The Tennessee River, a primary drainage of the southern Appalachians and significant sediment source for the Gulf of Mexico, is generally considered to be the product of captures that rerouted the river from a more direct gulfward course. Sedimentary and genetic evidence indicates that a paleo-Tennessee flowed into the Mobile Basin through the late Miocene, although alternate models propose other redirections of the river. We constrain the river course’s age by dating terraces near Pickwick, Tennessee, with cosmogenic 26Al/10Be isochron burial dating. We find that the river’s present path dates to at least the early Pliocene.
{"title":"The Pliocene-to-Present Course of the Tennessee River","authors":"W. Odom, D. Granger","doi":"10.1086/719951","DOIUrl":"https://doi.org/10.1086/719951","url":null,"abstract":"The Tennessee River, a primary drainage of the southern Appalachians and significant sediment source for the Gulf of Mexico, is generally considered to be the product of captures that rerouted the river from a more direct gulfward course. Sedimentary and genetic evidence indicates that a paleo-Tennessee flowed into the Mobile Basin through the late Miocene, although alternate models propose other redirections of the river. We constrain the river course’s age by dating terraces near Pickwick, Tennessee, with cosmogenic 26Al/10Be isochron burial dating. We find that the river’s present path dates to at least the early Pliocene.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"325 - 333"},"PeriodicalIF":1.8,"publicationDate":"2022-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45083601","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}
The Eocene Tallahatta Formation forms part of the Tallahatta-Winona aquifer, which is part of the lower Claiborne confining unit of the Mississippi Embayment. A thorough understanding of the distribution of natural resources within the Tallahatta is limited by a lack of detailed studies at the outcrop and pore scales. In this study, we integrate particle size, petrographic, lithofacies, and geochemical analyses to interpret depositional environments, sequence stratigraphy, provenance, and diagenetic history from outcrops in Grenada County, Mississippi. Lithofacies include unprotected sand flat, tidal channel, open tidal mudflat, protected tidal flat, lower and middle shoreface, offshore transition, and offshore shelf settings. Lithofacies associations indicate that the Tallahatta consists of at least two parasequences within a falling stage systems tract (FSST). This is the first report on FSST strata from the Tallahatta Formation. Petrographic and geochemical results show that sediments were sourced from Precambrian Laurentian basement, recycled Appalachian basin sediment, and the Appalachian hinterland. Upsection changes in geochemistry represent an increase in contributions from recycled Appalachian basin sediment and progressive weathering of Acadian orogeny elements. Geochemistry also suggests that potentially economically important Ti-rich minerals are concentrated in tidal flat facies and scarce in shoreface facies. Primary porosity and bioturbation exert the greatest influence on the high porosity within many lithofacies. Early diagenesis also included detrital clay coating of framework grains, deposition of fecal pellets, minor chemical weathering, and hematite precipitation. Fecal pellets were altered to glauconite and opal cement was precipitated shortly after deposition or during early burial. Shrinkage of fecal pellets during glauconitization introduced minor moldic porosity. Limited burial produced minor physical compaction and only slightly reduced porosity. Abundant hematite precipitation during exhumation greatly decreased porosity locally.
