Microbial contribution to deposition of upper carboniferous and lower Permian seamount-top carbonates,Akiyoshi, Japan

Summary Microbial organisms significantly contributed to the accumulation of shallow-marine carbonates in an open-ocean realm of the Panthalassan Ocean during Late Carboniferous-Early Permian time. The Jigokudai plateau in the northern part of the Akiyoshidai Plateau is the study area, where the limestone of the Upper Carboniferous Kasimovian Stage to the Lower Permian Artinskian Stage is well exposed. The fusulinid biostratigraphy as well as top-bottom geopetal fabrics revealed that the rocks of the study area are overturned. The thickness of this succession is approximated to 150 m. The succession is lithologically divided into the Lower Jigokudai and Upper Jigokudai formations. The lime-stones of these formations were deposited in a lagoonal setting. The Lower Jigokudai formation (95 m thick: Kasimovian to Asselian) is characterized by sand shoal facies represented by crinoid-Tubiphytes-fusulinid peloidal pack/grainstones and oolitic grainstones. Phylloid algal grain/packstones and microbial boundstones subordinately crop out. The Upper Jigokudai Formation (55 m thick: Sakmarian to Artinskian) is characterized by shoal and tidal flat facies represented by mollusk-fusulinid peloidal grain/rudstones, and peloidal grain/rudstones and peloidal lime-mudstones, respectively. Laterally discontinuous microbial bound-stones occur intercalated in mollusk-fusulinid peloidal grain/rudstones. This formation contains pendant and meniscus cements, and flat-pebble breccia indicative of an intertidal deposition and subaerial exposure. Various types of boundstone and organosedimentary structures constructed mainly by filamentous cyanobacteria,Tubiphytes obscurus tubular microproblematicum A, and other microproblematica were recognized. Significant facies types are (1) filamentous cyanobacteria-microproblematicum A bind/framestones, (2)Tubiphytes obscurus bindstones, (3) stromatolitic bindstones, (4) microbial laminites, (5) microbially linked structures, (6) oncoids, (7) microproblematica B-C framestones. The calcimicrobes, combined with synsedimentary cementation, formed small-scale and low-relief mounds of these facies, and greatly contributed to the deposition of the Kasimovian to Artinskian Panthalassan buildup.     

Facies evolution and sequence chronostratigraphy of a “mid”-Cretaceous shallow-water carbonate succession of the Apulia Carbonate Platform from the northern Murge area (Apulia,southern Italy)

The “mid”-Cretaceous carbonate succession of the Apulia Carbonate Platform cropping out in northern Murge area (Apulia, southern Italy) is composed of shallow-water carbonate rocks and is over 400 m in thickness. This paper focuses on the lithofacies analysis of this carbonate succession, its paleoenvironmental interpretation, and its sequence-chronostratigraphic architecture. Lithofacies analysis permitted to identify deposits which can be grouped into the following three facies belts: (1) terrestrial facies belt formed by: intraclast-supported paleosoils; solution-collapse breccias; (2) restricted facies belt made up of lithofacies deposited in protected peritidal environments; (3) normal-marine facies belt made up of lithofacies formed in moderate- to high-energy subtidal environments. The detailed study both in outcrops and in thin-sections revealed that, at the bed scale, lithofacies are cyclically arranged and form shallowing-upward small-scale depositional sequences comparable to parasequences and/or simple sequences. The following three small-scale sequence types have been distinguished: (1) subtidal sequences mostly made up of lithofacies formed in the normal-marine open subtidal domain; (2) peritidal sequences made up of lithofacies formed in the restricted peritidal domain; (3) peritidal sequences showing a cap formed by paleosoils. Small-scale sequences are not randomly arranged in the compiled succession but form discrete packages, or sets, that alternate in the sedimentary record. The repetition of such small-scale sequence packages in the succession has been the key to recognize large-scale sequences comparable to third-order depositional sequences. Although sedimentological data are often fragmentary due to late dolomitization, four large-scale sequences have been distinguished. The data support a generalized landward-backstepping of facies belts during transgression, which implies a gradual gain of accommodation culminating with the deposition of a package of small-scale sequences formed by normal-marine subtidal deposits. These mark periods of maximum accommodation space and form the maximum-flooding zones of large-scale sequences. A gradual seaward progradation of facies belts is recorded during highstand conditions, which implies a gradual loss of accommodation culminating with the deposition of a package of peritidal small-scale sequences capped by paleosoils or by solution-collapse breccias. The occurrence of terrestrial deposits marks periods of minimum accommodation on the platform and determines the sequence boundary of large-scale sequences. The large-scale sequences identified in this study fit with the main transgressive/regressive cycles published in the sequence-chronostratigraphic chart of European basins. As a consequence, it is interpreted that changes of the sea level recorded at the scale of European basins played an important role in determining the sequence-stratigraphic architecture of the studied succession. In spite of this, the occurrence of solution-collapse breccias, which implies a significant gap in carbonate sedimentation in between Early and Middle Cenomanian times, may also have an alternative interpretation. In particular, this deposit may represent the local fingerprint of the well-known tectonic phase which, during Late Albian-Early/Middle Cenomanian times, determined the subaerial exposure of large parts of Periadriatic carbonate platforms producing a marked regional unconformity.     

