Shallow-marine Carbonate Successions Research Articles

Controls on the terrigenous fractions in Early Kimmeridgian shallow-water carbonate deposits in Southern Iberia

AbstractTwo Kimmeridgian shallow-marine carbonate successions (mid-shelf), sharing a similar paleoclimatic framework (climatic zone), were previously explored using carbonate chemostratigraphy. Before, the goal was to detect signals related to paleoplatform bottom physiography, degree of connection with oceanic waters and overall circulation patterns. In this contribution, complementary bulk mineralogical composition and clay mineral fractions are investigated to contrast and complement previous information, aiming for a more complete overview of continent-ocean dynamics along shallow-carbonate platforms of southern Iberia. The goal is now to explore complex patterns of continental influence along proximal settings and their relative spatial distribution across differentiated settings without relevant difference in paleolatitude. The choice of both stratigraphic sections is based on this distinctness: Rocha Poço (Algarve Basin, Portugal) represents a more restricted and relatively proximal setting, in contrast to Puerto Lorente (South Iberian Paleomargin, S Spain) placed at a relatively more open and probably distal shallow-water context. Accordingly, quartz content was higher at Rocha Poço, especially at the lower siliciclastic interval of this section. Quartz contribution fades out at Puerto Lorente, where it was mainly controlled by short-lived terrigenous pulses. Clay mineral assemblages also differed, being more varied (smectite, illite and traces of kaolinite) and abundant at Rocha Poço, and generally leaner at Puerto Lorente. At the latter site, terrigenous pulses do not contribute to clay mineral abundance, only showing abundant illite at the topmost horizons. New information retrieved from mineralogical data provided evidence on depositional contrasts resulting from local differences in platform configuration, allowing a better understanding of mechanisms controlling the terrigenous fraction in the shallow-water carbonates analyzed.

Snowball earth: The African legacy

The stratigraphic association of late Precambrian glacial deposits with shallow-marine carbonate successions, which are biased toward the warmer parts of the surface ocean, was known in Africa and elsewhere since the 1930's, but geologists under-appreciated it as a paradox in physical climatology. A physical explanation for global freeze-up emerged from one-dimensional energy-balance climate models in 1969, after which it was widely discussed and refined. In 1981, it was suggested that a global freeze-up could be self-terminating after millions of years through slow accumulation of atmospheric CO2, a greenhouse gas. The idea of a self-reversing global freeze-up, or ‘snowball Earth,’ was first applied geologically by Kirschvink (1992) based on paleomagnetic evidence in South Australia. He cited no paper in physical climatology and geologists remained unaware of developments in that science until after 2000. A self-reversing global freeze-up involves strong predictions in duration, synchroneity and termination state that have now been tested in the geological record. Inflections in growth rate and disciplinary diversity of peer-reviewed papers in 1997 implies that climate physics and geology were galvanized to test both the Siderian (ca 2430 Ma) and Cryogenian (717−635 Ma) snowball Earth hypotheses by new field research conducted in South Africa and northern Namibia, respectively.

Ichnological evidence of marine incursions in the lacustrine–palustrine Bembridge Limestone Formation (Eocene), Isle of Wight, southern England

Trace fossils are described from the Eocene Bembridge Limestone Formation from the Isle of Wight and used to constrain the paleoenvironmental interpretation. The lacustrine–palustrine succession contains three limestone beds, which are separated by clay and marl. The middle and upper limestone beds reveal complex burrow systems developed at their top. Based on their characteristics, these burrow systems are assigned to the ichnotaxon Balanoglossites triadicus Mägdefrau, which is associated with the shallow superficial grooves Sulcolithos variabilis Knaust. B. triadicus is a common marine trace fossil mainly known from shallow-marine carbonate successions throughout the Phanerozoic. It is accompanied by other marine ichnotaxa such as Arachnostega gastrochaenae Bertling, Gastrochaenolites isp. aff. G. ornatus Kelly and Bromley, Spongeliomorpha iberica Saporta and Thalassinoides suevicus (Rieth). This ichnological evidence confirms the occurrence of short-term marginal-marine incursions in a predominantly lacustrine to palustrine environment.

