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Fossil communities in the Aguión formation (Lower Devonian) of the Arnao Platform (Asturias, NW Spain)
FOSSIL COMMUNITIES IN THE AGUION FORMATION (LOWER OF ARNAO PLATFORM (ASTURIAS, NW SPAIN)
MIGUEL ARBIZU, ISABEL lVIENDEZ-BEDIA & FRANCISCO SOTO ARBIZU M., MI~NDEZ-BEDIA I. & SOTO F. 1995. Fossil communities in the Agui6n Formation (Lower Devonian) of the Arnao Platform (Asturias, NW Spain). [Communaut6s fossiles dans la Formation d'AguiSn (D6vonien inf~rieur) de la Plate-forme d'Arnao (Asturies, NW de l'Espagne)]. GEOBIOS, 28, 5 : 567-571. Villeurbanne le 31.10.1995. Manuscrit d6pos6 le 10.02.1995 ; accept6 d6finitivement le 07.06.1995.
ABSTRACT This paper analyzes the evolution of faunal communities from the lower part of the Agui6n Formation (Lower Devonian, Upper Emsian) of the Arnao Platform (north side of the Cantabrian Zone, NW Spain). Three informal units of beds with different lithological and faunal characteristics are described. In these units several communities developed in different environmental conditions. In the basal calcareous unit communities were related to a patch-reef biostrome developed on bioclastic bars ; in the second marly-shaly unit the communities characterized sheltered environmental conditions ; the third unit of green and red marls, shows communities that were clearly related to platform environments with a high rate of terrigenous supply. KEY-WORDS
: FOSSIL COMMUNITIES,
AGUION
FORMATION,
UPPER
EMSIAN, CANTABRIAN
ZONE, NW SPAIN.
RESUME Dana cette 6tude on analyse l'6volution des communaut6s fauniques pr6sentes dans la pattie inf6rieure de la Formation d'AguiSn (Emsien sup6rieur) de la Plate-forme d'Arnao (versant N de la Zone Cantabrique, NW de l'Espagne). Trois unit6s informelles sont reconnues avec des caract6ristiques lithologiques et fauniques particuli~res ; l'6tablissement des communaut6s, leur d6veloppement et leur 6volution sont influenc6s par des variations du milieu. Dans la premiere de ces unit6s, calcaire, les communaut6s sont en rapport avec le d6veloppement, sur des barres bioclastiques, d'une structure biostromale de type "patch-reef' ; dans la deuxi6me unit6, schisto-marneuse, les communaut6s correspondent h des milieux prot6g6s ; celles de la troisi6me unit6, marnes rouges et vertes, sont en rapport avec des milieux de plate-forme h s6dimentation rapide. MOTS-CLI~S : COMMUNAUTI~S ESPAGNE.
FOSSILES,
FORMATION
D'AGUION,
EMSIEN
SUPERIEUR,
ZONE
CANTA_BRIQUE,
NW
RESUMEN En e! presente trabajo se realiza un an~lisis sint6tico de la evolucidn de comunidades f6siles presentes en !a parte inferior de la Formacidn Aguidn (Devdnico Inferior, Emsiense Superior) en la Plataforma de Arnao (vertiente N de la Zona Cant~brica, NO de Espafia). Se han distinguido informalmente tres unidades con caracterlst~cas litoldgicas y faunisticas diferentes en las que se establecen varias comunidades, cuyo desarrollo y evoluci6n estaban influenciados pot diferentes condiciones ambientales. En la primera de las unidades distinguidas, unidad calc~rea, las comunidades estaban relacionadas con el desarrollo, sobre barras biocl~sticas, de una estructura biostromal de tipo ,patch-reef' ; en la segunda unidad, unidad pizarrosa-margosa, las comunidades presentes eran propias de medios protegidos ; finalmente, las de la tercera unidad, unidad de margas rojas y verdes, estaban ligadas a medios de plataforma con aportes importantes y variables de material terr/geno. PALABRAS-CLAVE ESPAI~A.
: COMUNIDADES
FOSILES, FORMACION
AGUION,
EMSIENSE
SUPERIOR,
ZONA
CANT~BRICA,
NO
568
INTRODUCTION
quarry, close to the town of Avil6s, NW of Oviedo. The rocks that crop out in this quarry (Fig. 1C) belong to the Agui5n Formation, Lower Devonian (Upper Emsian).
