Past geological and geochemical changes suggest the existence of two transitions in Earth history at 2.5–2 and 1–0.5 Ga. Twenty-one changes during Transition One and eight during Transition Two signify global-scale shifts in terrestrial tectono-thermal systems. Transition-One changes include the onset of major zircon age peaks, increases in average plate speeds, increases in the number of collisional orogens and associated metamorphism, and continental sedimentation. In addition, changes in rock abundances and chemical composition of continents, of detrital sediments, and in seawater composition reflect an increase in volume of felsic component in the continental crust as well as increasing continental emergence. Changes in composition of oceanic basalts during this time reflect mantle cooling, oxidation, and the appearance of an enriched component in mantle magma sources. A rise in large igneous province (LIP) frequency may reflect enhanced mantle-plume activity responding to thermal-compositional disturbance in the deep mantle due to sinking slabs. Transition Two is characterized by the appearance and establishment of blueschists, ultrahigh-pressure (UHP) metamorphism, and greatly enhanced ophiolite preservation beginning at 750–600 Ma, and this transition may signal the first subduction of continental crust. Each transition is proposed to reflect a thermal threshold during which the lithosphere strengthens. Transition One is interpreted to record the onset, propagation, and eventual establishment of plate tectonics, and Transition Two the onset and establishment of subduction of a strong continental lithosphere.
{"title":"Two Major Transitions in Earth History: Evidence of Two Lithospheric Strength Thresholds","authors":"K. Condie","doi":"10.1086/711141","DOIUrl":"https://doi.org/10.1086/711141","url":null,"abstract":"Past geological and geochemical changes suggest the existence of two transitions in Earth history at 2.5–2 and 1–0.5 Ga. Twenty-one changes during Transition One and eight during Transition Two signify global-scale shifts in terrestrial tectono-thermal systems. Transition-One changes include the onset of major zircon age peaks, increases in average plate speeds, increases in the number of collisional orogens and associated metamorphism, and continental sedimentation. In addition, changes in rock abundances and chemical composition of continents, of detrital sediments, and in seawater composition reflect an increase in volume of felsic component in the continental crust as well as increasing continental emergence. Changes in composition of oceanic basalts during this time reflect mantle cooling, oxidation, and the appearance of an enriched component in mantle magma sources. A rise in large igneous province (LIP) frequency may reflect enhanced mantle-plume activity responding to thermal-compositional disturbance in the deep mantle due to sinking slabs. Transition Two is characterized by the appearance and establishment of blueschists, ultrahigh-pressure (UHP) metamorphism, and greatly enhanced ophiolite preservation beginning at 750–600 Ma, and this transition may signal the first subduction of continental crust. Each transition is proposed to reflect a thermal threshold during which the lithosphere strengthens. Transition One is interpreted to record the onset, propagation, and eventual establishment of plate tectonics, and Transition Two the onset and establishment of subduction of a strong continental lithosphere.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711141","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48500551","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Mukhopadhyay, R. Armstrong, J. Gutzmer, M. de Kock, N. Beukes
The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725±57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733±53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India.
{"title":"Stratigraphy, Depositional Setting, and SHRIMP U-Pb Geochronology of the Banded Iron Formation–Bearing Bailadila Group in the Bacheli Iron Ore Mining District, Bastar Craton, India","authors":"J. Mukhopadhyay, R. Armstrong, J. Gutzmer, M. de Kock, N. Beukes","doi":"10.1086/713683","DOIUrl":"https://doi.org/10.1086/713683","url":null,"abstract":"The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725±57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733±53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713683","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43476614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Christopher A. Vanderlip, R. Cox, D. Larsen, Jeff Mitchell, J. B. Harris, C. Cearley
A newly recognized thrust and nearby asymmetric anticline crop out 40 km north of Memphis, Tennessee, and they deform Eocene through Quaternary strata. These east–west-striking, south-verging structures are peripheral to the New Madrid seismic zone (NMSZ) of central North America, the source of M7+ earthquakes in 1811–1812. The thrust dips ∼20° N and has 55 m of throw in Eocene strata. An angular intraformational unconformity indicates most deformation was Eocene. The anticline’s limbs dip 7° N and 22° S and fold Eocene and Pleistocene strata. Pleistocene sediments are dropped at least 4 m into a graben along the fold axis. Holocene sediment is ponded upstream from the fold axis, suggesting Holocene activity. Based on outcrops, well logs, and seismic reflection, we interpret the anticline as a fault-tip fold above a splay of the thrust fault. We interpret these thrusts in the context of a previously published sandbox model of a restraining bend uplift, which we apply here to the Reelfoot Rift fault complex. Using the eastern rift margin as the strike-slip fault of the sandbox model, the periphery of the model uplift has an east–west-striking, south-verging oblique-slip thrust where the actual thrust and anticline crop out. These results suggest that young thrust faults may be common along the periphery of the NMSZ and similar active intraplate restraining bends, that the eastern margin of the Reelfoot Rift may have been a principal strike-slip fault of the restraining bend, and that the seismic zone was active as early as Eocene.