Stratigraphy and sedimentology of Muschelkalk carbonates of the Southern Iberian Continental Palaeomargin (Siles and Cehegín Formations,Southern Spain)

The Triassic sediments of the External Zones of the Betic Cordillera were deposited on the Southern Iberian Continental Palaeomargin. Two coeval Ladinian formations, namely the Siles Formation and the Cehegín Formation, are described to illustrate the facies and lithostratigraphic variability in the Muschelkalk carbonates. There has been some dispute over the number of carbonate units present in the Siles Formation. Our studies assign a tectonic origin to these recurrent carbonate units. Both formations comprise only one carbonate unit, which is correlated to the Upper Muschelkalk of the Catalan and Germanic basins and some Iberian Range sections. To characterize the sedimentological features of these formations, 14 facies were defined. The most widespread sediment was originally lime mud, although bioclastic deposits are also common. In the facies succession, a main transgressive-regressive sequence could be identified. According to the facies model proposed here, a muddy coastal and shallow-water platform prograded over mid ramp deposits. There is no evidence for a seawards reefal or oolitic-bioclastic sandy barrier. The most significant feature of this sedimentary interpretation is that these carbonate facies show clear characteristics of an epicontinental platform.     

Evolution of an isolated carbonate bank during Oligocene,Miocene and Pliocene times,Cayman Brac,British west Indies

Summary Modern carbonate sedimentation in the Caribbean Sea commonly occurs on banks that are surrounded and isolated by deep oceanic water. This depositional regime also occurred during the Tertiary, and many islands, such as Cayman Brac, have sequences that evolved in such settings. Cayman Brac is a small (about 39 km²) island, located on the Cayman Ridge, that has an exposed Oligocene to Pliocene succession which encompasses three unconformity-bounded formations. The upper Lower Oligocene Brac Formation is formed ofLepidocyclina limestones and sucrosis dolostones that locally contain numerous bivalves and gastropods. The overlying Lower to Middle Miocene Cayman Formation is formed of pervasively dolomitized mudstones to grainstones that contain an abundant, diverse biota of corals, gastropods, bivalves, foraminifera, and algae. Rhodolites are locally common. The Pliocene Pedro Castle Formation is formed of limestones, dolostones, and dolomitic limestones that contain a biota which is similar to that in the Cayman Formation. The unconformities between the formations represent substantial periods of time during which the previously deposited carbonates were lithified and eroded to produce karst terrains. All facies in the Brac, Cayman, and Pedro Castle formations on Cayman Brac developed on a bank that was no more than 20 km long and 3 km wide. There is no evidence of reef development other than isolated thickets ofStylophora and/orPorites and no systematic stratigraphic or geographic changes in the facies patterns of the formations. Comparison with modern Caribbean banks shows that the depositional regime was primarily controlled by water depth and energy levels. Limestones of the Brac Formation probably accumulated in low-energy conditions in water less than 10 m deep. The overlying Cayman Formation contains facies that formed in water 15 to 30 m deep with good cross-bank circulation. The Pedro Castle Formation formed in slightly shallower water (5–25 m) and lower energy conditions. The disconformities between the packages correlate with world wide eustatic drops in sea level.     

