News Release

New GSA Bulletin articles published ahead of print in July

Peer-Reviewed Publication

Geological Society of America

Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet; the Rae and Slave cratons; Vega Island, Antarctica; and continental geological evidence for Solar System chaotic behavior. You can find these articles at .

Rates and processes controlling periglacial alluvial fan formation: Implications for martian fans
Marisa C. Palucis; A.M. Morgan; J.V. Strauss; F. Rivera-Hernandez; J.A. Marshall ...
Abstract: Alluvial fans are found across a range of climates and are built from a combination of fluvial and debris flow processes. Correct identification of process is critical to reconstructing the climate and water histories of alluvial fans on Earth and Mars. Theory and data from subaerial Earth fans are often used to estimate paleoflow discharges and sediment fluxes for martian fans; however, most terrestrial work has been conducted on fans that are in hot, dry climates with runoff sourced from rainfall. This differs from the prevailing interpretation that martian fans were sourced from snowmelt under warming periglacial conditions. To characterize processes and rates of periglacial fan formation, we conducted a field-based study of the Black Mountain alluvial fan in the Aklavik Range, Canada. We observed active fluvial bedload transport as well as several small debris flows that had initiated from ice-filled gullies. Following a runoff event of ∼0.005 mm/hr to ∼0.2 mm/hr across the fan, we estimated sediment fluxes of ∼0.04 m3/hr. Under bankfull conditions, we estimated runoff rates between ∼0.01 mm/hr to ∼14 mm/hr and corresponding sediment fluxes of ∼0.3 m3/hr to ∼550 m3/hr. This suggests that moderate flow events, well below the maximum runoff production rates suggested for Mars, are capable of entraining and transporting appreciable amounts of sediment by fluvial processes. However, sedimentological and geomorphological observations suggest that ∼67% of the fan was deposited fluvially; the remainder was deposited by mass flows. Our results emphasize the need to take care in interpreting martian sedimentary processes and climate from fan surface morphology alone.
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A comprehensive genetic model for the world’s largest Sb deposit (Xikuangshan, China)
Shanling Fu; Ruizhong Hu; Jiantang Peng; Liyan Wu; Dongsheng Ma
Abstract: The Mesozoic (160−130 Ma), fault-controlled Xikuangshan Sb deposit within Devonian limestone strata of Hunan Province, Southern China is the world’s largest Sb deposit containing a proven reserve of ∼2.5 m Sb. Although mined for over a century, its genesis remains poorly understood. Here we use new He-Ar isotope data of hydrothermal stibnite and both new and existing C-O-Sr-Nd isotopes of hydrothermal calcite with known stages to decipher its genesis and the major constraints on mineralization intensity. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions trapped in stibnite are from 0.01 to 0.04 Ra (Ra: atmospheric 3He/4He ratio) and 304−1077, respectively, indicating the ore-forming fluids at Xikuangshan were dominated by air-saturated meteoric groundwater after interaction with crustal rocks. Ore-stage calcite C and O isotopes indicated that most CO 2 in the fluids was acquired from marine carbonate rocks by dissolution; whereas Sr and Nd isotopes differed from deposited Devonian country rocks but were similar to the underlying regional Proterozoic clastic rocks in the region. Calcite from early and late stages showed a strong positive correlation between δ18O and 87Sr/ 86Sr, consistent with the mixing between the circulating groundwater and compounds released from the Proterozoic rocks due to extensive fluid-rock interaction. The 3He/Q ratios of the fluid inclusions are low, varying from 4.3 to 18.5 × 10−15 cm 3 standard temperature and pressure (STP) J−1, indicating deep-seated magma could have provided heat by conduction but no volatiles into the ore-forming fluids. Based on these new results, we suggest that deep-seated granitic magma heated the down-going meteoric groundwater along fault zones, after which the groundwater extensively interacted with and extracted Sb from the Proterozoic Sb-rich rocks to form Sb-rich fluids. The Sb-rich fluids then ascended through regional faults and deposited Sb as stibnites at favorable structural traps to form the Xikuangshan Sb deposit. This study highlights that extensive water-rock interaction is essential to form the deposit, and more intensive water-rock interaction at an early stage allowed for early-stage mineralization yielding higher Sb reserves (>80%) at Xikuangshan.
