Facies patterns and depositional environments of Palaeozoic cephalopod limestones

Sedimentary Geology, 44 (1985) 263- 300

263

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

FACIES PATTERNS AND DEPOSIT1ONAL ENVIRONMENTS OF PALAEOZOIC CEPHALOPOD LIMESTONES

J. W E N D T and T. A I G N E R

lnstitut und Museum fftr Geologic und Paliiontologie der Unit,ersit~t T~bingen, Sigwartstrasse 10, D-7400 Tiibingen 1 (FR.G.) (Received August 21, 1984; accepted for publication December 17, 1984)

ABSTRACT Wendt, J. and Aigner, T., 1985. Facies patterns and depositional environments of Palaeozoic cephalopod limestones. Sediment. Geol., 44: 263-300. In the eastern Anti-Atlas (Morocco) a platform and basin topography was established during the late Devonian, probably as a result of early Variscan tensional tectonics. Cephalopod limestones were deposited on shallow pelagic platforms, platform slopes and shallow, slowly subsiding basins. On the platform a transition from land areas into nearshore quartzose brachiopod coquinas, crinoidal limestones, condensed cephalopod limestones and finally into nodular limestones is observed. The latter often become disintegrated into incipient debris flows which pass into nodular limestone/marl alternations of a shallow basin. Deeper basins with shale sedimentation lack cephalopod limestones. Similar facies types also occur in the late Devonian of the Montagne Noire (France), Rheinisches Schiefergebirge (West Germany), Moravian Karst (Czechoslovakia), Holy Cross Mountains (Poland) and in the early Carboniferous of the Cantabrian Mountains (Spain). Due to strong late Variscan compressional tectonics and limited outcrops, detailed facies patterns could not be mapped in these regions, but the same facies types as in the eastern Anti-Atlas suggest similar coast/platform, slope and shallow basin topographies. During cephalopod limestone deposition water depth on the platforms was in the order of several tens to about one hundred metres, as is inferred from repeated subaerial exposures and distinctive depositional and faunal/floral features. Water depth in the adjacent shallow basins might have reached several hundreds of metres, Cephalopod limestones represent a typical stage in the evolution of geosynclines, characterized by extremely low sedimentation rates (1-5 m m.y.--1). This stage is preceded by deposition of thick neritic clastics a n d / o r carbonates and is succeeded by deposition of deep-water clastics or flysch.

INTRODUCTION

Palaeozoic cephalopod limestones occur throughout Hercynian Europe and northwest Africa, from southern Poland and Czechoslovakia, through the Carnie Alps, Harz Mountains, Rheinisches Sehiefergebirge, the Montagne Noire, Pyrenees and Cantabrian Mountains to the Anti-Atlas chaines of southern Morocco (Fig. 1). In all these regions cephalopod limestone deposition begins in the early late Devonian, or locally in the early Devonian. By the end of the Visean/early 0037-0738/85/$03.30

© 1985 Elsevier Science Publishers B.V.

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Fig. l. Occurrence of principal Upper Devonian (UD, ud ) and Lower Carboniferous ( LC, h') cephalopod limestones in Europe and North Africa. Capitals: major occurrences; small letters: minor occurrences. Namurian a rather abrupt transition into deep-water clastics or carbonates is observed. Cephalopod limestones are characterized by the predominance of pelagic organisms (cephalopods, dacryoconarids, thin-shelled ostracods, conodonts), a much reduced sedimentation rate with intervening periods of non-deposition and little or no terrestrial influx. Stratigraphically they mark a transitional stage of the geosynclinal sequence between shallow-water carbonates or elastics and deep-water greywackes and flysch. During the late Palaeozoic of western Europe and northwest Africa they were deposited on slowly subsiding ridges and platforms, separated by rapidly subsiding basins or troughs. This topography may be attributed to early Variscan tensional tectonics preceding the late Carboniferous orogeny. In contrast to the well-known stratigraphic sequence and faunal zonation, coeval facies patterns and their palaeogeographic implications are much less well estab-

265

lished. Limited outcrops in central Europe and strong Variscan tectonic shortening have precluded detailed mapping of facies distributions, so only very generalized facies models have been developed. No such limitations exist in a similar palaeogeographic and palaeotectonic setting on the northern border of the Sahara Craton (eastern Anti-Atlas, Morocco), where a similar Upper Devonian facies sequence is developed. Perfect outcrops and weak structural deformation have allowed large-scale mapping of coeval rock units and the reconstruction of depositional environments. It is the aim of the present study to compare European Upper Devonian and Lower Carboniferous settings of cephalopod limestone deposition to the Moroccan palaeogeographical model. Special emphasis will be placed on facies patterns, sedimentation rates and the reconstruction of palaeoenvironments. EASTERN ANTI-ATLAS Marine Devonian sediments cover a large part of the northwestern Sahara Craton. In southern Morocco they crop out along the unfolded northern margin of the Tindouf Basin in the west and in more isolated anticlines and synclines of the Mader and Tafilalt in the east (Hollard, 1967). The overall thickness of the Devonian decreases from the northern margin of the Tindouf Basin (3000 m) across the Mader (2200 m) to the northern Tafilalt (300 m; Hollard, 1981). Towards the end of the Middle Devonian, a platform, and basin topography developed in the eastern Anti-Atlas, probably as a result of early Variscan tensional tectonics (Wendt et al., 1984). From east to west the following palaeogeographic units can be distinguished: Tafilalt Basin, Tafilalt Platform, Mader Basin, Mader Platform (Figs. 2 and 3). This platform and basin topography persisted during the late Devonian, until it was levelled by uppermost Famennian to Tournaisian deltaic deposits.

Tafilalt Basin In southern Morocco only the western edge of the Tafilalt Basin is exposed. To the east it is covered by Upper Cretaceous and Tertiary rocks, but late Devonian basinal sediments reappear in western Algeria, in the area of Ben Zireg and the Saoura Valley (Massa, 1965; Legrand, 1967). In Morocco the basinal sediments consist of grey marls and shales with sandy interbeds and intercalated red nodular limestones (Fig. 3). In contrast to the calcareous cephalopods of normal size which occur in the nodular limestones, small (body-chambered) hematized cephalopods are abundant in the marls. Upper Devonian sediment thickness decreases markedly from the south (400 m) to the north (30 70 m), suggesting that the Tafilalt Basin did not extend very much farther to the north beyond the area exposed. No obvious hiatuses were observed in the Upper Devonian sequence of the Tafilalt Basin. A transition is observed between rocks of this basin and the discontinuous condensed deposits of the Tafilalt Platform. The platform-to-basin transition is characterized by nodular limestones, incipient debris flows and slumping phenomena.

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Tafilalt Platform (Figs. 2-5) This platform extends 60 km in a north-south and 25-40 km in an east-west direction (Fig. 2). Its further extension to the north and south is masked by overlying Upper Cretaceous and Tertiary rocks, but a similar Upper Devonian condensed sequence is known in the eastern Moroccan Meseta (Hollard, 1967), suggesting a continuation of the Tafilalt Platform to the north. The platform sediments are mainly cephalopod limestones which appeared sporadically during the Middle Devonian, but become more widespread during the late Devonian. During the early Frasnian a few metres of well-bedded or nodular styliolinid limestones with intercalated marls, at some levels crowded with cephalopods, were deposited. Local hummocky cross-stratification and ubiquitous ironstained oncolites indicate shallow-water conditions. After a local hiatus and slight angular unconformity, dark grey fetid cephalopod limestones were deposited during the late Frasnian (Hollard, 1967; Buggisch and Clausen, 1972). Their thickness ranges from 0 - 2 m on the central part of the Tafilalt Platform, where they are locally missing due to pre-early Famennian erosion, to 10 15 m towards the platform margins. A general vertical succession of storm layers (subtidal), overlain by birdseye limestones with small-scale teepees (intertidal) followed by reworked limestone cobbles and early cements suggest deposition in shallow subtidal to

Fig. 6. Crinoidal wackestone (biomicrosparite). Note limonitic staining of crinoid ossicles and microborings (algal or fungal) in recrystallized shell (upper centre). Lower Famennian. Jebel Bou lfarherioun, thin section.

