Depositional characteristics and constraints on the mid-Valanginian demise of a carbonate platform in the intra-Tethyan domain,Circum-Rhodope Belt,northern Greece

Two platform-type carbonate successions of Berriasian to early Valanginian age are exposed in the eastern Circum-Rhodope belt which extends from the Chalkidiki Peninsula to the Thrace region in northern Greece. On the basis of new sedimentological and biostratigraphic results and analysis of published palaeomagnetic data, the Porto Koufos Limestones and Aliki Limestones are interpreted as deposits of a formerly unknown earliest Cretaceous carbonate platform in the Western Tethys realm. This Circum-Rhodope carbonate platform existed in tropical latitudes of the intra-Tethyan domain on the northern shelf area of the small Vardar oceanic basin. It was characterized by limited regional extent, remoteness from land, and short lateral transitions into deeper basin areas. Predominantly skeletal sediments with various microencrusters were produced along with variable amounts of lime mud, marine cements, peloids, intraclasts, aggregate grains, ooids and microbialites. The microfacies analysis of limestones formed around the Berriasian–Valanginian boundary indicates the configuration of a rimmed shelf with restricted lagoon, open lagoon, reef margin, fore-reef and upper slope depositional environments. During the early Valanginian a change from photozoan to heterozoan mode of carbonate production occurred mainly as a result of climate cooling. Deposition continued in protected lagoon, shoal and near-shoal settings implying a ramp-like morphology of the platform. Finally, a shift from skeletal to non-skeletal carbonate deposition took place as a consequence of high seawater carbonate saturation and possibly coeval increase of the marine trophic levels. A major sea level fall and climate cooling were the prime palaeoenvironmental controls that caused decline of the shallow-water carbonate factory and subsequent demise of the Circum-Rhodope carbonate platform in mid-Valanginian time that was followed by a long-term subaerial exposure and karstification which continued at least until the middle Eocene. The new results can be used for correlation with other shallow marine carbonates deposited in the intra-Tethyan domain during the earliest Cretaceous. Also, they appear to be of critical significance to decipher the Mesozoic geodynamic evolution of the Circum-Rhodope belt and adjacent tectonic zones.     

Carbonate platform growth and demise offshore Central Vietnam: Effects of Early Miocene transgression and subsequent onshore uplift

Miocene carbonate platforms cover a large part of the Central Vietnamese South China Sea margin. Early carbonate deposition took place on two regional platforms separated by a narrow depression developed along the trace of the East Vietnam Boundary Fault Zone. West of the East Vietnam Boundary Fault Zone, the Tuy Hoa Carbonate Platform fringes the continental margin between Da Nang and Nha Trang. Here, platform growth initiated during the Early Miocene and continued until Middle Miocene time when regional uplift led to subaerial exposure, termination of platform growth and karstification. East of the fault zone, the Triton Carbonate Platform was also initiated during the Early Miocene. Carbonate growth thrived during Early and part of Middle Miocene time and a thick, clean Lower and Middle Miocene carbonate succession cover the Triton Horst and the Qui Nhon Ridge. During the Middle Miocene, partial drowning resulted in the split-up of the Triton Carbonate Platform. Repeated partial drowning events throughout the Middle and Late Miocene resulted in westwards retreat of platform growth and eventual platform drowning and termination of carbonate deposition. Modern carbonate growth continues on isolated platforms hosting the Paracel Islands farther seawards. The onset of widespread carbonate deposition largely reflects the Early Miocene transgression of the area linked with early post-rift subsidence and the opening of the South China Sea. The mid-Neogene shift in carbonate deposition is interpreted as a consequence of regional uplift and denudation of central and south Indochina starting during Middle Miocene time when the Tuy Hoa Carbonate Platform became subaerially exposed. Stressed carbonate growth conditions on the Triton Carbonate Platform probably resulted from increased inorganic nutrient input derived from the uplifted mainland, possibly enhanced by deteriorated climatic conditions and rapid sea-level fluctuations promoting platform drowning.     

