Upper Silurian Trilobitesof Bohemian affinities from the West Asturian-Leonese zone (NW Spain)

FROM

UPPER SILURIAN TRILOBITES OF BOHEMIAN AFFINITIES WES T ASTURIAN-IJEONESE ZONE (NW SPAIN)

ISABEL R A B A N O & JUAN-CARLOS G U T I E R R E Z - M A R C O U.E.I. de Paleontologia, Instituto de Geologfa E¢on6mica (C.S.I.C.-U.C.M.), Facultad de Ciencias Geol6glcas, Universidad Complutense, 28040 Madrid, Espaf~a. MICHEL R O B A R D E T Laboratoire de Stratigraphie et Paldontologie, Universitd de Rennes I e t URA 1364 CNRS ~Paldozo~que matin gondwanien', F-35042 Rennes Codex. RABANO I., GUTIERREZ-MARCO J.C. & ROBARDET M. 1993. Upper Silurian Trilobites of Bohemian affinities from the West Asturian-Leonese zone (NW Spain). [Trilobites d'sffinlt~s bohdmiennes dans le Silurian sup~rieur de la zone Asturo-L~onaise (NO de l'Espagne)]. [Trilobites de Afinidades boh6micas en el Silfirico superior de la zona asturoccidental-Leonesa (NO de Espafia)]. GEOBIOS, 26, 3 : 361-376. 30.06.1993. Manuscrit d~pos~ le 13.04.1992 ; accept~ d~finitivement le 13.07.1992. ABSTRACT Fossiliferous localities within chloriteid slates of Upper Ludlow age i~om the West Asturian-Leonese Zone (N.W. Spain) in the Pefialba and Sil synclines are here reviewed. On a regional scale, the occurrence of similar facies and faunas within beth synclines indicates that the limit between the West Asturian-Leonese Zone and the Central-Iberian zone most probably runs within the Sil syncline. Trilobite faunas with representatives of the genera Crotalocephalus, Cerauroides, Cromus, Denckmannites ? and Lioharpes (Fritchaspis) have clear aflVmities with the Prionopeltis archiaci Assemblage of the Upper Ludlow in Bohemia. From a palaeogeographical point of view, the occurrence of Silurian trilobites with Bohemian affinities in N.W. Spain as well as in Pyrenees and Catalonia may suggest the possible existence of a North-Iberian Domain including these regions and southern France (Aquitaine, Montague Noire). Key-Words : Silurian, Ludlow, Spain, West Asturian-Leonese zone, Trilobites, Palaeogeography. R~SUM~ Des gisements fossihferes des schistes ~ chloritoide du Ludlow sup6rieur de la Zone Asturo-I~onaise (NO de l'Espagne) ont ~t6 r~tudi~s dans les synclinaux de Pefialba et du Sil. A l'~chelle r~gionale, l'existence de d~pSts et de faunes analogues dans les deux synclinaux indique qu'ils appartenaient ~ la m~me unit~ pal£~g~ographique et que la limite entre les Zones Asturo-L~onaise et Centre-Ib6rique se situe vraisemblablement au sein du synclinal du Sil. Les faunas de Trilobites, avec des repr6sentants des genres Crotalocephalus, Cerauroides, Cromus, Denckmannites ? et Lioharpes (Fritchaspis) pr6sentent des affinit6s certaines avec cellos de l'assemblage Prionopeltis archiaci du Ludlow SUl~rieur de Boh~me. Au point de vue pal~og6ographique, l'existence de faunes de Trilobites ~ aiTmit$s boh~miennes dans le NO de YEspague, clans las Pyr6n~es et en Catalogne, permet de discuter l'existence possible d'un Domaine Nord-Ib~rique regroupant ces r~gions et le Sud de la France (Aquitaine, Montagne Noire). Mots-cl~s : Silurien, Ludlow, Espagne, Zone Asturo-L~onaise, Trilobites, Pal6og6ographie. RESUMEN Se revisan dos yacimientos con trilobites del Ludlow superior, localizados en los sinclinales de Pefialba y Sil, que se ubican estratigr~ficamente en la base de las pizarras cbn cloritoide suprayacentas a l a s facies graptolfticas del Llandovery/Ludlow inferior. La correlaciSn de ambos horizentes fosiliferos demuestra la pertenencia de estas dos estructuras a una misma unidad paleogeogr~fica, cuya sedimentaci6n silfirica acusa los efectes del denominado "Eventd Kopanina Superior", con implicaciones que apoyan la ubicaciSn del lfmite entre los dominios Caurel-Pefialba y Truchas en el seno del sinclinal del,Sil (posible lfmite entre las zonas Asturoccidental-leonasa y

362 Centroib6rica del Macizo Hesp6rico). Las formas identificadas en los yacimientos estudiados, corresponden a los g~neros Crotalocephalus, Cerauroides, Cromus, Denckmannites? y Lioharpes (Fritchaspis), con especies caracteristicas de la Biozona Prionopeltis archiaci de Bohemia (parte superior de la Formaci6n Kopanina). Las afmidades bohfimicas de 6stas y otras asociaciones semejantes descritas en Pirineos y Cataluga, permiten discutir la posible existencia de un dominio biogeogrdfico nordib6rico que agruparia ademtts determinadas regiones del sur de Francia (Aquitania, Montafia Negra). Palabras-clave : Sildrico, Ludlow, Espafia, Zona AsmroccidentaI-Leonesa, Trilobites, Paleogeografia.

INTRODUCTION Silurian trilobites are relatively uncommon in the Iberian Peninsula. Fii~een species have been described by Kegel (1929), Gaertner (1930), Costa

(1940a,b,c), Hern~mdez-Sampelayo (1944), Gandl (1972), Degardin & Pillet (1983) and Oczlon (1989), most of t h e m in Upper Silurian rocks from Pyrenees and Iberian Chains (NE Spain). References concerning other regions are very few, without any trilobites being described or figured, and only about ten localities in the whole Hesperian Massif have been recorded, even though this region comprises the most extensive areas of Silurian outcrops in Western Europe. In the West Asturian-Leonese Zone (NW Spain, Fig. 1) Silurian trilobites were first mentioned by Hernfindez-Sampelayo (1942 p. 134-135) who found a specimen of "Harpes venulosus CORD" to the south-east of Filiel (Le6n province) in the core of the Pefialba Syncline (n ° 4, Fig. 1). According to the description given by this author, the graptolites occurring in these levels are a retiolitid and representatives of either Saetograptus or Pristiograptus, of lower Ludlow age (Gorstian).

F i g u r e 1 - Geological s k e t c h m a p of t h e W e s t Asturian-Leonese Zone. a : Proterozoic ; b : C a m b r i a n a n d Lower Ordovician ( m a i n l y Arenig) ; c : Middle Ordovician (Luarca F o r m a t i o n ) ; d : U p p e r Ordovician ; e : Silurian ; f : Carboniferous ; g : H e r c y n i a n g r a n i t o i d s ; h : Mesozoic and Cenozoic ; I : Navia-Alto Sil D o m a i n ; II : Mondo~edo N a p p e Domain ; III: Caurel Domain ; IV : Truchas Domain ( n o r t h e r n flank of Ollo de Sapo Anticlinorium) ; 1-7 : Silurian fossiliferous localities w i t h i n Vega s y n c l i n o r i u m (1), Pefialba Syncline (2 : La B a r o s a ; 3 : M o l i n a f e r r e r a ; 4 : Filiel), Sil Synclinorium (5 : V i l l a m a r t i n ; 6 : Alto de la Zorra) and T r u c h a s S y n c l i n o r i u m (7). Carte g~ologique sch~matique de la Zone Asturo-ldonaiss. a : Prot~rozo~que ; b : C a m b r i e n et Ordovieien inf~rieur (Arenig) ; c : Ordovicien m o y e n (Formation L u a r c a ) ; d : Ordovicien sup~rieur ; e : S i l u r i e n ; f : Carbonif~re ; g : g r a n i t o f d e s hercyniens ; h : Mdsozofque et C$nozo~tue ; I : D o r n a i n e de N a v i a et Alto-Sil ; 11 : D o m a i n e de la N a p p e de Mondor~eclo ; 111 " D o m a i n s d u Caurel ; I V : D o m a i n s de T r u c h a s (flanc N o r d de l ' A n t i c l i n o r i u m de l'OUo de Sapo) ; I - 7 : g i s e m e n t s fossilif~res s i l u r i s n s d u Synclin o r i u m de Vega (1), d u S y n c l i n a l de P e £ a l b a (2 : L a B a r o s a ; 3 : M o l i n a f e r r e r a ; 4 : Filiel), d u S y n e l i n o r i u m d u S i l (5 : V i l l a m a r t i n ; 6 : A l t o de la Zorra)) st d u S y n c l i n o r i u m de

Truchas (7).

