Search

Lateral facies heterogeneity and carbonate production perturbation within peri-Tethyan platforms were widespread phenomena during the Early Jurassic. These variations were either controlled by local factors affecting small areas or by processes prevailing on a global scale. Three geological sections are investigated in Lower Jurassic outcrops in the Traras Mountains, northwest Algeria, from synrift-settings to disentangle the influence of local and global factors. The detailed sedimentological study reveals 11 facies types, grouped into four facies associations ranging from alluvial fan to outer ramp setting. Whereas thickness variation and lateral facies distribution are mainly controlled on a regional scale by synsedimentary tectonics, resulting from the spreading of the Tethys, the irregular topography inherited from the palaeo-alluvial fan topography of the underlying Beni Menir Formation seems to be the main control on a local scale. This irregular topography which coexisted in very close proximity to the studied carbonate deposits provided a significant amount of siliciclastic input, partially disturbing carbonate production. The local erosion of this detrital material during the initial early Pliensbachian transgression, combined with other local factors such as a low rate of accommodation space renewal and high-energy conditions, resulted in the absence of the otherwise prevalent Lithiotis fauna in the studied sector. Siliciclastic input perturbating carbonate production during the late Pliensbachian to early Toarcian could instead have been induced by a global warming event that increased continental weathering at the global scale during the Pl/To boundary or T-OAE associated with storm intensification. Alternatively, in view of the lack of solid biostratigraphic attribution, high terrigenous input could also have been induced from a cooling event leading to a sea-level fall during the latest Pliensbachian (Emaciatum Zone) and/or the lowermost Toarcian (latest Tenuicostatum Zone). In addition to tectonics, this eustatic sea-level fall could also explain the absence of shallow carbonate production in the Traras Mountains during these periods. In the future, a multidisciplinary basin-scale approach may resolve these uncertainties related to the lack of biostratigraphic dating, thus helping to identify the key factor influencing the perturbation of carbonate production. [ABSTRACT FROM AUTHOR]

Climatically the Silurian was a most unstable period, as it is shows several strong δ13C excursions, with the Lau excursion being the strongest short-lived positive δ13C excursion of the entire Phanerozoic. The causes of these excursions remain a topic of debate. In order to determine how benthic communities responded to these climatic perturbations, a microfacies analysis of carbonate rocks from the Silurian period (Late Telychian to the uppermost Ludfordian) of Gotland was conducted. The development of reefs was significantly enhanced during periods characterized by carbon isotope excursions, compared to time intervals without. In periods of elevated δ13C values (≥ 3 ‰) the abundance of bryozoans and microbes increases, and in some areas, they form reefs (bryostromatolites), while corals and stromatoporoids were the main reef builders in periods of lower isotope ratios (0–3 ‰). Furthermore, a significant increase of phosphatic components, gypsum pseudomorphs and the microproblematicum Palaeomicrocodium is observed in times of increased isotope ratios. A model is proposed that explains the changes in the benthic communities over time. It is based on previously published models and explains the geochemical processes from coastal areas to the open ocean. Changes are assumed from a humid climate in times without isotope excursions to an arid climate in times of isotope excursions. It emphasizes processes that may have led to the strong δ13C shifts during the Silurian period, and includes the phosphorus cycle in sea water as an important factor in the development of the benthic communities. [ABSTRACT FROM AUTHOR]

The Pliensbachian–Toarcian transition was characterised by a drastic turnover from a cool climate to a period of rapid global warming. While the warming associated with the Early Toarcian Oceanic Anoxic Event is rather well-studied, the cause, intensity and extent of the preceding cooling in the late Pliensbachian are still discussed. Occurrences of glendonite play an important role in this debate, since glendonite is a pseudomorph after the cryophilic carbonate mineral ikaite. This study describes the first glendonite-bearing carbonate concretions from South Germany (Buttenheim clay pit, northern Franconian Alb), which represent the southernmost glendonite occurrence in the late Pliensbachian documented so far. Based on petrographical and sedimentological investigations as well as stable isotope analyses it is concluded that a low temperature was the main factor for ikaite formation in the studied section, suggesting that the late Pliensbachian cooling had a more far-reaching impact on the temperature of the European epicontinental sea than previously assumed. To explain the low temperatures required for ikaite precipitation, a model for the sea-ice driven formation of cold bottom-water masses on the continental shelf is proposed. The occurrence of several layers containing reworked hiatus concretions in the studied outcrop is interpreted as the result of recurrent sea-level falls caused by multiple glacial pulses characterising the overall cool climate in the late Pliensbachian. [ABSTRACT FROM AUTHOR]

Bryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef‐dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower‐like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust‐forming contributors, suggest that it could have been formed directly at the palaeo‐sea surface, probably in times of minor but high‐frequency sea‐level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ13C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ13C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control. [ABSTRACT FROM AUTHOR]

Palaeozoic stromatoporoids, throughout their 100-million + year history (Middle Ordovician to Late Devonian and rare Carboniferous), are better preserved than originally aragonite molluscs, but less well-preserved than low magnesium-calcite brachiopods, bryozoans, trilobites and corals. However, the original mineralogy of stromatoporoids remains unresolved, and details of their diagenesis are patchy. This study of approximately 2000 stromatoporoids and the literature recognises three diagenetic stages, applicable throughout their geological history. Timing of processes may vary in and between stages; some components are not always present. Stage 1, on or just below sediment surface, comprises the following: micrite filling of upper gallery space after death, then filling of any remaining space by non-ferroan then ferroan calcite in decreasing oxygen of pore-waters; partial lithification of associated sediment from which stromatoporoids may be exhumed and redeposited, evidence of general early lithification of middle Palaeozoic shallow-marine carbonates; microdolomite formation, with the Mg interpreted to have been derived from original high-Mg calcite (HMC) mineralogy (likely overlaps Stage 2). Stage 2, short distance below sediment surface, comprising the following: fabric-retentive recrystallisation (FRR) of stromatoporoid skeletons forming fabric-retentive irregular calcite (FRIC), mostly orientated normal to growth layers, best seen in cross-polarised light. FRIC stops at stromatoporoid margins in contact with sediment and bioclasts. FRIC geometry varies, indicating some taxonomic control. Evidence that FRIC formed early in diagenetic history includes syntaxial continuation of FRIC into some sub-stromatoporoid cavities (Type 1 cement), although others were pre-occupied by early cement fills (Type 2 cement) formed before FRR, preventing syntaxial continuation of FRIC into cavities. Likely contemporaneous with FRIC formation, stromatoporoids in argillaceous micrites drew carbonate from adjacent sediment during reorganisation of argillaceous micrite into limestone–marl rhythms that are also early diagenetic. Stage 3, largely shallow burial, comprises the following: dissolution and silicification, but these may have occurred earlier in stromatoporoid diagenetic histories (more data required); burial pressure dissolution forming stylolites. [ABSTRACT FROM AUTHOR]

The late Miocene is a crucial interval for global climate evolution as well as for the regional geodynamic evolution of the Central Mediterranean area. It spans the transition from the warm Mid Miocene Climatic Optimum, associated with the major Monterey Carbon Isotope Excursion, to the cooler Pliocene, characterized by a bipolar glaciation. Within this climatic transition, during the early Tortonian, a positive carbon isotope excursion related to a global carbon cycle perturbation is recorded, named Carbon Maximum 7 (CM7). In this study, two upper Serravallian–lower Tortonian carbonate ramps of the Central Mediterranean have been analysed: the Latium‐Abruzzi and the Apula ramps (Central Apennines, Central Italy). Carbon isotope ratios have been coupled with facies and microfacies analyses with the aims: (1) to identify the CM7 in the Central Mediterranean carbonate ramp successions; (2) to evaluate potential carbonate factory changes or demises related to the CM7; and (3) to discriminate the role of global vs regional factors in affecting the Mediterranean carbonate ramps. The two studied ramps show different evolutions due to regional geodynamics causes. The Latium‐Abruzzi ramp drowns in the Tortonian, while the Apula shows a regressive trend, with upper Tortonian middle ramp facies overlying the lower Tortonian outer ramp. Despite the complex geodynamic setting, a positive carbon isotope shift has been identified in both ramps and correlated with the CM7. This positive carbon isotope shift may be linked to the strengthening of the global ocean circulation during the late Miocene cooling. This strengthening of the circulation enhanced the coastal upwelling, bringing nutrient‐rich waters to the surface and triggering an enhanced primary productivity consistent with the CM7. [ABSTRACT FROM AUTHOR]