Facies models for middle to late devonian Shallow-Marine carbonates,with comparisons to modern reefs: a guide for facies analysis

Summary Shallow marine tropical Devonian carbonates commonly were deposited in two major geologic settings, i.e., shallow shelf with shelf margin reef, and gently sloping ramp that grades into peritidal to supratidal, in places evaporitic facies. The facies types within these two settings can be grouped into a few distinct zones on the basis of water, energy, texture, amount of micrite, porosity, fossil assemblages, and indicaton fossils. These zones have been integrated into a composite facies model for shallow marine, tropical Devonian carbonates. The facies zones are easily recognizable in hand specimen and core, and can be used for fast and accurate facies analysis. Some facies recognizable in hand specimen or core do not easily fit into the integrated model and represent facies of short-lived depositional events, such as hurricanes or slump deposits, or spatially restricted areas, such as channel fills. Such facies have to be interpreted on a case-by-case basis by comparison to the surrounding facies and depositional framework through time. Comparisons with Cenozoic reefs reveal a number of similarities. In particular, large metazoans in both Devonian and Cenozoic reefs display a range of growth forms that is not species-specific. Furthermore, several metazoans display comparable growth forms in equivalent facies zones. For example, dendroid stromatoporoids, such asStachyodes, and branching coral, such asPorites porites, occur in equivalent facies zones.     

Upper Ordovician sequences of western Estonia

The Upper Ordovician (uppermost Caradoc-Ashgill) section of western Estonia consists of a series of seven open-shelf carbonate sequences. Depositional facies grade laterally through a series of shelf-to-basin facies belts: grain-supported facies (shallow shelf), mixed facies (middle shelf), mud-supported facies (deep shelf and slope) and black shale facies (basin). Locally, a stromatactis mud mound occurs in a middle-to-deep shelf position. Shallow-to-deep shelf facies occur widely across the Estonian Shelf and grade laterally through a transitional (slope) belt into the basinal deposits of the Livonian Basin.

Each sequence consists of a shallowing-upward, prograding facies succession. Sequences 1 (Upper Nabala Stage) and 2 (Vormsi Stage) record step-wise drowning of underlying shelf units (lower Nabala) that culminated in the deposition of the most basinal facies (Fjäcka Shale) in the Livonian Basin. Sequences 3–6 comprise the overlying Pirgu Stage and record the gradual expansion of shallow and middle-shelf facies across the Estonian Shelf. The Porkuni Stage (sequence 7) is bracketed by erosional surfaces and contains the shallowest-water facies of the preserved strata. The uppermost part of the section (Normalograptus persculptus biozone) is restricted to the Livonian Basin, and includes redeposited carbonate and siliciclastic grains; it is the lowstand systems tract of the lowest Silurian sequence 8. Sequence 7 and the overlying basinal redeposited material (i.e., the lowstand of sequence 8) correspond to the latest Ordovician (Hirnantian) glacial interval, and the bracketing unconformities are interpreted as the widely recognized early and late Hirnantian glacial maximums.

The sequences appear correlative to Upper Ordovician sequences in Laurentia. Graptolite biozones indicated that the Estonian sequences are equivalent to carbonate ramp sequences in the western United States (Great Basin) and mixed carbonate-siliciclastic sequences in the eastern United States (Appalachian Basin–Cincinnati Arch region). These correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins.     

Donnees biostratigraphiques nouvelles sur l'oxfordien et le kimmeridgien du djebel nador de tiaret (avant-pays tellien,algerie); consequences paleogeographiques

In “Djebel Nador de Tiaret” area two calcareous and marly intercalations situated in the upper part of the “Argiles de Faidja” formation contain ammonites of late Oxfordian (Planula zone) and lower Kimmeridgian (Hypselocyclum and Divisum zones). These new biostratigraphic data allow precise correlations with ammoniteless formations. So, the installation of the carbonate platform can be placed in upper Kimmeridgian in all the Western Algeria foreland. Moreover the development of the facies “Argiles de Saïda” into the upper part of lower Kimmeridgian, shows that the closing of the tlemcenian trough begins later in Djebel Nador than in the Ilemcen mountains.     

Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: implications for basin analysis

Carbonate reef talus facies of the Koněprusy Limestone (Pragian, Lower Devonian, Barrandian) locally exhibit widespread impregnation by organic matter resulting in a partial to complete blackening of the limestones. Two contrasting types of impregnation are recognized: blackening of individual carbonate fossils and bioclastic layers within the limestone originated very early during diagenesis. The blackening is due to finely dispersed organic matter and possibly some iron sulphides and clay minerals that selectively adhered to the outer layers of corals, bryozoans, and crinoid fragments, leaving other fossils unaltered. These darkened fossils are similar to black pebbles—i.e., reworked, dark to black limestone clasts and bioclasts that are known to occur exclusively in shallow-water zones of both ancient and modern carbonates. The alteration of fossil fragments may have taken place in very shallow-water environments, possibly those of saline and reducing back-reef lagoons or supratidal-intertidal zones, with organic matter being derived from decayed algae and microbes, or early vascular terrestrial plant material. Following the coloration, the blackened fossils were removed from their original position by waves or storms and transported into relatively deeper-water reef slope settings to form graded, “salt-and-pepper”-colored bioclastic beds. The presence of blackened fossils in the carbonate succession may point to episodic emergence and indicates a vanished vegetated siliciclastic hinterland that may once have existed to the west or south from the present-day erosive edge of the Barrandian Devonian strata. Subvertical veins cutting the Koněprusy Limestone and filled with black solid bitumen and blackened calcite resulted from a subsequent but substantially later diagenetic event, which is a testament of aqueous and petroleum fluid migration through the succession during deeper burial. Microthermometric characteristics of the aqueous inclusions embedded in vein calcite indicate that the veins were precipitated by brines of low to moderate salinity (0.5–9.5 wt% NaCl equiv.) with temperatures in the range of 87–116°C. The bitumen in the veins is epi-impsonite (R_r?=?0.70–1.90%), which is interpreted as degraded petroleum residuum that experienced thermal alteration at around 120°C. The AFT modeling combined with fluid inclusion microthermometry and wider geological considerations indicate that the veins originated during the Variscan orogeny, most probably upon deep burial of the Lower Paleozoic strata in Carboniferous time.     

Tide-dominated Middle Devonian sequence from the northern part of the Holy Cross Mountains (Central Poland)

Summary The dolomitic Wojciechowice Formation distinctly differs from the remaining, mainly shaly Middle Devonian succession in northern part of the Holy Cross Mountains (Central Poland). The upper Member of the Formation (Crystalline Dolostone Mb.), in greater part dolomitized but also containing limestone beds, is composed of shallowing-upward cyclothems well exposed in Skaly quarry in the Bodzentyn syncline. The lower parts of the cyclothems, interpreted as subtidal facies, contain fossils characteristic for restricted environments. They are grouped into two assemblages. The first, with brachiopods (largeBornhardtina andEmanuella), massive stromatoporoids, and subordinate gastropods and amphiporoids is related to a deeper subtidal environment, while the second (mainly amphiporoids, gastropods, ostracodes and calcareous algae) is shallower subtidal. Towards the top of succession the fossil content radically decreases. The upper parts of cyclothems are composed mainly of different types of laminites. In these parts of the section, interpreted as intertidal/supratidal units, stromatolites, desiccation polygons, intraformational breccias, and common bioturbations are present. The whole succession was deposited in a low-energy environment, only intermittently affected by high-energy events. For their most spectacular example of this, aBornhardtina-coquinite, a tempestitic origin is proposed. The interval with cyclic sedimentation studied correlates with the dolomitized lower “Unit I” of the Stromatoporoid-Coral Kowala Formation from the southern part of the Holy Cross Mountains, which exhibits sabkha-type cyclicity. The differences in development of cyclothems in both regions resemble outer and inner part of an extensive platform, and correspond well with basic trends of the Lower-Middle Devonian transgression in the Holy Cross Mountains. The general succession of formations deposited during this process coincides with transgressive events on Johnson's eustatic curve for the Devonian.     

Biostratigraphic control of the latest-Ordovician glaciogenic unconformity in Alnif (Eastern Anti-Atlas, Morocco), based on brachiopods

In the Alnif region of the Eastern Atlas (Morocco), seven fossiliferous horizons within the Lower-Ktaoua and Upper-Tiouririne formations (Ktaoua Group), as well as in the glaciomarine microconglomeratic shales of the Upper Formation of the Second-Bani Group have yielded biostratigraphically significant brachiopods and other taxa, such as trilobites and echinoderms. Several brachiopod species with short stratigraphic ranges, well-known in south-western Europe, allow a precise chronostratigraphic control of a succession that displays important lateral lithological and facies changes, when compared with the type sections in the Central Anti-Atlas. They have also permitted a better consensus between the macrofauna-determined age and that based on micropaleontological analyses. For the first time, the occurrence of a Hirnantia Fauna within the microconglomeratic shales of the Upper Formation of the Second Bani Group is reported. The biostratigraphic conclusions restrict the age of the Latest Ordovician glaciation to the early Hirnantian.     