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The transition from continental to lithospheric breakup recorded in proto-oceanic crust: Insights from the NW South China Sea
Peng Chao; Gianreto Manatschal; Cuimei Zhang; Pauline Chenin; Jianye Ren ...
Abstract: The formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive ocean-continent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT.
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Middle Jurassic intracontinental evolution of East Asia: Insights from the Tianshifu-Dongyingfang basin of the Liaodong Peninsula, NE China
Zhiheng Ren; Shuwen Dong; Yueqiao Zhang; Xuanhua Chen; Wei Shi ...
Abstract: The Middle Jurassic intracontinental shortening event in East Asia, known as episode A of the Yanshanian Movement in the eastern North China Craton and the first phase of the Daebo Orogeny in the Korean Peninsula, respectively, has been thoroughly studied during the past two decades. However, the timing of its beginning, duration, and deformational processes are still highly controversial because few studies have probed the correlations between episode A of the Yanshanian Movement and the first phase of the Daebo Orogeny. The Tianshifu-Dongyingfang basin, located in the northern Liaodong Peninsula, is filled with complete Jurassic sequences that can be compared with those in the eastern North China Craton to the west of the Tan-Lu fault zone and the Korean Peninsula. Regional stratigraphic correlations suggest that the Tianshifu-Dongyingfang basin contains not only the syntectonic conglomerates, but also the lower and upper unconformities A1 and A2 of episode A of the Yanshanian Movement, which separated the syntectonic conglomerates from the underlying Early−Middle Jurassic coal measures and the overlying Late Jurassic red beds, respectively. Detailed field survey and structural analysis show that the development of syntectonic conglomerates and unconformities A1 and A2 were controlled by the imbricate thrust fault system in the basement, which resulted from the WNW−ESE compression inversed from the fault-slip data. The youngest detrital zircon U-Pb ages obtained near unconformities A1 and A 2 show that the WNW−ESE compression of episode A of the Yanshanian Movement in the Liaodong Peninsula initiated at ca. 171 Ma and ended at least by 163 Ma. Our new structural and geochronological data, combined with data from the neighboring areas, suggest that episode A of the Yanshanian Movement and the first phase of the Daebo Orogeny belong to a single intracontinental shortening event, the compressional deformation of which was derived from the westward, low-angle subduction of the Paleo-Pacific plate and transmitted to the Korean and Liaodong Peninsulas at ca. 172−170 Ma and to the eastern North China Craton to the west of the Tan-Lu fault zone at 170−165 Ma.
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Incipient collision of the Rae and Slave cratons at ca. 1.95 Ga
Jamie Cutts; Brendan Dyck
Abstract: Incipient collision between the Rae and Slave cratons represents a critical initial stage in the final assembly of the composite Laurentian craton. However, the timing and kinematics of this event remain poorly constrained. Here, we integrate phase equilibria modeling and Lu-Hf garnet and U-Pb monazite and zircon petrochronology to precisely date the timing of peak metamorphism along the Great Slave Lake shear zone, the central segment of the western Rae cratonic margin that was the locus of terminal Rae-Slave collision. Zircon and monazite U-Pb ages broadly constrain prograde-to-peak metamorphism to 1933−1913 Ma, whereas the timing of upper-amphibolite facies peak metamorphism (0.8−1.0 GPa and 750−800 °C) is best constrained by two garnet Lu-Hf ages of 1931 ± 12 and 1917 ± 6 Ma. Our results highlight the inherent strengths and limitations of garnet, monazite, and zircon petrochronology in the context of dating Paleoproterozoic-aged metamorphism. Based on the timing of crustal thickening constrained by Lu-Hf garnet geochronology, we posit that the Slave craton first collided with the western Rae margin at ca. 1950 Ma. Taken together with prior constraints on the timing of metamorphism elsewhere along the western Rae margin, we propose incipient collision was followed by the northwards zippering of the margins and the eventual ∼500 km of dextral translation of the Slave craton along the Great Slave Lake shear zone.
