In Squires C H ed Geology of Italy 2 pp 49 81 Tripoli Cocks L R M FORTEY R A 1990 Biogeography of Ordovician and Silurian faunas Geol Soc Mem 12 pp 97 104 COURTESSOLE R MAREK L PILLET J UBAGHS G VIZCAINO D 1983 Calymenina Echinodermata et Hyolitha de l Ordovicien inf rieur de la Mon tagne Noire France m ridionale M m Soc Et Sci Aude Carcassonne 62 p COURTESSOLE R PILLET J VIZCAINO D 1981 Nouvelles donn es sur la biostratigraphie de l Ordovicien inf r
42. nment of the Maurerie and Foulon formations perhaps induced by sea level lowstand during Cluse de Orb time Facies model B Tectonic uplift island coastline Based on our field observations and regarding to the complete Lower Areni gian sequence we recommend a clastic ramp model as an alternative interpreta tion of the Roquebrun series Fig 12 This model implies no significant break in depositional history at the boundaries of the Cluse de Orb Fm apart from lateral shift of adjacent environments Lithofacies A to F respectively are interpreted to represent different seg ments of a continuous shelf to shore profile implying a standard barred beach depositional system Elliot 1986 Reineck and Singh 1986 ichnofacies after Frey and Pemberton 1984 A muddy open shelf well below storm wave base sandy shale B open shelf upper Zoophycos facies receiving minor sandy input from storm induced suspension clouds but not directly affected by storm surge bottom erosion sandy shale with thin sandstones Phycodes Planolites C open shelf Cruziana facies more proximal than B and located above storm wave base subjected to storm ero sion and redeposition hummocky cross bedded sandstones with shale interbeds Phycodes Cruziana D shoreface nearshore sublittoral site of frequent redeposition Skolithos facies mainly by waves and storms HCS dominated quartzose sandstones Daedalus
43. o specific trace fossils have been found the lack of clear stratifica tion may be related to intense bioturba tion homogenizing the sediment Shelly faunas are diverse and include both ben thic brachiopods trilobites echino derms bivalves and planktic grapto lites hyoliths cephalopods organisms They are scarcely scattered throughout the sediment only in few discrete hori zons they are somewhat enriched com pared to the bulk of the deposit Within the shaley sediment the fossils are typi cally damaged by the slaty cleavage but in carbonate or at the surface dchric nodules real fossil breccias can be found The gray and black shales of the Landeyran Formation closely resemble this facies no or weak bedding sideritic concretions diverse fauna of compa rable composition but differ by the absence of sandy material The abrupt cessation of sandy input documented by the Foulon Landeyran transition cannot easily be explained by simple facies shift to more distal environments and will be discussed separately below Interpretation Occasional bedding planes bearing shell concentrations may indicate rare erosional events No other evidence of strong bottom currents or of storm wave action has been observed Compared to the other lithofacies types sandy input is weak This sandy shale facies probably reflects outer shelf conditions with water depths well below storm wave base Lithofacies B Shales with t
44. oss bedded sandstones and shales of the Cluse de Orb Fm Two subfacies can be distinguished which alternate with each other on a dm scale El Thinbedded 2 5 cm rippled sandstones with shale drapes Pl le Oscillation ripples wavy lamination fla ser bedding and thin beds of parallel lamination can be seen Rare decimeter scale sets of planar to concave crossbed ded sandstones Eschard in Courtessole et al 1985 Pl II may also be included in this lithofacies E2 Shales and sandy shales com monly dark gray to black with cm scale crossbedded sandstone lenses and load casted ripples Fig 6 Polygonal shrin kage cracks occur in the shaley deposits Occasionally interbedded sandstone beds 5 20 cm bear dm scale interferen ce ripples on their tops Trace fossils are scattered throughout the sediment but are rarely dense enough to overprint primary bedding textures Daedalus Skolithos and Planolites seem to be the only recognizable traces Body fossils are rare mainly restricted to small fragments and bioclasts In thin section microscopic fragments of grap tolites have been identified Noffke and Ligouis in prep Interpretation LF E is characterized by small scale vertical changes in hydraulic conditions predominance of wave formed textures frequent emer sion and a low degree of bioturbation Shrinkage cracks and megaripple bed ding indicate some tidal influence but the overall depositional patte