Limestone-marl alternations are often interpreted to reflect cyclic changes in the depositional environment, with time spans for the deposition of a limestone-marl couplet between thousands and tens of thousands of years. Data from halysitid coral colonies from Gotland (Silurian), Sweden, indicate a diagenetic origin of limestones and marls indifferent to original sedimentary differences. The specimens crosscut several layers of their surrounding limestonemarl alternation, which means that both limestone and marl layers are among corallites and surrounding the coral colony. We analysed thin sections, SEM samples and micro-CT scans from Halysites catenularius from Gotland to document (1) the spatial extent of corals, limestone, and marl, and (2) the chronological order of sedimentation and subsequent diagenesis. Our results indicate a sedimentary infill of the coral frame while the coral was still alive. Growth banding indicates growth rates of 3.8 mm per year. Early diagenetic mass loss by aragonite dissolution as well as sedimentary overburden during progressive burial caused the compaction in marl beds fragmenting the coral, whereas it is well preserved in the limestone beds. The time span of a limestone-marl couplet penetrating the colony lies in the order of a few decades. Thus, the missing time in the depositional record is rather reflected by hiatuses than condensed sedimentary cycles. The mismatch of environmental changes recorded in the coral colonies with the changes in lithology, and the spatial heterogeneity of sedimentation rates within one bed imply constraints for applying lithological changes in cyclostratigraphy. [ABSTRACT FROM AUTHOR]

Abstract Stromatoporoids were abundant components of reefs, reef complexes and associated facies for ca. 100 million years between Middle Ordovician and end-Devonian time. A lot of environmental information stored in their skeletons may be used to develop: a) understanding of stromatoporoid growth controls; and b) interpretations of sedimentary environments in which they lived. General patterns of stromatoporoid distribution are well known, but knowledge of detailed interactions between stromatoporoids and their environments is poorly developed and under-used, potentially of great value in analysis of facies and palaeoecology. From a pool of several thousand specimens examined over four decades, this study identifies four key attributes of stromatoporoids that may be applied to enhance broad-scale knowledge of these fossils and their environments: 1) Substrates: Most stromatoporoids grew directly on wackestone to packstone substrates comprising micrite, clay and bioclasts. Evidence from the relationship between stromatoporoids and sediments demonstrates they were able to grow on soft substrates, but also leads to interpretation of partial lithification of the sea floor (and/or stabilisation by microbial filaments that may not be preserved) in mid-Palaeozoic carbonate facies, with potential implications for models of oceanic carbonate cycling. 2) Growth interruption: Almost all stromatoporoids examined show growth interruption, mostly caused by sedimentation and movement. Stromatoporoids normally recovered quickly and completely from interruption, thus were resilient to interruption events; 3) Associated organisms: Stromatoporoids have abundant associated organisms in two groups: (i) epibiotic encrusters and borers; and (ii) endobiotic organisms embedded in their structure, alive as the stromatoporoids grew. Epibionts used stromatoporoid surfaces that are presumed dead in almost all cases; some are associated with interruption events, but in most cases those were overgrown by successive stromatoporoid growth. Endobionts (mostly corals, plus spirorbids and others) are common to abundant in many stromatoporoid taxa. Stromatoporoid growth was little affected by presence of endobionts but in many cases (commonly restricted to certain stromatoporoid taxa) there was a complex biological interaction valuable in understanding controls on stromatoporoid development. 4) Growth form and taxonomy: Stromatoporoid assemblages are low diversity in almost all cases, regardless of age and facies, with two or three taxa much more abundant than the others. Some stromatoporoid taxa are limited to certain growth forms, thus taxonomic information is very important for facies analysis and palaeobiological interpretations. Stromatoporoids occur commonly with rugose and tabulate corals, both of which could also live on unconsolidated fine-grained substrates, therefore sediment-tolerance cannot be the only reason for stromatoporoid ability to outgrow corals in reefs. Arguments in the literature for photosymbiosis in both stromatoporoids and Palaeozoic corals are currently inconclusive for both fossil groups. Nevertheless, the sum of evidence indicates stromatoporoids were fast-growing, resilient and flexible benthic organisms, in a range of water depths, capable of dealing with soft substrates and largely unaffected by presence of endobionts. These characteristics are interpreted to have made stromatoporoids successful during mid-Palaeozoic time and valuable as tools in facies analysis. Recognition of the four key attributes which encompass all aspects of stromatoporoid growth controls makes palaeobiological study of stromatoporoids, in palaeoenvironmental reconstructions where they occur, readily accessible to researchers. [ABSTRACT FROM AUTHOR]

Abstract: Based on the aragonite composition of extant and exceptionally preserved fossil cephalopods going back to the early Palaeozoic, it is commonly assumed that all externally shelled cephalopods had an aragonitic shell wall. We demonstrate herein that at least two taxa of Siluro‐Devonian orthoconic nautiloids (Dawsonoceras, Spyroceras) had an original bimineralic shell, which developed convergently with gastropods and bivalves. [ABSTRACT FROM AUTHOR]

The current understanding of the origins of modern carbonate muds and their early stages of transformations are reviewed. The fine-grained nature of such sediments makes them susceptible to intensive structural and chemical alteration at relatively shallow burial depths driven especially by mineral instability under microbially mediated reactions within the sediment associated with the decay of organic matter. Whereas the transformation of high Mg calcite (HMC) to low magnesian calcite (LMC) generally takes place incongruently and in situ , that of aragonite is predominantly via congruent dissolution. The loss of aragonite produces a huge potential flux of carbonate which is translocated within the sediment. Parallel to studies on Quaternary sediments, investigations in the stratigraphic record have shown that in low-energy settings the transformation of the aragonite component leads to the formation of diagenetic bedding (such as limestone-marl-alternations or LMAs), and related bipartite lithologies, some types of hardgrounds, secondary carbonate mudrocks, and to the significant modification of the original skeletal biota. There are uncertainties such as the exact sources for the precursor aragonite, where the released carbonates reprecipitate in the shallow sediment column, how much carbonate (and hence carbon) is back-fluxed to the water column, how this affects palaeoenvironmental proxies, and the environmental distributions of these processes. The evidence from the stratigraphic record, from settings where organic matter was able to accumulate promoting these transformations, is that these changes in carbonate-rich sediments took place very early, at shallow (decametre or less) burial depths, affecting what we now see from the micron to the outcrop scale. The complexity and inter-relationships of these fundamental processes has received more attention in recent years, though their implications still seem remarkably undervalued. [ABSTRACT FROM AUTHOR]

The Tarim Basin, located in NW-China, represents one of China's major palaeoplates. Its palaeogeographic position in the Ordovician, however, is debated. In this study we present biostratigraphically controlled stable carbon isotope data from some 600 samples spanning almost the entire Ordovician in order to provide a tool that can be used for future chemostratigraphic correlations. The data correlate well with results from South China, and several major global δ 13 C excursions such as the MDICE (Dw2/3), SAICE (Sa2), GICE (Ka1), and Whitewater excursion (Ka4) are documented for the first time from the Tarim Basin. Major sedimentary gaps are detected in the lower Floian between the Penglaiba and Yinshan formations, in the upper Darriwilian to lower Sandbian between the Dawangou and Saergan (or Kanling) formations, and in the latest Sandbian between the Kanling and Qilang formations as well as between the Tumuxiuke and Lianglitag formations. Three scenarios for the depositional history of the late Darriwilian to early Sandbian Saergan Formation, which represents an important hydrocarbon source rock in the Tarim Basin, are discussed with respect to their bio- and chemostratigraphic implications. The mid-upper Ordovician rocks recording high δ 13 C values have been deposited during transgressive or highstand intervals, whereas the regressive intervals are mostly missing. This results in a stacking of several positive excursions, and thus chemostratigraphic correlation without biostratigraphic control might produce misleading results, especially for shallow-water carbonates. [ABSTRACT FROM AUTHOR]