Ichnological analysis of Jurassic shallow to marginal marine deposits: example from Wagad Highland,Western India

Abstract

Jurassic deposits of shallow to marginal marine (delta) environments are widely reported from different continents of the world. This study shows inter-relationship of the animal-sediment behaviours in shallow and marginal marine conditions, suggesting an interpretation of the possible ichnodisparity. The Jurassic succession exposed at Washtawa Dome and Adhoi Anticline of Wagad highland, Kachchh comprises an approximately 341 m thick succession, divided into two formations – Lower Washtawa and Upper Wagad Sandstone. Eight sparsely to highly bioturbated sedimentary units show twenty-three identifiable ichnospecies from fifteen ichnogenera representing five ichnoassemblages broadly attributable to the Skolithos and the Cruziana ichnofacies, and developed in shallow-marine strandplain shoreface and delta depositional facies. The relative frequency statistical data reveals the dominant occurrence of feeding structures generated by polychaetes preferentially in quartz arenites. These structures represent sub-horizontal, sub-vertical and complex burrows, and show twelve and six categories of architectural designing Callovian-Oxfordian of shallow-marine shoreface and Oxfordian-Kimmeridgian shallow-water delta successions, respectively. The lower ichnodiversity and ichnodisparity associated with the Oxfordian-Kimmeridgian deltaic intervals represents a decrease in the exploitation of under-used ecospace, due to unfavourable environmental conditions rather than an expression of evolutionary radiation.     

Upper Cambrian-Ordovician successions overlying Timanian complexes: new evidence of acritarchs and brachiopods from Kolguev Island, Arctic Russia

Drillholes to several kilometres depth on Kolguev Island in the southern Barents Sea have sampled early Palaeozoic successions, known elsewhere in the Pechora Basin to overlie Neoproterozoic basement complexes. New studies on acritarch microfossils from the lowermost part of the Palaeozoic succession (c. 4500 m depth), reached by the Bugrino 1 and North-Western 202 boreholes, revealed diverse and biostratigraphically significant assemblages, which indicate the position of the Cambrian-Ordovician boundary in a sedimentologically continuous offshore marine succession. Upper Cambrian strata equivalent to the Peltura and Acerocare zones are distinguished on the basis of common taxa known from the neighbouring East European Platform and other areas in Baltica, Avalonia, and Gondwana. Invertebrate faunas, including brachiopods, problematic mollusc and phyllocarid arthropods, are revised taxonomically; they are indicative for the Tremadocian and Arenigian stages in the upper part of the succession. The Cambrian strata are insofar documented by fossil record only on the Kolguev Island, although their extension (of various series) in other areas of the Pechora Basin is claimed on the grounds of geophysical data and/or is inferred from geological successions. This new biostratigraphic evidence and facies development suggest that the Upper Cambrian-Tremadocian platformal deposits were likely widely distributed over the northeastern Baltica, as they were in the East European Platform and Baltoscandia. They were accumulated on a stable and passive margin of the craton with steady subsidence and a high rate of sedimentation.     

Role of substrate and landscape context in early succession: An experimental approach

Both local site conditions and landscape context influence the course of succession, but there is a lack of experimental studies on the relative importance of these two factors. It is hypothesised that convergence vs. divergence in succession is determined by the interplay of site factors, such as type of substrate and the nature of the surrounding landscape. In order to evaluate the role of substrate and surrounding landscape in the initial development of vegetation, experimental plots with tertiary clay, sand, peat, sterilised local soil and undisturbed local soil as a control were established in two contrasting regions, and the cover of all the species present was recorded annually for 10 years. In early succession, vegetation was affected by both the substrate and surrounding landscape, but their effects resulted in different trends. The importance of the substrate gradually decreased, while that of the landscape context increased. In the course of succession the vegetation between the two regions diverged and converged within each region. We concluded with regard to the divergence vs. convergence dichotomy in succession: if contrasting habitats occur in the same or similar landscapes, convergence is expected, whereas if similar or the same habitats are located in contrasting landscapes, divergence is expected. For the remaining combinations, i.e. contrasting habitats in contrasting landscapes or the same habitats in the same or a similar landscape, successions may exhibit no or only slight divergence or convergence.     