Carbon and oxygen stable isotope record of upper Kimmeridgian shallow-marine ramp carbonates (Iberian Basin, NE Spain): the imprint of different burial and tectonic histories

Bulk carbon and oxygen stable isotopes of ancient shallow-marine carbonates can record the effects of multiple palaeoenvironmental factors, but also the imprint of several post-depositional processes, which may alter the original marine isotopic composition. In this study, carbon and oxygen stable isotope analyses were performed on bulk carbonate, bivalve calcitic-shell (Trichites) and calcite vein samples from two stratigraphic sections (Tosos and Fuendetodos, present-day distance 15km), representing proximal inner- and distal mid-ramp environments, respectively, of the uppermost Kimmeridgian ramp facies deposited in the northern Iberian Basin (NE Spain). These successions underwent different diagenetic pathways that altered the primary marine isotopic composition in each section in different ways. Different burial histories, tectonic uplift and a variable exposure to meteoric diagenesis from the end of the Kimmeridgian to the Cenozoic (following Alpine tectonic uplift) are reflected in the different alteration patterns of the carbon and oxygen stable isotope signatures. A significant deviation to lower values in both δ13O and δ18O is recorded in those carbonates mostly exposed to meteoric diagenesis (distal mid-ramp Fuendetodos section), because of post-depositional tectonic uplift (telogenesis). On the other hand, the deposits mainly affected by burial diagenesis (proximal inner-ramp Tosos section) only record low δ18O with respect to expected values for pristine Kimmeridgian marine carbonates. The different burial and tectonic uplift histories of these deposits in each sector, due to their different tectonic evolution in this part of the basin, resulted in a variable degree of diagenetic resetting. However, in spite of the different diagenetic resetting reported of the carbon and oxygen stable isotope signatures in each section, these carbonates show similar cement types in termsof fabrics and cathodoluminescence properties. The diagenetic resetting reported for these carbonates prevents the use of the δ13O and δ18O records for addressing palaeoenvironmental interpretations, but instead highlights useful features regarding the variable diagenetic overprint of the studied shallow-marine carbonate successions concerning their specific post-depositional history.

Shallow-marine carbonate sedimentation in a tectonically mobile basin, the Muschelkalk (Middle Triassic) of Upper Silesia (southern Poland)

Abstract Earthquakes and resulting tsunamis are important geological phenomena, however their sedimentary paleorecord is difficult to identify because some fabrics can be generated by other processes and the criteria for discrimination of seismic and non-seismic triggers are not fully established. This paper presents several deformation and sedimentary structures in the Middle Triassic carbonates of Upper Silesia and discusses their seismogenic and tsunami origin and controls on the deformation type, intensity, preservation potential, and geographic distribution. The deformation type and intensity were controlled by the primary lithology, whereas the lateral continuity of deformed horizons depended on the lateral extent of hosted facies. The thickness and abundance of these structures increase towards the Krakow-Lubliniec Fault Zone which bounded the area from the east and is considered as the main epicentrum of earthquakes and tsunami initiation. The discussed fabrics have a universal significance and can potentially be found in other shallow-marine carbonate successions.

