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Exceptionally well-preserved fossil cryptic communities have been discovered in submarine cavities of the Lower and Middle Devonian (Emsian to Givetian) mud mounds in the Hamar Laghdad area (Anti-Atlas, Morocco). The cryptic biota encrusted the roofs of the cavities and grew predominantly oriented upside down. These unique coelobiotic communities were dominated by various solitary rugose corals, which very often displayed a “calice-in-calice” growth pattern. Apart from the rugosans, the cavities were inhabited by other sessile invertebrates: tabulate corals, cladochonids, crinoids and lithistid sponges. Many of the skeletons were encrusted by microbial structures. The high density of organisms overgrowing each other points to intensive competition for space in all studied Devonian submarine crypts. The late Emsian to latest Givetian cavities were colonised by various sets of coelobiotic inhabitants mostly originating from the surrounding deep-water marine environments. It is suggested that the local species pool was a decisive factor in determining the ecological succession and taxonomic structure of cryptic communities. The changes in taxonomic composition and diversity, over time and space, were primarily related to variations in local physical environment. Consequently, the communities in crypts affected by venting of high-temperature fluids or hydrocarbon seepage were of low diversity and included endemic elements and characteristically small specimens.

Middle Devonian (Eifelian/Givetian transition) brachiopod-hosted sclerobiont assemblage from Gondwana (Morocco, eastern Anti-Atlas, Mader Basin) has been studied for the first time. The analysis of hundreds of brachiopod shells revealed at least 26 sclerobiont taxa, making the studied palaeoecommunity one of the most diverse Middle Devonian sclerobiont assemblage reported to date. The palaeocommunity is dominated by encrusting organisms, in particular bryozoans, foraminifers, ascodictyids and microconchids, whereas bioeroders are represented by Clionolithes and Oichnus ichnospecies. As supported by microfacies, fossil preservation and the presence of encrusting Rothpletzella algae, the sclerobionts developed in a calm, euphotic, offshore marine environment situated below normal wave base. Prevalence of articulated shells without signs of either extensive mineral staining, or taphonomic alteration resulting from hydrodynamic processes, as well as preservation of some delicate encrusters (Vinella, Rothpletzella), indicate that the palaeocommunity did not undergo extensive time-averaging. The hosts were colonized syn vivo, as evidenced by the epibiont growth near the commissure margin, the presence of shell malformations and even embedded auloporid corals and cornulitid tubeworms. Competition for space among sclerobionts was apparently limited, as suggested by the very low number of overgrowth interactions, indicating that the shelly substrate available for colonization was abundant. The presence of the same dominant groups of sclerobionts on different host taxa indicates a lack of species-selectivity by these colonizing sclerobionts. It also suggests that the sclerobiont species pool in the environment was generally similar throughout the deposition of the sampled rock interval. As testified by the host brachiopod and bryozoan species, the sclerobiont assemblage from this locality of northern Gondwana had strong affinities with those from the southern margin of Laurussia, most likely reflecting strong connectivity between these regions provided by the South Equatorial – North Gondwana current system during the late Eifelian Kaˇc´ak transgressive episode.

International audience; Spectacular Early Devonian carbonate buildups exposed in the eastern Anti-Atlas of southern Morocco and widely known as the Kess-Kess mounds constitute a classical example of deep-water mud mounds interpreted to be related to hydrothermal venting. They developed on the Hamar Laghdad elevation, located near Erfoud, created by a submarine volcanic eruption and composed of peperites and tuffs displaying geochemical characteristics of typical intra-plate basalts. During late Emsian time, reactivation of magmatic processes caused doming of the volcanic complex and the overlying sedimentary strata, and in consequence, a network of radial and tangential faults originated. Most of the mounds developed over the cross-points of these faults. One of the characteristic features of the Kess-Kess mounds is the presence of small caves and rock shelters. They represent fragments of larger fissures, originally open and connected to the seafloor through vent outlets. This system has formed simultaneously with the formation of mounds or immediately after. The irregular and/or pipe-shaped geometry of fissures points to chemical corrosion processes rather than to a tectonic mechanism for their origin. Dissolution of carbonates appears to be in contradiction with enhanced carbonate production, characteristic for the rapid mound growth, but it can easily be explained by mixing of two different carbonate solutions, hydrothermal fluids and seawater. It is suggested that during the formation of the Kess-Kess mounds dissolution took place in zones dominated by hydrothermal fluid-rich mixtures, and precipitation progressed with dilution of the vent fluids by seawater.

Neodymium and strontium isotope compositions of clastic material in Lower Paleozoic rocks, in addition to detrital zircon and mica geochronology and faunal data are used to constrain the geotectonic provenance of the Malopolska and Łysogory terranes in southern Poland. Being characterized by markedly different geological histories during Early Paleozoic times, Malopolska and Łysogory were most often interpreted as exotic terranes, but their identity, provenance and accretionary history are still being discussed. This study presents the first systematic Nd and Sr isotope survey of the Lower Paleozoic siliciclastic rocks. Cambrian clastic rocks of Malopolska differ in their Nd and Sr isotope compositions from those of Łysogory. The former are characterized by a wide range of the eNd(505 Ma) values, from − 11.7 to − 5.7, Sr isotope ratios from 0.712 to 0.767, and model ages (T2DM) predominantly younger than 1.9 Ga, whereas the eNd(t) values of the latter vary within a fairly narrow range of the eNd(505 Ma), from − 11.0 to − 8.3, Sr isotope ratios from 0.722 to 0.754, and model ages are older than 1.9 Ga. New Nd data combined with detrital zircon ages provide evidence that Malopolska must have been supplied with clastic material from Amazonian sources during Cambrian times, being also progressively sourced from the Baltic Svecofennian crust in the Middle Cambrian. Detritus recognized in Łysogory has mostly been derived from the Svecofennian basement of Fennoscandia, but it has also a small addition from Cadomian sources. All provenance and biogeographic data suggest that Malopolska was situated close to the Amazonian Craton near the western end of the Cadomian Belt during the Early Cambrian, and it was not until the late Middle Cambrian that Malopolska collided with the Baltica margin. During the Late Cambrian Łysogory occupied a position between Baltica and the Cadomian Belt but it was probably not yet accreted to the Baltic crust at that time.

