Compostional variations and patterns of condont reworking in Late Devonian and early carboniferous calciturbidites (Moravia, Czech Republic)

Summary Compositional variations and grain-size properties of both carbonate constituents and conodonts as an alternative component group were used for interpreting the processes governing the deposition of upper Famennian and middle Tournaisian calciturbidites in Moravia, Czech Republic. Both the composition and grain-size properties of conodont element associations showed to be markedly dependant on facies type of their host sediment. Upper Devonian calciturbidite successions deposited on flanks of wide, Moravian-Silesian carbonate platform are composed mainly of echinoderm-and peloid-rich wacke/packstones and intraclastic float/rudstones (fine-grained calciturbidites, “normal” calciturbidites with Tab Bouma sequences, debris-flow breccias) with abundance of shelf-and shelf margin conodont taxa and epipelagic and “mesopelagic” conodonts. Upper Devonian calciturbidites deposited on slopes of volcanic sea-mounts are composed of echinoderm-and peloid-rich wacke/packstones and float/rudstones with increased proportion of intraclasts and volcanigenic lithoclasts (fine-grained calciturbidites, normal calciturbidites), yeilding abundant conodont associations with higher proportion of “mesopelagic” taxa compared to the platform-flank examples. Middle Tournaisian calciturbidite succession composed of crinoid-, peloid-, intraclast-and lithoclast-rich lime mudstones, wacke/packstones and float/rudstones (normal calciturbidites and debris-flow breccias) yielded conodont element associations rich in shelt-and shelf-margin taxa, “mesopelagic” conodonts and reworked Middle-and Upper Devonian conodonts. In general, the ratio of shelf-and shelf margin conodont taxa to “mesopelagic” taxa is distinctly lower in finegrained calciturbidites than it is in normal calciturbidites and debris-flow breccias. Grain-size properties (mean grain size and sorting) and percentage of fragmented conodont elements, too, are markedly dependant on the facies type: in fine-grained calciturbidites the values of mean grain-size and fragmentation are low and the sorting is good to very good whereas in normal calciturbidites and debris-flow breccias the values of mean grain-size and fragmentation are distinctly higher and the sorting is poorer. The interdependence of facies type and composition and grain-size properties of conodont element associations in gravity-flow deposits is explained as resultant from hydrodynamic sorting during turbidity current flow and final deposition of the bed. Compositional variations observed in our sections may thus be attributed to facies variability (coarsening-and thickening-upward trends) rather than to sea-level fluctuations (highstand shedding of carbonate platforms). On the other hand, significant enrichment in reworked conodont taxa in middle Tournaisian normal calciturbidites compared to scarcity and/or absence of such conodonts in essentially identical facies of upper Famennian age indicate sea-level to be the major control governing such compositional variations, with low relative sea-level stand in middle Tournaisian and high relative sea-level stand in upper Famennian. Thorough analysis of conodont evolution, palaeoecology and taphonomy, with emphasis on understanding the processes of deposition of their host rock, are recommended for any biostratigraphic and biofacies study to be done in carbonate sediments deposited under strong hydrodynamic regimes, such as calciturbidites, temperstites, debris-flow deposits, shelf-edge oolitic sands, tidal-channel facies etc.     

Drowning of a Lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco

Summary The high-plateau of the Jbel Bou Dahar, situated in the Central and Eastern High Atlas of Morocco, represents a Lower Jurassic carbonate platform that drowned at the beginning of the Toarcian. Three phases of platform evolution can be distinguished: During thepre-drowning phase (upper Sinemurian— upper Pliensbachian) the platform interior facies reflects a restricted-marine lagoonal environment, protected by scattered buildups and cemented debris at the platform margin. Upper and mid-slope are dominated by coarse-grained, poorly sorted limestones, deposited through debris flows during sea-level lowstands. Sea-level highstand deposits occur at the toe of slope and are formed by an alternation of fine-grained litho- and bioclastic pack- to grainstones (turbidites), marls and mud- to wackestones (hemipelagic oozes). A condensed section, reflecting an abrupt and fundamental environmental change along the entire platform, characterises thedrowning phase (upper Pliensbachian— lower Toarcian). Within the platform interior densely packed biosparites represent the switch to high-energy environments, causing erosion of the former pre-drowning lagoonal sediments. These erosional products were redeposited on the platform slope, leading to the formation of coarse-grained non-skeletal sparites and micrites. Both platform interior and slope successions show a series of cyclic variations in sediment composition that could have been triggered by small-scale sea-level fluctuations. In contrast to the abrupt facies change at the pre-drowning —drowning boundary, the transition to thepost-drowning phase (lower Toarcian—Aalenian) is gradual. During this phase, biopelmicrites and pure micrites were deposited in all platform sections, followed by the deposition of calcistiltites. The facies point to quiet-water conditions below storm-wave base and display a uniform deep-marine sedimentation. This analysis shows that the drowning of the Jbel Bou Dahar carbonate platform was caused by abrupt and fundamental changes in the shallow-water realm. After exposure of the platform, these changes prevented the carbonate factory from re-establishing itself and made it impossible for the platform to keep up with the subsequent rise in sea level. These local changes were probably triggered by high-frequency sealevel variations in combination with regional or even worldwide changes in ocean circulation patterns.     

