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141.
GUDRÚN LARSEN Bryndís G. Róbertsdóttir Bergrún A. Óladóttir JÓN EIRÍKSSON 《第四纪科学杂志》2020,35(1-2):143-154
Hekla volcano is a major producer of large, widespread silicic tephras. About 3000 years ago, the dominant eruption mode shifted from infrequent large (>1 km3) to more frequent moderate (<1 km3) eruptions. In the following two millennia ≥20 explosive silicic-to-intermediate eruptions occurred, and six or more basaltic. Three categories can be identified with dacite/andesite to basaltic andesite in the oldest eruptions through basaltic andesite to basalt in the youngest eruptions. Ten tephra layers of the first category have distinct field characteristics: a pale lower unit and a dark upper unit (two coloured or TC-layers). Colour separation is sharp indicating a stratified magma chamber origin. The lower unit is dominantly andesitic (61–63% SiO2), while the upper unit is basaltic andesite (53–57% SiO2). Volumes of the eight largest TC-layers range from 0.2 to 0.7 km3 as freshly fallen. Radiocarbon and soil accumulation rate dates constrain the TC-layers to between 3000 and 2200 years ago. Two of these (~2890 and ~2920 b2k) are likely to occur overseas. Low SiO2 in the last erupted tephra of the TC-layers is comparable to that of historical Hekla lavas, implying a final effusive phase. The Hekla edifice may, consequently, be younger than 3000 years. 相似文献
142.
Gwydion Jones Siwan M. Davies Richard A. Staff Neil J. Loader Sarah J. Davies Michael J. C. Walker 《第四纪科学杂志》2020,35(1-2):163-174
A tephra record is presented for a sediment core from Llyn Llech Owain, south Wales, spanning the early- to mid-Holocene. Seven cryptotephra deposits are discovered with three thought to correlate with known eruptions and the remaining four considered to represent previously undocumented events. One deposit is suggested to correlate with the ~6.9 cal ka bp Lairg A tephra from Iceland, whereas more distant sources are proposed as the origin for two of the tephra deposits. A peak of colourless shards in early-Holocene sediments is thought to tentatively correlate with the ~9.6 cal ka bp Fondi di Baia tephra (Campi Flegrei) and a second cryptotephra is tentatively correlated with the ~3.6 cal ka bp Aniakchak (CFE) II tephra (Alaska). The Fondi di Baia tephra has never been recorded beyond proximal sites and its discovery in south Wales significantly extends the geographical distribution of ash from this eruption. The remaining four cryptotephra deposits are yet to be correlated with known eruptions, demonstrating that our current understanding of widespread tephra deposits is incomplete. This new tephra record highlights the potential for sites at more southerly and westerly locations in northwest Europe to act as repositories for ash from several volcanic regions. 相似文献
143.
Michael C. Leaf Jessica S. A. Gay Matthew J. Newbould Owen R. Hewitt Steven L. Rogers 《Geology Today》2020,36(2):75-80
Algae is an informal term used to describe a broad group of simple organisms from the plant kingdom. The organisms included within this grouping are aquatic photosynthetic biota with an extensive range of life habits and forms. These organisms range from micron-sized unicellular forms to giant seaweeds and kelps, which can grow to several metres long. Both benthic and planktonic modes of life are known and display a wide variety of life cycles. 相似文献
144.
Fabian Boesl Max Engel Rodrigo C. Eco JaN B. Galang Lia A. Gonzalo Francesca Llanes Eva Quix Helmut Brückner 《Sedimentology》2020,67(3):1393-1410
Coastal boulder fields provide clues to long-term frequency-magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour-intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub-metre resolution (from Pléiades and WorldView-3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon-generated waves. During the investigated events, boulders with a-axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a-axes and b-axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra-high-resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a-axes and b-axes including their orientation, while precise values for c-axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder-specific Structure from Motion-based three-dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre-scale, or documentation of fine-grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre-scale and to better understand the hydrodynamics of extreme-wave events on a fringing reef coastline. 相似文献
145.
Pankaj Khanna Heath H. Hopson André W. Droxler Dylan A. Droxler Daniel Lehrmann Brian Kubik Jacob Proctor Pulkit Singh Paul M. Harris 《Sedimentology》2020,67(2):1135-1160
Although Late Cambrian microbial build-ups were recognized in the Point Peak Member of the Wilberns Formation in Central Texas (USA) nearly 70 years ago, only a few studies focused specifically on the build-ups themselves. This study focuses on the interpretation of the regional (15 measured sections described in literature representing an area of 8000 km2) and local (field and drone photogrammetry studies in a 25 km2 area from within south Mason County) microbial build-up occurrence, describes their growth phases and details their interactions with the surrounding inter-build-up sediments. The study establishes the occurrence of microbial build-ups in the lower and upper Point Peak members (the Point Peak Member is informally broken up into the lower Point Peak and the upper Point Peak members separated by Plectotrophia zone). The lower Point Peak Member consists of three <1 m thick microbial bioherms and biostrome units, in addition to heterolithic and skeletal/ooid grainstone and packstone beds. One, up to 14 m thick, microbial unit associated with inter-build-up skeletal and ooid grainstone and packstone beds, intercalated with mixed siliciclastic–carbonate silt beds, characterizes the upper Point Peak member. The microbial unit in the upper Point Peak member displays a three-phase growth evolution, from an initial colonization phase on flat based, rip-up clast lenses, to a second aggradation and lateral expansion phase, into a third well-defined capping phase. The ultimate demise of the microbial build-ups is interpreted to have been triggered by an increase of water turbidity caused by a sudden influx of fine siliciclastics. The lower Point Peak member represents inner ramp shallow subtidal and intertidal facies and the upper Point Peak member corresponds to mid-outer ramp subtidal facies. Understanding the morphological architecture and depositional context of these features is of importance for identifying signatures of early life on Earth. 相似文献
146.