Analyses of the Nd isotope composition of conodonts from the Anti-Atlas (Morocco), the Montagne Noire (France) and the Rhenohercynian domain (Germany) provide insight into the temporal and lateral variations in seawater geochemistry in the western part of the Variscan Sea during the Late Devonian. Most of the isotopic excursions accurately record changes in sea level. The eNd values decreased during regression phases when erosion accelerated supply of unradiogenic Nd from old continental sources. A rise in sea level generated a positive shift in eNd values due to input of more radiogenic oceanic water into the seawater reservoir of the shelf areas. The method has great potential for sea-level research, paleoceanography and correlation because sea-level changes can be precisely recognized even in successions with uniform, monotonous lithologies. However, it has a significant limitation being valid only in epeiric seas and shelf areas. We have used the Nd isotope record to reconstruct the eustatic sea-level changes in late Frasnian–early Famennian time. The onset of the organic-rich Kellwasser units coincides with negative trends in eNd values. This accords with oxygen deficiency at the sea bottom being initiated by short-term regressive events, which in turn would be consistent with cooling periods of glacial origin, as previous authors have suggested.

A fauna of bivalve molluscs is described from methane seep carbonates of the Middle Devonian (c. 390 Ma) Hollard Mound in the eastern Anti-Atlas, Morocco. We describe a new modiomorphid genus Ataviaconcha gen. nov. with the type species Ataviaconcha wendti sp. nov. This is a very large, semi-infaunal species occurring in large colonies similar to those formed by Recent chemosymbiotic cold-seep and hydrothermal vent bivalves. It is the second modiomorphid bivalve known from Palaeozoic chemosynthesis-based ecosystems, after the roughly coeval Sibaya ivanovi Little, Maslennikov, Morris & Gubanov, 1999, from the Sibay hydrothermal vent deposit in the Ural Mountains, Russia. The second and much less numerous bivalve species described in this paper is the solemyid Dystactella? eisenmanni sp. nov., belonging to a genus known also from Ordovician to Devonian marine environments distinct from cold seeps and hydrothermal vents. As with other fossil and Recent solemyids, it was an infaunal burrower, most likely livi...

The uppermost Eifelian Kostyukovichi Formation corresponds to a distinct marine transgressive-regressive cycle in predominantly continental to restricted-marine Middle Devonian succession of the intracratonic Belarusian Basin in a near-equatorial area of SE Euramerica. The presence of the conodont ensensis Zone points to the correlation with the global Kacak Episode whose onset coincided with the eustatic If transgression. The comprehensive study of the Kostyukovichi Fm. in 18 boreholes included sedimentological, palaeontological (conodonts, palynoflora, vertebrates) and geochemical analyses (C, O and Nd isotopes in carbonates and bioapatite). The results demonstrate that the sedimentation was characterised by intermittent hypoxic conditions and salinity fluctuations; it was controlled by increasing terrigenous and freshwater influx from the northern Scandinavian Land with carbonate sedimentation limited mostly to the early stage and southern part of the basin. The palynological study and occurrence of phytoclasts document terrestrial plant expansion, while the abundance of diverse fish taxa attests to the proliferation of vertebrates. These bioevents are attributed to transgression, climate humidization, and expansion of aquatic environments in surrounding land areas. Regional review suggests that such attributes of the Belarusian Kacak Episode as the initial If transgression, marine hypoxia, land plants blooms, carbonate decline and terminal regression, are characteristic also for other palaeogeographic settings of the episode worldwide. The Kacak Episode is here explained as a result of complex interrelations of various terrestrial and marine processes and events initiated by a global warming resulting in eustatic transgression and climate humidization. It is proposed that the important component of the terrestrial-marine teleconnections was the late Eifelian plant expansion presumably connected with the proliferation of progymnosperm arborescent plants with deepened root systems. It contributed to transiently increased denudation rates and, indirectly, to the development of marine hypoxia. The episode was terminated by the low-amplitude regression related to the enhanced CO2 sequestration and resulting climatic cooling.

One of the most striking features of modern chemosynthesis-based ecosystems surrounding methane seeps is the presence of abundant chemosymbiotic bivalves. However, such accumulations have rarely been reported from Palaeozoic to mid-Mesozoic seeps, and it is widely thought that general trends in the evolution of chemosynthetic communities paralleled those typifying most marine environments, with the bivalve prevalence starting in the Mesozoic and with Palaeozoic seeps being dominated by brachiopods. Here, we report a discovery of bivalve clusters in the oldest-known methane seep that hosted metazoan fauna, dated to the late Silurian. We identify the bivalves, externally very similar to modern chemosymbiotic forms, as members of the extinct family Modiomorphidae, known previously from a younger, Devonian seep. The bivalves inhabited the seep at a stage of increased fluid flow, when they co-occurred with atrypid brachiopods, and display a set of morphological characteristics suggesting a seep-obligate lifestyle. We conclude that bivalves colonised chemosynthesis-based ecosystems at least as early as brachiopods and apparently first developed specialized lineages able to thrive in seep-related habitats for a prolonged period of time. Rather than being simple ecological successors of brachiopods, rich bivalve communities represent an ancient and recurring theme in the evolution of chemosynthetic assemblages.

An unusual and exceptionally well preserved cryptic community of cnidarians, crinoids, sponges, and microbes developed in a submarine cavity of Middle Devonian age in the Hamar Laghdad area (Morocco). The biota encrusted the cavity roof and grew predominantly in an upside-down position, forming spectacular “hanging gardens.” The investigated assemblage differs strikingly from both its Paleozoic and modern analogues; it constitutes one of a very few known examples of fossil cryptic assemblages developed in relatively deep water settings, and is the first report of a cryptic paleoecosystem dominated by rugose corals. The results support the view that during the middle Paleozoic there was no distinct polarization between open-surface and cryptic faunas in deep-water environments, but keen competition for space already existed in Devonian cryptic assemblages. The regional species pool seems to have been the main determinant of the ecological succession and structure of this cryptic community.

Microbially induced Frutexites microstromatolites developed on corallites of the Givetian rugose coral “Amplexus” in the sedimentary cover of a submarine volcanic intrusion in the eastern Anti-Atlas of southern Morocco. The corals lived in proximity to submarine hydrothermal fluid emissions. Frutexites form irregular shrubs encrusting external walls of corallites. The shrubs, consisting of alternations of calcitic and hematitic laminae, grew predominantly on abandoned corallites. Some Frutexites grew within the sediment, whereas others developed entirely above the seafloor, or started their accretion in water and continued to grow during burial. Three types of Frutexites encrustations have been distinguished. They look similar macroscopically, but differ significantly in their microstructure and mineralogical characteristics, resulting primarily from spatial and temporal variations in redox conditions. Both hematitic and calcitic laminae are primary features of Frutexites. The shrubs accreted as a result of mineralization of microbial biofilms under fluctuating environmental conditions, caused by changes in pH, Fe2+-supply and/or oxygenation, presumably related to discharges of reducing hydrothermal fluids. The calcitic laminae are interpreted to have formed due to activity of heterotrophic (mainly sulphate-reducing) microbes, whereas the hematitic laminae developed as a result of enhanced activity of nitrate-reducers or due to periodical passive mineralization of biofilms with iron. Cathodoluminescence data provide evidence that the nitrate and sulphate reduction preceded the Mn(IV) and Fe(III) reduction, presumably due to increased accumulation of organic matter and a high stability of the iron oxides present.