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New age constraints support a K/Pg boundary interval on Vega Island, Antarctica: Implications for latest Cretaceous vertebrates and paleoenvironments
Eric M. Roberts; Patrick M. O’Connor; Julia A. Clarke; Sarah P. Slotznick; Christa J. Placzek ...
Abstract: A second K/Pg boundary interval in the northern sector of the Antarctic Peninsula on Vega Island has been proposed, yet current temporal resolution of these strata prohibits direct testing of this hypothesis. To not only test for the existence of a K/Pg boundary on Vega Island but also provide increased age resolution for the associated vertebrate fauna (e.g., marine reptiles, non-avian dinosaurs, and avian dinosaurs), the Vega Island succession was intensively re-sampled. Stratigraphic investigation of the Cape Lamb Member of the Snow Hill Island Formation, and in particular, the overlying Sandwich Bluff Member of the López de Bertodano Formation, was conducted using biostratigraphy, strontium isotope stratigraphy, magnetostratigraphy, and detrital zircon geochronology. These data indicate a Late Campanian−early Maastrichtian age for the Cape Lamb Member and present three possible correlations to the global polarity time scale (GPTS) for the overlying Sandwich Bluff Member. The most plausible correlation, which is consistent with biostratigraphy, detrital zircon geochronology, sequence stratigraphy, and all but one of the Sr-isotope ages, correlates the base of the section to C31N and the top of the section with C29N, which indicates that the K/Pg boundary passes through the top of the unit. A second, less plausible option conflicts with the biostratigraphy and depends on a series of poorly defined magnetic reversals in the upper part of the stratigraphy that also correlates the section between C31N and C29R and again indicates an inclusive K/Pg boundary interval. The least likely correlation, which depends on favoring only a single Sr-isotope age at the top of the section over biostratigraphy, correlates the section between C31N and C30N and is inconsistent with an included K/Pg boundary interval. Although our preferred correlation is well supported, we failed to identify an Ir-anomaly, spherules/impact ejecta, or other direct evidence typically used to define the precise position of a K/Pg boundary on Vega Island. This study does, however, confirm that Vegavis, from the base of the Sandwich Bluff Member, is the oldest (69.2−68.4 Ma) phylogenetically placed representative of the avian crown clade, and that marine vertebrates and non-avian dinosaurs persisted in Antarctica up to the terminal Cretaceous.
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Genetic mechanism of permeability anisotropy in conglomerate reservoir and its controlling factors
Feng-Qi Tan; Xian-Kun Li; Chun-Miao Ma; Jian-Rong Lv; Wen-Tao Liu
Abstract: The near-source, multistream, and rapidly changing sedimentary environments of conglomerate reservoirs lead to reservoir characteristics such as strong heterogeneity and complex modal pore structure, which create unique seepage pathways and strong permeability anisotropy, which in turn affect the displacement mode and oil recovery of crude oil in pores and throats. Thus, it is necessary to clarify the genetic mechanism and controlling factors of this permeability anisotropy, providing the basis and premise of sustainable and efficient development of conglomerate reservoirs. In this paper, a typical conglomerate reservoir from the Kexia Formation in the 7th Block of the Karamay oilfield (Xinjiang, China) was selected as the research object to solve the permeability anisotropy problem. According to the sealed coring data, eight conglomerate samples were cut preliminarily in the field and prepared finely in the laboratory to make standard cube samples with side lengths of 4.5 cm, and then these samples were investigated using casting thin section, computed tomography scanning, and constant velocity mercury injection experimental methods to determine the genetic mechanism of permeability anisotropy and its controlling factors at macro- and microscales. The results showed that sedimentation and hydrodynamic conditions controlled the distribution of permeability anisotropy, the separation, rounding, and arrangement of mineral particles, the development of sedimentary structural planes, and the different types of packing textures, which affected the seepage paths and resistance in different directions, resulting in great differences in permeability anisotropy. The permeability in the direction parallel to water flow was the largest, followed by the direction perpendicular to water flow, and the permeability in the vertical sedimentation direction was the lowest because of the combined influences of the tortuosity of seepage, the connectivity of pore throats, and the sedimentary structural planes. In addition, diagenesis in later stages can reduce the absolute value of permeability in different directions and the strength of permeability anisotropy by changing the micropore structure of the conglomerate reservoir, but it will not change the distribution of permeability anisotropy. Based on these observations, a generalized linear mixed model was used to analyze the micropore structure parameters and standard deviation of permeability in different directions from the eight conglomerate samples, and the results showed that the relative sorting coefficient had the greatest influence on the standard deviation of permeability, and the uniformity of pore-throat size distribution was the main microscale controlling factor on the strength of permeability anisotropy. When the pore-throat size distribution was less uniform, the difference in the amplitude of permeability anisotropy was more obvious. The average capillary radius, seepage tortuosity, and maximum pore-throat radius showed less importance than relative sorting coefficient, so they are considered secondary factors. The other micropore structure parameters showed some influence on the permeability anisotropy, but the degree of influence was limited.