Ordovician and Silurian ichnofossils from carbonate facies in Estonia: A collection-based review

Trace fossils are common in the Ordovician and Silurian shallow marine carbonate succession of Estonia, with 45 ichnofossil genera and five bioclaustration structures identified, representing 31 categories of architectural designs and nine categories of ethological classification. Diverse soft sediment traces, bioerosional traces and bioclaustrations occur both in the Ordovician and Silurian. Diversity of trace fossils is similarly high in the Late Ordovician and Silurian, but markedly lower in the Middle Ordovician. This could be explained by the fact that during the Late Ordovician, Baltica drifted to the subtropical climatic zone where ichnofauna is usually more diverse than in temperate climatic settings. In addition, the Great Ordovician Biodiversification reached its peak in the Late Ordovician for many groups of organisms, which further contributed to the increase in ichnodiversity. Distribution of trace fossils is also controlled by the type of sedimentation, so that the mixed carbonate-siliciclastic systems prevailing in the Late Ordovician and Silurian have higher ichnodiversity than the pure carbonate settings of Middle Ordovician age. Feeding and locomotion traces are relatively rare in the Ordovician and Silurian of Estonia with the exception of the feeding structure Arachnostega, which is formed inside of protective shells and therefore has abundant occurrences. Bioerosional trace fossils may be extremely common in places, with a large number of different genera in the Upper Ordovician, supporting the idea of the Ordovician Bioerosion Revolution.

Geostatistical Modelling of Cyclic and Rhythmic Facies Architectures

A Pluri-Gaussian method is developed for facies variables in three dimensions to model vertical cyclicity related to facies ordering and rhythmicity. Cyclicity is generally characterised by shallowing-upward or deepening-upward sequences and rhythmicity by the repetition of facies at constant intervals along sequences. Both of these aspects are commonly observed in shallow-marine carbonate successions, especially in the vertical direction. A grid-free spectral simulation approach is developed, with a separable covariance allowing a dampened hole-effect to capture rhythmicity in the vertical direction and a different covariance in the lateral plane along strata, as in space-time models. In addition, facies ordering is created by using a spatial shift between two latent Gaussian functions in the Pluri-Gaussian approach. Rapid conditioning to data is performed via Gibbs sampling and kriging using the screening properties of separable covariances. The resulting facies transiograms can show complex patterns of cyclicity and rhythmicity. Finally, a three dimensional case study of shallow-marine carbonate deposits at outcrop shows the applicability of the modelling method.

Cyclostratigraphy across a Mississippian carbonate ramp in the Esfahan–Sirjan Basin, Iran: implications for the amplitudes and frequencies of sea-level fluctuations along the southern margin of the Paleotethys

The Tournaisian–Visean carbonate successions of the Esfahan–Sirjan Basin (ESB) from Sanandaj–Sirjan Zone, Iran, have been used to generate a sequence stratigraphic model that enhances facies characterization and improves paleoenvironmental interpretation of shallow marine successions deposited along the southern margin of the Paleotethys. Detailed facies analysis allowed to differentiate seven facies, which, in order of decreasing abundance, are: (1) shaly and marly, F1; (2) peloidal mudstones/wackestones, F2; (3) peloidal/bioclastic packstones, F3; (4) intraclastic/bioclastic packstones/grainstones, F4; (5) oolitic/bioclastic packstone/grainstone, F5; (6) sandy intraclastic/bioclastic grainstones, F6; (7) sandy oolitic/bioclastic grainstones, F7. The different facies can be grouped into three facies associations that correspond to different environments of a carbonate platform with ramp geometry (homoclinal), from outer ramp (F1 and F2), mid-ramp (F3, F4 and F6) to inner ramp areas (F5 and F7). Meter-scale cycles are the basic building blocks of shallow marine carbonate successions of the Tournaisian–Visean ramp of the ESB. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. According to the recognized facies and the stacking pattern of high-frequency cycles across the ramp, five large-scale cycles in the southeastern outcrops (Tournaisian–Visean) and three large-scale cycles in the northwest outcrops (Visean) related to eustatic sea-level changes can be recognized. The overall retrogradational nature of the carbonate ramp, illustrated by both vertical facies relationships and the stacking patterns of high-frequency cycles within the third-order cycles, implies that the deposition of the Tournaisian–Visean successions mainly took place under a long-term transgressive sea-level trend. The stratigraphic architectural style of the sequences, characterized by the lack of lowstand deposits and exposure surfaces, associated with the evidence of progressive increase in the proportion of backstepping of facies belts across bounding surfaces and predominant subtidal characteristics, is in accordance with the long-term transgressive sea-level trend and greenhouse conditions during the Tournaisian–Visean. The continued transgression on this broad shelfal platform could lead to the shutdown of the shallow water carbonate factory, reduction in sediment supply or sediment transport towards the offshore setting and the development of give-up sequences. The association of transgressive events with the deposition of thick open-marine marls/shales is a common feature in Tournaisian to Visean times of the southern margin of the Paleotethys.

