Braided River Research Articles

Variable Ophiomorpha ichnofabric: Improving the understanding of mouth bar environments in fan-delta complex depositional settings from the Upper Cretaceous of NW South America

Fan-delta deposits, an important sedimentary component of fan-delta complexes, are scarcely described in the ancient record because it is very difficult to distinguish them from other types of coarse-grained deposits (e.g., alluvial, fluvial, or deep marine deposits). This obstacle is overcome when a well-record of physical/biogenic sedimentary structures or fossils unambiguously marks the influence of the receiving basin. However, this is usually not the case. Here, we present the Ophiomorpha ichnofabric record as a key proxy of marine influence within a generally fluvial-dominated system for improving the paleoenvironmental understanding and reconstructing animal-substrate interplay during the evolution of a fan-delta complex. The ichnological record, together with detailed sedimentological analysis, allows us to define three ichnofabric types, distributed in four levels. The stratigraphic distribution of these features points to distal alluvial environments towards the base, dominated by debris and sheet flows deposits and locally shallow braided channels; they evolve upward into well-developed braided fluvial system whose facies arrangement is related to channel fills, locally influenced by debris flows. Subsequently, a basinward migration of the sedimentary system is recorded through the occurrence of the first level with Ophiomorpha ichnofabric A, and then ichnofabrics A and B in levels 2 and 3, respectively, suggesting marine-influenced mouth bar settings. Toward the top, another steep basinward migration is evidenced by Ophiomorpha ichnofabric C in level 4, indicating wave-dominated processes in distal mouth bars environments. The record of marine-influenced and dominated levels may indicate relative sea-level fluctuations related to local or autogenic processes at the basin scale. Furthermore, the present study demonstrates the ability of Ophiomorpha tracemaker to colonize challenging sedimentary environments.

Sedimentary texture of crevasse splays formed by present-day and palaeofloods against the background of floodplain geomorphology and lithofacies exposed in channel cut banks (in the Vistula River valley between Warsaw and Płock, Poland)

Abstract On the floodplain of a sandy, braided river in the Central European Lowland, we studied the sedimentary texture of two crevasse splays, which were: (1) formed due to a levee breach in the 2010 flood, and widely documented by hydrological, bathymetric and LIDAR data, (2) developed under natural conditions when the channel was not embanked by artificial levees (dikes). We compare the sedimentological results with a geomorphological map, a model of the floodplain geomorphology showing different facies of fluvial deposition derived from a meandering river, and deposits in cut banks of the river channel. The statistical parameters of the grain size composition and geomorphic features of the splay, shaped by overbank flow through the broken embankment, are similar to the natural landform. Most of the cut banks (60% of alluvial deposits were mapped there) consist of lithofacies representing proximal floodplain; 30% were distal floodplain (muds), and 10% channel (coarse sand with gravel, pebbles and cobbles). We speculate about the palaeogeography of the Lower Vistula, and the origin of muds and coarse deposits in the banks of the channel. We link these unusual sediments with avulsion, deposition in side arms during ice-jam conditions, and dredging of the channel bottom by suction excavator, which reached the fluvioglacial, suballuvial layer on the bedrock protrusion.

Lithospheric Flexure Controls on Geomorphology, Hydrology, and River Chemistry in the Andean Foreland Basin

AbstractTectonics exerts a strong control over the morphology of Earth's surface that is apparent in active mountain belts. In lowland areas, subtle processes like lithospheric flexure and isostatic rebound can impact Earth surface dynamics, hydrologic connectivity, and topography, suggesting that geomorphic and hydrologic analyses can shed light on underlying lithospheric properties. Here we examine the effect of lithospheric flexure on the geomorphology, hydrology, and river water chemistry of the Rio Bermejo fluvial system in the east Andean foreland basin of northern Argentina. Results show that proximal to the mountain front, foredeep basin subsidence causes sedimentation along a braided channel belt that is superelevated relative to the surrounding flood basin. During floods, water flows from the superelevated channel into the groundwater reservoir, causing a net loss of discharge with distance downstream. Further downstream, forebulge uplift forces channel narrowing, high lateral migration rates, and incision up to 13 m into older river deposits. This incision locally allows groundwater flow into the river, causing a ∼20% increase in river solute load. Groundwater emerges from the forebulge into the backbulge, predominantly as spring‐fed channels. Here, channel migration rates decrease, suggesting a switch from net uplift to subsidence that reduces the depth to the groundwater table. This analysis shows that subtle lithospheric flexure can have significant effects on river channel morphology that determine hydrologic flow paths, and ultimately influence geochemical and ecological patterns. We suggest that these effects may elucidate lithospheric properties that are otherwise inferred from bulk geophysical observations.