{"title":"Sedimentological and Geochemical Analysis of the Eocene Tallahatta Formation in Northern Mississippi, USA","authors":"Husamaldeen Zubi, B. Platt, Jennifer N. Gifford","doi":"10.1086/720183","DOIUrl":"https://doi.org/10.1086/720183","url":null,"abstract":"The Eocene Tallahatta Formation forms part of the Tallahatta-Winona aquifer, which is part of the lower Claiborne confining unit of the Mississippi Embayment. A thorough understanding of the distribution of natural resources within the Tallahatta is limited by a lack of detailed studies at the outcrop and pore scales. In this study, we integrate particle size, petrographic, lithofacies, and geochemical analyses to interpret depositional environments, sequence stratigraphy, provenance, and diagenetic history from outcrops in Grenada County, Mississippi. Lithofacies include unprotected sand flat, tidal channel, open tidal mudflat, protected tidal flat, lower and middle shoreface, offshore transition, and offshore shelf settings. Lithofacies associations indicate that the Tallahatta consists of at least two parasequences within a falling stage systems tract (FSST). This is the first report on FSST strata from the Tallahatta Formation. Petrographic and geochemical results show that sediments were sourced from Precambrian Laurentian basement, recycled Appalachian basin sediment, and the Appalachian hinterland. Upsection changes in geochemistry represent an increase in contributions from recycled Appalachian basin sediment and progressive weathering of Acadian orogeny elements. Geochemistry also suggests that potentially economically important Ti-rich minerals are concentrated in tidal flat facies and scarce in shoreface facies. Primary porosity and bioturbation exert the greatest influence on the high porosity within many lithofacies. Early diagenesis also included detrital clay coating of framework grains, deposition of fecal pellets, minor chemical weathering, and hematite precipitation. Fecal pellets were altered to glauconite and opal cement was precipitated shortly after deposition or during early burial. Shrinkage of fecal pellets during glauconitization introduced minor moldic porosity. Limited burial produced minor physical compaction and only slightly reduced porosity. Abundant hematite precipitation during exhumation greatly decreased porosity locally.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"253 - 282"},"PeriodicalIF":1.8,"publicationDate":"2022-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46393402","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}
During the Cretaceous, the concurrence of changing paleoceanographic and paleoclimatic conditions, coupled with variations in eustatic sea level, contributed to episodes of globally widespread deposition of organic matter (OM)–rich marine sediments collectively termed oceanic anoxic events (OAEs). Here, we aim to investigate the response of a lower Aptian hemipelagic sequence from the northeastern Iberian margin in the context of OAE1a. Stable–carbon isotope (δ13Corg) data are consistent with the pattern reported for the end of carbon isotope segment C5 within OAE1a. Moreover, high sediment accumulation rates (bulk: ∼37.13 cm/ky, wet: ∼63.29 cm/ky) permit the establishment of refined details suitable for precise chemostratigraphic correlations. We recognized three distinct lithologic units. Within unit 1, variable pulses of fluvial fluxes explain the alternating lithology, with limestones depicting the least terrestrially influenced end member and marlstones representing episodes of highest terrigenous input. In the ensuing marlstone-dominated unit 2 interval, results show relatively higher OM, redox-sensitive trace elements (RSTEs), P, Fe, Al, Si, and Ti values than before, thus suggesting an increase in runoff with quasi-permanent eutrophic surface waters and continuous oxygen-deprived conditions, but without a fully anoxic phase, as benthic fauna, while relatively reduced, are present throughout. Unit 3 registers limestones impoverished in OM, Al, Si, Ti, P, and Fe, with a lower relative proportion of autochthonous to allochthonous OM, indicating a reduction in runoff and surface water fertility linked to drier climate conditions. Simultaneous changes in microfacies, with coarser packed biosparites, reduced planktonic foraminifera, and an increase in benthic taxa, imply shallowing of the basin, similar to that reported for sequences in the neighboring Basque-Cantabrian Basin synchronous with the negative δ13Corg shift heralding the end of segment C5.
{"title":"Demise of Organic Matter–Rich Facies and Changing Paleoenvironmental Conditions Associated with the End of Carbon Isotope Segment C5 of Oceanic Anoxic Event 1a in the North and Northeastern Iberian Peninsula","authors":"J. Socorro, F. Maurrasse","doi":"10.1086/718834","DOIUrl":"https://doi.org/10.1086/718834","url":null,"abstract":"During the Cretaceous, the concurrence of changing paleoceanographic and paleoclimatic conditions, coupled with variations in eustatic sea level, contributed to episodes of globally widespread deposition of organic matter (OM)–rich marine sediments collectively termed oceanic anoxic events (OAEs). Here, we aim to investigate the response of a lower Aptian hemipelagic sequence from the northeastern Iberian margin in the context of OAE1a. Stable–carbon isotope (δ13Corg) data are consistent with the pattern reported for the end of carbon isotope segment C5 within OAE1a. Moreover, high sediment accumulation rates (bulk: ∼37.13 cm/ky, wet: ∼63.29 cm/ky) permit the establishment of refined details suitable for precise chemostratigraphic correlations. We recognized three distinct lithologic units. Within unit 1, variable pulses of fluvial fluxes explain the alternating lithology, with limestones depicting the least terrestrially influenced end member and