辽宁西部晚中生代地层研究之进展及存在问题——辽宁西部晚中生代地层和鱼群研究之四

辽宁西部晚中生代地层的综合层序为:义县组、九佛堂组、沙海组/张老公屯组、阜新组和孙家湾组.中西部朝阳—喀左—建昌以及凌源—三十家子盆地的义县组大致相当于东部阜新—义县—锦州盆地的狭义义县组及其底部的北票尖山含化石沉积夹层;中西部的九佛堂组则可与东部的金刚山、吐呼鲁和九佛堂三个岩组相对比;张老公屯组火山岩在层序上大部分可能位于九佛堂组和阜新组之间.以北票鲟、中华弓鳍鱼和狼鳍鱼为代表的鱼类化石普遍见于中国北方热河群及其相当岩群的下部河湖相地层中,以粒鳞鱼、固阳鱼和副狼鳍鱼为代表的鱼类化石多见于上部的扇三角洲和湖沼相含煤地层中.热河群及相当岩群时代的确定以及陆相侏罗—白垩系界线的厘定受制于目前地层和生物化石的研究程度.从鱼群的面貌看,义县—九佛堂组、沙海—阜新组及相当地层的时代可能分别为晚侏罗世晚期—早白垩世早期和早白垩世中期.     

Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment

Summary The Silurian of Gotland is characterized by repeated changes in depositional facies development. The deposition of uniform sequences of micritic limestones and marls was interrupted four times by the growth of reef complexes and the formation of expanded carbonate platforms. Coinciding with these, often abrupt, facies changes extinction events occurred which predominantly affected nektonic and planktonic organisms. Ratios of carbon- and oxygen-isotopes covary with the facies development. Periods in which the deposition of limestonemarl alternations prevailed are characterized by relatively low C- and O-isotope values. During periods of enhanced reef growth isotope values are high. For these changes,Bickert et al. (1997) assume climatic changes between humid “H-periods”, with estuarine circulation systems and cutrophic surface waters with decreased salinity in marginal seas, and arid “A-periods”, with an antiestuarine circulation and oligotrophic, stronger saline surface waters. In order to separate local and regional influences on the isotopic development from the global trend, the interactions between facies formation and isotope record have to be clarified. For this purpose, the patterns of isotope values in the upper part of the Silurian sequence on Gotland (upper Wenlock —upper Ludlow) has been determined and stratigraphically correlated along four transects through different facies areas. Facies formation during this time interval was investigated by differentiation and mapping of twelve facies complexes in the southern part of Gotland. These include shelf areas, reef complexes with patch reefs and biostromes, backreef facies, and marginal-marine deposits. The good correspondence between the carbon-isotope records of the four transects suggests that local environmental conditions in the different facies areas did not influence the δ¹³C values. Therefore, a supra-regional or even global mechanism for the C-isotope variations is likely. In contrast to carbon istopes, the oxygen-isotope values of the four transects generally show parallel trends, but higher variabilities and in parts distinctly deviating developments with a trend to more negative values. These are interpreted as an effect of local warming in small shallow-water areas which developed during arid periods in reef complexes and backreef areas. The boundaries between A-periods and H-periods, as defined by δ¹³C values, which are interpreted as isochrones, can be mapped. From the upper Homerian to the Pridolian six parastratigraphic isotope zones are defined which only partly match the stratigraphic division ofHede (1942, 1960). The isotope stratigraphy results in an improved correlation between the shallow and marginal-marine areas in the eastern part of Gotland and the uniform shelf areas at the west coast of the island. Furthermore, a detailed relationship between the development of carbon and oxygen isotope ratios, the carbonate facies formation, and the succession of palaeontological events could be observed. At the transition from H-periods to A-periods, major extinction events occurred prior to the first increase of δ¹³C and δ¹⁸O values. Extinction events affected conodonts, graptolites, acritarchs, chitinozoans, and vertebrates and resulted in impoverished nektonic and planktonic communities. The reef-building benthos was less affected. Parallel to a first slight increase of isotope values, facies began to change, and reefs developed in suitable locations. The subsequent rapid increase of C- and O-isotope values occurred contemporarily with strong facies changes and a short-term drop of sea-level. Oligotrophic conditions in the later stages of A-periods led to strong reef growth and to an expansion of carbonate platforms. The transitions from A-periods to H-periods were more gradual. The δ¹³C values decreased slowly, but reef growth continued. Later the reefs retreated and were covered by the prograding depositional facies of shelf areas. The diversity of planktonic and nektonic communities increased again. The close relationship between facies formation, palaeontological events and isotope records in the Silurian suggests common steering mechanisms but gives no indication of the causes for the repeated extincion events related to H-period/A-period transitions. Especially the observation, that strong extinctions occurred prior to changes of isotope values and facies, points to causes that left no signals in the geological record. Hypothetical causes like collapse of trophical nets, anoxias, or cooling events are not evident in the sediment record or do not fit into the regular succession of period transitions.     