The colour temperature and placement of LED lighting near rivers can reduce attraction of flying adult caddisflies

Abstract Artificial light at night (ALAN), which can confuse and distract insects, has been implicated as a contributor of their decline worldwide. Changes in artificial light technology has led to a widespread conversion of outdoor lighting to light‐emitting diode (LED) bulbs, which are more energy efficient than older high‐intensity discharge or fluorescent lamps. However, many outdoor LED lights emit a proportion of short‐wavelength blue light, to which insects are commonly sensitive. The potential to reduce flying insect attraction to outdoor lights by using LEDs that emit different amounts of blue light, or by placing lights further from insect sources has not been well‐studied, particularly for freshwater insects. We tested whether four blue‐white LEDs that varied in their emittance of blue light intensity (3000, 4000, 5650, 6500 Kelvin [K] colour temperature) differed in their attractiveness to flying insects, particularly adult caddisflies (Trichoptera) over distances up to 100 m from a river. LEDs were deployed at a lit urban river and a predominantly unimpacted braided river in Canterbury, New Zealand. We expected a greater abundance and species richness of adult caddisflies to be attracted to LEDs that emit more blue light (cooler white LEDs [6500, 5650 K]) than those that emit less (warmer colour temperatures [3000 K, 4000 K]). We also expected the abundance and species richness of adult caddisflies in light traps to decline rapidly with increasing distance from the river, regardless of colour temperature. Moths, flies and a total of 26 caddisfly species were collected from both rivers. Significantly more caddisflies were attracted to higher than lower colour temperature LEDs. For example, a more than 1.6‐fold number of caddisflies were caught by the bluer‐white 6500 K LEDs than the warmer‐white 3000 K LEDs. This trend was consistent across most caddisfly taxa and both rivers. The number of caddisflies attracted to LEDs significantly declined with increasing distance from the river edge for the two most abundant caddisfly families and the four most abundant species; 65% of all caddisflies were caught by 20 m from the river margin. In order to reduce the attraction of caddisflies to outdoor lights such as streetlights, our study highlights the importance of considering a low LED colour temperature (3000 K or 4000 K) or those that emit less blue light and the benefit of positioning lights further back from waterways, where possible. Our results indicate that plans for urban outdoor lighting should consider the ecological implications of different colour temperature LEDs and their spatial position as project design considerations.

In front of the retreating ice-sheet: Fauna complex of central-western Poland in MIS 3–2 (Krosinko site)

A new locality in Central Europe (Krosinko, Poland) yields a rich fossil material of great taxonomical variety. Its location in the Warsaw–Berlin ice-marginal valley, where it forms a 1-m horizon at a the depth of 6–8 m, is suited to a taphonomic analysis aimed at reconstructing the paleoevents and depositional history of the late Pleistocene (MIS 3–2).Our analysis revealed that the material accumulated as the result of high-energy fluvial transport. This transport disturbed the original bone associations, as indicated by the lack of articulation, low refitting, and bone surface modification. During water movement, the bones were polished, abraded, pitted, and fragmented. Prior to that, however, taphonomic signatures were left behind by predators, humans, and plant roots. This is the first time that unquestionable burning marks have been found on woolly rhino remains from Poland.The secondary context of the faunal assemblage was confirmed by neodymium isotope studies of the faunal fragments and sediment. Given the results of sedimentological analysis from previous studies, these sediments can be associated with a high-energy braided river. The remains were moved along with paleobotanical material and flints in a westerly direction; this is suggested by the direction of water flows in the valley and the river waters supplying it.The deposition occurred within a single geologic event around 26 ka BP, as indicated by the youngest radiocarbon date of the remains, or ca. 19 ka BP, considering the geochronological constraints for the local LGM ice margin position in central and eastern Poland ∼22–18 ka, and the specified accumulation time of the sediments in which they were found. Secondary mineralization of manganese and iron on the bones was the final taphonomical process that impacted the remains.The combined taphonomic, palaeozoological, isotopic and geological results show for the first time for a Polish paleontological site how data integration improves interpretation and highlights the advantages of a multiproxy approach.