In the Cantabrian Zone, NW Spain, (Fig. 1A, B) the Asturo-Leonese facies of the Devonian (Brouwer 1964) is characterized by rich benthic faunas with brachiopods, bryozoans, crinoids, corals, stromatoporoids, etc. The abundance of fossils combined with excellent outcrop conditions permit study the fossil communities and their relationship with the sedimentary environment.
The stratigraphic succession (Fig. 1D) can be d i into three informal units (Alvarez-Nava & Arbizu 1986) : a lower calcareous unit, a middle marly-shaly unit and an upper unit of green and red marls, all in the 60 basal metres of the AguiSn Formation (Fig. 1E). These three units closely resemble the beds differentiated by Schmidt (1931) on the basis of geological data provided by Kegel, it being noted by AlvarezNava & Arbizu (op. cit.) that the polarity of the succession was incorrectly interpreted. vided
On the Asturian coast (north side of the Cantabrian Zone), there are several localities suitable for the analysis. One of these is on the Arnao Platform (Fig. 1B), a traditional site for Asturian geology. It is situated on the coast, in an old B
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Figure I - A, location of t h e Cantabrian Zone in the Iberian Peninsula. B, distribution of the devonian outcrops in t h e Cantabrian Zone and geographical setting of studied locality. C, geological map of Arnao Platform. D, stratigraphical s u c c e s s i o n o f t h e Agui5n Formation at Arnao Platform showing lithological units, t h e i r faunal composition and t y p e s of communities. E, lithostratigraphy and chronostratigraphy o f t h e Lower Devonian w i t h Asturo-Leonese facies in Asturias. A, situation de la
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569 The studies on the fossil assemblages and the palaeoecological analyses published by AlvarezNava & Arbizu (op. cir.) thus far have focused on certain aspects of the succession. More recent research on these materials by the present authors, and the reinterpretation of data from previous studies hmre allowed a global synthesis o f the evolution of faunal communities in the succession and t h e ~ interpretation in terms of changing environmentaI conditions.
variety of organisms, such as low domal alveoli~ rids, laminar bryozoans, compomld rugose corals and branching tabulates (P1. 69, fig. 3).
The three units of the Arnao Platform have been further divided into different horizons in relation to faunal contents.
D o m i n a t i o n s t a g e - During this stage, two different taxa contributed successively to the growth of the reef, forming a bindstone " mainly mediumsized flattened low domal alveolitids, were later replaced by large laminar bryozoans (P1. 69, fig. 4). In sheltered zones, below the alveolitids, there was profuse colonization by various different organisms, such as cemented brachiopods, cylindrical and laminar bryozoans, autoporids as well as small solitary rugose corals. In general, the type of matrix (packstone and wackestone) and the argillaceous material content together with the morphology of the organisms suggest a shallow water platform of moderate energy as the patchreef developed.
CALCAREOUS
MARLY-SHALY UNIT
FOSSIL ASSEMBLAGES PAL~OECOLOGICAL INTERPRETATION
AND
UNIT
It is 22 m thick and consists of two different horizons (Fig; 1C,D). The lower one, 17 m thick, is composed by dominantly bioclastic grey and red massive limestones with crinoid debris, bryozoans a n d with some corals. The upper horizon, consisting of slightly argillaceous grey limestones, is a biostrome interpreted as a patch-reef, e. 5 m thick, built up essentially by bryozoans and tabulate corals. In this calcareous unit the fossil assemblages represent communities showing an ecological succession comparable to that described by Walker & Alberstadt (1975) and M~ndez-Bedia & Soto (1984). It developed in four different stages : stabilization, colonization, diversification and domination ; the first one took place in the lower horizon of this unit. S t a b i l i z a t i o n s t a g e - In this stage the substrate became suitable for the subsequent settlement of reefa! fauna. T h e s e are interpreted as communities r e l a t e d to bioclastic bars from the lower levels of: the unit. The macrofossils are mainly small t a b u l a t e s and encrusting bryozoans, sparsely distributed on bioclastic bars formed through stacking of fragmental remains of other organisms, crinoids and bryozoans especially (P1. 69, fig. i): C o l o n i z a t i o n s t a g e - In this stage pioneer organisms, namely, branching tabulates and bryozoans, colonized bioclastic bars, giving rise to the later growth of a patch-reef (P1. 69, fig. 2). D i v e r s i f i c a t i o n s t a g e - The number of taxa increased during this stage. However, it is not well developed in the succession and could, perhaps, be represented by a series of levels with a greater
