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  • 1
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    Wiley-Blackwell
    Publication Date: 2018-03-06
    Print ISSN: 0037-0746
    Electronic ISSN: 1365-3091
    Topics: Geosciences
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  • 2
    Publication Date: 2018-03-06
    Description: The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widely-adopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be in preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths greater than 20 m being constituted by up to 12.2 m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetre-scale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels. This article is protected by copyright. All rights reserved.
    Print ISSN: 0037-0746
    Electronic ISSN: 1365-3091
    Topics: Geosciences
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  • 3
    Publication Date: 2018-02-28
    Description: Submarine lobes have various geometries and stacking patterns, whose differences are likely to be the result of variations in flow efficiency and degree of confinement. This study examines four contrasting units with differing flow efficiency and confinement, to evaluate their roles on bed geometries and stacking patterns. Three of these units occur in the Late Palaeozoic Paganzo Basin, north-west Argentina: the Las Lajas system is developed in a 0.8 km wide palaeofjord; the Cerro Bola system (of which two different units were studied) was deposited in a larger sub-basin, at least 20 km wide. The Paine C system of the late Cretaceous Magallanes Basin in Chile is confined by an incision surface 3 km wide. Seventy-eight individual beds in the four units have been chosen to calculate flow efficiency and degree of confinement. Individual flow efficiency has been estimated semi-quantitatively using the outcrop cross-sectional area of the bed (as a proxy for flow volume) and percentage of mud in the beds (as a relative estimate for that in the flows). The degree of confinement experienced by the flows was assessed semi-quantitatively by dividing the flow efficiency by the maximum preserved basin dimension. It is found that: (i) degree of confinement (efficiency divided by maximum preserved basin dimension) influences individual bed geometry, highly confined flows having a higher tabularity (smaller thinning rate); (ii) in highly confined settings, individual beds stack vertically, whereas in unconfined systems, they stack compensationally; (iii) highly confined or high efficiency flows have higher tabularity (smaller thinning rate), which implies that truly sheet-like systems are only developed in highly confined and high efficiency systems. The generic model of architecture of submarine lobes and turbidite sheet systems, as a function of flow efficiency and degree of confinement, could be applied widely to sheet-like systems both at outcrop and in the subsurface. This article is protected by copyright. All rights reserved.
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    Electronic ISSN: 1365-3091
    Topics: Geosciences
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  • 4
    Publication Date: 2018-02-27
    Description: Reconstruction of the palaeoenvironmental context of Martian sedimentary rocks is central to studies of ancient Martian habitability and regional palaeoclimate history. This paper reports the analysis of a distinct aeolian deposit preserved in Gale crater, Mars, and evaluates its palaeomorphology, the processes responsible for its deposition, and its implications for Gale crater geological history and regional palaeoclimate. Whilst exploring the sedimentary succession cropping out on the northern flank of Aeolis Mons, Gale crater, the Mars Science Laboratory rover Curiosity encountered a decametre-thick sandstone succession, named the Stimson formation, unconformably overlying lacustrine deposits of the Murray formation. The sandstone contains sand grains characterized by high roundness and sphericity, and cross-bedding on the order of 1 m in thickness, separated by sub-horizontal bounding surfaces traceable for tens of metres across outcrops. The cross-beds are composed of uniform thickness cross-laminations interpreted as wind-ripple strata. Cross-sets are separated by sub-horizontal bounding surfaces traceable for tens of metres across outcrops that are interpreted as dune migration surfaces. Grain characteristics and presence of wind-ripple strata indicate deposition of the Stimson formation by aeolian processes. The absence of features characteristic of damp or wet aeolian sediment accumulation indicate deposition in a dry aeolian system. Reconstruction of the palaeogeomorphology suggests that the Stimson dune field was composed largely of simple sinuous crescentic dunes with a height of ca 10 m, and wavelengths of ca 150 m, with local development of complex dunes. Analysis of cross-strata dip-azimuths indicates that the general dune migration direction and hence net sediment transport was towards the north-east. The juxtaposition of a dry aeolian system unconformably above the lacustrine Murray formation represents starkly contrasting palaeoenvironmental and palaeoclimatic conditions. Stratigraphic relationships indicate that this transition records a significant break in time, with the Stimson formation being deposited after the Murray formation and stratigraphically higher Mount Sharp group rocks had been buried, lithified and subsequently eroded. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 5
    Publication Date: 2018-02-21