Sedimentary and biofacies records in calciturbidites at the Devonian-Carboniferous boundary in Moravia (Moravian-Silesian Zone, Middle Europe)

Summary At the Devonian/Carboniferous boundary, major climatic and oceanographic changes influenced sedimentation on carbonate platforms and in peri-platfrom asreas. Three deep-water carbonate successions in Moravia, which were selected to represent different paleotectonic settings, have been studied with the aim of testing the influence of eustatic, climatic and tectonic controls on sedimentation and conodont paleoecology and taphonomy. On the slopes of the wide carbonate platforms of the Moravian Karst Development (Lesní lom and Grygov sections), an exemplary highstand shedding systems developed in the upper Famennian (expansa Zone), marked by a pronounced thickness of their respective calciturbidite successions and an abundance of shallow-water skeletal grains.Palamatolepis— andBispathodus-dominated conodont assemblages contain an admixture ofPolygnathus representing a transported, near-shore component. The eustatic sea-level fall in the praesulcata Zone and the lowstand conditions at the D/C boundary resulted in a decline of carbonate platform production and condensed deposition or nondeposition. In the Lesní lom section, a condensed sequence of turrbiditic calcarenites and shales (Middle praesulcata—lowermost sulcata Zone) was followed by lime mud calciturbidites (sulcata and duplicata Zones). In the conodont assemblages, the first event in the Lower praesulcata Zone was associated with the reduction of ‘mesopelagic’Palmatopic and a bloom of epipelagicPolygnathus communis. The second event in the Middle praesulcata Zone corresponds to the onset of polygnathidprotogranthodid biofacies, indicating a carbonate slope environment. In the Grygov section, a pronounced thickening and upward-coarsening succession of tubiditic calcilutites through calcarenites and intraclast breccias, with poor palmatolepid-bispathodid connodont assemblages (expansa Zone), indicates a progradation of the calciturbidite system associated with sea-level highstand. After a break in sedimentation, covering the interval from the Lower praseulcata to the base of Lower crenulata Zone, thick-bedded, fine-grained calciturbidites were deposited in the Lower crenulata Zone, and are associated with poor, mixed assemblages where siphonodellids and polygnathids predominate. At the isosticha-Upper crenulata/Lower typicus boundary, coasre grained, turbiditic calcarenites and breccias rich in clastic quartz grains and mixed conodont assemblages with reworked Frasnian and Famennian conodonts indicate a deep erosion of the source area, presumably due totectonic uplift (relative lowstand). In the Jesenec section, on the flanks of the volcanic seamount (the Drahany Development), a deep-water Upper Famennian condensed succession of calciturbidites and presumably winnowed pelagic limestones is marked by conodont assemblages of palmatolepid-bispathodid biofacies. More proximal calciturbidites with mixed deep-water and shallowwater conodonts prograde at the top of the Upper Famennian succession (Middle to Upper expansa Zone). A striking hiatus, covering the interval from the Early preaesulcata to the base of Lower crenulata Zone, resulted from extreme condensation and submarine bottom current erosion due to sea-level lowstand in the late Famennian and early Tournaisian. The renewed middle Tournaisian calciturbidite sedimentation with strong evidence of erosion at the source area indicates global eustatic rise and tectonic uplift of the Drahany Development seamounts (relative lowstand). The earlier occurrence of the uplift in the Jesenec area, relative to the Grygov section, shows the advance of tectonic processes over time in the Moravian-Silesian basin (orogenic polarity) as a consequence of Variscan orogenic movements.     