Interactions between diurnal winds and floodplain mosaics control the insect boundary layer in a river corridor

Insect flight along river corridors is a fundamental process that facilitates sustainable succession and diversity of aquatic and terrestrial insect communities in highly dynamic fluvial environments. This study examines variations in the thickness of the insect boundary layer (i.e., the pre-surface atmosphere layer in which air velocity does not exceed the sustained speed of flying insects) caused by interactions between diurnal winds and the heterogenous habitat mosaics in the floodplain of a braided river. Based on advective–diffusive theory, we develop and test a semi-empirical model that relates vertical flux of flying insects to vertical profiles of diurnal winds. Our model suggests that, in the logarithmic layer of wind, the density of insect fluxes decreases exponentially with the altitude due to the strong physical forcing. Inside the insect boundary layer, the insect fluxes can increase with the altitude while the winds speed remains nearly constant. We suggest a hypothesis that there is a close correspondence between the height of discontinuity points in the insect profiles (e.g. points with abrupt changes of the insect flux) and the displacement heights of the wind profiles (e.g. points above which the wind profile is logarithmic). Vertical profiles were sampled during three time-intervals at three different habitat locations in the river corridor: a bare gravel bar, a gravel bar with shrubs, and an island with trees and shrubs. Insects and wind speed were sampled and measured simultaneously over each location at 1.5-m intervals up to approximately 17 m elevation. The results support our working hypothesis on close correspondence between discontinuity and displacement points. The thickness of the insect boundary layer matches the height of the discontinuity points and was about 5 m above the bare gravel bar and the gravel bar with shrubs. Above the island, the structure of the insect boundary layer was more complex and consisted of two discontinuity points, one at the mean height of the trees’ crowns (ca. 15 m), and a second, internal boundary layer at the top of the shrubs (ca. 5 m). Our findings improve the understanding of how vegetation can influence longitudinal and lateral dispersal patterns of flying insects in river corridors and floodplain systems. It also highlights the importance of preserving terrestrial habitat diversity in river floodplains as an important driver of both biotic and abiotic (i.e., morphology and airscape) heterogeneity.

Petrology and geochemistry of sandstone from Lower Cretaceous Kikhla Formation, NW Libya: Implications for provenance and depositional setting.

This study aims to determine and investigate the provenance of the source rocks and the depositional setting of the Kikhla Formation, northwestern Libya. It is mainly composed of mature quartzitic sandstone alternating with conglomerate and clay. The study proved that the sandstone is yellow-white in color and commonly carries quartz granular and small fragments of silicified wood. The environmental deposition is under differing fluvial conditions and by a large braided river. Texturally, the Kikhla sandstone is poorly cemented, coarse to very coarse grained and strongly unimodal and is classified as litharenite and subarkose by the modal composition, which is supported by geochemical studies. According to petrography and geochemistry, the Kikhla sandstone was deposited in a passive continental margin basin, mainly from granitic sources. On the basis of major elements ratios, and petrographic interpretation, the source rocks are most likely recognized as Proterozoic granites that were exposed via rifting. The CIA values (73.28-93.97) indicate a high degree of chemical weathering, which could be attributed to the arid climate conditions in the source area and mainly controlled by the source-rock provenance, hydraulic sorting during transport and deposition, diagenesis and depositional environment. According to chemical investigations, sandstone exhibits significant concentrations of SiO2, Na2O> K2O, and Fe2O3, which is compatible with the modal data.

