Late Cambrian Research Articles

Cambrian correlation within the Perigondwana Faunal Realm

Stratigraphic positions of Redlichioidea trilobites found in the Lower Cambrian of Morocco, Spain, South France, Sardinia, Jordan, Doberlug and Gorlitz (both in Germany), and those from Central Asia, Himalayan region, Australia and Antarctica are correlated with that of the Chinese biostratigraphic scheme. During the Middle Cambrian, biostratigraphic correlation between the Mediterranean region (including Morocco and Central Europe) and China is very difficult because of the complete absence of Paradoxides-fauna in China. Based on shallow-shelf and open-shelf trilobite faunas, the Middle and Upper Cambrian biostratigraphy of the Near East, Central, South and Southeast Asia, Australia and Antarctica are correlated with that of both China's shallow-shelf and open-shelf biostratigraphic schemes. Late Cambrian trilobite faunas of eastern Asiatic affinities also occur in Morocco, Spain, South France and Turkey. So far, there is no trilobite fauna of the Late Cambrian Fengshanian Stage found in Europe.

Redetermination of the Zalantun Group in the ARong Qi Area of Da Hinggan Mountains (Northeastern China): Evidence from Petrology, Geochronology and Geochemistry

The Da Hinggan Mountains are an important area in the tectonic evolution of the Central Asian Orogenic Belt (CAOB), and there are disputes over the closure time of the Paleo-Asian Ocean and the amalgamation spatiotemporal relationship between the Ergun-Hinggan Massif and the Songnun Massif. The geochronology and geochemistry of the Late Cambrian-Late Silurian volcanic rock assemblages in the ARong Qi area at the eastern margin of the Da Hinggan Mountains are studied in this paper. The results suggest that the U-Pb zircon ages of the Late Cambrian, Late Ordovician and Late Silurian volcanic rock assemblages are 507.5 ± 1.0 Ma, 456.2 ± 1.0 Ma, 446.1 ± 0.95 Ma and 423.3 ± 1.4 Ma, respectively. The Late Cambrian-Late Silurian volcanic rocks are quasi-aluminous-peraluminous, belonging to calc-alkaline-shoshonite series, which is rich in HREE but has insignificant europium anomalies. There are abundant large ion lithophile elements (LILE) in the rock, and remarkable Nb, Ta and Ti negative anomalies. The previous data and the current study indicate that a continental margin arc tectonic setting existed in the ARong Qi-Zalantun region during the Early Paleozoic, which is inferred to be the product of the subduction-accretion-amalgamation of the plates along the eastern margin of the Ergun Massif during the Early Paleozoic.

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Geochronology and geochemistry of the dioritic rocks from the Inexpressible Island, Northern Victoria Land, Antarctica and their geological implications

