Geological Society of America Bulletin最新文献

{"title":"Transport and deposition of the 39.8 ka Campanian Ignimbrite large-scale pyroclastic density currents (Italy)","authors":"Aurora Silleni, Guido Giordano, M. Ort, R. Isaia","doi":"10.1130/b37500.1","DOIUrl":"https://doi.org/10.1130/b37500.1","url":null,"abstract":"Processes of transport and deposition of large-scale pyroclastic density currents, which transport 102−103 km3 of material, are still poorly understood. The volcanic explosivity index (VEI) 7 Campanian Ignimbrite pyroclastic density current is one such poorly understood case that is subject to lively scientific debates about its origin and processes. The Campanian Ignimbrite is related to the 39.8 ka eruption that formed the partly submarine and active Campi Flegrei caldera in Italy. The related pyroclastic density current extended over an area of more than 6000 km2 on land around the caldera, with a final runout of ∼75 km, and surmounted mountain ridges higher than 1000 m above sea level, even at distal reaches. We used physical characteristics of the ignimbrite to understand the transport and emplacement dynamics of the Campanian Ignimbrite pyroclastic density current. Thickness, mass, and clast-size distributions across paleotopography were used to infer that the main transport system was a dilute, density-stratified pyroclastic density current, which was at least 1.5 km thick, which was short-lived (possibly as short as ∼20 min), and which had a mass flow rate of ∼1011 kg/s. At its base, high-concentration undercurrents formed and interacted with the rough paleotopography, laying down a low-aspect-ratio ignimbrite sheet when on flat plains, but generating back-flows from ridges and channeling in paleovalleys. The deposit is predominantly a massive valley-pond ignimbrite facies, even across rough mountain terrains. The absence of veneer facies on steep slopes reflects the near-Newtonian rheology of the undercurrents, interpreted as fluidized dense granular flows. This work advances the interpretations of pyroclastic density current deposits, which do not necessarily directly reflect conditions in the transport system.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"118 s1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141002169","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stratigraphic and U-Pb zircon age constraints on the timing of the Yanliao Biota in northern China","authors":"Zu-Yang Zou, Qiang Ma, Yi-Gang Xu, Liang Liu, Xiaoping Xia, Chuangguo Yang","doi":"10.1130/b37497.1","DOIUrl":"https://doi.org/10.1130/b37497.1","url":null,"abstract":"The Yanliao Biota from northern China is one of the most famous Mesozoic terrestrial lagerstätten in the world, with well-preserved fossil records in the Jurassic volcanic-sedimentary Ningcheng and Jianchang basins. However, the temporal evolution of the Yanliao Biota remains controversial, mainly due to the confusing stratigraphic framework and the unclear age of fossiliferous deposits in the Ningcheng basin. To address this issue, we carried out detailed field investigations and secondary ion mass spectrometry (SIMS) U-Pb age dating of zircons on fossiliferous and related strata in the southern part of the Ningcheng basin. The Jurassic infill of this area is composed of, in ascending order, fossil-bearing tuffaceous clastic rocks (unit 1), andesite and pyroclastic rocks (unit 2), fossil-bearing clastic rocks interlayered with minor volcanic rocks (unit 3), and andesite and pyroclastic rocks (unit 4). Integration of stratigraphic and geochronological data indicates that the Yanliao Biota in the Ningcheng basin appeared no later than ca. 163 Ma and lasted until ca. 156 Ma. A comparison with other analogous basins in northern China (e.g., the Luanping, Jianchang, and Beipiao basins) leads to the conclusion that the Yanliao Biota evolved in two phases and lasted for more than 11 m.y.: the first phase from ca. 167 Ma to ca. 163 Ma in the middle Bathonian to middle Callovian, followed by the second phase between ca. 161 Ma and ca. 156 Ma in the lower and middle Oxfordian.