271

fig. 7. Thick-bedded cephalopod limestone with numerous cheiloceratids. Lower Famennian (lift),Jebel Amelane, polished slab.

supratidal environments (shallowing-upward sequence). Solution cavities filled with vadose cements indicate repeated subaerial exposure during the late Frasnian. The most obvious hiatus occurred during the latest Frasnian, in the north central part of the Tafilalt Platform. In this area, the Upper Frasnian cephalopod limestone shows an erosional top and prominent solution cavities filled with red Lower Famennian mudstone. Tectonic dykes filled with the same mudstone penetrate from the erosional surface down into Middle Devonian rocks. It can be concluded that tectonic uplift, which also caused angular unconformities at the F r a s n i a n / F a m e n n i a n boundary, is responsible for a short emergence of the northern Tafilalt Platform (Wendt et al., 1984). On the southernmost portion of the Tafilalt Platform, upper Famennian (V) rocks overlie those of the Silurian or older Palaeozoic (Hollard, 1974), suggesting emergence during most of the Devonian. Farther to the north, the early Famennian ( I I - I I I ) comprises four different facies belts which succeed one

272

Fig. 8. Thick-bedded cephalopod wackestone (biomicrosparite)with juvenile goniatite (a), gastropod (b), ostracod (c) and molluscan shell debris. Note molluscan shell fragment incrusted by colloform limonite (d) indicating slow deposition and reworking. Lower Famennian (Ilfl), Hamar Laghdad, thin section.

another in a roughly N - S direction (Figs. 2, 4 and 5): (1) Sandy brachiopod coquinas (3-25 m) cover the southern part of the platform. The large amount of detrital quartz (20-30%) indicates the proximity of land. This facies belt passes laterally into (2) thick-bedded crinoid-rich cephalopod limestones (Fig. 6). These were deposited on a shallow submarine ridge which prevented much transport of detrital quartz farther to the north. (3) A facies belt of thick-bedded red limestones, crowded with cephalopods occurs on the northern Tafilalt Platform (Figs. 7 and 8). Towards the platform margins this facies passes into (4) red and grey nodular limestone with a similar fauna (Fig. 9). The facies zonation during the early Famennian on the Tafilalt Platform is also reflected in a distinct distribution of faunal elements (Fig. 5): goniatites are rather common over the entire platform except in the sandy brachiopod coquinas where

273

Fig. 9. Nodular limestone (biomicrosparite)with molluscan shell debris and entomozoan ostracods. Note gradational boundary between nodule (upper right) and clayey matrix (lower left). Lower Famennian (il fl), Oued Rheris, thin section.

they occur only sporadically. In the thick-bedded cephalopod limestone facies they are associated with abundant orthoconic nautiloids which are distinctly current-oriented (Wendt et al., 1984). Brachiopods are essentially limited to the near-shore quartz-rich coquinas. The habitats of crinoids were probably current-swept shoals within the crinoidal limestone facies, whence their ossicles were transported into adjacent environments. In this facies crinoid holdfasts occur in isolated patches throughout the Famennian, generally associated with solitary rugose corals and the tabulate Cladochonus. Thin-shelled ostracods (entomozoans), trilobites and gastropods are generally found as scattered, mostly fragmented specimens, with a slight predominance in the more off-shore facies belts. During the late Famennian (IV-V) the facies pattern on the Tafilalt Platform became more uniform. The vertical transition from the above facies pattern of the

274

early Famennian into widespread red nodular limestones of the late Famennian probably reflects a levelling of the submarine topography and a slightly increased water depth during that interval. Only on the central part of the platform do crinoidal limestones continue into the Upper Famennian. Mader Basin and Mader Platform West of the Tafilalt Platform, Devonian rocks are almost completely eroded. Three isolated outcrops in the northwest show Upper Frasnian/Lower Famennian debris flows. From these, it can be inferred that the area west of the Tafilalt Platform represented a palaeoslope which was probably affected by synsedimentary fault scarps. The proximity of a slope is also indicated by incipient debris flow and slumping phenomena in Upper Frasnian and Lower Famennian nodular limestones, along the western border of the Tafilalt Platform. Cephalopod-free crinoidal limestones reappear in an isolated limb of a platform in the northern Mader which was probably connected to the Mader Platform (Fig. 2). Increased subsidence in the central Mader Basin resulted in the deposition of several hundred meters of shales with intercalated sandstones during the Lower Famennian (" Flysch de Bou Dib" of Hollard, 1967). Farther to the south, the shales contain abundant hematized goniatites. Towards the southern and western margin of the Mader Basin, the Lower Famennian shales are more reduced in thickness and contain thin (2-5 cm) beds of red and buff cephalopod limestone. On the Mader Platform, Upper Famennian (V) cephalopod limestones with giant gonioclymenids overly Silurian sandstones and shales (Hollard, 1974), indicating emergence of that area during most of the Devonian. Three isolated outcrops of Lower Famennian cephalopod limestone show that the eastern margin of this platform was still submerged (Figs. 2 and 3). Summarizing the palaeogeographic and sedimentological evidence from the Upper Devonian of the eastern Anti-Atlas, it becomes evident that condensed cephalopod limestones are restricted to platform areas with an extremely low rate of subsidence. Nodular limestones extend into shallow basinal areas with slightly increased sedimentation rates, becoming intercalated with shales. Platform-to-basin transitions are characterized by incipient disintegration of individual limestone layers through slumping and sliding. Complete disintegration of bedding into giant debris flows and megabreccias indicates steeper, probably tectonically controlled slopes. MONTAGNE NOIRE

Between Middle Devonian shallow-water limestones with local reefs and Upper Visean flysch, a reduced sequence of some tens to about 300 m of cephalopod limestones, shales and cherts is developed. Nodular limestones ("Griotte") which

275

pass laterally into calcareous shales are widespread in the Upper Devonian, but occur also, more locally, in the Lower Visean (Maurel, 1966; Tucker, 1974; Bourrouilh, 1981). The Upper Devonian Griotte is lithologically divided into the Frasnian Infragriotte (multi-coloured limestones, limestone breccias, shales, cherts), the Lower Famennian Vraies Griottes (red cephalopod limestones) and the Upper Famennian Supragriottes (red and pink bedded and nodular limestones; Fig. 10). Biostratigraphic correlation is difficult caused by the paucity of cephalopods; a conodont zonation has been established only in a few localities (Boyer et al., 1968). Carboniferous nappe tectonics and mass transport of huge exotic blocks (Engel et al., 1981) have produced a complex fragmentation of the pre-flysch depositional areas. The

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276

following facies types were recognized in the Infragriotte and Vraies Griotte (Fig. 10): (1) Hardgrounds and condensed facies. Biostratigraphically well established hiatuses, locally marked by thick ferromanganiferous crusts, seem to be most common in the "zone du pal6orelief" (Maurel, 1966) of the "Ecailles de Cabri6res". At Touriere, thick-bedded red cephalopod limestones of Famennian III a-age are overlain by crinoid-rich nodular limestones of early Visean age (Engel et al., 198l). The boundary is affected by small faults with throws up to 0.5 m and calcitic veins which do not continue up into the Visean (Fig. 11). There is no direct evidence that these areas of non-deposition were temporarily emergent, but land was probably not far away, as indicated by transported land plants in the Griotte (Ovtracht and Fournie, 1956). Local M n / F e deposits, once mined from karst cavities in the

Fig. 11. Discontinuity surface (arrowed) truncating Lower Famennian (Illa) cephalopod limestone (a) with calcitic veins (b), overlain by iron-stained Lower Tournaisian hardground (c) and crinoidal limestone (d). Touriere, Montagne Noire, thin section.

277

Montagne Noire and the French Pyrenees, were originally believed to be the result of a late Devonian/Tournaisien period of emergence (Jaeger et al., 1958; Perseil, 1969). Detailed conodont sampling by Boyer et al. (1974) has proved, however, that there was no hiatus at the Devonian/Carboniferous boundary and that the M n / F e mineralization is probably of Frasnian age, redeposited after the Hercynian nappe tectonics. Other typical indicators of an intermittent and extremely reduced sedimentation are iron-stained and iron-encrusted lithoclasts, shell debris and oncolites (Tucker, 1973a), sometimes overgrown by crinoids, Cladochonus and foraminifera (Fig. 12). (2) Black cephalopod limestones. Thin levels (10-20 cm) of dark grey to black bituminous limestone with intercalated marls occur mainly in the condensed facies of the "Ecailles de Cabri6res" but have been found also farther to the west (Boyer et al., 1968). They are limited to the latest Frasnian and have been compared to the Kellwasser Limestone of the Rheinisches Schiefergebirge by Buggisch (1972). The rich fauna (goniatites, current- or wave-oriented orthoconic nautiloids, bivalves, brachiopods, crinoids) is similar to that of the contemporaneous facies in the eastern Anti-Atlas, suggesting that the black cephalopod limestones of the Montagne Noire were also deposited in shallow water. (3) Crinoidal mudstones. Completely disarticulated crinoid ossicles prevail, but crinoid stems up to 15 cm long, indicating little transport, have also been observed.