Origin and resedimentation of rhodoliths in the Late Paleocene flysch of the Polish Outer Carpathians

This study analyses the rhodolith-bearing deposits in the largest and most rhodolith-rich outcrop of the Polish Outer Carpathian flysch, located in the Silesian Nappe, at the village of Melsztyn. The rhodoliths and sparse associated biota occur as resedimented components in a deep-marine succession of siliciclastic conglomerates and coarse-grained sandstones, deposited by high-density turbidity currents and debris flows. The sediment was derived from a fan-delta system located at the southern margin of the Silesian flysch basin. Stratigraphic data indicate that the succession represents the Upper Istebna Sandstone deposited during the Late Paleocene. The rhodoliths are composed mostly of coralline red algae with seven genera and eight species representing the family Sporolithaceae and the subfamilies Mastophoroideae and Melobesioideae. Rhodoliths show sub-spheroidal and sub-ellipsoidal shapes with encrusting, warty and lumpy growth forms. Lumpy growth forms show massive inner arrangements, whereas the encrusting growth forms are usually made of thin thalli and show more loosely packed inner arrangements. The rhodoliths grew on a moderately mobile siliciclastic substrate in a shallow-marine environment with a low net sedimentation rate. It is inferred that the growth of rhodoliths was favored during a relative sea-level rise. During the subsequent sea-level fall, the rhodoliths and associated siliciclastic deposits were resedimented by gravity flows into the deep-sea setting. The analyzed deposits, like other Paleocene–Eocene deposits of the Polish Outer Carpathians, provide no evidence of coeval widespread shallow-marine carbonate sedimentation along the margins of the Outer Carpathian flysch basins.     

Facies and faunal assemblage changes in response to the Holocene transgression in the Lagoon of Mayotte (Comoro Archipelago, SW Indian Ocean)

This paper documents the facies change in response to the Holocene transgression within five sediment cores taken in the lagoon of Mayotte, which contain a Type-1 depositional sequence (lowstand, transgressive and highstand deposits underlain by an erosive sequence boundary). Quantitative compositional analysis and visual examination of the bioclasts were used to document the facies changes. The distribution of the skeletal and non-skeletal grains in the lagoon of Mayotte is clearly controlled by (1) the rate and amplitude of the Holocene sea-level rise, (2) the pre-Holocene basement topography and (3) the growth-potential of the barrier reef during sea-level rise, and the changes in bathymetry and continuity during this period. The sequence boundary consists of the glacial karst surface. The change-over from the glacial lowstand is marked by the occurrence of mangrove deposits. Terrigenous and/or mixed terrigenous-carbonate muds to sandy muds with a mollusc or mollusc-ostracod assemblage dominate the transgressive deposits. Mixed carbonate-siliciclastic or carbonate sand to gravel with a mollusc-foraminifer or mollusc-coral-foraminifer assemblage characterize the early highstand deposits on the inner lagoonal plains. The early highstand deposits in the outer lagoonal plains consist of carbonate muds with a mollusc-foraminifer assemblage. Late highstand deposits consist of terrigenous muds in the nearshore bays, mixed terrigenous-carbonate sandy muds to sands with a mollusc-foraminifer assemblage on the inner lagoonal plains and mixed muds with a mollusc-foraminifer assemblage on the outer deep lagoonal plains. The present development stage of the individual lagoons comprises semi-enclosed to open lagoons with fair or good water exchange with the open ocean.     

Reef slope geometries and facies distribution: controlling factors (Messinian,SE Spain)

Sea-level fluctuations and changes in sediment grain size are widely thought to be the main factors controlling carbonate platform slope geometries. Two successive clinoform bodies from the Upper Miocene Cariatiz carbonate platform (SE Spain) were selected to analyze geometry and facies distribution in relation to sea-level oscillations. Facies occurring in these clinoform bodies are from top to bottom reef-framework, reef-framework debris, Halimeda breccia, Halimeda rudstone, and bioclastic packstone, as well as siltstone and marl. Slope geometry and facies, composition, and distribution, are significantly different in each clinoform body. These differences are the result of the interaction of several factors such as coral growth, in situ slope carbonate production, rockfalls and sediment gravity flows, hemipelagic rain, reworking of reef-slope facies and siliciclastic input. Changes in accommodation were related to sea-level fluctuations and controlled the relative impact of these factors. A sea-level fall took place in the time between deposition of the selected clinoform bodies and changed the hydrographical conditions of the basin. These changes influenced the presence of Halimeda and the grain-size distribution, and consequently the slope geometries. Reef-slope geometry is not exclusively controlled by changes in grain size. The stabilization by organic binding is proposed to be a significant factor controlling the slope deposition.     