{"title":"Newly Recognized Quaternary Surface Faulting and Folding Peripheral to the New Madrid Seismic Zone, Central United States, and Implications for Restraining Bend Models of Intraplate Seismic Zones","authors":"Christopher A. Vanderlip, R. Cox, D. Larsen, Jeff Mitchell, J. B. Harris, C. Cearley","doi":"10.1086/713686","DOIUrl":"https://doi.org/10.1086/713686","url":null,"abstract":"A newly recognized thrust and nearby asymmetric anticline crop out 40 km north of Memphis, Tennessee, and they deform Eocene through Quaternary strata. These east–west-striking, south-verging structures are peripheral to the New Madrid seismic zone (NMSZ) of central North America, the source of M7+ earthquakes in 1811–1812. The thrust dips ∼20° N and has 55 m of throw in Eocene strata. An angular intraformational unconformity indicates most deformation was Eocene. The anticline’s limbs dip 7° N and 22° S and fold Eocene and Pleistocene strata. Pleistocene sediments are dropped at least 4 m into a graben along the fold axis. Holocene sediment is ponded upstream from the fold axis, suggesting Holocene activity. Based on outcrops, well logs, and seismic reflection, we interpret the anticline as a fault-tip fold above a splay of the thrust fault. We interpret these thrusts in the context of a previously published sandbox model of a restraining bend uplift, which we apply here to the Reelfoot Rift fault complex. Using the eastern rift margin as the strike-slip fault of the sandbox model, the periphery of the model uplift has an east–west-striking, south-verging oblique-slip thrust where the actual thrust and anticline crop out. These results suggest that young thrust faults may be common along the periphery of the NMSZ and similar active intraplate restraining bends, that the eastern margin of the Reelfoot Rift may have been a principal strike-slip fault of the restraining bend, and that the seismic zone was active as early as Eocene.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713686","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43760228","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xin Zhou, J. Ji, Jing Zhou, Lhamo Yungchen, Wuxun Quan, J. Tu
The Jehol Biota is famous for its diversity of Early Cretaceous fossils preserved under large amounts of pyroclastic materials. However, the precise determination on the ages of volcanoclastic strata is challenging, because of their complex provenance. The solution to this problem may lie in the greatly improved geochronological methods, such as the 40Ar/39Ar geochronology. In this study, we use the laser fusion 40Ar/39Ar dating method to date volcanoclastic samples and apply statistical analyses of the acquired apparent ages to determine the volcanic eruption periods. From the 12 volcanoclastic samples analyzed in this study, we obtained 262 apparent ages that vary from 85.5 to 153.6 Ma. By fitting these apparent ages with a Gaussian mixture distribution model, we find three age clusters that are normally distributed, with each cluster statistically representing an eruption period. The mean values of the three normal distributions are 90.37±2.72 (1σ), 109.89±4.94, and 125.26±10.01 Ma. A comparison of our results with a statistical analysis of literature data indicates that volcanic activities occurred over a wide spatial scale across western Liaoning Province during the two older periods, whereas during the last time period, eruptions occurred more locally and belonged to the final stage of volcanic eruptions in the basin. The method and results reported in this study provide a good example of (1) dating volcanoclastic materials using the 40Ar/39Ar method to determine the time periods of volcanic activities and (2) investigating the stratigraphic correlation in complex volcanoclastic basins.
