Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling.Based on systematic observation and experimental analysis of outcrop and cor...Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling.Based on systematic observation and experimental analysis of outcrop and core samples,the deposition and development conditions and characteristics of marine shale are revealed,the geochemical and reservoir characteristics of marine shale are evaluated,and the layers of marine shale oil in the Mesozoic are determined.The following geological understandings are obtained.First,there are two sets of marine organic-rich shales,the Lower Jurassic Quse Formation and the Upper Triassic Bagong Formation,in the Qiangtang Basin.They are mainly composed of laminated shale with massive mudstone.The laminated organic-rich shale of the Quse Formation is located in the lower part of the stratum,with a thickness of 50–75 m,and mainly distributed in southern Qiangtang Basin and the central-west of northern Qiangtang Basin.The laminated organic-rich shale of the Bagong Formation is located in the middle of the stratum,with a thickness of 250–350 m,and distributed in both northern and southern Qiangtang Basin.Second,the two sets of laminated organic-rich shales develop foliation,and various types of micropores and microfractures.The average content of brittle minerals is 70%,implying a high fracturability.The average porosity is 5.89%,indicating good reservoir physical properties to the level of moderate–good shale oil reservoirs.Third,the organic-rich shale of the Quse Formation contains organic matters of types II1 and II2,with the average TOC of 8.34%,the average content of chloroform bitumen'A'of 0.66%,the average residual hydrocarbon generation potential(S1+S2)of 29.93 mg/g,and the Ro value of 0.9%–1.3%,meeting the standard of high-quality source rock.The organic-rich shale of the Bagong Formation contains mixed organic matters,with the TOC of 0.65%–3.10%and the Ro value of 1.17%–1.59%,meeting the standard of moderate source rock.Fourth,four shallow wells(depth of 50–250 m)with oil shows have been found in the organic shales at 50–90 m in the lower part of the Bagong Formation and 30–75 m in the middle part of the Quse Formation.The crude oil contains a high content of saturated hydrocarbon.Analysis and testing of outcrop and shallow well samples confirm the presence of marine shale oil in the Bagong Formation and the Quse Formation.Good shale oil intervals in the Bagong Formation are observed in layers 18–20 in the lower part of the section,where the shales with(S0+S1)higher than 1 mg/g are 206.7 m thick,with the maximum and average(S0+S1)of 1.92 mg/g and 1.81 mg/g,respectively.Good shale oil intervals in the Quse Formation are found in layers 4–8 in the lower part of the section,where the shales with(S0+S1)higher than 1 mg/g are 58.8 m thick,with the maximum and average(S0+S1)of 6.46 mg/g and 2.23 mg/g,respectively.展开更多
The oldest representative of the genus Megarhyphus Kovalev （Diptera： Anisopodidae） is described from the Lower Jurassic （Sinemurian） of England. A summary of our knowledge of Jurassic Anisopodidae is given.
The Lower Jurassic is subdivided in ascending order into the Wulong, Kangdui and Yongjia Formations on the north slope of Mount Qomolangma, with a total thickness of 1362 m. They are thought to have been deposited res...The Lower Jurassic is subdivided in ascending order into the Wulong, Kangdui and Yongjia Formations on the north slope of Mount Qomolangma, with a total thickness of 1362 m. They are thought to have been deposited respectively in the environments of the carbonate ramp fault-bounded basins and carbonate platform, with six sedimentary facies and six sub-facies. During the Early Jurassic, the Qomolangma area experienced strong faulting and subsidence, and was of a matured rift basin. The Lower Jurassic consists of eleven 3rd-order sequences, which can be grouped into three 2nd-order sequences and form a large transgressive-regressive cycle. The 3rd-order sequences and the corresponding sea-level changes recognized in the area can be correlated quite well with those set up in the western Tethys, and may have been caused by the eustatic fluctuations, while the 2nd-order sequences seem to be more closely related to the basement subsidence and the variation in sedimentary influx, indicating the evolution of the eastern Neotethys and the movement of the plates on its two sides.展开更多