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"159 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141002110","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Unveiling the connection between lode gold mineralization and the metamorphic core complex evolution from the large Yangzhaiyu gold deposit, North China Craton","authors":"Wen-Tian Li, Shao-Yong Jiang","doi":"10.1130/b37424.1","DOIUrl":"https://doi.org/10.1130/b37424.1","url":null,"abstract":"The Xiaoqinling gold province, located in the Neoarchean−Paleoproterozoic uplifted footwalls of the Xiaoqinling metamorphic core complex (XMCC), is one of China’s largest gold producers; however, achieving a consensus regarding their metallogenic model remains elusive. Scheelite is an indicator mineral that commonly occurs in lode gold deposits worldwide used to recognize deposit types and understand hydrothermal evolution and the origin of features. Xenotime, monazite, and rutile are common hydrothermal minerals in association with lode gold deposits worldwide. Here, we provide textual, in situ U-Pb geochronology of xenotime, monazite, and rutile, and in situ elemental and Sr-Nd isotopic compositions of scheelite within different stages from the large Yangzhaiyu lode gold deposit, aiming to elucidate its genesis and, for the first time, establish a holistic correlation between the lode gold mineralization and the evolution of the XMCC. Notably low εNd(t) values (−30.7 to −23.7), high 87Sr/86Sr ratios (0.72659−0.75914), and distinct rare earth elements, Sr, Mo, and As contents of scheelite confirm a metamorphic crustal source. Xenotime U-Pb dating and pre-ore (Stage I) scheelite reveal that ore-barren metamorphic fluids at ca. 140 Ma were oxidized with low Bi contents and buffered by greenschist facies metamorphism when the XMCC initiated. Monazite and rutile U-Pb dating combined with ore-stage scheelite geochemistry indicate a compositional shift in the more reduced auriferous metamorphic fluids, which dominated during major gold deposition periods (stages II and III) from 130 Ma to 120 Ma, characterized by significantly depleted Na and increased Bi contents. This resulted from the prograde greenschist-to-amphibolite metamorphism at mid-lower crustal depths as the result of the XMCC isostatic doming and the lithospheric mantle thinning after 130 Ma. This study highlights the crucial role of metamorphic core complexes in governing the timing, locations, and resources of the lode gold metallogenic system.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"80 s1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141002215","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Deep-time thermal history of the Great Unconformity in the Grand Canyon, USA: Combined zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronometers","authors":"O.G. Thurston, W.R. Guenthner, K.E. Karlstrom, M. Heizler, J.W. Ricketts, K.T. McDannell","doi":"10.1130/b37358.1","DOIUrl":"https://doi.org/10.1130/b37358.1","url":null,"abstract":"Deep-time thermochronology by the zircon (U-Th)/He (ZHe) method is an emerging field of study with promise for constraining Precambrian rock thermal and exhumation histories. The Grand Canyon provides an opportunity to further explore this method because excellent geologic constraints can be integrated with multiple thermochronometers to address important questions about the spatial variability of basement erosion below the sub-Cambrian Great Unconformity composite erosional surface. In this study, we synthesize new ZHe results (n = 26) and published (n = 77) ZHe data with new K-feldspar 40Ar/39Ar data and models (n = 4) from Precambrian basement rocks of the Grand Canyon, USA. We use HeFTy and QTQt thermal history modeling to evaluate the ability of the individual ZHe and K-feldspar 40Ar/39Ar thermochronometric data sets to resolve Precambrian thermal histories and compare those results with jointly modeled data using the QTQt software. We also compare Precambrian basement thermal histories of the eastern and western Grand Canyon, where the eastern Grand Canyon has ∼4 km of Grand Canyon Supergroup strata deposited and preserved, and the western Grand Canyon, where the Supergroup was either never deposited or not preserved. In all locations, models constrained only by ZHe data have limited resolving power for the past ∼600 m.y., compared to models that combine K-feldspar 40Ar/39Ar and ZHe data, which extends the recorded history into the Mesoproterozoic. Our model results suggest that two regional basement unroofing events occurred. A ca. 1350−1250 Ma cooling event is interpreted to record basement exhumation from depths of ∼10 km, and a second cooling episode (∼200−100 °C total) records exhumation from a depth of ∼3 km to 7 km to near-surface conditions between ca. 600 Ma and 500 Ma. Easternmost Grand Canyon models suggest that the preserved maximum ∼4 km thickness of the Grand Canyon Supergroup (with burial heating at ∼100 °C) approximates the total original Mesoproterozoic and Neoproterozoic stratal thickness. Whether these Supergroup rocks were present and then eroded in the western Grand Canyon, as suggested by regional geologic studies, or were never deposited is not constrained by thermochronological data.