The rugose coral ‘Amplexus’ occurs frequently in the sedimentary cover of the Devonian submarine volcanic intrusion in the eastern Anti-Atlas, southern Morocco. This study elucidates the palaeoecology of ‘Amplexus’, which forms very rich and mostly monospecific, spot assemblages within the Lower and Middle Devonian carbonates. Geological and isotopic evidences show that these associations developed at hydrothermal vents and at a cold seep site. The assemblages are always spatially associated with micritic carbonate bodies, occurring within bedded, hemipelagic deposits. The ‘Amplexus’ corals preferred locations in the close proximity of submarine hydrothermal fluid seepage, but they generally avoided places with the most elevated temperatures. The corals colonised also a hydrocarbon seep, probably only in its terminal phase of development, when the fluid flow was still at least periodically active. This was only possible as a result of the corals following a calice-in-calice growth, developed due to the environmental toxicity, which facilitated selective survival of larvae that settled in the shelter of empty calices. The ‘Amplexus’ corals appear to have constituted ecological opportunists, thriving in the nutrient-rich, venting- and seepage-affected areas that were hostile for other benthic organisms. It can be suspected that the unusual, extremely simplified morphology of ‘Amplexus’ made it particularly well adapted to living in environments typified by harsh and unstable conditions.

Conodonts collected from sections near the small towns of Thong Pha Phum and Mae Sariang in the westernmost part of Thailand are used to reconstruct the neodymium (Nd) isotopic composition of seawater during the Late Devonian. The study provides the first Devonian seawater signatures recognized within the Australian shelf of northeastern Gondwana and the adjacent Paleotethys Ocean. At Thong Pha Phum site, the seawater was characterized by very low e Nd values (from − 13.1 to − 18.2) and very high Sm/Nd ratios (between 0.36 and 0.66). In contrast, the seawater at the Mae Sariang site was characterized by much more radiogenic signatures (e Nd values from − 8.7 to − 11.1) and uniform, low Sm/Nd ratios (between 0.20 and 0.23). Extremely low e Nd values recognized at Thong Pha Phum attest to a passive margin continental setting and a paleogeographic position very close to a continental area where Paleoproterozoic and Neoarchean rocks were eroded. Thus, the isotopic data provide strong evidence that during Late Devonian time the Sibumasu terrane was situated in the proximity to the Archean cratons of Western Australia, presumably adjacent to the Carnarvon intracratonic basin. Moreover, Sibumasu may not have been situated in an outboard position on the shelf, as previously suggested, but could have been directly attached to the Australian continental crust. By contrast, low and uniform Sm/Nd ratios of seawater at Mae Sariang resemble those of the Variscan and the present-day oceanic seawaters. Therefore, a pelagic setting within the Paleotethys Ocean is postulated for the Mae Sariang succession. This conclusion is also constrained by minor temporal changes in e Nd values and suggests that the Paleozoic of Mae Sariang is not part of the Sibumasu terrane but belongs to the Inthanon Zone.

An 11 m thick condensed sequence of Late Devonian limestones in northwestern Thailand exhibits faunal associations and sedimentological-/microfacies data which are indicative of a pelagic facies setting. The entire long-ranging section is completely free from clastic input. Similar successions are known worldwide in a few sections only. The Mae Sariang section is characterised by low sedimentation rates as recognised by a number of hardgrounds, neptunian dikes and Fe/Mn crusts. The succession comprises a number of pelagic faunal elements e.g. conodonts, cephalopods and pelagic ostracodes. The fauna records rare macrofossils and the faunal diversity is low. The very condensed section ranges from the Late rhenana to praesulcata conodont biozones, but contains some global events as undoubtedly shown by biostratigraphical and carbon-isotope results (including major Kellwasser and Hangenberg biotic crises). In terms of plate tectonics this important succession most probably belongs to the Inthanon Zone comprising remnants of the Paleotethys Ocean.

A monospecific assemblage of myodocopid ostracods has been found in the Middle Devonian of a unique hydrothermal vent system of Hamar Laghdad (eastern Anti-Atlas, southern Morocco), which developed over a submarine eruption of basaltic rocks. This fauna has been recovered from a neptunian dyke representing the sediment infill of an Eifelian venting fissure discovered in the central part of Hamar Laghdad. The assemblage consists of thousands of relatively large shells forming a coquina layer in which the valves are disarticulated, perfectly washed, well sorted and stacked into each other. Their orientation points to transport due to fluids migrating in the vent system. The ostracod specimens, on the base of the general outline, the presence of rostrum and rostral incisure is assigned to the order Myodocopida. The new genus and species Hamaroconcha kornickeri is proposed, most likely representing a new family. Its carapace morphology gives no clear clue to its mode of life; however, a relatively long rostral incisure may suggest that frontal appendages of this species were sufficient for active swimming. Therefore, a nectobenthic lifestyle is postulated for H. kornickeri nov. gen., nov. sp. Isotope data from the sediment infill provide evidence for the activity of hydrothermal fluids.

The Illizi Basin is the easternmost one of a series of cratonic basins of the Algerian Sahara which extend along the northern margins of the Precambrian Reguibat and Hoggar massifs. Devonian rocks crop out along the erosional southern border of the basin and can be roughly divided into three major lithostratigraphic units: (1) Lower Devonian sandstones (upper part of the Tassilis externes), (2) Middle Devonian carbonates (Illizi Formation, introduced in this article) and (3) Upper Devonian shales (Tin Meras Formation). In the present study emphasis was laid on the Middle Devonian carbonates which were dated by conodonts and correlated in 12 sections over a W–E distance of about 350 km. The sequence shows a lateral transition from condensed series in the west, deposited on a submarine high (Amguid Ridge), into a shallow basin (Illizi Basin) which was additionally differentiated into minor ridges and sub-basins. The eastern portion of the basin is characterized by an enhanced clastic input from the Tihemboka Ridge which separates the Illizi Basin from the Murzuq Basin in western Libya. The differential subsidence during the Devonian shows that the Illizi Basin is primarily a depositional structure which subsequently became more accentuated after the Variscan orogenesis at the Carboniferous/Permian transition.