marlstones representing episodes of highest terrigenous input. In the ensuing marlstone-dominated unit 2 interval, results show relatively higher OM, redox-sensitive trace elements (RSTEs), P, Fe, Al, Si, and Ti values than before, thus suggesting an increase in runoff with quasi-permanent eutrophic surface waters and continuous oxygen-deprived conditions, but without a fully anoxic phase, as benthic fauna, while relatively reduced, are present throughout. Unit 3 registers limestones impoverished in OM, Al, Si, Ti, P, and Fe, with a lower relative proportion of autochthonous to allochthonous OM, indicating a reduction in runoff and surface water fertility linked to drier climate conditions. Simultaneous changes in microfacies, with coarser packed biosparites, reduced planktonic foraminifera, and an increase in benthic taxa, imply shallowing of the basin, similar to that reported for sequences in the neighboring Basque-Cantabrian Basin synchronous with the negative δ13Corg shift heralding the end of segment C5.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"133 - 170"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45682416","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}
M. Azer, A. A. Surour, A. Madani, M. Ren, A. A. El-fatah
In this work, we present new field observations, geochemical data, mineral chemistry, and interpretations from the late Neoproterozoic El-Bakriya mafic intrusion in the central Eastern Desert of Egypt to enhance our knowledge and understanding of the formation of the juvenile continental crust in the Arabian-Nubian Shield (ANS). The field relationships indicate that the gabbroic intrusion is younger than the syntectonic granodiorite but older than the postcollisional El-Bakriya granites. The El-Bakriya mafic intrusion is neither deformed nor metamorphosed and preserves typical primary mineralogical features and texture. The intrusion is composed of three main varieties of gabbro with prominent gradational contacts, namely, troctolite, olivine gabbro, and hornblende gabbro. Both megascopic and imperceptible layering is recognized in the intrusion. El-Bakriya gabbros are characterized by an enrichment in large-ion lithophile elements relative to high field strength elements and a noticeable alkaline affinity owing to fractional crystallization. Different varieties of gabbro show continuous linear composition trends in all major- and trace-element variation diagrams, indicating a cogenetic origin source. The initial magma was an alkaline mafic melt that derived from the mantle with minor crustal assimilation before its emplacement in the upper crust in a stable postcollisional cratonic setting. Mineralogical and geochemical data are combined evidence of an asthenosphere upwelling and lithosphere delamination scenario for the evolution of the El-Bakriya mafic intrusion. On the basis of geothermobarometric calculations, the gabbroic rocks crystallized at a pressure of 5–6.8 kbar and a temperature of 750°C–900°C. The petrological, geochemical, and mineralogical characteristics of the El-Bakriya mafic intrusion are akin to those of the Egyptian postcollisional younger gabbros. These gabbros are totally different from the metamorphosed oceanic and arc-related gabbros, which constitute the earlier juvenile crustal components that formed during the development of the ANS in the late Neoproterozoic.
{"title":"Mineralogical and Geochemical Constraints on the Postcollisional Mafic Magmatism in the Arabian-Nubian Shield: An Example from the El-Bakriya Area, Central Eastern Desert, Egypt","authors":"M. Azer, A. A. Surour, A. Madani, M. Ren, A. A. El-fatah","doi":"10.1086/719130","DOIUrl":"https://doi.org/10.1086/719130","url":null,"abstract":"In this work, we present new field observations, geochemical data, mineral chemistry, and interpretations from the late Neoproterozoic El-Bakriya mafic intrusion in the central Eastern Desert of Egypt to enhance our knowledge and understanding of the formation of the juvenile continental crust in the Arabian-Nubian Shield (ANS). The field relationships indicate that the gabbroic intrusion is younger than the syntectonic granodiorite but older than the postcollisional El-Bakriya granites. The El-Bakriya mafic intrusion is neither deformed nor metamorphosed and preserves typical primary mineralogical features and texture. The intrusion is composed of three main varieties of gabbro with prominent gradational contacts, namely, troctolite, olivine gabbro, and hornblende gabbro. Both megascopic and imperceptible layering is recognized in the intrusion. El-Bakriya gabbros are characterized by an enrichment in large-ion lithophile elements relative to high field strength elements and a noticeable alkaline affinity owing to fractional crystallization. Different varieties of gabbro show continuous linear composition trends in all major- and trace-element variation diagrams, indicating a cogenetic origin source. The initial magma was an alkaline mafic melt that derived from the mantle with minor crustal assimilation before its emplacement in the upper crust in a stable postcollisional cratonic setting. Mineralogical and geochemical data are combined evidence of an asthenosphere upwelling and lithosphere delamination scenario for the evolution of the El-Bakriya mafic intrusion. On the basis of geothermobarometric calculations, the gabbroic rocks crystallized at a pressure of 5–6.8 kbar and a temperature of 750°C–900°C. The petrological, geochemical, and mineralogical characteristics of the El-Bakriya mafic intrusion are akin to those of the Egyptian postcollisional younger gabbros. These gabbros are totally different from the metamorphosed oceanic and arc-related gabbros, which constitute the earlier juvenile crustal components that formed during the development of the ANS in the late Neoproterozoic.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"209 - 230"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44656940","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}
