Large hatchling size in Cretaceous nautiloids persists across the end-Cretaceous mass extinction: New data of Hercoglossidae hatchlings

The hatchling sizes of the Cretaceous nautiloids (Hercoglossa forbesianus and Cimomia angustus; Hercoglossidae) are reported here for the first time as ranging between 20.4-22.2 mm in diameter. These new data for Hercoglossidae and the previously reported data for Nautilidae and Cymatoceratidae suggest that all Cretaceous nautiloids had large hatchlings irrespective of their taxonomy relative to those in ammonoids. Based on the relationship between hatching events and the nepionic constriction in modern nautili, the hatchling size of nautiloids can be recognized by the constriction on the shell surface of the innermost whorl. The range of hatchling diameter in all Cretaceous nautiloids (9-35 mm) is comparable to those of modern nautili (20-32 mm), although the Cretaceous nautiloids are characterized by a wider range with the smaller minimum diameter. The large hatchling size in Cretaceous nautiloids stands in marked contrast to those in ammonoids, which had much smaller hatchling size (mostly <2 mm in diameter) and became extinct at the end of the Cretaceous. Such distinct differences in reproductive strategy (i.e., hatchling and egg sizes) suggest that these differences may have played a significant role in the fate of ammonoids and nautiloids through the K/T mass extinction, as previously hypothesized by several paleontologists.     

A correlation between the Large Igneous Provinces and mass extinctions: constraint on the end-Guadalupian mass extinction and the Emeishan LIP in South China,eastern Tethys

ABSTRACT

Well-documented outcrops around the Emeishan Large Igneous Province (ELIP) in South China, eastern Tethys, encompassing the end-Guadalupian mass extinction have been investigated. Correlatable sections recording the event exhibit very similar lithological characters, positive-then-negative C isotope excursions and massive biotic demise. Detailed analyses of the fossil record and carbon isotopic variations were carried out on the Guadalupian–Wuchiapingian Boundary sections over the inner, middle, outer zones of the ELIP and its margin. Due to a pronounced decrease in marine habitat area and the environmental and ecological change over this part of the Tethys, the biota crisis records show the loss of numerous tropical invertebrate taxa, and exhibit fewer genera and smaller testing sizes and low productivity. The biota crisis was a sustainable and gradual reduction in diversity over the Capitanian. The associated carbon isotopic data reveal unusually high δ¹³C(carb) values before the late Capitanian, representing higher primary productivity (or buried rate) and more ¹³C-enriched CO₂ released by hydrothermal carbonate breakdown from the upper crust into the sediments at that time. Subsequently, an accelerated negative excursion across the boundary and the gradual excursion with low carbon isotope amplitude favours an increased influx of light ¹²C sourced by the volcanism around the eastern Tethys. The very similar time–space relation between the biota crisis and the Emeishan volcanism confirms that volcanic eruptions may have triggered the biota crisis event in South China. Intensive volcanism could result in detrimental environmental and ecological stresses, habitat loss, organic material splitting, or the emission of light carbon and thermal fluid (or aerosol), implying that the losses of the shallow-marine invertebrates either occurred geologically instantaneously or in a series of closely spaced crises coinciding with the initial phase of ELIP formation. These findings in South China may reveal the causal relation between mass extinctions and LIPs in a global context.     

The fungal and acritarch events as time markers for the latest Permian mass extinction:An update

The latest Permian extinction(252 Myr ago)was the most severe in the geologic record.On land,widespread Late Permian gymnosperm/seed-fern dominated forests appear to have suffered rapid and almost complete destruction,as evidenced by increased soil erosion and changes in fluvial style in deforested areas,signs of wildfires,replacement of trees by lower plants,and almost complete loss of peat-forming and fire-susceptible vegetation.Permian-Triassic boundary strata at many sites show two widespread palynological events in the wake of the forest destruction:The fungal event,evidenced by a thin zone with95%fungal cells(Reduviasporonites)and woody debris,found in terrestrial and marine sediments,and the acritarch event,marked by the sudden flood of unusual phytoplankton in the marine realm.These two events represent the global temporary explosive spread of stress-tolerant and opportunistic organisms on land and in the sea just after the latest Permian disaster.They represent unique events,and thus they can provide a time marker in correlating latest Permian marine and terrestrial sequences.     