An up to 3,000-m-thick pile of Carboniferous rocks covers the northern fringe of the Precambrian Hoggar Massif (Touareg Shield) in southern Algeria, thus terminating the depositional history of the Palaeozoic in this sector of the North African Craton. The previous Devonian (Eifelian to Frasnian) palaeogeographic configuration of the area, characterized by ridges and shallow basins, is leveled by a widespread Famennian playa and lower Tournaisian delta sedimentation on a largely undifferentiated shelf. Tournaisian to Moscovian strata were deposited under open-marine, deltaic, shallow-subtidal, fluvial, and continental environments. The Carboniferous sequence can be subdivided into 12, largely interfingering, lithostratigraphic formations, which were dated by conodonts, ammonoids, foraminifers, and brachiopods, yielding a modified biostratigraphic framework of the area. The formations are stacked in four transgressive–regressive cycles, which include two major gaps, one during the middle Tournaisian, the other during the middle Visean to Serpukhovian. The oscillations of sea level can be traced into neighboring areas and seem coeval to early pulses of the Late Carboniferous/Early Permian glaciation of Gondwana. To a lesser degree they may reflect more local tectonic effects of the Variscan orogeny.

Large colonies of rugose coral Scruttonia kunthi occurring in the upper Famennian of Sudetes (southern Poland) reveal distinct growth banding in their skeletons. They were investigated for internal structural characteristics and stable isotopic composition. The skeletal tissue consists of alternating light and dark bands which differ in thickness, density and morphology of structural elements, and in occurrence of corallite contraction and rejuvenescense. Darker parts with densely arranged thick skeletal elements are thin in comparison to lighter parts. In addition, they include frequently offsets and contraction of corallites. A couplet of dense and less dense bands is interpreted to represent most probably an annual cycle. The calculated growth rate for Scruttonia kunthi varied from 6 mm/yr to 12 mm/yr. Growth-band formation was influenced environmentally. Oxygen isotopic data provide an evidence that high-density bands were formed in the season of higher environmental stress, with relatively warmer temperatures and higher sedimentation rates. Carbon isotopic signatures are very uniform, and thus enigmatic. They indicate that at least growth rate of the skeleton and seawater temperature had no influence on the coral δ 13 C.

Neodymium isotope data obtained from conodonts collected in Morocco and Poland attest a geochemical decoupling of seawater in the Variscan realm during the Late Devonian. The water masses on the shelves were characterized by unradiogenic e Nd values, from about − 7 to − 12. The surface water in the bordering oceans (the Rheic Ocean and the Variscan Sea) yielded more radiogenic e Nd values, ranging from about − 1 to − 6. A compilation of Nd and C isotope signatures from the Moroccan Meseta suggests that the shelf and the surface oceanic water differed also in their carbon isotope evolution. The geochemical decoupling constrains a restricted water exchange between shelves and the ocean. Oceanic waters entered far into the shelf areas only during the semichatovae transgression in the Frasnian Zone 11. Sea-level fluctuations constituted the major force that modified circulation, and thus controlled the mixing of shelf and surface oceanic seawater. The Nd conodont data reveal that the surface water of the Variscan oceanic water resembled the seawater of the modern Pacific Ocean rather than those of the Atlantic and Indian oceans. The geochemical character of the Variscan oceanic water is consistent with the geotectonic regime of the Variscan realm in Late Devonian and Carboniferous times.

The Ahnet and Mouydir regions of southern Algeria are part of one of the world's largest, almost undeformed exposures of Palaeozoic rocks which exemplify a hitherto poorly known early Variscan development of a Devonian basin and ridge system. This area includes a series of intracratonic basins along the northern margin of the West African Craton which consists (from W to E) of the Reggane Basin, Azel Matti Ridge, Ahnet Basin, Foum Belrem Ridge and Mouydir Basin. The depositional and palaeogeographic interpretation is based on 71 sections in this region, which for the first time were biostratigraphically calibrated by means of conodonts, goniatites and brachiopods. The structural evolution during Devonian times was probably controlled by reactivation of ancient N–S- to NW–SE-running faults in the Precambrian basement, which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf. A hiatus during Emsian times indicates widespread emergence during this interval. The entire area was flooded during the earliest Eifelian, when the first vestiges of the Azel Matti Ridge become evident by stratigraphic condensation. The palaeogeographic differentiation is most apparent during the Givetian, when a shoal with reduced carbonate sedimentation was established on the Azel Matti Ridge passing towards the west and east into basinal environments of the Reggane and Ahnet basins, respectively. The Foum Belrem Ridge is distinguished by increased subsidence during the early Givetian and by revived uplift during the late Givetian. In the Mouydir Basin further east, up to 1000 m of shales were deposited during the Givetian. The early Frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites. These lithologies occur repeatedly already during the Middle Devonian and document intermittent anoxic conditions. The basin and ridge topography is levelled by the shallowing-up sequence of up to 1400 m thick upper Frasnian and Famennian shales which grade into a deltaic sequence of uppermost Famennian/Tournaisian sandstones. The up to now only vaguely discriminated lithostratigraphic formations of the Devonian have been biostratigraphically defined in suitable type sections.

Anatomically preserved plant fragments are reported from Devonian marine deposits exposed in the Dra Valley of southern Anti-Atlas, Morocco. Associated conodont and tentaculite faunas indicate that the sediments yielding plants, which consist of black shales with intercalated calcareous concretions, are early Frasnian in age and most probably represent Zone 2 of the conodont zonation. This is the first record of Frasnian plants in North Africa. The specimens found all correspond to decorticated portions of axes. Six are referable to Callixylon, the organ genus corresponding to anatomically preserved axes of the progymnosperm tree Archaeopteris. Based on wood characters, especially ray structure, they are assigned to the species C. henkei, formerly described from the Famennian of Europe. One single specimen is compared to Xenocladia, a cladoxylopsid genus previously known from the Middle Devonian of Europe, USA and Kazakhstan. Interestingly, Archaeopteridales and Cladoxylopsida are two groups that dominate the younger plant assemblages of Famennian age recently described from the eastern Anti-Atlas. Callixylon henkei-type axes occur both in the Frasnian and in the Famennian deposits of the Anti-Atlas and they are all devoid of growth rings. These results are in accordance with a close position of Gondwana and Euramerica during Late Devonian times.