Formation of sedimentary opal-AG in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures and textural observations in thousands of opal nodules from Lightning Ridge (New South Wales, Australia) to further constrain the timing and depositional environment of opal-AG. Satellite imaging identifies river channels—now marked by pedogenic silcrete—associated with an inland river delta as the intermittent source of water that drove localized deep weathering and silicification in reactive volcanogenic sediments. Variably mineralized weathering profiles show evidence for wet/dry cycling that controlled redox and pH fluctuations critical in the conversion of detrital feldspar to kaolinite, release of silica, formation of opaline silica spheres, and opal mineralization during the initial intense phase of weathering. Subsequent less intense weathering under consistently oxidizing conditions modified the weathering profiles but produced little further opal. Textures in oriented opal nodules indicate that cavities filled rapidly under changing Eh-pH. Opal nodules formed when opaline silica spheres, nucleated and grown in perched groundwater bodies, accumulated and drained into cracks and dissolution cavities in underlying claystone. Drier conditions promoted sol-gel ripening processes that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest that it formed through oxidation, diffusion, and leaching in common opaline Si gel during periods of low water flux and was rate limited by the solidification of the Si gel. Ostwald ripening increased silica sphere size to produce the characteristic “play of color.” Opal formation largely ceased once acidification potential was lost or all feldspar had been converted to kaolinite.
{"title":"Formation of Sediment-Hosted Opal-AG at Lightning Ridge (New South Wales, Australia): Refining the Deep Weathering Model","authors":"J. Herrmann, R. Maas","doi":"10.1086/718833","DOIUrl":"https://doi.org/10.1086/718833","url":null,"abstract":"Formation of sedimentary opal-AG in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures and textural observations in thousands of opal nodules from Lightning Ridge (New South Wales, Australia) to further constrain the timing and depositional environment of opal-AG. Satellite imaging identifies river channels—now marked by pedogenic silcrete—associated with an inland river delta as the intermittent source of water that drove localized deep weathering and silicification in reactive volcanogenic sediments. Variably mineralized weathering profiles show evidence for wet/dry cycling that controlled redox and pH fluctuations critical in the conversion of detrital feldspar to kaolinite, release of silica, formation of opaline silica spheres, and opal mineralization during the initial intense phase of weathering. Subsequent less intense weathering under consistently oxidizing conditions modified the weathering profiles but produced little further opal. Textures in oriented opal nodules indicate that cavities filled rapidly under changing Eh-pH. Opal nodules formed when opaline silica spheres, nucleated and grown in perched groundwater bodies, accumulated and drained into cracks and dissolution cavities in underlying claystone. Drier conditions promoted sol-gel ripening processes that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest that it formed through oxidation, diffusion, and leaching in common opaline Si gel during periods of low water flux and was rate limited by the solidification of the Si gel. Ostwald ripening increased silica sphere size to produce the characteristic “play of color.” Opal formation largely ceased once acidification potential was lost or all feldspar had been converted to kaolinite.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"77 - 110"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41600916","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}
M. Ghoneim, A. Abdel-Karim, M. A. Anbar, Azza Nageib, S. El-shafei
Postcollisional magmatism is widely distributed in southern Sinai, the extreme northern part of the Neoproterozoic Arabian-Nubian Shield. This article deals with mineral and whole-rock chemistry of postcollisional syenogranites and associated volcanic rocks from three localities in southern Sinai: Iqna Sharay’a, Rusis-Rutig, and Um Shuki–Abu Khusheib. The studied volcanic rocks have compositions between rhyolites and dacites with minor andesite. The whole-rock chemical compositions of the investigated rock types together with the biotite chemistry are consistent with high-K calc-alkaline and alkaline/peralkaline magma. The studied syenogranites and most volcanic rocks are more akin to anorogenic alkaline within-plate environments. Only a few samples of Um Shuki–Abu Khusheib volcanic rocks display some characteristics of orogenic arc-type environments. The high-K calc-alkaline to alkaline affinity and the relative enrichments in large ion lithophile elements (especially K, Rb, and Ba) and light rare earth elements together with a significant negative Eu anomaly imply that the studied granites and volcanic rocks were generated by partial melting of lower to middle crustal materials accompanied by the underplated mafic magma produced in the lithospheric mantle (convective diffusion). This convective diffusion describes a specific scenario of active chemical interaction between mafic and silicic magmas in order to explain formation of voluminous high-K calc-alkaline and alkaline/peralkaline magmatism in postcollisional tectonic environments. The enhanced temperatures of A-type silicic magmas of more than 1000°C suggest that magma generation could occur even at the depth of the uppermost lithospheric mantle.