More recent data from this area are those given by Riemer (1966) and Nollau (1966, 1968). Riemer (1966) recorded two localities regarded as Upper Silurian in the Sil Syncline (Orense province, n ° 5 and 6 fig. 1) with poorly preserved specimens of Cromus sp., Encrinurus sp., Harpes sp. and Scutellum (Planiscutellum) sp. (SDZUY'S identifications in RIEMER, 1966). Nollau (1966, 1968) mentioned another fossil locality in the Pefialba Syncline, to the south of Molinaferrera (Le6n province, n ° 3, Fig. 1), about 3 km north-west of Herngmdez-Sampelayo's locality. In these rocks, regarded as probably of Wenlock age, the fauna (Sdzuy's identifications in Nollau 1966, 1968) includes Harpes cf. ungula (STERNBERG), Cheirurus ef. articulatus (MONSTER) and Cromus sp. The most recent reference is t h a t of Whhrle (1981 mentioned in Zeitz & Nollau 1984) who recorded Encrinurus sp. from a locality n e a r those of Riemer (1966) and considered it as of Ludlow age. The aim of the present paper is to present a detailed analysis of the Silurian trilobite assemblages previously recorded : the new and

363

k

;i:i: Figure 2 - Geological s k e t c h m a p of t h e core a n d s o u t h e r n flank of t h e Pefialba Syncline (alder geological m a p of S p a i n 1150 000, s h e e t n ° 192 Lucillo). a : C a m b r i a n and Ordovician f o r m a t i o n s of t h e n o r t h e r n flank ; b : u p p e r - m o s t p a r t of t h e Cabos Series ( A r m o r i c a n q u a r t z i t e facies) ; e : R u b i a n a F m . ; d : L u a r c a F m . ; e: Casaio F m . ; f : A q u i a n a F m . ; g : S i l u r i a n f o r m a t i o n s ; h : anticlinal axis ; i : N e o g e n e f o r m a t i o n s (conglomerates a n d d a y s ) ; j : Q u a t e r n a r y ; k : possible locations of H e r n ( m d e z - S a m p e layo's fossiliferous locality ; 1 : trilobite locality r e f e r r e d to i n t h i s text. Carte g~ologique sch~rnatique du coeur et du flane sud du Synclinal de Pe~alba. (d'apr~s la carte gdologique d'Espagne ~ 1/50 000, feuille n ° 192 Lueillo). a : Formations cambriennes et ordoviciennes du flanc N ; b : partie supdrieure de la Sdrie de los Cabos (facies Gr~s armoricain) ; e : Formation Rubiana ; d : Frn. Luarca ; e : Fm. Casaio ; f : Frn. Aquiana ; g : formations siluriennes ; h : axes anticlinaux ; i : formations ndog~nes (conglomdrats et argiles) ; j : formations quaternaires ; k : localisations possibles du gisement de Herndndez-Sampelayo ; l : gisement fossili]~re gtudid.

rather well preserved material found in the Pefialba and Sil Synclines allows the taxonomic attribution, age, correlations and palaeobiogeographic significance of these assemblages to be made with precision. L O C A T I O N AND S T R A T I G R A P H Y OF THE FOSSILIFEROUS LOCALITIES The area first investigated (Fig. 2) lies within the Pefialba Syncline (or Seara-Cabarcos Syncline of other authors) which constitutes the eastern prolongation of the Caurel Syncline, within the "Caurel Domain" (Martinez-Catal~n 1985) of the West Asturian-Leonese Zone (III, Fig. 1). As mentioned above, the Silurian rocks cropping out in the core of this syncline have yielded trilobites at two localities though these were not recorded precisely. The first locality (n ° 4, Fig. 1) near Filiel (Hern~ndez-Sampelayo 1942) has not been relocated despite a careful search in the Tolaniel and Valle Prado stream sections which comprise the most complete and accessible Silurian outcrops in the area around Filiel.

The second fossiliferous locality mentioned by Nollau (1966, 1968) has however been rediscovered owing to the location given in the later paper ("eastern bank of Rio Cabrito, south of Molinaferrera"). The locality (n ° 3 Fig. 1,1 ; Fig. 2) is situated 1600 m south-west of Molina- ferrera on the northern slope of Teleno Sierra (Lambert coordinates x = 378,550 ; y = 867,650 : sheet 192-Lucillo-of the 1/50 000 geological map of Spain) ; the fossiliferous beds occur in a small excavation in the eastern bank of Arroyo del Cabrito (about 75 m above the stream b e d ) j u s t above an escarpment surmounting water pipes for Roman gold mines. These rocks belong to the southern flank of the Pefialba Syncline (i.e. the northern flank of the Teleno Anticlinorium). In the Arroyo del Cabrito section the Silurian rocks comprise two distinct lithological units : - a lower unit mainly composed of ampelitic black-shales (about 100 m ; n ° 8, Fig. 4), with poor exposures within linear topographic depressions ; a thicker upper unit of siliceous slates and siltstones (n ° 10, Fig. 4), frequently chloritoidrich and of higher relief.

364

~o~ f [-:'~'] e

Figure 3 - Geological s k e t c h m a p of t h e n o r t h e r n flank o f t h e Sil Syncline (after geological m a p o f S p a i n 1150 000, s h e e t n ° 190 E1 Barco). a : L u a r c a Fro. b : A q u i a n a Fro. ; c : Silurian graptolitic black s h a l e s (Wenlock? - Ludlow) ; d : chloritoid slates w i t h fossiliferous b a s a l levels ; e : d i p p i n g ; f : N e o g e n e f o r m a t i o n s (conglomerates a n d clays) ; g : Q u a t e r n a r y ; h : Alto de la Zorra fossiliferous locality. Carte gdologique schdmatique du flanc nord du Synclinal du Sil (d'aprhs la carte gdologique d'Espagne ~ 1 / 5 0 000, feuille n* 190 El Barco). a : Frn. Luarca ; b : Fro. A q u i a n a ; c : schistes graptolites siluriens (Wenlock? - Ludlow) ; d : schistes & chlorito~Ie avec niveaux fossiliferes de base ; e : pendage des couches ; f : Formations ndog~nes (conglomdrats et argiles) ; g : Formations quaternaires ; h : gisement fossilifere de Alto de la Zorra.

The fossiliferous beds occur in the lowermost part of the latter unit, about 120 to 130 m above the top of the Aquiana limestones. It is almost impossible to give a precise stratigraphy of these rocks due to the lack of suitable and continuous exposures, the scarcity of lithological guide-levels and biostratigraphical data, and to the importance of the deformation which can affect the whole unit. This explains why the stratigraphical interpretations proposed in previous studies have been so at variance and also why the whole Silurian succession has been misinterpreted (as for example the proposed distinction (Nollau 1966, 1968) between Pobladura and Pefialba formations which actually both include Ordovician and Devonian limestones with tectonic repetitions). Precise biostratigraphical data concerning the Silurian rocks of the Pefialba Syncline are practically non-existent. In addition to the occurrence of trilobites, previous works have only mentioned some graptolites of Llandovery (Monograptus communis), LlandoveryWenlock (M. priodon), Wenlock (M. riccartonensis) and early Ludlow ages (Saetograptus chimaera, S. roemeri, "Colonograptus '° sp., retiolit{ds), but without giving any precise information about the fossiliferous localities and levels (Hern~ndez-Sampelayo 1942 ; Nollau 1966, 1968 ; Velando & Matas 1981). Likewise the upper part of the Silurian succession yielded at La Barosa (W of Carucedo Lake ; n ° 2, Fig. 1)

numerous nodules with orthoceratids, bivalves (Cardiolidae), articulate brachiopod fragments and possibly tabulate corals, but their age is not known precisely. However, the Silurian rocks of the ~vestern part of the syncline are overlain by a carbonate succession with Lower Devonian (Pragian-Zlichovian) fossils (P~rez-Estafin 1978 ; Truyols-Massoni 1986) : it there- fore appears that the Silurian-Devonian boundary most probably lies within the uppermost part of the rocks that have been up to the present attributed to the Silurian. We have also re-investigated the Sil syncline, in the same Caurel Domain (I/I, Fig. 1), where Riemer (1966) recorded Silurian Trilobites. The fossiliferous locality (n ° 6, Fig. 1) is located in the northern slope of Alto de la Zorra, 900 m to the NE of the hamlet of E1 Castelo, about 10 km to the ENE of E1 Barco de Valdeorras (Orense province : Lambert coordinates x = 338,800, y = 877,250 of the sheet n ° 190-E1 Barco- of the 1/50 000 geological map of Spain). This locality (Fig. 3) lies within grey siltstones, about 20 m thick, yielding an abundant fauna of echinoderms (crinoids), brachiopods, cephalopods, bivalves, solitary rugose corals and trilobites. These horizons form distinct topographical features and have been mapped by Apalategui et al. (1981) as part of the "Cuarcitas Azules" (equivalent to "Cuarcita de Yeres" of Zeitz & Nollau 1984) : at the present locality quartzitic levels are restricted to some thin intercalations, in the uppermost part of the fossiliferous sequence. PALAEONTOLOGICAL

REVIEW.