A co-occurrence of the clymeniid Postclymenia evoluta Schmidt, 1924 and the goniatite Acutimitoceras hilarum Korn, 2002 is reported from the Anti-Atlas of Morocco. Both species occur in the same limestone horizon within the Acutimitoceras prorsum Zone, that has yielded an exclusive conodont fauna of the Upper praesulcata Zone (latest Devonian, above the Hangenberg Black Shale). This record is firm evidence that some clymeniids survived the global Hangenberg Event, but soon later became extinct without descendants.

Anoxic conditions were widespread in NW Africa during the early Frasnian (Late Devonian) that resulted in deposition of organic-rich shales and limestones with total organic carbon (TOC) values of up to 14%. Organic richness and thickness of these sediments vary laterally, and organic-rich vs. organic-poor facies boundaries are likely to have been diachronous. A precise temporal–spatial reconstruction of this anoxic phase in NW Africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts. This contribution is based on eight outcrop sections at the margin of the central Algerian Ahnet Basin, where detailed spectral gammaray measurements were carried out using a handheld instrument. The pre-weathering organic richness in Frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that, based on well studies, is positively correlated with the total organic carbon content. Conodont biostratigraphic results from these sections suggest that the uraniumenriched interval (the anoxic interval) at the basin margin is most common in the basal Frasnian conodont Biozones 1–2, confirming previous results from the Anti Atlas in Morocco. In three of the eight localities studied the basal Frasnian has not been deposited and the Frasnian here commences with distinctly younger uranium-enriched intervals, including Zones 4–11. Well data from the eastern Algerian Berkine Basin is interpreted to indicate a significantly longer anoxic phase there. Million-yearscale diachroneities of the Frasnian anoxia, therefore, clearly exist across the North Africa shelf. It is assumed that the palaeorelief might have been a major factor in controlling the onset, duration, and intensity of anoxia in the region. D 2003 Elsevier B.V. All rights reserved.

Nd and Sr isotope data were used to characterize the sources of the Upper Triassic (Keuper) siliciclastic rocks of Silesia in southern Poland. This continental succession, consisting predominantly of fine-grained mud- stones and siltstones, yields a remarkably uniform Nd isotopic composition. Nd model ages T 2DM vary from 1.56 to 1.69 Ga and e Nd values are in the range from –8.9 to –11.2, documenting old crust contribution in the provenance. In contrast, the Sr isotopic composition ( 87 Sr/ 86 Sr) of the clastics exhibits a relatively large variation from 0.710 to 0.723. The isotopic compositions indicate that the southern part of the Germanic Basin in Silesia was supplied with clastic material from the Bohemian Massif. The axis of the drainage area must have crossed from SW to NE the Saxothuringian units of the East Sudetes and most probably also the area of the Tepla–Barrandian Unit. There is no indication of any sediment transport from the Moravo-Silesian Belt and the Fore-Sudetic Block. It seems, that the Palaeozoic rocks of the latter domain must have been buried completely during Late Triassic times.

The Devonian reef limestone complex of Rösenbeck near Brilon (Rhenish Mountains) shows numerous neptunian dykes and other hollows which have been filled with Carboniferous siliciclastic as well as fossil-rich carbonate sediments with ammonoids, conodonts, and chondrichthyan fish. While the shales of the infill can be interpreted as autochthonous sediments, the carbonates represent erratic blocks of sediments which were deposited in elevated areas but subsequently eroded and transported as erratic blocks into the karstic cavities. The biota of the carbonates demonstrates that two stratigraphic intervals are represented: (1) the Tournaisian–Viséan boundary interval, which can be identified by the presence of trilobites, and (2) the late Viséan (Asbian–Brigantian), proven by the presence of ammonoids and conodonts embedded in shales that probably have an early Serpukhovian age. Two carbonate facies can be distinguished in the Late Viséan blocks: (1) low-energy micritic sediments with Goniatites crenistria, which are probably equivalent to the crenistria Limestone and (2) high-energy (tempestite) sediments with numerous ammonoid specimens and in a more subordinate role other to fossil groups.

This study investigates stable isotope signatures of five species of Silurian and Devonian deep-water, ahermatypic rugose corals, providing new insights into isotopic fractionation effects exhibited by Palaeozoic rugosans, and possible role of diagenetic processes in modifying their original isotopic signals. To minimize the influence of intraskeletal cements on the observed signatures, the analysed specimens included unusual species either devoid of large intraskeletal open spaces ('button corals': Microcyclus, Palaeocyclus), or typified by particularly thick corallite walls (Calceola). The corals were collected at four localities in the Holy Cross Mountains (Poland), Mader Basin (Morocco) and on Gotland (Sweden), representing distinct diagenetic histories and different styles of diagenetic alteration. To evaluate the resistance of the corallites to diagenesis, we applied various microscopic and trace element preservation tests. Distinct differences between isotopic compositions of the least-altered and most-altered skeleton portions emphasise a critical role of material selection for geochemical studies of Palaeozoic corals. The least-altered parts of the specimens show marine or near-marine stable isotope signals and lack positive correlation between δ13C and δ18O. In terms of isotopic fractionation mechanisms, Palaeozoic rugosans must have differed considerably from modern deep-water scleractinians, typified by significant depletion in both 18O and 13C, and pronounced δ13C-δ18O co-variance. The fractionation effects exhibited by rugosans seem similar rather to the minor isotopic effects typical of modern non-scleractinian corals (octocorals and hydrocorals). The results of the present study add to growing evidence for significant differences between Scleractinia and Rugosa, and agree with recent studies indicating that calcification mechanisms developed independently in these two groups of cnidarians. Consequently, particular caution is needed in using scleractinians as analogues in isotopic studies of extinct coral lineages. Answering some of the pertinent palaeoecological questions, such as that of the possibility of photosymbiosis in Palaeozoic corals, may not be possible based on stable isotope data.