{"title":"Petrogenesis of Postcollisional High-K Calc-Alkaline and Alkaline Magmatism in Southern Sinai, Egypt: The Role of Crustal Anatexis Combined with Convective Diffusion","authors":"M. Ghoneim, A. Abdel-Karim, M. A. Anbar, Azza Nageib, S. El-shafei","doi":"10.1086/718832","DOIUrl":"https://doi.org/10.1086/718832","url":null,"abstract":"Postcollisional magmatism is widely distributed in southern Sinai, the extreme northern part of the Neoproterozoic Arabian-Nubian Shield. This article deals with mineral and whole-rock chemistry of postcollisional syenogranites and associated volcanic rocks from three localities in southern Sinai: Iqna Sharay’a, Rusis-Rutig, and Um Shuki–Abu Khusheib. The studied volcanic rocks have compositions between rhyolites and dacites with minor andesite. The whole-rock chemical compositions of the investigated rock types together with the biotite chemistry are consistent with high-K calc-alkaline and alkaline/peralkaline magma. The studied syenogranites and most volcanic rocks are more akin to anorogenic alkaline within-plate environments. Only a few samples of Um Shuki–Abu Khusheib volcanic rocks display some characteristics of orogenic arc-type environments. The high-K calc-alkaline to alkaline affinity and the relative enrichments in large ion lithophile elements (especially K, Rb, and Ba) and light rare earth elements together with a significant negative Eu anomaly imply that the studied granites and volcanic rocks were generated by partial melting of lower to middle crustal materials accompanied by the underplated mafic magma produced in the lithospheric mantle (convective diffusion). This convective diffusion describes a specific scenario of active chemical interaction between mafic and silicic magmas in order to explain formation of voluminous high-K calc-alkaline and alkaline/peralkaline magmatism in postcollisional tectonic environments. The enhanced temperatures of A-type silicic magmas of more than 1000°C suggest that magma generation could occur even at the depth of the uppermost lithospheric mantle.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"111 - 132"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46856057","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}
E. Garzanti, G. Bayon, Pedro A. Dinis, P. Vermeesch, G. Pastore, A. Resentini, M. Barbarano, L. Ncube, H. J. van Niekerk
Elemental geochemistry, Nd isotopes, clay minerals, and U-Pb zircon ages integrated by petrographic and heavy-mineral data offer a multiproxy panorama of mud and sand composition across the Zambezi sediment-routing system. Detrital zircon geochronology highlights the four major episodes of crustal growth in southern Africa: Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial/winter-dry climate of the Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling (Uppermost Zambezi), supply from Karoo basalts (Upper Zambezi), and first-cycle provenance from Precambrian basements (Lower Zambezi). Mildly negative for sediments derived from mafic granulites, gabbros, and basalts, εNd values are most negative for sand derived from cratonic gneisses. Intrasample variability among cohesive mud, very coarse silt, and sand is principally caused by the concentration of Nd-rich monazite in the fine tail of the size distribution. The settling-equivalence effect also explains deviations from the theoretical relationship between εNd and TNd,DM model ages, suggesting that monazite carries a more negative εNd signal than less dense and less durable heavy minerals. Elemental geochemistry and Nd isotopes reveal that the Mazowe-Luenha river system contributes most of the sediment reaching the Zambezi delta today, with minor supply from the Shire River. Sediment yields and erosion rates are much lower on the low-relief Kalahari Plateau than in rugged Precambrian terranes. On the plateau, mineralogical and geochemical indices testify to extensive breakdown of feldspars and garnet unjustified by the present dry climate. Detrital kaolinite is recycled by incision of Cretaceous–Cenozoic paleosols even in the wetter lower catchment, where inefficient hydrolysis is testified to by abundant fresh feldspars and undepleted Ca and Na. Mud geochemistry and surficial corrosion of ferromagnesian minerals indicate that, at present, weathering increases only slightly downstream the Zambezi River.
