Carbonate Platform Research Articles

An Upper Devonian Kadzielnia-type carbonate buildup in Skrzelczyce Quarry, southern Holy Cross Mountains (Poland)

In Skrzelczyce Quarry, located in the southern part of the Holy Cross Mountains in central Poland, a large Upper Devonian carbonate mound is exposed. The Skrzelczyce mound, developed as massive and faintly bedded limestones, measures nearly 100 m in lateral extent and over 20 m in thickness, and laterally passes into coral-rich biostromal limestones belonging to the Sitkówka Beds. Four facies have been identified within the massive limestone unit: stromatactis-bearing limestones with scarce macrofossils (M1), macrofossil-rich limestones (M2), stromatoporoid-microbial limestones (M3), and limestones with fenestral structures (M4). These facies indicate varying depositional conditions and microbial activity, reflecting a complex interplay of environmental factors. The mound’s lithological features and spatial facies distribution resemble the late development stage of Devonian atolls in the Ardennes. This analogy suggests that the initial mound setting at Skrzelczyce might have been at depths of 30–60 m, in a moderate energy zone. The mound, composed predominantly of polygenetic biomicrite with a significant presence of laminar stromatoporoids and corals, features various forms of stromatactis, some typical and others related to sediment winnowing or to shelter cavities. The foraminiferal assemblages, along with lithostratigraphic correlation, suggest that the Skrzelczyce mound may belong to the lower to middle Frasnian, although the precise age remains uncertain due to the lack of conodont dating. The Skrzelczyce mound aligns with the definition of the Kadzielnia Member, and particular facies from Skrzelczyce can be matched with those described from the Kadzielnia Quarry. The structure outcropping in Skrzelczyce is the second largest Upper Devonian carbonate buildup after Kadzielnia in the Holy Cross Mountains, and the largest on the southern edge of the Kielce carbonate platform.

Radiolarian and planktonic foraminiferal biochronology of the Soğukçam Limestone Group, Elmadağ Olistostrome, and Unaz Formation (Ankara region, central Türkiye): Insights into the Cretaceous evolution of the Sakarya Continent and overlying units

The Ankara region (central Türkiye) comprises a part of the Sakarya Terrane and the accretionary remnants inherited from the Izmir-Ankara-Erzincan (IAE) branch of the Northern Neotethys. The Sakarya Terrane is characterized by a pre-Jurassic basement overlain by Jurassic-Cretaceous sedimentary assemblages, collectively known as the Sakarya Continent Cover. In this study, we aim to elucidate the Jurassic-Cretaceous evolution of the Sakarya Terrane through detailed geological mapping in the regions of Haymana, Yakacık, north of Alagöz and west of Memlik, located to the west and southwest of Ankara city, central Türkiye. By analyzing radiolarian and planktonic foraminiferal assemblages, we provide precise dating for three key lithological units: the Soğukçam Limestone Group (part of the Sakarya Continent Cover), the Elmadağ Olistostrome, and the Unaz Formation (from the overlying units). The oldest rock unit exposed in the Haymana region, south of Ankara city, is the Bilecik Limestone Group, consisting of Tithonian to lower Berriasian platform carbonates. A drowning unconformity separates the Bilecik Limestone Group from the overlying middle Berriasian-uppermost Albian Soğukçam Limestone Group, which has been elevated to "group" status in this study. The Soğukçam Limestone Group is subdivided into two formations -the Seyran Formation and the Akkaya Formation- separated by a disconformity surface. The Seyran Formation, at the base, consists of middle Berriasian limestone breccia in a micritic pelagic matrix, and upper Berriasian-lower Aptian micritic clayey, cherty pelagic limestones with abundant and diverse radiolarians in its upper part. The disconformity between the Seyran and Akkaya formations represents a small depositional gap corresponding to the middle to late Aptian. The lower part of the Akkaya Formation contains lower to middle Albian debris flow deposits while the upper part is composed of clayey pelagic limestones rich in glauconite and planktonic foraminifers, dating to the latest Albian. Overall, the Akkaya Formation is roughly assigned to the Albian stage. The Akkaya Formation within the Soğukçam Limestone Group is unconformably overlain by the Coniacian Elmadağ Olistostrome, which also covers the ophiolitic mélange of the IAE Ocean (containing Turonian radiolarian blocks) to the west of the Memlik region. The basal part of the Elmadağ Olistostrome is characterized by an unstratified, loosely-packed, gray- to beige-colored carbonate matrix, while its upper part features stratified, highly-sheared gray- to beige-colored clayey carbonate/marl matrix with embedded carbonate blocks. These blocks, dated through radiolarians and planktonic foraminifers, range from the early Callovian to Coniacian. These blocks, observed in the Haymana, north of Alagöz, and Yakacık regions, primarily originated from the underlying Soğukçam Limestone Group. The Elmadağ Olistostrome was likely deposited in a peripheral foreland flysch basin, resembling trench-like settings, in front of the southward-moving nappes derived from the Neotethys Intra-Pontide Ocean. This olistostrome is unconformably overlain by the Unaz Formation, which represents the lowermost unit of the fore-arc basin within the Galatean Arc and is present in all four studied regions. The Unaz Formation mainly consists of thin-bedded, gray- to red-colored clayey limestones with abundant planktonic foraminifers, along with red- to pink-colored marls. In the northern part (west of Memlik), the Unaz Formation also includes basal clastics. Based on its characteristic planktonic foraminiferal assemblages, the formation is dated to the late Santonian and correlates well with the type locality in the Pontides, northwestern Türkiye. Following this sequence, a brief period of continental arc magmatism occurred in the Santonian-Campanian, accompanied by the deposition of fore-arc sediments (the Haymana Formation, consisting of clastics) during the Campanian-Maastrichtian interval. In this study, an abundant, well-preserved, and diverse radiolarian microfauna (146 taxa, including fourteen new species and four new subspecies from the early Hauterivian to early Aptian, Early Cretaceous) were recovered from the clayey micritic limestones of the Seyran Formation in the Soğukçam Limestone Group. The vertical distributions of these radiolarians and their age correlations with previous studies are also documented.