Global perturbation of the marine calcium cycle during the Permian-Triassic transition

A negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end-Permian extinction horizon in South China has been used to argue for an ocean acidification event coincident with mass extinction. This interpretation has proven controversial, both because the excursion has not been demonstrated across multiple, widely separated localities, and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion. Here, we further test potential controls on the Permian-Triassic calcium isotope record by measuring calcium isotope ratios from shallow-marine carbonate successions spanning the Permian-Triassic boundary in Turkey, Italy, and Oman. All measured sections display negative shifts in δ44/40Ca of up to 0.6‰. Consistency in the direction, magnitude, and timing of the calcium isotope excursion across these widely separated localities implies a primary and global δ44/40Ca signature. Based on the results of a coupled box model of the geological carbon and calcium cycles, we interpret the excursion to reflect a series of consequences arising from volcanic CO2 release, including a temporary decrease in seawater δ44/40Ca due to short-lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and, potentially, subsequently elevated carbonate saturation states caused by the persistence of elevated CO2 delivery from volcanism. Locally, changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions, but this effect alone does not explain the globally observed negative excursion in the δ13C values of carbonate sediments and organic matter as well. Only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data. The carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification, warming, and anoxia.

Tectonic and environmental factors controlling on the evolution of Oligo-Miocene shallow marine carbonate factories along a tropical SE Circum-Caribbean

The evolution of the Cenozoic Circum-Caribbean shallow marine carbonate factories and ecosystems has been for long attributed to major global climatic and environmental changes. Although temporal variations in the Cenozoic shallow marine carbonate factories in this region seem to follow global trends, the potential effects of regional processes, such tectonic activity and local environmental change, on the evolution of the shallow marine carbonate factories are not well established. Here we present detailed sedimentologic and stratigraphic information from Middle Oligocene - Middle Miocene (Chattian-Burdigalian) shallow marine carbonate successions of the Siamana Formation in the Cocinetas sub-basin, Alta Guajira Basin, Guajira Peninsula, northern Colombia. We document the potential effects of regional tectonics and local environmental deterioration on the evolution of the Oligocene-Miocene tropical shallow marine carbonate factories along the SE Circum-Caribbean. Our results show that mixed heterozoan-photozoan biotic associations dominated the shallow marine carbonate factories during the Chattian, while purely photozoan biotic associations constituted the primary carbonate factory during the Aquitanian-Burdigalian transition. The Chattian mixed heterozoan/photozoan biotic association is associated with the development of mixed carbonate/siliciclastic shelves along which detached patchy reef areas occur. The onset of the Aquitanian-Burdigalian purely photozoan biotic associations parallels the increase in coral diversity as well as the occurence of rimmed/detached carbonate platforms in the northern part of the basin. The development of the rimmed/detached platforms coincides with a time of increased basin subsidence and increased silicilcastic input along the southernmost part of the basin. A significant change in the carbonate factory occurs in the Late Burdigalian, when purely heterozoan (rodalgal) biotic associations constituted the main shallow marine carbonate factory. This shift in the carbonate factory was favored by a major extensional component that resulted in subsidence and relative sea level rise along the Caribbean - South American margin. This extensional component is evidenced by the occurrence of normal faults that controlled the thickness and lateral extension of shallow marine rodalgal biostromes. The onset of the rodalgal biostromes, which elsewhere is associated with excellent carbonate reservoirs, also parallels the initiation of the Urumaco Trough and a further increase in silicilastic input toward the Cocinetas sub-basin. It also parallels a major tectonic reactivation along the Caribbean - South American plate boundary which resulted from the initial closure of the Panama Seaway.