The middle Silurian 'big crisis' refers to a graptolite extinction event and faunal turnover at the onset of a double-peaked positive carbon isotope excursion. The crisis has been proposed to affect conodonts, giving rise to a sophisticated palaeoecological model of their stepwise extinction, known as the Mulde Event. However, the impact of the event on conodont faunas outside Gotland and the Silurian Baltic Basin remains unknown. Here, it is examined in the Widowo IG-1 core (E Poland). The middle Silurian succession in this core represents foreshoal, shoal, and lagoonal settings on a tropical carbonate ramp on the shelf of the Eastern European Craton. Three positive δ13Ccarb excursions have been identified; the two upper excursions correlate with the Mulde isotope anomaly and with two global eustatic regressions. Conodont species proposed to be affected by the extinction event either were not observed or ranged through the extinction interval. Changes in their frequencies are best explained by the sequence stratigraphic architecture of the late Wenlock strata in the Widowo IG-1 core. The little impact of the 'big crisis' may reflect facies homogeneity across the studied interval, supporting the hypothesis that the late Wenlock conodont turnover in epicontinental settings was primarily driven by eustatically controlled facies shifts. [ABSTRACT FROM AUTHOR]

A specimen of the orchocladinid spongeArchaeoscyphia rectilinearisde Freitas (1989) is presented here as the first representative of the Anthaspidellidae from Wenlock strata on Gotland (Sweden). Other coeval occurrences are not yet known from the Silurian of Baltica. This specimen forms a link between the recently described sponge assemblage from Llandovery (Telychian) strata on Gotland and those from Wenlock to Ludlow strata from Arctic Canada. In addition, the specimen fills a gap in the fossil record of Silurian non-stromatoporoid sponges, which are poorly known worldwide. [ABSTRACT FROM PUBLISHER]

The boundary between the Lower and Upper Visby formations on Gotland (Sweden), which roughly correlates with the Llandovery–Wenlock boundary, is characterised by a mass occurrence of the large solitary rugose coralPhaulactis angusta. This coral-rich layer can be followed for some 50 km along the north-west coast of Gotland. The mass occurrence coincides with the onset of a pronounced, globally recognised δ13C and δ18O excursions in the lower Sheinwoodian. Altogether 31 specimens ofP. angustafrom eight localities were examined. We demonstrate that the corals preferred to grow in an upright position. Attachment structures were not observed, and therefore a liberosessile life strategy is suggested. Some corallites show an abrupt change in growth direction accompanied by a decrease in diameter. These features mostly occur in later growth stages and may be explained by disturbances in the growth due to toppling or tilting of the corals. Many specimens were able to change their growth axes, but finally died after such an event. Most specimens in the outcrops show a flattened upper surface, which is interpreted as a result of post-mortem abrasion. Epibiontic bryozoan encrustation took place mostly while the corals where alive, whereas stromatoporoid encrustation andTrypanitesborings occurred post-mortem. The repeated growth, tilting and erosion of the corallites ofP. angustademonstrate that thePhaulactislayer represents a condensed interval, characterised by multiple pulses of sediment accumulation and erosion. [ABSTRACT FROM PUBLISHER]

The late Wenlock is characterized by two global regressive-transgressive eustatic cycles in association with a double-peaked positive carbon isotope excursion. The onset of the excursion coincides with an extinction event affecting graptolites (the lundgreni event) and proposed to affect conodonts (the Mulde Event) and proliferation of non-skeletal carbonates. In order to test the hypothesized relationships between eustatic and ecological changes, the tropical carbonate Homerian succession in Podolia has been examined with respect to conodont, sequence and δ13C stratigraphy. Four depositional sequences ( DS) have been identified. The onset of the δ13C excursion occurs at the boundary between DS1 and DS2, corresponding to a forced regression of proposed glacioeustatic origin. The following rapid eustatic transgression associated with the highest δ13C values of 5.2‰ includes a higher-order shallowing episode recorded in Podolia as normal regression and a boundary between DS2 and DS3. This interval is distinguished by the presence of oncoids and thrombolitic buildups. The latest Wenlock eustatic fall and the corresponding second peak of the δ13C excursion corresponds in Podolia to a stratigraphic gap. The first δ13C peak (top of DS1 and DS2) corresponds to the O. bohemica longa conodont Zone, the interval between the two peaks ( DS3) - to K. ortus absidata and C. murchisoni zones, and DS4 is tentatively placed in the lowermost Ludlow Series. The record of relative sea-level changes in Podolia is consistent with reconstructions based on successions in England and Sweden. The moderate drop in conodont taxonomic richness may reflect the primary depositional control over their proposed extinction. [ABSTRACT FROM AUTHOR]

Eleven bryozoan species were described from Nors Stenbrott (Gotland, Sweden), represented by six trepostomes, four cystoporates and one fenestrate. Three genera with three species are new: two cystoporatesAcantholunaria expansagen. n. sp. n. andCurviporamonostylatagen. n. sp. n., and a trepostomeStellatotrypa hirsutagen. n. sp. n. Two trepostome species are new:Leptotrypa perforatasp. n. andLeptotrypa enodissp. n. The described bryozoan fauna is dominated by thin encrusting taxa which are involved in intensive interactions with calcimicrobes in bryozoan-microbial crusts in the reef. The bryozoan-microbial crusts (bryoliths) show certain sequences by microbes, encrusting bryozoans and tabulate corals. Multiple self-overgrowth of bryozoans occurs frequently and is interpreted as a defensive tactic against microbial fouling. Bryozoans avoid vicinity to tabulate corals, apparently due to strong competition for substrate. The bryozoan fauna from Nors Stenbrott is highly endemic, only two species are known elsewhere (North America and Great Britain). [ABSTRACT FROM PUBLISHER]

Stable carbon isotope curves are used as a precise stratigraphic tool in the Paleozoic, even though they are commonly based on shallow-water carbonate record, characterized by low stratigraphic completeness. Identification of episodes of large-scale redeposition and erosion may improve δ 13 C carb -based correlations. Here, a series of at least three episodes of high-energy onshore redeposition are described from the Makarivka Member (new unit) of the Ustya Formation from the Homerian (middle Silurian) of Podolia, Ukraine. The Makarivka Member is emplaced within a tidal flat succession. Its most prominent part is divided into a lower polymictic conglomerate of sand- to boulder-sized clasts representing a range of subtidal facies, and an upper heterolithic unit composed of grainstone and mudstone laminae. The aim of the study is to identify the mechanism of deposition of the allochthonous conglomeratic material in this Member. Based on analogies with recent tsunami deposits, the conglomerate is interpreted to reflect the strongest landward-directed current in the tsunami run-up phase, and the heterolith — alternating high-density landward currents, stagnant intervals allowing mud and land-derived debris to settle, and backwash flows. The tsunamite was deposited during an interval of decreasing isotopic values of the Mulde excursion, a global δ 13 C excursion reaching + 5.2‰ in the studied sections. Clast redeposition in an interval characterized by rapidly changing δ 13 C carb offers the opportunity to evaluate the degree of temporal and spatial averaging caused by the tsunami. The clasts in the polymictic conglomerate show scattered δ 13 C carb values (− 0.3‰ to + 2.1‰) compared to homogenous (1.3‰ to 1.6‰) values in the matrix. The presence of clasts characterized by low δ 13 C carb values is explained by their decrease with bathymetry rather than erosion of pre-excursion strata, whereas high values characterize material entrained from the sea-floor and strata directly underlying the tsunamite. Close (1.3‰ and 1.5‰) average δ 13 C carb values suggest that the matrix of the conglomerate is potentially a product of clast grinding. [ABSTRACT FROM AUTHOR]