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Albian−Cenomanian granitoid magmatism in Eastern and Central Tibet as a result of diachronous, continental collision induced slab tear propagation
Xue Gao; Yildirim Dilek
Abstract: A discrete belt of Albian−Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau’s crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, whole-rock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K 2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04−0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their εHf (t) values correspond to wide ranging Hf isotope crustal model ages (TDMC) of 0.3−1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034−0.7105) and negative εNd(t) values of −8.8 to −4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145−120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian−Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.
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A newly defined, long-lived Paleozoic intra-oceanic arc in the South Tianshan (NW China): Implications for multiple accretionary tectonics in the southern Altaids
Limin Gao; Wenjiao Xiao; Zhou Tan; Xinshui Wang; Yuhong Guo
Abstract: The South Tianshan Orogenic Belt marks the final assembly of the southern Altaids and the Karakum-Tarim Cratons. Integrated petrological, geochemical, and geochronological studies were carried out on the Akeyazi high-alumina basaltic lavas (HABs) and their host sandstones from the southern margin of the Central Tianshan Block. Given their relative high alumina contents (Al2O3 > 16 wt%) and geochemical similarity to basalts from modern arcs (e.g., Aleutian, Kamchatka, Izu, and Honshu arc), the Akeyazi basaltic lavas are classified as HABs. The Akeyazi HABs are distinguished by strong enrichment in large ion lithophile elements (LILE) and negative Nb-Ta anomalies. Furthermore, their mid-oceanic ridge basalt (MORB)-like Sr-Nd-Hf (87Sr/86Sr(i) ∼0.705766, εNd(t) ∼+2.5, εHf (t) ∼+9.5), along with arc-like trace element patterns, indicate a petrogenesis derived from a mantle wedge metasomatized by pelagic, sediment-derived melts (2%−5% melts mixed, as confirmed by melt modeling). Zircon U-Pb ages of Akeyazi HABs and associated arc-related gabbros suggest that they were mainly erupted during ca. 415 Ma. Moreover, host sandstones of Akeyazi HABs exhibit a unimodal distribution with a peak at 480−410 Ma in the detrital zircon spectrum, which is notably distinct from those of the Central Tianshan Block, South Tianshan Accretionary Complex, and North Tarim Craton, each of which is multimodal. The maximum depositional age (MDA) of the meta-sandstones is 351.0 ± 4.3 Ma, with the weighted average of three oldest Paleozoic grains at 502 ± 26 Ma. Collectively, our data, for the first time, discloses an intra-oceanic arc setting from ca. 502−351 Ma during the northward subduction of the South Tianshan Ocean, which further reveals the history of multiple episodes of accretion in the southern Altaids.
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Asymmetric propagation mechanism of hydraulic fracture networks in continental reservoirs
Jingshou Liu; Lianfu Mei; Wenlong Ding; Ke Xu; Haimeng Yang ...