Bedload tracing with RFID tags in gravel‐bed rivers: Review and meta‐analysis after 20 years of field and laboratory experiments

AbstractSoon after their first deployment in rivers in the early 2000s, RFID tags rapidly became the reference technology for bedload tracing in rivers. We can estimate from the literature that during the last 20 years, more than 30,000 RFID tracers have been injected in gravel‐bed rivers all around the world to study bedload transport. Many field experiments have been reported in a great diversity of fluvial environments, complemented by many laboratory experiments and methodological developments. This paper proposes a review of these works, notably based on the compilation of more than 350 RFID surveys, complemented by 97 magnetic surveys, for a total of 125 study sites. The meta‐analysis of this database shows that RFID tracers have improved our understanding of sediment transport in fluvial environments with rapid bedload dispersion. It is also shown that central positions of tracer plumes are moving faster over time than tracer leading fronts, as attested by a general relation between maximum and mean distances of transport. The most recent methodological developments based on the use of active UHF RFID tags show that it is now possible to conduct efficient bedload tracing experiments not only in small streams, but also in large gravel‐bed rivers or very active braided channels. Other addressed topics include RFID deployment and survey in river channels, controlling factors of tracer mobility (flow conditions, grain‐size and shape, channel morphology), bedload monitoring approaches using RFID tracers, and applications of RFID tracers for evaluating human effects on bedload transport. Key challenges of bedload tracing with RFID tags are also proposed.

Early to Middle Jurassic sedimentation within the northern Sichuan Basin in response to exhumation of the South Qinling orogenic belt, central China

The northern Sichuan Basin of central China is an important region for investigating the continent-continent convergence that resulted from the closure of the Palaeo-Tethys Ocean. The Early-Middle Jurassic sediments of this basin record the palaeogeography, palaeoenvironments and tectonic evolution of the southern Qinling orogenic belt, especially the Dabashan thrust belt. Detailed sedimentological investigations, coupled with provenance analyses using zircon UPb geochronology, demonstrate that during Early Jurassic times, conglomerate megafan facies extended across the northern Sichuan Basin into the Micangshan area deformed by South Dabashan thrusting and reached even as far as the Mianlue suture belt. These megafan deposits were mainly reworked from the Silurian sediments of the North Dabashan thrust belt (also called the South Qinling orogenic belt), north of the Mianlue suture. These conglomerates record the initial thrust-driven uplift and exhumation of the Qinling orogenic belt that rapidly followed the diachronous closure of the Mianlue oceanic basin. Moreover, our results show that the detrital materials of meandering stream, lacustrine and braided river delta facies that accumulated in the Middle Jurassic mainly originated from the southern Qinling orogenic belt, Mianlue suture zone and northern South China block, suggesting the southward progradation of the Dabashan fold-thrust belt onto the northern Yangtze block and the southward migration of the basin centre. These findings support a model in which the Early to Middle Jurassic evolution of the northern Sichuan Basin may have been triggered by East Asian convergence.

Intelligent architecture modeling for multilevel fluvial reservoirs based on architecture interface and elements

At present, the architecture modeling method of fluvial reservoirs are still developing. Traditional methods usually use grids to characterize architecture interbeds within the reservoir. Due to the thin thickness of this type of the interlayers, the number of the model grids must be greatly expanded. The number of grids in the tens of millions often makes an expensive computation; however, upscaling the model will generate a misleading model. The above confusion is the major reason that restricts the large-scale industrialization of fluvial reservoir architecture models in oilfield development and production. Therefore, this paper explores an intelligent architecture modeling method for multilevel fluvial reservoirs based on architecture interface and element. Based on the superpositional relationship of different architectural elements within the fluvial reservoir, this method uses a combination of multilevel interface constraints and non-uniform grid techniques to build a high-resolution 3D geological model for reservoir architecture. Through the grid upscaling technology of heterogeneous architecture elements, different upscaling densities are given to the lateral-accretion bedding and lateral-accretion bodies to simplify the model gridding. This new method greatly reduces the number of model grids while ensuring the accuracy of lateral-accretion bedding models, laying a foundation for large-scale numerical simulation of the subsequent industrialization of the architecture model. This method has been validated in A layer of X oilfield with meandering fluvial channel sands as reservoirs and B layer of Y oilfield with braided river sands as reservoirs. The simulation results show that it has a higher accuracy of production history matching and remaining oil distribution forecast of the targeted sand body. The numerical simulation results show that in the actual development process of oilfield, the injected water will not displace oil in a uniform diffusive manner as traditionally assumed, but in a more complex pattern with oil in upper part of sand body being left behind as residual oil due to the influences of different levels of architecture interfaces. This investigation is important to guiding reservoir evaluation, remaining oil analysis, profile control and potential tapping and well pattern adjustment.

