Jobst Wendt

The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

Abstract The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.

Age and depositional environment of upper devonian (early Frasnian to early famennian) black shales and limestones (Kellwasser facies) in the eastern Anti-Atlas, Morocco

SummaryThe 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.ZusammenfassungDie sog. Kellwasserfazies besteht aus schwarzen bituminösen Kalken und Tonen, die auf pelagischen Plattformen und in benachbarten flachen Becken abgelagert wurden. Im Gegensatz zu ihren gut bekannten europäischen Äquivalenten ist die marokkanische Kellwasserfazies sehr fossilreich und enthält neben den vorherrschenden nektonischen und planktonischen auch verschiedene Gruppen benthonischer Organismen. Einer ersten kurzzeitigen Kellwasser-Phase im untersten Frasne folgt nach einer Einschaltung grauer Crinoiden- und Knollenkalke eine zweite, länger andauernde im oberen Frasne/unteren Famenne. Sie besteht aus langsam abgelagerten Cephalopoden-Tentakuliten-Styliolinen-Schillkalken und Debrisflows auf den Plattformen, die in Tone mit eingeschalteten feinkörnigen Sandsteinen und Kalklinsen in den Becken übergehen. Dieses Faziesmuster ist nicht nur ein Abbild der größerräumigen Paläogeographie sondern auch von lokalen kleinerräumigen Ablagerungsmustern, die durch unterschiedliche Absenkung, synsedimentäre Dehnungstektonik und submarine Erosion gebildet wurden. Von den paläogeographischen Konturen bestimmte Bodenströmungen mit ein- oder zweigipfligen Maxima sind auf den Plattformen deutlich und nehmen gegen die Becken hin ab. Die diskontinuierliche stratigraphische Abfolge der pelagischen Plattformen spiegelt bedeutende Meeresspiegelschwankungen wider, die sich in den kontinuierlichen Becken-Ablagerungen nur undeutlich abbilden. Großräumige Transgressionen erfolgten in der unterenasymmetricus-, der unteren und oberengigas-, der unterenmarginifera- und der unterenexpansa-Zone. Sie wurden abgelöst durch Regressionen in der mittlerenasymmetricus-bisA. triangularis-Zone, derrhomboidea-Zone und der (mittleren?)praesulcata-Zone. Im Gegensatz zu den vermuteten eustatischen Meeresspiegelschwankungen an der Frasne/Famenne-Grenze in Euramerica ist dieser Zeitraum in Marokko durch einen mehr oder weniger hohen Meeresspiegelstand gekennzeichnet.

Disintegration of the continental margin of northwestern Gondwana: Late Devonian of the eastern Anti-Atlas (Morocco)

The Devonian–early Carboniferous sequence in the eastern Anti-Atlas represents a complete record of the last stage of the depositional and tectonic evolution along the northwestern margin of Gondwana. As a consequence of early Variscan block faulting, a platform and basin topography was established during the Middle and Late Devonian. Platforms were covered by condensed cephalopod limestones; sedimentation in the basins was mainly argillaceous with calcareous and turbiditic intercalations. In the latest Famennian/early Tournaisian the whole area was covered by delta deposits in the south passing into turbidites and olistostromes toward a continental slope farther north. This sedimentary and structural evolution reflects the gradual foundering and disintegration of the northwestern continental margin of Gondwana prior to the collisional stage in the late Visean/late Carboniferous.

Facies patterns and depositional environments of Palaeozoic cephalopod limestones

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

Temporal–spatial reconstruction of the early Frasnian (Late Devonian) anoxia in NW Africa: new field data from the Ahnet Basin (Algeria)

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.

The Cassian patch reefs (lower Carnian, southern Alps)