西南特提斯川滇黔成矿区富锗铅锌矿床成矿理论研究新进展

川滇黔铅锌多金属成矿区位于特提斯成矿域与环太平洋成矿域交界地带的扬子陆块西南缘，并广泛分布了全球罕见的碳酸盐岩容矿的后成热液型富锗铅锌多金属矿床，其铅锌品位特高、储量大、富含稀散元素锗和贵金属银，经济价值巨大，备受广泛关注。针对制约该区深部找矿突破的成矿构造动力学背景及其响应机制、矿源—输运—聚集成矿过程、铅锌锗元素超常富集—巨量聚集成矿机制、矿床类型归属与铅锌多金属成矿系统等重大理论问题，历经20余年，通过区域地球物理剖面探测、矿田构造解析、典型矿床精细解剖及矿床地球化学、实验地球化学等系统研究，在成矿理论方面取得重要研究进展：① 提出了富锗铅锌矿形成于印支期碰撞造山过程的陆内走滑构造系统，解析并厘定了断褶构造组合样式及成矿构造体系，建立了构造分级成矿- 控矿、矿体空间定位模式；② 系统概括了矿化空间组合结构及矿床成矿规律，查明了“矿源—输运—聚集”成矿过程，揭示了铅锌共生分异机制及铅锌锗超常富集成矿机制；③ 提出了具有一定普适性的会泽型铅锌矿床新类型，建立了矿床成矿系统及流体“贯入”—交代成矿模型。在此基础上，提出了陆内走滑构造系统控矿—流体“贯入”- 交代成矿论（构造控矿—流体“贯入”成矿论）。应用该理论研发出适用于该类矿床深部勘查技术方法系列，取得了近些年来川滇黔地区一系列找矿突破和新进展。该研究进展不仅为川滇黔成矿区矿床勘查技术研发及深部找矿突破提供了理论支撑，而且将在扬子陆块西南缘乃至特提斯成矿域的类似矿床研究和找矿勘查部署中发挥重要的指导作用。     

Cyclostratigraphic calibration of cretaceous magnetic polarity events (Cismon, Southern Alps, Italy)

In an introductory section the problems of constructing a geologic time scale and the role of magnetic reversals and Milankovitch cycles in geochronology are outlined. Results of a detailed cyclostratigraphic and magnetostratigraphic study of the Valanginian/Hauterivian part of the pelagic limestone section at Cismon in the Southern Alps are presented and used in conjunction to estimate the duration of magnetic subchrons between CM10N and CM8. The new estimates are shorter than in most published time scales by a factor of two to three. More research along these lines may make a revision of the Early Cretaceous time scale necessary.     

Illawarra Reversal: The fingerprint of a superplume that triggered Pangean breakup and the end-Guadalupian (Permian) mass extinction

The Permian magnetostratigraphic record demonstrates that a remarkable change in geomagnetism occurred in the Late Guadalupian (Middle Permian; ca. 265 Ma) from the long-term stable Kiaman Reverse Superchron (throughout the Late Carboniferous and Early-Middle Permian) to the Permian–Triassic Mixed Superchron with frequent polarity changes (in the Late Permian and Triassic). This unique episode called the Illawarra Reversal probably reflects a significant change in the geodynamo in the outer core of the planet after a 50 million years of stable geomagnetism. The Illawarra Reversal was likely led by the appearance of a thermal instability at the 2900 km-deep core–mantle boundary in connection with mantle superplume activity. The Illawarra Reversal and the Guadalupian–Lopingian boundary event record the significant transition processes from the Paleozoic to Mesozoic–Modern world. One of the major global environmental changes in the Phanerozoic occurred almost simultaneously in the latest Guadalupian, as recorded in 1) mass extinction, 2) ocean redox change, 3) sharp isotopic excursions (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. In addition to the claimed possible links between the above-listed environmental changes and mantle superplume activity, I propose here an extra explanation that a change in the core's geodynamo may have played an important role in determining the course of the Earth's surface climate and biotic extinction/evolution. When a superplume is launched from the core–mantle boundary, the resultant thermal instability makes the geodynamo's dipole of the outer core unstable, and lowers the geomagnetic intensity. Being modulated by the geo- and heliomagnetism, the galactic cosmic ray flux into the Earth's atmosphere changes with time. The more cosmic rays penetrate through the atmosphere, the more clouds develop to increase the albedo, thus enhancing cooling of the Earth's surface. The Illawarra Reversal, the Kamura cooling event, and other unique geologic phenomena in the Late Guadalupian are all concordantly explained as consequences of the superplume activity that initially triggered the breakup of Pangea. The secular change in cosmic radiation may explain not only the extinction-related global climatic changes in the end-Guadalupian but also the long-term global warming/cooling trend in Earth's history in terms of cloud coverage over the planet.     