东南极北维多利亚地沿威尔逊、鲍尔斯地体边缘发育广泛的花岗质岩浆作用。本文选取了罗斯造山带北维多利亚地难言岛古生代闪长质岩石开展了锆石U-Pb年代学、Sr-Nd-Hf同位素地质学及全岩地球化学研究，分析探讨其岩浆的源区特征和岩石成因。锆石U-Pb同位素定年结果显示，难言岛闪长质岩石形成于约500Ma，属于晚寒武世罗斯造山后岩浆活动产物。岩石地球化学特征显示的SiO₂含量变化范围为54.03%~60.10%，总碱值为5.92%~6.91%，里特曼指数σ为2.67~3.68，A/CNK值为0.77~0.85，属于高钾钙碱性、准铝质岩石。在正常洋中脊玄武岩标准化蛛网图中，样品富集大离子亲石元素（LILE），亏损Nb、Ta、Ti；在球粒陨石标准化稀土元素配分图中，难言岛闪长质岩石显示轻稀土相对富集，（La/Yb）_N=12.99~14.60，Eu弱负异常（0.62~0.71）。样品的锆石ε_Hf（t）值为-10.7~-6.3；（⁸⁷Sr/⁸⁶Sr）_i范围0.7095~0.7097，（¹⁴³Nd/¹⁴⁴Nd）_i范围0.511502~0.511629，ε_Nd（t）范围-7.1~-10.3。结合前人资料，显示难言岛闪长质岩石来自于先前富集的陆下岩石圈地幔部分熔融，在罗斯汇聚的过程中被俯冲组分进一步交代。晚寒武世罗斯造山带处于后碰撞环境，熔融可能与后碰撞伸展环境下俯冲板块回卷、撕裂过程中软流圈上涌或其他减薄机制有关。;Extensive Early Paleozoic granitic magmatism developed along the margins of the Wilson and Bowers terranes in Northern Victoria Land, Antarctica. Based on zircon U-Pb geochronology, Sr-Nd-Hf isotope geology and geochemistry of the Paleozoic Inexpressible Island dioritic rocks in the Northern Victoria Land, Ross Orogenic Belt, this paper analyzed the initial time of the post-collisional extensional environment and the characteristics of magma source region. Zircon U-Pb isotope dating reveals that the Inexpressible Island diorites were formed at about 500Ma, representing Late Cambrian Ross magmatic activity. The samples show characteristics of a high potassic and magmatic arc affinity. The contents of SiO₂ and total alkali (K₂O+Na₂O) range within 54.03%~60.10% and 5.92%~6.91%, respectively. The σ and A/CNK values vary from 2.67 to 3.68 and 0.77 to 0.85, respectively, indicating that the rocks belong to metaluminous rocks. On the normal mid-ocean ridge basalt-normalized diagram, the samples are enriched in large ion lithophilic elements (LILE) and depleted in Nb, Ta and Ti. In the chondrite-normalized REE diagram, the Inexpressible Island diorite shows a relatively rich LREE with (La/Yb)_N=12.99~14.60 and a slight negative Eu anomaly (0.62~0.71). Furthermore, zircon ε_Hf(t) and ε_Nd(t) values of the measured samples range from -10.7 to -6.3 and -7.1 to -10.3, respectively, with correspondent (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i ratios of 0.7095 to 0.7097 and 0.511502~0.511629. Combined with previous data, the Inexpressible Island diorites can be ascribed to the melting of previously enriched subcontinental lithospheric mantle, which further metasomatized by a subduction component before. The mechanism that triggers partial melting could be slab rollback and convective thinning and/or delamination of overthickened lithosphere during Ross orogenic belt post-collision.

The unconformity caused by the Huaiyuan movement and the deep natural gas exploration field in the Ordos Basin, China

In order to clarify the hydrocarbon accumulation significance and exploration prospect of the unconformity caused by the Huaiyuan movement in the Ordos Basin, this paper studies the spatial distribution and structural plane characteristics of this unconformity and its relationships with hydrocarbon accumulation by observing field outcrops and cores and analyzing logging data, based on the previous research results and the interpretation results of 2D and new 3D seismic data. And the following research results are obtained. First, the unconformity was mainly formed in the Floian Age of Early Ordovician and widely occurs at the bottom of the Jiawang Formation and the top of the Sanshanzi Formation and the related tectonism lasts 30 Myr. Second, basal conglomerate and sandstone less than 1 m in thickness are developed above the unconformity at the edge of the basin, while thin mudstone, argillaceous dolomite (limestone) and marl are developed above the unconformity in the central part of the basin. Third, the unconformity structurally consists of three layers, including a basal conglomerate layer, a paleosoil layer and a fully–semi weathered carbonate layer from top to bottom, among which, the last one is 20–90 m in thickness with developed dissolution fractures and pores to form a quality reservoir. Fourth, the unconformity results in the development of a series of large valleys landforms, which incise the Lower Ordovician–Upper Cambrian. Fifth, the unconformity can act as a good channel for hydrocarbon migration, and it connects with the unconformity caused by the Caledonian movement within the paleo-uplift of the Ordos Basin, which is favorable for the the hydrocarbon of different sources in the west side of the basin eastwards to migrate, accumulate and form a gas reservoir. In conclusion, the deep Lower Paleozoic related to the Huaiyuan unconformity is expected to be an important natural gas exploration field in the Ordos Basin.

Trace element variations across Middle–Upper Cambrian carbonates: Implications for the paleoenvironment of eastern Laurentia