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"64 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141011705","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit","authors":"Yunfei Ren, Danling Chen, Xiao-hui Zhu, Haijie Wang, B. Bai","doi":"10.1130/b37448.1","DOIUrl":"https://doi.org/10.1130/b37448.1","url":null,"abstract":"Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39−2.28 Ga magmatism and 1.93−1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452−439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"353 8","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141006827","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Early−Middle Devonian paleomagnetic results from the Zhongba microterrane, Tibetan Plateau: Evidence for its origin from the northern margin of Greater India","authors":"Shuai Li, Yalin Li, Xiaodong Tan, Zhongpeng Han, Chengshan Wang, Zining Ma, Yunchuan Xu, Zihao Wang, Siqi Xiao, Jianbo Cheng, Zijian Li, Xingduo Ma, Zichun Guo, Song Huang","doi":"10.1130/b37147.1","DOIUrl":"https://doi.org/10.1130/b37147.1","url":null,"abstract":"The initial disintegration of Gondwana during the Paleozoic laid the foundation for the formation of the Tibetan Plateau in the Cenozoic. Determining the relative positions of the microterranes in Gondwana during the Paleozoic not only informs the subsequent drift and accretion processes of these microterranes but is also crucial to the paleogeographic reconstruction of Gondwana. However, the lack of paleomagnetic constraints on the Devonian paleogeography of the microterranes in the northern part of Gondwana makes this effort challenging. Here, we report paleomagnetic results for the first time from the Early−Middle Devonian sediments of the Zhongba microterrane. The site-mean direction is declination (Ds) = 310.7°, inclination (Is) = −67.2°, ks = 31.2, α95 = 8.3°, and n = 11, in stratigraphic coordinates. Positive fold and reversal tests, together with rock magnetism results and microscopic observations, strongly suggest that the remanence carriers are of depositional origin. The paleomagnetic results meet the paleomagnetic reliability criteria and therefore can be used for tectonic reconstructions. Our results constrain the paleolatitude of the Zhongba microterrane to be 50.0°S ± 11.7°S in the Early−Middle Devonian. Combined with published detrital zircon ages as well as paleomagnetic results and geological data, our data indicate that the Zhongba microterrane, which had no tectonic affinity with the Lhasa terrane, was part of the northern margin of Greater India during 408−388 Ma and coupled with the South Qiangtang terrane, Tethyan Himalayas, and other terranes to form the continuous northern continental margin of East Gondwana.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"328 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141006992","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Miocene vanishing of the Central American Seaway between the Panamá Arc and the South American Plate","authors":"F. Vallejo-Hincapié, A. Pardo-Trujillo, Angel Barbosa-Espitia, Daniela Aguirre, Sergio A. Celis, Carlos A. Giraldo-Villegas, Angelo Plata-Torres, Raúl Trejos-Tamayo, A. Salazar-Ríos, J. Flores, Marie-Pierre Aubry, Fabián Gallego, Eduardo Delgado, David S. Foster","doi":"10.1130/b37499.1","DOIUrl":"https://doi.org/10.1130/b37499.1","url":null,"abstract":"A controversy has developed in recent years regarding the timing of the closure of the Central American Seaway. This tectonic event significantly impacted oceanic circulation between the tropical Pacific and Atlantic oceans and resulted in the formation of a land bridge connecting the South and North American continents. The long-held view of a Pliocene age (ca. 3 Ma) for the closure of the Central American Seaway has been challenged by the proposal that the Panamá Arc collided with South America during the Middle Miocene (15−13 Ma) as a deep oceanic gap between them closed along the Uramita suture zone. However, direct geologic evidence from this suture zone to support either interpretation has been lacking. Here, we report on a comprehensive study of three stratigraphic transects across the Uramita suture zone, using a host of methodologies including sedimentological, ichnological, micropaleontological, U-Pb detrital geochronological, and provenance analyses. Our data reveal that lower offshore to slope conditions prevailed in the Central American Seaway along the suture zone during the latest Early to earliest Middle Miocene (16.4−15.1 Ma) and that oceanic conditions there ceased to exist between the Middle and Late Miocene. These results agree with the Middle Miocene age proposed for the Central American Seaway closure along the tectonic boundary. However, other deeper portions of the Central American Seaway persisted in western Colombia, which challenges the notion of a Central American Seaway confined to the suture zone between the Panamá Arc and South American Plate during the Middle Miocene.