Based on the latest results of near-source exploration in the Middle and Lower Jurassic of the Tuha Basin,a new understanding of the source rocks,reservoir conditions,and source-reservoir-cap rock combinations of the ...Based on the latest results of near-source exploration in the Middle and Lower Jurassic of the Tuha Basin,a new understanding of the source rocks,reservoir conditions,and source-reservoir-cap rock combinations of the Jurassic Shuixigou Group in the Taibei Sag is established using the concept of the whole petroleum system,and the coal-measure whole petroleum system is analyzed thoroughly.The results are obtained in three aspects.First,the coal-measure source rocks of the Badaowan Formation and Xishanyao Formation and the argillaceous source rocks of the Sangonghe Formation in the Shuixigou Group exhibit the characteristics of long-term hydrocarbon generation,multiple hydrocarbon generation peaks,and simultaneous oil and gas generation,providing sufficient oil and gas sources for the whole petroleum system in the Jurassic coal-bearing basin.Second,multi-phase shallow braided river delta–shallow lacustrine deposits contribute multiple types of reservoirs,e.g.sandstone,tight sandstone,shale and coal rock,in slope and depression areas,providing effective storage space for the petroleum reservoir formation in coal-measure strata.Third,three phases of hydrocarbon charging and structural evolution,as well as effective configuration of multiple types of reservoirs,result in the sequential accumulation of conventional-unconventional hydrocarbons.From high structural positions to depression,there are conventional structural and structural-lithological reservoirs far from the source,low-saturation structural-lithological reservoirs near the source,and tight sandstone gas,coal rock gas and shale oil accumulations within the source.Typically,the tight sandstone gas and coal rock gas are the key options for further exploration,and the shale oil and gas in the depression area is worth of more attention.The new understanding of the whole petroleum system in the coal measures could further enrich and improve the geological theory of the whole petroleum system,and provide new ideas for the overall exploration of oil and gas resources in the Tuha Basin.展开更多
Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of ...Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of the mainland Gavrovo-Tripotis platform, the most significant external platform of the Hetlenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner- carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces （elementary cycles）. Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies （tidal fiat progradation）. The lithofacies association includes dotomitic intraclastic-peloidal-bioclastic wackestones-packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna （bivalves, gastropods, ostracods and seldom benthic fora- minifers）, representing a shaUow subtidal to intertidal, moderately high-energy environment within an inner- platform setting （peritidal environment to restricted lagoon）. This lithofacies association has been intermit- tently subaeriaUy exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidat facies are capped by dolocretes controlled by root-activities （laminar dolocretes, petoi- dat-pisoid dolocretes and massive dotocretes）, marking the end of each depositional cycle, and, thus, dis- tinguishing the successive episodes of a prolonged subaerial exposure period and birth of pateosol horizons. Dotocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, ＂flower spar＂ to blocky sparry cement in primary pores, micritic coatings, crudely pelleted watts, atveotar-septat texture, infilts of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy （attocyctic processes）, in an inner- ramp-to-shelf carbonate setting with tidal fiat and restricted lagoon depositional environment. However, a combination of attocyctic with autocyctic processes controtting accommodation space and sediment accumutation, shoutd be atso taken in consideration. The studied high-frequency cyctes are interpreted to have been formed during the fatting stage and correspond to towstand systems tract （LST） sediments, which consists part of a regressive peak of the Lower/htiddte Jurassic Transgressive-Regressive Facies Low Frequency Cycte （2nd order）. Due to the absence of massive evaporites, dolomitization has been attributed to reflux of stightly increased-satinity marine fluids （i.e. penesatine）, that are driven into the undertying tagoonat-peritidat carbonate sediments during periods of tong term sea-fever fatl in a marginal marine setting.展开更多
