Brachiopod palaeoecology on a Thethyan Jurassic Seamount (Pliensbachian, Bakony Mountains, Hungary)

Palaeogeography, Palaeoclimatology, Palaeoecology, 57 (1986): 241 271

241

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

BRACHIOPOD PALAEOECOLOGY ON A TETHYAN JURASSIC SEAMOUNT (PLIENSBACHIAN, BAKONY MOUNTAINS, HUNGARY)

A. VOROS

Department of Geology and Palaeontology, Hungarian Natural History Museum, Budapest (Hungary) (Received J u n e 10, 1985; revised and accepted M a r c h 26, 1986)

ABSTRACT V6r6s, A., 1986. Brachiopod palaeoecology on a Tethyan Jurassic seamount (Pliensbachian, Bakony Mountains, Hungary). Palaeogeogr., Palaeoclimatol., Palaeoecol., 57: 241-271. The environmental distribution of brachiopods in the Pliensbachian of the Bakony area (Hungary) was studied on the basis of a very rich fauna (more t h a n 100 species and 6000 specimens) collected from 31 localities. The analysis was carried out using an "empirical approach", i.e. the discussion started from the ancient environment. A depositional model for the Pl.iensbachian of the Bakony area was outlined and five lithologies and corresponding depositional environments were differentiated: (A) Red, massive, manganiferous limestones - - seamount top; (B) Hierlatztype limestones - - seamount slope and foot; (C) Crinoidal limestones - - basin margin; (D) Spiculitic cherty limestones - - transition to basin interior; (E) Ammonitico rosso limestones - basin interior. Taphonomic studies of the different fossil assemblages revealed t h a t strong ecological mixing must be t a k e n into account in the seamount slope and basin margin environments. Environmental information obtained from fossils associated with brachiopods strengthened the view t h a t the five depositional environments outlined in the depositional model correspond to five general palaeoenvironments. The study of environmental distribution of brachiopods led to the following conclusions: (1) The seamount top is characterized by a lowdiversity fauna where, besides the dominant terebratulids, strophomenids play a significant role. A characteristic taxon is the genus Koninckodonta. (2) In the very rich fauna of the seamount slope the orders Terebratulida and Rhynchonellida play a n equally important role. (3) The basin areas are characterized by a low-diversity fauna dominated by large terebratulids. Characteristic taxa are Hesperithyris renierii and Lychnothyris rotzoana. (4) Short-looped terebratulids were better adapted to unfavourable conditions t h a n the rest of the brachiopod fauna. (5) Sulcate and "axiniform" morphological types do not show any definite relationship with environment or depth. INTRODUCTION

Early JurassiC times saw a secondary but important worldwide flourishing of the Phylum Brachiopoda. The rich brachiopod faunas are best documented in Europe where, at that time, two faunal provinces existed: a NorthwestEuropean and a Mediterranean one (Ager, 1967a; V6rSs, 1977). The NorthwestEuropean province corresponds to the contemporaneous European shelf and epicontinental seas, whereas the Mediterranean province was located more to 0031-0182/86/$03.50

© 1986 Elsevier Science Publishers B.V.

242 the south, embracing the peri-Adriatic-Alpine regions and some dislocated fragments in the Betic, Atlas, Pontian, Crimean and Caucasian Ranges. This southern region was interpreted by the author (VSrSs, 1977, 1984) as a "Mediterranean microcontinent" inside the Jurassic Tethys Ocean disconnected from the European and the African shelves. The Northwestern-European province can be characterized by shallowwater environments with very variable, clayey, sandy and carbonate sedimentation during the Pliensbachian. Palaeoecological analyses of brachiopod faunas from these environments have been carried out by Ager (1956, 1965), Tchoumatchenko (1972), Alm~ras and Moulan (1982) and others. As a result of these studies a clear relationship between the environment and the composition of the brachiopod fauna and between the "biotopes" and ~thanatocoenoses" has been pointed out. At the same time, the area of the Mediterranean province was not a uniform shelf sea but consisted of three main facies domains: (1) vast Bahamian-type carbonate platforms characterized by the "Lithiotis-facies"; (2) deep basins with dark marly sediments intercalated by redeposited, platform-derived carbonates; (3) submerged and dissected platform areas existing as "seamounts ''1 and "interseamount basins" with varied bio-detrital and muddy limestones. The Bakony Mountains in Hungary represent a part of this third facies domain. Because of the favourable outcrops and the very rich Pliensbachian brachiopod fauna found here, this area seems to be suitable for a palaeoecological analysis, first undertaken in the Mediterranean province. Though the area of study is small (20 x 40 kin), the results seem to be of wider interest because the seamount-interseamount arrangement was widespread in the Alpine and peri-Adriatic regions at that time. MATERIAL In the last two decades the Hungarian Geological Institute (Survey) carried out very detailed and extensive collections in the Bakony Mountains under the direction of Dr. J. Konda. This activity was epoch-making from the point of view of Jurassic stratigraphy and palaeontology of the region. The geographical distribution of the 31 Pliensbachian brachiopod localities studied is shown in Fig.1. (In a few cases, certain localities of little importance have been grouped together and figured as single points.) A very precise ammonoid biostratigraphy was developed by G~czy (1971a, b, 1972, 1974, 1976) for the most important sections. The simplified lithological columns of these precisely investigated sections (which were at the same time important brachiopod localities) were presented elsewhere (VSrSs, 1983a). The Pliensbachian localities of the Bakony yielded 6250 brachiopod speci1The term '%eamount" is used here in the sense of Jenkyns (1970) and Bernoulli (1971) and describes generally flat-topped submarine elevations with steep slopes. They are block-faulted horsts in contrast to the Recent oceanic seamounts and guyots of volcanic origin.

243

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Fig.1. Pliensbachian brachiopod localities in the Bakony. 1: V~roslSd. 2: Gomb~s. 3: Szentg~l (T-I). 4: Szentg~l (T-II). 5: Szentg~l (T-III). 6.' Kisnyerges. 7: KSzSskfit. 8: Fenyveskfit. ~. Kericser. 10: Papod (81+82). 11: Papod (84+86). 12: MohoskS. 13: KSz@ph~t. 14: BiidSskfit. 15: L6kfit. 16: K~v~shegy (A). 17: K~v~shegy (I). 18: Epl@ny. 1~. Somhegy. 2& KSrishegy (1 + 3). 21: KSrishegy (4 + 5). 22: Bocskorhegy. 23: Borzav~r. 24: Bakonycsernye. 25: Hamuh~za. mens. T h e f a u n a c o n s i s t s of 101 species of 33 genera. F o u r t e e n of t h e species a n d s e v e n of the g e n e r a a r e new; the new g e n e r a a n d one of the new species w e r e d e s c r i b e d by t h e a u t h o r (VSrSs, 1983b). T h e 101 species w e r e listed e l s e w h e r e (VSrSs, 1983a); t h e r e is no r e a s o n to r e p e a t the l o n g list here. T h e f a u n a is c o m p o s e d of p e d u n c u l a t e a r t i c u l a t e s , i n a r t i c u l a t e s a n d c e m e n t e d f o r m s are m i s s i n g a l t o g e t h e r . R h y n c h o n e l l i d s a n d t e r e b r a t u l i d s a r e the m o s t n u m e r o u s : t h e y a r e r e p r e s e n t e d by 44 a n d 39 species, r e s p e c t i v e l y . PALAEOECOLOGICAL ANALYSIS In its widest sense, p a l a e o e c o l o g y m e a n s e n v i r o n m e n t a l r e c o n s t r u c t i o n . D u r i n g this a c t i v i t y , a c c o r d i n g to t h e s u g g e s t i o n by R a u p a n d S t a n l e y (1971), it is r e a s o n a b l e to d i s t i n g u i s h w h e t h e r t h e o r g a n i s m s w e r e u s e d to r e c o n s t r u c t t h e p a s t e n v i r o n m e n t or the e n v i r o n m e n t w a s r e c o n s t r u c t e d f r o m o t h e r d a t a and is used as a f r a m e w o r k for i n t e r p r e t a t i o n of fossil species. T h i s d i s t i n c t i o n