Fig. 12. Part of ferromanganese nodule (oncolite) with encrusting tabulate Cladochonus (a) and crinoid holdfast (b). Lower Famennian cephalopod limestone (Vraies Griottes). Tombe d'Izarne, Montagne Noire (France), thin section.

278 The faunal association of this facies, which is most common in the Infragriotte, consists of other transported benthic organisms (Cladochonus and trilobites) associated with "pelagic" styliolinids and thin-shelled ostracods. (4) Debris flows. Nodular limestones (size of individual nodules 1-5 cm) are best developed in the Vraies Griottes. At certain levels most of the nodules are small body-chambered goniatites (mostly cheiloceratids). The complex multiphase infilling history of the goniatite shells, deciphered by Neumann and Schumann (1974) in the Montagne Noire and other Palaeozoic and Mesozoic nodular limestones, provides clear evidence that the nodules were not formed in situ but by intermittent processes of reworking and redeposition (Bourrouilh, 1981). The interpretation of many

Fig. 13. Large blocks, probably derived from submarine fault scarp, embedded in nodular limestone (debris flow) of Lower Famennian slope facies (Vraies Griottes). Mont Peyroux, Montagne Noire (France), Top of section is to the left. Hammer for scale.

279

nodular limestones as debris flow deposits is based on the chaotic admixture of heterogeneous pebbles, their poor sorting (grain size from a few millimetres to about 5 cm, locally up to 1 m; compare Figs. 13 and 14) and local inverse grading. At one locality a typical "Scheck" appearance (redeposited red limestone pebbles cemented by white sparry calcite) of a debris flow was observed (Fig. 15), similar to that of the well-known Middle Liassic debris flow deposit of the Salzburg area (Jenkyns, 1974). (5) Bedded and nodular limestones with marly partings. This is the most widespread lithology, well developed in the Infra- and Supragriotte. Individual beds are generally 5-20 cm thick, but may reach 1 m. The fauna consists of scattered goniatites, orthoconic nautiloids and shell debris (crinoids, trilobites, ostracods, styliolinids). Orientation patterns of orthoconic nautiloids, measured in the lower portion of the Supragriotte at Coumiac, indicate weak currents from NE-NNE.

Fig. 14. Close-up of bed shown in Fig. 13. Nodular limestone (debris flow).

280

Fig. 15. Upper Frasnian "Scheck" (debris flow cemented by white sparry calcite). Domaine de Libes. Montagne Noire (France), polished slab (scale is in cm).

Fig. 16. Spongeliomorpha-likeburrows on bedding-parallel section. Griotte, probably Frasnian, Montagne Noire. Coin for scale.

281

Sedimentary structures are generally obliterated by bioturbation and pressure solution. Sections cut parallel to bedding show horizontally bifurcating burrows of the Spongeliomorpha type (Fig. 16). This burrowing activity contributed to the development of the nodular structure of some of the thick-bedded limestones. It is often observed that these bioturbated limestones grade laterally and vertically into typical debris flows. (6) Grey marls with small pyritized goniatites and cherts occur in the Upper Frasnian and Lower Famennian. Intercalated levels of red nodular limestone suggest deposition in a shallow basinal environment similar to that of the Tafilalt Basin, but probably more restricted. These facies types can be interpreted to represent the following depositional environments (Fig. 10): shallow submarine platform (condensed cephalopod limestones with hardgrounds, bituminous cephalopod limestones, crinoidal limestones), slope (debris flows, nodular limestones partim, local slumpings), and shallow basin (nodular limestones partim, red cephalopod limestones with marly partings, grey marls and cherts). The more monotonous sequence of Upper Devonian calcareous schists was probably deposited in deeper water. At our present state of knowledge, the tectonic fragmentation of the area does not permit palaeogeographic reconstructions of the late Devonian in the Montagne Noire. CANTABRIAN MOUNTAINS

Upper Devonian cephalopod limestones extend from the Montagne Noire into the French and Spanish Pyrenees (Cavet, 1959; Mirouse, 1967) and reappear in the Palentine Basin of the eastern Cantabrian Mountains. The age of these nodular limestones (Carda~ao and Vidrieros Fm) ranges from late Givetian to early Tournaisian (Van Adrichem Boogaert, 1967; Higgins and Wagner-Gentis, 1982). Intercalations of quartz-sandy turbidites indicate that the nodular limestones were deposited in a slope-basin transitional area (Maas, 1974). Towards the west and south the nodular limestones are replaced by coarse clastics (Nocedo and Ermita Formations) which were deposited in litoral and deltaic environments around an emergent source area, the Asturian Geanticline (Wagner et al., 1971). During the Middle Tournaisian, in the Cantabrian Mountains (and in other areas of the Hercynian fold belt), black shales and cherts (Vegamian Fm) were deposited in restricted shallow depressions of the Ermita sandstone (for a different bathymetric interpretation of the Vegamian shales, see Frankenfeld, 1981). The Vegamian shales are overlain with a sharp lithologic boundary by red (rarely grey) cephalopod limestones (Alba Formation) of late Tournaisian to early Namurian age. In contrast to the small, tectonically isolated occurrences of the same age and lithology in the Montagne Noire, the Alba Formation covers an area of several thousand square kilometres (Fig. 17) and thus provides an excellent picture of facies patterns and depositional environments of cephalopod limestones.

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In many sections the Alba Formation can be divided into three members (Fig. 18): a lower division (Gete or Gorgera Member) of nodular limestones with marly partings and local crinoidal limestones; a middle division (Valdehuesa or Lavandera Member) of red shales and cherts; and an upper division (La Venta or Canal6n Member) of well-bedded and nodular limestones which grade vertically into the allodapic limestones of the Barcaliente Formation or into the shaley-turbiditic Olleros Formation. Only the lower portion of the Alba Formation, corresponding to the uppermost Tournaisian and lowermost Visean (anchoralis and typicus zone. see Higgins and Wagner-Gentis, 1982) is considered here. It comprises the following facies types (Fig. 18):

Fig. 19. Calcareous quartz-rich sandstone with poorly sorted angular to well-rounded quartz grains and reworked phosphatic clasts derived from underlying Middle Tournaisian Vegamian shales (arrowed). Base of Alba Formation. Puente de Alba. southern Cantabrian Mountains (North Spain). thin section.

285

(1) Quartz sandstones and conglomerates (Fig. 19). These occur locally at the base of the Alba Formation. Well-rounded quartz grains (0.1 0.5 mm diameter) with phosphatic and arenaceous pebbles reworked from the underlying Vegamian and Ermita Formations, are embedded in a calcareous (partly dolomitic) matrix. These arenaceous levels which represent the transgression of the late Tournaisian sea over uplifted and eroded Upper Famennian or lowermost Tournaisian strata, occur along the southern margin of the Cantabrian Mountains in the Porma-Bernesga area and locally also to the northwest and southeast (Higgins and Wagner-Gentis, 1982; Fig. 17). (2) Crinoidal limestones. These constitute the Baleas Formation of Wagner et al. (1971), the biostratigraphic position of which is somewhat controversial. Wagner et al. (1971) and Higgins (1971) assigned it to the Upper Tournaisian (anchoralis zone) making it contemporaneous with the lower portion of the Alba Formation. Higgins and Wagner-Gentis (1982), using the same conodont faunas, attribute the Baleas Formation to the Middle Tournaisian (cooperi zone). They envisage a deposition of this unit on a submarine ridge ("Pola de Gordon Swell") within the Vegamian basin. The sharp lithologic contrast between the Baleas Formation and the Vegamian Formation and the lack of known lateral and vertical transitions between the two formations makes this interpretation unlikely. The close facies similarities between the crinoidal limestones of the Baleas Formation and the crinoid-rich basal division

Fig. 20. Crinoidal wackestone with crinoid ossicles (a), partly with syntaxial rims (arrowed) and bryozoan fragment (b). Baleas Formation, Pola de Gordon, southern Cantabrian Mountains (North Spain), thin section.