The fossiliferous horizon at Molinaferrera has yielded several trilobites, brachiopods, crinoids, cephalopods, and rare solitary rugose corals. They are preserved as moulds in hard siliceous fissile slates, with some chloritoid content, and are flattened, showing slight tectonic deformation. The most abundant trilobite is an harpetid, characterized by its broad horseshoe brim, with a very wide external rim, small alae and one row of coarse pits at the genal roll/brim boundary (pl. 1, figs. 1-4 and 6-7). These features are consistent with the subgenus Lioharpes (Fritchaspis) VANEK, 1963, whose first record in Silurian rocks of SW Europe is given here. The Spanish material may be a new species, but the cephala do not provide enough information to allow the brim and glabellar structures to be assessed definitively.

365

Tr SlL

1Tr

SYNCLINORIUM (NORTH)

lV

PENALBA SYNCLINE

[loom.

-.

TRUCHAS

J@-

VEGA SYNCLINORIUM ( SOUTH - WEST )

(SOUTH)

" ? \ \ 12

-?"111"

'@

10

SYNCLINORIUM

8II;; ...............

--...,

¢

-.-. -. .- .

.

.

.

.

//

// / / /

/

/ /

// /

/

/

/•

/"

I

I

I

/ • •

/

@

/

// / /

//// /

/•

(~)

I [ I

\,

@

\

\ \

\

/" /"

/

'

/

/ /

e

/

~d ~g ~h~i IZ]m n ~o

I

I

i

F i g u r e 4 - C o r r e l a t i o n of t h e U p p e r O r d o v i c i a n a n d S i l u r i a n s u c c e s s i o n s i n t h e s o u t h e r n p a r t o f t h e W e s t A s t u r i a n - L e o n e s e Z o n e ( g e o g r a p h i c a l l o c a t i o n : s e e f i g u r e 1). 1 : L u a r c a Fro. ; 2 : C a s a i o Fro. ; S : R o z a d a i s Fro. ; 4 : L o s a d f l l a Fro. 5 : A g i l e i r a Fro. ; 6 : A q u i a n a F m . ; 7 : V e g a Q u a r t z i t e ; 8 : g r a p t o l i t i c b l a c k - s h a l e s ; 9 : Y e r e s Q u a r t z i t e ; 10 : c h l o r i t o i d s h a l e s ; 11 : siltstone-sandstone alternations ; 12 : D e v o n i a n l i m e s t o n e s a n d s h a l e s . A : u p p e r l i m i t o f t h e L u a r c a F m . ; B : Ordovician-Silurian boundary ; C : Silurian-Devonian boundary ; D : probable correlation• a : sandstones ; b : siltstone-sandstone alternations ; c : shales ; d : clast-boaring sfltstones and shales ; e : black-shales ; f : limestones ; g : siltstones with nodules ; h : t r i l o b i t e s ; i : S c y p h o c r i n i t e s ; j t o o : f i r s t g r a p t o l i t i c l e v e l s ; j : o f R h u d d a n i a n a g e ; k : of A e r o n i a n a g e ; 1 : of T e l y c h i a n a g e ; m : of W e n l o c k a g e ; n : o f L u d l o w a g e ; o : of P r i d o l i age. V e r t i c a l a r r o w s i n d i c a t e t h e s t r a t i g r a p h i c a l r a n g e o f t h e u n c o n f o r m i t i e s a t t h e b a s e of t r a n s g r e s s i v e f o r m a t i o n s • CorrElations entre les successions de r O r d o v i c i e n s u p d r i e u r et d u S i l u r i e n de la p a t t i e s u d de la Zone Asturo-l~onaise. (localisation g6ographique : voir f i g u r e 1). 1 : F o r m a t i o n L u a r c a ; 2 : Fro. Casaio ; 8 : Fro. R o z a d a i s ; 4 : Fin. L o s a d i l l a ; 5 : Fro. Agi2eira ; 6 : Fm. A q u i a n a ; 7 : Q u a r t z i t e de Vega ; 8 : ampdlites h Graptolites ; 9 : Q u a r t z i t e de Yeres ; 10 : schistes & chlorito~le ; 11 : schistes ~ intercalations grdseuses ; 12 : schistes et calcaires d~voniens. A : limite supdrieure de la Fro• L u a r c a ; B : limite O r d o v i c i e n - S i l u r i e n ; C : limite Silurien-D~vonien ; D : correlation probable• a : gr~s ; b : alternances sehisto-gr6seuses ; c : schistes ; d : pdlites & f r a g m e n t s ; e : amp~lites ; f : calcaires ; g : siltstones ~ n o d u l e s ; h : Trilobites ; i : S c y p h o c r i n i t e s ; j & a : p r e m i e r s n i v e a u x & Graptolites ; j : d u R h u d d a n i e n ( L l a n d o v e r y infdrieur) ; k : de l'Aeronien ( L l a n d o v e r y moyen) ; l : d u Telychien ( L l a n d o v e r y sup4rieur) ; m : d u Wenlock ; n : d u L u d l o w ; o : d u Pridoli. L e s fl~ches verticales i n d i q u e n t l'extension s t r a t i g r a p h i q u e d e s l a c u n e s pr~c~dant les f o r m a t i o n s transgressive&

The specimens figured by NoUau (1968 : pl. 7, figs. 1-2) as Harpes cf. ungula from the same locality, show the same features as those of L.

(Fritchaspis). Bohemoharpes (Unguloharpes) un-

gula (STERNBERG,1833) has been recorded in the Ludlow of Bohemia ( I ~ T L & I~iBYL, 1954 ; PRIB~L & VAN~K, 1986) and Eastern Alps (HERITSCH, 1929). Its supposed presence in the

366 Iberian Peninsula is based upon Almera (1891), who recognized the species near Barcelona (in the Catalonian Coastal Ranges). This reference has been later mentioned by Prantl & Ph3~yl, (1954), but without revision. It may be a Devonian harpetid because of its provenance from shales overlying La Creu Formation above the Silurian/ Devonian boundary (Julivert et al. 1987).

transiens (BoU~EK, 1935) from the Lower Pridoli Formation of Bohemia, differing from C. moraveci in the sigmoidal $3 furrows. Thus, the incomplete cranidium identified as Crotalocephalus (Pille~opeltis) aft. transiens by DEGARDIN & PILLET (1983) from Ludlow levels of the Spanish Central Pyrenees should be also regarded as C. moraveci.

Following Van~k (written communication, 1991), the record of Lioharpes (Fritchaspis) starts at the Prionopeltis archiaci Biohorizon in the uppermost levels of the Kopanina Formation (Upper Ludlow of Bohemia : unpubl, data), and continues up to the Upper Devonian. The discovery of Lioharpes at Molinaferrera makes its presence at the closer locality, of Filiel more likely, where Hern~udezSampolayo (1942) quoted "Harpes venulosus" (the type species of Lioharpes) most probably at the same levels as those studied herein.

Sdzuy (in Nollau, 1968) noted the presence of Cheirurus cf. articuIatus (MONSTER, 1840) from the same locality. This cheirurid was originally described from Ludlow rocks of Frankenwald (Germany), being later recognized in Bohemia as its junior synonym Cheirurus hawlei BARRANDE, 1852. The species is typical of the Prionopeltis archiaci Biohorizon, showing in its pygidium a striking prolongation of the pygidial spines in the first pair of pleurae. The pygidium ofpl. 1, fig. 11 has long and ensiform pygidial spines, confirming hence the assignation to Cerauroides articulatus.

The second trilobite identified at Molinaferrera (represented by two incomplete cranidia, one cephalon plus thorax and an hypostoma : pl. 1, figs. 8-9, 12-13) is a large cheirurine assigned to the Siluro-Devonian genus Crotalocephalus. It differs from other cheirurines, such as Crotalocephalina, by the transglabellar $2 and $3 furrows, and broader free cheeks and thoracic pleurae. The absence of sigmoidally curved $3 furrows relates the Spanish specimens to Crotalocephalus moraveci VOKAC & DOUBRAVA, 1990 from the Prionopeltis archiaci Biohorizon of the Bohemian Kopanina Formation (Upper Ludlow). The other Silurian species of the genus is C.