Correlating carbon and oxygen isotope events in early to middle Miocene shallow marine carbonates in the Mediterranean region using orbitally tuned chemostratigraphy and lithostratigraphy.

During the Miocene prominent oxygen isotope events (Mi‐events) reflect major changes in glaciation, while carbonate isotope maxima (CM‐events) reflect changes in organic carbon burial, particularly during the Monterey carbon isotope excursion. However, despite their importance to the global climate history they have never been recorded in shallow marine carbonate successions. The Decontra section on the Maiella Platform (central Apennines, Italy), however, allows to resolve them for the first time in such a setting during the early to middle Miocene. The present study improves the stratigraphic resolution of parts of the Decontra section via orbital tuning of high‐resolution gamma ray (GR) and magnetic susceptibility data to the 405 kyr eccentricity metronome. The tuning allows, within the established biostratigraphic, sequence stratigraphic, and isotope stratigraphic frameworks, a precise correlation of the Decontra section with pelagic records of the Mediterranean region, as well as the global paleoclimatic record and the global sea level curve. Spectral series analyses of GR data further indicate that the 405 kyr orbital cycle is particularly well preserved during the Monterey Event. Since GR is a direct proxy for authigenic uranium precipitation during increased burial of organic carbon in the Decontra section, it follows the same long‐term orbital pacing as observed in the carbon isotope records. The 405 kyr GR beat is thus correlated with the carbon isotope maxima observed during the Monterey Event. Finally, the Mi‐events can now be recognized in the δ18O record and coincide with plankton‐rich, siliceous, or phosphatic horizons in the lithology of the section.

Discontinuity surfaces in Upper Cretaceous to Paleogene carbonates of central Dalmatia (Croatia): Glossifungites ichnofacies, biogenic calcretes, and stratigraphic implications

Substrate-controlled ichnofacies and biogenic calcretes represent key features for identification and interpretation of discontinuities in the carbonate rock record, which are of great significance for stratigraphic interpretations and correlations. Intraformational firmground and composite surfaces, as well as a regional Cretaceous to Paleogene (K–Pg) subaerial unconformity, developed in Upper Cretaceous to Paleogene intra-platform peritidal successions in central Dalmatia, Croatia (Adriatic-Dinaridic Carbonate Platform, ADCP), were analyzed for their trace fossil and subaerial exposure features. Thalassinoides (probably T. paradoxicus) box-work burrow systems of the substrate-controlled Glossifungites ichnofacies characterize the two documented firmgrounds and one composite (polygenic) surface. Rhizogenic laminar calcretes developed subsequently inside burrows of the composite surface through diagenetic overprint of marine sediment that passively infilled the burrows. While the formation of the two firmgrounds was probably caused by cessation of precipitation and/or deposition of calcium carbonate due to relative sea-level fall, the recorded trace fossils associated with the composite surface indicate that this surface developed through both submarine firmground and subaerial exposure stages probably caused by several episodes of regression and transgression, and exemplifies the general complexity of hiatal surfaces in shallow-marine carbonate successions. The regional K–Pg subaerial unconformity is characterized by biogenic (beta microfabric) calcretes with rhizoliths including Microcodium aggregates, root tubules, as well as alveolar-septal structures. Laminar calcretes and pisoids, together with in situ and resedimented speleothems, and bauxitic deposits, were also recorded. The unconformity developed due to formation of a forebulge in front of the approaching Dinaridic orogen. Ichnological and subaerial exposure features, together with stratigraphic implications derived from the analyzed discontinuities, serve as examples that can be applied to discontinuities present in carbonate successions elsewhere.