45. oted from the litera ture Thoral 1933 1946 Dean 1966 Courtessole ef al 1981 1985 Distinct body fossil assemblages have been attri Foulon Formation 80m Fig 2 Lithostratigraphic sum mary column of the study area adapted from Courtessole ef al 1982 1985 Lowermost Ordovi cian deposits Barroubio and St Chinian Formations respectively crop oul to ihe west and uvrth west of the study area White gray shales and sandy shales black black bituminous shales and sandy shales dense stipple micaceous sandstones schema tic wide stipple complex inter stratifications of quartzose sandstones siltstones and sandy shales cf Fig 9 The Ordovician Devonian unconformity cuts down to the Maurerie Fm in the northern part of the study area Fig 2 Colonne lithostratigra phique d ensemble de la zone tudi e adapt e d apr s Courte sole et al 1982 1985 Les d p ts de la base de l Ordovicien inf rieur formations respectives de Barroubio et St Chinian affleurent l ouest et au nord ouest de la zone tudi e En blanc shales gris et argilites sableuses en noir schistes bitu mineux noirs et argilites sableu ses en pointill dense gr s mi cac s repr sentation sch mati que en pointill espac inter stratifications complexes de gr s quartzeux de siltites et d argi lites sableuses cf fig 9 La dis cordance Ordovicien D vonien coupe la
46. oturbation displaying nothing else than vertically arranged Daedalus cones side by side Fig 5 Coquinas of phosphatic lingulid shells are common in this lithofacies and gave rise to the name Gr s a Lingules referring either to the quartzitic deposits or to the complete Cluse de l Orb Fm Thoral 1941 They occur as graded shell beds at the base of some individual beds Other fossils are restricted to rare fragments and abraded clasts Interpretation The sudden increase in sandstone maturity from LF C to D probably reflects the transition from intermediate to high energy environ ments Frequent reworking and resedi mentation is also indicated by the replacement of the Phycodes Cruziana trace fossil assamblage by vertically oriented Daedalus and Skolithos traces indicative of depositional environments above normal wave base Seilacher 1967 Frey and Pemberton 1984 The sedimentary record is dominated by proximal storm deposits HCS rip cur rent channels Apart from Skolithos Facies bioturbation little trace has been left of fairweather redeposition at the sedimentary surface The common facies transition to and interfingering with wave rippled deposits of LF E see below indicates a coastal though pro bably sublittoral shoreface environ ment for LF D Lithofacies E Rippled sandstones and sandy shales with flaser texture PI 1e Description In LF E we include thin bedded variably rippled and cr
47. posits probably was situated at or near the shelf break of the continental margin Paris and Robardet 1990 Cocks and Fortey 1990 Two alternative stratigraphic set tings can be proposed to explain shoreli ne deposition at the outer shelf Some kind of tectonic uplift may be inferred as depositional nucleus for marginal marine deposition his hypothetical rise howe ver may have been uplifted slightly prior to Cluse de Orb time but became emer gent only during this time interval due to a temporary sea level lowstand Alterna tively the Cluse de l Orb Fm may repre sent a shelf margin lowstand deposit and form part of a model shelf edge sequence analogous to standard sequence stratigraphic models of modern continen tal margins Yet there is too little known about the depositional architecture of the Ordovician continental margin of Nor thern Gondwana to decide between these models A sea level lowstand during Cluse de Orb time is indicated in either of these facies models though the implied magnitude of relative sea level change is somewhat different A comparatively minor change may hold for offshore bar generation or emersion of a pre existing tectonic rise but a large fall in sea level and exposed shelves have to be inferred if the shelf margin facies model is adopt ed This lowstand is followed by rene wed transgression at the end of Cluse de l Orb time as indicated by the deepen ing upward architectu