Wetheredella is a calcareous encrusting microproblematicum known only from its two-dimensional expression in thin-sections. It occurs in Cambrian through Permian rocks, often in association with Rothpletzella and Girvanella, probable calcifying cyanobacteria. We show that Wetheredella can be produced by sectioning of the Paleozoic incertae sedis sclerobiont Allonema, which has until now been recognized based on its surface, 3D aspect. Therefore, we propose that Wetheredella should be regarded as a junior synonym of Allonema. Structures resembling Wetheredella have also been obtained in thin-sections through another encrusting microproblematicum, Ascodictyon, which, however, differs in the ultrastructure of its wall, which consists of parallel calcite crystals in contrast to radial crystals in Allonema. Allonema specimens used in the study were extracted from the Middle Silurian Mulde Brick-clay Member (Gotland, Sweden), representing an off-platform environment below the photic zone. This, together with the primary calcitic, radial wall structure of Allonema argues against a cyanobacterial or algal affinity, which had been considered for Wetheredella. The taxonomic position of Allonema remains unresolved, but the sclerobiont shares many common characteristics with some Paleozoic encrusting foraminifers. Combining the occurrence data on Allonema in its surface aspect and the ' Wetheredella' aspect recognized in thin-sections may shed more light on its affinity and paleoecological functions, particularly its association with cyanobacteria and its role as a framework-builder in Paleozoic reefs. [ABSTRACT FROM AUTHOR]

The Homerian (middle Silurian) Mulde Event is an extinction event affecting hemipelagic organisms associated with a positive stable carbon isotope excursion, and an increased proliferation of microbial deposits. The Event is recorded in the Bagovytsya section (Podolia, Ukraine), representing a carbonate ramp setting in the East European Craton, and recording the δ13Ccarbexcursion with values reaching +5.17%. The associated facies include oncolitic beds, mud-mounds and thrombolitic buildups dominated by cyanobacteria and microproblematica with proposed bacterial affinities. The associated faunal assemblage includes a high proportion of rostroconchs and abundant gastropods and trilobites. The co-occurrence of grazing organisms and microbial sediments suggests that the microbial preservation was not caused by decreased grazing rates. [ABSTRACT FROM AUTHOR]

Calcite pseudomorphs after gypsum are observed in a patch reef in the upper Slite Group (Wenlock) on north-east (NE) Gotland (Sweden). The idiomorphic pseudomorphs occur (a) in silt-sized carbonate sediment in former moldic pores, (b) in the micritic matrix of bioclastic rudstones and (c) in the thrombolitic matrix of the reef. The dominance of stenohaline organisms in the reef indicates normal marine salinities during reef growth. The development of evaporitic conditions and the formation of evaporitic minerals therefore postdate reef growth. The authigenic gypsum crystals are interpreted to be formed by downward percolating hypersaline brines which existed on an extremely shallow carbonate platform during the early Homerian (latestCyrtograptus lundgrenigraptolite Zone) regression. [ABSTRACT FROM AUTHOR]

Abstract: The Late Ordovician and Silurian are characterised by several strong, global positive δ13Ccarb excursions. Some of them exceed +5‰ and thus belong to the strongest perturbations of the carbon cycle in the Phanerozoic. The onset of the excursions is characterised by extinction and/or turnover events of several groups of marine invertebrates. The causal mechanisms of the carbon cycle perturbations, however, are still unknown and currently a matter of vigorous scientific debate. Our own investigations in the Hirnantian (latest Ordovician) have shown that the onset of the major δ13C excursion (HICE) is characterised by very high abundances of acritarchs showing abnormal, teratological forms. A critical review of published reports of abnormal acritarchs from the Late Ordovician to Early Devonian, and a correlation of their occurrences with the global stable carbon isotope curve, show that high abundances of teratological forms of acritarchs are often coeval to the run-up of δ13C excursions. High abundances of teratological forms in modern marine protists are commonly observed in environments with a high degree of environmental stress. In the fossil record, the challenge is to attribute abnormal forms of organisms to specific environmental circumstances. Our study implies that they are somehow related to the global carbon cycle, i.e., to carbon isotopic composition of the ambient sea water, and that they share a common extrinsic cause with the contemporaneous extinction and/or turnover events in other fossil groups. [Copyright &y& Elsevier]

The Digital Integrated Stratigraphy Project (DISP) aims to eliminate the stratigraphical ambiguity associated with sample position within a stratigraphical section. For example, it is often impractical or impossible to compare one author's measured section and data precisely against another author's geographically identical measured section and data due to discrepancies in the measured thicknesses of units, variations in the assignments of litho-and chronostratigraphical terms, and/or the precise line of measured sections between authors. The DISP will provide a solution to this problem by producing a precise 3-D digital rendering of the exposure using Terrestrial Laser Scanning (TLS) LiDAR, Real-Time Kinematic (RTK) Global Navigation Satellite Systems (GNSS), overlain digital photography, and GIS software to produce a cm to mm-scale accurate digital version of a given exposure. Once available online as a permanent web-based digital resource for stratigraphy, researchers will be able to access the program and digital models of scanned exposures where they will have themeans to log their sample positions directly onto the digital exposure while in the field, thus allowing unambiguous stratigraphical reference for future comparison. TheDigital Integrated Stratigraphy Project represents the next generation of stratigraphical standard and can serve as the protocol for the future of high-resolution Palaeozoic Earth history research. [ABSTRACT FROM AUTHOR]

Abstract: A detailed study of Late Ordovician–early Silurian acritarchs (Palaeozoic phytoplankton) from Anticosti Island (Québec, Canada) revealed an unusually high abundance of abnormal forms from the upper Hirnantian carbonate strata (uppermost Ordovician) of the Ellis Bay Formation in the western part of the island (member 6, Spinachitina taugourdeaui chitinozoan Biozone). The objective of this paper is to describe these abnormal forms in detail. Two species are particularly affected: Disparifusa psakadoria Loeblich and Tappan, 1978 presents abnormally hypertrophied central vesicles, whereas Peteinosphaeridium laframboisepointense nov. sp. has appendices that are fused along their length. The abnormal forms of acritarchs occur in rocks deposited during periods that are near time-equivalents to those of maximum ice-sheet extensions on Gondwana during the Hirnantian glaciation. Although this stratigraphic level corresponds to an interval of strong perturbations of the global carbon cycle, the exact factors causing the observed malformations remain unknown. [Copyright &y& Elsevier]

Abstract: The Late Ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the Phanerozoic. During this interval, Anticosti Island (Québec, eastern Canada) was located at low to intermediate palaeolatitudes (15–30° S) on the eastern margin of Laurentia. It displays a relatively complete section across the Ordovician–Silurian (O/S) boundary. Upper Ordovician to Lower Silurian strata of Anticosti studied here comprise the Vauréal (Katian), Ellis Bay (Hirnantian) and Becscie (latest Hirnantian–Rhuddanian) formations. Phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies (δ13C), with recent palynological data from Baltica and also with phytoplankton dynamics as described from near-polar, high latitude localities of the Gondwana region. Two positive excursions of the carbon isotope record have been identified in the lower and upper Ellis Bay Formation. Overall, phytoplankton trends are as follows: (1) as with chitinozoans, conodonts, brachiopods and corals, acritarchs from Anticosti display a major turn-over during the early Hirnantian (uppermost Vauréal Formation–lower Ellis Bay Formation), with the appearance of taxa that exhibit Silurian affinities (e.g., Tylotopalla sp., Ammonidium sp., Oppilatala sp., Evittia sp., Dilatisphaera spp.). High polymorphism for some common taxa is observed in the interval corresponding to the first δ13C positive excursion. Using sequence stratigraphy, this period may be related to a first glacially driven sea-level drawdown corresponding to terrestrial ice-sheet growth on Gondwana, such as recorded in Morocco. (2) Subsequently, during the late Hirnantian of Anticosti (uppermost Ellis Bay Formation), phytoplanktic communities suffered a crisis, exemplified by the disappearance of several typical, large Late Ordovician taxa (i.e., Orthosphaeridium spp., Peteinosphaeridium spp., large Baltisphaeridum spp., Sacculidium sp.). The lower Becscie strata, corresponding to the time of deglaciation at the O/S boundary interval, only contain a low diversity assemblage dominated by large acritarchs of the genus Hoegklintia and other morphologically related forms, and a few tolerant and long-ranging taxa (e.g., Evittia remota, Micrhystridium spp., leiospheres). Despite a possible taphonomic bias in the uppermost Ellis Bay Formation, the major crisis in the late Hirnantian coincides with the second δ13C excursion and a second, stronger glacially-driven sea-level fall corresponding to the time of the maximum extend of the ice sheets on Gondwana. (3) Such phytoplankton events have also been detected in Gondwanan ice-center area, but with “typical Ordovician taxa crises” occurring apparently later when plotted against positive carbon isotope excursions, as also observed in Baltica. This time-lag may be linked to a basin configuration of the studied areas, where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost Ordovician strata of Anticosti. In addition to their biostratigraphic and palaeobiogeographic utility, acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions. [Copyright &y& Elsevier]

