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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.

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanow and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.

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).

Anthropogenic impact on surface waters and other elements in the environment was investigated in the Noteć River basin in central Poland. The approach was to trace changes in the Sr isotope composition ((87)Sr/(86)Sr) and concentration in space and time. Systematic sampling of the river water shows a very wide range of (87)Sr/(86)Sr ratios, from 0.7089 to 0.7127. This strong variation, however, is restricted to the upper course of the river, whereas the water in the lower course typically shows (87)Sr/(86)Sr values around 0.7104-0.7105. Variations in (87)Sr/(86)Sr are associated with a wide range of Sr concentrations, from 0.14 to 1.32mg/L. We find that strong variations in (87)Sr/(86)Sr and Sr concentrations can be accounted for by mixing of two end-members: 1) atmospheric waters charged with Sr from the near-surface weathering and wash-out of Quaternary glaciogenic deposits, and 2) waters introduced into the river from an open pit lignite mine. The first reservoir is characterized by a low Sr content and high (87)Sr/(86)Sr ratios, whereas mine waters display opposite characteristics. Anthropogenic pollution is also induced by extensive use of fertilizers which constitute the third source of Sr in the environment. The study has an important implication for future archeological studies in the region. It shows that the present-day Sr isotope signatures of river water, flora and fauna cannot be used unambiguously to determine the "baseline" for bioavailable (87)Sr/(86)Sr in the past.