{"title":"A New 40Ar/39Ar Analysis Method of Volcanoclastic Strata to Determine Eruption Periods—Example of Xintaimen, China","authors":"Xin Zhou, J. Ji, Jing Zhou, Lhamo Yungchen, Wuxun Quan, J. Tu","doi":"10.1086/713685","DOIUrl":"https://doi.org/10.1086/713685","url":null,"abstract":"The Jehol Biota is famous for its diversity of Early Cretaceous fossils preserved under large amounts of pyroclastic materials. However, the precise determination on the ages of volcanoclastic strata is challenging, because of their complex provenance. The solution to this problem may lie in the greatly improved geochronological methods, such as the 40Ar/39Ar geochronology. In this study, we use the laser fusion 40Ar/39Ar dating method to date volcanoclastic samples and apply statistical analyses of the acquired apparent ages to determine the volcanic eruption periods. From the 12 volcanoclastic samples analyzed in this study, we obtained 262 apparent ages that vary from 85.5 to 153.6 Ma. By fitting these apparent ages with a Gaussian mixture distribution model, we find three age clusters that are normally distributed, with each cluster statistically representing an eruption period. The mean values of the three normal distributions are 90.37±2.72 (1σ), 109.89±4.94, and 125.26±10.01 Ma. A comparison of our results with a statistical analysis of literature data indicates that volcanic activities occurred over a wide spatial scale across western Liaoning Province during the two older periods, whereas during the last time period, eruptions occurred more locally and belonged to the final stage of volcanic eruptions in the basin. The method and results reported in this study provide a good example of (1) dating volcanoclastic materials using the 40Ar/39Ar method to determine the time periods of volcanic activities and (2) investigating the stratigraphic correlation in complex volcanoclastic basins.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713685","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46412520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Trọng, K. Zong, Yongsheng Liu, Yu Yuan, P. T. Hieu, L. Dung, P. Minh
Southeast Asia plays a key role in the evolution of the eastern Tethys, which is characterized by accretion and amalgamation of numerous terranes since early Paleozoic. In the present study, an early Paleozoic granitoid sequence from the Kontum massif in the central Indochina block was investigated to reveal the early Paleozoic accretionary orogenesis of the eastern Tethys. Early Paleozoic Kontum granitoids include diorites and granites. Early Ordovician (485–473 Ma) Ben Giang diorites show high Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as positive zircon ɛHf(t) values (+6.2 to +10), probably reflecting the melting of a metasomatized mantle wedge. Late Ordovician (457–453 Ma) Dien Binh hornblende-bearing diorites and granites exhibit low Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as negative zircon ɛHf(t) values (−8.7 to −4.2), which is interpreted to reflect the reworking of ancient continental mafic crust during subduction. In contrast, the Silurian (422 Ma) Dai Loc muscovite-bearing granites are characterized by relatively high A/CNK and heavy rare earth element contents, and a negative Eu anomaly, corresponding to a typical S-type granite affinity. Their negative zircon ɛHf(t) values (−6.0 to +0.6) suggest that a petrogenesis linked to reworking of crustal sediments. In combination with the widespread occurrence of coeval high-pressure metamorphism, we suggest that an advancing accretionary orogenesis was responsible for the generation of the early Paleozoic magmatic arc in the Kontum massif. Such magmatic arc systems dominated the early Paleozoic evolution of the northern Gondwana margin during the subduction of the Proto-Tethys.
{"title":"Early Paleozoic Arc Magmatism and Accretionary Orogenesis in the Indochina Block, Southeast Asia","authors":"N. Trọng, K. Zong, Yongsheng Liu, Yu Yuan, P. T. Hieu, L. Dung, P. Minh","doi":"10.1086/713727","DOIUrl":"https://doi.org/10.1086/713727","url":null,"abstract":"Southeast Asia plays a key role in the evolution of the eastern Tethys, which is characterized by accretion and amalgamation of numerous terranes since early Paleozoic. In the present study, an early Paleozoic granitoid sequence from the Kontum massif in the central Indochina block was investigated to reveal the early Paleozoic accretionary orogenesis of the eastern Tethys. Early Paleozoic Kontum granitoids include diorites and granites. Early Ordovician (485–473 Ma) Ben Giang diorites show high Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as positive zircon ɛHf(t) values (+6.2 to +10), probably reflecting the melting of a metasomatized mantle wedge. Late Ordovician (457–453 Ma) Dien Binh hornblende-bearing diorites and granites exhibit low Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as negative zircon ɛHf(t) values (−8.7 to −4.2), which is interpreted to reflect the reworking of ancient continental mafic crust during subduction. In contrast, the Silurian (422 Ma) Dai Loc muscovite-bearing granites are characterized by relatively high A/CNK and heavy rare earth element contents, and a negative Eu anomaly, corresponding to a typical S-type granite affinity. Their negative zircon ɛHf(t) values (−6.0 to +0.6) suggest that a petrogenesis linked to reworking of crustal sediments. In combination with the widespread occurrence of coeval high-pressure metamorphism, we suggest that an advancing accretionary orogenesis was responsible for the generation of the early Paleozoic magmatic arc in the Kontum massif. Such magmatic arc systems dominated the early Paleozoic evolution of the northern Gondwana margin during the subduction of the Proto-Tethys.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713727","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42245499","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.