About one hundred carbonate mud mounds, covering an area of 440 km 2 in the eastern Anti-Atlas of Morocco, constitute one of the largest mound agglomerations known so far from Lower Carboniferous settings. They occur within a 4000-m-thick succession of shales with intercalated bedded limestones, sandstones, and siltstones. According to conodont and goniatite biostratigraphy, mound formation started in the early Gnathodus texanus Zone and terminated during the G. bilineatus Zone of the Visean stage. Individual mounds are a few metres to 30 m high, have base diameters of up to 300 m and are concentrated in several parallel, WNW–ESE running belts. From their lithology and facies relationships, four types of mounds can be distinguished: (1) massive crinoidal wacke- or packstones without stromatactis; (2) massive crinoidal wacke- or packstones with rare stromatactis; (3) similar to (2), but allochthonous; and (4) biodetrital (skeletal) grainstone mounds. While carbonate deposition in types (1) to (3) was probably triggered by microbial precipitation, type (4) is the result of a predominantly mechanical accumulation of skeletal debris. Biota in the four types comprise a great variety of invertebrates, among which crinoids, sponges, and bryozoans are most common. Diagenesis of the mound carbonates was dominated by recrystallization of micritic matrix and organic remains and late burial cementation. Oxygen and carbon isotope data of brachiopod and crinoid ossicles, matrix, and early marine cements plot in a large field and do not allow definite conclusions about the composition of the ambient seawater. Microbial activity and the absence or scarcity of green algae, colonial corals and coralline sponges suggest deposition of the mounds in moderate water depth close to the lower limit of the photic zone. D 2001 Elsevier Science B.V. All rights reserved.

A provenance study of detrital zircon has been carried out in Middle Cambrian sandstones from the Lysogory Unit (Holy Cross Mountains) and the East European Platform (EEP, Baltica) to investigate the docking of terranes along the Trans-European Suture Zone (TESZ) during the Paleozoic. Paleontological evidence suggests that the Lysogory Massif is a peri-Gondwanan-derived terrane which did not reach the Baltic realm before Ordovician times. For a comparison with the adjacent sequence in the EEP, across the TESZ, we have analyzed samples from borehole Okuniew IG-1 (15 km east of Warsaw), which contains Baltic faunas. The age spectrum of the detritus in the Cambrian of the Lysogory Massif (19 analyses) reflects input of sources with ca. 600 Ma, ca. 1.8^2.0 Ga, and s 2.5 Ga zircon ages. In the EEP (Okniew borehole; 19 analyses), the U^Pb data demonstrate input from sources with ca. 530^700 Ma, 1.0^1.2 Ga, 1.5 Ga, ca. 1.8^2.0 Ga, 2.2 Ga, and s 2.5 Ga zircon ages. Additionally, in this borehole the crystalline basement has been dated at 1800 ˛ 6 Ma. These new data show that Late Neoproterozoic zircon ages are not restricted to periGondwanan sources and that provenance studies across the TESZ are more complex than expected. fl 2000 Elsevier Science B.V. All rights reserved.

Mashkovia is one of the provincial conodonts which developed during late Famennian time in the cratonic regions of Russia. In this study, the taxonomy of this genus is revised, based on diagnostic characters of the Pa elements, such as the morphology of the anterior part of the platform, the ornamentation and the shape of the secondary keels. As a consequence, four species, including M. silesiensis n. sp. now discovered in Upper Silesia of southern Poland, are distinguished. The apparent absence of Mashkovia from North America, Variscan Europe, Australia and Africa cannot be simply explained by using temperature or other global climatic factors as a reason for the provincialism. Currents and/or local palaeoecologic factors were probably more important in controlling the distribution of these conodonts.

Spectacular conical Early Devonian carbonate buildups up to 55 m high that crop out in the eastern Anti-Atlas of southern Morocco are microbially mediated carbonate mud mounds that were surfaced by soft-bottom communities dominated by small tabulate corals. They formed on the Hamar Laghdad elevation, which was created by a submarine volcanic eruption, and were associated with a network of synsedimentary radial and tangential faults that originated by uplift of the intrusive laccolithic body underlying the Kess-Kess Formation. These faults served as conduits for the migration of hydrothermal fluids to the sea floor. Most mounds developed over cross-points of radial and tangential faults. Vents were episodically active until the Famennian, but extensive vent carbonate production occurred only during the Emsian. Preliminary geochemical results document that mud-mound carbonates and calcite cements in neptunian dikes precipitated from brines comprising a mixture of hydrothermal fluids and seawater. In addition, carbon isotope compositions (13C as low as -18o/oo PDB) suggest a contribution from thermogenic methane derived presumably from underlying basaltic intrusives. Aerobic bacterial oxidation of methane is favored as the main process driving carbonate precipitation in, and rapid lithification of, the mounds.

The broad Middle Devonian carbonate platform of the Holy Cross Mountains was fragmented during the Frasnian and afterwards submerged as isolated blocks and transformed into a topographic depression. The section exposed in the Ostrowka quarry illustrates a well-documented drowning of the last small fragment (seamount) of this carbonate platform which submerged during Late Devonian and Early Carboniferous times. The recognized facies succession reflects a progressive, stepwise lowering of the sediment surface through time from a peritidal, lagoonal environment to a deep-water, anoxic basin. The Frasnian shallow-water carbonate platform is represented by lagoonal facies with peritidal sediments. Small-scale deepening-upward cycles punctuated with emersion surfaces are a feature of this stage. They resulted from a combination and continuous balance between carbonate production, subsidence and low-order, eustatic sea-level fluctuations. The carbonate platform aggraded at rates around 100–125 m/Ma. The shallow-water peritidal carbonates are separated from the post-platform deposits by an unconformity surface developed during subaerial exposure. The flooding of the faulted block of Galezice in the pre-late marginifera time and the subsequent initial drowning resulted primarily from a rapid sea-level rise. Small dimension of the seamount and its cemented upper surface were the most important factors that facilitated the sediment removal and thus suppressed the sediment accumulation. A condensed section was deposited on the top of the seamount. Its stratigraphic succession and rare cephalopod storm beds account for a model of redistribution of sediment over the swell, disintegrated by small-scale faulting. During drowning, the seamount traversed down through the photic zone at an average rate not higher than 20–25 m/Ma. Transformation of the swell into a basin accelerated in the late Tournaisian anchoralis Zone. It is manifested by an increase of deposition rate and by a lithology of alternating limestones and clays, with participation of fine-grained calciturbidites. The associated tephra deposits and syndepositional ferruginous hydrothermal mineralization were the result of extensional tectonics on the facies evolution. The drowning was completed in the Visean with deposition of black siliceous shales in the starved basin. The ‘death’ of the Devonian carbonate platform in the Galezice area illustrates well the famed ‘paradox of drowned platforms'’as the subsidence rate was much slower during the drowning than during the phase of the platform aggradation.