Statistical distribution of rainy events characteristics and instantaneous hyetographs generation (Merguellil watershed in central Tunisia)

In semi-arid regions, variability of rainfall inputs is high at different time and space scales. Thus, intense and highly variable hydro-meteorological events, which generate both potential resource and hazards, are of major interest. The Merguellil watershed is characterized by a large majority of rainfall in the form of storms, particularly violent in spring and autumn. Storms of summer are short and violent and can be the origin of important floods. Due to the strong rainfall variability, precipitation and hydrological regime are very irregular. This is what led us to think of a detailed study of rainy events. It was founded that two statistical distributions of parameters characterizing events are dominated (log normal, exponential).Thus, our objective consists in studying, in its globality, the collected pluviographic data in order to make a synthesis and analysis which should make it possible the generation of fictitious events to better control the rainfall risk and to study the response of the watershed faces to several types of rainfall. As result, we obtained hyetographs of the height studied rain gauges. These hyetographs show several types of events that are well in conformity with the observed reality. This generation is an advantageous tool in terms of both its robustness and its performance. Its relevance is based on the analysis of the fine time structure of recorded rainfall events. It can be made over several years, and would permit us to avoid being constrained by sample size. Generating rainfall events provides extensive time-related rainfall data that are consistent at all time steps, and can be seen as solution, even partially, of data lack in hydrological study.     

Detrital mode, mineralogy and geochemistry of the Sidi Aïch Formation (Early Cretaceous) in central and southwestern Tunisia: Implications for provenance, tectonic setting and paleoenvironment

The chemical and mineralogical composition of the Sidi Aïch Formation sandstones in central and southwestern Tunisia has been investigated in order to infer the provenance and tectonic setting, as well as to appraise the influence of weathering. The sixteen studied samples are mainly composed of quartz, feldspar, kaolinite and/or illite. Sidi Aïch sandstones are mainly arkosic, potassic feldspar-rich and immature. Much of the feldspar was transformed to kaolinite. Concerning the relation between sandstone detrital composition and their depositional setting, the Sidi Aïch Formation sandstone in the major studied localities, probably accumulated in relatively proximal small basins within the continental interior. However, for the Khanguet El Ouara study site, sandstones may have been deposited in a foreland basin which received recycled sediments from an adjacent orogenic belt.The source area may have included quartzose sedimentary rocks. The dominance of quartz and enrichment in immobile elements suggest that the depositional basins were associated with a passive margin. The petrography and geochemistry reflect a stable continental margin and sediments were derived from granitic and pegmatitic sources located in the southern parts of the Gafsa basin. High values for the chemical index of alteration (CIA) indicate that recycling processes might have been important. Particularly high CIA values in the Garet Hadid locality indicate more intense chemical alteration, either due to weathering processes or tectonic control.     

中国西南地区二叠纪乐平世陆生生物大灭绝期煤中砷和硒的演化及古环境意义

为了探讨中国西南地区二叠系乐平统(上二叠统)龙潭组中下部煤系不同煤组分中砷、硒的含量和演化及古环境意义,对近10年来采集于中国西南扬子地台的二叠系乐平统龙潭组主采煤层的原煤、亮煤、煤矸石、黄铁矿结核等样品中砷、硒元素含量进行了测定和分析,并与中国华北地台一些煤矿的上石炭统-下二叠统太原组、山西组的原煤、镜煤、亮煤、煤矸石、黄铁矿结核等样品中砷、硒含量进行对比分析。结果表明：西南地区乐平统龙潭组煤的砷、硒含量变化较大,但总体高于华北晚石炭世和早二叠世煤的砷、硒含量;西南二叠系乐平统龙潭组大多数的亮煤中砷、硒含量高于原煤全煤样(刻槽样)中的砷、硒含量,也远远高于同煤层煤矸石的含量,龙潭组部分亮煤的砷含量尤其高,为55～338 mg/kg,还发现砷含量为89 mg/kg的亮煤。但华北上石炭统-下二叠统的镜煤中的砷、硒含量与之相反,低于原煤全煤样的砷、硒含量,其中砷含量非常低,为063～129 mg/kg。说明西南地区上二叠统煤中的砷和硒与煤的有机质密切相关,可能主要来源于成煤古植物。在西南乐平世早、中期第Ⅰ幕陆生生物集群灭绝事件期间,陆生动物的食物--植物中有毒有害元素砷、硒含量明显增加,陆生环境或泥炭沼泽中可溶性砷、硒含量增加。     