244 corresponds to the two approaches used by Ager (1963): "applied palaeoecology" ("what the fossils can tell about the conditions under which the sediments were deposited") vs. "pure palaeoecology" ("what the sediments can suggest about the conditions under which the fossils lived"). These two approaches may be used in the same study as it was done e.g. by Ager (1967b). In his work on brachiopod palaeoecology Ager used "uniformitarian" and "empirical" approaches. In the present article the "empirical approach" will be used since the "uniformitarian" data are very scarce. The palaeoecology of the Pliensbachian brachiopod fauna of the Bakony will be discussed starting from the side of the ancient environment. These kinds of studies were published in great number about Palaeozoic brachiopod faunas (e.g. Bowen et al., 1974; Fiirsich and Hurst, 1974; Thayer, 1974; Faber et al., 1977; Vogel, 1980) but owing to the great distance in time and the differences in palaeogeography they are useful for the present purpose mainly from a methodological point of view. The environmental adaptation of mesozoic brachiopods was treated excellently by Ager (1965). The whole Pliensbachian brachiopod fauna of the Bakony fits well into the habitattype No.6, differentiated by Ager as "deeper and/or calmer sea-floors" therefore even this model cannot be used for a more detailed analysis. The present study must follow a new path. In the following (1) the depositional model for the Pliensbachian in the Bakony will be outlined; (2) after clearing up taphonomical problems, the ancient environment will be reconstructed by means of the associated fossils and the depositional model; and (3) the environmental distribution of brachiopods will be recorded and discussed. THE DEPOSITIONALMODEL Several authors wrote about the sedimentological and facies problems of the Jurassic of the Bakony area but detailed studies (mainly on microfacies ground) were carried out firstly by G~czy (1961) and SzabS-Drubina (1962). The work done by Konda (1970) was epoch-making in the knowledge of the Jurassic sedimentology of the Bakony. This very detailed analysis served as an indispensable basis for our depositional model (Gal~cz and VSrSs, 1972) in which we interpreted Konda's " c o n t i n u o u s " and "discontinuous" sequences as "basin" and " s e a m o u n t " sequences, respectively. Due to enlarging knowledge obtained by field and laboratory studies the present author has modified some details of the earlier depositional model to reach a more coherent picture of the Pliensbachian palaeogeography of the Bakony area. The Pliensbachian submarine topography was brought about by disintegration of a former carbonate platform. The " H a u p t d o l o m i t + D a c h s t e i n k a l k " carbonate platform of Norian and Rhaetian times was drowned and became dissected by normal faults in the Hettangian. The tectonic movements and block-faulting were repeated several times during the Sinemurian and Pliensbachian and resulted in a basin-and-seamount bottom topography.

245 The various limestones occurring in the Pliensbachian of the Bakony can be classified into five main types. The five lithologies correspond to five depositional environments which differed partly in depth and partly in distance from seamounts.

Lithologies (A) Red, massive, manganiferous limestones In the fine-grained carbonate matrix skeletal fragments of brachiopods, ammonoids, echinoderms, gastropods and (rarely) bivalves and solitary corals occur, generally stained or encrusted by ferro-manganese oxides. Mn-oxide nodules and crusts are frequent too. The rock is massive or thick-bedded; it occurs in condensed, discontinuous sequences or in neptunian dykes. Boundaries with under- and overlying rocks are generally sharp, frequently associated with hardgrounds. At these unconformities sometimes several stages are missing. In thin sections the rock is a biomicrite with packstone or wackestone texture. In the micritic matrix ammonoid, brachiopod and echinoderm skeletal debris, benthonic foraminifers, small ammonoids and gastropods and Mn-oxide coated extra- and intraclasts can be recorded. The majority of the biogenic particles show traces of bioerosion and are coated with Mn-oxides (Fig.2). The original aragonitic shell material (e.g. of ammonoids) became dissolved during early diagenesis and the voids were filled with internal micrite sediment and/or sparry calcite.

Depositional environment: This lithology and this kind of sequences are frequent in other areas of the Mediterranean Jurassic; they are regarded as characteristic formations of seamounts (Wendt, 1970; Bernoulli, 1971; Jenkyns, 1971a). This interpretation was also accepted for the Bakony (Gal~cz and VSrSs, 1972; VSrSs, 1974). The fragmentation of fossils, the extra- and intraclasts and the unconformities and gaps are due to increased current activity on the top of the seamounts. The water currents (apart from short episodes of deposition) swept away the lime mud. The long exposure favoured bioerosion and the encrusting activity of organisms (the majority of the skeletal particles can be interpreted as bioerosional fragments). Among bioerosional traces, algal borings occur only sporadically in the Pliensbachian of the Bakony (VSrSs, 1973a) thus the top of the majority of the seamounts were probably below the euphotic zone (i.e. more than 100m below sea level).

(B) Hierlatz-type limestones The rock is made up mainly of ammonite and brachiopod shells (locally crinoids) and is cemented by sparry calcite. Gastropods and bivalves occur subordinately. Red, pink or yellow, micritic void-fillings and geopetal structures are frequent; in local lenses micrite matrix predominates over sparite. All these provide the rock with a peculiar variegated appearance. Extraclasts

246

21

Fig.2. Red, manganiferous limestone (lithology A) in thin section (Ibex Zone, KSzSskdt). A. Oncoidal limestone (Hettangian) encrusted with ferromanganese oxides. The bioeroded extraclast served as a nucleus for a manganese nodule. Scale bar = 2 mm. B. Ammonite body chamber (centre) filled by lighter and surrounded by darker biomicrite. Note the dark rims (microbial bioerosion) on several grains and the partial dissolution of the ammonite shell. The erosional surface truncating the ammonite (top) is capped by Toarcian biomicrite. Scale bar--- 2 mm.

f r o m o l d e r J u r a s s i c r o c k s occur, s o m e t i m e s in m a s s e s (scarp breccias). M a n g a n e s e - o x i d e s t a i n i n g s or c r u s t s a r e v e r y r a r e on t h e shell f r a g m e n t s a n d e x t r a c l a s t s . T h e r o c k f o r m s m a s s i v e or thick-bedded, g e n e r a l l y s e v e r a l m e t e r s t h i c k sequences; s c a r p b r e c c i a s p r e v a i l a t t h e base. T h e u n d e r l y i n g s u r f a c e is u n e v e n or is d i s s e c t e d b y fissures; t h e o v e r l y i n g beds u s u a l l y follow c o n t i n u ously. I n t h i n s e c t i o n t h e r o c k is a b i o s p a r i t e or b i o m i c r i t e w i t h g r a i n s t o n e or p a c k s t o n e t e x t u r e . S o m e t i m e s a l m o s t i n t a c t a m m o n i t e a n d b r a c h i o p o d shells p r e d o m i n a t e b u t e c h i n o d e r m s ( m a i n l y c r i n o i d ossicles) a n d b e n t h o n i c s m a l l e r f o r a m i n i f e r s a r e also n u m e r o u s . O s t r a c o d s , s p o n g e spicules, f r a g m e n t s of c o r a l s a n d c a l c a r e o u s s p o n g e s o c c u r s p o r a d i c a l l y . S o m e of t h e s k e l e t a l f r a g m e n t s a n d e x t r a c l a s t s a r e bioeroded. A t l e a s t t w o p h a s e s c a n be disting u i s h e d in t h e p r e c i p i t a t i o n of t h e c e m e n t . T h e first g e n e r a t i o n is a void-lining r a d i a l - f i b r o u s c a r b o n a t e c e m e n t i n g t h e grains; t h e s e c o n d g e n e r a t i o n is a voidfilling r h o m b o h e d r a l , b l o c k y c a l c i t e (Fig.3). M i c r i t e infillings p r e c e d e a n d s o m e t i m e s s u c c e e d t h e f i r s t - g e n e r a t i o n cement. All t h e s e f a c t s s u g g e s t v e r y r a p i d s u b m a r i n e lithification.

247

Fig.3. Hierlatz-type limestone (lithology B) in thin section (negative print; Ibex Zone, Kericser). The ammonite phragmocone in center shows the two phases of cementation. The micrite infiltration preceded the first-generation cement (see geopetal structure in upper center). Scale bar = 2 mm.

Depositional environment: No recent data have been published about the sedimentology and facies-interpretation of Hierlatz limestones at the typelocalities in the Alps. Some earlier authors regarded this rock as a heap of skeletal material accumulated by waves at the sea-shore. This possibility, however, has to be rejected by reason of the excellent preservation of the fossils. J e n k y n s (1971b) interpreted Sicilian crinoidal limestones that he studied as cemented sand-waves formed by currents on the tops of seamounts. This idea may be applied to some Sinemurian Hierlatz-type limestones of the Bakony but the Pliensbachian sequences studied here need a different model. Konda (1970) has shown that these rocks were deposited in transitional zones between areas with ~'continuous" and "discontinuous" sequences. G~czy (1971a) suggested t h a t the fossil assemblage of the Hierlatz limestones at the Kericser locality was redeposited from an adjacent submarine high to a deeper basin. Following this idea, our i n t e r p r e t a t i o n was t h a t the depositional environments of the Hierlatz limestones were on the flanks and at the foot of seamounts, where the amount of redeposited skeletal material locally surpassed t h a t of the background limemud (Gal~cz and VSrSs, 1972; VSrSs, 1974). The local occurrence of algal borings (VSrSs, 1973a) proves that some of the seamounts reached up to the euphotic zone, thus the depth of deposition of the Hierlatz limestones might have had a wide range from one to several hundred meters.