Conodont colour alteration (CAI) data have been applied in the reconstruction of palaeothermal conditions in the Cracow-Silesia region, southern Poland. The analysis of conodont CAI values supplemented by electron spin resonance (ESR) data provided a complex maturity pattern which coincides well with the tectonostratigraphical divisions of this region during the Variscan and Alpine diastrophic cycles. The Palaeozoic rocks in the Upper Silesian Coal Basin attained the observed thermal maturity during Late Carboniferous or Permian time just after the tectonic inversion of the basin. Vertical maturity trends suggest that palaeo-geothermal gradients were significantly higher than the present values. The irregular maturity pattern recognized in the Cracow-Myszkow Zone is related to the thermal overprint of intrusive igneous rocks on the maturity field resulting from the relatively shallow burial. This pattern has not been modified by Mesozoic hydrothermal events. The low maturity level of Devonian and Early Carboniferous rocks in the substrate of the Carpathian Foredeep is a product of the subsidence during Miocene time. The implications for hydrocarbon generation and further exploration are discussed. The CAI data are consistent with other indicators of organic metamorphism previously used in this region. The study highlights also features about the conodont colour alteration, such as the influence of silica mobilization on conodont colour, which limit the application of the CAI method.

The abundant conodont fauna of a black limestone and shale lithology (Kellwasser facies) of late Frasnian/early Famennian age provides a significant example of environmental control on conodont distribution and abundance. Conodont collections from the Upper gigas and Upper crepida zones of the eastern Anti-Atlas were examined. Biofacies maps for the conodont genera Palmatolepis, Polygnathus, Icriodus, Ancyrodella, and Ancyrognathus were constructed and their facies distribution patterns analysed. No significant restriction in conodont distribution was observed to be caused by the rise and extension of oxygen-depleted water over the platform during the late Frasnian/early Famennian. The abundance of conodonts, however, appears to have been related to optimal food supply in the oxygenated portion of the water column. Conodont biofacies patterns observed in the Upper Devonian of the eastern Anti-Atlas do not fit a simple ecological model involving distance from shore and depth of water. The faunas of two intervals investigated exhibit a similar level of species diversity, the pattern of which evidently follows the platform-basin configuration, but the majority of species shows a more or less random distribution. The onset of black sediment deposition was characterized by the dominance of Polygnathus and Icriodus, whereas towards its termination during the Upper crepida Zone time interval the genus Palmatolepis clearly predominated. This difference is a consequence of both a rise of sea-level and of evolutionary changes in the composition of the conodont fauna during the early Famennian.

The so-called Kellwasser facies consists of black bituminous limestones and shales which were deposited on pelagic platforms and in adjacent shallow basins. In contrast to the well-known European counterparts, Moroccan Kellwasser lithologies are extremely fossiliferous and, in addition to the predominating nektonic and planktonic, also contain different groups of benthic organisms. A first short-time Kellwasser episode during the lowermost Frasnian is succeeded, after an interval of grey nodular and crinoid limestone deposition, by a second longer one during the late Frasnian/early Famennian. On the platforms the upper member consists of slowly deposited cephalopod/tentaculitid/styliolinid coquinas and debris flows. Towards the basins they pass into shales with intercalated fine-grained sandstones and limestone lenses. This facies pattern reflects not only the regional palaeogeography but also local depositional environments on the platforms which were formed by differential subsidence, synsedimentary tensional tectonics and submarine erosion. Uni- or bidirectional bottom currents which reflect the overall palaeogeographic pattern are very pronounced in the platform environments but decrease towards the basins. The discontinuous stratigraphical record of the pelagic platforms images major sea-level oscillations which left only minor traces in the continuous basinal sequences. Widespread transgressions occurred during the Lowerasymmetricus, the Lower and Uppergigas, the Lowermarginifera and the Lowerexpansa Zones. They are separated by sea-level lowstands during the Middleasymmetricus/A. triangularis, therhomboidea and the (Middle?)praesulcata Zones. In contrast to the inferred fluctuations at the Frasnian/ Famennian transition in Euramerica, this interval in Morocco is represented by a more or less continuous highstand of sea-level.

The thermal and burial history of the eastern Anti-Atlas during Late Palaeozoic time is investigated using conodont colour alteration data and the electron spin resonance technique (ESR). Both the conodont colour alteration index (CAI) patterns and the results of ESR investigations indicate that the Devonian rocks of the eastern Anti-Atlas attained their present-day maturation level during Carboniferous time prior to Variscan uplift. The observed CAI values from 3 to 5 reflect heating to temperatures of 120°C and 310°C respectively. The conodont CAI pattern observed in the Devonian rocks is a product of burial and loading by thick Carboniferous cover but the maturation level is too high to be accounted for by burial beneath the known stratigraphical overburden under normal heat flow conditions. The analysis of palaeotemperatures inferred from conodont CAI values shows variation in heat flow during Carboniferous time, with an average geothermal gradient of 30°C/km in the Bechar Basin and higher gradients in the region of the mader and Tafilalt. The style of the observed CAI patterns suggests that depositional trends in the Carboniferous cover, now largely removed by erosion, followed the Devonian subsidence patten.

