Bedrock Erosion Research Articles

Insights into temporal earthquake clustering from the Settlement Fault, southeastern Otago, Aotearoa New Zealand

ABSTRACT We combine previous studies, fieldwork, lidar data, and trenching to examine late Quaternary activity of the SE-dipping ≥23 km long Settlement Fault in the southeastern South Island. Trenching of a scarp crossing a small alluvial fan exposed a > 3 m thick sequence of folded sandy to silty fan alluvium. The uniformity of folding and four Optically Stimulated Luminescence dates indicate that these sediments were deposited relatively rapidly at ∼20 ka and subsequently accommodated 2.5 m of throw following ≥1 ruptures on an underlying buried reverse fault. Other key constraints on late Quaternary Settlement Fault activity are: (1) stranding of a shore platform at ∼3.7 ka, due to 1.5–2 m of fault uplift, and (2) a flat bedrock erosion surface overlain by loess and correlated to the last interglacial maximum (∼125 ka), indicating 0–4 m of fault uplift. From incorporating this information into a stochastic model of fault displacement accumulation we derive a slip rate of ∼0.02 mm/yr since 125 ka, and ∼0.27 mm/yr since 20 ka. We link the Settlement Fault’s current phase of heightened activity to a similarly timed increase in Akatore Fault activity, despite the tips of these faults being separated by between 9 and 25 km.

Experimental Study on the Anti-Scouring Characteristics of Bedrock in Engineering Reservoir Areas That Are Conducive to Sustainable Development

High-speed water flow conditions can cause erosion of the bedrock in engineering areas. Due to the lack of accurate evaluation of bedrock scour and erosion rates, there has been a consumption of manpower and resources without achieving satisfactory engineering outcomes. Therefore, studying the scouring and erosion effects of water flow on bedrock is of significant importance for maintaining the sustainable development and safety of engineering projects. Using the bedrock prototype from the Xiaonanhai site in the upper reaches of the Yangtze River, a model test device was developed to conduct anti-scour tests on the bedrock. The study quantitatively examined the basic physical properties, incipient erosion velocity, and erosion rates of different types of bedrock. The study found that the prototype bedrock under natural exposure, submerged immersion, and alternating wet and dry conditions showed a trend of decreased tensile strength, with the alternating wet and dry conditions being the most detrimental to maintaining the physical properties of the rock mass. The anti-scour velocity of silty claystone and clayey siltstone samples increased with the increase in tensile strength, and the erosion rate increased with the increase in shear stress. If the shear stress is kept constant, the erosion rate decreases with the increase in tensile strength. The erosion rate is inversely proportional to the ratio of the bedrock’s tensile strength to the riverbed shear stress, with the fitting relationship showing a piecewise linear distribution. The research results can provide guidance for the safe production of engineering involving bedrock erosion in engineering reservoir areas that are conducive to sustainable development.

The Abundance and Persistence of Plunging Flows in Bedrock Canyons

AbstractLandscape scale bedrock erosion is the integration of bedrock erosion at the reach scale, which is driven by particle impacts from sediment transport caused by near‐bed hydraulics. Plunging flow hydraulics have been identified in bedrock canyons and cause velocity profile inversions, which enhance near‐bed velocities, sediment transport, and the potential for bedrock erosion. Observations of plunging flows are limited, and the frequency and statistical properties of this hydraulic phenomenon have not been investigated. Here, we define metrics to identify velocity inversions and use them to detect instances of plunging flows through a 375 km reach of the Fraser River where channel morphology is controlled by bedrock. Isolated plunging flows are identified as well as plunging flow complexes where a series of plunges cause the core of maximum velocity to remain depressed in the water column for a prolonged distance. A significant relationship between plunging flows and bedrock exposure is identified, and plunging flows occupy more than half of the bedrock confined reaches. Stronger plunging flows are correlated with deeper and narrower channels with higher maximum shear stresses. Plunging flows are also concentrated in steeper reaches, which likely represent knickzones in the river profile. We use particle abrasion‐based bedrock erosion models to show that plunging flows drive reach‐scale incisions in bedrock rivers, creating deep bedrock pools. These pools dominate the incision into the bedrock, which sets the base level for their drainage areas and in turn sets the pace of landscape evolution.

Assessing future ice-sheet variability for long-term safety of deep geological repositories