Microbial deposits in the aftermath of the end‐Permian mass extinction: A diverging case from the Mineral Mountains (Utah,USA)

The Lower Triassic Mineral Mountains area (Utah, USA) preserves diversified Smithian and Spathian reefs and bioaccumulations that contain fenestral‐microbialites and various benthic and pelagic organisms. Ecological and environmental changes during the Early Triassic are commonly assumed to be associated with numerous perturbations (productivity changes, acidifica‐tion, redox changes, hypercapnia, eustatism and temperature changes) post‐dating the Permian–Triassic mass extinction. New data acquired in the Mineral Mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures. These data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the Smithian–early Spathian. The studied section records a large‐scale depositional sequence during the Dienerian(?)–Spathian interval. During the transgression, depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral–microbial limestones, shallow subtidal marine sponge–microbial reefs to deep subtidal mud‐dominated limestones. Storm‐induced deposits, microbialite–sponge reefs and shallow subtidal deposits indicate the regression. Three microbialite associations occur in ascending order: (i) a red beds microbialite association deposited in low‐energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure; (ii) a Smithian microbialite association formed in moderate to high‐energy, tidal conditions where microbialites include stromatolites and associated carbonate grains (oncoids, ooids and peloids); and (iii) a Spathian microbialite association developed in low‐energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes. Data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase in accommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and the traditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned.     

Discovery of rhyolitic tuffaceous slate in the southwestern margin of Yangtze Craton: Zircon U-Pb ages (2491 Ma) and tectonic-thermal events

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area, central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks, which are closely related to the early tectonic evolution of the Earth. However, its formation era, sedimentary filling sequence, and geotectonic characteristics have always been in dispute. In this study, several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone. This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village, Tongchang Township, Yimen County. LA-ICP-MS zircon dating was conducted, achieving the crystallization age of magma of 2491 ± 15 Ma and the metamorphic ages of about 2.3 Ga, 2.0 Ga, and 1.8 Ga for the first time. Meanwhile, according to in-situ Hf isotope analysis, the zircon ε_Hf(t) values were determined to range from −3.0 to 7.6, with an average of 2.7. Furthermore, the first-stage Hf model age (T_DM1) was determined to be 2513−2916 Ma, indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean. Therefore, it is believed that the strata of the slate were deposited in the Late Neoarchean, instead of the Mesoproterozoic as determined by previous researchers. Accordingly, it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group. Instead, they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information. Furthermore, the magma ages of 2491 ± 15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton, and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic. The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland. Meanwhile, the metamorphic ages of 2.3 Ga, 2.0 Ga, and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga, 1.95 Ga, and 1.85 Ga of the northern margin of the Yangtze Craton, indicating that the strata experienced Paleoproterozoic tectonic-thermal events. The study area is located on the eastern margin of Qinghai-Tibet Plateau, and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous. The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.©2021 China Geology Editorial Office.     

Carbonate turbidite sequences deposited in rift-basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)

Syn-rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault-bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates. The most abundant redeposits are bio- and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high- and low-density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow-water carbonates and skeletal debris from nearby submarine highs. After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge-shaped with a horizontal upper boundary and consists of a thinning- and fining-upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin-bedded turbidites and hemipelagic limestones. The thick-bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick-bedded turbidites, thinning- and fining-upward cycles are common. The overlying thin-bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine-grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin-plain association developed comprising fine-grained and thin-bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.     

广西来宾二叠系瓜德鲁普统-乐平统界线蓬莱滩剖面沉积微相演变及其对生物灭绝事件的响应

在中二叠世(瓜德鲁普世)末期发生了一次全球范围的生物灭绝事件,大约有34%的海洋无脊椎动物消失,同时发生了全球规模的海退事件。该事件在中国反应为东吴运动,致使华南大部分地区瓜德鲁普统—乐平统之交产生不整合。广西来宾地区位于扬子地台南缘,在中—晚二叠世时期处于滇黔桂盆地的东部。由于其独特的古地理位置,来宾地区发育了连续的中—晚二叠世海相沉积,是研究此次生物大灭绝和环境演变的绝佳位置。本文对广西来宾蓬莱滩瓜德鲁普统—乐平统(G—L)界线剖面沉积微相和生物化石进行了综合研究。研究表明,该剖面1～7层共发育5种主要的沉积微相组合,既礁基相组合、礁核相组合、覆礁相组合、礁滩相组合和深水斜坡相组合,生物礁发育于一个海退序列中。生物碎屑的丰度和类型在7a层突然急剧降低,与该剖面碳同位素的负偏和汞异常的出现一致,但实际上在这一层位只有个别牙形石和菊石消失,生物屑丰度的剧变并不能代表灭绝线,而是对海平面剧变的沉积响应。     