A new family of planthoppers Fulgoromorpha from the Lower Jurassic of southern China is described to comprise Qiyangiricania cesta Lin,1986.The new family differs in venation pattern from the other Jurassic representa...A new family of planthoppers Fulgoromorpha from the Lower Jurassic of southern China is described to comprise Qiyangiricania cesta Lin,1986.The new family differs in venation pattern from the other Jurassic representatives of Fulgoroidea,presenting particular model of tegmen venation,not found among extinct and recent planthoppers.The Mesozoic stage of Fulgoroidea evolution is discussed.The extinct taxon Ricaniites fulgens（Brodie,1845）from the Purbeck of United Kingdom is excluded from the Hemiptera.展开更多
The Lenghu tectonic belt is located at or near the northern margin in the Qaidam Basin,which is an intracontinental composite basin evolved during the Mesozoic and Cenozoic epochs.The Lower Jurassic is considered a go...The Lenghu tectonic belt is located at or near the northern margin in the Qaidam Basin,which is an intracontinental composite basin evolved during the Mesozoic and Cenozoic epochs.The Lower Jurassic is considered a good source rock with a wide distribution.Since the deposit of the Lower Jurassic hydrocarbon source rock,the basin has undergone many tectonic events of subsidence and uplift.The source rocks in the Lengke-1 well,have experienced a complicated tectonic-burial history during which different hydrocarbon generation(HG) evolutions have taken place.We have investigated the history of burying,heating and hydrocarbon generation of the Lower Jurassic source rocks,not just on the basis of tectonic disturbance and deep burial,but also from new studies in fluid inclusion measurements and the application of numerical simulation with EASY%Ro.Our study reveals the evolution process,tectonic episodes and the strength of the HG of the Jurassic source rock.We conclude that twice HG processes have taken place since the Lower Jurassic formation and infer from that the important conclusion that more HG of the Lower Jurassic source rock took place during the Eocene-Miocene epochs.Finally,we discuss the oil and gas tarp formation and destruction in the Lenghu tectonic belt and point out that more attention should be paid to the thrust faults,which formed during the late Himalayan epoch.展开更多
The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies ...The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies the Triassic Minjur Formation and it in turn is overlaid by the Middle Jurassic Dhruma Formation. Thirteen lithofacies types can be identified that permit the recognition of five lithofacies associations in a mixed clastic/carbonate platform. These lithofacies range from low-energy peritidal, intertidal, and back-shoal to moderate-and high-energy shoal and foreshoal lithofacies associations. The Marrat Formation exhibits three depositional sequences, each sequence is grouped into a transgressive systems tract(TST) and a highstand systems tract(HST) and then bounded by sequence boundary surfaces(SBSs). The TSTs are generally identified in clastic tidal-flat beds and back-shoal wackestones, while the HST is generally recorded in the carbonate tidal-flat and shoal. The vertical succession of facies associations from peritidal to foreshoal depositional environments is indicative of a deepening upward and retrogradational systems tract, from Lower to Upper Toarcian. The correlation between the studied sections reveals a general shallowing towards the south and the similarities between the studied sequences and others in the Arabian Gulf, the northern Neo-Tethys Plate, and Gondwanaland countries.展开更多
The marl and limestone alternations of the Lower Jurassic Ain Ouarka and Ain Rhezala formations(Pliensbachian-Toarcian)in the western Saharan Atlas,Northwest Algeria,yield a diverse micro-and mac-rofauna,including mod...The marl and limestone alternations of the Lower Jurassic Ain Ouarka and Ain Rhezala formations(Pliensbachian-Toarcian)in the western Saharan Atlas,Northwest Algeria,yield a diverse micro-and mac-rofauna,including moderately numerous crinoids,which are represented by remains of isocrinids,i.e.,Bal-anocrinus ticinensis Hess and columnals of the genus Percevalicrinus.So far,the latter genus has been observed from the Upper Jurassic-Lower Cretaceous strata of Eurasia,North America,and the African continent.Thus,the present find is the oldest record of this crinoid genus,and the second one from the southern Tethyan margin.In this paper,it is shown that Percevalicrinus,which is traditionally regarded as a representative of the subfamily Balanocrininae,displays several features of the subfamily Isocrininae.The crinoid assemblage and associated facies and invertebrate fauna are typical of a low-energy deep outer shelf/ramp(below the storm wave-base)setting.展开更多
基金Supported by the PetroChina Science and Technology Major Project(2021DJ08)National Natural Science Foundation of China(42241203).