The measured ranges of 52 conodont taxa present in 7 Eifelian sections of the eastern Anti-Atlas in southern Morocco have been as- sembled by graphic correlation method into a chronostratigraphic framework. The Anti- Atlas regional composite developed in this study provides much higher stratigraphic res- olution than the traditionally used conodont zonation. It allows a subdivision of the Eifelian into 65.7 composite standard units, derived from the Jebel Ou Driss section, which was se- lected as the standard reference section. The framework appears to have a linear fit to time, giving the first image of the relative duration of the conodont zones in the upper Emsian to lower Givetian interval. The costatus, kockeli- anus, and Lower varcus chrons appear to have lasted more than two times longer than the partitus, australis, and/or hemiansatus chrons. An alternative upper Eifelian-lower Givetian conodont zonation with zones of more equal duration is proposed. Several conodont index taxa in the Anti-Atlas area display an almost synchronous appearance, independent of fa- cies conditions and shelf paleobathymetry. The analysis by graphic correlation reveals significant differences in rate of sediment accu- mulation between the Tafilalt Platform, which had condensed sedimentation (2.5 m/m.y.), and the Mader Basin, characterized by rates up to 40 m/m.y. area, the Devonian rocks offer excellent condi- tions for detailed biostratigraphical studies. Their depositional history reflects the process of disin- tegration of the northern margin of the Sahara cra- ton (Wendt, 1985, 1988). In contrast to the rather uniformly developed Upper Silurian and Lower Devonian sedimentary rocks, differential subsi- dence affected the area during Eifelian time, and thus a platform to basin topography of the shelf began to originate. This phase is biostratigraphi- cally relatively well constrained. Conodonts were found in many sections, providing a base for cor- relation of Eifelian rocks in the Anti-Atlas (Bul- tynck and Hollard, 1980; Bultynck and Jacobs, 1981; Bultynck, 1985, 1987, 1989, 1991). The fauna dominated by Polygnathus elements is lo- cally very abundant, but some stratigraphically important Eifelian index taxa are rare or absent, so that the position of some zonal boundaries (e.g., the base of the Eifelian) cannot be precisely rec- ognized. This is why Bultynck (1987) proposed two alternative conodont zonations for the upper Eifelian-lower Givetian interval. In order to provide a more accurate basis for correlation of Eifelian rocks and events in north- ern Africa, we used the graphic correlation method to develop a composite standard section based on conodont data from the eastern Anti-At- las. Our regional chronostratigraphic framework (Anti-Atlas regional composite) provides a much more linear biostratigraphic subdivision than any previously used conodont zonation. It offers, moreover, the best possible stratigraphic resolu- tion, useful for detailed analysis of the sedimen- tary evolution of the study area during the Middle Devonian. This contribution constitutes a first ap- proach in the construction of the Eifelian com- posite standard with worldwide applicability. In conventional biostratigraphy, fossil zones are used to achieve correlation. The graphic cor- relation technique, however, produces a frame- work that is founded on biostratigraphy, but inte- grates all the biostratigraphic events onto a composite standard. This method was developed by Shaw (1964), and was subsequently extended (e.g., Miller, 1977; Sweet, 1979; Edwards, 1984, 1989) to be applicable to nonpaleontologic and nonunique, but chronostratigraphically signifi- cant, events (including magnetic polarity data, key lithological marker beds, various geophysical log- type data, and stable isotope logs). It constitutes a powerful tool for the analysis of synchroneity or nonsynchroneity of geologic events. Integrated with sequence stratigraphy and sedimentological data, Shaw's method of graphic correlation is one of the standard techniques applied in modern basin analysis (e.g., Neal et al., 1994). GEOLOGIC SETTING

In the southern Ahnet Basin of Algeria a unique Middle Devonian underwater scenery is revealed, which consists of 191 mud mounds, 15 mud ridges, and 2 mud "atolls" distributed over nine discrete areas of a shallow-pelagic carbonate shelf. All the buildups are totally exhumed, thus perfectly exhibiting their original morphologies, slope angles, elevations, and the pattern of the onlapping strata as well as their relationship to the surrounding off-buildup sequences. Types of buildups include (1) small mud mounds (1-5 m high, 10-20 m diameter), (2) large mud mounds (up to 40 m high, up to 200 m diameter), (3) lens-shaped mounds(1-30 m thick, 10-15° slopes), (4) mud ridges (up to 100 m high, 150-8500 m long) and (5) semicircular mud "atolls" (2500-2800 m long, 20-30 m high). The buil ups were constructed during a short time span in the early Givetian (part of the Lower varcus zone), probably not exceeding a few hundred thousand years. Sedimentation rates in coeval off-buildup areas were at least 3-10 times lower. The lithology of the buildups is a massive boundstone with a high proportion of irregular, spar-filled cavities (stromatactis) but only very scattered potential frame builders. The carbonate mud is considered to be an in situ, cyanobacterial precipitate, although evidence of such is poor. Organic components of the buildups are not significantly different from those of the off-buildup lithologies, and include numerous crinoid ossicles, some tabulate and solitary rugose corals, brachiopods, mollusc debris, trilobites, and very rare sponge spicule and bryozoans. The total absence of stromatoporoids, large tabulate and colonial rugose corals, green algae, and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone (100-200 m). They are onlapped by a discontinuous sequence, a few meters thick, of skeletal packstones and grainstones, and locally, orthoceratid floatstones that were deposited later in the Givetian. A widespread rise of sea level in the early Frasnian buried both buildups and adjacent areas under shales and siltstones. Postdepositional alteration of the mud buildups includes: (1) formation of stromatactis cavities, probably by dewatering and internal erosion, (2) cementation of stromatactis cavities and interparticle pore spaces by early high-Mg calcite and late, blocky low Mg calcite, (3) neomorphic transformation of Mg-calcitic mud and aragonitic skeletal debris, (4) occasional formation of neptunian dikes, (5) slight erosion of buildup surfaces, (6) compaction and slumping of onlapping capping beds, and (7) local dolomitization. The formation and growth of the mud build-ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds. By continuous vertical and lateral growth, closely spaced mounds amalgamated into mud ridges and mud "atolls". Regular NW-SE and N-S patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late Precambrian tectonic lineaments, which reappear in the Hercynian orogeny. The isotopic signature of matrix and early ements of mound rocks, however, does not support the idea of fluid migration and hydrothermal CO2 supply during buildup growth.

Spectacular, totally exhumed, carbonate mud buildups of Givetian age can be found in the southern Ahnet basin of the central Sahara. Their geometries range from isolated mud mounds to mud ridges up to 8 km long. The latter are the result of lateral coalescence of closely spaced, individual mounds. Mounds up to 40 m high and ridges up to 85 m high show original slopes of 25° to 65°. Frame-builders in the massive core of the buildups are restricted to scattered small tabulate and solitary rugose corals. The total absence of stromatoporoids, colonial rugose corals, and algae, as well as the lack of debris at the toes of the buildups, shows that they were constructed in deep water below the photic zone and storm wave base. The arrangement of the buildups in north- to northwest-trending clusters and ridges suggests that their distribution was controlled by early Variscan extensional movements inherited from Precambrian patterns.

Conodonts display higher specific gravity (2.84-3.10) than common minerals in sedimentary rocks. For several decades this property has been exploited to separate these microfossils from sediment residues using density separation techniques. Two heavy liquids commonly used, bromoform and tetra-bromoethane, are toxic (Sax, 1979; HaufTand Airey, 1980) and constitute a danger to human health. Tetrabromoethane is a known carcinogen (Brem et al., 1974). Both of these liquids should be abandoned in favor of nontoxic sodium polytungstate (Callahan, 1987; Merrill, 1987b; Krukowski, 1988). There are, however, some technical problems with its use because of relatively high viscosity and a tendency to recrystallize during separation. Another big disadvantage is also its price at $100 per kilogram. These problems may be considerably overcome by using polytungstate in the way described by Savage (1988).