The last Silurian trilobite identified from the Pefialba syncline is represented by a cranidium and two pygidia of an encrinurid (pl. 1, fig. 5 & 10). It has been identified as Cromus aft. bohemicus BARRANDE, 1852 on account of the outline and structure of the spinose pygidium, with its large axis and sagittal tubercles on rings 1, 4, 7, 10 and 12?, deeply incised apodemes, and flat ribs produced into long spines, which overhang a narrow border and are bent towards the horizontal plane. The Spanish pygidia have 12-14 axial rings and 10 pleural ribs (plus a last pair, fused in a postaxial rib), showing therefore

PLATE 1 Upper Ludlow trilobites from the Pefialba syneline (Molinaferrera locality). Trilobites du Ludlow supdrieur du Synclinal de Pe~alba (gisement de Molinaferrera). Fig. 1-4, 6-7 - Lioharpes (Fritchaspis) nov. sp. 1. Latex cast of incomplete eephalon, dorsal view. Moulage latex d'un c6phalon incomplet, vue dorsale ; IGEP 7020, x 1,6. 2. Internal mould of ventral part of cephalon ; IGEP 7021, x 1,6. 3. Latex cast of cephalon, ventral view ; IGEP 7022, x 1,6. 4. Latex cast of incomplete cephalon, dorsal view ; IGEP 7023, x 1,6. 6. D6tail of brim and genal roll boundary, latex cast ; IGEP 7024, x 3,4. 7. D~tall of brim and rim prolongation, latex cast ; IGEP 7025, x 2,5. 1. Moulage latex d'un cdphalon incomplet, vue dorsale. 2. Moule interne de la partie ventrale du c~phalon. 8. Moulage latex du cdphalon, vue ventrale. 4. Moulage latex d'un cdphalon incomplet, rue dorsale. 6. Ddtail du limbe et de la limite de la rdgion juxta-glabellaire, moulage latex. 7. Ddtail du limbe et du prolongement de la bordure, moulage latex. Fig. 5, 10 - Cromus aft. bohemicus BARRANDE, 1852. 5. Latex cast of eranidium ; IGEP 7030, x 2,6. 10. Latex cast of flattened pygidiurn ; IGEP 7031, x 3,5. 5. Moulage latex du cranidium. 10. Moulage latex d'un pygidium aplati. Fig. 8-9, 12-13 - Crotalocephalus moraveci VOKAC & DOUBRAVA, 1990. 8. Hypostoma ; IGEP 7035, x 1,6. 9. Incomplete cranidium ; IGEP 7036, x 1,6. 12. Cephalon plus thorax ; IGEP 7037, x 2,6. 13. Latex cast of incomplete cranidium ; IGEP 7038, x 2,5. 8. Hypostome. 9. Cranidium incomplet. 12. Cephalon et thorax. 13. Moulage latex d'un cranidium incomplet. Fig. 11 - Cerauroides articulatus (MONSTER, 1840). Latex cast of pygidium showing long and ensiform spines ; IGEP 7041, x 1,6. Moulage latex d'un pygidium montrant de longues dpines ensiformes.

Geobios

n" 26, fasc. 3

P1. 1

I. Rabano, J.C. Gutierrez-Marco & M. Robardet

368 fewer constituents than those described by Snajdr (1985) for C. bohemicus. In Bohemia the latter is typical of the uppermost levels of the Kopanina Formation (Upper Ludlow ; Prionopeltis archiaci Biohorizon). The number of pygidial rings and ribs of C. aft. bohemicus fits well within the range of Encrinuraspis beaumonti (BARRANDE, 1846), with 13-16 axial rings and 10 (11) paired ribs. However, the species of the E. beaumonti plexus have blunt-ended pygidial pleurae, whereas in Cromus they are spinose. This character is easily recognizable in most of our specimens, but sometimes as the spines are not located on the same plane as the pygidial border, they seem to be missing in some internal moulds. This is probably what happened with the pygidium figured by Nollau (1968 : pl. 7, fig. 3) as Cromus sp. ; this being conspecific with our Cromus aft. bohemicus from the same locality.

Cromus aft. bohemicus shows a mixture of features of different genera and species, and the same can occur with Upper Silurian encrinurids from Morocco : "Cromus aft. beaumonti" (Corn~ et al. 1985 : Western Coastal Meseta), "Cromus sp. aft. beaumonti vel C. bohemicus" (Destombes et al. 1985 : E of Casablanca), "Encrinurus sp. aft. beaumonti" (Alberti 1969 : E of Rabat) or "E. (C.) sp. gr. beaumonti (Destombes et al. 1985 : Central part of High Atlas). All these records come from younger beds than those from Bohemia (lower part of the Kopanina Formation) with the true Encrinuraspis beaumonti. Some of these could be conspecific with Cromus aft. bohemicus as identified herein, especially those from the Moroccan Coastal Meseta where Destombes et al. (1985) found t h e m together with Harpes in levels of supposed basal Pridoli age. With regard to other enerinurid records in the Iberian Peninsula, the pygidium identified by Degardin & Pillet (1983) as Cromus sp. aft. kromulsi CHLUP~, 1971 from Ludlow shales of the Spanish Central Pyrenees, may well be conspecific with C. aft. bohemicus. The ascription to C. kromulsi can only be based upon sculptural criteria and morphological details of the pygidial spines, which cannot be observed in the only pygidium recovered by these authors. They also noted that it is very close ("tr~s voisin") to C. bohemicus. Another Pyrenean species of doubtful relationships with the Leonese form is "Encrinurus '° rialpensis GAERTNER, 1931, of supposed Wenlock age, but it is accompanied by a typical Ludlow association with Acanthalomina rninuta (BARRANDE), Ananaspis fecunda (BARRANDE), Phacopidella (or Denckmannites) and Otarion. Because of the narrow glabella and

Figure 5 - Silurian trilobites from the Sil syncline (La Zorra locality) Trilobites siluriens du Synelinal du Sil (gisement de La Zorra). a-b : Harpetidae gen. et sp. indet. ; a : latex cast of lower lamella with pronounced girder and r i m ; IGEP 7000, x 1,7. b : latex cast of incomplete lower lamella of b r i m ; IGEP 7001, x 2. c - d : C r o m u s aft. b o h e m i v u s BARRANDE, 1852 ; latex cast (c) and let1 p a r t of i n t e r n a l mould (d) of pygidium, showing spinose pleurae ; IGEP 7008, x 3,5. a : moulage latex de la lame infdrieure avec bordure et bord interne du limbe, b: moulage latex d'une lame infdrleure du limbe incomplete, c.d : Moulage latex (c) et partie gauche du moule interne (d) du py~dium, montrant les pl~vres dpineuses.

pygidial axis, the ribs not produced into spines, and the large and sessile eyes, the species may belong most probably to Encrinuraspis. From a biostratigraphic point of view, the association of Crotalocephalus moraveci, Cerauroides cf. articulatus, Lioharpes (Fritchaspis) nov. sp. and Cromus aft. bohemicus is closely comparable with the Prionopeltis archiaci Biohorizon recognized in the uppermost levels of the Bohemian Kopanina Fm. This Biohorizon includes the Pristiograptus fecundus to P. fragmentalis graptolite biozones correlated with the late Ludfordian (Upper Ludlow) (see Havlicek & Storch 1990, text-fig. 5). A similar trilobite association dominated by harpetids and encrinurids and also of Bohemian affinities has been re-discovered in the locality of

369 Alto de la Zorra (northern flank of the Sil synchne), where we have identified : Cromus aft. bohemicus with characteristics identical to the samples from Molinaferrera (Fig. 5, c-d) ; diverse indeterminate harpetids (Fig. 5, a-b), some of them of very large size (sagittal length of cephalon and the genal and brim prolongations of approximately 80 mm) ; and a phacopid, probably Denckrnannites. RIEMER (1966) mentioned the presence of Planiscutellum which we have not been able to confirm. The general composition of the trilobite association and the presence of C. aft. bohemicus would indicate that these rocks are equivalent to the Molinaferrera level which also contains identical crinoids and rugose corals. Moreover the trilobite beds of the Sil syncline represent a lateral equivalent of the Yeres Quartzite which directly overlies graptolite shales of Gorstian (Lower Ludlow) age, as it can be seen along the road North of Salas de la Ribera (Jaeger in Riemer 1966, Willefert & Matte in Abril Hurtado et al. 1982) and West of Yeres (Teller & Gutidrrez-Marco unpublished). This confirms a Ludlow age for the trilobite beds at Alto de la Zorra, as not older than the late Gorstian. In the Silurian of Bohemia Planiscutellum is known from the Llandovery to the late Wenlock but the m a x i m u m diversity and abundance of this styginid is found in the Ludlovian Kopanina Formation (Snadjr 1960, 1989). As regards the other fossils found with the trilobites, Nollau (1968) mentioned at Molinaferrera the presence of crinoids, brachiopods (Orthis and "Obulus '°, sic), orthoceratids and monograptids. Amongst the crinoids, the most abundant are discoidal columnals (Cyclocyclopa) up to 15 m m in diameter and 1-1.5 m m high, with a very wide lumen representing half of the diameter of the articular facet. They occur as disarticulated fragments (isolated or in clusters) or as homeomorphic pluricolumnals with up to 20 ossicles. One of these is connected with a calyx fragment with 12 cycles of radial plates (35 m m long). Amongst the sporadic brachiopods, small spiriferid fragments and strophomenids occur. One of these has kindly been identified by Dr. P.R. Racheboeuf (written communication 1991) as a ventral valve of an undetermined strophochonetid whose outline, external ornamentation and arrangement of spines resemble those of the Upper Silurian (Ludlow-Pridoli) genus Hypselonetes RACHEBOEUF,1981. Apart from some flattened fragments of orthocone nautiloids, no inarticulate brachiopods nor

graptolites have been found. The grain size of the siltstone, together with its low chloritoid content, seems too coarse for the preservation of graptolites. However we have observed some straight narrow hollows without any structure indicative of an organic origin, but which would probably correspond to the graptolites mentioned by former authors. The rare solitary corals are small conical forms (maximum height 12 ram) with a very wide base. In Alto de la Zorra, Riemer (1966) mentioned the existence of Cardiola cornucopiae (GOLDFUSS, 1837) whose acme (KH~ 1979, table 1) in the uppermost Ludlow is coincident with the age we have attributed to the trilobite association. The benthic faunas at this locality are much more abundant and diverse than at Molinaferrera. The same crinoids and solitary rugose corals occur, but the crinoid ossicles show a more remarkable diversity with other col-renal morphotypes