The investigated Middle–Upper Cambrian carbonates span, from bottom to top, the uppermost Hawke Bay (40 m), the March Point (∼83 m thick), and the lowermost Petit Jardin (∼47 m) formations of the Port au Port Group (western Newfoundland, Canada). A multi-technique evaluation of the preservation of carbonates was applied by using several petrographic and chemical screening tools. The variations in several proxies of bioproductivity (e.g., P, Ni, and Cu), input of weathering products (Mn, Fe, Al, and ∑REE), and paleoredox (e.g., Th/U) have been utilized to study the paleoenvironmental conditions during the investigated time interval. Distinct positive shifts in the profiles of Al, ∑REE, Mn, and Fe have been associated with the negative shifts recorded by the δ13C profile and correlated with the DICE (Middle Cambrian) and base of the SPICE (lowermost Upper Cambrian) events. They reflect contributions from detrital weathered material during sealevel falls. Similar correlated positive shifts are also documented by the P, Ni, and Cu profiles, thus suggesting an increase in the riverine inputs of nutrients associated with the drop of sealevel and increase of weathering activities. In addition, the changes in the bioproductivity and weathering proxies were associated with relative rising in the Th/U ratios (0.1–8.3), which reflects variations in the redox state towards relatively more oxidizing conditions.

Evidence for fault activity during accumulation of the Furongian Chaomidian Formation (Shandong Province, China)

The Late Cambrian (Furongian) Chaomidian Formation accumulated in an epeiric sea that covered a large part of the North China Plate and extended from China to Korea. The depositional environment of the formation is commonly considered to have been affected by storms that broke up numerous limestone layers, but that was tectonically quiet. It is here argued, however, that some features of the formation – more in particular the many dozens of breccia layers and the occurrence of slid-down limestone blocks that are embedded in autochthonous oolites – can be explained satisfactorily only if some significant fault activity took place during its accumulation. The faulting may have been due to differential loading and subsidence, but an endogenic origin seems more probable. This implies that the structural history of the study area in Shandong Province may need re-consideration.

Characteristics of natural gas accumulation in the Cambrian weathered crust in southwestern Ordos Basin

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.

P-047. Elevated placental histone H3K4 methylation via upregulated histone methyltransferases in preeclampsia and its possible involvement in hypoxia-induced pathophysiology

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.

Facies and depositional environment of the Upper Cambrian of the central Alborz, members 3 and 4 of the Mila Formation (Deh-Molla Formation), with emphasis on their ribbon rocks

کامبرین پسین همراه با تغییرات مهمی در اقیانوس‌ها و خشکی‌ها از جمله نهشت رخساره‌های خاصی در بخش‌های مختلف دنیا است که به دلیل ماهیت مطبق و نوارمانند به نام سنگ‌های نواری شناخته می‌شوند. توالی‌های کامبرین بالایی البرز با نام پیشنهادی سازند ده‌ملا شناخته می‌شوند. در مطالعه کنونی، این توالی‌ها در ناحیه البرز مرکزی با تأکید بر روی سنگ‌های نواری آن مورد بررسی رسوب‌شناختی قرار گرفته‌اند و سعی شده است با آنالیز رخساره‌ای و تفسیر شرایط محیطی، درک مناسبی از ویژگی‌های رسوب‌شناختی کامبرین بالایی البرز به دست آید. در این راستا، سازند ده‌ملا به نه واحد لیتواستراتیگرافی و 12 ریزرخساره رسوبی تفکیک و مشخصات رسوب‌شناختی آن به تفصیل مورد بررسی قرار گرفته است. نتایج نشان می‌دهد که این رخساره‌ها در یک پلاتفرم کربناته اپیریک نهشته شده‌اند ولی در طول زمان تغییرات مهمی در فیزیوگرافی و توزیع رخساره‌ای حوضه رخ داده است، به نحوی که تکامل پلاتفرم مذکور را می‌توان به سه مرحله تقسیم نمود. در مرحله یک رخساره‌های عمدتا جزر و مدی در کمربندهای رخساره‌ای وسیع نهشته شده‌اند و محیط رسوبگذاری در معرض تلاطم‌های بزرگ قرار نداشته است. در مرحله دوم در اثر پیشروی سریع دریا، پلاتفرم دچار غرق‌شدگی شده و رسوبات شیلی سبز بر روی نهشته‌های پیشین نهشته شده‌اند. در مرحله سوم، محیط رسوبگذاری به شدت تحت تأثیر عوامل آشفته‌ساز محیطی همچون طوفان، جریانات دریایی و جزر و مد قرار داشته است و این وضعیت منجر به تغییرات شدید رخساره‌ای و نهشت سنگ‌های نواری شده است. سنگ‌های نواری طیفی از شرایط زیرجزر و مدی تا جزر و مدی نشان می‌دهند.