Pennsylvanian–Early Permian cyclothemic succession on the Yangtze Carbonate Platform, South China

Abstract Pennsylvanian (Late Carboniferous)–Early Permian cyclothems are documented for the first time from the shallow-marine carbonate succession on the Yangtze Carbonate Platform, South China. The Zongdi section in southern Guizhou Province comprises bioclast-rich grey limestones with dark-coloured intervals yielding macro- and microfabrics related to emergent events. We recognized 26 depositional sequences (consisting essentially of basal peritidal sediments, middle shallow subtidal sediments and upper peritidal sediments affected by meteoric diagenesis and subaerial exposure) separated by 25 lowstand events. These are interpreted as having been formed by alternating marine transgression and regression resulting from variations in the Gondwanan ice sheets. They are compared with the Midcontinent (Kansas-type) cyclothems in North America. However, the Zongdi cyclothems are relatively thin and consist entirely of very shallow-marine sediments formed above wave base, whereas the Midcontinent cyclothems reflect deposition far below wave base. The Zongdi depositional environment was an upper shelf area, on a platform within the epeiric sea on the South China Craton, where sedimentation occurred in a relatively small (shallow) accommodation space during the highest stands of sea level. They were formed by major (large-scale) transgressions that submerged high-shelf areas, and most of them correspond to the major cyclothems of the North American Midcontinent Basin. The fusuline biostratigraphical framework reveals an uneven stratigraphical pattern of the Zongdi cyclothems, reflecting subtle fluctuations of tectonic subsidence on the epeiric Yangtze Carbonate Platform. Less erosion and karstification on the tops of cyclothems in the Zongdi section, together with the presence of partly calcareous palaeosols, indicates that a dry climate prevailed on the South China Block during the Pennsylvanian–Early Permian when the Zongdi cyclothems were formed.

Neritic isotope and sedimentary records of the Eocene–Oligocene greenhouse–icehouse transition: The Calcare di Nago Formation (northern Italy) in a global context

From the Middle Eocene to Early Oligocene, the Earth experienced the most significant climatic cooling of the Cenozoic era. The Eocene–Oligocene transition (EOT) represents the culmination of this climatic cooling, leading to the onset of the Antarctic glaciation and, consequently, to the beginning of the present-day icehouse world. Whereas the response of deep-sea systems to this climate transition has been widely studied, its impact on the shallow-water carbonate realm is poorly constrained. Here, the sedimentary expression of the EOT in two shallow-marine carbonate successions (Nago and San Valentino, northern Italy) belonging to the Calcare di Nago Formation is presented. The chronostratigraphic framework was constructed by integrating litho-, bio-, and isotope-stratigraphic data (C and Sr isotopes), allowing to correlate these shallow-marine successions with pelagic sections in central Italy (Massignano), Tanzania (TDP Sites 12 and 17), and the Indian Ocean (ODP Site 744). Within several sections in northern Italy, including Nago and San Valentino, a Priabonian (Late Eocene) transgression is recorded. Oxygen isotopes of ODP Site 744 show a coeval negative shift of 0.4‰, suggesting a glacio-eustatic origin for this transgression. In the Nago and San Valentino sections, no prominent sequence boundary has been detected that would indicate a rapid sea-level drop occurring together with the positive shift in δ18O defining the EOT-1 cooling event. Instead, a gradual shallowing of the depositional environment is observed. At TDP Sites 12 and 17, the EOT-1 is followed by a negative shift in δ18O of around 0.4‰, which correlates with a relative deepening of the environment in the studied sections and suggests a melting pulse between EOT-1 and the Oligocene isotope event 1 (Oi-1). The positive δ18O shift related to the Oi-1 translates in San Valentino into a change in carbonate factory from a photozoan association dominated by larger benthic foraminifera, corals, and red algae to a heterozoan association dominated by bryozoans. The same bryozoan facies occurs in several Italian localities near the Eocene–Oligocene boundary. This facies is interpreted to represent an analogue of modern cool-water carbonates and results from a cooling pulse of at least regional scale, associated to the Oi-1 event.