Recent recalibration of the Silurian timescale and improved global chronostratigraphic correlation of Silurian strata significantly altered the Silurian 87Sr/86Sr curve and the temporal extent of available data. Whereas previous Silurian 87Sr/86Sr composites showed a generally monotonic increase throughout the Silurian, revisions to the Silurian timescale now require a major increase in the rate of change in 87Sr/86Sr at or near the onset of the Gorstian Age of the Ludlow Epoch. Similarly, improved chronostratigraphic correlations between Silurian outcrops on Anticosti Island, Canada, and Gotland, Sweden, indicate that the middle part of the Telychian Age, which is roughly 10%–15% of the totalduration of the Silurian period, is undersampled and underrepresented in Silurian 87Sr/86Sr composites. A revised Silurian 87Sr/86Sr curve based on 241 new and published analyses confirms the significant increase in the rate of change of 87Sr/86Sr toward more radiogenic values near the base of the Ludlow Series. On the basis of these data, we propose that the rapid trend toward more radiogenic 87Sr/86Sr values is indicative of increased weathering of old sialic crust exposed during the Silurian uplift of portions of Baltica, Laurentia, and Avalonia. Importantly, however, the actual rate of change of 87Sr/86Sr will remainequivocal until the durations of Silurian epochs and ages are better constrained. [ABSTRACT FROM AUTHOR]

Abstract: About 500 samples from 11 Ordovician sections in South China have been analysed for their δ13Ccarb composition. Four sections are located in the Yangtze platform region, the remaining seven sections in the Jiangnan slope region. The sections span more or less the entire Ordovician, but the sample resolution in its lower part (Tremadocian to Dapingian) is lower than in its upper part (Darriwilian to Hirnantian). Overall, the data show a good correlation with the published global isotope curve. At the Cambrian–Ordovician boundary positive values are observed. In the succeeding Tremadocian the values decrease and remain negative into the early Floian. In the late Floian, the values increase to slightly positive. The correlation of Dapingian rocks between the Yangtze and the Jiangnan regions is somewhat problematic which might be due to high siliciclastic input in the Yangtze section. In the Darriwilian a small positive excursion is observed in the Yangtze platform area, however, it is missing in the Jiangnan slope area. This excursion probably correlates with the mid-Darriwilian excursion (MDICE) reported from Baltoscandia. Both chemostratigraphy and conodont biostratigraphy indicate a possible gap between the Shihtzupu Formation and the Pagoda Formation, i.e. most of the Sandbian is probably not represented in the Upper Yangtze region. The earliest Katian is characterised by a pronounced positive excursion correlating with the well-known global Guttenberg excursion (GICE). In the late Katian at least one additional positive excursion was determined. The precise biostratigraphy, however, is not clear, and therefore it is not known which of the known four to five remaining Katian δ13C excursions identified in Baltica correlate(s) with the one(s) observed in the present study, and further biostratigraphic study is required. The lower part of the Hirnantian excursion (HICE) is probably not preserved, with only small remnants of the HICE observed near the base of the Kuanyinchiao Bed. [Copyright &y& Elsevier]

The base of the Middle Ordovician (i.e. Dapingian Stage) has been defined at the first appearance datum (FAD) of conodont Baltoniodus? triangularis at Huanghuachang, Yichang, China, but the precise correlation of the boundary to regions of other facies remain to be resolved. Herein we review the biostratigraphy and chemostratigraphy of the Huanghuachang Global Standard Stratotype-Section and Point (GSSP) section, and present our latest stratigraphic work on the nearby Chenjiahe section in Yichang, and the Hengtang Quarry section, Jiangshan, Zhejiang, which is regarded as representative of slope facies. The conodont and graptolite biostratigraphy as well as chemostratigraphy of the Chenjiahe section indicate that the base of the Middle Ordovician also falls within the graptolite Azygograptus suecicus Zone, and coincides with a high or maximum δ13C value within a minor positive carbon isotope excursion, suggesting that the base boundary can be readily recognized across the entire Yangtze Gorges area. The integrated graptolite and conodont biostratigraphy and chemostratigraphy of the Hengtang section, Jiangshan, indicates that the basal boundary probably falls within the graptolite Isograptus caduceus imitatus Zone that overlies the Azygograptus suecicus Zone, and coincides with a remarkable drop of δ13C. This difference indicates that a multi-disciplinary approach is critical to identify the base boundary in those regions where the Baltoniodus? triangularis is absent [ABSTRACT FROM AUTHOR]

Abstract: Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans, the Ordovician and Silurian periods witnessed three subsequent major biotic events: the Great Ordovician Biodiversification Event, the end-Ordovician extinction (the first animal extinction and second largest of the five mass extinctions of the Phanerozoic), and the Early Silurian post-extinction recovery. There are currently no simple explanations for these three major events. Combined extrinsic (geological) and intrinsic (biological) factors probably drove the biodiversifications and radiations, and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography, palaeoclimate and sea-water chemistry. The present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the Ordovician and Silurian, including sea-level changes, geochemical proxies (δ13C, δ18O, 87Sr/86Sr) of the ocean waters, and the evolution of the atmosphere (oxygen and carbon dioxide content). [ABSTRACT FROM AUTHOR]

Abstract: Scanning electron microscope examinations of polished and etched surfaces of sediments from the Silurian carbonate platform of Gotland, Sweden, revealed the presence of numerous, morphologically diverse “calcispheres” (Calcitarcha). Some of these spherical calcareous microfossils display wall structures that are surprisingly similar to those of calcareous dinoflagellate cysts. In analogy to the interpretation of the biological affinities of Palaeozoic acritarchs as cysts of organisms that might have been the ancestors of organic-walled dinoflagellates, the Calcitarcha from Gotland can be compared and may possibly be related to organisms that may have been the ancestors of calcareous cyst-producing dinoflagellates that so far have not been observed before the Late Jurassic. [Copyright &y& Elsevier]

Due to the lack of biostratigraphically useful graptolites in the crinoidal and reefal Chicotte Formation on Anticosti Island (Québec, Canada), the precise chronostratigraphic position of the formation is not known. New stable carbon isotope and conodont data and a comparison with the detailed conodont distribution and δ13C development in the Viki core (Estonia) suggest an early Telychian age (Pterospathodus eopennatus to P. a. angulatus zones) for at least the lower half of the formation. This interval is characterized by a small but distinct positive excursion in δ13C, with peak values of + 3.2‰ and + 2.8‰ VPDB in the lowermost 10?15 m of the Chicotte Formation on Anticosti and in the Viki core, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Sinoporella leei was first described and illustrated by Yabe in 1949 as a new species and was ascribed to a new subgenus of the genus Triploporella (Steinmann). This alga was not illustrated again until Mu published 25-years later a photomicrograph of a longitudinal-tangential section. The recent finding of new specimens in Permian strata of the Guangxi region (South China) allows us to better illustrate this alga, and to improve our understanding of its morphology to the extent that we find it necessary to emend the current diagnosis. The new observations focus mainly on the morphology of the second-order laterals, on their relationship to the first-order laterals, and the connection of the first-order laterals to the main stem. These data suggest that Sinoporella represents a self-standing, discrete genus that differs from Triploporella. We also consider the lack of cystophores (cyst containers sensu De Castro) and the possible occurrence of individual cysts in the first order laterals as characteristics that do not accord with Barttolo''s emended diagnosis of Triploporella. [Copyright &y& Elsevier]