Abstract: Hydraulic fracturing technology is relatively mature in North America, but under complex geological conditions, such as those in China, the application of this technology still faces great challenges. At present, techniques for the numerical simulation of hydraulic fracture networks are mainly based on the prediction of the fracture half-height and half-length, which cannot capture the heterogeneity of continental low-permeability sandstone reservoirs in China and the distribution of the asymmetric hydraulic fracture network present in them. Therefore, determining the asymmetric propagation mechanism of hydraulic fracture networks is very important for improving the recovery rates of continental reservoirs. In this paper, taking the Ordos Basin in China as an example, the spatial distribution of the stress field of a heterogeneous continental reservoir is precisely predicted by reservoir mechanical heterogeneity modeling. By using a microseismic monitoring method, the 3-D morphology of the hydraulic fracture network is determined. Through the coupling of multisource data, the frequency distributions of the determined in situ stress magnitudes in different hydraulic fracturing stages are obtained. The propagation direction of the hydraulic fracture network changes under the control of the horizontal stress difference (Δσ) and the presence of natural fractures. The smaller Δσ is, the greater the deflection of the hydraulic fracture propagation direction. The asymmetric propagation of these fractures is related to the frequency distribution of Δσ. As the frequency of Δσ approaches a normal distribution, the two wings of the hydraulic fracture network become basically equal in length, and as Δσ deviates more from a normal distribution, the difference between the two wings of the hydraulic fracture network increases. These research results will provide new insight for modeling, exploring, and developing continental reservoirs.
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Continental geological evidence for Solar System chaotic behavior in the Late Cretaceous
Huaichun Wu; Linda A. Hinnov; Shihong Zhang; Ganqing Jiang; Tianshui Yang ...
Abstract: The Earth’s geologic record of Milankovitch cycles closely tracks Solar System solutions for the past 50 million years. Prior to 50 million years ago (Ma), however, the solutions lose accuracy rapidly due to chaotic behavior of the Solar System. Here we reconstruct a 10.173 million year-long record from 82.358 Ma to 92.531 Ma of Earth’s orbital parameters from a continental lacustrine sequence in the Songliao Basin, China constrained by four in situ high-resolution radioisotopic U-Pb ages and magnetic reversal stratigraphy. Analysis of thorium and ostracode shell abundance records from the Songliao Basin reveal evidence for two chaotic secular resonance transitions in the orbital motions of Earth and Mars from 85.2 Ma to 91.55 Ma. The evidence validates similar observations in western North American marine stratigraphy. A unique phasing between the observed orbital eccentricity and obliquity modulations may explain the anoxic events that occurred in both marine and continental environments during this time. Taken together, the continental and marine stratigraphic evidence demonstrates a strong global reach of Late Cretaceous Milankovitch cycles, and provides an important constraint on Solar System chaoticity and the calculation of accurate orbital solutions prior to 50 Ma.
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Tectonic evolution of the Beishan orogen in central Asia: Subduction, accretion, and continent-continent collision during the closure of the Paleo-Asian Ocean
Jie Li; Chen Wu; Xuanhua Chen; Andrew V. Zuza; Peter J. Haproff ...
Abstract: The Beishan orogen is part of the Neoproterozoic to early Mesozoic Central Asian Orogenic System in central Asia that exposes ophiolitic complexes, passive-margin strata, arc assemblages, and Precambrian basement rocks. To better constrain the tectonic evolution of the Beishan orogen, we conducted field mapping, U-Pb zircon dating, whole-rock geochemical analysis, and Sr-Nd isotopic analysis. The new results, when interpreted in the context of the known geological setting, show that the Beishan region had experienced five phases of arc magmatism at ca. 1450−1395 Ma, ca. 1071−867 Ma, ca. 542−395 Ma, ca. 468−212 Ma, and ca. 307−212 Ma. In order to explain the geological, geochemical, and geochronological data from the Beishan region, we present a tectonic model that involves the following five phases of deformation: (1) Proterozoic rifting that separated the North Beishan block from the Greater North China craton that led to the opening of the Beishan Ocean, (2) early Paleozoic north-dipping subduction (ca. 530−430 Ma) of the Beishan oceanic plate associated with back-arc extension followed by collision between the North and South Beishan microcontinental blocks, (3) northward slab rollback of the south-dipping subducting Paleo-Asian oceanic plate at ca. 450−440 Ma along the northern margin of the North Beishan block that led to the formation of a northward-younging extensional continental arc (ca. 470−280 Ma) associated with bimodal igneous activity, which indicates that the westward extension of the Solonker suture is located north of the Hongshishan-Pengboshan tectonic zone, (4) Late Carboniferous opening and Permian north-dipping subduction of the Liuyuan Ocean in the southern Beishan orogen, and (5) Mesozoic-Cenozoic intracontinental deformation induced by the final closure of the Paleo-Asian Ocean system in the north and the Tethyan Ocean system in the south.