A new high-resolution δ13C record for the Early Triassic: Insights from the Arabian Platform

A new carbon isotope record with conodont biostratigraphy is presented for the entire Early Triassic from the Musandam Peninsula, United Arab Emirates (UAE). This is a near-continuous and exclusively shallow marine carbonate succession that allows analysis of a high-resolution primary carbon cycling signature in the absence of significant depth-dependent or lithologic controls. The Musandam carbon isotope record can be broadly correlated with global isotopic events but also resolves additional features, including the presence of significant negative events during the previously identified positive excursions at both the Dienerian/Smithian and Smithian/Spathian boundaries. A further positive event is revealed during the development of the mid-Spathian negative excursion, a feature not previously reported in any other section. These new short-lived events are probably related to the occurrence of the more widely recognized Early Triassic excursions, and may represent fluctuations in the driving mechanisms superimposed on the continued instability of the global carbon cycle in the aftermath of the end-Permian extinction. Together, these features highlight additional complexities to the Early Triassic carbon cycle perturbations than previously documented.

Preservation of forcing signals in shallow water carbonate sediments

No consensus has been reached on whether the metre-scale cycles that commonly occur in peritidal carbonates are predominately a product of external relative sea-level variations (allocycles) or an intrinsic property of carbonate production generated via the interaction of non-linear processes (autocycles). For any forcing signal such as eustatic sea-level change, to be detectable in stratigraphy its effects must be preserved. Here, a deterministic, three-dimensional geological process model is used to explore how such cycles are preserved in the geological record in the presence of autocyclic processes. Each simulation produced cycle thickness distributions that are statistically indistinguishable from a theoretical Poisson process, regardless of whether auto- or allo-cycles dominated. Spectral analysis of depositional time series constructed from idealised geological sections showed that all detectable signals occurred within the Milankovitch forcing frequency bands, even when no Milankovitch forcing was present. Thus, it is deduced that from any geological section alone, external forcing signals are detectable but are not distinguishable from autocyclically produced signals. Interestingly, there is no correlation between the percentage of sediment preserved and the accuracy with which signals are detectable in the preserved sediment: in some model realisations, even with preservation as low as 40%, the correct forcing signal can be detected accurately while, conversely, sections with preservation as high as 90% can have poor signal preservation. The reverse can also be true in other models. It is therefore concluded that distinguishing allocyclic and autocyclic forcing in shallow marine or peritidal carbonate successions is likely to be extremely difficult except in cases of extraordinary sedimentary preservation and dating accuracy.

Late Neoproterozoic to Early Palaeozoic evolution and hydrocarbon prospectivity of the NW Tarim Basin, China

Abstract Recent contraction in the NW Tarim Basin, China, has exhumed a thick (2–3 km) sequence of Upper Neoproterozoic to Lower Palaeozoic sediments that provide a unique insight into the early evolution of the basin. The sedimentary sequence was examined in outcrop and consists of a lower, 500-m-thick fluvial–lacustrine clastic and volcanic succession, conformably overlain by a 2000-m-thick shallow marine carbonate succession which records a major rifting event that initiated in the Late Neoproterozoic. This rifting event probably corresponds to the break-up of East Gondwana and the separation of the Tarim Block from a conjugate margin equivalent in NW Australia. The generation and infilling of rift basins creates a number of potential hydrocarbon plays, although analysis of individual play elements indicates a relatively high risk, despite the prevalence of hydrocarbons derived from the same rift sequence elsewhere in the basin.