The Natiscotec outcrop on Anticosti Island, Canada (Ellis Bay Formation, Laframboise Member, Hirnantian, Late Ordovician), exposes a patch reef some 20–30 m in diameter, 2–3 m thick, with abundant rugose corals, as well as favositids and heliolitids. Reef capping and flanking sediments include typical Hirnantian brachiopods, such as Hirnantia, Hindella, and Eospirigerina. Within the reef peloidal microbialites encrusting bryozoan colonies are common. The bryozoan fauna includes three cystoporates, seven trepostomes, and three phylloporines. Two genera and two species are new: the cystoporate Natiscotecella tenuis n. gen. and n. sp. and the phylloporine Dilaminocladia natiscotecensis n. gen. and n. sp. Three more species are also new: the cystoporates Ceramopora clara n. sp. and Acanthoceramoporella spinigera n. sp. and the trepostome Revalotrypa honguedensis n. sp. Furthermore, we identify the three trepostomes Atactoporella aff. ortoni (Nicholson, 1874), Hallopora elegantula (Hall, 1852), and Monotrypella cf. aequalis Ulrich, 1882, and a phylloporine Parachasmatopora porkunensis Lavrentjeva, 1985. Four species are identified at generic level and in open nomenclature: the three trepostomes Lioclemella sp., Calloporella sp., and Trepostomata sp. and a phylloporine ? Ralfinella sp. The bryozoan fauna shows some affinities with the Late Ordovician fauna of Scandinavia. Stable carbon isotope investigations from brachiopod shells of the same outcrop yield values for δ13C of up to +6.7‰, which represent the highest values reported from the Anticosti succession so far. This indicates that the stratigraphic position of the outcrop is at or close to the peak of the globally recognized Hirnantian δ13C excursion. [ABSTRACT FROM AUTHOR]

The Upper Jurassic (Tithonian) plattenkalk successions in the Solnhofen/Eichstätt area consist of alternations of thin-bedded, laminated, fine-grained, very pure limestones (so-called ‘flinz beds’) and softer interlayers with slightly lower carbonate contents that are also laminated and show a foliaceous weathering appearance (‘fäule beds’). These successions are world famous for their exceptionally well-preserved fossils. In contrast to the well-studied wealth of fossils, little is known about the origin and diagenesis of the host rock. The reason for this discrepancy might lay in the monotonous appearance of these fine-grained mudstones that require electron microscopical examination. Study of samples from the Solnhofen–Eichstädt area implies that flinz and fäule beds have undergone differential diagenesis. The ultrastructure of the flinz beds is characterized by interlocking microspar crystals, whereas the fäule beds show smaller and less interlocking crystals. The ratios of diagenetically inert trace elements lack clear differences between the two interlayered lithologies. While most authors agree that the flinz–fäule rhythm reflects rhythmically changing environmental conditions, primary rhythms can be taken as proven only where statistically significant differences in diagenetically resistant proxies are found. The absence of clear primary differences between flinz and fäule beds, however, leaves the question of primary differences unsolved. It is concluded that diagenesis has had a strong influence on the genesis of the lithological rhythm, and that any primary rhythm underlying the diagenetically mature rhythm is less clear than generally assumed. [ABSTRACT FROM AUTHOR]

Since the establishment of astrochronology, calcareous rhythmites are frequently used as the basis of high-resolution chronostratigraphy. In particular for the Neogene, calcareous rhythmites serve as stratotypes and for absolute dating of stratigraphic boundaries (Global Stratotype Sections and Points – GSSPs). However, the exact mechanisms responsible for the formation of the rhythmic intercalation of lithologies in such successions are complex and not easily reconstructed. To a large extent this is the effect of diagenetic modifications of the original sediment. Here, two examples of Neogene calcareous rhythmites are studied; the Monte dei Corvi section and the Monte Gibliscemi section. The first is the GSSP location of the Serravallian–Tortonian boundary, whereas the second is an auxiliary stratotype for the same boundary. During the past years, astrochronological approaches were applied to these successions to considerably increase time resolution compared to the elaborated biostratigraphic database. The present study focuses on micropetrographic, trace element and clay mineralogical methods in order to gain a better understanding of the genesis of the rhythmites. In the Monte Gibliscemi section, sediment parameters that are robust against diagenetic change clearly reflect primary differences, i.e. cyclical environmental changes. In contrast, no clear primary signal is determined for the Monte dei Corvi section on a couplet scale, leaving the origin of the rhythm ambiguous. This impedes the interpretation of the latter and the comparability between the two successions, and also compromises any bed-by-bed correlation between the two. The unclear origin of the rhythmites of Monte dei Corvi introduces uncertainty into the applicability of astrochronology to this succession. [ABSTRACT FROM AUTHOR]

In order to investigate whether or not equivalents to modern calcareous plankton existed in Palaeozoic times, extremely well-preserved successions have to be investigated. The Silurian strata on Gotland (Sweden) are exceptionally well preserved because they have not experienced deep burial conditions and tectonic stress, due to their position on the stable Baltic Shield. Scanning electron microscope investigations of polished, slightly etched rock surfaces revealed the presence of a variety of calcareous micro- and nannofossils. Among these organisms, many can be termed ‘calcispheres’ (60–100 µm in diameter), whereas others due to their size range (nannofossils) are informally termed herein as ‘nannospheres’ (10–25 µm in diameter). The systematic attribution of these fossils is unknown ( incertae sedis). Mesozoic calcispheres are usually attributed to calcareous cysts of dinoflagellates following comparisons of ultrastructure with modern species. The abundance of different calcispheres in the Silurian sediments of Gotland and the observation that many of the calcareous microfossils occur in distinctly different facies as well as their spherical shape indicate that they probably belong to calcareous micro- and nannoplankton. We therefore conclude that calcareous plankton most probably existed already during the Palaeozoic, but can only be observed under conditions of exceptional preservation. [ABSTRACT FROM AUTHOR]

Abstract: The Silurian is characterised by strong environmental changes, as indicated by several pronounced positive δ 13C and δ 18O excursions. The mechanisms responsible for these isotopic shifts are a matter of much debate. The purpose of the present study is a quantitative high-resolution evaluation of the palynomorph distribution across the Late Ludlow (Ludfordian) isotope excursion on Gotland. Marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved. Moreover, the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms. To correlate our results with the carbon isotope stratigraphy, we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells. Our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development. Low abundances of acritarchs occur in times of high stable isotope values and vice versa, indicating that an increase in marine productivity cannot have been the reason for the positive δ 13C excursion. The results are in good agreement with climatic models for the Silurian assuming alternating humid and arid climatic conditions in low latitudes. Times of high stable isotope values correspond to arid climatic conditions in low latitudes, with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities, whereas times of low stable isotope values correlate with humid climate, high nutrient input, and abundant and diverse acritarch communities. The major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms (graptolites, conodonts) have been affected. This indicates that environmental changes connected with the stable isotope excursion first affected deeper-water settings and later the photic zone. The absolute abundance of terrestrial spores closely mirrors the marine δ 13C and δ 18O development, showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes. The high abundance of spores during the isotope excursion might be explained by increased aeolian input. [Copyright &y& Elsevier]