Stratigraphy and tectonics of a time-transgressive ophiolite obduction onto the eastern margin of the Pelagonian platform from Late Bathonian until Valanginian time,exemplified in northern Evvoia,Greece

The obduction of an ophiolite sheet onto the eastern Pelagonian carbonate platform complex of the Hellenides began during the Late Bathonian and ended with the final emplacement of the ophiolite during Valanginian time. The early stages of obduction caused subaerial exposure of the platform, recorded by an unconformity of Callovian age, which is marked by laterites overlying folded and faulted, karstic substrates. The laterites have distinct ophiolitic geochemical signatures, indicating that emergent ophiolite had been undergoing lateritic weathering. This unconformity coincides with widespread western Tethyan, Callovian gaps, indicating that the obduction in the Hellenides was probably related to far-reaching plate tectonic processes. Resumed gravitational pull and rollback of the subducted, oceanic leading edge of the temporarily exposed ophiolite. Platform drowning continued into Tithonian–Valanginian time, documented initially by reefal carbonates and then by below-CCD, carbonate-free radiolarian cherts and shales. Subsequently, siliciclastic turbidites, which apparently originated from uplifted Variscan basement, were deposited together with and over the radiolarite as the ophiolite nappe sheet advanced. The nappe substrate underwent tectonic deformations of varying intensity, while polymictic mélange and syntectonic sedimentary debris accreted beneath the ophiolite and at the nappe front. The provenience of the ophiolite nappe complexes of northern Evvoia most probably has to be looked for in the Vardar ocean.     

Tithonian–Early Valanginian evolution of deposition along the proto-Caribbean margin of North America recorded in Guaniguanico successions (western Cuba)

In the Guaniguanico Mountains of western Cuba, the Late Jurassic–Early Cretaceous limestones occur in three stratigraphic successions, which have accumulated along the proto-Caribbean margin of North America. The Late Jurassic subsidence and shallow-water carbonate deposition of the Guaniguanico successions have no counterpart on the northeastern Maya block, but some distant similarities with the southeastern Gulf of Mexico may exist. Four facies types have been distinguished in the Tithonian–Lower Valanginian deposits of the Guaniguanico tectonic units. Drowning of the Late Jurassic carbonate bank of the Sierra de los Organos occurred at the Kimmeridgian/Tithonian boundary. During this boundary interval, sedimentation in the west Cuban area and southwestern margin of the Maya block (Mexico) has evolved in a similar way in response to a major second-order transgression.The Lower Tithonian ammonite assemblages of the Guaniguanico successions indicate, in general, the neritic zone. Presence of juvenile gastropods and lack of adult specimens suggest unfavorable environment for these molluscs, probably related to low oxygenation levels. The Early Tithonian transgressive phase terminated about the lower boundary of the Chitinoidella Zone. The Late Tithonian “regressive” phase is weakly marked, whereas the latest Tithonian–earliest Berriasian strata were deposited during a deepening phase. The latter transgressive phase has ended in the Late Berriasian Oblonga Subzone. We correlate the bioturbated pelagic biomicrites of the Tumbitas Member of the Guasasa Formation with a significant fall of the sea level during the latest Berriasian–Early Valanginian. The average sedimentation rate for the Tumbitas Member biomicrites was about three times faster than for the Berriasian Tumbadero Member limestones. Sedimentation rates for the Tumbitas Member and the Valanginian limestones at the DSDP Site 535 in the southeastern Gulf of Mexico were similar. In the Los Organos succession, the Late Valanginian transgressive interval is associated with radiolarian limestones and black chert interbeds in the lower part of the Pons Formation. In the Southern Rosario succession, the pelagic limestones pass into the radiolarian cherts of the Santa Teresa Formation indicating a proximity of CCD during Late Valanginian–Hauterivian times.     

The Early Cretaceous drowning unconformities of the Apulia carbonate platform (Gargano Promontory, southern Italy): local fingerprints of global palaeoceanographic events.