The earliest Cretaceous demise of the Slivnitsa carbonate platform: an interplay of three controlling factors

The flat-topped Slivnista carbonate platform was subaerially exposed during the earliest Valanginian and intense karstification occurred until the latest Valanginian producing various breccia types. The platform demise was caused by a relative sea-level fall but a drastic paleoenvironmental change around the Berriasian/Valanginian boundary also contributed to the shutdown of the carbonate factory. This is revealed by a rapid change from photozoan through heterozoan and then to non-skeletal mode of carbonate production. A similar biological crisis is known from other carbonate platforms across the Western Tethys and was probably associated with climate cooling, humidification and increase of marine trophic levels. The late development of the Slivnitsa platform was affected by synsedimentary extensional tectonics that led to differential uplift and emergence. This tectonic activity was most likely related to the Neo-Cimmerian (Late Cimmerian) phase.

Isotopic and Sedimentologic Proxies of Palaeoexposure Events and Oceanic Anoxic Event 2 (OAE2) in the Cenomanian–Turonian Neritic Carbonates (the Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran

ABSTRACTThe shallow marine carbonate deposits of the Sarvak Formation have been deposited in an active tectonic setting under warm and humid climatic conditions during the Cenomanian–Turonian (C–T). The focus of this study is on the sedimentological and geochemical analysis of this formation in the Abadan Plain, Zagros Basin. Petrographic studies led to the identification of five microfacies deposited in the restricted lagoon, tidal flat, reef, shoal and open marine environments. They indicate deposition of the Sarvak Formation on a homoclinal ramp–like carbonate platform. Sequence stratigraphy studies have led to the identification of two third‐order sequences named Cenomanian sequence (DSS‐1) and Turonian sequence (DSS‐2), along with six fourth‐order sequences. The upper boundary of DSS‐1, known as the C–T disconformity (CT‐ES), is subjected to meteoric diagenetic processes, as evidenced by meteoric dissolution (karstification), collapsed brecciation and the development of palaeosols. The upper SB of DSS‐2 is marked by the mid‐Turonian disconformity (mT‐ES), characterised by silicification, brecciation, meteoric dissolution and iron oxide staining of core samples. The significant decreases in δ18Ocarb and δ13Ccarb values indicate a strong effect of meteoric diagenesis beneath the palaeoexposure surfaces. The calculation of sea surface temperatures (SSTs) based on the δ18Ocarb values measured from the unaltered carbonates indicated that the average SST for the C–T is 31.8°C and 30.9°C in K‐01 and K‐02 wells, respectively. A decrease of about 1.5°C is measured during the C–T transition, which indicate a cooling trend of the Late Cretaceous in the Tethyan realm. The carbon isotopic pattern observed in the analysed sections shows correlation with previous studies conducted in the Tethyan region. Additionally, the isotopic composition indicative of Oceanic Anoxic Event 2 (OAE2) is identified around the C–T boundary. Nevertheless, the signature of OAE2 has been somewhat obscured by diagenetic processes associated with C–T palaeoexposure.