This is 12 m thick and consists of brown and grey shales and marls (Fig. 1C,D), with large brachiopods (different species of P l i c o s t r o p h e o d o n t a , E u rysp~rifer, L e p t o s t r o p h i e l l a , M e g a n t e r i s , etc.) most abundant at the base and with numerous bryozoans (especially fenestellids) and crinoids (P1. 69, fig. 5). The excellent preservation of the entire fauna together with the type of associated sediments suggest quiet conditions, characteristic of sheltered areas (sublagoon?). UNIT OF GREEN AND RED MARLS This is 24 m thick and consists of red and green marls (Fig. 1C,D) with abundant crinoids, particularly T r y b l i o c r i n u s f l a t h e a n u s GEINITZ, as well as bryozoans and a few brachiopods. Arbizu et al. (1993) carried out a palaeoecological study of the unit and distinguished four communities dominated by one or two taxa. Argillaceous contents remain constant for each community within the following values : - F e n e s t e l l a Community - 20 to 28 % argillaceous material. - I s o t r y p a Community : 28 to 43 % argillaceous material. - T r y b l i o c r i n u s and A n a t h y r i s Community : 43 to 56 % argillaceous material. - 7~:ybliocrinus Community : 56 to 70 % argillaceous material. Due to variations in the terrigenous supply, the same communities became established several times within the unit. The first community was dominated by well-preserved large flattened fenestellids (Fenestella sp.) occurring essentially as described by Johnson (1960) in his model I, in which organisms must have grown in a well-oxy-
570 g e n a t e d q u i e t e n v i r o n m e n t a n d developed on a clastic s e d i m e n t s u b s t r a t e supplied periodically w i t h m a t e r i a l s i m i l a r to t h a t forming t h e substrate. T h e second c o m m u n i t y (P1. 69, fig. 6) was domin a t e d b y closely spaced conical fenestellids (Isotrypa sp.). T h e lower n u m b e r of t a x a a n d the absence of f l a t t e n e d fenestellids provide evidence of a h i g h e r t e r r i g e n o u s i n p u t a n d a n increase in m u d d i n e s s of t h e water. T h e last two c o m m u n i t i e s (P1. 69, fig. 7,8) developed in a n e n v i r o n m e n t c h a r a c t e r i z e d by a grad u a l i n c r e a s e i n t e r r i g e n o u s supply. This is evid e n c e d b y t h e s m a l l e r n u m b e r a n d size of t h e b r y o z o a n s , w h i c h a d o p t e d g r a d u a l l y closer conical shapes, a n d b y t h e l a r g e r a m o u n t of Trybliocrin u s f l a t h e a n u s , w h i c h h a d a gas exchange s y s t e m t h a t allowed it to live in m u d d y e n v i r o n m e n t s ( S c h m i d t 1931). In the last c o m m u n i t y , large inf a u n a l bivalves w e r e also p r e s e n t . Hence, t h e env i r o n m e n t would h a v e b e e n similar to t h a t of J o h n s o n ' s (1960) model II, c h a r a c t e r i z e d b y a cont i n u o u s s u p p l y of argillaceous m a t e r i a l , causing a r a p i d burial. In s u m m a r y , t h e c o m m u n i t i e s r e c o r d e d in t h e calcareous u n i t s e e m r e l a t e d to t h e d e v e l o p m e n t of a b i o s t r o m a l p a t c h - r e e f on bioclastic bars; t h o s e of t h e m a r l y - s h a l y u n i t c h a r a c t e r i z e d shelt e r e d e n v i r o n m e n t s . Finally, the c o m m u n i t i e s in t h e u n i t of g r e e n a n d r e d m a r l s s e e m to h a v e b e e n typical of p l a t f o r m e n v i r o n m e n t s w i t h h i g h l y v a r i a b l e r a t e s of t e r r i g e n o u s supply. T h e v e r t i c a l f a u n a l c h a n g e observed in the calcar e o u s u n i t could r e p r e s e n t a n autogenic succession w h e r e a s t h e succession of fossil assemblages p r e s e n t in t h e o t h e r u n i t s are i n t e r p r e t e d as a n e n v i r o n m e n t a l l y i n d u c e d a s s e m b l a g e sequence.