Mineralogical and Grain Size Analysis of Bai Hassan Formation Terraces in Al-Teeb Area, SE Iraq

Bai Hassan Formation is exposed as remnants of beds from previous formations due to severe erosion that were removed. Results from grain size analysis revealed that most of the studied sediments were coarse-grained sediments ranging from coarse sand to gravel. The study shows that the sediments are poorly sorted. The values of skewed coefficients ranged from fine to strongly skewed, and the negative values indicate the occurrence of hard changes in the environment. Meanwhile, the kurtosis values for the studied sediment indicate that the differences in these values are due to changes in the flow and sediment sorted in a relatively high energy environment. Light minerals like quartz, and feldspars, and rock fragments are identified. Heavy minerals such as chlorite, amphibole, pyroxene, mica, epidote, zircon, garnet, tourmaline, rutile, kyanite, and staurolite can provide information about the geological processes and environments that occurred in a particular area. These minerals can be indicative of specific rock types, metamorphic or igneous formations, that are derived from high Zagros mountains. The collision between the Arabian Plate and the Eurasian Plate resulted in the formation of the Zagros Fold and Thrust Belt, leading to the uplift and deformation of the Zagros Mountains. The Bai Hassan Formation in Iraq originated during the Miocene epoch from fluvial and alluvial fan environments. Sediments carried by rivers and streams from the elevated mountains in Iraq and Iran were deposited, forming conglomerates, sandstones, and mudstones that reflect various depositional settings such as braided river channels and floodplains indicating a rapidly sinking basin with freshwater and lacustrine environments.

Volcanosedimentary fill of the early Cretaceous Yingcheng formation and response to the end of continental rifting in the Songliao Basin: Constraints from well SK-2, northern Xujiaweizi fault depression

Well SK-2 of the International Continental Scientific Drilling Project (ICDP) obtained continuous core samples with a length of 365.32 m in the Early Cretaceous Yingcheng Formation, showing a stratigraphy of interbedded rhyolites and coarse clastic rocks in the Xujiaweizi fault depression. The Yingcheng Formation in well SK-2 is divided into three sequences. Sequences 1 (S1) and 3 (S3) consist of coarse clastic rocks, and four types of conglomerate lithofacies are identified. The conglomerate lithofacies of S1 is characterized by gravity flows. In comparison, the conglomerate lithofacies of S3 is characterized by a progradational braided channel, reflecting a proximal provenance and a shallowing upward setting. Sequence 2 (S2) rocks are rhyolites, and geochemical analysis shows these to have mainly originated from the middle-upper crust. On the basis of volcanic rocks in wells Shengshen 2–1 and Shengshen 203 and seismic profiles crossing these wells, we identify two types of volcanic edifices: stratovolcanoes in the east and shield volcanoes in the west. The Early Cretaceous strike-slip faults, including the Xuzhong and Xudong faults, play an important role in controlling this area. In comparison with extensive, dark, and fine-grained clastic rocks of the Shahezi Formation, the felsic and coarse clastic rocks of the Yingcheng Formation in well SK-2 mark a sedimentary response to strike-slip tectonics and indicate the ending fault movement of the Xujiaweizi fault depression, and perhaps even the end of the Early Cretaceous Songliao Basin rifting stage.