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"42 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140655922","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Revisiting the stratigraphic development of the postglacial, transgressive paleo-Changjiang (Yangtze River) mouth: A journey from tide-dominated estuary to delta","authors":"Shihao Liu, John A. Goff, Wei Feng, Yan Liu, Xin Shan, Weifen Hu, Yufeng Chen, Ya Ping Wang","doi":"10.1130/b37368.1","DOIUrl":"https://doi.org/10.1130/b37368.1","url":null,"abstract":"The transition between a tide-dominated estuary and a tide-dominated delta, which is observed in modern settings and repeatedly throughout the geologic record, is a critical process for understanding sea-level and environmental changes. This transition remains poorly documented, however, leaving a knowledge gap about the sedimentary and stratigraphic processes involved. To help bridge the gap, we reexamined the postglacial transgressive system beneath the modern Changjiang (Yangtze River) Delta, built by one of the world’s largest rivers, using new chirp data supplemented by correlation with previously documented boreholes. Our results revealed a previously undocumented tide-dominated estuary-to-delta transition around the Younger Dryas, with the pretransition evolution dominated by the retrogradation of the tide-dominated estuary, which fills most of the accommodation space in the lowstand incised valley. This transition occurred when the middle and seaward parts of the estuary reached the subaqueous modern Changjiang Delta area and underwent extensive tidal erosion. Overlying the tidal ravinement, a series of ridge-and-swale structures formed due to tidal/fluviotidal channelization on the mud-dominated heterolithic substrate, primarily consisting of delta-front sediments characterized by flat, stratified reflections (bedding) with shallowing-upward trends. Ridge orientation rotates clockwise from north to south, with intervening swales bifurcating from a common apex point, and these features are flanked by inclined and sigmoidal levee-like channel fills dominated by homogeneous mud (acoustically transparent material). We reinterpret these features as tide-dominated delta islands and distributary channels, rather than the previously interpreted erosional mud ridges in the seaward part of the estuary, as they align with patterns observed in classic tide-dominated deltas. These patterns also suggest a shift from delta-front to delta-plain environments coinciding with shoreline advance ca. 10 k.y. B.P., but this was preceded by the evolution of a backstepping delta. Our study highlights the interplay between sediment supply and sea-level fluctuations in controlling the large-scale stratigraphic and environmental evolution of tidal-dominated river mouth systems. In the transgressive, paleo-Changjiang mouth, this interplay drove the system to evolve through stages of retrogradational estuary, backstepping delta, and progradational delta, and each environment differs significantly from the pattern observed in the modern Changjiang Delta. This study provides important insights for evaluating the evolution and sequence-stratigraphic reconstruction of other tidal depositional systems.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"97 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140665555","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Subduction history of the Paleo-Asian oceanic slab along the northern margin of the North China Craton: A case study of zircons and cherts from central Jilin Province, NE China","authors":"Chen Song, F. Pei, Wenliang Xu, Jingyang Wei, Bingqian Ding, Tianjiao Sang, Kai Song, Zhiwei Wang, Hongchen Dong","doi":"10.1130/b36704.1","DOIUrl":"https://doi.org/10.1130/b36704.1","url":null,"abstract":"This paper presents U-Pb ages and Hf isotope data for detrital zircons and whole-rock geochemical data for siliceous-clastic sedimentary rocks of the Shitoukoumen Formation, central Jilin Province, NE China. The new data combined with published data for the northern North China Craton enable us to constrain the temporal changes in crustal thickness along the Solonker−Xar Moron−Changchun−Yanji suture belt (SXCYB). The Shitoukoumen Formation contains ophiolitic lenses, radiolarian-bearing