Depositional environment and constraining factors on the facies architecture of the Qom Formation, Central Basin, Iran

In the Central Iran Basin, the mixed carbonate–siliciclastic deposits of the C member of the Qom Formation were deposited on a carbonate platform which is dominated by rhodalgal associations occurring in tropical–subtropical environment. The biogenic rhodalgal association is dominated by bryozoa, coralline red algae, bivalves and echinoids together with smaller amounts of photo-dependent biota including large benthic foraminifera and corals. The abundance of heterozoan association and the bloom of suspension-feeding organisms are the result of an increase in nutrient availability which has profound controlling effect on the biotic system. The low occurrence of symbiont-bearing benthic foraminifera and coral, typical of stable, oligotrophic condition, represents their low tolerance to unstable, nutrient-rich environment. In the investigated Oligocene–Miocene shallow marine carbonate succession, 10 different microfacies were distinguished through depositional texture and biotic components. The rock sequences investigated are referred to an open shelf carbonate platform in which the depositional environments range from outer shelf to inner shelf conditions.

Discontinuity surfaces recorded in shallow-marine platform carbonates: an example from the Early Jurassic of the Velebit Mt. (Croatia)

Discontinuity surfaces of different types and scales are common in successions of shallow-marine carbonate platforms because sediments there are deposited close to the sea level and therefore are sensitive to any significant physico-chemical changes of environmental factors. Discontinuity surfaces indicate breaks in sedimentation under subaqueous or subaerial conditions. Most discontinuities in shallow-marine carbonate successions are on a bed-scale, and can be determined only by analysis of sedimentologic, diagenetic, taphonomic, and ichnologic features of the rock. The study of small-scale discontinuities has been carried out on two Lower Jurassic successions of the Velebit Mt. Depending upon their common features and environment of formation, three groups of discontinuities are distinguished on simple bedding planes: subaerial exposure surfaces, erosion surfaces, and omission surfaces. The distribution of discontinuity types in both successions is evaluated. Exposure surfaces prevail in both sections, and four units (relatively thin intervals of the sedimentary record) with abundant subaerial exposures are recognized. Dated by biostratigraphy, these units are of earliest Sinemurian, middle Early Sinemurian, earliest Pliensbachian, and late Early Pliensbachian age. Omission surfaces are the least common type of discontinuity. Thickness variations of high-frequency peritidal and shallow subtidal shallowing-upward cycles, highlighted by the Fischer plots show a very similar long-term trend for the two sections. The units with common subaerial exposure surfaces coincide with the falling limb of the Fischer plots and the section with common omission surfaces coincides with the rising limb of the plots. The studied discontinuities are formed by autocyclic and/or allocyclic processes operating on the shallow platform, but the units with abundant subaerial exposures invoke allogenic forcing of the sedimentary record. The use of the units with abundant discontinuities instead of a single surface has proven useful for the correlation of the studied shallow-platform deposits because one type of discontinuity may change laterally into another type or features of different discontinuity types can be superimposed.

Late Cretaceous calcareous nannofossil and planktonic foraminiferal bioevents of the shallow-marine carbonate platform in the Mitla Pass, west central Sinai, Egypt

The first detailed biostratigraphic analyses of the Coniacian–middle Campanian shallow-marine carbonate successions exposed in the Mitla Pass, west central Sinai, Egypt have revealed the stratigraphic distribution of diverse calcareous nannofossil and planktonic foraminiferal species. Thirty-six calcareous nannofossils and thirty-two planktonic foraminifera are identified, indicating a Coniacian to middle Campanian age and four Tethyan planktonic foraminiferal and five calcareous nannofossil zones. A comparison of these bioevents from different palaeolatitudes shows considerable variation in age. Three sequence boundaries coincident with the Turonian/Coniacian, Coniacian/Santonian and Santonian/Campanian stage boundaries are recognized. A fourth sequence boundary is marked by a major upper Campanian to early Ypresian (early Eocene) unconformity. These sequence boundaries are primarily related to regional tectonism associated with the Syrian Arc Fold System and secondarily to eustatic sea-level fluctuations.