Lithofacies and microfacies and depositional environment model of the Cenozoic carbonate platform: an example from the Upper Jafnayn Formation of Jafnayn area in north-east Oman

Lithofacies and microfacies and depositional environment model of the Cenozoic carbonate platform: an example from the Upper Jafnayn Formation of Jafnayn area in north-east Oman

Aborted oceanic basin during the Mesozoic rifting of the South-Iberian Palaeomargin and interactions with the sedimentation processes

ABSTRACT During the Mesozoic, the South-Iberian Palaeomargin experienced successive phases of extensional tectonics related to the fragmentation of Pangea and the aperture of the Central Atlantic. During the Pliensbachian, the rifting favoured the fracturing and collapse of the Early Jurassic carbonate platform and delimited a subdomain characterized by high subsidence called Median Subbetic (Betic Cordillera, S Spain). The progression of the rifting and the related extension along ~ 102 Ma, from late Pliensbachian to Santonian, favoured the activity of lithospheric scale faulting and extensional basic magmatism in the Median Subbetic with alkaline affinity. The highest volume of lava emissions, represented by pillow-lava flows interbedded with hemipelagic sediments, occurred during the Middle Jurassic. The pillow-lavas show aphanitic texture with Ca-rich plagioclase and clinopyroxene, whereas subvolcanic rocks are aphanitic with high content of plagioclase to phaneritic with plagioclase, olivine and biotite. The rocks present low SiO2 (46.6–53.5 wt.%) and relatively high K2O (locally reaching 8.7 wt.%), and the content of trace elements generally ranges between those from E-MORB and OIB composition. Values of some ratios such as (Gd/Yb)N, (La/Yb)N, (La/Sm)N, and the Nb content increase in the Upper Cretaceous pillow-lava flows, whereas Yb and Y contents decrease, that point to more alkaline affinity. Low Yb and Y contents point to the presence of garnet as residual phase in the mantle source. We interpret that during the early Late Cretaceous, hyperextensional phase of the continental rifting throughout an aborted oceanic basin occurred and allowed lithospheric deeper asthenospheric mantle up-lift and the partial melting. The phases of extensional tectonic controlled the configuration of this palaeomargin in complex tectono-palaeogeographic subdomains, the sedimentary cycles, and the volcanic activity during the Jurassic and Cretaceous. Finally, the transition from passive margin to a convergent one began during the Campanian and no more volcanism is recorded in the Median Subbetic.

Microfacies Characteristics of Late Pennsylvanian Cyclothems on the Carbonate Platform Margin in Guizhou, South China.

Late Pennsylvanian cyclothems are documented from the carbonate platform margin in Guizhou, South China, providing a unique opportunity to study glacio-eustatic fluctuations and their impact on reef development. This paper focuses on a shallow-water, reef-bearing succession and a deep-water succession in the Houchang area of Guizhou. Fourteen microfacies, grouped into seven associations, represent distinct depositional environments. These microfacies associations exhibit vertical cyclicity, interpreted as cyclothems, similar to those observed globally, which are attributed to the waxing and waning of the Gondwana ice sheet. The cyclothems are primarily composed of sediments below the wave base within a shallow-water platform margin and deep-water settings. Those cyclothems show strong correlations with those observed in South China, Ukraine, and the North American Midcontinent, suggesting a potential connection to global glacio-eustatic processes. A brief and rapid sea-level rise during the late Kasimovian may correspond to a recently recognized global warming event. A microfacies analysis indicates that these cyclothems reflect glacial-type sea-level fluctuations ranging from 15 to 35 m. Notably, the reef-bearing cyclothems correspond to intermediate, major cyclothems identified in South China and the Midcontinent from the late Moscovian to early Kasimovian stages. The global cyclothem correlations and reef development patterns in South China suggest that intermediate, major cycles were the primary controls on reef growth and demise, while minor cycles influenced biostromes and community succession within the reefs. These findings underscore the pivotal role of the Late Paleozoic Ice Age (LPIA) in shaping reef development in far-field regions during the Late Pennsylvanian.