(Cyclopentastellatopa, PentagonopentasteUatopa, Cyclopentagonopa) currently being studied by Dr. R. Prokop (Prague). Articulate brachiopods are abundant and are ascribed to four different genera of Athyrididina, Atrypidina and Spiriferidina Dr. V. Havlicek (Prague) has kindly identiffed them as Pseudoprotathyris ? sp., Lissatrypa sp., Gracianella graciosa HAVLICEK & STORCH, 1990 (one specimen) and Alaskospira ? sp. All these forms are common in Bohemia, where G. graciosa is confined to the lowermost part of the Pridoli Formation, but Pseudoprotathyris and Alaskospira are somewhat older and have never been recorded above the Kopanina Fm. (Havlicek & Storch 1990). For these reasons a late Ludlow age, close to the Ludlow-Pridoli boundary is most probable. At this locality we found also several orthocone nautiloids with a narrow central siphuncle and rare indeterminate pterioid bivalves. From a palaeoecological point of view, the whole fossil association apparently corresponds to a relatively offshore environment, below wave base but within the occasional reach of surface currents. The trilobite exuviae are almost always disarticulated with the exception of a cephalon plus thorax of Crotalocephalus and of Lioharpes at Molinaferrera, and of an unclassified pygidium plus thorax in Alto de la Zorra. The absence of strong turbulence is also indicated by the preservation of pluricolumnals and articulated fragments of crinoids, of brachiopods with connected valves and of a balanced number of enerinurine cranidia and pygidia (cf. Snadjr 1985 p. 12). The strophochonetids found at Molinaferrera characterize a firm substrate (Rache-

370 boeuf 1990) which is compatible with the silty nature of the rocks. All the brachiopods from Alto de la Zorra are epibenthic forms and the poorly mineralized shells of the bivalves probably indicate an epiplanktie mode of life. The general disarticulation of these shells more probably corresponds to weak articulation structures rather than to water turbulence. The trilobite association is characterized by the abundance of harpetids and the presence of encrinurids and cheirurids of large geographical distribution. Harpetids are generally vagrant benthic to nektobenthic forms relatively independent of lithofacies, but with distribution areas which exclude interspecific competition (Pribyl & Vanek 1986). The geographical distribution of Cromus aft. bohemicus is probably the widest within the genus (if Pyrenean and Moroccan occurrences are confirmed), analogous to that of the Encrinuraspis beaumonti plexus in the Upper Silurian (Snadjr 1985). Amongst the cheirurines, Crotalocephalus occurs abundantly in the Devonian hemipelagic "Hercynian magnafacies" and thus probably also in the Silurian. For these reasons the occurrence of C. moraveci is relatively rare in the shallower limestone lithofacies of the Prionopeltis archiaci Assemblage of Bohemia (Vokac & Doubrava 1990) and much more frequent in the offshore shale lithofacies of northern Iberian Peninsula. Cerauroides articulatus is also a pelagic cheirurid of wide distribution in central Europe. STRATIGRAPHICAL CORRELATIONS AND SILURIAN PALAEOGEOGRAPHY OF THE WEST ASTURIAN-LEONESE ZONE The Silurian stratigraphy of NW Spain is rather poorly known except in some structural units of the northern part of the West Asturian-Leonese Zone (WALZ) where abundant biostratigraphical data are based upon numerous graptolite occurrences (Villaodrid and Rececende synelines, Santa Eulalia de Oscos thrust - sheet : cf. Marcos 1973 and 1/50 000 geological maps). In the southern part of the WALZ the stratigraphical scheme is much less precise and based upon rather generalised studies and some local and sometimes questionable stratigraphical data. The recent revision of the lowermost part of the Silurian succession in this area (Gutierrez- Marco & Robardet 1991) has allowed precise conclusions to be drawn about the Ordovician-Silurian.boundary and about the first assemblages of Rhud-

danian graptolites to be documented in Northern Spain. The graptolitic black shale facies extends up to the Lower Ludlow and is overlain by a thick sequence of chloritoid shales and siltstones with nodules and sporadic bands of quartzites and decalcified limestones (specially developed in the upper half). This unit extends from the Upper Ludlow-Pridoli up to the lowermost Devonian and is overlain in the Pefialba syncline by fossiliferous limestones and shales of Pragian age (Fig. 4, HI). In the Sil syncline (Fig. 4, II) this chloritoid shale unit (Salas Beds of Zeitz & Nollau 1984) is thicker (more than 1 000 m) and its upper third may possibly represent a lateral equivalent of the Lower Devonian rocks from the Pefialba syncline. In the eastern part of the Sil syncline, the thick sequence of chloritoid shales begins with quartzite intercalations (Yeres Quartzite), clearly reflected in the topography, and laterally passing westwards to the fossiliferous levels with trilobites found in Alto de la Zorra and other localities. The rest of the succession has yielded numerous nodules some of which contain orthocone and cyrtocone nautiloids, crinoids, brachiopods and rare conularids, but these fossils cannot be precisely identified and therefore are of limited stratigraphic value. However in the area of Alto de la Zorra, Apalategui et al. (1981) mentioned the occurence of Scyphocrinites elegans in the lower half of the chloritoid shales, which might suggest a Pridolian age. Previously Riemer (1966) mentioned Scyphocrinus ? with Monograptus transgrediens ?, M. haupti ?, M. ultimus ? and Linograptus posthumus at Jagoaza (N of E1 Barco de Valdeorras). The graptolites listed by Riemer include the index species of different Pridolian graptolite biozones which cannot be found in the same level, but we have not been able to relocate Riemer's fossiliferous locality. As a result of the present study, we have been able to infer from the Ludlovian a direct correlation between the fossiliferous levels of the Sil and Pefialba synclines which display the same characteristics and were obviously parts of the same depositional area. This is an important point concerning the palaeogeographical subdivisions within the WALZ and the positioning of the limit between the Caurel-Pefialba (Ill, Fig. 1) and Truchas (IV, Fig. 1) Domains. This limit is not easily recognized but is sometimes regarded as the boundary between the West AsturianLeonese Zone and the Central-Iberian Zone of the Hesperian Massif (Martinez-Catalfin 1985 ;

371 Barros-Lorenzo 1989). The occurrence of similar Upper Silurian trilobites and facies within both the Pefialba and the Sil synclines indicates that the major limit m u s t be located to the South, probably within the Sil syncline. The Silurian core of this structure is a complex area which conceals several blocks passing gradually into the Pefialba shoal area. These structures have influenced sedimentation during Late Ordovician times (P~rez Estafin & Marcos 1981) and apparently persisted during the Silurian and up until the Early Devonian. This structural and sedimentary pattern resulted in the more or less pronounced erosion of the Ordovician successions and in the diachronous development of the Silurian graptohte black shales in the Truchas and Sil synclines, whose age apparently denotes a South-North progression of the transgressive facies (Fig. 4). In Early Devonian times, calcareopelitic a l t e r n a t i o n s with benthic and pelagic f a u n a s - w e r e deposited on the Pefialba shoal (P~rez Estadn 1978, Truyols-Massoni 1986), whereas the probable equivalents in the Sil syncline (uppert part of the Salas Beds) are much more pelitic with rare and discontinuous calcareous layers.

French Armorican Massif and in some parts of North Africa. In contrast, the area studied is characterized by the presence and diversification of pelagic faunas with Bohemian affinities as are similar faunas from the Pyrenees and Morocco. In these regions the graptolite shales are succeeded by monotonous pelitic sediments enriched in diagenetic chloritoid (ottrelite according to Riemer 1966). Within the Iberian Peninsula, the only similar deposits are the so-called "Xistos raiados" (laminated silty shales) of the Ossa Morena Zone (SW Spain and Portugal), regarded as uppermost Silurian and/or Lower Devonian (Ruiz et al. 1984, Jorquera de Guindos et al. 1990, Oliveira et al. 1991). The Silurian successions of the Ossa Morena Zone (Robardet 1982) and of the Catalonian Coastal Ranges (Julivert et al. 1985) were the only sequences of Mediterranean type previously described in the Iberian Peninsula and are closely analogous to the best known Silurian successions of Sardinia, Thuringia and Carnic Alps (Jaeger & Robardet 1979). However the Upper Silurian of the Pefialba and Sil synclines also shows Mediterranean faunal affinities which had not previously been emphasized.

The relationship between these areas and the Alto Sil Domain during the Silurian-Devonian is not known in detail. However this subsiding trough, where the Upper Ordovician turbidites of the Agfieira Formation were deposited, probably still persisted at the beginning of the Silurian as illustrated by the apparent continuity between the two Systems and by the early development of the graptotitic black shale facies in the Rhuddanian (P~rez-Estafin 1978 ; Guti~rrez-Marco & Robardet 1991). It m u s t also be mentioned that the Eastern Cantabrian Zone with the SiluroDevonian Hercynian magnafacies is an allochthonous unit (Palentine Domain or PisuergaCarri6n Unit) which probably originated from part of the WAI.Z (see Garcia Alcalde et al. 1988, Rodrfguez Fern~mdez & Heredia 1988 and others) possibly located in the eastern prolongation of the Alto Sil Domain.