Project PB-92-1008 : 'lArrecifes dev6nicos de la Zona Cant~brica (Cordillera Cant~tbrica, NO de Espafia) : Modelos de formaci6n, composiciSn y evoluciSn". We thank Dr. R.H. Wagner (Jardin Bot~_nico de Cordoba, Espafia) for his help in checking the English and comments. We also thank the referees, Drs. B. Mistiaen and P. Skelton, whose constructive critical reviews helped in the improvement of this paper.
REFERENCES ALVAREZ-NAVA H. & ARBIZU M. 1986 - ComposiciSn y desarrollo de un arrecife Emsiense en la Plataforma de Aruao (Asturias, NO de Espafia). Memorias I Jornadas de Paleontologia : 33-51. ARBIZU M., ALVAREZ-NAVA H., MI~NDEZ-BEDIA I. & GARCiA-LOPEZ S. 1993 - Las comunidades biSticas de las "Capas con Trybliocrinus" (Dev6nico Inferior) en la Plataforma de Arnao (Asturias, Noroeste de Espafia). Revista Espa~ola de Paleontologfa, Nflmero Extraordinario : 71-77. BROUWER A. 1964 - Deux faci6s dans le D6vonien des Montagnes Cantabriques m6ridionales. Breviora GeoIogica Astftrica, 8 (1-4) : 3-10. JOHNSON R.G. 1960 - Models and methods for the analysis of the mode of formation of fossil assemblages. Bulletin Geological Society of America, 71 : 10751086. M]~NDEZ-BEDIA I. & SOTO F. 1984 - Paleoecological succession in a Devonian organic buildup (Moniello Fm, Cantabrian Mountains, NW Spain). Geobios, 8: 150-157. SCHMIDT W. E. 1931 - Crinoiden und Blastoiden aus dem jfingsten Unterdevon Spaniens. Palaeontographica, 76 : 1-33. WALKER K. R. • ALBERSTADTL.P. 1975 - Ecological succession as an aspect of structure in fossil communities. Paleobiology, 1 : 238-257.
M. ARBIZU, I. M]~NDEZ-BEDIA & F. SOTO Acknowledgements
- This article has been partially
financed by the CICYT, within the framework of
Departamento de Geologia Universidad de Oviedo 33005 Oviedo, Spain
PLATE 69 Fig. 1-4 - Calcareous unit. 1 : stabilization stage consisting of crinoidal bars with some corals (in centre of photograph) and bryozoans. 2 : colonization stage with branching bryozoans and tabulate corals x 0.8. 3 : diversification stage with low domal alveolitids, laminar bryozoans and branching tabulate corals. 4 : domination stage with laminar bryozoans. Unitd calcaire. 1 : stade de stabilisation rdprdsentg par des barres encrinitiques avec quelques coraux et bryozoaires. 2 : stade de colonisation avec des bryozoaires et des coraux tabulds ramifids x 0,8. 3 : stade de diversification avec des alveolitidds reprgsentds par des formes en d6me has, des bryozoaires lamellaires et des coraux tabulds ramifids. 4 : stade de domination avec des bryozoaires lamellaires. Fig. 5 - Marly-shaly unit with large brachiopods (in centre of photograph), fenestellids and crinoids. Unitd schisto-marneuse, avec de grands brachiopodes, des fenestellidds et des crino~des. Fig. 6-8 - Unit of green and red marls. 6 : Isotrypa community. 7 : A n a t h y r i s and T r y b l i o c r i n u s community. 8 : T r y b l i o c r i n u s community. Unitd des marnes rouges et vertes. 6 : communautd ~ Isotrypa. 7 : communautd A n a t h y r i s et Trybliocrinus. 8 : communautd h Trybliocrinus.