{"title":"Source of Detritus in Subducted Turbidites, Tectonic Mélange, Port Macquarie Block, Southern New England Orogen, Australia—A Geochemical Perspective","authors":"R. Offler, R. Boyd","doi":"10.1086/713684","DOIUrl":"https://doi.org/10.1086/713684","url":null,"abstract":"A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713684","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47398629","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Up to 2 km of siliciclastic and chemical sedimentary rocks, metamorphosed to greenschist facies, are preserved in the Baraboo Hills of southern Wisconsin. The strata compose two sedimentary successions separated by an angular unconformity. The lower succession includes the Baraboo quartzite, Seeley slate, and Freedom Formation, and the upper includes the Dake quartzite and Rowley Creek slate. Despite being studied for more than 100 y, the exposed section is only recently subdivided into informal members based on identification of sedimentary lithofacies and geologic mapping. This study integrates detrital zircon and paleocurrent analyses with sedimentologic and stratigraphic observations derived from detailed field mapping of the Baraboo and Dake quartzites. Our purpose is twofold: (1) characterize lithofacies of the Baraboo quartzite to interpret the environments of deposition for this unit and (2) evaluate sediment provenance and constrain depositional age. Lithofacies, paleocurrent, and detrital zircon U-Pb analyses of the Baraboo quartzite record fluvial braid plain, eolian, and fluvial-, tide- and wave-influenced deltaic depositional environments with sediments sourced from the north and deposited after 1714 ± 17 Ma. Lithofacies compose two backstepping alluvial to marine successions separated by a thick, laterally continuous coastal dune deposit. North-directed paleocurrents and detrital zircon ages confirm the Dake quartzite is a distinct unit with maximum depositional age of less than 1630.1 ± 8.6 Ma. Deposition of the Dake quartzite reflects basement uplift that postdates deposition of the Baraboo quartzite. The hiatus represented by the unconformity beneath the Dake quartzite is unconstrained and may represent more than 100 My.
{"title":"Revised Provenance, Depositional Environment, and Maximum Depositional Age for the Baraboo (","authors":"E. Stewart, L. Brengman, E. Stewart","doi":"10.1086/713687","DOIUrl":"https://doi.org/10.1086/713687","url":null,"abstract":"Up to 2 km of siliciclastic and chemical sedimentary rocks, metamorphosed to greenschist facies, are preserved in the Baraboo Hills of southern Wisconsin. The strata compose two sedimentary successions separated by an angular unconformity. The lower succession includes the Baraboo quartzite, Seeley slate, and Freedom Formation, and the upper includes the Dake quartzite and Rowley Creek slate. Despite being studied for more than 100 y, the exposed section is only recently subdivided into informal members based on identification of sedimentary lithofacies and geologic mapping. This study integrates detrital zircon and paleocurrent analyses with sedimentologic and stratigraphic observations derived from detailed field mapping of the Baraboo and Dake quartzites. Our purpose is twofold: (1) characterize lithofacies of the Baraboo quartzite to interpret the environments of deposition for this unit and (2) evaluate sediment provenance and constrain depositional age. Lithofacies, paleocurrent, and detrital zircon U-Pb analyses of the Baraboo quartzite record fluvial braid plain, eolian, and fluvial-, tide- and wave-influenced deltaic depositional environments with sediments sourced from the north and deposited after 1714 ± 17 Ma. Lithofacies compose two backstepping alluvial to marine successions separated by a thick, laterally continuous coastal dune deposit. North-directed paleocurrents and detrital zircon ages confirm the Dake quartzite is a distinct unit with maximum depositional age of less than 1630.1 ± 8.6 Ma. Deposition of the Dake quartzite reflects basement uplift that postdates deposition of the Baraboo quartzite. The hiatus represented by the unconformity beneath the Dake quartzite is unconstrained and may represent more than 100 My.