    Description: The East China Sea Shelf Basin was a back-arc basin located at the active continental margin of the western Philippine Sea Plate. This study explores facies and architectural changes from tide-influenced deltas to tide-dominated estuaries in transgressive–regressive cycles, as well as their controlling factors. Cores, wireline well-logs and seismic data allow the sedimentary architectures and models of the depositional systems to be reconstructed. In the Xihu Depression of the East China Sea Shelf Basin, the stratigraphic sequences of the Eocene Pinghu Formation are interpreted to be dominated by repeated phases of deltaic progradation, but with intervening transgressive phases only thinly developed as bioturbated, open-marine shelf deposits. The sequences of the overlying Oligocene Huagang Formation, in contrast, are interpreted as stacked, tide-dominated estuary units, alternating with only poorly preserved regressive half cycles because of repeated, strong estuary down-cutting. The intervening unconformity in the succession corresponds to the Yuquan tectonic movements, which triggered a change from extensional to compressional settings in the Xihu Depression. In the Late Eocene, extension of the Xihu Depression led to moderately high rates of subsidence (163 m/Ma), and short-term sea-level falls led to multiple phases of deltaic progradations. After the Yuquan Movement, Early Oligocene compression brought overall lower rates of subsidence (110 m/Ma), as well as sea-level rise and stacked estuary development with significant tidal influence in the infill. The interaction of tectonics, sea-level change and sediment supply determined the nature of the depositional systems on the shelf during the entire period, whereas the sedimentary processes were key to reworking and shaping the facies distribution, geomorphology and architectures in the back-arc basin. This research provides an insight into spatial and temporal characterization of deltaic and estuarine systems, contributing to a better understanding of the mechanisms controlling a change in dominant coastline type, despite continued strong tidal influence. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 6
    Publication Date: 2018-02-21
    Description: In modern systems, submarine channel–lobe transition zones show a well-documented assemblage of depositional and erosional bedforms. In contrast, the stratigraphic record of channel–lobe transition zones is poorly constrained, because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings. Several locations from an exhumed fine-grained base of slope system (Unit B, Laingsburg depocentre, Karoo Basin) show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions. The regional stratigraphy, lack of a basal confining surface, wave-like morphology in dip section, size and facies characteristics support an interpretation of subcritical sediment waves within a channel–lobe transition zone setting. Some sediment waves show steep (10 to 25°) unevenly spaced (10 to 100 m) internal truncation surfaces that are dominantly upstream-facing, which suggests significant spatio-temporal fluctuations in flow character. Their architecture indicates that individual sediment wave beds accrete upstream, in which each swell initiates individually. Lateral switching of the flow core is invoked to explain the sporadic upstream-facing truncation surfaces, and complex facies distributions vertically within each sediment wave. Variations in bedform character are related to the axial to marginal positions within a channel–lobe transition zone. The depositional processes documented do not correspond with known bedform development under supercritical conditions. The proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions, and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 7
    Publication Date: 2018-02-18
    Description: Extensive evaporites in Lower Mississippian successions from palaeoequatorial regions are commonly used as evidence for an arid to semi-arid palaeoclimate. However, in this study, detailed studies of evaporites and their context refute this interpretation. Detailed sedimentological and petrographical analysis of the Lower Mississippian of northern Britain, is combined with archived log data from more than 40 boreholes across southern Scotland, northern England and Northern Ireland, and published literature from Canada. Two key cores from the Tweed Basin and the northern margin of the Northumberland – Solway Basin contain 178 evaporite intervals and reveal twelve distinct forms of gypsum and anhydrite across seven facies that are associated with planar laminated siltstone and intercalated thin beds of ferroan dolostone. Nodular gypsum and anhydrite, typically in intervals less than 1 to 2 m thick, are integral components of the succession. Nodular evaporite occurs within about 1 m of a palaeosurface, but most evaporite deposits represent ephemeral brine pans to semi-permanent hypersaline lakes or salinas on a floodplain that was subjected periodically to storm surges introducing marine waters. Formation of evaporites under a strongly seasonal climate in a coastal wetland is supported by palaeosol types and geochemical proxies, and from palaeobotanical evidence published previously. Although 65% of modern equatorial areas experience a strongly seasonal climatic regime, salinas and sabkhas are a minor component today in comparison with the evidence from these Lower Mississippian successions. This implies that the earliest terrestrial environments were complex and dynamic providing a diverse range of habitats in which the early tetrapods became terrestrialized and represent a setting that is rarely preserved in the geological record. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 8
    Publication Date: 2018-02-03