Geobios nO 28, fasc. 5
PL 69 M, Arbizu, L M6ndez-Bedia & F. Soto
2
3
4
6
MIGUEL ARBIZU, ISABEL lVIENDEZ-BEDIA & FRANCISCO SOTO ARBIZU M., MI~NDEZ-BEDIA I. & SOTO F. 1995. Fossil communities in the Agui6n Formation (Lower Devonian) of the Arnao Platform (Asturias, NW Spain). [Communaut6s fossiles dans la Formation d'AguiSn (D6vonien inf~rieur) de la Plate-forme d'Arnao (Asturies, NW de l'Espagne)]. GEOBIOS, 28, 5 : 567-571. Villeurbanne le 31.10.1995. Manuscrit d6pos6 le 10.02.1995 ; accept6 d6finitivement le 07.06.1995.
ABSTRACT This paper analyzes the evolution of faunal communities from the lower part of the Agui6n Formation (Lower Devonian, Upper Emsian) of the Arnao Platform (north side of the Cantabrian Zone, NW Spain). Three informal units of beds with different lithological and faunal characteristics are described. In these units several communities developed in different environmental conditions. In the basal calcareous unit communities were related to a patch-reef biostrome developed on bioclastic bars ; in the second marly-shaly unit the communities characterized sheltered environmental conditions ; the third unit of green and red marls, shows communities that were clearly related to platform environments with a high rate of terrigenous supply. KEY-WORDS
: FOSSIL COMMUNITIES,
AGUION
FORMATION,
UPPER
EMSIAN, CANTABRIAN
ZONE, NW SPAIN.
RESUME Dana cette 6tude on analyse l'6volution des communaut6s fauniques pr6sentes dans la pattie inf6rieure de la Formation d'AguiSn (Emsien sup6rieur) de la Plate-forme d'Arnao (versant N de la Zone Cantabrique, NW de l'Espagne). Trois unit6s informelles sont reconnues avec des caract6ristiques lithologiques et fauniques particuli~res ; l'6tablissement des communaut6s, leur d6veloppement et leur 6volution sont influenc6s par des variations du milieu. Dans la premiere de ces unit6s, calcaire, les communaut6s sont en rapport avec le d6veloppement, sur des barres bioclastiques, d'une structure biostromale de type "patch-reef' ; dans la deuxi6me unit6, schisto-marneuse, les communaut6s correspondent h des milieux prot6g6s ; celles de la troisi6me unit6, marnes rouges et vertes, sont en rapport avec des milieux de plate-forme h s6dimentation rapide. MOTS-CLI~S : COMMUNAUTI~S ESPAGNE.
FOSSILES,
FORMATION
D'AGUION,
EMSIEN
SUPERIEUR,
ZONE
CANTA_BRIQUE,
NW
RESUMEN En e! presente trabajo se realiza un an~lisis sint6tico de la evolucidn de comunidades f6siles presentes en !a parte inferior de la Formacidn Aguidn (Devdnico Inferior, Emsiense Superior) en la Plataforma de Arnao (vertiente N de la Zona Cant~brica, NO de Espafia). Se han distinguido informalmente tres unidades con caracterlst~cas litoldgicas y faunisticas diferentes en las que se establecen varias comunidades, cuyo desarrollo y evoluci6n estaban influenciados pot diferentes condiciones ambientales. En la primera de las unidades distinguidas, unidad calc~rea, las comunidades estaban relacionadas con el desarrollo, sobre barras biocl~sticas, de una estructura biostromal de tipo ,patch-reef' ; en la segunda unidad, unidad pizarrosa-margosa, las comunidades presentes eran propias de medios protegidos ; finalmente, las de la tercera unidad, unidad de margas rojas y verdes, estaban ligadas a medios de plataforma con aportes importantes y variables de material terr/geno. PALABRAS-CLAVE ESPAI~A.
: COMUNIDADES
FOSILES, FORMACION
AGUION,
EMSIENSE
SUPERIOR,
ZONA
CANT~BRICA,
NO
568
INTRODUCTION
quarry, close to the town of Avil6s, NW of Oviedo. The rocks that crop out in this quarry (Fig. 1C) belong to the Agui5n Formation, Lower Devonian (Upper Emsian).
In the Cantabrian Zone, NW Spain, (Fig. 1A, B) the Asturo-Leonese facies of the Devonian (Brouwer 1964) is characterized by rich benthic faunas with brachiopods, bryozoans, crinoids, corals, stromatoporoids, etc. The abundance of fossils combined with excellent outcrop conditions permit study the fossil communities and their relationship with the sedimentary environment.