286 of the Alba Formation, the observed vertical transition between the two formations and the presence of the same conodont faunas strongly support the presumption that they are partly synchronous. Sanchez de la Torre et al. (1983) included all bioclastic limestones of late Famennian to late Tournaisian age in the Baleas Formation. The crinoidal limestones of the Baleas Formation are red or pink grainstones to mudstones, with abundant crinoid and bryozoan remains (Fig. 20) and less abundant brachiopods, tabulates (Cladochonus), trilobites and ostracods. The organic remains are generally fragmentary, often coated or stained with limonite and perforated by endolithic algae (or fungi ?). This feature indicates slow sedimentation and strong reworking on a shallow turbulent platform. In the basal portion of the crinoidal limestone, poorly sorted angular to well-rounded quartz may reach 20%. This lithology of the Baleas Formation was found only in a narrow belt along the southern margin of the Cantabrian Mountains, in the Porma-Bernesga area, where it is transgressive over Frasnian or lowermost Tournaisian neritic sandstones (Fig. 17). (3) Internal breccias. This term was proposed by Ft~chtbauer and Richter (1983) for brecciated shallow water limestones with good fitting, caused by tensional movements along shelf margins. In the Cantabrian Mountains internal breccias were found at one locality only (Pola de Gordon) in the transitional zone between crinoidal and nodular limestones (debris flows). The host rock consists of crinoidal limestone (Baleas Formation) which is irregularly dissected by sandstone dykes. The

Fig. 21, Bedding plane of debris flow in the lower part of the Alba Formation (Gete Member). Puente de Alba, southern Cantabrian Mountains (North Spain).

287

latter are derived from the underlying Ermita sandstone, which at that time was still mobile. Laterally the breccia passes into massive crinoidal limestone, (4) Debris flows (Fig. 21). Despite their wide distribution, especially in the Gete Member, debris flows have never been mentioned from the Alba Formation. The only exception seems to be a brief description and figure of an obvious debris flow from the southeastern Cantabrian Mountains, interpreted by Koopmans (1962) as the result of "early diagenetic reworking". Individual pebbles in the debris flows range from a few millimetres to about 10 cm in diameter. They are generally all of the same lithology (microsparitic mudstone) but differ in the abundance and composition of organic debris, suggesting slightly different source areas. Contacts with the surrounding more clayey matrix are indistinct, indicating a semi-lithified state for the pebbles during mass transport. Sharp pebble-matrix boundaries are the result of later pressure solution. Some of the pebbles are body chambers of goniatites, whose intricate depositional history was deciphered by Neumann and Schumann (1974). Individual debris flows are generally up to half a metre thick, but may reach up to 15 m in the southeast (Villabellaco area). They grade vertically and laterally into nodular limestones with marly partings. Thorough mapping of the debris flows should provide clues to the depositional topography of the Alba Formation. (5) Slumpings. Slumps are rare in the lower part of the Alba Formation and have been observed only in the chert (Valdehuesa) member. Due to limited outcrops the direction of the sediment displacement could not be determined. (6) Nodular limestones. The most widespread facies type of the Alba Formation is a well-bedded (0.1-0.5 m) red or pink (in the Palentine Basin grey) mudstone with irregular bedding planes, a nodular internal structure and dark red shaley partings. The fauna is dominated by pelagic organisms (goniatites, conodonts, thin-shelled ostracods, radiolarians). Benthic organisms (small solitary corals, tabulates, crinoids, gastropods, trilobites, brachiopods) are much less abundant. Occasionally shell debris and lithoclasts are stained and encrusted by limonite, thus indicating a provenance from source areas with condensed sedimentation. Detrital quartz is common at the base of the Gete Member, but almost absent in higher levels. (7) Cherts and clays. Red cherts with intercalated clays are restricted to the middle portion of the Alba Formation (Valdehuesa Member). The fauna of this facies is very poor (abundant radiolarians, rare unidentifiable goniatites). The Valdehuesa Member is patchily distributed and missing in the more condensed sections of the Esla area, the Palentine Basin and parts of the southern Cantabrian margin (Higgins and Wagner-Gentis, 1982).

Late Tournaisian / early Visean palaeogeography The above facies types reflect a coast-platform-slope-basin topography during the late Tournaisian/early Visean. The late Tournaisian coast was located close to the

288 southern margin of the Cantabrian Mountains, where it is now largely buried under younger sediments. However, in the northwest, towards the Asturian Geanticline, and at the southeastern margin of the Palentine Basin coarse near-shore sediments were also desposited (Higgins and Wagner-Gentis, 1982). The crinoidal limestones of the Baleas Formation were deposited on a shallow ridge or platform probably tending E-W, which still received a modest influx of clastics from the south. Widespread occurrence of debris flows indicates a platform/coast-to-basin transition towards the north and a significant submarine topography within the Alba basin. Slopes were probably very gentle, giving rise to only short-distance sediment mass movements. Only locally, steeper flexures of submarine fault scarps produced internal breccias. The Alba basin accumulated a very small amount of sediment (average sedimentation rate 2 m m.y. 1) without any obvious lithologic breaks. Higgins and Wagner-Gentis (1982), however, postulated several hiatuses based upon the absence of certain conodont zones. The decrease of debris flows suggests that the submarine topography was levelled towards the end of the early Visean. Only a few slumps were observed in the chert-clay (Valdehuesa) member of that interval. Thus, only the topmost division of the Alba Formation (La Venta Member) may have reached a "levelling stage" (Kullmann and SchOnenberg, 1975) in the geotectonic evolution of the Cantabrian area. During the early Namurian, renewed tectonic activity led to a further break up of the Alba Basin into structural highs with shallow water limestones and deeper basins with sandy or calcareous turbititic sedimentation (Reuther, 1977). RHENOHERCYNIAN ZONE The Rheinisches Schiefergebirge and the Harz Mountains are the classical areas in which a most refined late Devonian goniatite and conodont zonation has been established. Upper Devonian patterns of cephalopod limestone and shale/turbidite distribution, now largely obscured by late Variscan compressional tectonics, have been related to a complicated platform/rise and basin topography (Tucker, 1973b, 1974; Franke et al., 1978; Babin et al., 1980). Platforms and rises of various origins (shelf, reef, volcanic, crystalline, tectonic--see Franke and Walliser, 1983) are covered by a thin veneer of Upper Devonian (locally also Lower Carboniferous) cephalopod limestone. Numerous hiatuses in the sequence on the ridges reflect extremely reduced and discontinuous sedimentation. The presence of Upper Dev o n i a n / Lower Carboniferous sedimentary dykes penetrating down into underlying reef limestone, may be attributed to weak tensional tectonic or volcanogenic movements rather than to subaerial karstification (Franke, 1973). Detailed facies maps of the cephalopod limestones cannot be reconstructed, due to the limited outcrops, but facies types similar to those of the Anti-Atlas, Montagne Noire and Cantabrian Mountains are present. Most common are red or grey cephalopod limestones, which pass laterally into nodular limestones and shales with calcareous

289

nodules, deposited in the platform to basin transitions. Crinoidal limestones are reported from the shallowest areas of submarine rises (submerged reefs, see Franke, 1973), some of which might have been temporarily exposed above sea level (Clausen et al., 1978). In the Belgian Famennian, pink thick-bedded crinoid wackestones with Stromatactis are interpreted as shallow water crinoid mounds which pass laterally into nodular limestones (Dreesen, 1982). Black bituminous limestones and shales (Kellwasser Limestone) occur locally in the late Frasnian (Buggisch, 1972) within the condensed sequences on the pelagic ridges and in the more basinal facies. They have been attributed to a world-wide interval of oxygen-deficiency (Engel et al., 1983). It should be noted, however, that they are much thinner and of a much poorer benthic fauna than the contemporaneous bituminous limestones of the Tafilalt Platform which were clearly deposited in extremely shallow and agitated waters. The phase of condensed cephalopod limestone deposition on submarine platforms and rises ends in the late Visean with the widespread deposition of greywacke turbidites. MORAVIAN KARST The late D e v o n i a n / e a r l y Carboniferous sedimentation in the Sudetic zone stands in close relation to that in the Rhenohercynian zone: thick G i v e t i a n / F r a s n i a n shallow water (partly reefal) limestones with brachiopods, stromatoporoids, tabu-

Fig. 22. Debris flow intercalated in well-bedded cephalopod limestone. Upper Frasnian, Jedovnice, Moravian Karst (CSSR). Coin for scale.