A Method for Defining Sedimentary Characteristics and Distributions and Its Application in Qinnan Depression, Bohai Bay Basin

A new method incorporating geophysical analysis and geological analysis is proposed to define the sedimentary characteristics and distributions in basins with few drilling wells to promote the exploration of reservoirs. This method is applied to a study, through which its principles, closed-loop workflow and technologies are introduced in detail and the sedimentary characteristics and distributions of the study area are accurately defined. During the application process of the method, a compatible geological model is established, based on which the seismic data are interpreted and the results derived from the interpretation are further verified via seismic forward modeling. The study results exhibit a successive sand-rich deposition from the retrogradational gully-filling gravity flow deposition including near-shore fans, slope fans and basin-floor fans delimited by different slope break belts in transgressive sequences to the progradational delta deposition in a retrogressive sequence including braided river deltas with a long extension distance and fan deltas developed along a steep slope belt. And the potential reservoirs are located at the point-out sites of sand bodies with lower average P-wave velocities than those of muddy sediments. The proposition and application of this method are of great significance for oil and gas exploration.

Spatial responses of water quality to river density and connectivity alterations on the Taihu Plain.

With the advancement of urbanization, the structure and connectivity of river networks have been changed by the interference of human activities, resulting in a series of water environment problems. Numerous studies have indicated that river networks are associated with water quality; unfortunately, few studies have revealed the contributions of the structure and connectivity of river networks to variations in water quality. Taking one water conservancy region with dense and braided rivers on the Taihu Plain as an example, we depicted the spatial aggregations of water quality using the Getis-Ord Gi* index, quantified the variations in polluted regions using the standard deviational ellipse method, and quantified the influence of river density and connectivity on water quality during the different seasons. The results showed that (1) the water quality during the flood season was better than that during the non-flood season, especially in the western region; (2) the spatial aggregations of most water quality indicators were higher and the polluted regions increased in size during the flood period compared to the non-flood period; and (3) the relative contribution rates of the river density and connectivity exhibited mean values of 62.5% (61.2%) and 37.5% (38.8%) in the flood (non-flood) period. Our results provide theoretical support for enhancing water environment management.

Seismic Response Variance of Depositional Sequences: Implications for Reservoir Prediction in Lacustrine Basin

In recent years, lithologic oil and gas reservoirs have become an important target in continental hydrocarbon-bearing basins. Geophysical prospecting technology using seismic data is an indispensable tool for oil and gas exploration. However, while previous work has paid much attention to the seismic responses of reservoirs (sandstones), the seismic responses of depositional sequences composed of sandstone–mudstone cycles are not well understood in reservoir prediction. This problem seriously restricts efficient oil–gas exploration and development. The Cretaceous Baxigai Formation in the Yingmaili area, west of the Tabei Uplift, is an important exploration target for lithologic oil and gas reservoirs in the Tarim Basin. The Baxigai Formation is deeply buried with thin thickness. The Baxigai Formation in the study area is divided into a lower sandstone section and an upper mudstone section. Braided river delta sand bodies are developed in the lower sandstone section, and braided river delta sand bodies and beach bar sand bodies are developed in the upper mudstone section. According to the difference in the depositional sequences in different zones, five types of the vertical combination style of sandstone and mudstone were identified. Through seismic forward modeling, the seismic response variance of the five kinds of sequence models was established. Then, the amplitude attributes were extracted via wavelet decomposition to reflect the distribution of sandstone–mudstone in different zones. This could help predict the vertical and horizontal distributions of different depositional sequences and the sandstones in these sequences. During the sedimentary period of the upper mudstone section of the Baxigai Formation, the beach bar sand bodies were distributed along the northeast coast. The thin sand bodies pinched out along the up-dip direction to form favorable lithologic traps, which has important significance for lithologic reservoir exploration.