Drowning successions which cap carbonate platforms and flanks bear palaeoenvironmental information which is useful for genetic stratigraphy; they constitute predictive key-markers in regional to global correlations. An Early Cretaceous platform-to-basin transition has been investigated in Apulia (southern Italy) and two drowning unconformities, dated as early Valanginian and late early Aptian, have been documented. They occur at the base of thick pelagic tongues wedging toward the platform and mark the base of two depositional sequences showing distinct transgressive–regressive cycles. Timing of drowning processes, based on biostratigraphy and dynamic stratigraphy, allows the correlation of unconformities with global-scale palae- oceanographic events marked, among others, by positive spikes of well-established δ¹³C curves. Drowning signatures in the Apulia carbonates fit the stratigraphic, palaeoecological and possibly geochemical evidence found in global records at the same stratigraphic levels. Moreover, it is proposed that the observed drowning events were caused by palaeoceanographic crises affecting factory productivity.     

Sequence stratigraphy of Lower Cretaceous deposit on the eastern Russian Plate

The generalized eustatic and tectonoeustatic models developed by the author are tested on Lower Cretaceous deposits of the eastern part of the Russian Plate. The models are applicable to facies analysis of sections of epicontinental basins with mainly slope sedimentation. They demonstrate possible variations in section lithology depending on the rate of eustatic changes and the intensity and direction of epeirogenic movements. It has been revealed that the Lower Cretaceous sections in the east of the Russian Platform formed as a result of the synchronous global eustasy and regional epeirogeny. Superposition of the global eustatic curve onto the Lower Cretaceous chronostratigraphic chart of the eastern part of the platform showed that global eustasy, periodically concealed by regional epeirogeny, played a crucial role in the Early Cretaceous history of the study area. Regional epeirogenic and eustatic curves were constructed. The epeirogenic curve demonstrates the contribution of vertical tectonic movements to the overall eustatic-epeirogenic result recorded on a regional eustatic curve. The latter was constructed from the analysis of the spatial and temporal changes in the stratigraphic position of formations and strata and transgressive surfaces ranking. Eustatic cycles of different ranks, from elementary (systems tracts) to regional scale, have been recognized. In the rank of largest lithostratigraphic units, three sequences are revealed: Valanginian (RP-1K), Upper Hauterivian–Upper Aptian (RP-2K), and Albian (RP-3K), which reflect the crucial stages of the Early Cretaceous evolution of the eastern Russian Plate. The eustatic-epeirogenic processes during accumulation of formations and strata from Early Berriasian to Late Albian (145.5–99.6 Ma) are considered. It is shown that the division of the studied composite section into sequences permits precise prediction of diverse solid minerals.     

Berriasian drowning of the Plassen carbonate platform at the type-locality and its bearing on the early Eoalpine orogenic dynamics in the Northern Calcareous Alps (Austria)

The Plassen carbonate platform (Kimmeridgian to Early Berriasian) developed above the Callovian to Tithonian carbonate clastic radiolaritic flysch basins of the Northern Calcareous Alps during a tectonically active period in a convergent regime. Remnants of the drowning sequence of the Plassen Formation have been discovered at Mount Plassen in the Austrian Salzkammergut. It is represented by calpionellid-radiolaria wacke- to packstones that, due to the occurrence of Calpionellopsis oblonga (Cadisch), are of Late Berriasian age (oblonga Subzone). Thus, the Plassen Formation at its type-locality shows the most complete profile presently known, documenting the carbonate platform evolution from the initial shallowing upward evolution in the Kimmeridgian until the final Berriasian drowning. The shift from neritic to pelagic sedimentation took place during Berriasian times. A siliciclastic-influenced drowning sequence sealed the highly differentiated Plassen carbonate platform. The former interpretation of a Late Jurassic carbonate platform formed under conditions of tectonic quiescence cannot be confirmed. The onset, evolution and drowning of the Plassen carbonate platform took place at an active continental margin. The tectonic evolution of the Northern Calcareous Alps during the Kimmeridgian to Berriasian time span and the reasons for the final drowning of the Plassen carbonate platform are to be seen in connection with further tectonic shortening after the closure of the Tethys Ocean.     