290

lates and rugose corals, pass gradually into some tens of metres of grey to red cephalopod limestones of late Frasnian/early Famennian to early Tournaisian (locally middle Visean) age. The boundary between these units is not isochronous (Dvorak, 1973), so coral limestones must pass laterally into the basal portion of the cephalopod limestones. The nodular structure of the latter (Krtiny Limestone) is partly a result of sediment mass movements in a semi-lithified state (debris flows, Fig. 22) which indicates deposition on a slope between a stable carbonate platform in the southeast and more strongly subsiding basinal areas in the northwest. The depositional history, complicated by synsedimentary faulting, can be well deciphered in the Mokra quarry northeast of Brno where the highly variable thickness of Famennian cephalopod limestones is controlled by NW SE trending faults. The limestones are often slump-folded with the slump folds facing towards the N N E (J. Dvorak, pers. commun., 1984). The nodular Krtiny Limestones pass l'aterally into grey mudstones with shaley partings (Hady Limestone) which in turn grade into basinal shales and radiolarites of the Ponikev Beds (Chlupac, 1964; Zukalova and Chlupac, 1982). Apart from local hiatuses, indicated by the absence of certain conodont zones, the condensed platform sequence is continuous until the late Visean, when it became buried by thick shaley flysch, laterally interfingering with coarse debris flows (Dvorak, 1973). HOLY CROSS MOUNTAINS

In the Upper Devonian of the southwestern Holy Cross Mountains (Poland) a carbonate platform runs approximately E-W, flanked by northern and southern basins (Szulczewski, 1971). Growth of the shallow water carbonate platform (with occasional stromatoporoid mounds) ceased abruptly in the earliest Famennian. During the late Frasnian, cephalopod limestones, interfingering with stromatoporoid-coral-Renalcis detrital limestones, accumulated on the flanks of the carbonate platform. The detrital limestones are disconformably overlain by a highly condensed sequence of cephalopod limestones, which range from the early Famennian into the late Tournaisian. It is not clear if the obvious break in sedimentation between the platform carbonates and the cephalopod limestones is due to uplift and local erosion or to drowning below the level of active reef and platform growth. Evidence against the latter view comes from the co-existence of cephalopod limestones and oolite shoals in the early Famennian. Sedimentary dykes of various age (early Famennian to late Tournaisian) occur through the whole sequence. They are obviously the result of block-faulting and tensional tectonics (Szulczewski, 1973). which probably also gave rise to the large-scale platform-basin topography of the late Devonianflearly Carboniferous. The condensed cephalopod limestones are predominantly mudstones and wackestones with cephalopod and crinoid debris. Limited outcrops of this facies, which is encountered only in a very restricted area (about 5 km in a N - S and 20 km in an

291

E-W direction) have not allowed the reconstruction of the palaeogeographic distribution of individual facies types. To the south and north, however, the Upper Devonian condensed facies passes into slope and basinal lithologies (Lysogory facies), characterized by slumping, debris flows and proximal turbidites (Szulczeswki, 1968). PALAEOBATHYMETRY

There is no general agreement on the absolute depth in which Palaeozoic and Mesozoic cephalopod limestones were deposited (Table 1). Several facts may account for the controversial palaeobathymetric interpretation: (1) generally, cephalopod

TABLE 1 Estimates of depositional depth of Palaeozoic and Mesozoic cephalopod limestones Author Ovtracht and Fournie, 1956 Tucker, 1973b

Formation, area

Marbre Griotte, Pyrenees, Montagne Noire, S-France Cephalopodenkalk, Rhenohercynian Syncline, West G e r m a n y Tucker, 1974 Cephalopodenkalk, West Germany; Griotte, Montagne Noire, Southern France Bandel, 1974 Cephalopod limestone, Carnic Alps, Austria Buchroithner Steinberg cephalopod limestone, et al., 1979 Graz, Austria Franke and Cephalopod limestones, Variscan Walliser, 1983 zone, central Europe Van Adrichem Alba Formation, Cantabrian Boogaert, 1967 Mountains, North Spain Reuther, 1977 Alba Formation, Cantabrian Mountains, North Spain Garrison and Adnet Limestone, Northern Fischer, 1969 Alps, Austria Jenkyns and Condensed cephalopod limestone Torrens, 1971 (seamount facies), West Sicily Wendt, 1970 Condensed cephalopod limestones, Tethyan Bernoulli, 1972 Ammonitico Rosso, central Mediterranean and North Atlantic Wendt, 1973 Hallstatt Limestone, Jugoslavia and Greece Schlager, 1974 Cephalopod Limestones, western Tethys

Age

Depth of deposition

Upper Devonian

very shallow, transgressive some 10's to some 100's of metres some 10's to some 100's of metres

Upper Devonian Upper Devonian

Devonian--Lower Carboniferous Upper Devonian

200-4000 m

Lower Carboniferous

relatively deep

Lower Jurassic

neritic to 4000 m

Jurassic

some lO's of metres

below photic zone and wave base Upper D e v o n i a n / L o w e r not exceeding 100 m Carboniferous Lower Carboniferous shallow, transgressive

M i d d l e - U p p e r Triassic, 50-100 m Jurassic Jurassic several 100's to 1000 m Middle Triassic

100 m

Upper Triassic, Jurassic

below A C D

292

limestones show only a few palaeontological and sedimentological features which can be attributed to a specific depositional environment; (2)especially in the Palaeozoic, limited outcrops and tectonic shortening prevent reliable palaeogeographic reconstructions of depositional areas; (3) diagenetic changes, such as pressure solution, have strongly altered if not obscured many original depositional features; and (4) modern environments comparable to those in which condensed cephalopod limestones accumulated, are not known. As was pointed out above, cephalopod limestones were deposited in three major

TABLE 2 Principal sedimentologic, palaeontological and palaeogeographic/depositional Devonian/Lower

characteristics of Upper

C a r b o n i f e r o u s c e p h a l o p o d limestones (platform-facies only). Lower

Upper Devonian Tafilalt

Montagne

Rheinisches

Moravian

Holy Cross

Platform

Noire

Schiefergeb.

Karst

Mountains

Carboni ferous Cantabrian Mountains

M a i n facies b e l t s quartz sandstone

c

a

a

a

a

o

crinoidal limestone

c

r

r

?

a

o

limestone

c

r

r

r

o

a

nodular limestone

o

r

c

c

o

r

cephalopods

c

c

c

o

o

o

styliolinids

c

c

c

c

c

a

entomozoan ostracods

r

r

o

o

o

o

crinoids

c

r

r

r

r

o

brachiopods

c

r

r

r

r

r

calcareous algae

a

a

?

a

a

a

stromatolites/oncolites

r

r

a

a

a

a

endolithic algae/fungi

o

r

r

?

?

o

Spongeliomorpha b u r r o w s

c

c

o

?

?

o

sandstone/carbonates

c

o

c

c

c

o

transgressive base

o

a

~

a

a

o

condensed cephalopod

Fauna/flora

Palaeogeographic and depositional features underlying neritic

gradual transition from u n d e r l y i n g neritic facies

r

c

r

c

a

r

micritic envelopes

a

a

a

a

a

a

distinct current patterns

c

r

?