48. re of the Foulon Fm and possibly by increased abundance of glaucony at the base of this Forma erence S tion Dabard and Chauvel 1991 The formation of this mineral which is also present near the base of the Cluse de l Orb Fm Dabard and Chauvel 1991 is not restricted to transgressive environ ments however and its occurrance may therefore represent other environmental changes not caused by sea level change Reineck and Singh 1986 Pettijohn er al 1987 Einsele 1992 Several short term sea level changes are superimposed onto this main regression transgression cycle documented by smaller scale cyclicities throughout the sequence Syndepositional tectonic activity is indicated by drastically increased accu mulation rates at the beginning of the Arenigian A further hint of tectonic activity can be seen in the sudden cessa tion of sandy input at the base of the Landeyran Fm Facies successions d This interpretation is of course even more speculative than those concerning depositional environment Due to imit ed outcrop area and pre Devonian ero sion these indications of Early Ordovician tectonic activity remain cir cumstantial Nevertheless southern France represents a white spot on Ordovician paleotectonic maps and there fore we may hope that even these tenta tive clues will contribute to the deciphering of the early paleozoic tecto nic evolution of the Hercynian Foldbelt A
49. re predominantly of LF D and E see Fig 9 Note that the shale sandstone alternations of the Cluse de l Orb Fm are composed of LF F with minor portions of LF E but those of Maurerie and Foulon formations belong to LF A to C respectively Outcrop bars designate 1 Route de Laurenque 2 Setso Valley 3 Rieuberliou Valley 4 Landeyran Valley cf Fig 1 In a sequence stratigraphic interpretation model C the Cluse de l Orb Fm is proposed to represent a lowstand shelf margin deposit superseded by trans gressive thinning upward Foulon shelf deposits The maximum flooding deposit is probably posi tioned within the Foulon Fm at the turning point from thinning upward to thickening upward architecture but may alternatively be hidden within the monotonous sand free shales of the Lan deyran Fm Fig 11 Distribution des logs composites et des faci s pour les 900 m sup rieurs de la section tu di e Les types de lithofaci s varient de A plateau continental ouvert sur le large le plus souvent distal F lagunaire le plus souvent proximal Les gr s Lingula quartztiques pointill espac t moignent de faci s complexes allant principalement des lithofaci s D E voir Fig 9 On notera que les alternances de schiste et de gr s de la Cluse de la formation de l Orb se composent du litho faci s F avec de petites parties du lithofaci s E mais que celles des formations de Maurerie et du Foulon appartiennent respecti
50. rn is wave dominated Deposition above fair wea ther wave base in a littoral environment foreshore back barrier or lagoonal sho reline seems probable Some deposits of LF EI may have been formed in a shal low sublittoral upper shoreface setting either Lithofacies F Sandy shales and siltstones with thin micaceous quartzarenite beds PI 1f Description In the Cluse de l Orb Fm LF D and E alternate with shale sandstone alternations up to 20 60 m thick These shaley sequences consist mainly of thin bedded sandy shales and siltstones with interbedded dm thick micaccous quartzarcnites 5 10 10 m Gutter casts and scours are common at the bases of these sandstone beds which frequently exhibit HCS This facies is rather similar to LF C in GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE overall appearance Nevertheless clear differences can be seen in the conspi cously lower frequency of massive sand stone beds and the occurrence of micaceous quartzarenites which are absent from LF C Rapid deposition of even meter scale sandstones is indicated by soft sediment deformation textures load balls convolute bedding and dish structures in some of these beds Sedi ment liquefaction and load casting however may also be due to periodic wave loading in shallow water during major storms Walker 1984 a possibili ty supported by equal distance b
51. rts of the sequence see d below Contrary to the smooth facies transitions within each cycle cycle and subcycle boundaries are clearly visible as abrupt changes in sandstone thicknesses and frequency and may represent either hia tuses or sudden shifts in depositional environment b Cluse de l Orb Formation The Cluse de Orb Fm is made up of lithofacies D E and F The transitions between these lithofacies are less gra dual than those between LF A to C des cribed above and decimeter to meter scale interfingering of different 10 facies is common Fig 9 As previously pointed out by Eschard in Courtessole et al 1985 there is a pronounced large scale cyclicity in the thickness range of 50 80 m and hence comparable to the Maurerie cycles produced by the alternation of two main facies associa tions 1 20 40 m thick units of thick bed ded micaceous quartzarenites with only minor intercalations of shale and sand stone Facies R4 of Eschard Gr s a Lingules sensu stricto Fig 10a These quartzitic units consist of meter scale alternations or subcycles of LF D E and F Fig 9 2 20 30 m thick units of sandy shales and siltstones with intercalated sandstones Fig 10b foreground pre dominantly of LI F Facies R1 and R2 of Eschard amalgamated hummocky cross stratified beds up to 1 2 m in thick ness are occasionally intercalated LF D Facies R3 of Eschard Locally bedding is dist