Abstract. In many regions considered for deep geological repositories (DGR) for nuclear waste, repeated glaciations could occur within the time frame relevant to their long-term post-closure safety (up to 1 million years; Myr). Ice sheets can affect the long-term safety of a DGR by elevating the hydrostatic pressure at DGR depth, altering groundwater flow and chemistry, influencing the frequency and severity of earthquakes, and causing surface bedrock erosion. Therefore, DGR safety assessments must account for uncertainties in future ice-sheet variability, including the timing, frequency and duration of ice sheets at the DGR site. Using coupled ice sheet-climate models to constrain uncertainties in ice-sheet variability over the next 1 Myr is not feasible due to the long timescales involved and substantial computational requirements. Instead, we propose a simplified methodology to assess future ice-sheet variability at potential DGR sites using reconstructions of past ice sheets and global simulations of future climate change. The simulations are conducted using a conceptual climate model driven by changes in insolation and atmospheric CO2 concentrations resulting from anthropogenic emissions. The model is calibrated with 500 000 years (500 kyr) of climate proxy data inferred from deep-ocean sediments. Applying this methodology to the planned Swedish DGR site intended for disposal of spent nuclear fuel in Forsmark suggests that the onset of the next glaciation at the site will not occur until 100 kyr after present, even in the absence of anthropogenic CO2 emissions. However, anthropogenic emissions have the potential to delay the next glaciation in Forsmark by several hundred thousand additional years. Following the initial glaciation, our results suggest that the frequency and duration of subsequent glaciations in Forsmark resemble those observed over the last 800 kyr. Considering uncertainties in anthropogenic emissions and future climate evolution, a wide range of possible future glacial developments is identified. At the extremes of this uncertainty range – developments with a low likelihood of occurrence but relevant for evaluating the robustness of the DGR – we find that Forsmark could experience ice-sheet coverage for nearly half of the next 1 Myr or remain almost entirely ice-free throughout this period. The proposed methodology is easy to implement and applicable to any potential DGR site with a recorded history of glaciations.

Waterfalls Alter Reach‐Scale Fluvial Erosion Rates: Evidence From Field Data and Process Modeling

AbstractWaterfalls are often interpreted as transient, upstream‐propagating features that mark changes in external conditions. Thus, waterfalls are commonly used to infer past tectonic and climatic forcing, making understanding the controls on waterfall erosion central to predicting how external perturbations move through landscapes. Surprisingly, there exist few direct field measurements of waterfall erosion, and existing waterfall retreat measurements are rarely paired with measurements of waterfall morphology and frequency, which, theory suggests, modulate retreat rates. This lack of data limits our ability to test existing theory and explore how waterfalls alter reach‐scale bedrock erosion rates. Here, we use cosmogenic 10Be accumulated in bedrock riverbeds to measure erosion rates in fluvial reaches with varying waterfall frequency and morphology. We find that waterfall‐rich reaches erode one to five times faster than the landscape average, and that reach‐averaged erosion rates increase with increasing waterfall frequency. We develop a new, process‐based model combining waterfall and planar‐channel erosion to explore mechanistic controls on the relative erosion rate between waterfall‐rich and waterfall‐free reaches. This model predicts that reach‐averaged erosion rates increase with waterfall frequency at low sediment supply, consistent with our field measurements, but that waterfalls can also slow reach‐averaged erosion rates for high sediment supply, large grain sizes, low water discharge, or large plunge pools. Our work is consistent with previous suggestions that waterfall erosion rates may decrease in low drainage areas and can influence long‐profile morphology.

Controls on Erosion and Cyclic Step-Formation Upstream of Waterfalls.

Waterfall retreat transmits base-level perturbations upstream, thereby providing markers of changing climate and tectonics. In homogeneous rock, waterfalls often retreat either by direct waterfall-face erosion or incision from repeating ('cyclic') steps formed above waterfalls. We lack knowledge on the conditions driving these different erosion styles, limiting our ability to predict waterfall retreat. We address this knowledge gap through flume experiments assessing how changing flow hydraulics modulates bedrock erosion. We show that, under large discharges, changes in flow hydraulics cause spatial variability in particle impact velocity, leading to cyclic step formation. As discharge decreases, both the magnitude and spatial variability of particle impact velocity decreases, causing more uniform erosion, limiting cyclic step development and potentially allowing direct erosion of the waterfall face to become the dominant retreat mechanism. These results suggest climate change and water-resource management can alter the rate and style of waterfall retreat.

Assessing proxy methods for measuring bedrock erodibility in fluvial impact erosion

AbstractThe erodibility of bedrock and rock masses is an important parameter for understanding landform development, landscape evolution modelling and engineering applications. Yet, complex geotechnical properties and the difficulty of directly quantifying erodibility limit the theoretical understanding and prediction of erosion processes. Several proxy methods have been suggested to assess bedrock erodibility by fluvial impact erosion. Yet, none of these proxy methods have been rigorously benchmarked with direct laboratory or field measurements. Here, we assess the usefulness of proxy methods described in the literature in the quantitative prediction of fluvial impact erosion. We compare four proxy methods – Mohs' hardness, the Schmidt hammer rebound value, Annandale's erodibility index and the Selby score – to erodibility laboratory data measured using erosion mills. We assess these methods using three statistical parameters: Kendall's tau and Spearman's rho rank correlation coefficients, and the adjusted R2 from an exponential fit. We distinguish between three applications, which require increasing correlation strength. These are (i) trend detection (sorting groups of data by their relative erodibility), (ii) quantitative ranking (relative erodibility of groups of data can be quantitatively assessed), and quantitative prediction (erodibility for individual sites can be quantitatively assessed). Mohs' hardness, Schmidt hammer measurements and Annandale's method are suitable for trend detection, while Selby's method is not. None of the methods is suitable for quantitative prediction. As such, none of the methods is a suitable proxy for estimating erodibility in fluvial bedrock erosion at a particular location. For quantitative ranking, we suggest to use either Mohs' hardness or Schmidt hammer measurements, because of (i) the correlation with mill‐measured erodibility, (ii) their ease and quickness of application in the field and (iii) the minimum of required training. When applying these methods, investigators should obtain data both from the same and from different lithological units at many sites. Then, the results can then be used for bulk assessment, but not for individual sites.