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Mesoproterozoic oxygenation event: From shallow marine to atmosphere
Baozeng Xie; Jian-ming Zhu; Xiangli Wang; Dongtao Xu; Limin Zhou ...
Abstract: Oxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1−1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%−1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ∼20‰ to ∼50‰ and a subsequent decline to ∼10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca+Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.
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The influence of Late Ordovician volcanism on the marine environment based on high-resolution mercury data from South China
Yong Wang; Jingqiang Tan; Wenhui Wang; Lian Zhou; Peng Tang ...
Abstract: Volcanic ash interbeds in the Late Ordovician strata in South China record high-frequency volcanic activity. To assess the impact of these volcanic events on the climate and marine biological evolutionary crisis during the Late Ordovician Mass Extinction (LOME), we present high-resolution measurements of mercury (Hg) concentrations and organic carbon isotope ratios (δ13Corg) in an Ordovician−Silurian sedimentary succession (Muchanggou section). The results show that high-frequency volcanic ash layers and Hg enrichments developed synchronously in the Katian graptolite P. pacificus (Lower Subzone) and T. typicus biozones and are coupled with a negative shift of ∼1‰ in δ13Corg, which reflects the perturbation of the surface carbon and mercury cycles by intense volcanic activity. Based on volcanic Hg fluxes, it is estimated that more than 1.1 × 1013 tons of CO2 were emitted by the Katian volcanic activities in South China, which interrupted the growth of the polar ice sheet and the rapid cooling of tropical oceans in the mid-Boda event. As volcanism weakened, increased weathering led to the growth of the polar ice sheet and cooling in the Hirnantian. Both regional and global biodiversity declined rapidly with the end of volcanism, which suggests that the marine ecological crisis was related to a series of disruptions in biogeochemical cycles in the post-volcanic period.
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Giant deep submarine depressions: A combined dissolution-mechanical process along carbonate margins
T. Mulder; T. Cavailhes; V. Hanquiez; H. Gillet; A. Recouvreur ...
Abstract: Submarine depressions are common features on the eastern Bahamian seafloor but the genesis of the deepest ones (>4000 m in water depth) is not well understood. We conducted a morphometric analysis and compared them to a worldwide database of rounded depressions, whatever their genesis is. The deep Bahamian depressions are large elongated structures, among the largest on Earth, with a width greater than 1000 m and a depth sometimes greater than 200 m. They extend at the toe of the Blake Bahama Escarpment (BBE), one of the tallest escarpments on Earth. Some of them align parallel to the BBE. Other depressions align along large submarine canyon axes. When aligned along canyon axes, the depressions closest to the canyon head are flanked by a topographic high interpreted as a slope-break deposit, i.e., sediment deposited after flow expansion following a hydraulic jump. Turbidity currents in the carbonate canyon system are not permanent processes, but are rather triggered during sea-level highstands when the carbonate platform is flooded. In addition, some depressions are not located in canyon axes. Consequently, the size and location of the depressions are not likely explained by a simple plunge pool mechanical erosion. Rather, our data suggest that all depressions could be initiated by giant karstic dissolution structures (dolines or sinkholes). Under interpretation, those located in canyon axes are sporadically refreshed by carbonate-laden turbidity currents. The height of the outsized chutes marking the crossing of the BBE by the canyon mouth generates a hydraulic jump allowing sediment deposition toward the bottom of the depression. Large depressions observed at this location in the Bahamas were the result of an initial dissolution phase related to retreat of the BBE and the more recent sediment-laden flow activity. The depressions orientated along canyon axes facilitated the regressive erosion that formed the canyons. At present, the depressions located at canyon mouths act as regular plunge pools.