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293 environments which grade into one another: pelagic platforms, slopes+ and basins with reduced sedimentation. Depth estimates must primarily be based on the absolute depth of the shallowest depositional areas, i.e. the pelagic platforms. Several palaeontological, sedimentologic and palaeogeographic characteristics indicate a shallow water origin of these areas (Table 2). Contrary to the opinion of Wilson (1975, p. 354), red and pink encrinites can generally be interpreted as shallow water sediments, especially when the presence of crinoid holdfasts and long, articulated stem sections indicates little transport. The remaining groups of benthic organisms are generally too poor in species and individuals to clearly indicate a specific water depth+ but at least they record well-aerated bottom conditions. It seems that calcareous algae do not occur in cephalopod limestones. The identity of enigmatic algae reported from this facies (Schneider, 1970; Tucker and Kendall, 1973: Zimmerle and Krebs, 1976) has never been established. Stromatolites have been reported only from the Tafilalt Platform (Wendt et al., 1984). More abundant are small oncolites (1 5 cm diameter) which have been found in the Frasnian of the Tafilalt Platform and the F r a s n i a n / L o w e r Famennian of Montagne Noire. Their cyanophycean nature cannot be proved (some of them were described as manganese nodules by Tucker, 1973a), but attached crinoid holdfasts and Cladochonus suggest a sublitoral origin of these coated grains. The intense burrowing of many cephalopod limestones cannot be attributed to any diagnostic organism, but it is similar in form to the Spongeliomorpha type, which is most common in intertidal to shallow subtidal sediments (F~rsich, 1973; Sheehan and Schiefelbein, 1984). Orientation patterns of orthoconic nautiloids may indicate strong bottom currents or wave accumulations. Such patterns are common on the shallowest areas of the Tafilalt Platform during the late Frasnian and the early Famennian (Wendt et al., 1984). They we;e observed also in the Supragriotte of the Montagne Noire and in the Alba Formation of the Cantabrian Mountains. In Ordovician and Silurian cephalopod limestones similar patterns clearly indicate nearshore environments (Dixon, 1970; Sundquist, 1982). The most reliable indicator of the palaeobathymetric position at which cephalopod limestones accumulated, is provided by a critical evaluation of their vertical and lateral facies transitions. In the eastern Anti-Atlas the lateral transition from quartz-rich brachiopod coquinas to crinoidal limestones, condensed cephalopod limestones and finally to nodular limestones reflect a gentle deepening of the Tafilalt Platform from nearshore areas across a shallow platform into slope areas. In the other regions discussed in this paper, facies patterns have not been mapped, but the often observed gradual transition from underlying neritic platform or deltaic deposits into pelagic cephalopod limestones implies a shallow water environment for the base of the latter. Still better evidence is the transgression of cephalopod limestones over previously subaerially exposed surfaces, as is observed on the northern Tafilalt Platform and locally along the southern margin of the Cantabrian Mountains. Karstification of some G i v e t i a n / e a r l y Frasnian reefs and their subse-

294

quent submergence in the Rheinisches Schiefergebirge, however, is judged controversially (Franke, 1973; Clausen et al., 1978; Poty, 1980). To summarize, there is good evidence that the absolute depth of platforms on which cephalopod limestones accumulated, ranged from sea level to about 100 m. The absence of calcareous algae and micritic envelopes and the occasional presence of endolithic algae (or fungi?) suggest that the bulk of these sediments was deposited in the lower part of the photic zone. Assuming a slope of 2 °, sufficient for sediment gravity transport (Lewis, 1971), basins adjacent to these platforms with occasional (Tafilalt Basin) or starved (Alba Basin) cephalopod limestone deposition probably reached depths of some hundreds of metres. SEDIMENTATION RATES Plots of average sediment thickness (not corrected for compaction) against absolute time (after Harland et al., 1982) show three stages of Devonian/early Carboniferous geotectonic evolution (Fig. 23): (1) The initial stage is characterized by high rates of sedimentation (20-500 m m.y. - ~) of shallow-water carbonates and clastics, keeping pace with subsidence. It is most typically represented in the early to middle Devonian of the Rheinisches Schiefergebirge, Moravian Karst and Holy Cross Mountains. On the Tafilalt Platform and in the Montagne Noire, subsidence was already markedly decreased during this interval. In the central Cantabrian Mountains this stage continues into the early Tournaisian, while the eastern Cantabrian Mountains (not represented in Fig. 23) show a similar development to that of the Montagne Noire and the Pyrenees. (2) It has been known for a long time that the intermediate stage of cephalopod limestone deposition, locally interrupted by black shales and cherts, with minimum sedimentation rates (1-5 m m.y. ~), occurred in the time span from the early Frasnian to the middle Visean. Only in the central Cantabrian Mountains did cephalopod limestones have their greatest distribution at a slightly later interval, from the late Tournaisian to the early Namurian. The most comprehensive term for this stage of geosynclinal development is Aubouin's (1962) "p6riode de vacuit6", while "bathyal lull" (Goldring, 1962), "levelling stage" (Kullmann and Sch6nenberg, 1975) and "phase of relief equalization" (Walliser, 1980) imply bathymetric or topographic interpretations which can no longer be sustained. (3) In the individual basins between pelagic ridges and platforms, the final stage starts in the late Frasnian (not represented in Fig. 23). The onset of widespread flysch deposition (Aubouin's "p~riode de comblement") during the middle to late Visean (Namurian in the Cantabrian Mountains) marks the rapid drowning of pelagic platforms to bathyal depths. A somewhat different development is observed on the Tafilalt Platform, where cephalopod limestones were buried by a thick prodelta/delta sequence in the latest Famennian, while in eastern Algeria Upper Visean flysch covers Frasnian/Famennian cephalopod limestones after a long period of non-deposition.

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296 CONCLUSIONS

(1) Cephalopod limestones occur in many areas of the Variscan foldbelt, along the southern margin of the Laurasia Craton in Europe and along the northwestern margin of the Gondwana (Sahara) Craton in North Africa. They are most widespread during the late Devonian and early Carboniferous, but occur more sporadically during the early and middle Devonian. (2) Pelagic organisms (cephalopods, styliolinids, entomozoan ostracods, conodonts) predominate over benthic organisms (crinoids, brachiopods, gastropods. pelecypods, rugose corals, tabulates, bryozoans), but in certain nearshore environments the latter may constitute the main faunal element. (3) Cephalopod limestones were deposited on shallow platforms or ridges of varied origin (neritic-reefal, volcanic, crystalline, tectonic), carbonate or clastic shelves, platform slopes and shallow, locally starved basins. In all these cases, the presence of continental crust in the various settings is well documented. The existence of oceanic crust (Palaeo-Tethys) between the Laurasia and Gondwana Cratons can be postulated from a plate tectonic model of the Variscan foldbelt, but its existence is not yet well established on the basis of rock evidence. (4) Cephalopod limestones present a succession of facies belts which correspond to their palaeogeographic settings; these may grade into one another. Quartzose sandstones, sandy shell coquinas, crinoid-coral-bryozoan-bearing mudstones or wackestones and condensed cephalopod wackestones were deposited on shallow pelagic platforms, some of which might have been connected to land areas. Many nodular limestones can be interpreted as debris flows indicating deposition on gently inclined slopes. Similar nodular and bedded limestones with shaley partings, often interbedded with thick marly beds, were laid down in shallow, sometimes starved basins. (5) Prior to cephalopod limestone deposition, underlying neritic carbonate platforms and clastic shelves may have been subaerially exposed. After a relative rise in sea level, water depth on the platforms reached only several tens to about one hundred metres during cephalopod limestone deposition. Depth of the adjacent nonturbiditic basins was of the order of several hundreds of metres. (6) Sedimentation rates of condensed cephalopod limestones range from 1 to 5 m m.y. ~. They represent an intermediate stage in the evolution of Variscan geosynclines between the initial phase of thick shallow water carbonates and clastics and the final stage of flysch sedimentation. ACKNOWLEDGEMENTS

Field work since 1972 was conducted in part with J. Loeschke, J. Neugebauer (both Tt)bingen) and D. Schumann (Darmstadt), having been supported by the special research group (SFB 53) Ti)bingen. Examination of the Polish and