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713687","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44373895","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Numerous late Mesozoic (Late Jurassic to Early Cretaceous) granitic plutons are present along the southern margin of the North China Craton (s-NCC), which belongs to the northernmost tectonic unit of the eastern Qinling orogenic belt. This tectonic unit records multiple tectonothermal and metallogenic events that occurred after continental collision of the North and South China Cratons in the late Early to Middle Triassic. This study reports the zircon U-Pb ages and the whole-rock and apatite geochemical compositions of the Taishanmiao granite, one of the largest late Mesozoic plutons in eastern Qinling. Three phases of this granite, emplaced between ∼125 and ∼113 Ma, are characterized by uniformly high SiO2 and K2O but low CaO, TFe2O3, and MgO contents and enriched Nd isotopic composition with initial εNd values of −13.5 to −12.7. Geochemical features reveal that these are A-type granites derived from partial melting mainly of ancient crust. Temperatures obtained from Ti-in-zircon thermometry and oxygen fugacity (fO2) data indicate that the magma(s) formed at high crystallization temperatures (∼800°–840°C) and fO2 levels (fayalite–magnetite–quartz [FMQ] buffer −1.5 to +1.5). Comparison with neighboring granites, such as the ∼150–130 Ma adakitic Heyu granite and the ∼115 Ma Donggou ore-bearing granite porphyry, implies that the Heyu granite formed from thickened crust and that the Donggou granite porphyry and Taishanmiao granite likely originated from a consistent magma system during the crustal thinning that occurred ∼125–113 Ma, except the porphyry rock was more evolved. This finding implies that their uniform magma chamber could provide sufficient metals and associated materials and even energy for the Donggou Mo deposit.
{"title":"Petrogenesis of the Taishanmiao A-type Granite in the Eastern Qinling Orogenic Belt: Implications for Late Cretaceous Tectonic Transition and Mineralization","authors":"Jun He, Yue Qi, Xinjie Fan, Fukun Chen","doi":"10.1086/713726","DOIUrl":"https://doi.org/10.1086/713726","url":null,"abstract":"Numerous late Mesozoic (Late Jurassic to Early Cretaceous) granitic plutons are present along the southern margin of the North China Craton (s-NCC), which belongs to the northernmost tectonic unit of the eastern Qinling orogenic belt. This tectonic unit records multiple tectonothermal and metallogenic events that occurred after continental collision of the North and South China Cratons in the late Early to Middle Triassic. This study reports the zircon U-Pb ages and the whole-rock and apatite geochemical compositions of the Taishanmiao granite, one of the largest late Mesozoic plutons in eastern Qinling. Three phases of this granite, emplaced between ∼125 and ∼113 Ma, are characterized by uniformly high SiO2 and K2O but low CaO, TFe2O3, and MgO contents and enriched Nd isotopic composition with initial εNd values of −13.5 to −12.7. Geochemical features reveal that these are A-type granites derived from partial melting mainly of ancient crust. Temperatures obtained from Ti-in-zircon thermometry and oxygen fugacity (fO2) data indicate that the magma(s) formed at high crystallization temperatures (∼800°–840°C) and fO2 levels (fayalite–magnetite–quartz [FMQ] buffer −1.5 to +1.5). Comparison with neighboring granites, such as the ∼150–130 Ma adakitic Heyu granite and the ∼115 Ma Donggou ore-bearing granite porphyry, implies that the Heyu granite formed from thickened crust and that the Donggou granite porphyry and Taishanmiao granite likely originated from a consistent magma system during the crustal thinning that occurred ∼125–113 Ma, except the porphyry rock was more evolved. This finding implies that their uniform magma chamber could provide sufficient metals and associated materials and even energy for the Donggou Mo deposit.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713726","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49377974","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Julien Michel, C. Lanteaume, Alexandre Lettéron, J. Kenter, M. Morsilli, J. Borgomano
The present study provides the baseline status of the spatial distribution of carbonate platforms for the Oligo-Miocene interval. The resulting global trend quantitatively shows the decreasing growth potential of shallow-marine carbonates toward higher paleolatitudes. Such a global trend provides a geological context and external constraints for local and regional interpretations of specific case studies. Furthermore, the direct relationship between carbonate accumulations and paleoclimatic regions shows that, using such a qualitative and quantitative data set for calibration, paleoceanographic models could be utilized for the prediction of the global distribution of carbonate stratigraphic architecture.