A Guzhangian (late Middle Cambrian) fauna from the Gidgealpa 1 drillhole, Warburton Basin, South Australia

A Cambrian Series 3 (Guzhangian, Boomerangian) fauna is described from Core 12 of the Gidgealpa 1 drillhole from the Warburton Basin, northeastern South Australia. There are four agnostoid taxa: Ammagnostus laiwuensis, Lejopyge calva, L. armata and an unassigned pygidium, and six polymerid trilobite taxa: Fuchouia sp., Dorypyge sp., Solenoparia changi sp. nov., Pianaspis sp., an unassigned cranidium and an unassigned pygidium. This fauna suggests a correlation with the Goniagnostus nathorsti Zone or the lower part of the Lejopyge laevigata Zone (Boomerangian on the northern Australian biostratigraphic scheme), with the latter preferred. These are the first published descriptions of fossils of this age from the Warburton Basin. Xiaowen Sun [Xiaowen_s@yahoo.com.au], Sun Petroleum Geoservices, 4 Jolie Grove, Box Hill, Victoria 3128, Australia; Christopher J. Bentley [ bigfossil@bigpond.com], 30 Albert Street, Clare, South Australia, 5453, Australia; James B. Jago [jim.jago@unisa.edu.au], University of South Australia–STEM, Mawson Lakes, South Australia 5095, Australia.

Peri-Gondwanan acritarchs, chitinozoans, and miospores from Paleozoic succession in the High Zagros Mountains, southern Iran: Regional stratigraphic significance and paleogeographic implications

Acritarchs and chitinozoans were documented from Ordovician strata and miospores from Lower Permian beds in the Zardkuh area. Ninety-one palynomorph species, comprising acritarchs (63 species of 31 genera), chitinozoans (16 species of 12 genera), and miospores (12 species of 10 genera) were recognized that form 10 local assemblage zones, comprising Zones A–F= acritarchs, G–I= chitinozoans, and J= miospores. Zone A occurs in the uppermost Ilebeyk Formation, indicating Late Cambrian; Zones B–E are present in the Zardkuh Formation, suggesting Early Ordovician (Tremadocian–Floian)–Middle Ordovician (Dapingian) and Zone F occurs in the Seyahou Formation, presenting Late Ordovician (Hirnantian). Likewise, three Gondwanan chitinozoan Zones G–I were established in this succession. Zones G–H occur in the Upper Shale Member of the Zardkuh Formation, supporting an Early-Middle Ordovician (Floian–Dapingian), and Zone I is restricted to the Seyahou Formation, suggesting Late Ordovician (Hirnantian) age. Finally, Zone J occurs in the Faraghan Formation, confirming Early Permian. Based on palynomorph zones, two hiatuses are present within the Paleozoic sequences of the study area. The first hiatus occurs between the Zardkuh and Seyahou formations, encompassing the Middle Ordovician (Dapingian) and much of the Late Ordovician (Sandbian and Katian). The second hiatus appears between the Seyahou and Faraghan formations, spanning the Silurian, Devonian, Carboniferous, and earliest Permian (Asselian). The palynomorph taxa, including Arbusculidium, Striatotheca, Coryphidium, Vavrdovella, Eremochitina, Velatachitina, and miospore taxa, confirm that the study area belongs to peri-Gondwana paleoprovince. Four new species are Vavrdovella mawlawii sp.nov., Vogtlandia zardkuhensis sp.nov, Pseudocoryphidium zagrosensis gen et sp.nov, and Pterospermella saadii sp.nov.

Kinematic evolution of fold-and-thrust belts in the Yubei-Tangbei area: Implications for tectonic events in the southern Tarim Basin