    Description: Geochemical variations across laminated tufas and travertines may reflect the growth style of the carbonate build-up and not just climate-related changes. This work presents the study of a carbonate deposit, formed on a ravine wall in Gran Canaria Island (Spain), from a broken pipe system used for irrigation of banana plantations. The deposit is a few tens of metres long and has a stepped morphology formed by successive cascade-barriers and pools. Main facies are framestones of coated stems, laminated bindstones, phytoclastic wackestones and silty mudstones, all of them displaying micritic or coarse crystalline textures. Lamination from a framestone with coarse crystalline texture was studied from a petrological-sedimentological and geochemical perspective, and water palaeotemperatures were calculated. Lamination displaying five orders of magnitude, from daily to annual or higher, was controlled by the discontinuous supply of water. Lamination consists of crystalline laminae-discontinuity couplets at all observed scales. Estimated mean precipitation rates are 0.7 mm/year, but discontinuity of sedimentation at all lamination orders may have involved greater precipitation rates. Whereas elemental geochemistry suggests variable conditions not far from chemical equilibrium, stable isotopes suggest that calcite precipitated under disequilibrium conditions. However, the small dimensions of the deposit and the relatively high flow velocities allowed lack of δ 13 C and δ 18 O isotope fractionation in CO 2 -HCO 3 - nor in HCO 3 - -calcite, leading to independent temperature calculations, both with mean values of 25°C. Isotopic trends found throughout lamination cannot be explained by strong changes in water temperature nor in δ 13 C DIC or δ 18 O w . The correction made to eliminate these isotopic trends yielded narrower temperature ranges. This paper discusses the accuracy of temperature estimations despite the difficulties coming from disequilibrium and how isotopic trends through time could be explained by the growth of the deposit and not by climate-related changes. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 9
    Publication Date: 2018-02-03
    Description: This work presents a detailed study of CONTOURIBER and Integrated Ocean Drilling Program 339 sediment data targeting sand-rich contourites in the Eastern Gulf of Cadiz. All of the collected sediments are interpreted as contourites (deposited or reworked by bottom currents) on the basis of oceanographic setting, seismic and morphometric features and facies characteristics. A variety of sandy and associated facies are found across the study area including: (i) bioturbated muddy contourites; (ii) mottled silty contourites; (iii) very fine mottled and fine-grained bioturbated sandy contourites; (iv) massive and laminated sandy contourites; and (v) coarse sandy/gravel contourites. The thickest sands occur within contourite channels and there is a marked reduction in sand content laterally away from channels. Complementary to the facies descriptions, grain-size analysis of 675 samples reveal distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (〈20 μ m) show normal grain-size distributions, poor to very poor sorting, and zero or low skewness. These are deposited by settling from weak bottom currents with a fine suspension load. Muddy to fine sandy contourites (20 to 200 μ m) trend towards better sorting and initially finer and then coarser skew. These are typical depositional trends for contourites. As current velocity and carrying capacity increase, more of the finest fraction remains in suspension and bedload transport becomes more important. Clean sandy contourites (〉200 μ m) are better sorted. They result from the action of dominant bedload transport and winnowing at high current speeds. The results highlight the importance of bottom current velocity, sediment supply and bioturbational mixing in controlling contourite facies. Despite growing interest in their hydrocarbon exploration potential, contourite sands have remained poorly understood. This research therefore has important implications for developing current understanding of these deposits and aiding the correct interpretation of deep marine sands and depositional processes. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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  • 10
    Publication Date: 2018-02-02
    Description: Sorted bedforms are widely present in sediment-starved littoral and inner shelf settings; they are indicators for hydrodynamic conditions and a primary contributor for the subsurface structure. This study investigated the morphology and migration of sorted bedforms on the inner shelf of Long Beach Barrier Island, New York, USA, by repeat geophysical and geological surveys in 2001, 2005 and 2013 (following superstorm Sandy) involving swath bathymetry, backscatter, chirp seismic reflection data and grab sampling. Swath data revealed that the western sector, comprising the western 75% of the survey region, is dominated by NNE–SSW-oriented, 0.5 to 1.0 km wide sorted bedforms with highly asymmetrical cross-sections, with steeper slopes and coarser sands on the eastern (stoss) flanks. Many secondary bedforms were also observed (north–south to north-east/south-west oriented lineation structures) at the western edges of coarse sand zones. The eastern sector displays an unusual sorted bedform pattern that is dominated by coarse-grained substrate, with isolated patches of fine-grained sands oriented north-east/south-west and are 0.15 to 1.0 km in length and ca 30 to 200 m in width, similar in scale and orientation to the secondary bedforms in the western sector. Comparison analysis of the swath data sets indicates that the primary transverse sorted bedform morphology within the western sector was largely stable over this time frame, although the swales were deepened following the storms. The coarse/fine sand boundaries did migrate, however, moving ca 1 to 5 m eastward between 2001 and 2005, and ca 5 to 20 m westward between 2005 and 2013; the higher migration rates (up to 2.5 m/yr) in the latter time period may be attributable to large storm forcing (for example, Irene, Sandy). Significant north-westward migration of the secondary bedforms and coarse sand patches in the western sector as well as fine sand patches in the eastern sector were also observed; these features are far more mobile than the primary sorted bedforms, possibly because they are fine sand drifts that do not erode into the coarse substrate. Seismic reflection data revealed a transgressive ravinement beneath sorted bedforms, merging with the sea floor at the bottom of swales. The authors hypothesize that long-term topographic migration of transverse sorted bedforms contributes to the formation and evolution of the ravinement. This article is protected by copyright. All rights reserved.
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    Topics: Geosciences
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