文摘Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling.Based on systematic observation and experimental analysis of outcrop and core samples,the deposition and development conditions and characteristics of marine shale are revealed,the geochemical and reservoir characteristics of marine shale are evaluated,and the layers of marine shale oil in the Mesozoic are determined.The following geological understandings are obtained.First,there are two sets of marine organic-rich shales,the Lower Jurassic Quse Formation and the Upper Triassic Bagong Formation,in the Qiangtang Basin.They are mainly composed of laminated shale with massive mudstone.The laminated organic-rich shale of the Quse Formation is located in the lower part of the stratum,with a thickness of 50–75 m,and mainly distributed in southern Qiangtang Basin and the central-west of northern Qiangtang Basin.The laminated organic-rich shale of the Bagong Formation is located in the middle of the stratum,with a thickness of 250–350 m,and distributed in both northern and southern Qiangtang Basin.Second,the two sets of laminated organic-rich shales develop foliation,and various types of micropores and microfractures.The average content of brittle minerals is 70%,implying a high fracturability.The average porosity is 5.89%,indicating good reservoir physical properties to the level of moderate–good shale oil reservoirs.Third,the organic-rich shale of the Quse Formation contains organic matters of types II1 and II2,with the average TOC of 8.34%,the average content of chloroform bitumen'A'of 0.66%,the average residual hydrocarbon generation potential(S1+S2)of 29.93 mg/g,and the Ro value of 0.9%–1.3%,meeting the standard of high-quality source rock.The organic-rich shale of the Bagong Formation contains mixed organic matters,with the TOC of 0.65%–3.10%and the Ro value of 1.17%–1.59%,meeting the standard of moderate source rock.Fourth,four shallow wells(depth of 50–250 m)with oil shows have been found in the organic shales at 50–90 m in the lower part of the Bagong Formation and 30–75 m in the middle part of the Quse Formation.The crude oil contains a high content of saturated hydrocarbon.Analysis and testing of outcrop and shallow well samples confirm the presence of marine shale oil in the Bagong Formation and the Quse Formation.Good shale oil intervals in the Bagong Formation are observed in layers 18–20 in the lower part of the section,where the shales with(S0+S1)higher than 1 mg/g are 206.7 m thick,with the maximum and average(S0+S1)of 1.92 mg/g and 1.81 mg/g,respectively.Good shale oil intervals in the Quse Formation are found in layers 4–8 in the lower part of the section,where the shales with(S0+S1)higher than 1 mg/g are 58.8 m thick,with the maximum and average(S0+S1)of 6.46 mg/g and 2.23 mg/g,respectively.
文摘The oldest representative of the genus Megarhyphus Kovalev （Diptera： Anisopodidae） is described from the Lower Jurassic （Sinemurian） of England. A summary of our knowledge of Jurassic Anisopodidae is given.
基金The research was jointly supported by the State ScienceTechnology Commission and the State Education Commission of China
文摘The Lower Jurassic is subdivided in ascending order into the Wulong, Kangdui and Yongjia Formations on the north slope of Mount Qomolangma, with a total thickness of 1362 m. They are thought to have been deposited respectively in the environments of the carbonate ramp fault-bounded basins and carbonate platform, with six sedimentary facies and six sub-facies. During the Early Jurassic, the Qomolangma area experienced strong faulting and subsidence, and was of a matured rift basin. The Lower Jurassic consists of eleven 3rd-order sequences, which can be grouped into three 2nd-order sequences and form a large transgressive-regressive cycle. The 3rd-order sequences and the corresponding sea-level changes recognized in the area can be correlated quite well with those set up in the western Tethys, and may have been caused by the eustatic fluctuations, while the 2nd-order sequences seem to be more closely related to the basement subsidence and the variation in sedimentary influx, indicating the evolution of the eastern Neotethys and the movement of the plates on its two sides.