The Cretaceous of the Spanish Central System: a review of the lithostratigraphy, palaeontology and sequence stratigraphy, including new educational perspectives

The Southern part of the Spanish Central System is formed by both terrigenous and carbonate Cretaceous rocks that have been affected by intense dolomitization. They are part of the sediments deposited on the western margin of the Iberian Basin during the Cenomanian–Maastrichtian, the depositional architecture of which has been reconstructed through a detailed sequence stratigraphical study and analysis of its fossil record. It is formed by three large transgressive–regressive cycles in which the carbonate platform moves towards the West, while the terrigenous facies of its base decreases and the overlying clay facies, with gypsum, increases. The integration of sedimentological and palaeontological data and the identification of a set of parasequences has allowed the differentiation of 13 depositional sequences. This research has been essential to carry out modern geological mapping and provide society with a model that improves the economic geology and geotechnical exploration of these areas. Furthermore, this scientific progress has also impacted on the interest of society towards the geosciences and, especially, palaeontology. This has widened the scope for new research fields related to scientific outreach for the community and learning opportunities at different educational levels.

GEOLOGICAL FEATURES OF SYRIAN OIL FIELDS

Over the past 60 years, exploration in Syria has focused on the major late Paleozoic and Mesozoic sedimentary basins. Source and reservoir rocks were deposited during major regional extensional periods in the late Paleozoic and Mesozoic. Oil and gas fields are concentrated mainly in the eastern and northeastern regions of the country, including the Palmyra, Deir-ez-Zor and Homs areas. The main oil fields are concentrated in the north-eastern and eastern regions of the country, in the oil and gas regions of Rmeilan, Tishreen, Jibsin, Tim and Ward. Most of the gas fields are located in the Palmyra region. The reserves of recoverable oil in Syria are estimated at 350 mln m3, and the reserves of recoverable gas – at 250 bln m3. According to some estimates, the recoverable resources of shale oil on land are about 7 bln m3. On the territory of Syria, with a total area of 2190 km², there are huge gas reserves – 600–700 bln m3 and oil reserves – 50 mln m3. New objects can be explored in Paleozoic and Mesozoic petroleum systems. Stratigraphic traps may include Paleozoic siliciclastic successions, isolated Cretaceous carbonate buildups or Jurassic-Cretaceous carbonate platforms and karsts, and marine basin-floor siliciclastic fans.

The oldest fossil record in the Iberian Peninsula; lower Ediacaran acritarchs of the Tentudía Formation, Ossa-Morena Zone (OMZ), Southwest Iberian Massif.

AbstractA diverse assemblage of acritarchs, that represents the oldest fossil record of the Iberian Massif, is described from Ediacaran metasedimentary rocks of the Tentudía Formation included in the Ossa Morena Zone stratigraphy (Southwestern Iberia). The microfossils found include Assesserium pyramidalis, Cavaspina sp. A, Ceratosphaeridium sp. A, Dictyotidium sp. A, Multifronsphaeridium sp. A, Schizofusa zangwenlongii, Tanarium megaconicum Tanarium sp. A, and Tanarium ? sp. B, This assemblage is diagnostic of the mainly lower Ediacaran Doushantuo-Pertatataka acritarchs (c. 633—565 Ma.). The lithological diversity of the Tentudía Formation suggests that these Ediacaran sedimentary rocks were deposited in a diversity of environments (platform, slope and submarine fans, including mass transport deposits in a back-arc basin that extended into the southern part of the Central Iberian Zone. The mass transport deposits are represented in the Ossa-Morena Zone by the Salvaleón Olistostrome composed by turbidites including chert clast probably derived from the shelf. The migration of the mass transport deposits into the Central Iberian Zone (Orellana Formation) indicates the migration of the back-arc basin depocenter. Later inversion of the back-arc basin is marked by the deformation of the lower Ediacaran sedimentary rocks Serie Negra Group in the Ossa-Morena Zone and Domo Extremeño Group in the Central Iberian Zone. In the upper Ediacaran-Terreneuvian a carbonate platform (Ibor Group) was established in the southern part of the Central Iberian Zone unconformably overlying the previously deformed Domo Extremeño Group. The most proximal areas in the Ossa-Morena Zone are represented by the Malcocinado Formation which overlies the Serie Negra Group and the distal areas of the southern part of the Central Iberian Zone are represented by distal shelf and slope deposits of the Cijara Formation, which are overlain by mass transport deposits that include carbonate olistolites from the Ibor Group (such as the Fuentes and Membrillar olistostromes).