248

(C) Red or yellow crinoidal limestones The echinoderm coquina contains a few brachiopods and very scarce ammonoids and gastropods. The echinoderm skeletal elements have a size range from microns (fragments) to centimeters (crinoid calyces and echinoid spines). The bedding is variable from massive and thick-bedded to thin-bedded or platy; intercalations with other lithologies are frequent. Cross-bedding occurs rarely; silicification is prevalent. The rock forms relatively thick (tens of meters) and continuous sequences generally with transitions from the underlying and to the overlying formations. In thin section it is a biosparite with a grainstone texture. Besides the predominant echinoderm skeletal material, the rock contains brachiopod and mollusc shell fragments, sponge spicules and a few ostracods and foraminifers. Biomicrite intraclasts occur frequently. The intragranular spaces, as is usual in crinoidal limestones, are filled with syntaxial rim cement.

Depositional environment: Redeposited crinoidal limestones interbedded with basinal limestones or marls are widespread and well-known in the Mediterranean Lower Jurassic. These are interpreted as turbidites or fluxoturbidites, depending on their sedimentological features and size (Bernoulli and Jenkyns, 1970; Colacicchi and Pialli, 1971; Jenkyns, 1971b; K~ilin and Triimpy, 1977; Catalano and D'Argenio, 1983). In the present case graded bedding is rare and the basinal crinoidal limestone bodies can be traced only up to about 2 km distance from the seamounts. Therefore the term fluxoturbidite seems to be proper for them. The massive, homogeneous crinoidal limestones correspond to the proximal parts of the fluxoturbidites whereas the thinner interbeds in more basinal sediments represent the distal parts. The depth of deposition might have been several hundreds of meters.

(D) Grey or pink spiculitic cherty limestones This fine grained limestone shows nodular or wavy bedding; its fossils are ammonites, brachiopods, scarce gastropods and bivalves. Calcitic shells (e.g. brachiopods) are frequently replaced by silica. Chert nodules and lenses are widespread; in some sequences the cherts appear as thick and coherent beds. This lithology occurs in relatively thick (several tens of meters) and continuous sequences; it may be intercalated with crinoidal limestones and/or "ammonitico rosso". In thin section the limestone is a biomicrite with packstone or wackestone texture. The most abundant grains are sponge spicules, but echinoderm fragments, benthonic foraminifers and ostracods are also present. The original siliceous material of the sponge spicules is sometimes dissolved and replaced by calcite.

Depositional environment: The Lower Jurassic cherty limestone series known from other Alpine or Mediterranean areas have always been interpreted as allodapic sequences deposited in relatively deep basins within carbonate platforms or between seamounts (Bernoulli, 1964, 1967; Bernoulli and Jenkyns,

249

1970; Tollmann, 1976). The chert was formed during diagenesis by mobilization of the material of siliceous sponge spicules. The spicules were redeposited from shallower areas but some sponges might have lived even in the basins. The depth of deposition could have been several hundreds of meters.

(E) Red, nodular ammonite limestones ("ammonitico rosso") This compact, fine-grained limestone has a nodular or "flaser"-bedding with clayey interlaminations and coatings around nodules. Frequent fossils are ammonites and other cephalopods, brachiopods are subordinate, bivalves and gastropods are rare. This rock develops in continuous sequences underlain, as a rule, by spiculitic cherty limestones; interfingerings between the two lithologies are common. In thin sections it is a biomicrite with wackestone or mudstone texture. Ammonite shell fragments, embryonic ammonites and small benthonic foraminifers are the most frequent skeletal grains.

Depositional environment: The "ammonitico rosso" is a well-known and characteristic but, as regards its depth of deposition, the most debated formation of the Mediterranean Jurassic (see Hallam, 1975). Sound palaeogeographical studies result in widely different results (fewer than hundred meters: Wendt, 1970; Sturani, 1971; Massari, 1979 vs. several thousand meters: Garrison and Fischer, 1969). This strongly suggests a heterogeneity, i.e. not all limestones are "ammonitico rosso" which are red and nodular. The stromatolitic types with definite signs of the euphotic zone need to be separated. The depositional depth of ammonitico rosso (in the restricted sense) was recently estimated between 800 and 1100 m in areas with continental crust (Winterer and Bosellini, 1981). The nodular structure of the limestone may be due partly to bioturbation (Fiirsich, 1973; Dommerguees et al., 1981) partly to diagenetic processes (Jenkyns, 1974) or to a combination of the two. In the Pliensbachian of the Bakony, the geographical distribution and the intercalations show that the "ammonitico rosso" has been formed in the basins farthest from the seamounts but not necessarily deeper than the spiculitic cherty limestones. The "ammonitico rosso" appears to have been deposited in such places and periods where and when the seamount-derived allodapic material has not influenced the sedimentation.

Interpretation The depositional environments and the spatial relationships of the five lithologies described above are shown in a palaeobathymetric diagram (Fig.4). The physiographic domains, which are at the same time different depositional environments, are the following: (A) Seamount top. The fine-grained material is swept away, the larger grains (e.g. skeletons) are rolled to and fro by water currents. Sediment accumulation is possible only in local depressions or in episodic neptunian dykes. Estimated depth: 100-200 m.

250

Fig.4. Bathymetrical diagram showing the depositional environments and spatial relationships of the five Pliensbachian lithologies of the Bakony. Heavy black arrows mark bottom currents and fluxoturbidites; thin arrows show gravitational roll-down of larger particles and skeletons of organisms. (Not to scale.) (B) Seamount slope and foot. The steep escarpment formed by normal faulting in several steps are largely non-depositional. The larger shells and skeletal fragments of organisms which lived on the top or slope of the seamount are carried down by gravitation and accumulate on the steps or at the foot of the seamount. Occasionally, fluxoturbidites start to flow toward the basin. Estimated depth range: 100-500 m. (C) Basin margin. The peripheral areas of the interseamount basins receive the proximal (mainly crinoidal) parts of the fluxoturbidites arriving from the seamounts. Estimated depth: 500-600 m. (D) and (E) Basin interior. The distal (mainly spiculitic) parts of the fluxoturbidites are deposited and the "ammonitico rosso" limestones accumulate here, respectively. Reworking by bioturbation may be strong. Estimated depth: 500-600 (800?)m. The above depositional model has a good analogy with a series of palaeogeographical reconstructions developed for other areas of the Mediterranean Jurassic where the sedimentation proceeded also on submarine swells and in intervening basins (Southern Alps: Winterer and Bosellini, 1981; Central Apennines: Bernoulli, 1967; Colacicchi and Pialli, 1971; Southern Apennines: Ippolito et al., 1975; Sicily: Catalano and D'Argenio, 1978). The great similarity comes from the uniform, passive subsidence history of the Peri-adriatic continental margin of the Tethys (Bernoulli and Jenkyns, 1974; Laubscher and Bernoulli, 1977; D'Argenio et al., 1980). However, critical evaluation of data and field trips made to the areas mentioned have convinced the author that the analogy is by no means perfect. The few kilometers wide basins of the Bakony with sediments less than 100 m thick are not to be compared to the vast Alpine,

251 Apenninic or Sicilian basins containing more t h a n thousand meters thick Liassic sequences (e.g. Lombardy, Lagonegro, Imerese basins). It seems that the basins in the Bakony have been formed by dissection of the surface of a single large seamount or submarine plateau. TAPHONOMY Questions of post-mortem transport will be discussed in this section in order to define the degree of reworking and of exotic admixture in the fossil assemblages of the five depositional environments. The reworking mechanisms of the sediments (including the skeletal particles) were outlined above. Thus, it was supposed t h a t the isolated skeletal elements of the sponges and crinoids (producing the largest amount of biogenic material) were redeposited as components of fluxoturbidites in their place of accumulation [depositional environments (C) and (D)]. Hardly any sponge spicules occur on the seamount top (A) and crinoid ossicles are also subordinate there, consequently both groups must have been living on the seamount slopes (B) or in deeper areas. More positive conclusions can be drawn from the study of bivalves and brachiopods.

Bivalves From many sections and localities studied, bivalves were also collected. The seamount communities were dominated by Pectinidae, Entoliidae, Limidae and by taxodonts which may belong to Isoarca or Catella; in the basins relatively large and smooth heterodonts occur more frequently. The most important taphonomic information recorded and evaluated numerically at each locality was the degree of disarticulation. This was taken as a measure of post-mortem reworking. A total of 402 bivalve specimens were studied from nine localities. Only four of the five lithologies (i.e. depositional environments) were represented at these localities; no bivalves were counted from the lithology (C) (crinoidal limestone). The results are shown in Table I. In lithologies (A) and (B) practically all bivalves are disarticulated. This can be explained by long exposure (lack of burial) and/or by bottom currents on the seamount top (A) and by gravitational transport (roll-down) on the seamount TABLEI Degree of disarticulation of bivalves in different lithologies Lithology

Specimens

Articulated

%

Disarticulated

%

A B D E

82 222 40 58

1 -11 23

1.2 -27.5 39.7

81 222 29 35

98.8 100.0 72.5 60.3

252

slope (B). The considerably lower degree of disarticulation found in the basinal lithologies (D and E) proves that the post-mortem reworking was much weaker here. At the same time, the partial disarticulation of the dominant infaunal bivalves calls attention to the significant role of bioturbation in basin areas.