Microfossil assemblages and relative sea-level fluctuations in a lagoon at the Oxfordian/Kimmeridgian boundary (Upper Jurassic) in the eastern part of the Paris Basin

The Late Oxfordian–Early Kimmeridgian interval of the eastern part of the Paris Basin is characterized by a carbonate succession deposited in shallow-marine platform environments. The Gudmont-Villiers section is represented by deposits ranging from barrier to typical lagoonal environments often poor in macrofossils. Previously unpublished calcareous microfossils are more abundant and provide alternative paleoenvironmental indicators. They also provide a biostratigraphical framework across the Oxfordian–Kimmeridgian boundary. The evolution of microfossil associations (algae and benthic foraminifera) in the lower part of the section, based on statistical analyses, is correlated to the sea-level variations. The first highly diversified association composed of small agglutinated and calcitic foraminifera (miliolids, textulariids, Spirillina, Trocholina, Molherina basiliensis etc.) characterizes high sea-level deposits; a second association richer in large agglutinated foraminifera (Alveosepta jaccardi, Everticyclammina, Nautiloculina oolithica) is significantly abundant in low sea-level deposits. A third association characterizes beds with a significant occurrence of encrusting microorganisms and algae (Lithocodium aggregatum, Troglotella incrustans, Cayeuxia piae, dasycladaceans). The upper part of the section is marked by more argillaceous beds and by the occurrence of one opportunist taxon (Lenticulina). This study shows that the microfauna-flora evolution in an internal carbonate platform environment constitute an efficient tool to determine variations in the relative sea level.     

Late Cretaceous sedimentary evolution of a northern sector of the Adriatic Carbonate Platform (Matarsko Podolje,SW Slovenia)

Cretaceous shallow-marine carbonate rocks of SW Slovenia were deposited in the northern part of the Adriatic Carbonate Platform. A 560-m-thick continuous Upper Cenomanian to Santonian carbonate succession has been studied near Hru?ica Village in Matarsko Podolje. With regard to lithological, sedimentological, and stratigraphical characteristics, the succession has been divided into nine lithostratigraphic units, mainly reflecting regressive and transgressive intervals of larger scale. During the latest Cenomanian and Early Turonian, hemipelagic limestones were deposited on top of shallow-marine lagoon and peritidal Upper Cenomanian deposits indicating relative sea-level rise. Subsequently, the deeper marine depositional setting was gradually filled by clinoform bioclastic sand bodies overlain by peritidal and shallow-marine low-energy mainly lagoonal lithofacies. Similar lithofacies of predominately inner ramp/shelf depositional settings prevail over the upper part (i.e., Coniacian to Santonian) of the succession. In the area, the Upper Cetaceous carbonate rocks are separated from the overlying Lower Eocene (Upper Paleocene?) carbonate sequence by regional unconformity denoted by distinct paleokarstic features. On the Adriatic Carbonate Platform the deeper marine carbonate setting, developed at the Cenomanian/Turonian boundary, is usually correlated with OAE2 and related eustatic sea-level rise. Similarly, subsequent reestablished shallow-marine conditions are related to Late Turonian long- and short-term sea-level fall. However, we are suggesting that deeper marine deposits were deposited in a tectonically induced intraplatform basin formed simultaneously with the uplift of the northern and northeastern marginal parts of the Adriatic Carbonate Platform.     

Mid-oceanic carbonate platforms as oceanic dipsticks: examples from the Pacific

Framebuilders of Cenozoic coral reefs are limited by their photic requirements to the contemporaneous sea-level, and therefore shallow water reef facies are reliable paleo sea-level indicators. Sea-level lowstands leave no record on coral reefs in areas subject to tectonic uplift, such as the Huon Peninsula, New Guinea, but are recorded by coral reefs in areas subject to tectonic subsidence. A eustatic sea-level fall which exceeds the rate of subsidence subaeriallyexposes the upper section of the reef complex, creating a meteoric ground water system whose diagenetic imprint on the reef carbonates offers a good indicator of a sea-level stillstand. Cenozoic reef platforms thus may contain records of sea-level fluctuations, whether eustatic and global, or tectonic and local. Those reef platforms which developed on seamounts formed in mid-oceanic plate settings are particularly useful for the study of eustatic sea-level changes because their subsidence history is relatively simple, and the tectonic factor can be accounted for when estimating the eustatic sea-level component. Conventional petrographic and biostratigraphic methods used to delineate erosional unconformities in Cenozoic carbonate sections are often deficient. We demonstrate here that stable oxygen and carbon isotopes of the carbonates can reveal the location of both the exposure surface and the paleo water table with greater confidence on account of the specific imprint of meteoric diagenesis. In addition, the⁸⁷Sr/⁸⁶Sr isotope technique offers a promising dating tool of disconformities linked to sea-level lowstands with a resolution superior to the conventional biostratigraphic techniques. Although oxygen, carbon, and strontium isotopes monitor different aspects of global sea-level changes, when used in conjunction they provide deeper insights into the past than either one could achieve alone. Examples from previous and ongoing studies of Pacific mid-oceanic carbonate platforms illustrate the potential of the isotope techniques to unravel sea-level changes. At Midway Atoll, stable carbon and oxygen isotopes along with lithologic and biostratigraphic data suggest a sharp eustatic sealevel fall during the Early Miocene and a series of rapid, brief eustatic fluctuations during the Pliocene-Quaternary. The frequency and timing of the latter is supported by sea-level data from Enewetak Atoll obtained on the basis of detailed strontium isotopes and lithology. The Enewetak data also indicate a series of rapid, brief eustatic fluctuations around the Early-Middle Miocene boundary. At Niue, a carbonate platform about 500 km south of Samoa, oxygen, carbon, and strontium isotope records cover the critical interval of the Miocene-Pliocene boundary and show two distinct disconformities. The mid-oceanic carbonate platforms offer a testing ground of Vail-Haq type eustatic sea-level curves derived primarily from sections along passive continental margins and continental interiors. We show that Neogene sea-level data obtained from Midway, Enewetak, and Niue differ from Vail and Hardenbol's contemporaneous sea-level curve and support Haq et al.'s version.     