The Yubei-Tangbei area in the southern Tarim Basin is one of the best-preserved Early Paleozoic northeast-southwest trending fold-and-thrust belts within this basin. This area is crucial for the exploration of primary hydrocarbon reservoirs in northwestern China. In this study, we constructed the structural geometric morphology of the Yubei-Tangbei area using geophysical logs, drilling, and recent two- and three-dimensional (2-D and 3-D) seismic data. The Early Paleozoic fault-propagation folds, the Tangnan triangle zone, fault-detachment folds, and trishear fault-propagation folds developed with the detachment of the Middle Cambrian gypsum–salt layer. According to a detailed chronostratigraphic framework, the growth strata in the Upper Ordovician–Lower Silurian layer formed by onlapping the back limb of the asymmetric fault-propagation folds, which therefore defines the timing of deformations. The changes in kink band hinges and amplitudes in the Permian–Carboniferous and Cenozoic folding strata suggest that the evolution of the fold-and-thrust belts followed a sequential evolution process rather than a simultaneous one. Above the pre-existing Precambrian basement structure, the Yubei-Tangbei fold-and-thrust belts can be divided into four tectonic evolution stages: Late Cambrian, Late Ordovician to Early Carboniferous, Carboniferous to Permian, and Cenozoic. The northwestern-verging Cherchen Fault is part of the piedmont fold-and-thrust system of the southern Tarim foreland basin. We interpreted its strata as a breakthrough trishear fault-propagation fold that developed in three phases: Mid–Late Ordovician, Silurian to Middle Devonian, and Triassic to present. These tectonic events are responses of the Altyn-Tagh and Kunlun collisional orogenic belts and the Indian-Eurasian collision. The inherited deformation and structural modification in the southern Tarim Basin may be an indicator of the growth and evolution of peripheral orogens.

Ribbon rocks revisited: the upper Cambrian (Furongian) Hwajeol Formation, Taebaek Group, Korea

Ribbon rocks revisited: the upper Cambrian (Furongian) Hwajeol Formation, Taebaek Group, Korea

Facies architecture and depositional model for a fine‐grained hybrid‐energy delta: An example from the Upper Cambrian to Lower Ordovician Barik Formation, Central Oman

Recognizing ancient deltas' depositional architecture is challenging due to the complex interplay of the fluvial, waves, and tidal processes. The integration of field‐based sedimentological and trace fossils studies of the Late Cambrian‐Early Ordovician Barik Formation in Central Oman allowed better identification of different lithofacies architectures and depositional process signatures. The Barik Formation is a major tight‐gas reservoir in the Oman Sedimentary Basins, where it is over 800‐m thick. The only outcrop analogue is in the Qarn Mahatta Humaid area, where ca. 50‐m thickness of the formation is exposed and extends laterally over 5 km. This study revealed the presence of four lithofacies associations, including (a) mouth bar/shoreface, (b) tidal flat, (c) tidal channel, and (d) delta distributary channels using 12 logged sections. The depositional features identified in the field include tidal, wave/storm, and fluvial. The tidal signatures are represented by mud‐drapes, large‐scale reactivation surfaces, sigmoidal cross‐bedding, and climbing and flaser bedding, predominantly in the mixed mud‐sand tidal flats of the lower delta plain and delta distributary channels of the upper to lower delta plains. The wave/storm signatures are represented by wave ripples and small‐scale hummocky cross‐stratification in the mouth bar/shoreface of the delta front platform. The fluvial and subaerial signatures are represented by the occurrence of the scour surfaces with lag deposits of channel‐fill and the development of desiccation mud‐cracks. The Barik Formation delta sandstones form amalgamated sheet‐like bodies prograding to the north‐northeast and deposited along a ramp‐type low relief basin. This prograding delta represents a highstand system tract. This study presents an example of the ancient fine‐grained hybrid‐energy prograding delta, where fluvial processes operated beside the tidal and wave/storm processes over a broad and low relief shelf in a vegetation‐free system.

Geology, mineralization and short wave infrared alteration mapping of the Khan Altai Au deposit, Mongolia

AbstractKhan Altai Au deposit is located near the N margin of the Neoproterozoic‐Cambrian Lake terrain in SW Mongolia, about 5 km from its contact with the Proterozoic‐Archean Baydrag craton, and 18 km SE of the Khantaishir ophiolite, which was emplaced in the Late Cambrian. The host rocks are strongly deformed and metamorphosed to lower greenschist facies, and of uncertain age. They comprise a sequence of rhyolitic volcaniclastics and porphyritic flows interbedded with laminated siltstone, basaltic andesite and faulted against dolomite. Low‐grade gold mineralization (up to 2 ppm Au) is hosted mainly in rhyolitic volcanics and extends over an area of about 1,100 × 160 m with a vertical extent of about 200 m. It is associated with disseminated pyrite (2–10% by vol) but includes high‐grade zones (up to 183 ppm Au over 1 m) related to cm‐wide quartz‐native Au‐pyrite veins. The Au mineralized zone also encompasses VMS mineralization (currently of minor extent), characterized by massive pyrite‐sphalerite lenses and quartz‐chalcopyrite stringer zones. A larger VMS deposit (Ereen Budagt, about 10 Mt sulfide ore) is found 6 km to the SSE in a similar geological setting. The main alteration assemblage is quartz‐white mica‐albite, but with minor carbonate, chlorite, epidote‐actinolite and pyrophyllite‐diaspore‐dickite alteration. Gold mineralization is related to zones of strong tectonic foliation and formation of phengitic white mica, with an outward zonation to high Al white mica. Pyrite is typically euhedral, and exhibits concentric growth zones, as well as quartz pressure shadows to enclosing foliation, consistent with syngenetic growth during metamorphism and deformation. Other sulfides include arsenopyrite, sphalerite and possible marcasite. Preliminary LA‐ICPMS mapping shows pyrite rims are enriched in Au, As, Co, Cu, Ni, Pb, Ag, Mo and Se, the pyrite core is enriched in Co, Bi, Te, and rhyolitic host rock is enriched in K, Ba, V and Tl. Whole rock geochemistry of basaltic andesite to rhyolite, shows N‐MORB characteristics for basaltic andesite, as well as a subduction signature for all rocks, and high MgO (~8%), TiO2 (~1%) and low Ni, Cr content in basalt, compatible with a back arc tectonic setting.