The association of harpetid, encrinurine, cheirurine and phacopine trilobites described above allows a comparison with the Prionopeltis archiaci to Denckmannites-Cromus late Ludlow assemblages of Bohemia (Chlupac 1987) occurring in the uppermost levels of the Kopanina Formation and correlated with the Benthic Assemblages 3 to 4 of Boucot's (1975) classification. In Bohemia the trilobite assemblages are associated with different facies, from the special conditions of the cephalopod limestone facies representing the most offshore tongues of the carbonate development (Denckmannites-Cromus Assemblage), to the light biodetrital limestones of shallow water and high energy environment bordering volcanic elevations in the Barrandian area (Chlupac 1987).

The Upper Silurian succession discussed here strongly contrasts with the shallower deposits of Cantabrian or Central-Iberian type, where the black shales are rapidly succeeded by sandy alternations, or by thick sandstone formations (Furada, San Pedro, Alcolea and Luesma Formations) gradually passing up into the Devonian. This type of succession is characterized by distinct trilobite biofacies, dominated by homalonotids and asteropygineacastavine phacopids, also occurring in the

In the region studied herein limestone deposition did not occur with the exception of some sporadic, thin and discontinuous intercalations and of rare calcareous nodules. The graptolitic black shales, representative of offshore sedimentation, are succeeded by the thick unit of chloritoid shales with the trilobite levels in the lowermost part. This lithological change may be attributed to a short regressive event terminating in the latest Silurian and which can be correlated with the so-called "Cardiola event" (SchSnlaub 1986) or "Upper Kopanina event" (Chlupac & Kukal 1988), which introduces green shales or slightly

372 condensed cephalopod limestone facies over a wide area in Europe and North Africa where the "Mediterranean type" of Silurian sedimentation Occurs.

DISCUSSION Sedimentary and faunal evidence can be used in the discrimination of palaeogeographic domains, and consequently can provide constraints on the reconstruction and the geodynamic interpretation of the Variscan Belt in Iberian Peninsula and more generally in Southern Europe (Paris & Robardet 1990 ; Ribeiro et al. 1990 ; Robardet & Guti6rrez Marco 1990 ; Robardet et al. 1990 ; Quesada 1991). The Bohemian affinities of the late Ludlow trilobite assemblage described above can be discussed in this context. During Cambrian and Ordovician times the West Asturian-Leonese Zone was characterized especially by very high sedimentary rates indicative of strong subsidence. This is illustrated by the "abnormal" thickness, and apparent continuity of the whole succession when compared with contemporary deposits in other Iberian regions (P~rez-Estafn et al. 1990). Ordovician faunas from NW Spain do not show strong differences from those of Central Iberia ; there are, however, some distinctions as regard the diversity of trilobites and the relative abundance of some genera (Rabano 1984 p. 276). The occurrence in the WALZ of such mesopelagic graptolites as Pterograptus, which are unknown in other Iberian regions, indicates relatively deeper environments (Robardet & Guti6rrez- Marco 1990). Most of these distinctive faunal and sedimentary characteristics also occur in the Iberian Chains (NE Spain) which are generally regarded as the eastern extension of the West AsturianLeonese Zone (Julivert et al. 1974, Carls 1983). Consequently Julivert (1978) proposed an early Palaeozoic palaeogeographie reconstruction where the northern part of the Iberian Peninsula (including the Cantabrian Zone, the West Asturian-Leonese Zone, the Iberian Chains) was individualized and regarded as part of a larger domain extending into Southern France (Aquitaine, southern Massif Central and the Montagne Noire). A rather similar palaeogeographic pattern was also proposed in a general reconstruction of the early Palaeozoic palaeogeography of the Variscan regions (Paris & Robardet 1990).

On the other hand, many conflicting models have been proposed to explain the geodynamic evolution of the West European Variscan Belt, with the possible existence of a distinct Northern Iberian Domain, regarded either as a microcontiner~t (Riding 1974) or as part of a larger unit also including Southern France (Autran & Cogn~ 1980 ; Bard et al. 1980 ; Badham 1982). In such models, the northern limit of this unit is regarded as coincident with the South Armorican Shear Zone or with the geophysical discontinuity off the French Atlantic coast. However no clear southern limit can be defined within the Iberian Peninsula. Moreover, the sedimentary and faunal affinities of the Mid-North Armorican regions which are clearly with Central Iberia in the Ordovician, are best documented with the Iberian Chains (i.e. the eastern extension of the WALZ) in the Devonian (Carls 1988). This complicated pattern has led to doubts about the existence of a distinct Northern Iberian Domain (Robardet & Guti6rrez-Marco 1990) or as to whether most of the Iberian Peninsula was actually a single palaeogeographic unit during pre-Variscan times (Ribeiro et al. 1990 ; Quesada 1991 ; Quasada et al. 1991). The results of the present study and especially the Bohemian affinities of the late Ludlow trilobite assemblages from the W,~T.Z thus raise new questions concerning : - the similarities between the Silurian-Devonian successions from the Caurel-Pefialba Domain and those of the Cumbres-Hinojales Unit of the Ossa Morena Zone, which denote similar environments, particularly for the chloritoid shales. the biogeographical relations during the Silurian period between the WALZ and other Iberian areas with sequences of Mediterranean type. On that point it m u s t be remembered that the trilobite associations from Pyrenees display clear Bohemian affinities from the Wenlock (Aulacopleura, "Kettneraspis", Otarion : Dalloni 1930 ; Cavet 1957 ; Degardin & Pillet 1983) until the Early Devonian (Feist et al. 1985 ; Arbizu 1986). In this region the Ludlow trilobites (Degardin & Pillet 1983) are of the same type as those from the Pefialba and Sil synelines : Pilletopeltis aft. transiens 6= Crotalocephalus moraveci), Cromus aft. krolmusi (= C. aft. bohemicus). Data concerning Lower Silurian trilobites are relatively rare but they indicate biogeographic affinities between both regions : Dalmanites batalleri HERNANDEZ SAMPELAYO 1944 (= Dalmanites (Preodontochile) camprodonensis DEGARDIN • PILLET 1983) described from Upper Telychian graptolite shales in the

373 Catalonian Pyrenees also occurs in different contemporaneous localities from the Castillian Branch of the Iberian Chains (El Pobo de Duefias and Checa : I. l ~ b a n o unpublished) which is regarded as the eastern extension of the West Asturian Leonese Zone. At present it is not possible to propose a precise early Palaeozoic palaeogeographic model for SW Europe: However we consider that the Bohemian affinities of the Silurian trilobites from the West Asturian-Leonese Zone and their similarity with Pyrenean and Catalonian coeval faunas provide s u p p l e m e n t a r y arguments in favour of the biogeographical originality of the Northern Iberian Peninsula.

Acknowledgements - We would like to thank Dr. J. Van~k (Prague) for information on Bohemian Upper Silurian trilobites and help with the identification of the Spanish Silurian trilobites. Drs. V. Havl~'cek (Prague) and P. Racheboeuf (Lyon) kindly determined the Silurian brachiopeds. We also appreciated the help during field work of Prof. L. Teller (Warsaw), and M. Hacar and the geological team of EGEO, S.A. Thanks are also due to Monique Le Moigne for typing the manuscript and to Dani~le Bernard and Michel Lautram for drawing the figures. Dr. Euan Clarkson (Edinburgh) kindly corrected the English text and, as Dr. Ivo Chlupfi~, made a constructive critical review of the manuscript. This work was carried out within the framework and with the fmancial support of the projects : "The Ordovician-Silurian boundary in France and the Iberian Peninsula" (Programe "Acciones Integradas" between Spain and France, 1990-1992) and AMB92-1037-C02-01 of the Spanish CICYT.

REFERENCES ABRIL HURTADO J., PLIEGO DONES D. & RUBIO NAVAS J. 1982 - Memoria explicativa de la Hoja n ° 191 (SilvAn) del Mapa Geol6gico de Espafia E. 1 : 50.000 (2a Serie). IGME, Madrid: 56 p. ALBERTI G.tLB. 1969 - Trilobiten des jiingeren Siluriums sowie des Unter- und Mitteldevons. I. Mit Beitr~igen zur Silur-Devon-Stratigraphie einiger Gebiete Marokkos und 5berfrankens.