{"title":"The Segmented Zambezi Sedimentary System from Source to Sink: 2. Geochemistry, Clay Minerals, and Detrital Geochronology","authors":"E. Garzanti, G. Bayon, Pedro A. Dinis, P. Vermeesch, G. Pastore, A. Resentini, M. Barbarano, L. Ncube, H. J. van Niekerk","doi":"10.1086/719166","DOIUrl":"https://doi.org/10.1086/719166","url":null,"abstract":"Elemental geochemistry, Nd isotopes, clay minerals, and U-Pb zircon ages integrated by petrographic and heavy-mineral data offer a multiproxy panorama of mud and sand composition across the Zambezi sediment-routing system. Detrital zircon geochronology highlights the four major episodes of crustal growth in southern Africa: Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial/winter-dry climate of the Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling (Uppermost Zambezi), supply from Karoo basalts (Upper Zambezi), and first-cycle provenance from Precambrian basements (Lower Zambezi). Mildly negative for sediments derived from mafic granulites, gabbros, and basalts, εNd values are most negative for sand derived from cratonic gneisses. Intrasample variability among cohesive mud, very coarse silt, and sand is principally caused by the concentration of Nd-rich monazite in the fine tail of the size distribution. The settling-equivalence effect also explains deviations from the theoretical relationship between εNd and TNd,DM model ages, suggesting that monazite carries a more negative εNd signal than less dense and less durable heavy minerals. Elemental geochemistry and Nd isotopes reveal that the Mazowe-Luenha river system contributes most of the sediment reaching the Zambezi delta today, with minor supply from the Shire River. Sediment yields and erosion rates are much lower on the low-relief Kalahari Plateau than in rugged Precambrian terranes. On the plateau, mineralogical and geochemical indices testify to extensive breakdown of feldspars and garnet unjustified by the present dry climate. Detrital kaolinite is recycled by incision of Cretaceous–Cenozoic paleosols even in the wetter lower catchment, where inefficient hydrolysis is testified to by abundant fresh feldspars and undepleted Ca and Na. Mud geochemistry and surficial corrosion of ferromagnesian minerals indicate that, at present, weathering increases only slightly downstream the Zambezi River.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"171 - 208"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42015257","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}
T. Ahmad, I. Bhat, Tsuyoshi Tanaka, M. Bickle, Y. Asahara, H. Chapman, H. Sachan
The Tso Morari Crystalline Complex (TMC), eastern Ladakh, is marked by the presence of eclogites as boudins and lenses within the Puga Formation. These eclogites are composed of garnet, omphacite, amphibole, phengite, glaucophane, quartz, and iron oxide, with rare coesite inclusions in garnet reflecting ultrahigh-pressure metamorphic characteristics. Geochemically, TMC eclogites have high Fe-Ti basaltic compositions and classify as subalkaline tholeiites. Rare earth element and multielement diagrams display enriched patterns similar to enriched mid-ocean ridge basalt coupled with perturbed large ion lithophile elements and higher whole-rock (87Sr/86Sr) ratios (0.70884 to 0.72721) reflecting the possible influence of postcrystallization processes rather than variable interaction with host granite gneisses (87Sr/86Sr ratio: ∼0.73901). To evaluate the existing protolith possibilities, we calculated εNd(t=289Ma) values (+1.9 to +9.5) and εNd(t=140Ma) values (+1.1 to +8.9) of TMC eclogites; both indicate their derivation from depleted-mantle sources. The εNd(t=289Ma) values of the early Permian enriched Panjal volcanics of Kashmir Valley (−5.3 to +1.3) and Phe volcanics of Zanskar Himalaya (−7.4 to −1.1) are very different from TMC eclogites. However, the εNd(t=289Ma) values of TMC eclogites are similar to the depleted Panjal volcanics (+0.3 to +4.3). Similarly, the εNd(t=140Ma) values of the TMC eclogites closely resemble those of the adjoining Ladakh ophiolites, such as the Nidar-Spongtang-Shergol-Dras ophiolitic mafic rocks (+5.1 to +9.9). These observations partly negate the existing hypothesis of enriched Panjal and Phe volcanics for being the protolith for the TMC eclogites. Thus, we propose that the protolith for the TMC eclogites could be represented by the subducted portion of the early Permian depleted Panjal volcanics and Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks, subducted to eclogite-grade metamorphism (around ~53 Ma) and were subsequently tectonically accreted to the obducting Indian continental crust during their exhumation.