Mesozoic and Cenozoic vertical movements in the Atlas system (Algeria, Morocco, Tunisia): An overview

The E-W trending Atlas System of Maghreb consists of weakly shortened, intra-continental fold belts associated with plateau areas (“Mesetas”), extending between the south-westernmost branch of the Mediterranean Alpine Belt (Rif-Tell) and the Sahara Platform. Although the Atlas system has been erected contemporaneously from Morocco to Algeria and Tunisia during the Middle Eocene to Recent, it displays a conspicuous longitudinal asymmetry, with i) Paleozoic outcrops restricted to its western part; ii) highest elevation occurring in the west, both in the Atlas System and its foreland (Anti-Atlas); iii) low elevation corridors (e.g. Hodna) and depressed foreland (Tunisian Chotts and Sahel area) in the east. We analyse the origin of these striking contrasts in relation with i) the Variscan heritage; ii) crustal vertical movements during the Mesozoic; iii) crustal shortening during the Cenozoic and finally, iv) the occurrence of a Miocene-Quaternary hot mantle anomaly in the west. The Maghreb lithosphere was affected by the Variscan orogeny, and thus thickened only in its western part. During the Late Permian-Triassic, a paleo-high formed in the west between the Central Atlantic and Alpine Tethys rift systems, giving birth to the emergent/poorly subsident West Moroccan Arch. During the late Middle Jurassic-Early Cretaceous, Morocco and western Algeria were dominantly emergent whereas rifting lasted on in eastern Algeria and Tunisia. We ascribe the uplift of the western regions to thermal doming, consistent with the Late Jurassic and Barremian gabbroic magmatism observed there. After the widespread transgression of the high stand Cenomanian-Turonian seas, the inversion of the Atlas System began during the Senonian as a consequence of the Africa-Eurasia convergence. Erosion affected three ENE-trending uplifted areas of NW Africa, which we consider as lithospheric anticlines related to the incipient Africa-Europe convergence. In contrast, in eastern Algeria and Tunisia a NW-trending rift system developed contemporaneously (Sirt rifting), normal to the general trend of the Atlas System. The general inversion and orogenesis of the Atlas System occurred during two distinct episodes, Middle-Late Eocene-Oligocene and Late Miocene-Pliocene, respectively, whereas during the intervening period, the Africa-Europe convergence was mainly accommodated in the Rif-Tell system. Inversion tectonics and crustal thickening may account for the moderate uplift of the eastern Atlas System, not for the high elevation of the western mountain ranges (Middle Atlas, High Atlas, Anti-Atlas). In line with previous authors, we ascribe part of the recent uplift of the latter regions to the occurrence of a NE-trending, high-temperature mantle anomaly, here labelled the Moroccan Hot Line (MHL), which is also marked by a strip of late Miocene-Quaternary alkaline magmatism and significant seismicity.     

Late Pleistocene stratigraphy, sedimentology and paleoenvironmental evolution of the Tarija-Padcaya basin (Bolivian Andes)

We illustrate the results of geomorphological, stratigraphical and sedimentological analysis of the Tarija-Padcaya basin, a wide depression in the eastern side of the Bolivian Cordillera. The basin is well known for the rich mammal fauna discovered since the beginning of the 19th century. The sedimentary infilling belongs to the Tolomosa Formation, corresponding to a major synthem subdivided into three main sub-synthems, mostly made of fluvial and alluvial fan sediments locally weathered by paleoalfisols (Ancon Grande sub-synthem), glacial and fluvio-glacial sediments (Puente Phayo sub-synthem) and finally alluvial fan and alluvial plain sediments (San Jacinto sub-synthem). Radiocarbon dating provides a chronology for the last sub-synthem and testifies that the sequence encompasses the Last Interglacial-Glacial cycle and constitutes a good proxy record for Late Pleistocene climatic changes. The occurrence of glacial deposits in the deeper part of the sedimentary filling suggests a major ice advance during MIS 4 and, together with glacial geomorphological evidence, points to further glacial erosion during the Last Glacial Maximum (LGM). The importance of glacial deposition and erosion opens the question of correlation with the events reported in the nearby Altiplano, where glacial deposits have been recognized only along the slopes of the higher volcanoes. In the Altiplano the LGM has been claimed to be characterized by an absence of deposition or deep erosion, due to extreme dryness, but the Tarija record suggests an erosional event of a scale that would imply the occurrence of a large ice cap.     