297

Czechoslovakian Upper Devonian was made possible by invitations to the senior author from the Warsaw and Ostrava Universities. J. Dvorak (Brno, CSSR), M. Szulczewski (Warsaw) and Z. Vasicek (Ostrava, CSSR) introduced J.W. to the Polish and Moravian Upper Devonian. R. Feist (Montpellier) suggested suitable locations in the Montagne Noire. The authors are especially indebted to Drs M. Bensaid and M. Dahmani (Minist6re de l'Energie et des Mines, Direction de la Gdologie, Rabat, Morocco) for issuing a working permit and permission to export samples. W. Graf, W. Ries, H. Vollmer and W. Wetzel (all Ti)bingen) gave technical assistance. R. Thomas (Lancaster, Pa., USA) critically read the manuscript. The authors are grateful to all those who assisted in this work and to the organizations which made this project possible. REFERENCES Aubouin, J., 1962. Propos sur les gdosynclinaux. Bull. Soc. G6ol. Fr., (7)3: 629-702. Babin, C., Cocks, L.R.M. and Walliser, O.H., 1980. Faci6s, faunes et paldog6ographie antd-carbonifere de l'Europe. 26 Congr. G6ol. Int., Coll., C6:191 202. Bandel, K., 1974. Deep-water limestones from the Devonian-Carboniferous of the Carnic Alps, Austria. Spec. Publ. Int. Assoc. Sedimentol., 1:93 115. Bernoulli, D., 1972. North Atlantic and Mediterranean Mesozoic facies: a comparison. Initial Reports of the Deep Sea Drilling Project, Vol. 11, U.S. Govt. Printing Office, Washington, D.C., pp. 801 871. Boschma, D. and Van Staalduinen, C.J., 1968. Mappable units of the Carboniferous in the southern Cantabrian Mountains (NW-Spain). Eeidse Geol. Meded., 43:221 232. Bourrouilh, R., 1981." Orthoceratitico-Rosso'" et "Goniatitico-Rosso": Faci6s marqueurs de la naissance et de l'6volution de pal6omarges au Paleozoique. In: A. Farinacci and S. Elmi (Editors), Proc. Rosso Ammonitico Symposium, Tecnoscienza, Roma, pp. 39 59. Boyer~ F., Krylatov, S., Ee F+vre, J. and Stoppel, D., 1968. Le Ddvonien supdrieur et la limite Ddvono-Carbonif+re en Montagne Noire (France). Lithostratigraphie-Biostratigraphie. Bull. Cent. Rech. Pau SNPA, 2 : 5 33. Boyer, F., Krylato~, S. and Stoppel, D., 1974. Sur le probl6me de l'existence d'une lacune sous les lydiennes -5 nodules phosphatds du Dinantien des Pyrdndes et de la Montagne Noire. Geol. Jahrb., Reihe B, 9: 3-60. Buchroithner, M.F., Ebner, F. and Surenian, R., 1979. Die Entwicklung der Steinbergkalke (Oberdevon, Grazer Pal~ozoikum) an ihrer Typuslokalit~,t. Mitt. Naturwiss. Vet. Steiermark. 109: 71-84. Buggisch, W., 1972. Zur Geologie und Geochemie der Kellwasserkalke und ihrer begleitenden Sedimente (Unteres Oberdevon). Abh. Hess. Landesamtes Bodenforsch., 62:1 67. Buggisch, W. and Clausen, C.D., 1972. Conodonten- und Goniatiten-Faunen aus dem oberen Frasnium und unteren Famennium Marokkos (Tafilalt, Antiatlas). Neues Jahrb. Geol. Palaeontol. Abh., 141: 137-167. Cavet, P., 1959. La Pal6ozoique de la zone axiale des Pyrdnees orientales franqaises entre le Roussillon et I'Andorre (6tude stratigraphique et paldontologique). Bull. Serv. Carte Geol. Fr., 55:303 518. Chlupac, I., 1964. Fortschritte in der Stratigraphie des M~hrischen (ostsudetischen) Devons. Geol. Rundsch., 54:1003 1025. Clausen, C.D., Grebe, H, Eeuteritz, K. and Wirth, W., 1978. Zur Altersstel[ung und pal~ogeographischen Bedeutung des Palgokarstes auf der Warsteiner Carbonatplattform. Neues Jahrb. Geol. Palaeontol. Monatsh., 1978:577 589. Dixon, O.A., 1970. Nautiloids and current ripples as paleocurrent indicators in Upper Ordovician limestones, Anticosti Island~ Canada. J. Sediment. Petrol., 40: 682-687.

298 Dreesen, R., 1982. Storm-generated oolitic ironstones of the Famennian ( F a l b - F a 2 a ) in the Vesdre and Dinant synclinoria (Upper Devonian, Belgium). Ann. Soc. G6ol. Belg,, 105: 105-129. Dvorak, J., 1973. Synsedimentary tectonics of the Palaeozoic of the Drahany Upland (Sudeticum. Moravia, Czechoslovakia). Tectonophysics, 17: 359-391. Engel, W., Feist, R. and Franke, W., 1981. Le Carbonif6re ant6-st6phanien de la Montagne Noire: rapports entre mise en place des nappes et s6dimentation. Bull. Bur. Rech. G6ol. Minieres, (2) sect. 1, 4: 341-389. Engel, W., Franke, W. and Langenstrassen, F., 1983. Palaeozoic sedimentation in the northern branch of the mid-European Variscides--essay of an interpretation. In: H. Martin and F.W. Eder (Editors), lntracontinental Fold Belts. Springer Berlin, pp. 9-41. Franke, W., 1973. Fazies, Bau und Entwicklungsgeschichte des lberger Rifles (Mitteldevon bis Unterkarbon lII, NW-Harz, W-Deutschland). Geol. Jahrb., Reihe A, 11:3 127. Franke, W. and Walliser, O.H., 1983. "Pelagic" carbonates in the Variscan Belt--their sedimentary and tectonic environments. In: H. Martin and F.W. Eder (Editors), Intracontinental Fold Belts. Springer, Berlin, pp. 77-92. Franke, W., Eder, W., Engel, W. and Langenstrassen, F., 1978. Main aspects of geosynclinal sedimentation in the Rhenohercynian zone. Z. Dtsch. Geol. Ges., 129:201 216, Frankenfeld, H., 1981. Krustenbewegungen und Faziesentwicklung im kantabrischen Gebirge (Nordspanien) vom Ende der Devonriffe (Givet/Frasne) bis zum Tournai. Clausthaler Geol. Abh., 39: 1-91.

Fi~chtbauer, H. and Richter, D.K., 1983. Carbonate internal breccias: a source of mass flows at early geosynclinal platform margins in Greece. Soc. Econ. Paleontol. Mineral., Spec. Publ., 33: 207-215. Fi~rsich, F.T., 1973. A revision of the trace fossils Spongeliomorpha, Ophiomorpha and Thalassinoides, Neues Jahrb. Geol. Palaeontol. Monatsh., 1973:719 735. Garrison, R.E. and Fischer, A.G., 1969. Deep-water limestones and radiolarites of the Alpine Jurassic. Soc. Econ. Paleontol. Mineral., Spec. Publ., 14: 20-56. Goldring, R., 1962. The bathyal lull: Upper Devonian and Lower Carboniferous sedimentation in the Variscan geosyncline. In: K. Coe (Editor), Some Aspects of the Variscan Fold Belt. Manchester Univ. Press, Manchester, pp. 75-91. Harland, W.B., Cox, A.V., Llewellyn, P.G., Pickton, C.A.G., Smith, A.G. and Walters, R., 1982. A Geologic Time Scale. Cambridge Univ. Press, Cambridge, 131 pp. Higgins, A.C., 1971. Conodont biostratigraphy of the late Devonian-early Carboniferous rocks of the south central Cantabrian Cordillera. Trab. Geol., 3: 179-192. Higgins, A.C. and Wagner-Gentis, C.H.T., 1982. Conodonts, goniatites and biostratigraphy of the earlier Carboniferous from the Cantabrian Mountains, Spain. Palaeontology, 25: 313-350. Hollard, H., 1967. Le D6vonien du Maroc et du Sahara nord-occidental. In: D.H. Oswald (Editor), International Symposium on the Devonian System. Alberta Soc. Pet. Geol., Calgary, Aha., 1: 203-244. Hollard, H., 1974. Recherches sur la stratigraphie des formations du D6vonien moyen, de l'Emsien sup6rieur au Frasnien, dans le Sud du Tafilah et dans le Mader (Anti-Atlas oriental). Notes et Mdmoires Serv. G6ol. Maroc, 264: 7-68. Hollard, H., 1981. Principaux caract6res des formations d6voniennes de rAnti-Atlas. Notes M6m. Serv. Geol. Maroc, 308: 15-21. Jaeger, J.L., Meunier, A. and Ovtracht, A., 1958. G6ologie du secteur mangan6sif6re du Minervois. Bull. Soc. Geol. Fr., (6) 8: 267-279. Jenkyns, H.C., 1974. Origin of red nodular limestones (Ammonitico Rosso, Knollenkalke) in the Mediterranean Jurassic: a diagenetic model. Spec. Publ. Int. Assoc. Sedimentol., 1: 249-271. Jenkyns, H.C. and Torrens, H.S., 1971. Palaeogeographic evolution of Jurassic seamounts in western Sicily. Ann. Inst. Geol. Publ. Hung., 54 (2): 91-104.