Carbon-isotope stratigraphy of the Middle–Upper Cambrian in eastern Laurentia: Implications for global correlation

The examined interval of the March Point and Petit Jardin formations (total thickness ~130 m) spans the Middle−Upper Cambrian (lower Port au Port Group) at the Belle Isle Strait, western Newfoundland. The deposits consist of dolomitized shallow-water carbonates ranging from microbial mudstones to oolitic grainstones. A multi-technique evaluation proved the petrographic and geochemical preservation of the investigated micritic carbonates. The δ13C and δ18O values (−1.2 ± 1.1‰ VPDB and −6.7 ± 1.3‰ VPDB, respectively) exhibit an insignificant correlation (R2 = 0.16) and similarly the δ13C values with their Mn/Sr ratio (R2 = 0.01). This supports the preservation of at least near-primary δ13C signatures that can be utilized to construct a high-resolution δ13C profile for reliable global correlations. The δ13C profile exhibits two major negative excursions, the lower excursion (~4‰) immediately below the lower boundary of the March Point Formation (Hawke Bay/March Point boundary) and the upper excursion (~2‰) almost at the lower boundary of the Petit Jardin Formation where these excursions are likely associated with short-term sealevel changes. The trilobite biozonation scheme suggests the correlation of the lower excursion with the DICE (Drumian Carbon Isotope Excursion) event and the upper excursion with the base of the SPICE (Steptoean Positive Carbon Isotope Excursion) counterpart and both events have been globally documented on different paleocontinents.

Quantifying the middle–late Cambrian trilobite diversity pattern in South China

The middle–late Cambrian is a key interval between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE) that marks a major biologic evolutionary transition in the geologic record. Diversity (i.e. richness) patterns in this interval show that macro-biota experienced evolutionary setbacks for the first time. However, relatively little attention has been paid to this interval in previous macroevolutionary studies and very different conclusions about the diversity patterns in coarse time-resolution have been documented. As a dominate group of marine benthic organisms in the Cambrian, trilobites provide insightful information to understand the history of biodiversity in the Cambrian marine ecosystem from the Cambrian Explosion to the GOBE. South China is a region with relatively good preservation of Cambrian fossil materials and sections, and 4 out of 6 Cambrian GSSPs were established in South China. In this paper, we use constrained optimization (CONOP) to correlate 19 sections from South China to reduce the Signor-Lipps effect and integrate the results with high-resolution chronostratigraphic data to evaluate the middle–late Cambrian trilobite diversity pattern in South China. This study, for the first time, constructs a high time-resolution (about 60 kyr) trilobite species-level richness curve with estimated time from Age 4 to the Paibian. A double-peaked species diversification is separated by a minor extinction in Stage 4; this is followed by a gradual decline of species diversity over the entire Wuliuan Age. A radiation of three times increase of species diversity in the Drumian and Guzhangian is exhibited and the diversity remains high for most of the Guzhangian Stage; this is followed by a great extinction at the end of the Guzhangian with around 50% species lost. Detailed comparison with previous composite studies and fossil material from single sections revealed that the Signor-Lipps effect could produce large biases in diversity pattern. In addition, a precise analysis of species evolution features compared with environmental indices provides a perspective about trilobite evolutionary mechanisms associated with environmental changes related to the carbon cycle and biotic competition.