{"title":"Oligocene and Miocene Global Spatial Trends of Shallow-Marine Carbonate Architecture","authors":"Julien Michel, C. Lanteaume, Alexandre Lettéron, J. Kenter, M. Morsilli, J. Borgomano","doi":"10.1086/712186","DOIUrl":"https://doi.org/10.1086/712186","url":null,"abstract":"The present study provides the baseline status of the spatial distribution of carbonate platforms for the Oligo-Miocene interval. The resulting global trend quantitatively shows the decreasing growth potential of shallow-marine carbonates toward higher paleolatitudes. Such a global trend provides a geological context and external constraints for local and regional interpretations of specific case studies. Furthermore, the direct relationship between carbonate accumulations and paleoclimatic regions shows that, using such a qualitative and quantitative data set for calibration, paleoceanographic models could be utilized for the prediction of the global distribution of carbonate stratigraphic architecture.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712186","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47083151","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
New investigations of the Morrison Formation in the western Oklahoma panhandle reveal that the formation is approximately 60 m thick near Kenton, considerably less than historical measurements. We provide a new isopach map of the Morrison Formation at the basin margin and divide the formation into three new members based on geological, geochemical, petrographic, and paleontological characteristics. Each member is defined by unique depositional facies recording a variable climatic signal and the eventual progradation of the distributive fluvial system to the basin margin. Analogous depositional facies and lithologies of these units suggest they are lateral facies successions of the well-established members of the Colorado Plateau. The Cimarron Member is composed of numerous small, ephemeral, clastic lakes at the distal margin of the alluvial braid plain of the Tidwell Member. The Boise Member consists of numerous perennial limestone lakes at the distal edge of the Salt Wash Member distributive fluvial system. At the top of the member, the lake coalesced into a large lake, herein termed Lake Stovall. The Kenton Member records the progradation of the Brushy Basin Member distributive fluvial system to the basin margin in Oklahoma. Kenton Member fluvial deposition was by small, isolated, anastomosing fluvial channels and their associated splays. The gradual progradation of facies to the basin margin signifies that deposition in the foreland basin was geographically extensive and protracted. Dinosaur fossils excavated during the 1930s are placed into a stratigraphic framework. All quarries lie in the basal 6 m of the Kenton Member. Depositional facies and taphonomic data imply the dinosaurs died during a series of severe droughts. Bone dispersal and burial resulted from ephemeral flood splay events.
{"title":"Stratigraphy and Sedimentology of the Morrison Formation in the Western Panhandle of Oklahoma with Reference to the Historical Stovall Dinosaur Quarries","authors":"D. Richmond, Tyler C. Hunt, R. Cifelli","doi":"10.1086/712368","DOIUrl":"https://doi.org/10.1086/712368","url":null,"abstract":"New investigations of the Morrison Formation in the western Oklahoma panhandle reveal that the formation is approximately 60 m thick near Kenton, considerably less than historical measurements. We provide a new isopach map of the Morrison Formation at the basin margin and divide the formation into three new members based on geological, geochemical, petrographic, and paleontological characteristics. Each member is defined by unique depositional facies recording a variable climatic signal and the eventual progradation of the distributive fluvial system to the basin margin. Analogous depositional facies and lithologies of these units suggest they are lateral facies successions of the well-established members of the Colorado Plateau. The Cimarron Member is composed of numerous small, ephemeral, clastic lakes at the distal margin of the alluvial braid plain of the Tidwell Member. The Boise Member consists of numerous perennial limestone lakes at the distal edge of the Salt Wash Member distributive fluvial system. At the top of the member, the lake coalesced into a large lake, herein termed Lake Stovall. The Kenton Member records the progradation of the Brushy Basin Member distributive fluvial system to the basin margin in Oklahoma. Kenton Member fluvial deposition was by small, isolated, anastomosing fluvial channels and their associated splays. The gradual progradation of facies to the basin margin signifies that deposition in the foreland basin was geographically extensive and protracted. Dinosaur fossils excavated during the 1930s are placed into a stratigraphic framework. All quarries lie in the basal 6 m of the Kenton Member. Depositional facies and taphonomic data imply the dinosaurs died during a series of severe droughts. Bone dispersal and burial resulted from ephemeral flood splay events.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712368","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46992105","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：