Microfacies and cycle stacking pattern in Liassic peritidal carbonate platform strata, Gavrovo-Tripolitza platform, Peloponnesus, Greece

Platform carbonate sediments of Liassic age cropping out in the area of the Pigadi-Fokianos Gulf (SE of Leonidion, Peloponnesus) have been investigated in order to determine their depositional environment. Facies analysis allowed the recognition of several microfacies types and their cyclic stacking pattern. The carbonates were deposited in a restricted inner platform environment (lagoon-peritidal domain) and are arranged into small-scale shallowing-upward cycles. Palaeosol horizons containing typical pedogenic features are developed on the top of the peritidal facies or are directly superimposed on subtidal deposits, forming diagenetic caps. This implies repeated sea-level fluctuations and periodic emersion episodes. The presence of orbitally forced cyclicity though is mostly probable, cannot be clearly documented by the available data. The studied carbonates are comparable with other coeval analogous peritidal cycles of the same age along the southern margin of the Tethys.     

Depositional sequences in the Kimmeridgian of the Vocontian Basin (France) controlled by carbonate export from shallow-water platforms

Detailed sedimentological analyses and sequential and cyclostratigraphic interpretations in the Kimmeridgian of the Swiss Jura and the Vocontian Basin lead to a high-resolution correlation from the platform to the basin where the biostratigraphy is well established. Several orders of depositional sequences are defined. Their duration is estimated from the time frame given in the sequence-chronostratigraphic chart of Hardenbol et al. (1998). It is suggested that an elementary sequence formed in tune with the 20 ky precession cycle. Small-scale and medium-scale sequences correspond to the 100 and 400 ky eccentricity cycles, respectively. The platform-to-basin correlation shows that the composition of the hemipelagic and pelagic deposits depends to a large part on cyclical variations of carbonate production in shallow-marine environments and subsequent export to the basin. The distribution of thick versus thin marl-limestone alternations and carbonate-dominated versus marl-dominated intervals observed in the basinal sections is explained by the superposition of high- and low-frequency sea-level changes that controlled the carbonate productivity on the platform and the export potential of carbonate mud to the basin.     

Late Smithian microbial deposits and their lateral marine fossiliferous limestones (Early Triassic,Hurricane Cliffs,Utah, USA)

Recurrent microbialite proliferations during the Early Triassic are usually explained by ecological relaxation and abnormal oceanic conditions. Most Early Triassic microbialites are described as single or multiple lithological units without detailed ecological information about lateral and coeval fossiliferous deposits. Exposed rocks along Workman Wash in the Hurricane Cliffs (southwestern Utah, USA) provide an opportunity to reconstruct the spatial relationships of late Smithian microbialites with adjacent and contemporaneous fossiliferous sediments. Microbialites deposited in an intertidal to subtidal interior platform are intercalated between inner tidal flat dolosiltstones and subtidal bioturbated fossiliferous limestones. Facies variations along these fossiliferous deposits and microbialites can be traced laterally over a few hundreds of meters. Preserved organisms reflect a moderately diversified assemblage, contemporaneous to the microbialite formation. The presence of such a fauna, including some stenohaline organisms (echinoderms), indicates that the development of these late Smithian microbial deposits occurred in normal-marine waters as a simple facies belt subject to relative sea-level changes. Based on this case study, the proliferation of microbialites cannot be considered as direct evidence for presumed harsh environmental conditions.     