{"title":"Tso Morari Eclogites, Eastern Ladakh: Isotopic and Elemental Constraints on Their Protolith, Genesis, and Tectonic Setting","authors":"T. Ahmad, I. Bhat, Tsuyoshi Tanaka, M. Bickle, Y. Asahara, H. Chapman, H. Sachan","doi":"10.1086/719333","DOIUrl":"https://doi.org/10.1086/719333","url":null,"abstract":"The Tso Morari Crystalline Complex (TMC), eastern Ladakh, is marked by the presence of eclogites as boudins and lenses within the Puga Formation. These eclogites are composed of garnet, omphacite, amphibole, phengite, glaucophane, quartz, and iron oxide, with rare coesite inclusions in garnet reflecting ultrahigh-pressure metamorphic characteristics. Geochemically, TMC eclogites have high Fe-Ti basaltic compositions and classify as subalkaline tholeiites. Rare earth element and multielement diagrams display enriched patterns similar to enriched mid-ocean ridge basalt coupled with perturbed large ion lithophile elements and higher whole-rock (87Sr/86Sr) ratios (0.70884 to 0.72721) reflecting the possible influence of postcrystallization processes rather than variable interaction with host granite gneisses (87Sr/86Sr ratio: ∼0.73901). To evaluate the existing protolith possibilities, we calculated εNd(t=289Ma) values (+1.9 to +9.5) and εNd(t=140Ma) values (+1.1 to +8.9) of TMC eclogites; both indicate their derivation from depleted-mantle sources. The εNd(t=289Ma) values of the early Permian enriched Panjal volcanics of Kashmir Valley (−5.3 to +1.3) and Phe volcanics of Zanskar Himalaya (−7.4 to −1.1) are very different from TMC eclogites. However, the εNd(t=289Ma) values of TMC eclogites are similar to the depleted Panjal volcanics (+0.3 to +4.3). Similarly, the εNd(t=140Ma) values of the TMC eclogites closely resemble those of the adjoining Ladakh ophiolites, such as the Nidar-Spongtang-Shergol-Dras ophiolitic mafic rocks (+5.1 to +9.9). These observations partly negate the existing hypothesis of enriched Panjal and Phe volcanics for being the protolith for the TMC eclogites. Thus, we propose that the protolith for the TMC eclogites could be represented by the subducted portion of the early Permian depleted Panjal volcanics and Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks, subducted to eclogite-grade metamorphism (around ~53 Ma) and were subsequently tectonically accreted to the obducting Indian continental crust during their exhumation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"231 - 252"},"PeriodicalIF":1.8,"publicationDate":"2022-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60725866","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}
Tafoni are a type of cavernous weathering that is found in a variety of rock types and locations around the world. Tafoni have been documented in a number of climatic zones ranging from hot and cold deserts to moist coastal environments. Despite the widespread distribution of tafoni, the major processes controlling tafoni weathering are not well understood and are still a matter of discussion. This study addresses the frequent distribution of well-developed tafoni in the cold, arid environment of the inland mountain range of central Dronning Maud Land, Antarctica. The aim is to document and characterize the nature of tafoni present in Gjelsvikfjella (2°E) eastward to Filchnerfjella (8°E) and to discuss formation processes. The cavities occur in groups and are typically spherical to oval shaped. They range in diameter and depth from 1 dm up to 1.5 m. The cold, arid environment of this region favors mechanical weathering mechanisms such as freeze-thaw actions and wind abrasion. Furthermore, the structural, textural, and mineralogical properties of the parent rock can potentially have a strong control on weathering and cavity development. Observed tafoni are typically formed in massive granitoid intrusives and granitic gneisses and migmatites. Chemical dissolution of pyroxene to iddingsite and radiation from rare earth element–bearing accessory minerals cause microfracturing, which facilitates freeze-thaw actions and accordingly enhances the weathering.