Cambrian-Ordovician continental magmatic arc at the northern margin of Gondwana: Insights from the Schladming Complex, Eastern Alps

The Austroalpine Unit is one of the major tectonic units in the Alps and contains distinct Pre-Mesozoic basement units, which underwent in part pre-Variscan (Ordovician), Variscan and Alpine events and preserved information of pre-Variscan magmatism and ocean slab relics. The Schladming Complex is an important part of the Austroalpine basement unit, its orthogneiss and paragneiss were intruded by a large number of the Cambrian to Ordovician magmatic rocks, which recorded the tectonic evolution related to the Proto-Tethys, but for which there have been very few studies carried out, especially in geochronology and geochemistry. In this study, for the first time we systematically investigated the magmatic rocks of the Schladming Complex by zircon U--Pb chronology, zircon Lu--Hf isotopic and geochemical to understand the tectonic setting and evolution of the Austroalpine Unit during the Early Paleozoic. Our study shows that the granodioritic gneisses (539–538 Ma) and the fine-grained amphibolite (531 ± 2 Ma) represent a bimodal magmatism. Geochemically, the granodioritic gneisses belong to A2-type granite and originated from the lower crust, the fine-grained amphibolites have an E-MORB affinity and derived from the lithospheric mantle. This association further implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian. A medium-grained amphibolite gives an age of 495 ± 5 Ma, exhibits ocean island basalt-like geochemical features and positive εHf(t) values (+5.3 ~ +10.9) indicating that the medium-grained amphibolite derived from a mantle source. The zircon U--Pb dating of monzonite granitic gneiss and plagioclase gneiss yields ages of 464 ± 4 Ma for and 487 ± 3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive εHf(t) values of +5.9 ~ +7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite.We propose that the Schladming Complex in the Eastern Alps has been a part of the active continental margin of Gondwana during Cambrian to Ordovician. With the subduction of the Proto-Tethyan Ocean to the south, a back-arc rift developed along the northern margin of the Gondwana in the Early Cambrian, subsequently, resulted in the opening of the Crypto-Rheic Ocean and the break-off of the Cadomian terrane from Gondwana in the Late Cambrian. Meanwhile, the I-type granites, related to the subduction, intruded into the northern margin of the Cadomian terrane. In the Early Ordovician the Cadomian terrane collided back to the Gondwana with the closure of the Crypto-Rheic Ocean due to the continuing southward subduction of the Proto-Tethyan Ocean. Subsequently, a number of S-type granites intruded due to the post-collisional extension during Early to Late Ordovician.

Tracing fault‐related fluids using in situ trace elements: A case study in Upper Cambrian dolostone in Gucheng area, Tarim Basin, NW China

The Gucheng area, located in the east of the Tarim Basin, is one of the important gas exploration targets, and the Upper Cambrian dolostone is the main pay layer. The dolostone is considered to be altered by hydrothermal fluids, but the hydrothermal altering degrees are still unclear. In this study we report the petrography and in situ trace elemental data, and try to discuss the hydrothermal fluids and their impact on the Upper Cambrian dolostones. Based on the petrography and CL properties, five types of dolomites are identified. MD1 and MD2 are the matrix dolomites. The crystal size of MD1 range from approximately 30 to 100 μm, while the MD2 is larger than 200 μm. Cement dolomite (CD) occurring only in vugs and fractures is subdivided into three types depending on the CL properties. CD‐zone1 is the core of CD with dull CL colours. CD‐zone2 comprises a bright red CL colour zone and a dull CL colour zone, and CD‐zone3 is the rim of CD dolomite with bright red CL colours. MD2 and CD‐zone1 show light rare earth elements (LREE) depletion, middle rare earth elements (MREE) enrichment and negative Eu anomalies. The REEY patterns of CD‐zone3 dolomite are marked by LREE enrichment and positive Eu anomalies. CD‐zone2 is the transitional mineral phase that shows slightly LREE depletion and negative Eu anomalies. The REEY patterns of CD‐zone1 suggest that CD‐zone1 is likely formed by local redistribution of older dolomites in a closed system. The REEY patterns of CD‐zone3 may indicate a magmatic associated hydrothermal fluid. Based on the petrography and in situ trace elemental data, the Permian magmatic fluid, which is recorded in CD‐zone2 and CD‐zone3 dolomites, may be only limited within fractures, while unlikely significantly altered the matrix dolomite in the Upper Cambrian.