Cretaceous oceanic anoxic events (OAEs) recorded in the northern margin of Africa as possible oil and gas shale potential in Tunisia: An overview

Abstract

Significant attention has been given, during the last decade, to Palaeozoic unconventional oil and gas shale in northern Africa where the productive Palaeozoic basins are located. New tentative strategies shed light on Mesozoic unconventional plays represented by Cretaceous shale reservoirs. In most petroleum systems the proven Cretaceous oceanic anoxic events (OAEs), represented mainly by the Fahdene and Bahloul Formations, played the main role in hydrocarbon generation with good distribution. Their deposition was restricted to the early Aptian (Bedoulian), the early/late Albian, and the Cenomanian–Turonian transition. Additional black shales have been detailed for the first time in Tunisia which relate to the Valanginian Weissert and late Hauterivian Faraoni events. Biostratigraphic and complete geochemical reviews have been undertaken from published papers and unpublished internal reports to better assess these important source intervals. These black shale levels, pertaining to OAEs, were deposited in almost deep marine environments during short-lived periods of anoxia (ca. 2 Ma). In the course of this review, thickness, distribution, and maturity maps have been established for each level using well data from published sources.     

The Mukun basin: Settings, paleoenvironmental parameters, and factors controlling the early mesoproterozoic terrestrial sedimentation (Lower Riphean section of the Anabar uplift, Siberia)

Quartz arenites of the Lower Riphean Mukun Group were formed 1.58 Ga ago exclusively in terrestrial settings of the alluvial and eolian-fluvial sedimentation. The subordinate fluvial-sabkha sediments were deposited in intracontinental lacustrine basins. Analysis of paleohydrological and granulometric data, distribution of sedimentary fluxes, and influence of the biotic factor on sedimentogenesis served as a basis for the facies-paleogeographic and basinal-stratigraphic reconstructions. These materials provided grounds for defining a peculiar eolian-fluvial type of microbial sedimentation and revealing the scale and mechanisms of the influence of terrestrial microbial ecosystems on the formation of the sedimentation basins and their sedimentary successions. It is shown that total microbial fixation and biostabilization of sediments resulted in the suppression of fluvial transport, the removal of a significant share of sedimentary material from them, and the formation of peculiar eolian-fluvial landscape, i.e., a system of very shallow ponds connected by intermittent water streams and separated by changeable eolian fields. Development of such landscapes was responsible for the accumulation of thick terrestrial quartz arenite sequences. The microbial type of eolian-fluvial sedimentation characteristic of the Early Proterozoic and initial Riphean basins determined proportions of sandy sediments on continents and shelves.     

Relationship between the Ordovician and Carboniferous–Permian collisional events in the southeastern Tunka bald mountains, East Sayan (southwestern framing of the Siberian Platform)

Granites from the Tunka pluton of the Sarkhoi complex, located in the eastern Tunka bald mountains (East Sayan), have been dated at the Middle Ordovician (462.6 ± 7.8 Ma) by LA ICP MS. The granites of the Sarkhoi complex within the studied area cut a foldthrust structure consisting of deformed fragments of the Vendian (Ediacaran)–Early Cambrian cover of the Tuva–Mongolian microcontinent (Upper Shumak metaterrigenous formation, Gorlyk carbonate formation). The red-colored conglomerates and sandstones of the Late Devonian–Early Carboniferous(?) Sagan-Sair Formation overlie the eroded surface of the Tunka pluton granites in the eastern Tunka bald mountains. The Sagan-Sair Formation, in turn, is overlain along a low-angle thrust by a group of tectonic sheets, which comprises the volcanic and carbonate sediments of the Tolta Formation, biotitic schists, and plagiogneisses with garnet amphibolite bodies. Two nappe generations have been revealed on the basis of the described geologic relationships, the Middle Ordovician age of the Tunka pluton granites, and numerous Late Paleozoic Ar–Ar dates of syntectonic minerals from the metamorphic rocks in the area. The first thrusting stage was pre-Middle Ordovician, and the second, Late Carboniferous–Permian. The Lower Paleozoic thrust structure resulted from the accretion of the Tuva–Mongolian microcontinent to the Siberian Platform. The Late Paleozoic nappes resulted from intracontinental orogeny and the reactivation of an Early Paleozoic accretionary belt under the effect of the Late Paleozoic collisional events.