The stratigraphic succession (Fig. 1D) can be d i into three informal units (Alvarez-Nava & Arbizu 1986) : a lower calcareous unit, a middle marly-shaly unit and an upper unit of green and red marls, all in the 60 basal metres of the AguiSn Formation (Fig. 1E). These three units closely resemble the beds differentiated by Schmidt (1931) on the basis of geological data provided by Kegel, it being noted by AlvarezNava & Arbizu (op. cit.) that the polarity of the succession was incorrectly interpreted. vided
On the Asturian coast (north side of the Cantabrian Zone), there are several localities suitable for the analysis. One of these is on the Arnao Platform (Fig. 1B), a traditional site for Asturian geology. It is situated on the coast, in an old B
A
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Figure I - A, location of t h e Cantabrian Zone in the Iberian Peninsula. B, distribution of the devonian outcrops in t h e Cantabrian Zone and geographical setting of studied locality. C, geological map of Arnao Platform. D, stratigraphical s u c c e s s i o n o f t h e Agui5n Formation at Arnao Platform showing lithological units, t h e i r faunal composition and t y p e s of communities. E, lithostratigraphy and chronostratigraphy o f t h e Lower Devonian w i t h Asturo-Leonese facies in Asturias. A, situation de la
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Zone Cantabrique clans la P6ninsule Ibgrique. B, distribution des affleurements ddvoniens dans la Zone Cantabrique et situation gdographique de la localitd Stud,de. C, schdm a g6ologique de la Plateform, d'Arnao. D, succession stratigraphique de la Formation d'Agui6n sar la Plateform, d'Arnao, avec les unitgs lithologiques, leur composition faunique et leurs types de communaut6s. E, lithostratigraphi, et chronostratigraphie du D6vonien infdrieur asturien en facies asturo-lgonais.
569 The studies on the fossil assemblages and the palaeoecological analyses published by AlvarezNava & Arbizu (op. cir.) thus far have focused on certain aspects of the succession. More recent research on these materials by the present authors, and the reinterpretation of data from previous studies hmre allowed a global synthesis o f the evolution of faunal communities in the succession and t h e ~ interpretation in terms of changing environmentaI conditions.
variety of organisms, such as low domal alveoli~ rids, laminar bryozoans, compomld rugose corals and branching tabulates (P1. 69, fig. 3).
The three units of the Arnao Platform have been further divided into different horizons in relation to faunal contents.
D o m i n a t i o n s t a g e - During this stage, two different taxa contributed successively to the growth of the reef, forming a bindstone " mainly mediumsized flattened low domal alveolitids, were later replaced by large laminar bryozoans (P1. 69, fig. 4). In sheltered zones, below the alveolitids, there was profuse colonization by various different organisms, such as cemented brachiopods, cylindrical and laminar bryozoans, autoporids as well as small solitary rugose corals. In general, the type of matrix (packstone and wackestone) and the argillaceous material content together with the morphology of the organisms suggest a shallow water platform of moderate energy as the patchreef developed.