Abhandlungen der senckenbergischen naturforschenden Gesellschaft, 520 : 1-692. ALMERA J. 1891 - Descubrimiento de otras dos faunas del Sil6rico inferior en nuestros contornos, determinaci6n de sus niveles y el de la fauna de los filadios rojo-purpdreos de Papiol. Cr6nica Cientffica, 14, 339 : 465-473. APALATEGUI ISASA O. & SANCHEZ CARRETERO R. 1991 - S/ntesis y correlaci6n de unidades en el borde meridional de la Zona de Ossa Morena

(Zorn) : Implicaciones geol6gicas. Bolettn Geol6-

gico y Minero, 102, 3 : 339-347. APALATEGUI ISASA O., ABRIL HURTADO J. & RODRIGUEZ FERNANDEZ L . R . 1 9 8 1 - Mapa y Memoria explicativa de la Hoja n ° 190 (Barco de Valdeorras) del M a p a Geol6gico de Espafia E: 1 : 50.000 (2a Serie). IGME, Madrid : 44 p. ARBIZU M. 1986 - E1 g~nero Odontochile HAWLEy CORDA (Trilobita) en el Dev6nico inferior de la Cordillera Cant~brica y el Pirineo. Revista de la Sociedad Geol6gica de Espaga, 1 : 93-99. AUTRAN A. & COGNE J. 1980 - La zone interne de l'orog~ne varisque dans l'Ouest de la France et sa place dans le d~veloppement de la chaiue hercynienne. M~moires du Bureau de Recherches G~ologiques et Mini~res, 108 : 90-111. BADHAM J.P.N. 1982 Strike-slip orogens-an explanation for the Hercynides. Journal of the Geological Society London, 139 : 493-504. BARD J.P., BURG J.P., MATTE Ph. & R1BEIRO A. 1980 - La chalne hercynienne d~Europe occidentale en termes de tectonique des plaques.

Mdmoires du Bureau de Recherches C~ologiques et Mini~res, 108 : 233-246. BARROS LORENZO J.C. 1989 Nuevos datos geol6gicos y cartogr~ficos sobre el flanco sur del Sinclinorio de Truchas (Ourense-Le6n, NW. de Espafia). Cuadernos do Laboratorio xeol6xico de Laxe, 14 : 93-116. BOUCOT A.J. 1975 - Evolution and Extinction Rate Controls (Developments in Paleontology and Stratigraphy 1). Elsevier : 1-427. CARLS P. 1983 - La Zona asturoccidental-leonesa en AragSn y el Macizo del Ebro como prolongaci6n del Macizo CantAbrico. In COMBA J.A. (ed.) : Geologfa de Espa~a, Libro jubilar J.M. Rfos t. HI Contribuciones sobre temas generales. IGME Madrid : 11-32. CARLS P. 1988 The Devonian of Celtiberia (Spain) and Devonian paleogeography of SW Europe. In McMILLAN N.J., EMBRY A.F. & GLASS D.J. (eds) : Devonian of the World.

Canadian Society of Petroleum Geologist, 1 : 421-466. CAVET P. 1957 - Le Pal~ozo~que de la zone axiale des Pyrenees orientales fran~aises entre le Roussillon et l'Andorre. Bulletin du Service de la Carte gdologique de la France, 55, 254 : 1-216. CHLUPA~ I. 1987 - Ecostratigraphy of Silurian trilobite assemblages of the Barrandian area, Czechoslovakia. Newsletters on Stratigraphy, 17, 3 : 169-186. CHLUP.~ I. & KUKAL Z. i988 - Possible global events and the stratigraphy of the Barrandian Palaeozoic (Cambrian-Devonian, Czechoslovakia). Sborn£k Geologickych Ved. Geologie, 43: 83-146.

374

CORNEE J.-J., COSTAGLIOLA C., LEGLISE H., WILLEFERT S. & DESTOMBES J. 1985 - Pr~cisions stratigraphiques sur l'Ordovicien sup~rieur et le Silurien du synclinal d'Oulad Abbou (Meseta marocaine occidentale). Manifestations volcaniques au Silurien. Annales de la Soci~td gdologique du Nord, 104 : 141-146. COSTA J.C. da 1940a - Nova esp~cie fSssil do Gotlandiano do Alegrete. Boletim do Museu e

Laborat6rio Mineraldgico e Geoldgico da Universidade de Lisboa, 7-8 : 3-11. COSTA J.C. da 1940b - Subsidio para o estudo do g~nero Homalonotus. Boletim da Facultade de CiZncias da Universidade do Porto, 19 : 6-12. COSTA J.C. da 1940c - Trilobites dos "Gr~s Superiores". Las Ciencias, 7, 3 : 1-7. DALLOI~I M. 1930 - Etude g~ologique des Pyrenees catalanes. Annales de la Facult~ des Sciences de MarseiUe, 26, 3 : 1-374. DEGARDIN J.M. & PILLET J. 1983 - Nouveaux Trilobites du Silurien des Pyrenees centrales espagnoles. Annales de la Socidt~ gdologique du Nord, 103: 83-92. DESTOMBES J., HOLLARD H. & WILLEFERTS. 1985 Lower Paleozoic rocks of Morocco. In HOLLAND, C.-H. (~d.) : Lower Paleozoic rocks of the World. John Wiley, London, 4 : 91-336. FEIST R., DELVOLVE J.-J. & CYGAN C. 1985 Trilobites d'affinit~ boh~mienne dans l'Eod~vonien des Pyrenees Centrales franqaises. Geobios, 18, 6 : 883-890. GAERTNER H.R.V. 1930 - Obersilurische F a u n e n aus des spanischen Pyren~ien. Nachrichten yon

der Gesellschaft der Wissenschaften zu Gi~ttingen, Mathematisch-Physicalische Klasse, 4, 2 : 179-188. GANDL J. 1972 - Die Acastavinae und Asteropyginae (Trilobita) Keltiberiens (NE-Spanien).

Nachrichten yon der Gesellschaft der Wissenschaften zu GSttingen, Mathematisch-Physicalische Klasse, 530 : 1-184. GARCIA ALCALDE J.L., MONTESINOS J.R., TRUYOLS-MASSONI M., GARCIA LOPEZ S., ARBIZU M.A. & SOTO F. 1988 - E1 Silf~rico y el DevSnico del Dominio Palentino (NW de Espafia). Revista de la Sociedad Geoldgica de Espaga, 1, 1-2 : 7-13. GUTIERREZ-MARCO J.M. & ROBARDET M. 1991 D~couverte de la zone ~ Parakidograptus acuminatus (base du Llandovery) darts le Silurien du Synclinorium de Truchas (Zone asturo-leonaise, Nord-Ouest de rEspagne) : consequences stratigraphiques et pal~og~ographiques au passage Ordovicien-Silurien.

Comptes Rendus de l'Acaddmie des Sciences de Paris, 2, 312 : 729-734. HAVLICEK V. & STORCH P. 1990 - Silurian brachiopods and benthic communities in the

Basin (Czechoslovakia). Rozpravy Ustredniho Ustavu geologickdho, 48 : 1-275.

Prague

HERITSCH F. 1929 - F a u n e n aus dem Silur des Ostalpen. Abhandlungen der Geologischen Bundesanstalt, 23, 2 : 1-183. HERNANDEZ-SAMPELAYO P. 1942 - Explicaci6n del nuevo Mapa Geol6gico de Espafia. Tomo II. E1 Sistema Siluriano. Memorias del Instituto Geoldgico y Minero de Espa~a, 45, 1 : 1-848. HERNANDEZ-SAMPELAYO P. 1944 - De la fauna Gotlandiense. Dalmanites batalleri SAMP. Correcci6n de Phacops longicaudatus MURCH. Dalmanites longicaudatus, e~mienda de Font y Sagii& Notas y Comunicationes del Instituto Geoldgico y Minero de Espa~a, 13 : 4-8. JAEGER H. & ROBARDET M. 1979 - Le Silurien et le D~vonien basal dans le nord de la province de Seville (Espagne). Geobios, 15, 5 : 687-714. JORQUERA DE GUINDOS A., DELGADO PASTORJ. & APALATEGUI ISASA 0. 1990 Memoria explicativa de la Hoja n ° 874 (Oliva de la Frontera) del Mapa Geol6gico de Espafia E. 1 : 50.000 (2a Serie). IGME, Madrid : 39 p. JULIVERT M. 1978 - Algunas bases para una correlaci6n paleogeogr~fica entre los Macizos Hercinianos del Occidente de Europa.