To better understand palaeoecological signatures in Palaeozoic acritarch assemblages, the distribution of palynomorphs has been quantitatively studied in eight localities from the Gorstian, lower Ludlow (Late Silurian) of Gotland, Sweden. The localities are situated along an inshore–offshore transect comprising shallow marine lagoonal environments to distal shelf facies. Process-bearing acritarchs and sphaeromorphs constitute the main components within the palynomorph assemblages. The lateral distribution of palynomorphs exhibits characteristic features at three different levels as follows. (1) With regard to the overall composition of the palynomorph assemblages, the abundance of process-bearing acritarchs increases towards the distal shelf, while the abundance of sphaeromorphs decreases. (2) At the generic level, the acritarchs Micrhystridium and Dilatisphaera are more abundant in the proximal facies, while Evittia, Percultisphaera and Oppilatala are more common in distal environments. (3) At an infrageneric level, Micrhystridium morphotypes with shorter processes are mainly present in proximal environments, while those with longer and ramified processes occur in more distal shelf environments. The palynomorph distribution along the inshore–offshore transect highlights the potential of acritarchs and prasinophytes as palaeoenvironmental indicators. [Copyright &y& Elsevier]

Characteristically, limestone–marl alternations are conspicuous in outcrop because of differential weathering, the marls being predisposed to stronger deterioration. Superimposed on the bedding rhythms are in many cases lower-frequency meter-scale rhythms, so-called bundles. These bundles are eye-catching from a distance, and are typified by variations in thickness of limestone beds and marl interbeds. The palaeoenvironmental significance of the bundles received markedly less attention in the literature than the origin of the small-scale couplets. Here we focus on the palaeoenvironmental significance of the bundles by reconstructing the initial mineralogical composition. As an example, we present a classical limestone–marl alternation from the Upper Jurassic (Oxfordian–Kimmeridgian) of SW Germany that is typified by two orders of bundles superimposed on the limestone–marl couplets.Our method of reconstruction of the original mineralogical composition of the precursor sediment of limestone–marl alternations is based on a diagenesis model that assumes dissolution of aragonite and reprecipitation as calcite cement as the major mechanism of early diagenesis. Calculated initial clay, aragonite, and calcite contents show rhythmic variations on a scale of several meters that correspond to the bundles as seen in the outcrop. Limestone-dominated parts of the bundles correspond to intervals of elevated aragonite content in the precursor sediment, but not necessarily to elevated calcite contents. Additionally, an overlain fluctuation in initial mineralogy on the order of tens of meters (mega-bundles), reflecting the former local subdivision of the SW German Upper Jurassic into “Weißjura alpha”, “beta”, and “gamma”, is observed. The limestone-dominated “Weißjura beta” (lower Kimmeridgian) is the interval with highest initial aragonite content.Our new method allows for quantification of initial mineralogical composition. As aragonite and calcite usually have different sources (shallow-water benthic versus planktic), this approach potentially adds significant new possibilities for palaeoceanographic interpretation of calcareous rhythmites. In the Jurassic example, the observed variations in reconstructed aragonite content are interpreted as variations in flooding events and related export of aragonitic material from marginal shallow-water platforms. [Copyright &y& Elsevier]

For a long time, the Silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the Late Ordovician glaciation. During the past decade, knowledge about the Silurian has increased markedly and today it is known that the conditions in the Silurian were much more variable than previously assumed. Detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values. In low latitudes, these periods of high C- and O-isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms. The sediments deposited during these excursions contain impoverished fossil assemblages, especially with respect to conodonts, graptolites, and trilobites. A conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the Llandovery/Wenlock boundary. This event is called the ‘Ireviken Event’ after its type locality on the island of Gotland, Sweden (Jeppsson, L., 1987. In: Palaeobiology of Conodonts. Ellis Horwood Ltd., Chichester, pp. 129–145). Here, isotope data from nine sections at the NW coast of Gotland are presented that cover the time interval of the Ireviken Event. The δ13C mean values rise from +1.4‰ to +4.5‰, and the δ18O values increase from −5.6‰ to −5.0‰. The relative timing of stable isotope development, extinctions, and facies development is discussed. It is shown that first extinctions precede the isotope excursion. This indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes. Other events characterised by similar relationships between positive isotope excursions, mass extinctions, and facies development are found in younger parts of the Silurian (late Wenlock and Ludlow), in the late Ordovician, the late Cambrian, and, with some reservations, in the Proterozoic. The similarities between these events indicate analogous controlling mechanisms. For the Silurian, climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors. A palaeoceanographic/climatic model was postulated which is consistent with most of the sedimentological, palaeontological, and geochemical data. Here we demonstrate that this model may be applicable also for the older events. [Copyright &y& Elsevier]

Ancient limestone-marl alternations are concentrated in settings analogous to loci of aragonite accumulation in the modern world. They typically occur on shelves in the tropical-subtropical climate belt, are far more abundant on passive continental margins than on active ones, and are rare in upwelling zones. In recent studies, aragonite was proposed to play an important role in differential diagenesis typical of most limestonemarl alternations. The coincidence of depositional settings of ancient limestone-marl alternations and modern aragonite accumulation is a strong case for this hypothesis. If confirmed, it could provide a valuable tool for broad-scale paleoenvironmental interpretations. An additional, different type of limestone-marl alternations resulted from the Cretaceous explosion in productivity of calcitic plankton: these pelagic ones are fundamentally different in their style of diagenesis. [ABSTRACT FROM AUTHOR]

Scanning electron microscopy of polished, slightly etched rock surfaces of samples from the Silurian of Gotland, Sweden, revealed the presence of excellently preserved calcareous microfaunas and microfloras. This paper illustrates several morphotypes of calcareous micro-organisms of unknown biological affinity, attributed to the new family Ovummuridae. Specimens of Ovummurus duoportius Minoura and Chitoku, a problematical microfossil, which was first described from the Upper Pennsylvanian of Kansas, USA, were found in addition to comparable micro-organisms attributed to the following new taxa: Minourella gotlandica gen. et sp. nov., Arouxina pluricellata gen. et sp. nov. and Samtlebenella circumcamerata gen. et sp. nov. In addition, a few other problematical micro-organisms, probably belonging to the same family, are described under open nomenclature. The enigmatic microfossils are compared with other known calcareous organisms, their possible biological affinities are discussed, and the different stages of diagenetic alteration and destruction are illustrated. [ABSTRACT FROM AUTHOR]

This study presents the first record of stable carbon and oxygen isotopes of well-preserved belemnites and bulk sediment from the Upper Jurassic pelagic carbonates of the Ouarsenis Mountains (northwestern Algeria), which were deposited at the southern margin of the Tethys ocean. Cathodoluminescence and scanning electron microscopy was used to eliminate potentially diagenetically altered specimens (brachiopods, aptychi, and some belemnites). Ultimately, 55 belemnite specimens were used to reconstruct water temperatures for the Middle Oxfordian to the Early Tithonian. Their δ18O carb values ranged from −0.35‰ to −3.88‰, suggesting warm and mostly stable temperature conditions during the Late Jurassic. Using a δ18O sea value of 0‰ for the tropical palaeolatitude of 15–20°N, the δ18O carb values of the well-preserved belemnite rostra translate into average temperatures of 27.8 °C in the Middle Oxfordian (Gregoryceras transversarium Zone), 27.0 to 27.5 °C in the Late Oxfordian (Epipeltoceras bimmamatum and Idoceras planula zones, respectively), 26.2 °C in the Early Kimmeridgian, and 27.3 °C in the Early Tithonian (Hybonoticeras hybonotum Zone). • Low latitudes display stable climatic conditions throughout the Late Jurassic. • Calculated sea surface temperatures throughout the Late Jurassic are 27 °C ± 0.8 °C. • Late Jurassic latitudinal temperature gradients are comparable to present-day. [ABSTRACT FROM AUTHOR]