CALCAREOUS
MARLY-SHALY UNIT
FOSSIL ASSEMBLAGES PAL~OECOLOGICAL INTERPRETATION
AND
UNIT
It is 22 m thick and consists of two different horizons (Fig; 1C,D). The lower one, 17 m thick, is composed by dominantly bioclastic grey and red massive limestones with crinoid debris, bryozoans a n d with some corals. The upper horizon, consisting of slightly argillaceous grey limestones, is a biostrome interpreted as a patch-reef, e. 5 m thick, built up essentially by bryozoans and tabulate corals. In this calcareous unit the fossil assemblages represent communities showing an ecological succession comparable to that described by Walker & Alberstadt (1975) and M~ndez-Bedia & Soto (1984). It developed in four different stages : stabilization, colonization, diversification and domination ; the first one took place in the lower horizon of this unit. S t a b i l i z a t i o n s t a g e - In this stage the substrate became suitable for the subsequent settlement of reefa! fauna. T h e s e are interpreted as communities r e l a t e d to bioclastic bars from the lower levels of: the unit. The macrofossils are mainly small t a b u l a t e s and encrusting bryozoans, sparsely distributed on bioclastic bars formed through stacking of fragmental remains of other organisms, crinoids and bryozoans especially (P1. 69, fig. i): C o l o n i z a t i o n s t a g e - In this stage pioneer organisms, namely, branching tabulates and bryozoans, colonized bioclastic bars, giving rise to the later growth of a patch-reef (P1. 69, fig. 2). D i v e r s i f i c a t i o n s t a g e - The number of taxa increased during this stage. However, it is not well developed in the succession and could, perhaps, be represented by a series of levels with a greater
This is 12 m thick and consists of brown and grey shales and marls (Fig. 1C,D), with large brachiopods (different species of P l i c o s t r o p h e o d o n t a , E u rysp~rifer, L e p t o s t r o p h i e l l a , M e g a n t e r i s , etc.) most abundant at the base and with numerous bryozoans (especially fenestellids) and crinoids (P1. 69, fig. 5). The excellent preservation of the entire fauna together with the type of associated sediments suggest quiet conditions, characteristic of sheltered areas (sublagoon?). UNIT OF GREEN AND RED MARLS This is 24 m thick and consists of red and green marls (Fig. 1C,D) with abundant crinoids, particularly T r y b l i o c r i n u s f l a t h e a n u s GEINITZ, as well as bryozoans and a few brachiopods. Arbizu et al. (1993) carried out a palaeoecological study of the unit and distinguished four communities dominated by one or two taxa. Argillaceous contents remain constant for each community within the following values : - F e n e s t e l l a Community - 20 to 28 % argillaceous material. - I s o t r y p a Community : 28 to 43 % argillaceous material. - T r y b l i o c r i n u s and A n a t h y r i s Community : 43 to 56 % argillaceous material. - 7~:ybliocrinus Community : 56 to 70 % argillaceous material. Due to variations in the terrigenous supply, the same communities became established several times within the unit. The first community was dominated by well-preserved large flattened fenestellids (Fenestella sp.) occurring essentially as described by Johnson (1960) in his model I, in which organisms must have grown in a well-oxy-
570 g e n a t e d q u i e t e n v i r o n m e n t a n d developed on a clastic s e d i m e n t s u b s t r a t e supplied periodically w i t h m a t e r i a l s i m i l a r to t h a t forming t h e substrate. T h e second c o m m u n i t y (P1. 69, fig. 6) was domin a t e d b y closely spaced conical fenestellids (Isotrypa sp.). T h e lower n u m b e r of t a x a a n d the absence of f l a t t e n e d fenestellids provide evidence of a h i g h e r t e r r i g e n o u s i n p u t a n d a n increase in m u d d i n e s s of t h e water. T h e last two c o m m u n i t i e s (P1. 69, fig. 7,8) developed in a n e n v i r o n m e n t c h a r a c t e r i z e d by a grad u a l i n c r e a s e i n t e r r i g e n o u s supply. This is evid e n c e d b y t h e s m a l l e r n u m b e r a n d size of t h e b r y o z o a n s , w h i c h a d o p t e d g r a d u a l l y closer conical shapes, a n d b y t h e l a r g e r a m o u n t of Trybliocrin u s f l a t h e a n u s , w h i c h h a d a gas exchange s y s t e m t h a t allowed it to live in m u d d y e n v i r o n m e n t s ( S c h m i d t 1931). In the last c o m m u n i t y , large inf a u n a l bivalves w e r e also p r e s e n t . Hence, t h e env i r o n m e n t would h a v e b e e n similar to t h a t of J o h n s o n ' s (1960) model II, c h a r a c t e r i z e d b y a cont i n u o u s s u p p l y of argillaceous m a t e r i a l , causing a r a p i d burial. In s u m m a r y , t h e c o m m u n i t i e s r e c o r d e d in t h e calcareous u n i t s e e m r e l a t e d to t h e d e v e l o p m e n t of a b i o s t r o m a l p a t c h - r e e f on bioclastic bars; t h o s e of t h e m a r l y - s h a l y u n i t c h a r a c t e r i z e d shelt e r e d e n v i r o n m e n t s . Finally, the c o m m u n i t i e s in t h e u n i t of g r e e n a n d r e d m a r l s s e e m to h a v e b e e n typical of p l a t f o r m e n v i r o n m e n t s w i t h h i g h l y v a r i a b l e r a t e s of t e r r i g e n o u s supply. T h e v e r t i c a l f a u n a l c h a n g e observed in the calcar e o u s u n i t could r e p r e s e n t a n autogenic succession w h e r e a s t h e succession of fossil assemblages p r e s e n t in t h e o t h e r u n i t s are i n t e r p r e t e d as a n e n v i r o n m e n t a l l y i n d u c e d a s s e m b l a g e sequence.