The Cretaceous megabreccias of the Gargano Promontory (Apulia, southern Italy): their stratigraphic and genetic meaning in the evolutionary framework of the Apulia Carbonate Platform

Huge megabreccias occur at the eastern margin of the Cretaceous Apulia Carbonate Platform (Gargano Promontory, southern Italy). Their stratigraphic and genetic meaning are controversial in the debated geological evolution of the Apulia Platform. New stratigraphic analyses have revealed that three distinct megabreccia levels occur within the coarse debrites that were previously interpreted to be the result of repeated collapses of a scalloped platform margin during the late Albian–Cenomanian. Each level has peculiar chronostratigraphic distribution, geometry, composition and genetic features. They are the Posta Manganaro Megabreccias (late early Aptian to late Albian pp. ), Monte S. Angelo Megabreccias (early–middle Cenomanian) and Belvedere di Ruggiano Megabreccias (middle Turonian). These deposits overlie regional, tectonically enhanced unconformities of late early Aptian, late Albian and late Cenomanian age. These megabreccias, which were formed, respectively, during drowning, prograding and exposure events of the Apulia Platform, reflect important turning points in its Cretaceous geodynamic evolution.     

Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin

The sedimentary history of the Nepal Tethys Himalaya began with deposition of thick carbonates in the Cambro?–Ordovician, followed by a mixed siliciclastic–carbonate epicontinental succession recording two major deepening events in the Early Silurian and Late Devonian. Fossiliferous carbonate ramp deposits in the Tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins (“rift stage”). Break-up in the mid-Early Permian, locally associated with effusion of tholeiitic lava flows, was followed by a transgressive sandy to shaly, locally coal-bearing or bioclastic unit capped by condensed pelagic carbonates in the Middle to Late Permian (“juvenile ocean stage”). Subsidence of the cooling stretched crust led close to bathyal water depths in the Olenekian, but then slowed down in the Middle Triassic to increase again sharply in the Late Triassic owing to renewed extensional tectonic activity and sediment loading during up- and out-building of the Indian continental terrace. Deposition of tropical platform carbonates finally became widespread in the middle Liassic (“mature passive margin stage”). The initial fragmentation of Gondwana in the Middle Jurassic led to rejuvenation of the Indian craton and deposition of quartzo-feldspathic hybrid arenites, capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late Bathonian/middle Callovian. Next, a discontinuous pelagic grey marly limestone unit was followed by the ammonoid-rich offshore Spiti Shale in the Late Jurassic. The final disintegration of Gondwana began in the Berriasian, when quartzose siliciclastics derived again from the rejuvenated Indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the Valanginian? followed in the Hauterivian to Albian by more felsic differentiates such as the trachyandesites exposed in the Lesser Himalaya 120 km to the south. A widespread drowning episode, fostered by waning volcaniclastic supply during a global eustatic rise, is documented by a major glauconitic horizon deposited at middle southern latitudes in the late Albian, overlain by “Scaglia-like” pelagic limestones in the latest Albian. The final part of sedimentary history, during the rapid northward flight of India and its collision with Eurasia, is not documented anywhere in Nepal due to later erosion of Upper Cretaceous to Lower Tertiary strata.     

Microfacies and age of the Ceahlău Massif carbonate olistoliths (Eastern Carpathians,Romania): Remnants of a lowermost Cretaceous carbonate platform

We carried out a comprehensive facies/microfacies, micropalaeontological and biostratigraphical analysis of several carbonate olistoliths incorporated within a widely developed Albian conglomeratic sequence from the Eastern Carpathians of Romania. The majority of the sampled olistoliths contain a rich assemblage of benthic foraminifera and calcareous green algae. All of the described microfossils represent common lowermost Cretaceous taxa not previously reported from these carbonate elements or from this region. Based on benthic foraminifera assemblages the age of the studied olistoliths is upper Berriasian–lower Valanginian, contrary to the general belief that they are Barremian–Aptian in age. The dominant microfacies types mainly reflect deposition in shallow-water environments and show similarities with synchronous platform carbonates of the central-western Neotethys Ocean. The micropalaeontological and sedimentological data support new interpretations concerning the source area of these carbonate elements and provide new information concerning the evolution of the lowermost Cretaceous carbonate platforms of the Carpathians.     