299

Koopmans, B.N., 1962. The sedimentary and structural history of the Valsurvio Dome, Cantabrian Mountains, Spain. Leidse Geol. Meded., 26: 121-232. Kullmann, J. and Sch6nenberg, R., 1975. Geodynamische und pal~.6kologische Entwicklung im Kantabrischen Variszikum (Nordspanien). Ein interdisziplin~es Arbeitskonzept. Neues Jahrb. Geol. Palaeont. Monatsh., 1975: 151-166. Legrand, P., 1967. Le Devonien du Sahara algerien. In: D.H. Oswald (Editor), International Symposium on the Devonian System. Alberta Soc. Pet. Geol., Calgary, Alta., 1 : 245 284. Lewis, K.B., 1971. Slumping on a continental slope inclined at 1 ° - 4 °. Sedimentology, 16: 97-110. Maas, K., 1974. The geology of Liebana, Cantabrian Mountains, Spain; deposition and deformation in a flysch area. Leidse Geol. Meded., 4 9 : 3 7 9 465. Margat, J., Destombes, J. and Hollard, H., 1962. Mdmoire explicatif de la carte hydrogeologique au 1/50000 de la plaine du Tafilalt. Notes Mdm. Serv. Geol. Maroc, 150bis: 1 242. Massa, D., 1965. Observations sur les series siluro-devoniennes des confins algdro-marocains du Sud (1954-1955). Notes M6m. Co. Fr. Pet., 8:1 187. Maurel~ M., 1966, Etudes gdologiques sur le Ddvonien et le Carbonif6re infdrieur du versant meridional de la Montagne Noire. Th6se Fac. Sci. Univ. Montpellier, 194 pp. Mirouse, R. 1967. Le D6vonien des Pyren6es occidentales et centrales (France). In: D.H. Oswald (Editor), International Symposium on the Devonian System. Alberta Soc. Pet. Geol., Calgary, Alta., 1: 153-170. Neumann, N. and Schumanm D., 1974. Zu Fossilerhaltung, besonders der Goniatiten, in roten Knollenkalken vom "Ammonitico Rosso"-Typ. Neues Jahrb. Geol. Palaeont. Monatsh., 1974: 294-314. Ovtracht, A. and Fournie, L., 1956. Signification pal6og~ographique des griottes d~voniennes de la France m6ridionale. Bull. Soc. G6ol. Fr., (6) 6: 71-80. Perseil, E.A., 1969. Caract6res min6ralogiques de quelques types de gisements mangan6sif6res de la France meridionale. Bull. Soc. Geol. Fr., 7 (10): 408-412. Poty, E., 1980. Evolution and drowning of paleokarst in Frasnian carbonates at Vis6, Belgium. Meded. Rijks Geol. Dienst, 32-7:53 55. Raven, J.G.M., 1983. Conodont biostratigraphy and depositional history of the Middle Dewmian to Lower Carboniferous in the Cantabrian zone (Cantabrian Mountains, Spain). Leidse Geol. Meded., 52:265 339. Reuther, C.D., 1977. Das N a m u r im s~dlichen Kantabrischen Gebirge (Nordspanien). Krustenbewegungen und Faziesdifferenzierung im Ubergang Geosynklinale Orogen. Clausthaler Geol. Abh., 28: 1-122. Sanchez de la Torte, L., Agueda Villar, J.A., Colmenero Navarro, J.R., Garcia-Ramos, J.C. and Gonzales Lastra, J., 1983. Evolucion sedimentaria y paleogeografica del Carbonifero en la Cordillera Cantabrica. 10 Congr. Int. Estratigrafia y Geologia del Carbonifero, Madrid, pp. 133-150. Schlager, W., 1974. Preservation of cephalopod skeletons and carbonate dissolution on ancient Tethyan sea floors. Spec. Publ. Int. Assoc. Sedimentol., 1 : 4 9 70. Schneider, J., 1970. Foraminiferen als Epibionten auf Conodonten aus dem Ober-Devon des Kellerwalds (Rheinisches Schiefergebirge) und des Harzes. GOttinger Arb. Geol. Palaeontol., 5 : 8 9 98. Sheehan, P.M. and Schiefelbein, D.R.J., 1984. The trace fossil Thalassinoides from the Upper Ordovician of the eastern Great Basin: deep burrowing in the early Paleozoic. J. Paleontol., 58: 440-447. Sundquist, B., 1982. Wackestone petrography and bipolar orientation of cephalopods as indicators of littoral sedimentation in the Ludlovian of Gotland. Geol. Foren. Stockh. F o r h , 104: 81-90. Szulczewski, M., 1968. Slump structures and turbidites in Upper Devonian limestones of the Holy Cross Mrs. Acta Geol. Pol., 18: 303-324. Szulczewski, M., 1971. Upper Devonian conodonts, stratigraphy and facial development in the Holy Cross Mrs. Acta Geol. Pol., 21:1 129. SzulczewskL M., 1973. Famennian Tournaisian neptunian dykes and their conodont fauna from Dalnia in the Holy Cross Mrs. Acta Geol. Pol., 23: 15-59.

300 Tucker, M.E. 1973a. Ferromanganese nodules from the Devonian of the Montagne Noire (S. France) and West Germany. Geol. Rundsch.. 62:137 153. Tucker, M.E., 1973b. Sedimentology and diagenesis of Devonian pelagic limestones (Cephalopodenkalk) and associated sediments of the Rhenohercynian Geosyncline, West Germany. Neues Jahrb. Geol. Palaeontol. Abh., 142: 320-350. Tucker, M.E., 1974. Sedimentology of Palaeozoic pelagic limestones: the Devonian Griotte (Southern France) and Cephalopodenkalk (Germany). Spec. Publ. Int. Assoc. Sedimentol., 1 : 71-92. Tucker, M.E. and Kendall, A.C., 1973. The diagenesis and low-grade metamorphism of Devonian styliolinid-rich pelagic carbonates from West Germany: possible analogues of Recent pteropod oozes. J. Sediment. Petrol., 43:672 687. Van Adrichem Boogaert, H.A., 1967. Devonian and Lower Carboniferous conodonts of the Cantabrian Mountains (Spain) and their stratigraphic application. Ceidse Geol. Meded., 39: 129-192. Wagner, R.H., Winkler Prins, C.F. and Riding, R.E., 1971. Lithostratigraphic units of the lower part of the Carboniferous in northern Leon, Spain (with a note on some goniatite faunas by C.H.T. Wagner-Gentis). Trab. Geol., 4: 603-663. Walliser, O.H., 1980. The geosynclinal development of the Variscides with special regard to the Rhenohercynian zone. In: H. Closs, K. von Gehlen, H. lilies, E. Kuntz, J. N e u m a n n and E. Seibo]d (Editors), Mobile Earth. Boldt, Boppard, pp. 185-195. Wendt, J., 1970. Stratigraphische Kondensation in triadischen und jurassischen Cephalopodenkalken der Tethys. Neues Jahrb. Geol. Palaeontol. Monatsh., 1970: 433-448. Wendt, J., 1973. Cephalopod accumulations in the Middle Triassic Hallstatt-Limestone of Jugoslavia and Greece. Neues Jahrb. Geol. Palaeontol. Monatsh., 1973:624 640. Wendt, J., Aigner, T. and Neugebauer, J., 1984. Cephalopod limestone deposition on a shallow pelagic ridge: the Tafilah Platform (upper Devonian, eastern Anti-Atlas, Morocco). Sedimentology, 31: 601 625. Wilson, J.L., 1975. Carbonate Facies in Geologic History. Springer, Berlin, 471 pp. Zimmerle, W. and Krebs, W., 1976. Die obere Tonstein- und Kalkfolge des Oberdevon und Unterkarbon. Geol. Jahrb., Reihe A, 27: 123-136. Zukalova, A. and Chlupac, 1., 1982. Stratigrafick/a klasifikace nemetamorfovan6ho devonu moravskoslezsk6 oblasti. Cas. Mineral. Geol., 27: 225-241.