52. se TTE E E E era ess AATE cr e tee saunas eee ere Des ttes rre sure onction PR rE EE N O AINAN ad RENEA E P E E E A E ET AE T E eae tees RETRO EOE TO EERE EE ORTH HAE EERE MEER ESE Ree ED RUSE REESE EERE EVE HEE SAM EERE RARE HERE RAAEEOPHMREEERED REE ESE EHESS i i i H i BULLETIN D ABONNEMENT retourner aux ditions ESKA 27 rue Dunois 75013 PARIS T l 1 44 06 80 42 Fax 1 44 24 06 94 ett ON A ll ERR tt A GERER amp COMPRENDRE uatre fois par an cette s rie des Annales des Mines pose un regard lucide parfois cri tique sur la gestion au concret des entre prises et des affaires publiques et vous propose Des dossiers Nouveaux l nder Innovation Pourquoi licencie t on Des entretiens e Michel CROZIER chercheur et enseignant e Yves DUBREIL comment r ussir un projet impossible Jean CHOUSSAT coute Pankylose administrative e Francois PAQUEMENT crises d Est crises d Ouest Des t moignages Jacques MELEZE syst mique mode d emploi e Yuri SALIENKO Tchernobyl l croulement d un mythe Didier DEFERT le patron fran ais et l actionnaire am ricain Des articles th oriques Des d bats ditions Oy ESKA 7 Je m abonne pour une ann e ee R alit s Industrielles G rer amp Comprendre 11 n Tarif de Base France 815 FF C tranger 950 FF 7 Je m abonne pour un an
53. sition of tempestites took place less frequent than in LF C thus implying a comparatively protected site of deposition GEOLOGIE DE LA FRANCE N 4 1994 Fig 6 Flaser and linsen hedding in shaley sand stones Lithofacies E Cluse de Orb Fm l Orb valley near Maynard Fig 6 Stratification tenti culaire dans des gr s argi leux Lithofaci s E Cluse de la Fm de l Orb Vall e de l Orb pr s de Maynard Fig 7 Cruziana traces at the base of a thin quartzitic sandstone bed intercalated in shale Lithofacies F Cluse de l Orb Fm l Orb valley near Maynard Fig 7 Traces de Cruziana la base d une mince couche de gr s quartzitique intercal dans les shales Lithofaci s F Cluse de la Fm de l Orb Vall e de l Orb pr s de Maynard Fig 8 Tectonically inverted thinning upward sequence of the basal Foulon Fm Thick sandstone beds of Lithofacies C to the left grade to the right into thin bedded sandsto ne shale alternations Lithofa cies B and finally into sandy Shales rigu at hanuner Five smaller scale subcycles of about 1m thickness are visible in this part of the section Rieuberliou outcrop Fig 8 S quence tectoni quement invers e vers le haut de la formation basale du Foulon Couches de gr s pais du Lithofaci s C gauche passant aux alternances gr s shales en minces niveaux droite Lithofaci s B et enfin aux argilites subleuses droi
54. st Ordovician rocks IHH Landeyran Fm ee 2 Foulon Fm Cluse de l Orb Fm A Maurerle Fm black Setso Member Fig 1 A B Tectonic sketch map of eastern Montagne Noire and location of the study area a autochthon parautochthon F Mts de Faug res Nappe P Pardailhan Nappe Arthaud 1970 Engel et al 1981 C Simplified geological map of the Ordovician rocks of the Roquebrun Synform after Noffke 1992 The whole sequence is tectonically inverted Numbers 1 4 refer to outcrops cited in Fig 11 Fig 1 A B Carte tectonique sch matique de l est de la Montagne Noire et localisation de la zone tudi e A autochtone parautochtone F Nappe des Mts de Faug res P Nappe de Pardailhan Arthaud 1970 Engel et al 1981 C Carte g ologique simplifi e des roches de l Ordovicien du synforme de Roquebrun d apr s Noffke 1992 La s quence dans son ensemble est tectoniquement invers e Les num ros 4 renvoient aux affleurements cit s la figure 11 1991 Noffke 1992 though the given thickness data may have limited accura cy only uppermost 20m or so of the Maurerie Fm consist of lithologically similar but black coloured sandstones and shales Setso Member Dean 1966 l hin bed The oldest deposits of the study area belong to the Maurerie Formation Gr s et Schistes de la Maurerie Courtessole ef al 1982 These are gray sandy shales with intercalated micaceous medium grained sandstones