Characteristics of peritidal facies and evidences for subaerial exposures in Dachstein-type cyclic platform carbonates in the Transdanubian Range,Hungary

In the Late Triassic, an extremely large carbonate platform system (Dachstein-type platforms) developed on the margin of the Neotethys. On the wide inner platform cyclic peritidal, lagoonal successions were deposited. In the Transdanubian Range (Hungary), the lower part of the 1.5–2-km-thick cyclic succession (Upper Tuvalian–mid-Norian) is pervasively dolomitised, the upper part (Upper Norian–Rhaetian) is non-dolomitised; there is a transitional interval between them made up of partially dolomitised cycles. The peritidal–lagoonal cycles are commonly bounded by well-developed disconformity surfaces reflecting subaerial erosion that punctuated the marine carbonate accumulation. Truncation of the cycles was preceded by pervasive cementation of the previously deposited cycle. In the early stage of the platform evolution, tidal flat dolomitisation under semi-arid conditions led to the consolidation of the previously deposited sediments. The truncation surfaces were commonly covered by dolocretes. During the more humid Late Norian–Rhaetian period, the early cementation was followed by karstification, accumulation of wind-blown dust and pedogenesis. Erosion during regularly recurring subaerial exposure that commonly reached the previously deposited subtidal beds suggests eustatic control of the cyclicity and supports the application of an allocyclic model, even if the Milankovitch signal is imperfect.     

Tectonically driven carbonate ramp evolution at the southern Tethyan shelf: the Lower Eocene succession of the Galala Mountains,Egypt

The succession of the Galala Mountains at the southern Tethyan margin (Eastern Desert, Egypt) provides new data for the evolution of an isolated carbonate platform in the Early Eocene. Since the Late Cretaceous emergence of the Galala platform, its evolution has been controlled strongly by eustatic sea-level fluctuations and the tectonic activity along the Syrian Arc-Fold-Belt. Previous studies introduced five platform stages to describe platform evolution from the Maastrichtian (stage A) to the latest Paleocene shift from a platform to ramp morphology (stage E). A first Early Eocene stage F was tentatively introduced but not described in detail. In this study, we continue the work at the Galala platform, focussing on Early Eocene platform evolution, microfacies analysis and the distribution of larger benthic foraminifera on a south-dipping inner ramp to basin transect. We redefine the tentative platform stage F and introduce two new platform stages (stage G and H) by means of the distribution of 13 facies types and syn-depositional tectonism. In the earliest Eocene (stage F, NP 9b–NP 11), facies patterns indicate mainly aggradation of the ramp system. The first occurrence of isolated sandstone beds at the mid ramp reflects a post-Paleocene-Eocene thermal maximum (PETM) reactivation of a Cretaceous fault system, yielding to the tectonic uplift of Mesozoic and Palaeozoic siliciclastics. As a consequence, the Paleocene ramp with pure carbonate deposition shifted to a mixed carbonate-siliciclastic system during stage F. The subsequent platform stage G (NP 11–NP 14a) is characterised by a deepening trend at the mid ramp, resulting in the retrogradation of the platform. The increasing deposition of quartz-rich sandstones at the mid ramp reflects the enhanced erosion of Mesozoic and Palaeozoic deposits. In contrast to the deepening trend at the mid ramp, the deposition of cyclic tidalites reflects a coeval shallowing and the temporarily subaerial exposure of inner ramp environments. This oppositional trend is related to the continuing uplift along the Syrian Arc-Fold-Belt in stage G. Platform stage H (NP 14a–?) demonstrates the termination of Syrian Arc uplift and the recovery from a mixed siliciclastic carbonate platform to pure carbonate deposition.     

The Frasnian-Famennian mass extinction: insights from high-resolution sequence stratigraphy and cyclostratigraphy in South China