Foraminifera and sediments cored from the bottom of the Faanu Mudugau Blue Hole, Ari Atoll, Maldives: Diversity, taphonomy and environmental reconstruction of an inhabitable substrate

The Maldives Archipelago is located above the largest carbonate platforms on Earth in the Indian Ocean. The Faanu Mudugau Blue Hole, located in the Ari Atoll (Maldives), is the only one so far discovered in the Indian Ocean and has several peculiarities.We provide for the first time a comprehensive study of the benthic foraminiferal assemblages recovered from a push core collected from the deepest part of the blue hole; a detailed taxonomy allowed the reconstruction of both abundance and diversity of foraminifera through the recovered core. Despite the acidic bottom conditions and an increase in hydrogen sulfide concentration, the benthic foraminifera of the Faanu Mudugau Blue Hole are well preserved and mostly unaltered. The systematic study gives evidence that benthic foraminifera, retrieved at the bottom of the blue hole at ∼80 m depth, are instead typical of the lagoon in a coral reef environment. The most abundant taxon is the genus Amphistegina, a symbiont-bearing larger benthic foraminifer that commonly lives on macroalgae on well-irradiated seafloors. The predominance of hyaline taxa based on the triplot and the evaluation of the Foram Index, which indicate a good oxygenation condition in the bottom waters, suggest that benthic foraminifera of the Faanu Mudugau Blue Hole have all been transported.Sediment granulometry, the taphonomic characteristics of foraminiferal shells, and the physical-chemical parameters of the water column confirm the hypothesis that all retrieved benthic foraminifera are transported within the blue hole from the nearby coral reef lagoon, and neither any of the individuals nor any of the taxa identified are living on the blue hole seafloor, making this environment inhospitable for protists. Transport is primarily triggered by sea surface currents, mostly related to seasonal monsoons and tides, which are typical of the Maldivian Archipelago.

Mg-C-O isotopes and elements reveal the origin of dolostone in the Middle Jurassic Buqu Formation

The origin of dolostone in the Middle Jurassic Buqu Formation of the Plateau Basin has been a subject of prolonged debate. This study combines detailed petrological observations with analyses of Mg-C-O isotopes and elements to constrain the origin of dolostones in the Buqu Formation. Petrography and cathodoluminescence (CL) examination identified three types of matrix dolostones: very finely to finely crystalline dolostone (D1), finely to medium crystalline dolostone (D2), and medium to coarsely crystalline dolostone (D3). The analysis of the diagenesis sequence reveals that D1 originated from the dolomitization of grainstone in the early diagenetic phase, whereas D2 and D3 resulted from the recrystallization of D1 during the later burial phase. The presence of high Na (>100 ppm), low Fe (<1000 ppm), low Mn (<250 ppm), positive Ce anomaly, LREE enrichment, stable δ26Mg (−2.28‰ to −2.04‰), and δ13C (1.02‰ to 2.95‰) indicates that the early dolomitization fluid was oxidized seawater. As the crystal size increases (D1→D2→D3), the progressively rising Mn content and significantly negative δ18O (−10.72‰ to −7.81‰) suggest that the dolostone has experienced modification and alteration by buried pore water in the later stages. The fluctuations in relative sea level during the sedimentary deposition of the Buqu Formation were reconstructed through the utilization of Na, Sr/Cu, Sr/Ba, Rb/Sr, ∑REE, and δ13C. It was observed that the δ26Mg of dolostone closely mirrored the variations in sea level. The consistent trend of change confirms that sea level fluctuations control the formation and distribution of early dolostone. Frequent sea level rise and fall prompted the limestone deposited on the carbonate platform to be continuously transformed into dolostone, which accumulates over a long period to form large-scale thick dolostone. After the formation entered the burial stage, under the combined action of high Mg/Ca ratio pore water, high temperature, and high pressure, the early dolostone experienced the adjustment of burial dolomitization. This research offers a typical case study on the application of Mg-C-O isotope and elements to determine the origin of dolostone. This will aid in a more comprehensive understanding of the formation process of dolostone in ancient rock records.