The Calcarenite di Gravina is a poorly lithified, heterogeneous bio-lithoclastic Plio-Pleistocene limestone formed in a temperate shallow-water setting. Since prehistoric times it has been used for construction, as witnessed by the "Sassi", the old city center of Matera (southern Italy). Here, the relatively distal, fine-grained bioclastic grain-packstone is characterized by large bioturbation structures of crustaceans. The aim of this work is to understand how bioturbation controls texture, porosity, permeability, and the cementation of the calcarenite. Crustaceans such as Callianassa are known to produce vertical burrows and sort grains. A bioturbated calcarenite level, outcropping in the Parco Scultura- Cava Paradiso in La Palomba (Matera northern area), has been selected as case study. Large rock samples including fossil traces and surrounding rocks have been analysed. Optical microscope and SEM observations show a close connection between sediment sorting and cementation, and 3D analyses by means of CT scans indicate that the internal structure and the connected porosity of the samples is a function of the cementation. Moreover, the porosity network is reconstructed in 3D in order to derive the real connected porosity variations and their implications for the evolution in terms of diagenesis and alteration of the rocks. Indeed, a cementation gradient from the external lining of the burrows to the internal part of the trace fossil is observed. Cementation is less pronounced outside of the trace fossil. Cluster analysis of point-counting results allows to identify four sub-facies characterized by differences in composition, cementation and porosity. One of them is present in the interior of the burrows while the other two characterize the external lining (crust) and storage chamber of crustacean burrows. The last sub-facies occurs in the area surrounding the trace fossils and shows a weaker cementation. The presence of a cementation gradient inside the trace fossil is explained through a diagenetic model that considers the contribution of different factors. The geochemical environments allow the CaCO 3 precipitation inside the burrows starting from the lining to the internal area thanks to the microbial actions, water circulation and relative oxygenation of fluids. Dissolution of aragonitic components probably represents the principal source of CaCO 3. Variation in cementation and porosity are quite moderate, but capable of generating major differences in the resistance to weathering. From this study it emerges how the diagenetic processes lead to significant changes in the bioturbated fabric, making the burrows less permeable than the surrounding sediment. This is confirmed by the selective erosion that brings to light very well-preserved traces of Callianassidae even on building stones. • Microfacies analysis of bioturbated calcarenite by means of optical and electron microscopy. • Qualitative and Quantitative subdivision of 4 sub-facies in bioturbated soft limestone. • High resolution study of 3D porosity network performed by means of Micro-Computer-Tomography. • Cementation of Gravina Calcarenite is favoured inside the burrow traces. • Diagenetic evolution affected petrophysical properties of soft limestone. [ABSTRACT FROM AUTHOR]

Artificially cemented sandstones were produced to assess the impact of detrital texture and composition on the precipitation and distribution of early calcite cement, and cement-related degradation in porosity. To simulate early-calcite cementation, loose sediment of variable composition (siliciclastic and calcareous) and grain size was exposed to a calcite supersaturated solution for 35 to 58 days at 23°C. Identification and distribution of the newly precipitated crystals was performed with high resolution 2D optical and scanning electron microscopy. The experimental results show the precipitation of grain-coating, pore-bridging and pore-filling granular calcite cement with up to 100 μm crystal size. Despite a positive correlation between the amount of detrital carbonate grains and calcite crystals, calcite cement does not preferentially nucleate on bioclast surfaces, irrespectively of their favourable mineralogy. Siliciclastic grains show high calcite cement coverage with altered feldspar, particularly plagioclase, displaying coverage of 94.3%. Grain size variations within the sand packs have influence on the precipitation pattern of calcite with coarse-grained layers (500–710 μm) showing minor calcite cementation (6.3%), while medium- (250–500 μm) to fine-grained layers (125–250 μm) comprise average calcite cement contents of 16.3% and 28.2%, respectively. The findings of this study enhance our knowledge regarding the precipitation processes of calcite in porous material with heterogeneous reacting mineral phases, shapes and pore connectivity. [ABSTRACT FROM AUTHOR]

The age and dolomitization processes in the Paleo-oil reservoir zone, which is composed of massive dolostones found in the Qiangtang Basin (SW China), are still debated. In this research, the Long'eni-Geluguanna Area was selected. Macroscopic information, thin sections, and geochemical methods were used to investigate the dolomitization characteristics and the processes that controlled dolomitization. Five types of replacive dolomites and two types of dolomite cement were observed. Some of the dolomites displayed ghosts of primary sedimentary structures. Saddle dolomites were prevalent, occurring in the interparticle and moldic pores of the limestone which should have been filled at an early diagenetic stage. Ten microfacies types were identified. The foraminifera assemblage provides evidence that the studied interval is of Early Jurassic age. The δ13C values are similar to the contemporaneous seawater signature. The REE+Y patterns of limestones and dolostones exhibit similarities to that of seawater. The mean Na and Sr values are comparable to those of other near-normal seawater dolomites. The δ18O values of all lithologies are markedly depleted. The dolomitization started penecontemporaneously, with deposition. A general sand shoal setting with patch reefs developed. The dolomitizing fluids, near-normal seawater, was probably formed by slight evaporation on top of the shoal. Saddle dolomites in the interparticle and moldic pores might indicate hydrothermal activity, which also caused the recrystallization of some pre-existing dolomites. The recrystallization might have slightly increased the crystal size, demolished the ghost structures, formed saddle dolomites, and altered the REE+Y patterns. The recrystallization extent diminished with increasing distance from the fluids-providing fracture. Furthermore, the existence of protected areas within the sand shoal settings could enhance the vertical and horizontal heterogeneity of dolostone reservoirs. [ABSTRACT FROM AUTHOR]

The algal genus Vitinellopsis nov. gen. is well represented in the Silurian limestones of Gotland (Sweden). This new taxon is interpreted as a calcified codiacean alga (Chlorophyta, Bryopsidales). In the studied material, the calcareous wall of sparry calcite, probably originally aragonitic, appears well preserved. The subcylindrical thallus, with rounded ends, rarely bifurcated, measures up to 27 mm long and 2.5 mm wide. The medullar zone exhibits 4–6 longitudinal, subtly undulating siphons; the subcortical zone shows inflated, vesiculiform radial siphons, and the cortical zone displays small, relatively short acrophorous siphons. The status of the tribe Vitinelleae nomen novum , formerly Bothryelleae, is discussed. Given its abundance, ease of identification, and short stratigraphic range, Vitinellopsis gotlandica nov. gen., nov. sp. may prove useful for regional correlations, facies and paleobiogeographic studies. [ABSTRACT FROM AUTHOR]

Compared to modern counterparts, bioerosion is rare in Paleozoic reefs, especially macro-bioerosion. The unique and enigmatic Silurian reefs from Gotland (Sweden), composed of bryozoans and microbial laminates, show evidence of a large amount of bioerosion. The samples contain Trypanites trace fossils, as well as a large number of undescribed macroborings. Small articulated bivalve shells are preserved in some of these macroborings, identified from thin-sections. Three-dimensional images from micro-computed tomography (microCT) reveal an additional bivalve, which is occupying a bioerosion trace. This specimen is possibly contained in a different boring that can be classified as possibly clavate-shaped. Furthermore, evidence of nestling, such as a subsequent modification of the ichnofossils, the presence of bivalves that are much smaller than the trace, or the presence of additional specimens, is missing; therefore, it is most likely that the bivalves made the borings. This is evidence for the existence of deep-boring bivalves in the Silurian. [ABSTRACT FROM AUTHOR]

Asymmetric, pendant cements are considered good indicators for early lithification in the vadose zone. In the present study, asymmetric cements are recorded in thin-sections of a Lower Jurassic limestone from the Traras Mountains (northwest Algeria). Geopetal fabrics, however, indicate that these seemingly "pendant cements" are, in some places, oriented upwards, i.e., they have grown in the opposite direction from that expected, or they grew from grains towards the pore centers. These observations disprove their origin as gravitational cements precipitated from pendant water droplets on the undersides of grains as in the vadose zone. In contrast, a formation in the marine phreatic zone seems more probable. Under high-energy conditions, and after an early lithification stage with isopachous cements in the subtidal zone, strong tidally driven horizontal pore-water flow allowed sufficient seawater to pass through the slightly cemented but still highly permeable rock. Those grain sides, which were oriented towards the pore center, where faster flowing water prevailed, were more exposed to CaCO3-supersaturated percolating seawater and therefore the cements precipitated here show their greatest thickness. In relatively more protected areas around the margins of the pores, asymmetric cements are rarely developed. The resulting rock exhibits an unusual, heterogeneous cementation with preferential centripetal nucleation areas. [ABSTRACT FROM AUTHOR]