Two sequences (SFr, SFa), each 1-1.2 Myr in duration, are recognised in the strata across the Frasnian-Famennian (F-F) transition both in carbonate platform and interplatform basinal successions in South China. The sequence boundary between the two sequences is placed a little below the top of the Frasnian. The sequences are basically composed of coarsening-upward/bed-thickness increasing-upward cycles and shallowing-upward cycles (parasequences) in basinal and platform deposits respectively, which stack into cycle-sets (typically six to eight cycles). 10 and 12 cycle-sets are identified in sequences SFr and SFa respectively. These cycle-sets can be further grouped into larger-scale composite cycle-sets (herein termed mesocycle- and megacycle-sets with two and four cycle-sets respectively). This vertical cycle-stacking pattern and the hierarchy of cyclicity suggest a Milankovitch style of forcing such that the cycles and cycle-sets were formed in response to the orbital perturbations of precession (16-18 kyr) and eccentricity (∼100 kyr in duration), respectively. In the basinal cycles, smaller-scale rhythmic stratification beds (typically six to eight beds in a cycle) are extensive, and were likely caused by millennial-scale climatic forcing. In the lower sequence, SFr, the latest highstand deposits consist of calciturbidites and debrites in deep-water strata and fenestral limestones in shallow-water strata, representing a major (third-order) sea-level fall. Within these deposits, four cycle-sets are further identified in both coeval deep-water and platform successions. Succeeding deeper-water organic-rich facies, within which three cycles occur, are the transgressive deposits of the overlying Famennian sequence (SFa). These cycles represent three higher-frequency (16-18 kyr) sea-level fluctuations and accompanying anoxia, superimposed on a major third-order sea-level rise. The F-F boundary is placed at the top of the first cycle, based on conodont data. Thus, a major sea-level fall and then a rise occurred in the F-F transitional period. Faunal and sedimentological data reveal a massive biotic decline in concert with the major sea-level fall, and a further biotic demise coinciding with the major sea-level rise and its three superimposed higher-frequency sea-level fluctuations and accompanying anoxia. The F-F biotic crisis was therefore characterised by two episodes of step-down extinction. On the basis of Milankovitch orbital rhythms, the first major biotic extinction took place over ∼400 kyr, and the subsequent event was ∼50 kyr in duration, i.e. ∼450 kyr for the entire event. At the same time as the massive decline of normal-marine fossils during the latest Frasnian sea-level fall, there was widespread cyanobacterial growth and a thriving of planktonic calcispheres, suggesting eutrophic conditions. This situation could have caused a severe biotic loss, as a result of the deterioration of surface water clarity and formation of anoxic bottom waters due to over-consumption of oxygen through respiratory demands and decomposition by the cyanobacteria and phytoplankton. The subsequent rapid sea-level rise with superimposed higher-frequency sea-level fluctuations and accompanying anoxia could have caused rapid elevation of anoxic bottom waters and expansion of eutrophic surface waters over shallow-water platforms due to enhanced upwelling ocean currents and improved ocean circulation. This situation would have exerted further stresses upon the already-weakened biota, leading to a further biotic demise. However, a small number of organisms such as pelagic tentaculitids, small mud-adapted brachiopods and gastropods did survive into the Famennian, although with very low diversity.     

Microbial carbonates and corals on the marginal French Jura platform (Late Oxfordian,Molinges section)

Molinges was located on an Upper Jurassic ramp system of low-energy regime that developed at the southern margin of the French Jura platform. The sedimentary succession is characterized by the transition from a mixed siliciclastic-carbonate to a carbonate depositional setting that occurred during a long-term shallowing-upward trend. The disappearance of siliciclastics is explained by a climatic change, from humid and cold to drier and warmer conditions, previously identified in Late Oxfordian adjacent basins. The base of the section shows marl-limestone alternations of outer ramp. In its middle part, the section displays oncolitic marls, coral-microbialite beds and oncolitic limestones that deposited in a mid ramp position. Finally, the upper section part is made of oolitic limestones of inner ramp. In outer- to mid-ramp settings submitted to terrigenous inputs, the stacking pattern of deposits and facies evolution allow the identification of elementary, small-, medium-, and large-scale sequences. Small amplitudes of sea-level variations probably controlled rapid shifts of facies belts and reef window occurrences. In small-scale sequences, the coral beds developed during periods of sea-level rise. The decreasing rate of sea-level rise is marked by the downramp shift of the oncolitic limestone belt that led to the demise of coral-microbialite beds. These bioconstructions are mainly represented by thin biostromes in which corals never reach great sizes. The coral assemblages mainly include the genera Enallhelia, Dimorpharaea, Thamnasteria, and some solitary forms (Montlivaltia and Epistreptophyllum). They suggest relatively low-mesotrophic conditions in marine waters during the edification of the primary framework. Relatively cold water temperatures and periods of more elevated nutrient contents are probably responsible of the reduced coral development and the formation of a large amount of microbialites.     