Understanding the effect of long term and short term hydrological components on landscape ecosystem

Modifications in river morphology are witnessed due to the hydrological fluctuations leading to braided rivers and sandbar developments. Understanding the hydrological alterations could offer crucial insight into the ecosystem's landscape pattern. To comprehend flood and drought in an agricultural ecosystem, we investigated the impact of short-term water level changes from 1999 to 2015. Numerical simulations were carried out on the fluctuant streamflow at different seasons in a braided river using a two-dimensional hydrodynamic model. The number of consecutive wet days (CWD), average wet days (AWD), consecutive dry days (CDD), and average dry days (ADD) were computed. The Standardized Streamflow Index (SSFI-4) was determined at a seasonal time scale, which is used to quantify the streamflow-based hydrological droughts. The calibration and validation of the hydrodynamic model was done with the measured velocity. To understand the planform geometry of the river, satellite images were derived for the historical period from 1976 to 80 till 2016–17. The Planform Index (PI) and Sinuosity Index (SI) were used to assess and classify the river into straight, braided and meander categories. Speedy modifications were observed across the longitudinal profile, with PFI varying between 3.72 and 42.8. Sandbars being an imperative asset for floodplain communities and dwellers, the effect of these fluctuations was studied in different landscape classes. We analyzed the changes in the spatial and temporal characteristics of dynamic landscape patterns between 2004 and 2014 at quinquennial and decadal scales. We found that a substantial-high water level from a 2010 flood led to a drastic change in the landscape pattern of the ecosystem. In addition, consecutive dry days witnessed over the sandbar regions affect the agriculture cropping patterns. Post flooding, the expansion of land under agriculture could be inferred from the fact that sediment was fertile and deposited after the flood recession; agriculture was dominantly practiced in the region. Overall, the study demonstrated that various hydrological components typically impact the spatial distribution characteristics of landscape patterns.

Morphometry of the cold-climate Bory Stobrawskie Dune Field (SW Poland): Evidence for multi-phase Lateglacial aeolian activity within the European Sand Belt

Abstract Aeolian dunes have been widely identified in the European Sand Belt, which was formed during the Pleniglacial and Late Glacial when cold and dry climatic conditions were favorable for intense Aeolian processes. In this study, we mapped and analyzed the fixed Bory Stobrawskie Dune Field (SW Poland) to determine factors that drive the evolution of dunes, expressed by the occurrence of different dune types and their spatial patterns. The study identified the longitudinal zonation within the dune field, as shown by the changeable proportion of specific dune types comparable to low-latitude dune fields. However, climatically controlled periodic and low sand supply combined with a changing vegetation cover caused the non-continuous and multi-phase evolution of the dune field. Additionally, we found that a dense pattern of streams has controlled the extent of the dune field. The trapping of sand by rivers led to a limitation of the dune field expansion; on the other hand, the supply of sand into rivers led to overloading of the fluvial system, affecting their transformation into braided rivers.

Geochemical characteristics of the braided river reservoir in block 19 of the sulige gas field

The sand body structure and geochemical characteristics of braided river reservoirs are the key geological factors affecting gas production and development effects. The Sulige gas field in the Ordos Basin is an important large-scale gas-producing layer. Owing to the control of sedimentary facies, the geological structure of the sand body changes greatly and its connectivity is poor. The geological characteristics have not yet been elucidated, and this is an important problem restricting the development of the Sulige Gas Field. To solve this problem, this study focuses on the braided river reservoir of the Shihezi Formation in Block 19 of the Sulige Gas Field, conducts geological surveys in the study area, analyzes the geological and geochemical characteristics of the reservoir, and obtains samples through drilling. Through a thin-section test, gas-water two-phase experiment, and simulation test, the braided river reservoir configuration and pore and gas-water characteristics are obtained. The results show that the reservoir lithology in the study area is mainly composed of quartz sandstone, lithic sandstone, and quartzy lithic sandstone, with a porosity of 3%–13% and a permeability of (0.05–0.7) × 10−3 m2. The reservoir has low porosity and low permeability. After drilling samples were obtained, 32 thin-section rock samples were selected. The pore types of the block reservoir mainly (82.9%) consisted of intragranular and intergranular dissolved pores. The difference in pore structure was mainly reflected by the size and distribution of the throat. The distribution of physical properties was 6%–10%, the gas saturation was 61%, the NMR effective porosity was 7.49%, the permeability was 4.08 × 102 μm2, and the physical properties were relatively good. In terms of the study area, the average thickness of the single braided channel in the lower section of He 8 was 4.7 m, the average width of the channel was 963 m, and the composite channel was distributed in a potato shape, parallel to the direction of the main flow. The average length of the channel was 2,147 m and the average width was 844 m. As the porosity increased, the efficiency of gas-driven water also increased, and there was a linear positive correlation between porosity and gas-driven water efficiency. With the increase in movable water saturation, the water-air ratio became larger and water production was greater. In low-amplitude structures and under low-permeability background conditions, for reservoirs with good local pore structure and physical properties, the water remaining at the bottom of the reservoir or sand body was controlled by the accumulation conditions or the weak structural differentiation after accumulation. In terms of the gas and water produced simultaneously in the study area, gas production was less than 2 × 104 m3/d and water production was relatively large at more than 10 m3/d; gas and water were mainly distributed in the downdip part of the main channel structure or in the island lens-shaped permeable sand bodies trapped by the surrounding tight layers. The study results provide theoretical data support for the exploration and production of the Sulige Gas Field.