Revision of the Zone 13 graptolite biostratigraphy in the Marathon, Texas, standard succession and its bearing on Upper Ordovician graptolite biogeography

One of the long-standing problems in North American graptolite biostratigraphy is the distinct differences in assemblages of post-Climacograptus bicornis age between the classical graptolite sequences in the New York - Quebec and Marathon, west Texas, regions. These have been attributed either to faunal provincialism or to the presence of a major hiatus between the Woods Hollow and Maravillas formations in Texas. New collections from the key Marathon Picnic Grounds section contain diagnostic Late Ordovician graptolites that confirm the existence of a major stratigraphic gap below the Maravillas Formation. The lower Maravillas Formation (Zone 13) has a Late Ordovician, low-diversity Pacific Province graptolite fauna that includes the biostratigraphically diagnostic species Climacograptus nevadensis, C. tubuliferus, Orthograptus fastigatus and Dicellograptus ornatus. Zone 13 graptolite assemblages from the Marathon region correlate with the C. tubuliferus to D. ornatus zones in the Trail Creek, Idaho, succession, the Ea4-Bo2 interval in Victoria, the O. fastigatus Zone in the Canadian Arctic Islands, the O. quadrimucronatus to D. ornatus zones of the Canadian Cordillera, and the D. complanatus to D. anceps zones in Scotland. The hiatus between the Woods Hollow and Maravillas formations spans an interval corresponding to at least the Eal Ea3 interval in Australia, the C. americanus to upper A. manitoulinensis zones in the New York - Quebec succession, and the D. clingani and P. linearis zones in Scotland. These results agree with the magnitude of the hiatus previously indicated by conodont biostratigraphy. Late Ordovician graptolite distribution patterns in North America can be explained by an extension of Cooper, Fortey & Lindholm's (1991; Lethaia 24) Lower Ordovician graptolite biofacies model into the Upper Ordovician, which incorporates both lateral water-mass specificity and depth stratification. Using this model, we recognize in Laurentia two separate biofacies among tropical-zone Late Ordovician Pacific Province graptolite faunas, a cosmopolitan Oceanic biofacies, and a cratonic Laurentian biofacies. The lower Maravillas Formation graptolite fauna is clearly part of the Oceanic biofacies, whereas the coeval Appalachian faunas represent the Laurentian biofacies. □Graptolites, Ordovician, biostratigraphy, Texas, biofacies, biogeography.     

Tectonically controlled late cretaceous terrestrial to neritic deposition (Northern Calcareous Alps, Tyrol, Austria)