AbstractThe fauna of the upper Cassian Formation is composed mainly of reef-building and reef-dwelling organisms which occur as reeeposited material in basinal sediments, but have not been found as original reef bodies. Such bodies have now been discovered in the uppermost Cassian Formation of the central Dolomites from the Sella Group in the west to the Monti Cadini in the east. Generally they are small-scale patch reefs, not exceeding a few metres in thickness and lateral extent, which are intercalated in well-bedded detrital and micritic limestones. locally, larger biostromes spread out from the margins of the Cassian Dolomite buildups. Four types of faunal communities have been encountered in these reefs:1.The thrombolite-calcareous algae community, composed of small patchy cryptalgal structures binding poorly sorted debris and associated with other Cyanophyta, sessile formainifera and scattered calcareous sponges and corals. This type is the most common within the calcareous and marly-tuffaceous facies of the Cassian Formation.2.The calcareous sponge-coral community, composed mostly of calcareous sponges (stromatoporoids, some pharetronids) and, to a lesser extent, colonial corals and thrombolites. This community corre-sponds well to the Cassian reef fauna, best known from erratic blocks at Alpe di Specie, but has been found in situ only at one locality.3.The Spongiomorpha-Solenopora community, associated with scattered calcareous sponges and colonial corals, forming a thin biostrome at one locality.4.Coral communities, composed predominantly of colonial Scleractinia; found only in small or stratigraphically illdefined outcrops and in erratic blocks. The Cassian patch reefs and biostromes mark the end of a basinal evolution which began in the Lower Ladinian, and the onset of newly expanding carbonate buildups of Cassian Dolomite. These buildups and the sponge-coral patch reefs might have been the source for the allochthonous reef fauna of the Cassian Formation which interfingers with both shallow water environments.ZusammenfassungDie Fauna der oberen Cassianer Schichten besteht überwiegend aus Riff-bauenden und Riff-bewohnenden Organismen, die in Beckenbereiche verfrachtet, aber bisher nicht als Riffkörper in situ gefunden wurden. Solche Riffe wurden jetzt in den obersten Cassianer Schichten der zentralen Dolomiten im Gebiet zwischen der Sella-Gruppe und den Monti Cadini entdeckt. Es handelt sich meist um kleine patch reefs von wenigen Metern Mächtigkeit und Ausdehnung, die in gut gebankte detritische und mikritische Kalke eingeschaltet sind. Stellenweise breiten sich größere Biostrome von den Rändern der Cassianer Dolomitmassive her aus. Vier Typen von Faunenvergesellschaftungen sind in den Riffen vertreten:1.Die Thrombolit-Kalkalgen-Vergesellschaftung bestehend aus kleinen fleckenhaften Thromboliten, die schlecht sortierten Schutt verkitten und mit anderen Cyanophyceen, sessilen Foraminiferen und vereinzelten Kalkschwämmen und Korallen vergesellschaftet sind. Dieser Typus ist der häufigste in der kalkigen und mergelig-tuffitischen Fazies der Cassianer Schichten.2.Die Kalkschwamm-Korallen-Vergesselschaftung, bestehend aus Kalkschwämmen (Stromatoporen, einige Pharetroniden) und untergeordnet. Stockkorallen und Thromboliten. Diese Gemeinschaft entspricht gut der Cassianer Riff-Fauna, die am besten aus erratischen Blöcken der Seelandalpe bekannt ist, wurde aber in typischer Ausbildung nur an einer Stelle gefunden.3.Die Spongiomorpha-Solenopora-Vergesellschaftung, mit vereinzelten Kalkschwämmen und Stockkorallen; an einer Lokalität ein geringmächtiges Biostrom bildend.4.Korallen-Vergesellschaftungen, vorwiegend gebildet von kolonialen Scleractiniern; gefunden nur in sehr begrenzten oder stratigraphisch unsicheren Vorkommen sowie in erratischen Blöcken. Die Cassianer patch reefs und Biostrome kennzeichnen das Ende einer Beckenentwicklung seit dem Unterladin und den Beginn einer erneuten Ausbreitung von Cassianer Dolomit-Massiven. Sowohl diese wie die Kalkschwamm-Korallen-patch reefs können das Liefergebiet für die allochthone Riff-Fauna der Cassianer Schichten gewesen sein, die sich mit diesen beiden Flachwasser-Arealen verzahnen.

Conodont biofacies patterns in the Kellwasser Facies (upper Frasnian/lower Famennian) of the eastern Anti-Atlas, Morocco

Abstract 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.

Shell directions as a tool in palaeocurrent analysis

Abstract Conical shells (mostly orthoconic nautiloids, locally gastropods and rugose corals) were used to determine current directions in Ludlovian to upper Famennian cephalopod limestones in the eastern Anti-Atlas of Morocco, Ougarta Aulacogen and Ahnet Basin (both Algeria). Data plots established on 50,413 measurements from 217 localities document rather consistent current patterns which show only minor variations through subsequent intervals. A conspicuous feature are currents derived from pelagic platforms and directed towards adjacent basins. Shell accumulations decrease markedly towards platform margins yielding less distinctive information on current directions which, due to lack of shells, cannot be established in the basins proper. Orientation patterns of styliolinids show such a puzzling variation in adjacent samples that their use for current analysis is doubtful. The same is true for the presumed down-stream position of goniatite apertures which shows a highly variable pattern which is rarely consistent with that of concomitant orthoconic nautiloids. The direction of orthocones in cephalopod limestones onlapping lower Givetian mud mounds and ridges in the Ahnet Basin of Algeria shows a radial pattern which is the result of a mere gravitational deposition of shells on the steep slopes of these buildups. Apart from this exception the applicability of conical shells for current analysis is confirmed.

Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)

The Ahnet and Mouydir regions of southern Algeria are part of one of the worlds 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.

An exhumed Palaeozoic underwater scenery: the Visean mud mounds of the eastern Anti-Atlas (Morocco)

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.