Brachiopods The most important numerical data were on the degree of disarticulation; in addition, size-frequency distribution was also considered.

Size-frequency distribution During this study, size categories were used for representation; the quantities of specimens belonging to the respective size categories are given as a percentage of the whole brachiopod fauna. The size-categories used are as follows: I: length below 10 mm; II: 10-15 mm; III: 15-20 mm; IV: 20-30 mm, V: above 30 mm. Size-frequency distributions are shown in Fig.5. Bimodality is

A

B

C

D

E

'

I

II

III

IV

I

II

III

IV

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Fig.5. Summary of taphonomieal data gained from brachiopods and the supposed mechanisms of post-mortem reworking.

253 seen on the seamount top (A); the maximum in the highest size-category is caused by the dominance of large specimens of Securithyris adnethensis in one locality (in the Epl~ny neptunian dyke). Apart from this, a gradual size increase can be seen from the seamounts to the basin interior. This distribution has some taphonomical importance. Namely, one may imagine that the brachiopod shells found in the basin areas were redeposited from the seamounts just like the majority of sediments enclosing them. In this case, parallel with the decrease in grain-size of sediments away from the seamounts, a decrease in size of brachiopod shells would be expected. However, the observed fact (i.e. that the largest brachiopods prevail in the basin interior) clearly shows that the benthonic fossils of these areas should be regarded as indigenous. In other words, the fossil communities of the basinal depositional environments (mainly D and E) were preserved in the same environment where they lived.

Disarticulation The two valves of a brachiopod are much less prone to be disarticulated than those of a bivalve. One of the causes is that in bivalves the ligament automatically opens the valves after relaxing or decaying of the adductor muscles whereas in brachiopods muscular effort is needed to open the valves. Therefore, in most cases, they remain closed after death (Ager, 1967b; Thayer, 1975). A further difference is that bivalve shells are fastened together only by the ligament while in the brachiopods with cyrtomatodont articulation the teeth are hook-like and are curved into the sockets so keeping together the valves even after the decay of organic tissues (Sheehan, 1978). Accordingly, in the Pliensbachian faunas of the Bakony, the proportion of disarticulated valves is much lower in brachiopods than in bivalves. Data of brachiopod disarticulation are shown in Table II. The degree of disarticulation averaged in different depositional environments can be seen in Fig.5. The real proportion of disarticulated valves is presumably higher since collectors always prefer double valves to single ones. It is expected, however, that this error appears proportionally in every collection and does not change the differences between localities or depositional environments. The degree of disarticulation is about 30% on the seamount top (A); this is due to water movements rolling the shells to and fro. The 10% value found on the seamount slope (B) is surprisingly low considering the complete disarticulation of bivalves in the same depositional environment. This is a clear manifestation of the anatomical difference between bivalves and brachiopods: the downslope rolling was enough for disarticulation of bivalves whereas the majority of brachiopod valves were held together by the cyrtomatodont hinge during this relatively short transport. Hardly any further reworking was possible in the place of accumulation - - as evidenced by the rapid cementation of the Hierlatz-type limestones. In the crinoidal limestones [depositional environment (C)] the degree of disarticulation is extremely high (about 80%). This can be reasonably explained by the transport in fluxoturbidites. Toward the basin, in the spiculitic cherty limestones [depositional environment (D)]

254 TABLEII Degree of disarticulation of brachiopods at different localities in the Bakony Locality

Lithology

Specimens

Disarticulated

~/o

KisnyergesA KSzSsk~t Epl6ny Fenyvesk6t Kericser V~roslSd K6rishegy (1 + 3) KSrishegy (4+ 5) KSz6ph~t BtidSsk6t Bocskorhegy Hamuh~za Bakonycsernye

A A A B B C C C D D D E E

72 104 680 658 3953 19 26 57 8 55 61 118 7

20 48 179 73 287 16 18 44 3 12 26 72 4

27.8 46.1 26.3 11.1 8.7 84.2 69.2 77.2 37.5 23.5 43.3 61.0 57.1

the proportion of disarticulated valves drops again in accordance with the decreasing influence of fluxoturbidites. The majority of this brachiopod assemblage must have lived and been fossilized in the same environment. The increasing degree of disarticulation found in the basin interior [(E); "ammonitico rosso"] may be due to the relatively slow sedimentation (slow burial) and to the strong bioturbation.

Taphonomic conclusions for the depositional environments (A) Seamount top: the fossil assemblage is strongly reworked but essentially indigenous; o ccu r r ence of exotic elements is unlikely. (B) Seamount slope and foot: the fossil assemblage was t ransport ed from various parts of the seamount top and slope (i.e. from very different habitats) and was mingled with the relatively deep-water assemblage of the foot of the seamount. (C) Basin margin: the most strongly t ransport ed and reworked fossil assemblage; its composition is similar to the previous one but it is impoverished both in specimens and in taxa. (D) Basin (transitional area): the majority of fossils was buried in situ; there are probably few t r ans por t ed fossils. (E) Basin interior: indigenous fossil assemblage without exotic elements; reworking went on in place. ENVIRONMENTAL INFORMATION OBTAINEDFROM ASSOCIATED FOSSILS The five lithologies and depositional environments discussed above served as five different bottom types and environments for the fossil biota. As

255 appeared from the taphonomic analysis, a high degree of "ecological mixing" is expected in lithologies (B) and '(C). This must be kept in mind during the study of enviromental data provided by other fossil groups occurring together with brachiopods.

Foraminifers Benthonic smaller foraminiferids occur in all five lithologies, especially frequently in (A) and (E). A precise identification was not attempted in thin sections but it was clearly seen that they were very variable calcareous (not agglutinated) forms. Presence of a rich benthos on the seamount top (A) is in accordance with the supposed, relatively shallow, bathymetric position. It is surprising, however, that the benthonic foraminiferal assemblage of the deeper basin [(E), ammonitico rosso] does not seem to be much poorer. According to Murray (1973) the density of Recent benthonic smaller foraminiferid assemblages does not decrease with increasing depth (at least down to 1000 m), only the taxonomical composition changes. Some deep-water species appear below 200 m and show increasing abundance with depth. On these grounds the rich benthonic foraminiferal assemblage of the "ammonitico rosso" does not contradict with the supposed greater (500-600 m) depth.

Bivalves Detailed taxonomic work on the Pliensbachian bivalves of the Bakony has not yet been done, therefore a precise palaeoecological evaluation is not possible, either. Nevertheless, the bivalves provide very important palaeoecological data because the shell morphology proclaims, usually at first sight, if the animal had an epibenthonic or endobenthonic mode of life. It is obvious that endobenthonic burrowers might have lived in great number only in soft bottoms whereas the dominance of epibenthonic forms is expected rather on hard (rocky) bottoms. The proportions of epifaunal and infaunal elements found in different lithologies are shown in Table III. [No bivalves were available from lithology (C)]. The distribution is in perfect harmony with the depositional model: the nearly 100% epibenthonic dominance found in lithologies (A) and (B) testifies to the predominantly rocky bottom of the seamount top and slope whereas the endobenthos dominant in lithologies (D) and (E) serves the evidence of soft bottom in the basins.

Gastropods Data on '~facies-dependence" of Pliensbachian gastropods in the Bakony were published by Szab5 (1979, 1980). The results were presented in a seamount-basin depositional model adopted from Gal~cz and VSrSs (1972) and from VSrSs (1974) and were in accordance with that.