The palaeobiogeographic significance of the Nötsch area (Austria) during the Middle and Late Mississippian based on rugose corals

The Carboniferous of Nötsch (South Austria) is composed of three formations: the Erlachgraben Formation, the Badstub Breccia, and the Nötsch Formation, that yielded abundant corals, several of them being new for that region. The assemblage is composed of 11 rugose coral species (Siphonophyllia sp., Pseudozaphrentoides juddi, Lublinophyllum? sp., Dibunophyllum bipartitum, Arachnolasma cylindrica, Palaeosmilia murchisoni, Aulokoninckophyllum carinatum, Siphonodendron martini, Diphyphyllum furcatum, Solenodendron furcatum, and Solenodendron horsfieldi), two tabulate species (Multithecopora sp. and Palaeacis sp.) and one heterocoral species (Hexaphyllia mirabilis). In addition, five rugosans that are not in our collection have been identified by previous authors (Clisiophyllum sp., Pseudozaphrentoides sp., Caninia sp., “Palaeosmilia isae”, and Lophophyllidium sp.). The rugose and tabulate species are described and figured. A palaeobiogeographic analysis comparing the Mississippian assemblages from Nötsch and other Austrian outcrops with other domains in Central Europe has been performed using hierarchical clustering with Simpson and Dice similarity indices. The statistical comparison of the rugose coral assemblages at the genus level allows a better perception of the distribution of the shallow water carbonate platforms in that part of the Western Palaeotethys during the Visean and Serpukhovian. The results are incorporated in a schematic palaeogeographical map of the studied area for the late Visean.

Geological Sites of the Anergui Region: Description and Place in the Alpine Geological History of the M'goun UNESCO Global Geopark (Morocco)

The M'goun UNESCO Global Geopark is located in the central part of the High Atlas Mountains in Morocco. Numerous studies have highlighted the richness of the geological heritage of this area. Despite these increased efforts in characterization, some areas, such as Anergui, remain underexplored. This mediation work aims to describe the most significant geological landscapes of this region and place them within the broader geological history framework of the UNESCO M'Goun Geopark, thereby enriching the list of geosites within this geopark and promoting geological awareness in the area. This study focused on describing 16 geosites selected based on scientific, educational and geotourism values. These geosites stand out for their geodiversity, showcasing both deep-seated and surface geological phenomena. They provide an excellent framework for understanding the alpine geological history of the M'Goun Geopark. Each site bears witness to one or several key stages of this history, highlighting the extension of the High Atlas basin, the development of carbonate platforms, Jurassic diapirism and magmatism, Late Cretaceous and Tertiary compression, Tertiary and Quaternary uplift, and erosion, resulting in a variety of geological features relating to deformation and alteration.

Facies Analysis and Depositional Environment of the Euphrates Formation in Selected Sections, Southwestern Iraq

The current study aims to investigate the microfacies, diagenetic processes, and depositional environments of the twenty-three rock samples from the Lower Miocene Euphrates Formation of two outcrops collected at Bahar-Najaf in Najaf governorate and Al-Shinafiya in Diwaniyah governorate, southern Iraq. Thin sections of these samples were prepared; hence, the nature of microfacies and related depositional environments was established. The formation contains abundant benthonic foraminifers and other fossils like miliolids, echinoderm, coral, gastropods, and pelecypods. Seven distinct microfacies were identified: lime mudstone, miliolid packstone, Pelletal Grainstone, Lime Wackestone, Peloidal Grainstone, Oolitic grainstone Microfacies, and fossiliferous packstone packed with bioclasts. Diagenetic processes evidenced in this study are dissolution, neomorphism, cementation, micritization, and dolomitization. The Euphrates Formation was deposited in a shallow marine setting with fluctuating energy levels, ranging from low-energy lagoons to moderate and high-energy shoals and fore-reef environments. Various microfacies indicate a complex depositional environment on a carbonate platform influenced by biota and sedimentary processes. These observations support the inference that the Euphrates Formation accumulated in shallow open marine, inner barrier, and shoal settings, providing a detailed interpretation of its para-depositional and diagenetic development.