基金Supported by the“Tianshan Talent”Project of Xinjiang(2022TSYCLJ0070)CNPC Technology Project(2023ZZ18)。
文摘Based on the latest results of near-source exploration in the Middle and Lower Jurassic of the Tuha Basin,a new understanding of the source rocks,reservoir conditions,and source-reservoir-cap rock combinations of the Jurassic Shuixigou Group in the Taibei Sag is established using the concept of the whole petroleum system,and the coal-measure whole petroleum system is analyzed thoroughly.The results are obtained in three aspects.First,the coal-measure source rocks of the Badaowan Formation and Xishanyao Formation and the argillaceous source rocks of the Sangonghe Formation in the Shuixigou Group exhibit the characteristics of long-term hydrocarbon generation,multiple hydrocarbon generation peaks,and simultaneous oil and gas generation,providing sufficient oil and gas sources for the whole petroleum system in the Jurassic coal-bearing basin.Second,multi-phase shallow braided river delta–shallow lacustrine deposits contribute multiple types of reservoirs,e.g.sandstone,tight sandstone,shale and coal rock,in slope and depression areas,providing effective storage space for the petroleum reservoir formation in coal-measure strata.Third,three phases of hydrocarbon charging and structural evolution,as well as effective configuration of multiple types of reservoirs,result in the sequential accumulation of conventional-unconventional hydrocarbons.From high structural positions to depression,there are conventional structural and structural-lithological reservoirs far from the source,low-saturation structural-lithological reservoirs near the source,and tight sandstone gas,coal rock gas and shale oil accumulations within the source.Typically,the tight sandstone gas and coal rock gas are the key options for further exploration,and the shale oil and gas in the depression area is worth of more attention.The new understanding of the whole petroleum system in the coal measures could further enrich and improve the geological theory of the whole petroleum system,and provide new ideas for the overall exploration of oil and gas resources in the Tuha Basin.
基金co-financed by the University of Athens Program KAPODISTRIAS/Special Account for Research Grants of the National and Kapodistrian University of Athens(S.A.R.G)
文摘Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of the mainland Gavrovo-Tripotis platform, the most significant external platform of the Hetlenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner- carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces （elementary cycles）. Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies （tidal fiat progradation）. The lithofacies association includes dotomitic intraclastic-peloidal-bioclastic wackestones-packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna （bivalves, gastropods, ostracods and seldom benthic fora- minifers）, representing a shaUow subtidal to intertidal, moderately high-energy environment within an inner- platform setting （peritidal environment to restricted lagoon）. This lithofacies association has been intermit- tently subaeriaUy exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidat facies are capped by dolocretes controlled by root-activities （laminar dolocretes, petoi- dat-pisoid dolocretes and massive dotocretes）, marking the end of each depositional cycle, and, thus, dis- tinguishing the successive episodes of a prolonged subaerial exposure period and birth of pateosol horizons. Dotocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, ＂flower spar＂ to blocky sparry cement in primary pores, micritic coatings, crudely pelleted watts, atveotar-septat texture, infilts of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy （attocyctic processes）, in an inner- ramp-to-shelf carbonate setting with tidal fiat and restricted lagoon depositional environment. However, a combination of attocyctic with autocyctic processes controtting accommodation space and sediment accumutation, shoutd be atso taken in consideration. The studied high-frequency cyctes are interpreted to have been formed during the fatting stage and correspond to towstand systems tract （LST） sediments, which consists part of a regressive peak of the Lower/htiddte Jurassic Transgressive-Regressive Facies Low Frequency Cycte （2nd order）. Due to the absence of massive evaporites, dolomitization has been attributed to reflux of stightly increased-satinity marine fluids （i.e. penesatine）, that are driven into the undertying tagoonat-peritidat carbonate sediments during periods of tong term sea-fever fatl in a marginal marine setting.
基金supported by the National Natural Science Foundation of China(Grant Nos.40872015, J0630967)Major Basic Research Program of China(Grant No.2006CB806400)+1 种基金The research resulted in this paper was also supported by the scientific exchange project of Chinese Academy of Sciences and Polish Academy of Sciences "Palaeobiodiversity of Fulgoromorpha and Cicadomorpha (Insecta:Hemiptera) of Daohugou Biota",the Chinese-Polish Scientific and Technological Cooperation project "Fulgoromorpha and Cicadomorpha (Insecta:Hemiptera) of the Daohugou Biota:morphology, classification,evolution"The paper is a contribution to the research grant "Extinct and extant genus in the palaeontological record of recent families of insects" from the Ministry of Science and Higher Education of Poland NN 303 2979 37 awarded to JS for the years 2009-2012
文摘A new family of planthoppers Fulgoromorpha from the Lower Jurassic of southern China is described to comprise Qiyangiricania cesta Lin,1986.The new family differs in venation pattern from the other Jurassic representatives of Fulgoroidea,presenting particular model of tegmen venation,not found among extinct and recent planthoppers.The Mesozoic stage of Fulgoroidea evolution is discussed.The extinct taxon Ricaniites fulgens（Brodie,1845）from the Purbeck of United Kingdom is excluded from the Hemiptera.