256

TABLE III Proportions of epifaunal and infaunal bivalves in different lithologies Lithology

Specimens

Epifaunal

~o

Infaunal

A B D

82 222 40

78 209 12

95.1 94.1 30.0

4 13 28

4.9 5.9 70.0

E

58

20

34.5

38

65.5

Ammonites

The Pliensbachian ammonites of the Bakony have been treated in detail by G6czy in his numerous works. The ammonites did not strictly belong to the benthos, yet they supply very useful data on the ancient environment (e.g. Scott, 1940). G6czy (1961), following the traditional idea, proved that the preferred habitat of the Jurassic Phylloceratina and Lytoceratina was the relatively deep (bathypelagic) sea in contrast to the suborder Ammonitina which was predominant in shallower seas. The proportion of the two groups of different habitat can be used to estimate water depth: a greater share of Phylloceratina + Lytoceratina means greater depth (G~czy, 1971a, 1972, 1974). In his monograph on Carixian ammonites of the Bakony G6czy (1976) published data of taxonomic composition of ammonites faunas of numerous sections. A summary of the data extracted from this work is shown in Table IV. [Only very few, poorly preserved ammonites are known from lithology (C) therefore it is not included into Table IV.] In lithologies (A) and (B), corresponding to the seamount top and slope, respectively, the proportion of P h y l l o c e r a t i n a + L y t o c e r a t i n a is well below 50%. The significant difference between faunal compositions of the seamount (A, B) and basin (D, E) areas is in accordance with the depositional model. PALAEOENVIRONMENTALSYNTHESIS As was seen above, the environmental information provided by the fossil associations harmonizes well with the depositional model and provides valuable additions. Hence, the five depositional environments shown in the depositional model (Fig.4) correspond to five general paleoenvironments. The most important criteria and relationships of the five palaeoenvironments are summarized in Table V. ENVIRONMENTALDISTRIBUTIONOF BRACHIOPODSIN THE PLIENSBACHIANOF THE BAKONY The five palaeoenvironments outlined above appear in the field as different facies types. In order to reveal the environmental distribution of brachiopods

257 TABLE IV Proportions of deeper-water ammonites in different lithologies (after G~czy, 1976) Lithology

Phylloceratina ÷ Lytoceratina %

A B D E

40 21-23 66-75 78-85

TABLE V Review of Pliensbachian environments in the Bakony Lithology

Depositional environment

A

Red, massive, manganiferous limestone

Seamount top

B

Hierlatz type limestone

Seamount slope and foot

hard rocky bottom

deep sublittoral to shallow bathyal

C

Red or yellow, crinoidal limestone

Basin margin

soft sandy bottom

bathyal

D

Spiculitic cherty limestone

soft muddy and sandy bottom

bathyal

E

Ammonitico rosso limestone

soft muddy bottom

bathyal

Basin interior

Bottom-type

Depth-zone deep sublittoral

it is n e c e s s a r y to s k e t c h o u t t h e P l i e n s b a c h i a n f a c i e s - p a t t e r n a n d p a l a e o g e o g r a p h y of t h e B a k o n y a r e a .

Pliensbachian palaeogeography of the northern Bakony O w i n g to t h e i r limited t h i c k n e s s a n d to s u b s e q u e n t erosion, J u r a s s i c r o c k s a p p e a r as r e l a t i v e l y s m a l l a n d i s o l a t e d o u t c r o p s in the B a k o n y . B e l o w y o u n g e r ( C r e t a c e o u s , E o c e n e or M i o c e n e ) s e d i m e n t a r y c o v e r J u r a s s i c s t r a t a w e r e p e n e t r a t e d b y n u m e r o u s boreholes. B o t h t h e a r e a of s u r f a c e o u t c r o p s a n d t h e n u m b e r of b o r e h o l e s a r e t h e g r e a t e s t in t h e w e s t e r n p a r t of t h e n o r t h e r n B a k o n y . T h i s is t h e r e g i o n w h e r e t h e i n f o r m a t i o n s e e m s to be e n o u g h to r e v e a l t h e f a c i e s - p a t t e r n , t h e r e f o r e h e r e it w a s a t t e m p t e d to o u t l i n e t h e P l i e n s b a c h i a n p a l a e o g e o g r a p h y . (This r e s t r i c t i o n in t h e a r e a of t h e m a p h a s a s e r i o u s

258

H.58 5 krn

I

A: Red,manganlferout S: "Hterlstz~ limestones, C : Crlnoldsl

limestones

O

limestones

in boreholes

Jurassi~ outcrops

breccias

~'~ elt~

l imstones

D:Spiculitic, cherty

Rock-types

J

inferred

facies -boundaries

Neogenestrike-Slip fault

Fig.6. Map showing basic data used in the reconstruction of the Pliensbachian facies-pattern in the Bakony.

disadvantage: the "ammonitico rosso" occurs in this region only in local lenses; its extensive development can be traced only farther to the east.) First a map was compiled to summarize the basic data (Fig.6). After plotting the facies-symbols, the picture is seen to be very discontinuous. In many cases it would be erroneous to extrapolate or to connect facies belts which seem to be continuous, since this area is dissected by younger (Cretaceous and Neogene) strike-slip faults, sometimes with several kilometers displacement (Telegdi-Roth, 1935; M6sz~ros, 1983). The most significant fault, which crosses the trends of Jurassic facies-belts, is shown on the map. Two seamounts and several partial basins seem to take shape in the area (cf.

259

A

:

B,

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I

i

C

:

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D,

E

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I

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~

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Fig.7. Density, diversity and taxonomical composition of the Pliensbachian brachiopod fauna of the Bakony in different environments.

Fig.8). A fairly large seamount is situated in the southwest, which might be called the "Hajag-Papod seamount" after the range of hills where its sediments occur. A little more to the north a smaller seamount area appears but, considering the strike-slip movements, this could be an integral part of the former. Another seamount region can be recognized east of the village Epl~ny; this may be called the "Amos seamount" again after a major hill there. The "L6kfit basin", with its symmetrically arranged facies-belts, extends between the two seamount regions. The northern part of the map is dominated by the "Zirc-basin"; the wide crinoidal limestone facies-belt on its western side points to a more westerly situated seamount. The Zirc and L6kt~t partial basins were

260

probably connected and formed a continuous basin area in Pliensbachian times.

Changes in faunal density and diversity Changes in specimen number, species number and diversity values of the Pliensbachian brachiopod fauna of the Bakony are shown in Fig.7. Diversity indices were obtained from the graph given by Williams (1964). The curves constructed from specimen number and species number data have similar trends: after a relatively low value on the seamount top (A) they reach a maximum on the seamount slope (B) then they fall down to a very low value in the basin areas. The "diversity-curve" follows nearly the same trend but it reaches a surprisingly high value in the basin margin (C). This anomaly may be due to the strongly transported and mixed nature of the fossil assemblages of the crinoidal limestones. The geographical distribution of diversity index values forms a similar picture (Fig.8). The values are between 1 and 4 on the seamounts, between 10

. . , , . ° . . . .

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~ B.

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I

- -.

Fig.8. Brachiopod diversity values at selected localities. 1: K6rishegy (4 + 5). 2: KSrishegy (1 + 3). 3: Bocskorhegy. 4: Epl~ny. 5: L6kflt. 6: BiidSskfit. 7: Kericser. 8: Fenyveskfit. 9: K5zSsk/zt. 10: Kisnyerges. Legend: same as in Fig.6.

261

and 14 on the seamount slopes, between 5 and 12 on the basin margins and around 3 in the more open basin areas. Faunal diversity is controlled by two main groups of environmental factors, namely by "resources" and by "stability" (Stanton, 1979). "Resources" embrace all environmental factors limiting life conditions or causing competition between organisms. "Stability" means the permanence of the environment and it favours high faunal diversity in a given area. In the present case the low diversity found in the basin areas (D and E) can be explained by the limitation of the "resources". One of the possible limiting factors was food-supply which shows a decrease with increasing water-depth and with increasing distance from seamounts (VSrSs, 1973b, 1974). Another factor was the hard bottom necessary for attachment of brachiopods. Solid objects occurred in very limited numbers in the muddy basin areas. On the other hand, the shallower, rocky seamount environments must have been very favourable in respect of food-supply and firm substratum as well. The maximum diversity found on the seamount slope (B) is in accordance with this idea but the relatively low diversity of the seamount top (A) assemblages seems to contradict this. The striking difference was caused by ecological and taphonomic factors. One of the ecological factors was the strong current activity on the seamount top. As McCammon (1973) showed, in areas of too strong bottom currents, no matter how favourable the substratum was, brachiopods were absent because their larvae were unable to settle. Very strong bottom currents might be disadvantageous in respect of feeding too because, though the supply was sufficient, the suspended matter was swept away at high speed. On the other hand, seamount slopes were in a favourable position as the suspended particles were raining down here in the lee. The dissected rocky bottom of the seamount slopes offered a great number of niches for diversification of organisms. These are the ecological factors causing high diversity in the seamount slope environment. Taphonomic causes are also to be kept in mind, however, because the fossil assemblage found in lithology (B; Hierlatz limestones, accumulated mainly at the foot of the seamounts) was transported from different parts of the seamount slope and from the seamount top. The ecological mixing exaggerates the diversity values obtained for this environment. The same goes for the anomalously high diversity of the basin margin (C) here an even higher degree of transport and mixing of fossil assemblages must be taken into account.