Cuadernos do Seminario de Estudos Cerdmicos de Sargadelos, 27 : 159-191. JULIVERT M., DURAN H., RICKARDS R.B. & CHAPMAN A.J. 1985 - Siluro-Devonian graptolite stratigraphy of the Catalonian Coastal Ranges. Acta Geologica Hispanica, 20, 3/4 : 199-207. JULIVERT M., DURAN H. GARCIA LOPEZ S., GIL IBARGUCHI J.I., TRUYOLS MASSONI M. & VILLAS E. 1987 - Pre-Carboniferous rocks in the Catalonian Coastal Ranges : volcanism, stratigraphic sequence and fossil content. In FL~GEL, H.W., SASSI F.P. & GRECULA P. (eds.): Pre-Variscan and Variscan events in the Alpine-Mediterranean mountain belts. Mineralia Slovaca. Monography, 1987 : 313-322. JULIVERT M., FONTBOTE J.M., RIBEIRO A. & CONDE L. 1974 - Mapa Tect6nico de la Peninsula Ib6rica y Baleares-Escala 1 : 1.000.000, IGME Madrid. KEGEL W. 1929 - Das Gotlandium in den Kantabrischen Ketten Nordspmliens. Zeitschrift der deutschen geologischen Gesellschaft, 81 : 35-62. KRIZ J. 1979 - Silurian Cardiolidae (Bivalvia). Sbornfk Geologickych Ved. Paleontologie, 22 : 5-517. MARCOS A. 1973 - Las series del Paleozoico inferior y la estructura herciniana del occidente de Asturias (NW. de Espafia). Trabajos de Geologia, Universidad de Oviedo, 6 : 1-113. MARTINEZ-CATALAN J.R. 1985 - Estratigraf/a y estruetura del domo de Lugo (Sector Oeste de

375 la zona Asturoccidental-leonesa). Corpus Geologicum GaUaeciae Segunda Serie, 2 : 1-291.

correlations with West African sequences. In DALLMEYER R.D. & L]~CORCH]~ J.P. (eds) : The

NOLLAU G. 1966 - E1 desarrollo estrfitigrafico del Paleozoico en el Oeste de la Provincia de Le6n.

West African Orogens and Circum Atlantic Correlatives. Springer Verlag : 267-293.

Notas y Comunicaciones del Instituto Geol6gico y Minero de Espa~a, 88 : 31-48.

RABANO I. 1984 Trilobites ordov/cicos del Macizo Hesp~rico espafiol : u n a visi6n bioestratigr~ifica. Cuadernos de Geologia Ibdrica, 9 : 267-287. RACHEBOEUF P. 1990 - Les Brachiopodes Chonetac~s dans les assemblages benthiques siluriens et d~voniens. Palaeogeography, Palaeoclimatology, Palaeoecology, 8 1 : 141-171. RIBEIRO A., QUESADA C. & DALLMEYER R.D. 1990 - Geodynamic Evolution of the Iberian Massif. In DALLMEYERR . D . & MARTINEZ-GARCIA(eds.): Pre-Mesozoic Geology of Iberia. SpringerVerlag: 399-409. RIDING R. 1974 - Model of the Hercynian foldbelt. Earth and Planetary Science Letters., 24 : 125-135. RIEMER W. 1966 - Datos p a r a el conocimiento de la estratigrafia de Galicia. Notas y Comu-

NOLLAU G., 1968 - Stratigraphie, Magmatismus und Tektonik der Montes de L~on zwischen Astorga und Ponferrada in Nordwest-Spanien. Geotektonische Forschungen, 27, 1-2 : 71-146. OCZLON M. 1989 - Fazies und F a u n a im Silur und Devon des "Valle" (Provinz Sevilla, SWSpanien), mit einer iiberregionalen Betrachtung der Mitteldevon-Schichliike. Diplomarbeit U n i v e r s i ~ t Heidelberg (unpubl.) : 94 p. OLIVEIRA J.T., OLIVEIRA V. & PI~ARRA J.M. 1991 Traqos gerais da evolucfio tectono-estratigrfifica da Zona de Ossa-Morena, em Portugal.

Cuadernos do Laboratorio Xeol6xico de Laxe, 1 6 : 221-250. PARIS F. & ROBARDET M. 1990 - Early Palaeozoic palaeobiogeography of the Variscan regions. Tectonophysics, 177 : 193-213. PEREZ-ESTAUN A. 1978 - Estratigraf/a y estrnctura de la R a m a Sur de la Zona Asturoccidental-Leonesa. Memorias del Instituto Geol6gico y Minero de Espa~a, 92 : 1-149. PEREZ-ESTAUN A. & MARCOS ~ 1981 - La Formaci6n Agiieira en el sinclinorio de Vega de Espinareda : aproximaci6n al modelo de sedimentaci6n durante el Ordovlcico superior en la zona Asturoccidentalleonesa (NW. de Espafia).

Trabajos de Geologia, Universidade d e Oviedo, 11 : 135-145. PEREZ-ESTAUN A., BASTIDA F., MARTINEZCATALAN J.R., GUTIERREZ-MARCO J.C., MARCOS A. & PULGAR J.A. 1990 - West AsturianLeonese Zone : Stratigraphy. In DALLMEYER R.D. & MARTINEZ-GARCIA E. (eds.) : Pre-Mesozoic Geology of Iberia. Springer-Verlag : 92-102, PRANTL F. & P~BYL A. 1954 - O ceskych zfistupcich celedi Harpedidae (Hawle et Corda) Trilobitae. Rozpravy Ustredniho Ustavu Geolo~ickdho, 18 : 1-170. PRIBYL A. & VAN~.K J. 1986 - A study of the morphology and phylogeny of the family Harpetidae HAWLE & CORDA 1847 (Trilobita). Sbornik Ndrodniko muzea v Praze, 42B : 1-72. QUESADA C. 1991 - Geological constraints on the Paleozoic evolution of tectonostratigraphic terranes in the Iberian Massif. Tectonophysics, 185 : 225-245. QUESADA C., BELLIDO F., DALLMEYER R.D., GILIBARGUCHI I., OLIVEIRA J.T., PEREZ-ESTAUN A., RIBEIRO A., ROBARDET M. & SILVA J.B. 1991 Terranes. within the Iberian Massif :

nicaciones del Instituto Geol6gico y Minero de Espaga, 81 : 7-20. ROBARDET M. 1982 - The Silurian-earliest Devonian succession in South Spain (Ossa Morena Zone) and its paleogeographical signification.

International Geological Correlation Project n ° 5 Newsletter, 4 : 72-77. ROBARDET M. • GUTIERREZ-MARCO J.C. 1990 Sedimentary and Faunal Domains in the Iberian Peninsula During Lower Paleozoic Times. In DALLMEYER R.D. & MARTINEZGARCIA E. (eds.) Pre-Mesozoic Geology of Iberia. Springer-Verlag : 383-395. ROBARDET M., PARIS F. & RACHEBOEUF P.R. 1990 - Palaeogeographie evolution of southwestern Europe during Early Palaeozoic times. In MeKERROW W.S. & SCOTESE C.R. (eds) Palaeozoic Palaeogeography and Biogeography. Geoogical Society, London, 12 : 411-419. RODRIGUEZ FERNANDEZ L.R. & HEREDIA N. 1988 Evoluci6n tectonosedimentaria de una Cuenca de Antepais ligada a u n a cadena arqueada : el ejemplo de la Unidad del Pisuerga-Carri6n (Zona Cantfibrica, NO de Espafia). Simposios H Congreso GeolSgico de Espaga, Granada : 65-74. RuIz LOPEZ J.L., FERNANDEZ CARRASCOJ., COULLAUT J.L. & APALATEGUI O. 1984 - Memoria explicativa de la Hoja n 896 (Higuera la Real) del Mapa Geol6gico de Espafia E. 1 : 50.000 (2a Serie). IGME, Madrid : 47 p. SCHONLAUB H.P. 1986 Significant geological events in the Paleozoic record of the Southern Alps (Austrian part). In WALLISER O. (~d.) : Global Bio-Events. Lecture Notes in Earth Sciences, 8 : 163-167.

376 SNAJDR M. 1960 - Studie o celedi Scutelluidae (Trilobitae). Rozpravy Ustredniho Ustavu v Geologickdho,, 26 : 1-263. SNAJDR M. 1985 - Bohemian representatives of the subfamily Encrinurinae (Trilobita). Sbornfk v geologickych Ved, Paleontologie., 27 : 9-46. SNAJDR M. 1989 Planiscutellum and Avascutellum from the Bohemian Silurian (Trilobita, Styginidae). Vestnik Ustredniho ~tstavu geologickdho, 64, 2 : 115-119. TRUYOLS-MASSONI M. 1986 - Nowakia acuaria (Praguiense, Dev6nico Inferior) de la Zona Asturoccidental-Leonesa (NO de Espafia). Breviora GeologicaAsturica, 27 : 12-16. VANEK J. 1963 - Nov~ rody celedi Harpidae Hawle et Corda, 1847 (Trilobita) z Barrandienu. Casopis Ndrozniho Muzea, Oddil Prirodovedny, 132 : 226-229.

VELANDO F. & MATAS J. 1981 - Memoria explicativa de la Hoja n 192 (Lucillo) del Mapa Geol6gico de Espafia E. 1:50.000 (2a Serie). IGME, Madrid : 31 p. VOt~C V. & DOUBRAVA M. 1990 - Crotalocephalus moraveci sp. nov. (Trilobita) from the Silurian of Bohemia. Vestnik Ustredniho (tstavu geologickdho, 65, 1 : 61-64. W0HRLE R. 1981 - Stratigraphie und Tektonik im

Sil-Synklinorium bei Puente de Domingo Fldrez (NW-Spanien). Diplomarb. Univ. Erlangen (unpubl.) : 109 p. ZEITZ U. & NOLLAU G. 1984 - Ordoviz und Silur im Sil-Synklinorium stidlich Ponferrada (Provinz Le6n), NW-Spanien. Zeitschrift der Deutschen geologischen Gesellschaft, 135 : 211222.