基金supported by the Natural Oil & Gas Stratagem Tap of China
文摘The Lenghu tectonic belt is located at or near the northern margin in the Qaidam Basin,which is an intracontinental composite basin evolved during the Mesozoic and Cenozoic epochs.The Lower Jurassic is considered a good source rock with a wide distribution.Since the deposit of the Lower Jurassic hydrocarbon source rock,the basin has undergone many tectonic events of subsidence and uplift.The source rocks in the Lengke-1 well,have experienced a complicated tectonic-burial history during which different hydrocarbon generation(HG) evolutions have taken place.We have investigated the history of burying,heating and hydrocarbon generation of the Lower Jurassic source rocks,not just on the basis of tectonic disturbance and deep burial,but also from new studies in fluid inclusion measurements and the application of numerical simulation with EASY%Ro.Our study reveals the evolution process,tectonic episodes and the strength of the HG of the Jurassic source rock.We conclude that twice HG processes have taken place since the Lower Jurassic formation and infer from that the important conclusion that more HG of the Lower Jurassic source rock took place during the Eocene-Miocene epochs.Finally,we discuss the oil and gas tarp formation and destruction in the Lenghu tectonic belt and point out that more attention should be paid to the thrust faults,which formed during the late Himalayan epoch.
基金The authors extend their appreciation to Researchers Supporting Project (No. RSP2023R139)。
文摘The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies the Triassic Minjur Formation and it in turn is overlaid by the Middle Jurassic Dhruma Formation. Thirteen lithofacies types can be identified that permit the recognition of five lithofacies associations in a mixed clastic/carbonate platform. These lithofacies range from low-energy peritidal, intertidal, and back-shoal to moderate-and high-energy shoal and foreshoal lithofacies associations. The Marrat Formation exhibits three depositional sequences, each sequence is grouped into a transgressive systems tract(TST) and a highstand systems tract(HST) and then bounded by sequence boundary surfaces(SBSs). The TSTs are generally identified in clastic tidal-flat beds and back-shoal wackestones, while the HST is generally recorded in the carbonate tidal-flat and shoal. The vertical succession of facies associations from peritidal to foreshoal depositional environments is indicative of a deepening upward and retrogradational systems tract, from Lower to Upper Toarcian. The correlation between the studied sections reveals a general shallowing towards the south and the similarities between the studied sequences and others in the Arabian Gulf, the northern Neo-Tethys Plate, and Gondwanaland countries.
基金supported by the National Science Centre,Poland(www.ncn.gov.pl)-Grant No.2020/39/B/ST10/00006.
文摘The marl and limestone alternations of the Lower Jurassic Ain Ouarka and Ain Rhezala formations(Pliensbachian-Toarcian)in the western Saharan Atlas,Northwest Algeria,yield a diverse micro-and mac-rofauna,including moderately numerous crinoids,which are represented by remains of isocrinids,i.e.,Bal-anocrinus ticinensis Hess and columnals of the genus Percevalicrinus.So far,the latter genus has been observed from the Upper Jurassic-Lower Cretaceous strata of Eurasia,North America,and the African continent.Thus,the present find is the oldest record of this crinoid genus,and the second one from the southern Tethyan margin.In this paper,it is shown that Percevalicrinus,which is traditionally regarded as a representative of the subfamily Balanocrininae,displays several features of the subfamily Isocrininae.The crinoid assemblage and associated facies and invertebrate fauna are typical of a low-energy deep outer shelf/ramp(below the storm wave-base)setting.