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Geophysical extent of the Wyoming Province, western USA: Insights into ancient subduction and craton stability
Paul A. Bedrosian; Carol D. Frost
Abstract: A new 3-D resistivity model, estimated from inversion of magnetotelluric data, images crustal and upper-mantle structure of the Wyoming Province and adjacent areas. The Archean province is imaged as a coherent resistive domain, in sharp contrast to active tectonic domains of the western U.S. Prominent high-conductivity belts define the northern, eastern, and southern margins of the Wyoming Province and are interpreted as sutures marking the remnants of Paleoproterozoic orogens. The model results suggest the northern boundary of the Wyoming Province is located 150 km south of its traditional placement and adjacent to a composite orogen separating the Wyoming Province and Medicine Hat block. The eastern province boundary is clearly imaged along the Black Hills, whereas the western margin is obscured by Cenozoic extension and magmatism. An internal boundary within the Wyoming Province is interpreted to represent a Neoarchean suture; in stark contrast to Proterozoic sutures, though, it is not marked by a high-conductivity belt. This difference in conductivity is speculated to reflect changes in the subduction process through time. The absence of high-conductivity along Archean sutures appears to be global in nature and related to reduced continental freeboard in the Archean which limited continental weathering and the delivery of carbon-rich sediments to the seafloor. Although the entire Wyoming Province has been proposed to have undergone lithospheric modification that lessened its stability, the resistivity model suggests a thick lithospheric root remains in place except along its western margin. These results suggest that Archean cratons may be more resistant to lithospheric modification by influx of heat and fluids associated with extension and plumes than previously thought, and that metasomatism does not necessarily weaken the lithosphere and set a craton on the path to destruction.
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Relationship between stylolite morphology and the sealing potential of stylolite-bearing carbonate cap rocks
Jun Wu; Tailiang Fan; Enrique Gomez-Rivas; Qian Cao; Anna Travé ...
Abstract: We analyzed the sealing effectiveness of cap rocks bearing different types of stylolites using a combination of petrographic, petrophysical, pore structure, and sealing capacity characterization techniques. This study was based on examples of carbonate cap rocks that seal ultradeep hydrocarbon reservoirs of the Tarim Basin (China). Samples from both drill cores and their outcrop analogues were investigated to quantify how morphology influences the sealing capacity of different types of stylolite-bearing rocks. The study cap rocks consisted of mudstone, wackestone to packstone, grainstone, and dolomitic limestone. Four types of stylolites were identified: rectangular layer, seismogram pinning, suture and sharp peak, and simple wave-like types. The difference in the sealing capacity of carbonate cap rocks is attributed to their pore structure connections and the types of stylolites they develop. Samples bearing simple wave-like stylolites showed the best sealing capacity, followed by those with rectangular layer and suture and sharp peak types, whereas carbonates hosting seismogram pinning types had the lowest sealing capacity. The impact of stylolite segments on the rock sealing properties, however, differed from one segment to another. Rectangular layer−type stylolites could be divided into three distinct segments (with good, moderate, and poor sealing, respectively). Both the seismogram pinning and suture and sharp peak stylolite types were divided in two parts, with the former one having moderate and poor sealing and the latter exhibiting good and moderate sealing. The simple-wave−like type had a good sealing capacity all along the pressure-solution seam. The most effective sealing barriers for vertical fluid flow form when (1) calcite and siliceous cements are pervasively distributed in the vicinity of stylolites, forming highly cemented zones with lower porosity and permeability than their surrounding host rocks; (2) stylolites are enriched in insoluble residues; and (3) rare microfractures and dissolution vugs are found along the stylolites. This work provides useful examples for the prediction of the sealing potential of stylolite-bearing carbonate rocks according to stylolite morphology in other geologic settings.
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