55. te du marteau Cinq sous cycles de plus petite chelle d environ m d paisseur sont visibles dans cette partie de la section affleurement de Rieuberlou Facies successions a Maurerie and Foulon formations The sedimentary successions of the Maurerie and Foulon formations are rather similar in that they consist of cyclic successions of lithofacies A B and C Several distinct thickening upward cycles each 60 100 m in thick ness can be recognized in the lower member of the Maurerie Fm On the other hand thinning upward cycles can be seen in the lower Setso Member and in the basal parts of the Foulon Fm The basal Foulon thinning upward cycle is only c 5 6 m thick but is further subdivi ded by 0 5 1 m thick smaller scale sub cycles Fig 8 Comparable high frequency cycles have only been observ ed in the Foulon and if the small scale facies variations of the D E F associa tion see below are referred to as cycles in the Cluse de Orb Fm Though all facies boundaries within these cycles are rather indistinct gradual facies shifts from LF A to LF B and from LF B to LF C in thickening upward cycles are present The first appearance of persistent sandstone beds as well as the onset of storm induced erosion and redeposition as indicated by scours and HCS supports the view that different lithofacies indicate different depositional environments These envi ronments however clearly were juxta SEDIMENTOL
56. the intermittent sandy shalcs Interpretation The graded and crossbedded sandstones indicate combi ned suspension traction transport Basal scour or other predepositional erosion is rarely documented Sandy input is sparse and sporadic probably representing short depositional events punctuating long periods of muddy background sedi mentation The crossbedded sandstones may have tormed when storm generated turbidity currents transported sandy material from nearshore environments to the onter shelf where it settled helow storm wave base Nelson 1982 Walker 1984 Lithofacies C Hummocky crossbedded sandstones with intermittent shales Pl 1c Description This facies consist of medium to coarse grained sandstone beds 10 100 cm in thickness that either GEOLOGIE DE LA FRANCE N 4 1994 Fig 3 Phycodes burrows at the base of a thin bioturbated sandstone bed Lithofacies B Setso Member Maurerie Fm Roquebrun Fig 3 Terriers de Phy codes la base d un mince lit de gr s bioturb Lithofaci s B Membre de Setso Fm de Maurerie Roquebrun Fig 4 Gutter cast infilling small meandering channel in shale The sandstone shows faint horizontal laminations within the channel and low angle crossbedding HCS in the middle of the bed Litho facies C Foulon Fm Rieu berliou Section scale in cm Fig 4 Petit chenal d cri vant des m andres dans le remplissage d une rigole creus e
57. theast of Roquebrun H rault where Ordovician rocks represent the central parts of a several kilometer wide synformal struc ture the Synforme de Roquebrun This structure is part of the inverted lower limb of the Mont Peyroux nappe one of three southward verging nappes of the GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE eastern Montagne Noire Mattauer and Proust 1963 Arthaud et al 1966 Arthaud 1970 Gu rang Lozes and Burg 1990 Fig 1 Deformation and nappe emplacement took place during the mid Carboniferous when terranes of Gondwanan crust were accreted to the southern European continental margin in the course of the Hercynian orogeny Gebauer and Griinenfelder 1974 Engel et al 1981 Vernay 1983 Matte 1986 Stratigraphy Ordovician deposits of the western Mont Peyroux nappe consist of silici clastic sediments of Tremadocian and Lower Arenigian age truncated by an angular unconformity and superseded by early Devonian clastics and carbonates Thoral 1933 1935a 1941 Geze 1949 Dean 1966 Feist and Sch nlaub 1974 Courtessole et al 1982 Fig 2 Within the study area only the upper 1 500 m of this Lower Ordovician sediment pile is preserved dated as lowermost Arenigian Didymograptus deflexus zone Cour tessole et al 1981 1982 1985 A revised scheme of litho and bios tratigraphical subdivisions has been work ed out by
58. tic bars of 10 20m thickness Lithofacies D and E light colour separated by sandy shales with tem pestitic sandstones Lithofacies F 30 80 m Rieuberliou section b Transition from Lithofacies D thick beds upper left to Lithofacies E light coloured thin beds left and dark coloured sandy shales of Lithofacies F foreground l Orb river near Maynard Both sections are tectonically inverted Fig 10 Alt rations des lithofaci s D E et F la Cluse de la formation de l Orb a barres quartzitiques de 10 20 m d paisseur Lithofaci s D et E en clair s par es par des argilites sableuses avec tempestites gr seuses Lithofaci s f 30 80 m section de Rieuberliou b Transi tion du lithofaci s D lits pais en haut gauche au lithofaci s E lits minces et clairs gauche et argilites sableuses de couleur sombre du lithofaci s F au premier plan rivi re de l Orb pr s de Maynard Les deux sections sont tectoniquement invers es 1966 Courtessole et al 1981 1982 1983 1985 An ad hoc interpretation therefore may be a shift to more distal and deeper water environments from induced by rapid subsidence or by a sud den sea level rise However relative sea level rise alone cannot explain the com plete absence of sandy material until the source area also subsided below sea level Otherwise at least small amounts of sand should be expected to be incor porated to the muddy outer shelf