Project PB-92-1008 : 'lArrecifes dev6nicos de la Zona Cant~brica (Cordillera Cant~tbrica, NO de Espafia) : Modelos de formaci6n, composiciSn y evoluciSn". We thank Dr. R.H. Wagner (Jardin Bot~_nico de Cordoba, Espafia) for his help in checking the English and comments. We also thank the referees, Drs. B. Mistiaen and P. Skelton, whose constructive critical reviews helped in the improvement of this paper.
REFERENCES ALVAREZ-NAVA H. & ARBIZU M. 1986 - ComposiciSn y desarrollo de un arrecife Emsiense en la Plataforma de Aruao (Asturias, NO de Espafia). Memorias I Jornadas de Paleontologia : 33-51. ARBIZU M., ALVAREZ-NAVA H., MI~NDEZ-BEDIA I. & GARCiA-LOPEZ S. 1993 - Las comunidades biSticas de las "Capas con Trybliocrinus" (Dev6nico Inferior) en la Plataforma de Arnao (Asturias, Noroeste de Espafia). Revista Espa~ola de Paleontologfa, Nflmero Extraordinario : 71-77. BROUWER A. 1964 - Deux faci6s dans le D6vonien des Montagnes Cantabriques m6ridionales. Breviora GeoIogica Astftrica, 8 (1-4) : 3-10. JOHNSON R.G. 1960 - Models and methods for the analysis of the mode of formation of fossil assemblages. Bulletin Geological Society of America, 71 : 10751086. M]~NDEZ-BEDIA I. & SOTO F. 1984 - Paleoecological succession in a Devonian organic buildup (Moniello Fm, Cantabrian Mountains, NW Spain). Geobios, 8: 150-157. SCHMIDT W. E. 1931 - Crinoiden und Blastoiden aus dem jfingsten Unterdevon Spaniens. Palaeontographica, 76 : 1-33. WALKER K. R. • ALBERSTADTL.P. 1975 - Ecological succession as an aspect of structure in fossil communities. Paleobiology, 1 : 238-257.
M. ARBIZU, I. M]~NDEZ-BEDIA & F. SOTO Acknowledgements
- This article has been partially
financed by the CICYT, within the framework of
Departamento de Geologia Universidad de Oviedo 33005 Oviedo, Spain
PLATE 69 Fig. 1-4 - Calcareous unit. 1 : stabilization stage consisting of crinoidal bars with some corals (in centre of photograph) and bryozoans. 2 : colonization stage with branching bryozoans and tabulate corals x 0.8. 3 : diversification stage with low domal alveolitids, laminar bryozoans and branching tabulate corals. 4 : domination stage with laminar bryozoans. Unitd calcaire. 1 : stade de stabilisation rdprdsentg par des barres encrinitiques avec quelques coraux et bryozoaires. 2 : stade de colonisation avec des bryozoaires et des coraux tabulds ramifids x 0,8. 3 : stade de diversification avec des alveolitidds reprgsentds par des formes en d6me has, des bryozoaires lamellaires et des coraux tabulds ramifids. 4 : stade de domination avec des bryozoaires lamellaires. Fig. 5 - Marly-shaly unit with large brachiopods (in centre of photograph), fenestellids and crinoids. Unitd schisto-marneuse, avec de grands brachiopodes, des fenestellidds et des crino~des. Fig. 6-8 - Unit of green and red marls. 6 : Isotrypa community. 7 : A n a t h y r i s and T r y b l i o c r i n u s community. 8 : T r y b l i o c r i n u s community. Unitd des marnes rouges et vertes. 6 : communautd ~ Isotrypa. 7 : communautd A n a t h y r i s et Trybliocrinus. 8 : communautd h Trybliocrinus.
Geobios nO 28, fasc. 5
PL 69 M, Arbizu, L M6ndez-Bedia & F. Soto
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