Reassessment of the earliest documented stegosaurian fossils from Asia

In 1929, the famous Swedish palaeontologist Carl Wiman documented the first unequivocal stegosaurian dinosaur fossils from Asia. His material comprised an isolated dermal spine, together with a dorsal vertebra that was briefly described but never figured. Since then these remains have languished in obscurity, being noted in some stegosaur review articles but often ignored altogether. However, recent auditing of the Museum of Evolution palaeontological collection at Uppsala University in Sweden has led to the rediscovery of Wiman's original specimens, as well as two additional previously unrecognised stegosaurian dorsal vertebrae. All of these bones derive from the Lower Cretaceous (Berriasian–Valanginian) Mengyin Formation of Shandong Province in eastern China, and are morphologically compatible with the stratigraphically proximal stegosaurian taxon Wuerhosaurus from the Valanginian–Albian Tugulu Group in the Xinjiang Uyghur Autonomous Region of Western China. Wiman's seminal stegosaurian fossils thus expand current palaeobiogeographical distributions, and contribute to the otherwise enigmatic record of Early Cretaceous stegosaurian occurrences globally.     

Upper Jurassic Rock Depositional Settings in the Baidar Valley and Evolution of the Crimean Carbonate Platform

The paper presents results of the lithological study of Upper Jurassic limestones, flyschoids and limestone breccias on the southern side of the Baidar Valley in the Crimean Mountains. Study of the microfacies revealed that the limestones are represented by deposits on lagoons, platform edge shoals, reefs, and forereef aprons on the carbonate platform slope. Flyschoids include deposits in the distributive turbidite channels and hemipelagic sediments in the deep-water part of the basin. Limestone breccias were formed by gravitation flows on the carbonate platform toe-of-slope and slope. The presence of gravitation deposits in the Upper Jurassic carbonate complexes of the Crimean Mountains can testify to the primary clinoform structure of this sedimentary sequence. Comparison of the obtained sedimentological data made it possible to reconstruct the facies model of the Crimean carbonate platform and main episodes of its formation. Development of the carbonate shelf was related to two transgressive-regressive cycles. A dome-shaped reef was formed away from the coast at the initial (Oxfordian) stage. The carbonate platform was formed at the early Kimmeridgian lowstand stage when sediments were deposited in the internal part of the platform adjacent to land. In the late Kimmeridgian and early Tithonian, configuration of the carbonate platform profile resembled a distally steepened ramp, and its active progradation and shelf expansion took place in the course of transgression. Regression in the late Tithonian–early Berriasian led to regressive transformation of the ramp into platform with a flattened shallow-water shelf. Tectonic deformations at the Jurassic/Cretaceous transition promoted the formation of megabreccias on the carbonate platform foreslope. The tectonically reworked rock sequence of the “extinct” carbonate platform was overlapped transgressively by the upper Berriasian or lower Valanginian, relatively deep-water deposits of the Cretaceous platform cover.     

Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (Valanginian–Campanian) passive margin,northern Gulf of Mexico