Sediment is sorted in river bends under the influence of gravity that pulls the heavier grains downslope and secondary flow that drags the finer grains upslope. Furthermore, when dunes are present, sediment is also sorted vertically at the dune lee side. However, sorting functions are poorly defined, since the relation to transverse bed slope and the interaction between lateral and vertical sorting is not yet understood for lack of data under controlled conditions. The objective of this study is to describe lateral sorting as a function of transverse bed slope and to gain an understanding of the interaction between lateral and vertical sorting in river bends. To this end, experiments were conducted with a poorly sorted sediment mixture in a rotating annular flume in which secondary flow intensity can be controlled separately from the main flow velocity, and therefore transverse bed slope towards the inner bend and dune dimensions can be systematically varied. Sediment samples were taken along cross-sections at the surface of dune troughs and dune crests, and over the entire depth at the location of dune crests (bulk samples), which enabled comparison of the relative contribution of vertical sorting by dunes to lateral sorting by the transverse bed slope. The data show that lateral sorting is always the dominant sorting mechanism in bends, and bulk samples showed minor effects of vertical sorting by dunes as long as all grain-size fractions are mobile. An empirical bend sorting model was fitted that redistributes the available sediment fractions over the cross-section as a function of transverse bed slope. Comparison with field data showed that the model accurately reproduces spatially-averaged trends in sorting at the bend apex in single-thread channels. The bend sorting model therefore provides a better definition of bend sorting with conservation of mass by size fraction and adds to current understanding of bend sorting. The implication for numerical modelling is that bend sorting mechanisms can be modelled independently of dunes, allowing the application of the active layer concept. 相似文献
147.
João Paulo B. Gomes Rodrigo B. Bunevich Sandra N. Tonietto Daisy B. Alves Julice F. Santos Fiona F. Whitaker 《Sedimentology》2020,67(5):2282-2309
Authigenic clays are an important control on reservoir quality in lacustrine carbonates but remain challenging to predict. Lacustrine depositional systems respond to climatic variations in rainfall, surface runoff and groundwater input, and evaporation, and result in rapid and frequent changes in lake volume; this is expressed through changing water depth and shoreline position. In the upper portion of the Early Palaeocene Yacoraite Formation of the Salta Basin in Argentina, extensive lacustrine deposits were deposited during the sag phase of rifting. Prior high-resolution stratigraphic studies have suggested that climatic factors control microbial carbonate sequences within a ‘balanced fill’ lake, with variation in the lake level having a major influence on facies association changes. This study characterizes the evolution of facies and mineralogy within the Yacoraite Formation, focusing on the distribution of clay minerals, making a link between the high, medium and low-frequency sequence stratigraphic cycles. The low-frequency transgressive hemicycle of the upper portion of the Yacoraite Formation is comprised of abundant siliciclastic facies, suggesting a wetter period. Microbialites occurring in this interval are coarse-grained and agglutinated. Detrital clay minerals such as illite and chlorite and associated siliciclastic sediments were input to the lake during high-frequency transgressive periods. During high-frequency regressive hemicycles, sedimentation was dominated by carbonate facies with Ca-rich dolomite and the authigenic clays are comprised of chlorite/smectite mixed-layers. By contrast, the low frequency regressive hemicycle records fine-grained agglutinated microbialite with horizons of fibrous calcite, more stoichiometric dolomite, barite and authigenic magnesian smectite. This indicates elevated ion concentrations in the lake under intense evaporation during an arid period. Understanding the conditions that are favourable for formation and preservation of authigenic clays within the lacustrine environment can improve understanding of reservoir quality in comparable economically important deposits. 相似文献
148.