Summary The Turonian to Santonian terrestrial to neritic succession (Lower Gosau Subgroup) in the Northern Calcareous Alps of the eastern part of the Tyrol, Austria, provides an example for deposition on a compartmentalized, narrow, microtidal to low-mesotidal, wave-dominated, mixed siliciclastic-carbonate shelf. The shelf was situated in front of a mainland with a relatively high, articulated relief, and underwent distinct changes in facies architecture mainly as a result of tectonism. The investigated succession was deposited above a deeply incised, articulated truncation surface that formed when the Eo-Alpine orogen, including the area of the future Northern Calcareous Alps, was uplifted and subaerially eroded. Distinct facies associations were deposited from (1) alluvial fans and fan deltas, (2) rivers, (3) siliciclastic lagoonal to freshwater marsh environments, (4) areally/temporally limited carbonate lagoons, (5) transgressive shores, (6) siliciclastic shelf environments, and (7) an aggrading carbonate shelf. During the Turonian to Coniacian, the combination of high rates of both subsidence and sediment accumulation, and a narrow shelf that was compartmentalized with respect to (a) morphology of the substratum, (b) fluviatile input of siliciclastics and contemporaneous input of carbonate clasts from fan deltas, (c) deposition of shallow-water carbonates, and (d) water energy and-depth gave rise to an exceptionally wide spectrum of facies as a distinguishing feature of the succession. With the exception of facies association 7, which formed only once, depositional sequences in the Turonian to Coniacian interval contain all of the facies associations 1 to 6. During Turonian to Coniacian times, the shelf was microtidal to low-mesotidal, and was dominated by waves, storm waves and storm-induced currents. In vegetated marshes, schizohaline to freshwater marl lakes existed. Transgressions occurred onto fan deltas and in association with estuaries, or in association with gravelly to rocky shores. The transgressive successions, including successions deposited from transgressive rocky carbonate shores, are overlain by regressive successions of shelf carbonates or shelf siliciclastics. Deposition of shallow-water carbonates generally occurred within lagoons and over short intervals of time. A „catch-up” succession of shelf carbonates about 100 m thick accumulated only in an area protected from siliciclastic input. In its preserved parts, the Turonian to Coniacian succession does not record deposition adjacent to major active faults. Lateral changes in thickness result mainly from onlap onto the articulated basal truncation surface. Subsidence most probably was controlled by major detachment faults outside the outcrop area, and/or was distributed over a wide area in association with secondary faults above the major detachments. During Coniacian to Early Santonian times, both the older substratum and the overlying Turonian-Coniacian succession were subaerially exposed, faulted and deeply eroded. The following Early Santonian transgression ensued with rocky carbonate shores ahead of a sandy, narrow shoreface-inner shelf environment and a deeper shelf with intermittentlydysaerobic mud. The transgression was associated with the influx of cooler and/or nutrient-rich waters, and heralds an overall deepening. Still during the Early Santonian, the deepening was interrupted by another phase of subaerial exposure. Subsequently, a short phase of shelf deposition was terminated by deepening into bathyal depths.     

Strontium isotope stratigraphy of the Gabbs Formation (Nevada): implications for global Norian–Rhaetian correlations and faunal turnover

The Luning and Gabbs formations in west‐central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Müller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index‐species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare ⁸⁷Sr/⁸⁶Sr measurements of well‐preserved carbonate shell material from Nevada to the well‐known and biostratigraphically constrained ⁸⁷Sr/⁸⁶Sr record from Tethys, where a negative excursion in ⁸⁷Sr/⁸⁶Sr is noted across the Norian–Rhaetian boundary. Our new ⁸⁷Sr/⁸⁶Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in ⁸⁷Sr/⁸⁶Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.     

Refining the late Silurian sea-level history of the Prague Syncline—a case study based on the Přídolí GSSP (Czech Republic)

A 50-m-thick section in the Po?áry quarry, Prague Syncline (Czech Republic) spanning the upper Silurian (uppermost Ludlow and P?ídolí) to the lowermost Devonian (Lochkovian) has been studied using sedimentological and physical stratigraphical (gamma-ray spectrometry) methods combined with conodont biostratigraphy. Conodont data demonstrate the presence of local conodont biozones: “Ozarkodina” crispa (uppermost Ludlow)—Zieglerodina zellmeri (base of P?ídolí)—Zieglerodina ivochlupaci—Delotaxis detorta—“Ozarkodina” eosteinhornensis s.s.—Zieglerodina klonkensis—Icriodus hesperius-optima (lowermost Lochkovian). The studied section represents a transgressive–regressive facies succession characterized by a transition from distal calciturbidites deposited in a distally steepened carbonate platform during transgression and sea-level highstand (crispa to lower ivochlupaci zones) to mixed calciturbidites/coarse-grained bioclastic limestones (falling-stage systems tract, upper ivochlupaci to lower klonkensis zones). The upper part of the succession consists of bioclastic limestones corresponding to a subtidal setting between storm-wave and fair-weather wave base (lowstand systems tract in upper klonkensis and hesperius-optima zones). This interpretation corresponds to the late Silurian global sea-level pattern as recorded in other regions. Thus the depositional system is interpreted as mostly driven by eustasy with short-lived periods of influence from local tectonics.