An integrated sequence stratigraphic study based on outcrop, core and wireline log data documents the combined impact of Cretaceous eustacy and oceanic anoxic events on carbonate shelf morphology and facies distributions in the northern Gulf of Mexico. The diverse facies and abundant data of the Comanche platform serve as a nearly complete global reference section and provide a sensitive record of external processes affecting Cretaceous platform development. Regional cross‐sections across the shoreline to shelf‐margin profile provide a detailed record of mixed carbonate–siliciclastic strata for the Hauterivian to lower Campanian stages (ca 136 to 80 Ma). The study window on the slowly subsiding passive margin allows the stratigraphic response to external forcing mechanisms to be isolated from regional structural processes. Three second‐order supersequences comprised of eight composite sequences are recognized in the Valanginian–Barremian, the Aptian–Albian and the Cenomanian–Campanian. The Valanginian–Barremian supersequence transitioned from a siliciclastic ramp to carbonate rimmed shelf and is a product of glacial ice accumulation and melting, as well as variable rates of mid‐ocean ridge volcanism. The Aptian–Albian supersequence chronicles the drowning and recovery of the platform surrounding oceanic anoxic events 1a and 1b. The Cenomanian–Campanian supersequence similarly documents shelf drowning following oceanic anoxic event 1d, after which the platform evolved to a deep‐subtidal system consisting of anoxic/dysoxic shale and chalk in the time surrounding oceanic anoxic event 2. Each period of oceanic anoxia is associated with composite sequence maximum flooding, termination of carbonate shelf sedimentation and deposition of condensed shale units in distally steepened ramp profiles. Composite sequences unaffected by oceanic anoxic events consist of aggradational to progradational shelves with an abundance of grain‐dominated facies and shallow‐subtidal to intertidal environments. Because they are products of eustacy and global oceanographic processes, the three supersequences and most composite sequences defined in the south Texas passive margin are recognizable in other carbonate platforms and published eustatic sea‐level curves.     

The response of two Tethyan carbonate platforms to the early Toarcian (Jurassic) oceanic anoxic event: environmental change and differential subsidence

Chemostratigraphic analyses (⁸⁷Sr/⁸⁶Sr, δ¹³C_carb) of limestones from two Jurassic platform‐carbonate sequences in Italy (Trento and Campania–Lucania Platforms) illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well‐calibrated sections elsewhere in Europe. Chemostratigraphic correlations between the platform‐carbonate successions and appropriate intervals from well‐dated reference sections allow the application of high‐resolution stratigraphy to these shallow‐water peritidal carbonates and, furthermore, elucidate the facies response to the Early Toarcian Oceanic Anoxic Event (OAE). Lower Jurassic (Toarcian) levels of the western Trento Platform (Southern Alps, Northern Italy) contain spiculitic cherts that appear where rising carbon‐isotope values characterize the onset of the OAE: a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near‐surface waters. There is a facies change to more clay‐rich facies at the level of the abrupt negative carbon‐isotope excursion, also characteristic of the OAE, higher in the section. The Campania–Lucania Platform (Southern Apennines, Southern Italy) records a change to more clay‐rich facies where carbon‐isotope values begin to rise at the beginning of the OAE but the negative excursion, higher in the section, occurs within oolitic facies. Although, in both examples, the Early Toarcian OAE can be recognized by a change to more clay‐rich lithologies, this facies development is diachronous and in neither case did the platform drown. Although the Trento Platform, in the south‐west sector studied here, was adversely affected by the OAE, it did not drown definitively until Late Aalenian time; the Campania–Lucania Platform persisted throughout the Jurassic and Cretaceous. Differential subsidence rates, which can be calculated using comparative chemostratigraphy, are identified as a crucial factor in the divergent behaviour of these two carbonate platforms: relatively fast in the case of the Trento Platform; relatively slow in the case of the Campania–Lucania Platform. It is proposed that where water depths remained as shallow as a few metres during the OAE (Campania–Lucania Platform), dissolved oxygen levels remained high, nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable, whereas more poorly ventilated and/or more nutrient‐rich waters (Trento Platform) adversely influenced platform growth where depths were in the tens of metres range. The stage was thus set for drowning on the more rapidly subsiding western margin of the Trento Plateau and a pulse of oolite deposition post‐dating the OAE was insufficient to revitalize the carbonate factory.     