Changes in taxonomic composition Spatial variation in the proportion of the four brachiopod orders occurring in the Pliensbachian fauna of the Bakony is shown in Table VI. Representatives of the order Strophomenida (the genera Amphiclinodonta and Koninckodonta following the classification of Rudwick, 1970) seem to be restricted to the seamounts. As for the other three orders, the inverse relationship between the quantity of Rhynchonellida and Terebratulida is apparent: terebratulids

262 T A B L E VI

Percentage of brachiopod orders in different environments Environments

Strophomenida Rhynchonellida

Spiriferida Terebratulida

A

B

C

D

E

11.9 5.0 1.4 81.7

0.8 37.3 5.0 56.9

-63.7 11.8 24.5

-10.0 3.4 86.6

-1.6 5.6 92.8

w h i c h are dominant on the seamount top (A) and in the basin areas (D, E) are almost equalled on the seamount slope (B) and are overshadowed in the basin margin (C) by rhynchonellids. Spiriferids remain subordinate everywhere. The above trend is followed by and is mainly due to the changes in proportion of the suborder Terebratulidina (i.e. the short-looped terebratulids) to the whole fauna. Percentages of Terebratulidina in different environments

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Fig.9. Percentage of suborder Terebratulidina in brachiopod faunas at selected localities. N u m b e r i n g of localities: same as i n Fig.8. Legend: same as i n Fig.6.

263 are p r e s e n t e d in Fig.7. The c u r v e shows a n a l m o s t perfect m i r r o r i m a g e of the diversity curve. A r e a s o n a b l e i n t e r p r e t a t i o n of this inverse r e l a t i o n s h i p c a n be t h a t short-looped t e r e b r a t u l i d s were m u c h b e t t e r a d a p t e d to the u n f a v o u r a b l e c o n d i t i o n s of t h e s e a m o u n t top and of the basin i n t e r i o r t h a n the rest of the fauna. This r e l a t i o n s h i p is seen also in the g e o g r a p h i c a l d i s t r i b u t i o n (Fig.9). The p r o p o r t i o n of T e r e b r a t u l i d i n a to the w h o l e b r a c h i o p o d f a u n a seems to be a useful tool in p a l a e o e n v i r o n m e n t a l analysis. Distribution of characteristic taxa A g r e a t n u m b e r of P l i e n s b a c h i a n b r a c h i o p o d species of the B a k o n y are r e p r e s e n t e d by o n l y a few specimens per species; m a n y o t h e r species o c c u r just in one or two localities. The s t u d y of e n v i r o n m e n t a l d i s t r i b u t i o n was reasonable in cases of widely-distributed t a x a r e p r e s e n t e d by a g r e a t n u m b e r of specimens from several localities. Sixteen t a x a (mostly species) o c c u r r i n g in more t h a n four localities are listed in Table VII. The p i c t u r e e m e r g i n g from specimen n u m b e r d a t a is s t r o n g l y biased by the g r e a t differences in f a u n a l d e n s i t y in different e n v i r o n m e n t s . M o r e t h a n two-thirds of the b r a c h i o p o d specimens came from e n v i r o n m e n t (B; Hierlatz limestones); t h e r e f o r e n e a r l y all t a x a s h o w a m a x i m u m a b u n d a n c e there. A m o r e useful c o m p a r i s o n is possible by c o n s i d e r i n g the role of t a x a played w i t h i n the f a u n a s of different e n v i r o n m e n t s . F i g u r e 10 shows the s h a r e of i n d i v i d u a l t a x a w i t h i n the w h o l e f a u n a of the r e s p e c t i v e e n v i r o n m e n t , exTABLE VII Distribution (specimen numbers) of characteristic braciopod taxa in different environments Environments

Koninckodonta spp. Phymatothyris cerasulum (Zittel, 1869) Spiriferina sicula (Gemmellaro, 1874) Bakonyithyris apenninica (Zittel, 1869) Septocrurella sp., aft. uhligi (Haas, 1884) Viallithyris gozzanensis (Parona, 1880) Stolrnorhynchia gemmellaroi (Parona, 1880) Apringia altesinuata (BSse, 1898) Apringia paolii (Canavari, 1880) Linguithyris aspasia (Zittel, 1869) Securithyris adnethensis (Suess, 1855) Bakonyithyris pedemontana (Parona, 1893) Lokutella liasina (Principi, 1910) Spiriferina gryphoidea (Uhlig, 1880) Lychnothyris rotzoana (Schauroth, 1865) Hesperithyris renierii (Catullo, 1827)

A

B

C

D

E

101 136 1 2 10 15 5 2 22 153 386

30 78 74 80 82 152 54 164 251 1005 256

-2 --1 1 -1 -12 5

-1 ----3 2 -87 5

-------1 7 95

--

75

2

1

--

56

2

5

38

4

3

2

1

1

4

3

6

---

--

--

--

--

1 --

264

Koninckodonta P. cerasulum S. sicula B. apenninica S, sp.,aff, uhligi V gozzanensis S. gemmellaroi A. altesinuata A. paolii L. aspasia S. adnethensis B. pedemontana L. liasina S. gryphoidea L. rotzoana H, renierfi

m

I i I I I I I

m

I

I

I

I

I

i

<2%

2-10,~

-10%

Fig.10. Distribution of characteristic taxa in different environments. The share of individual taxa within the whole fauna of the respective environment is expressed in percentages (cf. Table VII).

pressed in percentages. Some of the species have wide and uniform distribution (e.g. Linguithyris aspasia and Securithyris adnethensis). The small, concavoconvex genus Koninckodonta and, to a lesser degree, the species Phymatothyris cerasulum seem to be characteristic to the seamount top. The very rich assemblage of the seamount slope does not contain any definitely characteristic species. The basin areas are characterized mainly by large-sized species such as Liospiriferina gryphoidea, Lychnothyris rotzoana and Hesperithyris renierii. Basinal distribution of these two latter species and occurrences of the genus Konickodonta (restricted to seamounts) are shown in Fig.ll. The clear basinal affinity of Lychnothyris rotzoana and Hesperithyris renierii is very remarkable and surprising because these two species belong to the frequent and characteristic fossils of the so-called "Lithiotis-facies" of the Bahamian-type carbonate platforms in the Liassic of the Southern Alps and Apennines (Broglio Loriga and Neri, 1976). It is an apparent contradiction that these two, allegedly shallow-water forms, occur in the deeper basins and not on the relatively shallow seamounts in the Bakony.

265

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-:ZZZZZZ:z. F i g . l l . O c c u r r e n c e s of s o m e c h a r a c t e r i s t i c taxa. N u m b e r i n g of localities: s a m e as in Fig.8. Legend: s a m e as in Fig.6.

Distribution of special morphological types The bulk of the Pliensbachian brachiopod fauna of the Bakony is composed by "sulcate" and "axiniform" morphological types. These curious forms are characteristic of the Mediterranean province and are almost totally absent from the contemporaneous sediments of the European shelf (Ager, 1967a). The sulcate shell form is very advantageous in deep and/or calm environments where food-supply is limited (Ager, 1965; Vogel, 1966). The deep sinus of the anterior margin increases the divergence between the inhalant and exhalant feeding-currents and enhances the utilization of suspended organic matter. Observations made on Recent brachiopods support this idea: the sulcate rhynchonellid Neorhynchia lives in a 2560-4513 m depth range, the sulcate terebratulids Notorygmia and Abyssothyris were found in 2140-4600 m and 1893-5631 m depth ranges, respectively (Cooper, 1972, 1982; Zezina, 1976). In the Bakony fauna the sulcate type is represented by the following genera (specimen numbers in parentheses): Pisirhynchia (390); KericsereUa (10); Septocrurella (119); Rhapidothyris (17); Viallithyris (202); Linguithyris (1664); Antiptychina (33); Bakonyithyris (553)• This is 2988 specimens altogether, 47.8% of the total fauna.