cherts, and volcanic-sedimentary units of various ages. A radiolarian-bearing volcanic-sedimentary unit exhibits a Bouma sequence, in which detrital zircons from a rhyolitic tuff and siliceous-clastic sedimentary rocks yielded youngest ages of 278−273 Ma, which constrain the depositional age of the unit. The geochemical compositions of the cherts suggest the pelagic deposits contain some continent-derived clastic material and may be near-trench gravity flow deposits. The dominantly depleted zircon εHf(t) values of the sedimentary rocks in the Shitoukoumen Formation record extensive crustal growth during the Paleozoic. The variations in the crustal thicknesses during the Paleozoic to early Mesozoic record a tectonic transition from subduction initiation, mature continental arc, and finally to Paleo-Asian ocean closure. The thickest crust (∼58 km) at ca. 245 Ma might have been related to an orogenic peak during the final closure of the Paleo-Asian Ocean along the SXCYB.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"47 19","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140662173","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Paleomagnetic investigation of the basal Maieberg Formation (Namibia) cap carbonate sequence (635 Ma): Implications for Snowball Earth postglacial dynamics","authors":"Thales Pescarini, Ricardo I.F. Trindade, Paul F. Hoffman, Lucy Gomes Sant’Anna","doi":"10.1130/b37378.1","DOIUrl":"https://doi.org/10.1130/b37378.1","url":null,"abstract":"In this study, we investigate the paleomagnetism of the basal Maieberg Formation (Namibia) cap carbonate sequence to elucidate its magnetic properties and paleolatitude of deposition, establish global correlations, and contribute to the understanding of Snowball Earth postglacial dynamics. Two distinct magnetization components, C1 and C2, were identified. C1 is interpreted as a depositional or post-depositional remanent magnetization carried by detrital pseudo-single domain (PSD) magnetite, while the C2 component is a thermochemical remanent magnetization carried by fine authigenic single domain (SD)−PSD magnetite. The deposition paleolatitude provided by C1 is 33.3° ± 3.2°, which gives an initial quantitative approximation of the paleolatitude for the underlying Marinoan Ghaub diamictites. The thickness of the Keilberg Member cap dolostone is anomalously high for the paleolatitude calculated with C1, which suggests that other factors besides the influence of the paleolatitude on carbonate oversaturation may have influenced the sedimentary production of cap dolostones and the overall thickness of the flooding cap carbonate sequence. Possible explanations could include the influence of alkalinity input combined with local tectonic subsidence during a long glacial period with unusually low sedimentation rates, which appear to be in a favorable configuration for the substantial thickness of the Keilberg Member. Paleomagnetic field reversals at the Keilberg cap dolostone and analogous units globally suggest a longer duration of marine transgression than energy-balance deglaciation models and sedimentological-geochemical observations have constrained. Factors such as ocean warming, thermal expansion, and local glacio-isostatic adjustments imply extended marine transgressions beyond the deglaciation period. Still, magnetostratigraphic estimates for postglacial transgressive sequences require longer time scales by a factor of five or more. Thus, the conflict arising between estimates derived from paleomagnetic data and the constraints imposed by climate physics underscores uncertainties regarding an unconventional field state or a remanence acquisition mechanism within these cap carbonates that is not fully understood. Importantly, if such a phenomenon proves to be primary and global, the widespread occurrence of these stratigraphically compressed reversals would support the precise temporal correlation between Marinoan cap dolostones. The C2 pole correlates with Cambrian remagnetization poles observed in carbonates from West Gondwana, which now extend to the Congo craton. The remanence acquisition of C2 likely stems from diagenesis-related low-temperature authigenic magnetite formation after the conversion of iron-rich smectite to iron-poor illite. Cooling associated with the Kaoko orogen’s exhumation and tectonic uplift possibly locked the magnetic system at ca. 520 Ma, supported by the C2 pole position on the West Gondwana apparent polar wander path,","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"125 45","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140669230","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}