Meandering rivers that anabranch in the upper Midwest (USA): Prevalence, morphological characteristics, and power regimes

Channel planform is the result of present and past hydrologic, sedimentary, and geologic conditions, and represents an adjustment of fluvial systems to these conditions. An important yet overlooked aspect of planform diversity of rivers in intensively managed agricultural landscapes of the upper Midwest (USA) is the co-occurrence of single-thread meandering and multi-thread anabranching reaches. Recent work on the role of river corridors in critical-zone dynamics of intensively managed landscapes has revealed the existence of juxtaposed meandering and anabranching reaches, yet the prevalence of anabranching reaches within meandering rivers in the upper Midwest is unknown. In addition, the morphological characteristics of these reaches and whether the stream power domain of these reaches differs from that of meandering reaches are also unclear. This paper quantifies the prevalence, planform morphology, and stream power domain of anabranching reaches along numerous lowland meandering rivers in the upper Midwest. Anabranch prevalence is explored for five representative watersheds using Google Earth aerial imagery and airborne LiDAR data. Less than 1 % of the total stream length in these watersheds (∼42,000 km) exhibits anabranching, but no watershed is devoid of anabranching reaches. Most anabranching reaches are characterized by a two-channel looping configuration, although complex anabranches with more than three channels exist at a few locations. Mean bifurcation angles (70° ± 29°) of the anabranching reaches are higher than those of braided rivers (40°-60°), with maximum angles as high as 150°, whereas mean confluence angles (60° ± 28°) fall within the range of junction angles in stream networks (45°-75°). Stream power analysis indicates that the occurrence of anabranching reaches along meandering rivers in the upper Midwest is significantly related to local increases in channel slope. The stream power domain of most anabranching reaches in the upper Midwest falls within that of meandering rivers, although some reaches have power similar to that of braided or wandering gravel-bed rivers. When considered collectively, multi-thread reaches juxtaposed with meandering reaches do not fit neatly into existing classifications of anabranching rivers.

Unconformities and Gold in New Zealand: Potential Analogues for the Archean Witwatersrand of South Africa

Possible young analogues for regionally extensive unconformities (100 to 400 km2) in the gold-bearing Witwatersrand Supergroup (Archean, South Africa) occur in the South Island of New Zealand. Extensive marine unconformities in New Zealand show progression from an unconformity surface to conglomerate to clean well-sorted sandstone to marine mudstone, as is also found in the major Witwatersrand auriferous reef horizons. The hosting young sedimentary basins of the South Island rest on thin or thick crust on inboard and outboard foreland settings, with variable alluvial gold budgets. They expose the Cretaceous–Oligocene Waipounamu Erosion Surface unconformity that formed when most of New Zealand was subsiding, and Pleistocene–Holocene unconformities related to global sea level changes. The Witwatersrand gold-bearing reef sediments are a good match for such marine transgressions, but not alluvial fans or braided streams. Most Witwatersrand gold is immediately above planar unconformity surfaces and not restricted to, or concentrated in, erosion channels that are incised through the reefs. However, in modern alluvial fans or braided streams, gold is almost entirely in erosion channels on a smaller scale than the Witwatersrand gold reef packages and not spread across the planar unconformities. Alluvial fans and braid plains in New Zealand dilute gold with large volumes of gravel.