SULCATE TYPES

SULCATE + "AXINIFORM" (altogether)

Prionorhynchia, LokuteUa, Cuneirhynchia, Securithyris, Papodina, Securina

"AXINIFORM" TYPES

Pisirhynchia, Kericserella, SeptocrureUa, Rhapidothyris, ViaUithyris, Linguithyris, Antiptychina, Bakonyithyris

45.1 66.8

572

21.7

386

186

3107

656

2451

specimen

specimen

~o

B

A

Environments

77.8

16.4

61.4

~o

62

43

19

specimen

C

Specimen numbers and percentages of special morphological types in different environments

TABLE VIII

60.8

42.2

18.6

~/o

104

10

94

specimen

D

86.6

8.3

78.3

~o

104

95

9

specimen

E

83.2

76.0

7.2

~/o

267 Ager (1965) used the term "axiniform" for triangular, axe-shaped brachiopods characterized by extremely wide and straight anterior commissure and by well-developed planareas. The expanded anterior margin was regarded as a form of adaptation to deep or calm environment with limited food-supply. Data obtained from Recent brachiopods do not contradict this view: the "axiniform" Hispanirhynchia and Dyscolia live in depth ranges 439-2150 m and 595-1922 m, respectively (Zezina, 1976; Cooper, 1981). In the Bakony fauna this morphological type is represented by the following genera (specimen numbers in parentheses): Prionorhynchia (14); Lokutella (233); Cuneirhynchia (16); Securithyris (955); Papodina (25); Securina (2). It totals up to 1245 specimens what is 19.9% of the total fauna. Summing up, the sulcate and "axiniform" morphological types make 67.7%, more than two-thirds of the Pliensbachian brachiopod fauna of the Bakony. Recent analogies suggest a 400-5600 m depth range, i.e. bathyal and abyssal environments for these forms. It is very important to examine whether the distribution of these morphological types reflects the palaeobathymetrical pattern of the Bakony area. Table VIII summarizes specimen numbers and percentages of sulcate and "axiniform" types in different environments. (A few smaller localities, where the facies-interpretation was not clear, were left out of calculation.) Considering the expectations, the result may be qualified as negative. Percentages of both the sulcate and the "axiniform" types scatter very widely, sometimes they reach a minimum where a maximum should be, and vice versa. The total percentages of the sulcate + " a x i n i f o r m " types show a distribution which seems to be closer to the trend expected on the basis of the depositional model but even here, the minima and maxima are not on their expected place. Furthermore, there is no significant difference between the percentages of the "shallowest" and "deepest" environments (A: 66.8%; B: 83.2%). Detailed studies on the environmental distribution of the Pliensbachian brachiopod fauna of the Bakony does not support the view that the sulcate and "axiniform" types characterize relatively deep and/or calm water. On the contrary, the assemblages of the relatively shallow, current-swept seamount top and slope are also made up mainly by these special forms (66.8 and 77.8%). The apparent contradiction may be solved if we consider that the sulcate and ~'axiniform" brachiopods are characteristic and dominant elements in the Mediterranean faunal province (Ager, 1967a). These forms were adapted to and were able to live in deep-water but within the faunal province dominated by them, they might conquer the shallower areas too. CONCLUSIONS Studies on the environmental distribution of Pliensbachian brachiopods in the Bakony have led to the following results: (1) Seamount top environments are characterized by a low-diversity fauna

268

where, besides the dominant terebratulids, strophomenids play a significant role. Characteristic taxon is the genus Koninckodonta. (2) The seamount slope fauna is very rich (both dense and diverse). The orders Terebratulida and Rhynchonellida play an equally important role. (3) The fauna found in the basin margin can be regarded as an impoverished variant of the former fauna. (4) The basin areas are characterized by a sparse and low-diversity fauna, dominated by large terebratulids. Characteristic taxa are Hesperithyris renierii and Lychnothyris rotzoana. (5) Members of the suborder Terebratulidina, i.e. the short-looped terebratulids, seem to have been better adapted to unfavourable conditions (e.g. limited food-supply, muddy bottom) than the rest of the brachiopod fauna. (6) Sulcate and "axiniform" morphological types, which were regarded earlier as characteristic deep-water elements, do not show any definite relationship with depth or environment. ACKNOWLEDGEMENTS

The author is indebted to Prof. B. G~czy and Dr. J. Konda (Budapest) for supporting his studies in every respect. Thanks are due to Prof. B. D'Argenio (Naples) and R. Catalano (Palermo) for cordiality and help during field studies in the Apennines and in Sicily in the frame of an Italo-Hungarian cooperation. Prof. D. V. Ager (Swansea) encouraged the author to submit this paper and kindly took care of the manuscript. REFERENCES Ager, D. V., 1956. The geographical distribution of brachiopods in the British Middle Lias. Q.J. Geol. Soc. London, 112: 157-187. Ager, D. V., 1963. Principles of paleoecology. McGraw-Hill, New York, N.Y., 371 pp. Ager, D. V., 1965. The adaptation of Mesozoic brachiopods to different environments. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1(2): 143-172. Ager, D. V., 1967a. Some Mesozoic brachiopods in the Tethys region, in: C. G. Adams and D. V. Ager (Editors), Aspects of Tethyan Biogeography. Syst. Assoc. Publ., 7: 135-151. Ager, D. V., 1967b. Brachiopod palaeoecology. Earth-Sci. Rev., 3: 157-179. AlmAras, Y. and Moulan, G., 1982. Les tArAbratulidAs liasiques de Provence. PalAontologie, biostratigraphie, palAoAcologie, phylogAnie. Doc. Lab. gaol. Lyon, 86: 1-365. Bernoulli, D., 1964. Zur Geologie des Monte Generoso (Lombardische Alpen). Beitr. Geol. Karte Schweiz, N.F., 118: 1-134. Bernoulli, D., 1967. Probleme der Sedimentation in Jura Westgriechenlands und des zentralen Apennin. Verh. Naturforsch. Ges. Basel, 78(1): 35-54. Bernoulli, D., 1971. Redeposited pelagic sediments in the Jurassic of the Central mediterranean area. Ann. Inst. Geol. Publ. Hung., 54(2): 71-90. Bernoulli, D. and Jenkyns, H. C., 1970. A Jurassic basin: the Glasenbach Gorge, Salzburg, Austria. Verh. Geol. Bundesanst., 4: 504-531. Bernoulli, D. and Jenkyns, H. C., 1974. Alpine, Mediterranean and Central Atlantic Mesozoic facies in relation to the early evolution of the Tethys. In: R. H. Dott and R. H. Shaver (Editors), Modern and Ancient Geosynclinal Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ., 19: 129-160.

269 Bowen, Z. P., Rhoads, D. C. and McAlester, A. L., 1974. Marine benthic communities in the Upper Devonian of New York. Lethaia, 7(2): 93-120. Broglio Loriga, C. and Neri, C., 1976. Aspetti paleobiologici e paleogeografici della facies a "Lithiotis" (Giurese inf.). Riv. Ital. Paleontol., 82(4): 651 706. Catalano, R. and D'Argenio, B., 1978. An essay of palinspastic restoration across western Sicily. Geol. Rom., 17: 145-159. Catalano, R. and D'Argenio, B., 1983. Infraliassic strike-slip tectonics in Sicily and Southern Apennines. Rend. Soc. Geol. Ital., 5: 5-10. Colacicchi, R. and Pialli, G., 1971. Relationship between some peculiar features of Jurassic sedimentation and paleogeography in the Umbro-Marchigiano basin (Central Italy). Ann. Inst. Geol. Publ. Hung., 54(2): 195-207. Cooper, G. A., 1972. Homeomorphy in Recent deep-sea brachiopods. Smithson. Contrib. Paleobiol., 11:1 25. Cooper, G. A., 1981. Brachiopoda from the Gulf of Gascogne, France (Recent). Smithson. Contrib. Paleobiol., 44: 1-35. Cooper, G. A., 1982. New brachiopods from the Southern Hemisphere and Cryptopora from Oregon (Recent). Smithson. Contrib. Paleobiol., 41: 1-43. D'Argenio, B., Horv~th, F. and Channell, J. E. T., 1980. Palaeotectonic evolution of Adria, the African promontory. In: J. Aubouin, J. Debelmas and M. Latreille (Editors), Geology of the Alpine Chains born of the Tethys. M~m. B.R.G.M., 115: 331-351. Dommergues, J. L., Elmi, S., Mouterde, R. and Rocha, R. B., 1981. Calcaire grumeleux du Carixien Portugais. In: A. Farinacci and S. Elmi (Editors), Rosso Ammonitico Symp. Proc., Roma, pp.199-206. Faber, P., Vogel, K. and Winter, J., 1977. Beziehungen zwischen morphologischen Merkmalen der Brachiopoden und Fazies, dargestellt an Beispielen des Mitteldevons der Eifel und Sfidmarokkos. Neues Jahrb. Geol. Palaeontol., Abh., 154(1): 21-60. Fiirsich, F. T., 1973. Thalassinoides and the origin of nodular limestones in the Corallian Beds (Upper Jurassic) of Southern England. Neues Jahrb. Geol. Palaeontol. Monatsh., 3:136 156. Ffirsich, F. T. and Hurst, J., 1974. Environmental factors determining the distribution of brachiopods. Palaeontology, 17(4): 879-900. Gal~cz, A. and VSrSs, A., 1972. Jurassic history of the Bakony Mountains and interpretation of principal lithological phenomena. Bull. Hung. Geol. Soc., 102(2): 122-135 (In Hungarian with English abstract). Garrison, E. R. and Fischer, A. G., 1969. Deep-water limestones and radiolarites of the Alpine Jurassic. In: G.M. Friedman (Editor), Depositional Environments in Carbonate Rocks, a Symposium. Soc. Econ. Paleontol. Mineral. Spec. Publ., 14: 20-56. G~czy, B., 1961. Die jurassische Schichtenreihe des TfizkSves-Grabens von Bakonycsernye. Ann. Inst. Geol. Publ. Hung., 49(2): 507-567. G~czy, G., 1971a. The Pliensbachian of Kericser Hill, Bakony Mountains, Hungary. Ann. Univ. Sci. Budap., Sect. Geol., 14: 29-52. G~czy, B., 1971b. The Pliensbachian of the Bakony Mountains. Acta Geol. Acad. Sci. Hung., 15: 117 125. G~czy, B., 1972. Ammonite fauna from the Lower Jurassic standard profile at LSk~t, Bakony Mountains, Hungary. Ann. Univ. Sci. Budap., Sect. Geol., 15: 47-76. G~czy, B., 1974. The Lower Jurassic ammonite faunas of the Southern Bakony, Ann. Univ. Sci. Budap., Sect. Geol., 17: 181-190. G~czy, B., 1976. Les Ammonitines du Carixien de la Montagne du Bakony. Akad~miai KiadS, Budapest, 223 pp. Hallam, A., 1975. Jurassic Environments. Cambridge Univ. Press, Cambridge, 269 pp. Ippolito, F., D'Argenio, B., Pescatore, T. and Scandone, P., 1975. Structural-stratigraphic units and tectonic framework of Southern Apennines. In: C. Squyres (Editor), Geology of Italy. Earth Sciences Society of the Libyan Arab Republic, Tripoli, pp. 317-328. Jenkyns, H. C., 1970. Growth and disintegration of a carbonate platform. Neues Jahrb. Geol. Palaeontol. Monatsh., 6: 325-344.