Early Aptian development and OAE 1a-linked demise of a carbonate platform in the western Tethys: Lower Cretaceous of Sierra Mariola (South Iberian Paleomargin, SE Spain)

A carbonate platform succession of early Aptian age is described from the Sierra Mariola, which is located within the Prebetic Zone of the Betic External Zones (BEZ) of the Southern Iberian Palaeomargin, in SE Spain. The facies and stratigraphic architecture of the studied succession are described and analysed from multiple logged sections to characterize different depositional environments.The cyclic stacking of facies observed suggests a possible orbital forcing of climate transmitted to sedimentation. Superbundles are recognized describing two sequences of Regressive-Transgressive (R-T) evolution. Analysis of carbon-isotope data identifies both the interval before the hyperthermal event of the early Aptian Oceanic Anoxic Event (OAE 1a) and the onset of the latter event, with an abrupt change in sedimentation marking the demise of the carbonate platform. The field outcrop thus preserves a record of the transition from a greenhouse world to a hothouse event.

Field geology under the sea with a remotely operated vehicle: Mona Rift, Puerto Rico

We implemented concepts of field geology at great ocean depths by constructing virtual outcrops from a string of overlapping video frames collected by remotely operated vehicles (ROVs). This lower-cost alternative to drilling boreholes allows stratigraphic extension into the offshore and regional interpretation of marine seismic profiles. The imagery was collected along a dive transect on the western wall of Mona Rift, a deep and narrow rift northwest of Puerto Rico, between water depths of 1560 m and 3927 m. The northern coast of Puerto Rico and its large offshore area are underlain by a mid-Eocene and younger forearc basin topped by a thick carbonate platform. There are no drill holes offshore, and tying seismic lines across the shoreline there is problematic. We describe our virtual outcrop and constrain its age and stratigraphy using seven rock samples collected by ROV and compare the outcrop’s stratigraphy to deep boreholes and outcrops on land. Our formation descriptions and ages agree, for the most part, with those on land, but we identified a 100-m-thick section that is represented on land by an unconformity. Our stratigraphic interpretation indicates lateral variations in formation thicknesses and establishes a cross-section for additional sampling of the Eocene–Pliocene geology. It also suggests that Mona Rift has formed since the mid-Pliocene. The presence or absence of ferromanganese (Fe-Mn) crust on rocks along the transect may be correlated with the smoothness of the rock surface.

Sequence stratigraphic analysis of the late Permian Changhsingian platform marginal reef, Western Hubei, South China

This study presents a comprehensive analysis of the late Permian platform marginal reefs in the Sichuan Basin, focusing on reefal lithofacies and sequence stratigraphic patterns. Field outcrop observations and rock sample analyses from the Jiantianba reef were conducted to establish an evolution model of sponge reef development and spatial distribution. Four stages of marginal carbonate platform were documented: open platform, gently sloping reef, steeply sloping reef, and reef bank system. Distinct lithofacies were identified in these stages, reflecting different depositional environments and growth rates. The gently sloping reef was composed of filled skeleton framestone, filled skeleton bafflestone, and micrite organism limestone, indicating limited reef-building capacity. In contrast, the lithofacies of steeply sloping reefs were composed of open skeleton framestone, open skeleton bafflestone, binding skeleton bafflestone, and benthic organism bindstone, indicating stronger reef-building ability. Based on depositional features and carbon isotopic trends, the reef strata were divided into two sequences. Sequence 1 corresponds to the formation of unit 1, and sequence 2 can be further divided into units 2 and 3. In unit 1, reefs developed in a relatively deeper-water setting. It was characterized by rich micrite limestone, forming a gentle margin. Unit 2 witnessed reef development in shallower waters. Early marine cementation and microbial clots were prevalent, contributing to form a steep margin. In the early stage of unit 3, reefs primarily developed in a tidal-controlled environment. Subsequently, reef strata experienced a transition to a wave-influenced environment, leading to the formation of a reef bank system. In general, sequence 1 mainly formed in a heterozoan-dominated factory, and reefs contributed to a relatively gently sloping platform margin. In contrast, sequence 2 formed in a photozoan-dominated factory, and reefs contributed to a relatively steeply sloping platform margin.