59. urbed by soft sediment deforma tion textures ball and pillow structures minor slumps Facies R7 of Eschard Since any one lithofacies may be present in either unit the transition from one facies association to another is due to changes in relative lithofacies abun dance within the unit This implies that in spite of the quite different environ mental energy regimes documented lithofacies D to F have to be ascribed to intimately linked environments which may have easily been converted into each other though the probability of each to develop would have been dissi milar at different times GEOLOGIE DE LA FRANCE N 4 1994 SEDIMENTOLOGY OF LOWER ORDOVICIAN CLASTIC SHELF DEPOSITS MONTAGNE NOIRE FRANCE ES EE SO PE ET TESST ENS TEP REPS TS PTE NTE ES SEC c Boundaries of the Cluse de l Orb Fm There are sudden leaps in sandstone maturity from the uppermost Maurerie Fm to the Cluse de l Orb Fm as well as from the uppermost Cluse de l Orb Fm to the basal Foulon Fm These changes are mainly produced by the appearance and disappearance of micaceous quartza renites which are absent from preceding and later Ordovician deposits of the area In Lithofacies E and F of the Cluse de l Orb Fm however micaceous sand stones similar in composition to Maure rie and Foulon sandstones occur and mineralogical and geochemical studies have revealed no evidence for different sources of these two sandstone litholo gies Dabard an
60. vement aux lithofaci s A et C Les barres affleurantes d signent 1 Route de Laurenque 2 Vall e de Setso 3 Vall e de Rieuberliou 4 Vall e de Landeyran cf fig 1 Dans l interpr tation d une s quence stratigraphique mod le C nous proposons de consid rer la formation de la Cluse de l Orb comme un d p t de marge continentale de niveau bas recouverts par les d p ts de plate forme transgressifs du Foulon allant en s amincissant vers le haut Le d p t de plus hautes eaux se situe probablement l int rieur de la formation du Foulon au point de passage d un amincissement un paississement vers le haut mais peut aussi se trouver masqu l int rieur des schistes monotones exempt de mat riel ar nac de la formation de Landeyran 2 Cluse de l Orb and Foulon forma tions constructed by two hierarchical orders of cycles displaying LF D to F Cluse de l Orb Fm and A to C Foulon Fm respectively High frequency sub cycles of one to a few meters are grou ped into larger scale cycles of 6 10 m Foulon Fm or 50 80 m Cluse de VOrb Fm 3 Landeyran Formation consisting of homogeneously developed sand free shales Whereas facies boundaries within these sequences can be ascribed to late ral shifts of adjacent depositional envi ronments the transition from one sequence to another is marked by a dis tinct break in facies succession The Maurerie Cluse de l Orb boundary though poorly preserved
61. xus de l Ar ni gien inf rieur Seule la comparaison avec d autres r gions permettra de dire s il s agit d un v nement local ou global introduction In early Paleozoic time southwestern France was part of the Gondwana conti nent at its northern continental margin off the present African coast Matte 1986 Paris and Robardet 1990 Paleo geographic reconstructions van der Voo 1988 Scotese and McKerrow 1990 indicate Ordovician paleolatitudes of c 60 70 S and thus a position adjacent to or within the southern subpolar low pressure belt A high latitude position has also been suggested from pelagic tri lobites Cocks and Fortey 1990 and graptolites Atlantic cool water graptoli te province Berry and Wilde 1990 as well as from neritic biota North Gond wana Province Paris and Robardet 1990 Cocks and Fortey 1990 The Ordovician deposits of the Mon tagne Noire consist mainly of monoto nous sandstone shale alternations and some quartzitic sandstones formerly known as Gr s Lingules in their upper portion A panafrican basement source area has been inferred from petrography Dabard and Chauvel 1991 as well as from radiometric ages of detrital mica and zircons Gebauer and Griinenfelder 1974 1977 Gebauer et al 1989 Early workers Thoral 1935a b 1941 Geze 1949 proposed a shallow marine to littoral environment of deposi tion especially for the quartzose depo sits of the Gr s a Lingul

Sedimentology of - Géologie de la France

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Sedimentology of - G&eacute;ologie de la France

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Sedimentology of - Géologie de la France