Huan Li Ladislav A. Palinkaš Noreen J. Evans Koichiro Watanabe 《Geological Journal》2020,55(2):1409-1430
The Huangshaping world-class W–Mo–Cu–Pb–Zn deposit formed during Jurassic intra-continental extension in the central Nanling region (South Hunan), South China. In order to assess the role of fluids in ore genesis and the origin of polymetallic mineralization, three types of mineralized porphyries (quartz porphyry, granophyre, and granite porphyry) were studied. The multi-method approach included whole-rock Sr–Nd isotopic geochemistry and determination of the trace element content and gaseous and aqueous composition of ore minerals from the three stages of intrusion and ore emplacement. Geochemical results show that the Sr–Nd isotopes in these altered porphyries were strongly affected by fluid metasomatism. The 87Sr/86Sr and 147Sm/144Nd ratios for the quartz porphyry and granophyre range from 0.72894 to 0.83093 and 0.1524 to 0.2080, respectively, whereas higher and considerably more variable ratios were determined for the granite porphyry (0.90396 to 1.51943 and 0.2391 to 0.2914). These observations imply that granite porphyry-associated W–Mo mineralization underwent more intensive fluid–rock interaction. The relatively low REE concentrations and high abundances of CH4 and H2 in the Cu ores associated with the quartz porphyry unambiguously suggest a deep-sourced magmatic fluid for the Cu mineralization. The high REE concentrations, pronounced negative Eu anomalies, M-type tetrad effects, and high CO2, H2O, and Ca2+ contents of the W–Mo ores associated with the granite porphyry imply that the W–Mo polymetallic mineralization was formed in a relatively oxidizing, high-temperature environment with strong metasomatism. In contrast, the slightly negative Eu anomalies, W-type tetrad effects, low CH4, and high F− and Cl− concentrations of the Pb–Zn ores indicate involvement of a sediment-derived basinal brine and reducing mineralization conditions. Collectively, a three-stage genetic model is proposed for the Huangshaping polymetallic deposit. Corresponding to initiation, development, and cessation of Jurassic intra-continental extension, magmatic water, metasomatized fluids, and basinal brines played key sequential roles in Cu, W–Mo, and Pb–Zn mineralization. 相似文献
149.
The present study provides a better understanding of the nature of the upper Palaeocene–lower Eocene succession within the framework of the sequence stratigraphy. It is based on the qualitative and quantitative analyses of the foraminiferal assemblages of the Esna and El-Rufuf formations at the Kharga Oasis, Western Desert. Seven planktonic foraminiferal zones were recognized; P4c and P5 zones of the late Palaeocene (Thanetian) age and E1–E5 zones of the early Eocene (Ypresian) age. The Palaeocene/Eocene (P/E) boundary is recorded at the base of the Dababiya Quarry Member of the Esna Formation that coincides with the P5/E1 zonal boundary. A minor hiatus has been documented across this boundary equivalent to the missing lowermost part of the Dababiya Quarry Member. The foraminiferal investigations (e.g., planktonic percentage, benthic foraminifera cluster analyses, diversity, and abundance) as well as field work and the lithofacies examination have been integrated to subdivide the studied succession into five third-order depositional sequences separated by five sequence boundaries. These sequences are primarily formed in response to the greater impact of the global sea-level oscillations, as well as the partial effect of the regional tectonics, which may be related to the influence of the Syrian Arc system. The regional tectonics led to occasional discrepancies in the timing and extent of some changes in the relative sea-level and mismatching of some boundaries. 相似文献
150.
Anderson Conceição Mendes Eduardo Francisco da Silva Arthur Iven T. Fonseca Afonso C.R. Nogueira Thays Ferreira da Silva Jornada do S.S. do Nascimento Haroldo F.E. Igreja 《Geological Journal》2020,55(6):4398-4414
The Monte Alegre and Itaituba formations record the transition from a fluvial-aeolian to a carbonate platform system during the Pennsylvanian on the southern margin of the Amazonas Basin, North Brazil. Sandstones of these formations were analysed for their petrofacies, provenance, tectonic configuration and diagenetic history to determine their origin. The Monte Alegre sandstones are fine- to medium-grained, well-sorted, bimodal quartz-arenites with a normal grain-supported framework of straight and concavo-convex contacts. They present 97% of ZTR (zircon-tourmaline-rutile) heavy minerals. The Itaituba sandstones are fine- to very fine-grained, well-sorted, sub-arkoses with an open to normal framework supported by carbonate cement. The heavy minerals are represented by zircon, tourmaline, rutile, kyanite and garnet, with almandine and pyrope as the main types derived from high-grade amphibolite- to- granulite-facies metasediments, in addition to intermediate-acidic igneous rocks. The petrofacies analysis revealed that the sandstones of these formations belong to the tectonic regime of the continental block, Interior Craton. The Amazonas Basin basement on the southern edge consists of intrusive suites and cratonic rocks of the Tapajós and Iriri-Xingu provinces, which are represented by the Cuiú-Cuiú Complex and the Iriri Group. These groups of rocks were the main sources of sandstones of these formations. This conclusion indicates that these rocks were uplifted, exposed and eroded during and/or after the Middle Carboniferous. The imprints of early and deep burial diagenesis of these sandstones, including different stages of compaction, cementation and heavy minerals elimination, produced differences in sandstones porosity and heavy minerals assembly. 相似文献