Carnian (Triassic) aridization on the Levant margin: evidence from the M1 member, Mohilla Formation, Makhtesh Ramon, south Israel

The Carnian Pluvial Event was followed by extreme aridity around the Mediterranean region, but the relationship between these climatic modes and the evaporite deposits of the north Arabian margin has not been established. The Mohilla Formation in Israel was deposited on the rifted Levant margin of the Neotethyan seaway during the Carnian (Late Triassic). The lowermost member, M1 (carbonate, shale, minor evaporite), records sea-level and climate change of the transition from open-marine deposits to the thick evaporite of the overlying M2 member. Field, petrographic and cathodoluminescence investigation of the M1 member exposed at Makhtesh Ramon, southern Israel, enabled tracking of changing paleoenvironments. The M1 member can be subdivided into six sea-level controlled sedimentary cycles, each terminating in subaerial exposure. Open-marine, lagoonal, and supratidal belts are represented, but environments become increasingly restricted upwards. Three of the exposure horizons are marked by well-developed early diagenetic features, including dissolution porosity and meteoric cements. These changing facies belts and the superposed diagenetic modes formed under an oscillating climate regime of three relatively humid episodes dominated by deposition of carbonate, alternating with evaporitic sediments indicating aridity. Deposition of the M1 member can be correlated with the transition in the western Tethys from the Carnian pluvial episode to aridity. This transition in the Levant region is characterized by an oscillatory climate that culminated in a thick evaporite unit from the extreme arid phase. The oscillations may be attributed to shifting global climate belts, overprinted by monsoonal strengthening and weakening, in the equatorial belt.     

Carbonate breccias in the lower-middle Ordovician Maggol Limestone (Taebacksan Basin,South Korea): Implications for regional tectonism

Summary Carbonate breccias occur sporadically in the Lower-Middle Ordovician Maggol Limestone exposed in the Taebacksan Basin, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these breccias. This study, however, indicates that the majority of these breccias are solution-collapse breccias, which are causally linked to paleokarstification. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician. Extensive subaerial exposure of platform carbonates resulted in paleokarst-related solution-collapse breccias in the upper maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity elsewhere beneath the Middle Ordovician sequence, most notably North America and North China. Due to its global extent, the early Middle ordovician paleokarst unconformity (‘the Sauk-Tippecanoe sequence boundary’) has been viewed as a product of second-order eustatic sea level drop during the early Middle Ordovician. Although we recognizes a paleokarst breccia zone in the upper Maggol Limestone beneath the Middle Ordovician sequence, the early Middle Ordovician sequence boundary appears to be a conformable transgressive surface or a drowning unconformity, rather than a major paleokarst unconformity. The paleokarst breccia zone in the upper Maggol Limestone is represented by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. The paleokarst breccia zone in the upper Maggol Limestone was a likely consequence of repeated high-frequency sea level fluctuations of fourth- and fifth-order superimposed on a second-and third-order eustatic fall in sea level that was less than the rate of tectonic subsidence across the platform. It suggests that second- and thirdorder eustatic sea level drop may have been significantly tempered by substantial tectonic subsidence near the end of maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quarzite lenses as well as debris flow carbonate breccias. With the continued tectonic subsidence, subsequent rise in the eustatic cycle caused drowning and deep flooding of carbonate platform, forming a conformable transgressive surface or a drowning unconformity on the top of the paleokarst breccia zone. This tectonic implication contrasts notably with the slowly subsiding carbonate platform model for the Taebacksan Basin as previously intepreted. Here we propose that the Taebacksan Basin evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician. This study also provides a good example that the falling part of the eustatic sea-level cycle may not produce a significant event at all in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence.     

Characteristics of the Lofer cyclicity in the type locality of the Dachstein Limestone (Dachstein Plateau,Austria)

In the present paper, the results of our studies in the type locality of the Dachstein Limestone are summarised in order to contribute to the correct interpretation of the Lofer cycles. In the sections studied on the Dachstein Plateau, the boundaries of the Lofer cycles are usually erosional disconformities showing karstification features. Penetration by karstic solution was not more than a few decimetres, since during the recurrent sea-level drops the platform only slightly emerged above sea level. The reddish or greenish argillaceous carbonate interlayers (facies A) cannot be interpreted as in situ palaeosol horizons. They are tidal flat deposits consisting predominantly of subtidal carbonate mud redeposited by storms that was mixed with reworked airborne fine carbonate particles and argillite and/or reworked lateritic soil, which were accumulated on the subaerially exposed platform. Rip-ups from consolidated sediment, blackened intraclasts and skeletons of tidal flat biota may have also contributed to the sediment of facies A. Erosional boundaries of most of the investigated cycles, and definite features of karstic solution beneath the disconformities, suggest periodical drops of sea level followed by a renewed transgression. This appears to confirm the allocyclic model for the explanation of the origin of the Lofer cycles.