{"title":"Cavernous Decay of Granite and Granitic Gneiss, Central Dronning Maud Land, Antarctica","authors":"A. Engvik, S. Elvevold, P. Myhre","doi":"10.1086/718805","DOIUrl":"https://doi.org/10.1086/718805","url":null,"abstract":"Tafoni are a type of cavernous weathering that is found in a variety of rock types and locations around the world. Tafoni have been documented in a number of climatic zones ranging from hot and cold deserts to moist coastal environments. Despite the widespread distribution of tafoni, the major processes controlling tafoni weathering are not well understood and are still a matter of discussion. This study addresses the frequent distribution of well-developed tafoni in the cold, arid environment of the inland mountain range of central Dronning Maud Land, Antarctica. The aim is to document and characterize the nature of tafoni present in Gjelsvikfjella (2°E) eastward to Filchnerfjella (8°E) and to discuss formation processes. The cavities occur in groups and are typically spherical to oval shaped. They range in diameter and depth from 1 dm up to 1.5 m. The cold, arid environment of this region favors mechanical weathering mechanisms such as freeze-thaw actions and wind abrasion. Furthermore, the structural, textural, and mineralogical properties of the parent rock can potentially have a strong control on weathering and cavity development. Observed tafoni are typically formed in massive granitoid intrusives and granitic gneisses and migmatites. Chemical dissolution of pyroxene to iddingsite and radiation from rare earth element–bearing accessory minerals cause microfracturing, which facilitates freeze-thaw actions and accordingly enhances the weathering.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"63 - 76"},"PeriodicalIF":1.8,"publicationDate":"2022-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44779091","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}
Mao-Lan Zhang, Z. Gong, Yu Zhou, Yanmei Liu, Jun Li, Chengmin Huang
Paleosols that developed on large igneous province flood basalts provide a valuable opportunity to quantitatively reconstruct the terrestrial paleoclimate during a given period. This study uses a newly constructed basaltic soil–climate function to obtain terrestrial paleoclimatic data following the termination of the Emeishan basalt eruption in the late Lopingian (∼253 Ma). The constructed relationships are mean annual temperature of 32.81e−0.4166X (R2=0.55, standard error [SE]=4.6°C) and mean annual precipitation of −875.5ln(X)+1792 (R2=0.69, SE=328 mm), where X=SiO2/(Al2O3+Fe2O3) (the Saf index). We collected samples from nine paleosol profiles developed on the last-phase Emeishan basalts and conducted field observations as well as micromorphological, mineral composition, and geochemical analyses. On the basis of these paleosols, we reconstructed the regional mean temperature and precipitation to be 18.9°C±4.6°C and 1546±328 mm, respectively. The regional climate was cooler than those in modern equatorial regions of similar latitudes. The reduced CO2 concentration in the atmosphere resulting from rapid weathering of Emeishan basalts in a tropical climate could have played a dominant role in causing the regional cool terrestrial paleoclimate following the termination of the Emeishan basalt eruption.
{"title":"Cooler Equatorial Climate in the Late Lopingian Estimated from Paleosols Developed on Emeishan Basalts","authors":"Mao-Lan Zhang, Z. Gong, Yu Zhou, Yanmei Liu, Jun Li, Chengmin Huang","doi":"10.1086/718351","DOIUrl":"https://doi.org/10.1086/718351","url":null,"abstract":"Paleosols that developed on large igneous province flood basalts provide a valuable opportunity to quantitatively reconstruct the terrestrial paleoclimate during a given period. This study uses a newly constructed basaltic soil–climate function to obtain terrestrial paleoclimatic data following the termination of the Emeishan basalt eruption in the late Lopingian (∼253 Ma). The constructed relationships are mean annual temperature of 32.81e−0.4166X (R2=0.55, standard error [SE]=4.6°C) and mean annual precipitation of −875.5ln(X)+1792 (R2=0.69, SE=328 mm), where X=SiO2/(Al2O3+Fe2O3) (the Saf index). We collected samples from nine paleosol profiles developed on the last-phase Emeishan basalts and conducted field observations as well as micromorphological, mineral composition, and geochemical analyses. On the basis of these paleosols, we reconstructed the regional mean temperature and precipitation to be 18.9°C±4.6°C and 1546±328 mm, respectively. The regional climate was cooler than those in modern equatorial regions of similar latitudes. The reduced CO2 concentration in the atmosphere resulting from rapid weathering of Emeishan basalts in a tropical climate could have played a dominant role in causing the regional cool terrestrial paleoclimate following the termination of the Emeishan basalt eruption.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"23 - 44"},"PeriodicalIF":1.8,"publicationDate":"2021-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42393913","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}
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