U–Pb geochronology of carbonate-hosted Pb–Zn ores reveals plate-tectonic evolution of eastern Asia during the early Paleozoic

The Mississippi Valley–type (MVT) deposits reached their maximum abundance during the final assembly of Pangea. The intense orogenic activity during this assembly in relatively low latitudes created abundant opportunities for the migration of sedimentary brines into the interior carbonate platforms landward of the orogenic belts, leading to the formation of MVT deposits. Thus, dating the MVT deposits can potentially aid in the reconstruction of the plate tectonic evolution during the assembly of Pangea. The Yangtze Craton hosts significant carbonate-hosted Zn–Pb deposits (> 60 Mt Pb + Zn metals), accounting for 30% of China’s Zn–Pb resources. However, determining the timing of zinc and lead mineralization in these reservoirs is challenging. This study employs LA-ICP-MS U–Pb geochronology on calcites to date Zn–Pb deposits hosted in Lower Cambrian limestone in the Huayuan orefield. Three generations of calcite formation were dated: the first recorded the pre-ore deposition of Lower Cambrian limestone at 517 ± 10 Ma, the second marked a hydrothermal event linked to stratiform sphalerite ore formation at 501.4 ± 8.4 Ma, and the third was associated with discordant breccia-hosted Zn–Pb mineralization at 397.5 ± 9.6 Ma. Our results indicate that Paleozoic carbonate-hosted Pb–Zn mineralization in the Yangtze Craton is linked to (1) the final assembly of Gondwana in the late Cambrian-early Ordovician (520–480 Ma); and (2) the intracontinental orogeny response to Jiangnan Uplift (420–400 Ma). This study highlights the temporal relationship between low temperature carbonate-hosted mineralization and orogenic events that are consistent with classic MVT models worldwide. It also contributes geochronological data for the reconstruction of plate-tectonic evolution during Pangea assembly. Furthermore, it demonstrates the potential of in situ U–Pb calcite geochronology to date ore deposits lacking syn-ore minerals suitable for traditional dating methods.

Widespread coastal upwelling along the marginal Yangtze Platform (South China) during the early Cambrian: Implications for hyper-enrichment of organic matter

Early Cambrian Earth history witnessed significant changes in marine environments and biological evolution contemporaneous with extensive accumulation of organic-rich shale. However, major factors controlling hyper-enrichment of organic matter (OM) of lower Cambrian shale deposits remain controversial. Black shale of the lower Cambrian Niutitang Formation (NTT) of the marginal Yangtze Platform (South China) deposited during the Cambrian middle Age 2 are especially organic-rich, with total organic carbon (TOC) concentrations of as much as 13.0 wt%. Geochemical evidence suggests that cool, dry paleo-climatic conditions that prevailed on the Yangtze Platform during the Cambrian Fortunian-late Age 2, induced vigorous coastal upwelling. Paleo-productivity level assessments indicate that the magnitude of primary productivity contemporaneous with deposition of the NTT shale deposits exceeded that of modern upwelling systems (e.g., Peruvian Margin). The widespread occurrence of phosphate nodules within these deposits and decreased Co-EF × Mn-EF values of associated lower Cambrian black shale successions deposited along the margin of the Yangtze carbonate platform suggest extensive coastal upwelling. Widespread and strong coastal upwelling in tandem with elevated surface water primary productivity and anoxic (even euxinic) bottom water conditions are manifested by deposition of OM hyper-enriched black shale during Cambrian Fortunian to middle Age 2 time. However, weakened seasonal upwelling that appears to have prevailed during the late Cambrian Age 2 was accompanied by accumulation of NTT shale deposits of diminished TOC content. In summary, this study provides robust evidence of extensive coastal upwelling along the marginal Yangtze Platform during early Cambrian time that favored accumulation of OM hyper-enriched shale. These results help to elucidate the distribution of high-quality lower Cambrian natural gas source rocks in South China.