270 Jenkyns, H. C., 1971a. The genesis of condensed sequences in the Tethyan Jurassic. Lethaia, 4(3): 327-352. Jenkyns, H. C., 1971b. Speculations on the genesis of crinoidal limestones in the Tethyan Jurassic. Geol. Rundsch., 60(2): 471-488. Jenkyns, H. C., 1974. Origin of red nodular limestones (Ammonitico Rosso, Knollenkalke) in the Mediterranean Jurassic: a diagenetic model. In: K. J. Hsii and H. C. Jenkyns (Editors), Pelagic Sediments on Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 249-271. K~ilin, O. and Triimpy, D. M., 1977. Sedimentation und Pal~iotektonik in den westlichen Siidalpen: Zur triasisch-jurassischen Geschichte des Monte Nudo-Beckens. Eclogae Geol. Helv., 70(2): 295-350. Konda, J., 1970. Lithologische und Fazies-Untersuchung der Jura-Ablagerungen des BakonyGebirges. Ann. Inst. Geol. Publ. Hung., 50(2): 161-260. Laubscher, H. and Bernoulli, D., 1977. Mediterranean and Tethys. In; A. E. M. Nairn, W. H. Kanes and F. G. Stehli (Editors), The Ocean Basins and Margins. Plenum, New York, N.Y. pp. 1-28. Massari, F., 1979. Oncoliti e stromatoliti pelagiche nel Rosso ammonitico veneto. Mere. Sci. Geol. Padova, 32: 1-21. McCammon, H. M., 1973. The ecology of Magellania venosa, an articulate brachiopod. J. Paleontol., 47(2): 266-278. M~sz~ros, J., 1983. Structural and economic-geological significance of strike-slip faults in the Bakony Mountains. Rel. Ann. Inst. Geol. Publ. Hung., 1981:485-502 (In Hungarian with English abstract). Murray, J. W., 1973. Distribution and Ecology of Living Benthic Foraminiferids. Heinemann, London, 271 pp. Raup, D. M. and Stanley, S. M., 1971. Principles of paleontology. Freeman, San Francisco, Calif., 388 pp. Rudwick, M. J. S., 1970. Living and Fossil Brachiopods. Hutchinson, London, 199 pp. Scott, G., 1940. Palaeoecological factors controlling the distribution and mode of life of Cretaceous ammonoids in the Texas area. J. Paleontol., 14: 1164-1203. Sheehan, P. M., 1978. The hinging mechanism of brachiopods - - taphonomic considerations. J. Paleontol., 53(3): 748. Stanton, R. J., 1979. Diversity. In: R. W. Fairbridge and D. Jablonski (Editors), The Encyclopedia of Paleontology. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp. 268-274. Sturani, C., 1971. Ammonites and stratigraphy of the "Posidonia alpina" beds of the Venetian Alps. Mem. Ist. Geol. Mineral. Univ. Padova, 28: 1-190. SzabS, J., 1979. Lower and Middle Jurassic gastropods from the Bakony Mts. (Hungary). Part I. Euomphalidae (Archaeogastropoda). Ann. Hist. Nat. Mus. Natl. Hung., 71: 15-31. SzabS, J., 1980. Lower and Middle Jurassic Gastropods from the Bakony Mountains (Hungary). Part II. Pleurotomariacea and Fissurellacea (Archaeogastropoda). Ann. Hist. Nat. Mus. Natl. Hung., 72: 49-71. SzabS-Drubina, M., 1962. Examen lithologique des formations jurassiques de la Montagne Bakony. Rel. Ann. Inst. Geol. Publ. Hung., 1959:99-153 (In Hungarian with French and Russian abstract). Tchoumatchenko, P., 1972. Thanatocoenoses and biotopes of Lower Jurassic branchiopods in Central and Western Bulgaria. Palaeogeogr., Palaeoclimatol., Palaeoecol., 12: 227-242. Telegdi-Roth, K., 1935. Daten aus dem nSrdlichen Bakony-Gebirge zur jungmesozoischen Entwicklungsgeschichte der ~'Ungarischen Zwischenmasse". Math. Naturwiss. Anz. Ung. Akad. Wiss., 52:205-252 (In Hungarian with German abstract). Thayer, C. W., 1974. Marine paleoecology in the Upper Devonian of New York. Lethaia, 7(2): 121-155. Thayer, C. W., 1975. Diductor muscles of brachiopods: active or passive? Paleobiology, 1(1): 44-47. Tollmann, A., 1976. Analyse des klassischen nordalpinen Mesozoikums. Deuticke, Wien, 580 pp. Vogel, K., 1966. Eine funktionsmorphologische Studie an der Brachiopodengattung Pygope (Malm bis Unterkreide). Neues Jahrb. Geol. Palaeontol., Abh., 125(1-3): 423-442. Vogel, K., 1980. Uber Beziehungen zwischen morphologischen Merkmalen der Brachiopoden und

271 Fazies im Silur und Devon: die Bedeutung der Wassertiefe. Z. Dtsch. Geol. Ges., 131(3): 781 792. VSrSs, A., 1973a. Traces of boring algae from the Liassic of Kericser (Bakony Mts., Hungary). Bull. Hung. Geol. Soc., 103(1): 63-66 (In Hungarian with English abstract). VSrSs, A., 1973b. Speculations on food supply and bathymetry in the Mediterranean Jurassic sea. Ann. Univ. Sci. Budap., Sect. Geol., 16: 213-220. VSrSs, A., 1974. Bathymetric distribution of some Mediterranean Jurassic brachiopods (Bakony Mts., Hungary). Ann. Univ. Sci. Budap., Sect. Geol., 17:279 286. VSrSs, A., 1977. Provinciality of the Mediterranean Lower Jurassic brachiopod fauna: causes and plate-tectonic implications. Palaeogeogr., Palaeoclimatol., Palaeoecol., 21(1): 1-16. VSrSs, A., 1983a. The Pliensbachian brachiopods of the Bakony Mts. (Hungary): a stratigraphical study. Fragm. Mineral. Palaeontol., 11: 29-39. VSrSs, A., 1983b. Some new genera of Brachiopoda from the Mediterranean Jurassic. Ann. Hist. Nat. Mus. Natl. Hung., 75: 5-25. VSrSs, A., 1984. Lower and Middle Jurassic brachiopod provinces in the western Tethys. Ann. Univ. Sci. Budap., Sect. Geol., 24: 207-233. Wendt, J., 1970. Stratigraphische Kondensation in triadischen und jurassischen Cephalopodenkalken der Tethys. Neues Jahrb. Geol. Palaeontol. Monatsh., 7: 171-189. Williams, C. B., 1964. Patterns in the Balance of Nature. Academic Press, London, 324 pp. Winterer, E. L. and Bosellini, A., 1981. Subsidence and sedimentation on Jurassic passive continental margin, Southern Alps, Italy. Am. Assoc. Pet. Geol. Bull., 65(3): 394-421. Zezina, O. N., 1976. Ekologiia i rasprostranenie sovremennykh brakhiopod. [Ecology and distribution of Recent brachiopods.]. Nauka, Moscow, 138 pp. [In Russian].