Early cretaceous radiolarian assemblages from the East Sakhalin Mountains

Three-dimensional radiolarian skeletons isolated from rock matrix in HF solution and then studied under scanning electron microscope substantiate the Early Cretaceous age of volcanogenic-cherty deposits sampled from fragmentary rock successions of the East Sakhalin Mountains. Accordingly the Berriasian age is established for jasper packets formerly attributed to the Upper Paleozoic-Mesozoic Daldagan Group; the Valanginian radiolarians are identified in cherty rock intercalations in the Upper Paleozoic (?) Ivashkino Formation; the Berriasian-Barremian assemblage is macerated from cherty tuffites of the Jurassic-Cretaceous Ostraya Formation; and the Aptian-early Albian radiolarians are characteristic of tuffaceous cherty rocks sampled from the Cretaceous Khoe Formation of the Nabil Group. Photographic documentation of radiolarian skeletons specifies taxonomic composition and age of the Berriasian, Valanginian, Berriasian-Valanginian, Barremian, and Aptian-Albian radiolarian assemblages from the East Sakhalin Mountains, and their evolution as related to abiotic events is considered. Coexistence of Tethyan and Pacific species in the same rock samples evidence origin of radiolarian assemblages in an ecotone. Consequently, the assemblages are applicable for intra- and interregional correlations and paleogeographic reconstructions.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt, South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. 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 in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent 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. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.     

Cretaceous and Jurassic intrusions in Crimean Mountains: The first data of U–Pb (SIMS SHRIMP) dating

U–Pb geochronological studies of igneous rocks of the Crimean Mountains were carried out for the first time. The ages of magma crystallization determined for gabbro–dolerites of the Dzhidair and Pervomaiskii intrusions point to the injection of these rocks during the Middle Jurassic Aalenian–Bajocian stage of magmatism. The Berriasian–Valanginian and Aptian age of sill-like bodies within the mass of volcanogenic–sedimentary rocks presumes the necessity to reconsider the common notion of an exclusively Albian magmatic event during the Cretaceous. High-precision U–Pb dating of magma intrusions allowed us to verify the age of Middle Jurassic magmatism and to distinguish the new Early Cretaceous Berriasian–Valanginian magmatism stage of basic composition.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt,South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. 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 in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent 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. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.     

The Jurassic Prehodavci Formation of the Julian Alps: easternmost outcrops of Rosso Ammonitico in the Southern Alps (NW Slovenia)

The Julian Alps are located in NW Slovenia and structurally belong to the Julian Nappe where the Southern Alps intersect with the Dinarides. In the Jurassic, the area was a part of the southern Tethyan continental margin and experienced extensional faulting and differential subsidence during rifting of the future margin. The Mesozoic succession in the Julian Alps is characterized by a thick pile of Upper Triassic to Lower Jurassic platform limestones of the Julian Carbonate Platform, unconformably overlain by Bajocian to Tithonian strongly condensed limestones of the Prehodavci Formation of the Julian High. The Prehodavci Formation is up to 15 m thick, consists of Rosso Ammonitico type limestone and is subdivided into three members. The Lower Member consists of a condensed red, well-bedded bioclastic limestone with Fe–Mn nodules, passing into light-grey, faintly nodular limestone. The Middle Member occurs discontinuously and consists of thin-bedded micritic limestone. The Upper Member unconformably overlies the Lower or Middle Members. It is represented by red nodular limestone, and by red-marly limestone with abundant Saccocoma sp. The Prehodavci Formation unconformably overlies the Upper Triassic to Lower Jurassic platform limestone of the Julian Carbonate Platform; the contact is marked by a very irregular unconformity. It is overlain by the upper Tithonian pelagic Biancone (Maiolica) limestone. The sedimentary evolution of the Julian High is similar to that of Trento Plateau in the west and records: (1) emergence and karstification of part of the Julian Carbonate Platform in the Pliensbachian, or alternatively drowning of the platform and development of the surface by sea-floor dissolution; (2) accelerated subsidence and drowning in the Bajocian, and onset of the condensed pelagic sedimentation (Prehodavci Formation) on the Julian High; (3) beginning of sedimentation of the Biancone limestone in the late Tithonian.