Depositional architecture and evolution of Quaternary submarine canyon-fan system in the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea

Depositional architecture and evolution of Quaternary submarine canyon-fan system in the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea

Facies variations in gravelly cyclic steps deposited from turbidity currents: Miocene fan delta front deposits compared with a modern active fan delta, central Japan

AbstractGravelly cyclic steps formed by turbidity currents have recently been widely recognised in the geological record. However, the comparison between modern and ancient gravelly cyclic steps remains challenging. In this study, the facies variations of gravelly cyclic steps deposited from turbidity currents in an outcrop of Miocene fan delta front deposits in central Japan are compared with the geomorphic evolution of analogous cyclic steps on the active fan delta front in modern geological systems. These cyclic steps share common characteristics in setting, grain size and dimensions, allowing direct comparison between ancient and modern examples. Repeat bathymetric surveys of the modern Kurobe River fan delta have revealed various types of upslope migrating bedforms interpreted as cyclic steps. Most of these are short‐lived due to erosion of thalwegs by powerful surge‐type turbidity currents triggered by slope failures or burial of thalwegs by deposition, possibly from river‐fed hyperpycnal flows. Such erosion and burial events occur annually on the fan delta front, resulting in compensational cycles. Sedimentary facies of the Miocene fan delta front deposits include conglomerate with a gently upslope dipping erosional base, backset‐stratified sandstone and foreset‐stratified sandstone, which are interpreted as deposits of hydraulic jumps in high‐density turbidity currents in scours on the stoss sides of cyclic steps. Parallel‐stratified conglomerate sandstone is an additional component. Although previous facies models of gravelly cyclic steps have focussed on deposits on stoss sides, a comparison of modern and ancient examples suggests that facies on the lee sides could be parallel‐stratified conglomerate sandstone and undulating bedforms, reflecting supercritical flow conditions. The present study also suggests that hyperpycnal and surge‐type high‐density turbidity currents may deposit different types of facies and play different roles in the construction and destruction of cyclic steps. The present study has significant implications for facies models of gravelly cyclic steps.

Estimation of Denudation Parameters and River Capture Events From Neural Network Inverse Modeling of River Profiles and Thermo‐ and Geochronology Data

AbstractEarth's topography represents the cumulative effects of tectonics and surface processes modulated by climate and lithology. These factors shape landscapes through time. River profiles can be inverted to estimate the rock uplift histories or lithology‐specific erodibilities. However, river systems are dynamic and evolve in response to spatial and temporal internal dynamics, such as river capture events. Here, we present a modeling framework to infer denudation rates from the inversion of river profiles and thermo‐ and geochronology data. We achieve this by coupling a landscape evolution model and an efficient inverse modeling scheme to infer poorly resolved erosional and tectonic parameters. An application of the approach is presented for the Neckar catchment, southwest Germany, characterized by stark lateral variation in bedrock erodibility and rock uplift, and that have demonstrably undergone multiple river capture events. Different end‐member scenarios are explored in the simulations. First, we test uniform and spatial variability in rock uplift rate and bedrock erodibility, and second, temporal variations in rock uplift rate and base level. Finally, we simulate river capture events by adding upstream sections (drainage area) at specific times and locations within the fluvial network. We find that spatial variation in rock uplift rate is necessary to reproduce the Neckar's river profile while honoring analytical observations. Simulations integrating river captures allow improved river profile predictions of specific tributaries of the Neckar catchment, leading to potentially more realistic erodibility and rock uplift history estimates. The time and location of the capture events determined from the modeling agree with previous estimations from geological evidence.

Pleistocene glacial advances and exposure age scatter in the Olympus Range, Antarctica: A study of cosmogenic 36Cl/3He in dolerites and 10Be in sandstones

Pleistocene glacial advances and exposure age scatter in the Olympus Range, Antarctica: A study of cosmogenic 36Cl/3He in dolerites and 10Be in sandstones

Soil diversity at Jezero crater and Comparison to Gale crater, Mars

Soil diversity at Jezero crater and Comparison to Gale crater, Mars

A mathematical model for bedrock incision in near‐threshold gravel‐bed rivers

AbstractGravel‐bed rivers that incise into bedrock are common worldwide. These systems have many similarities with other alluvial channels: they transport large amounts of sediment and adjust their forms in response to discharge and sediment supply. At the same time, the occurrence of bedrock incision implies behaviour that falls on a spectrum between fully detachment‐limited ‘bedrock channels’ and fully transport‐limited ‘alluvial channels’. Here, we present a mathematical model of river profile evolution that integrates bedrock erosion, gravel transport and the formation of channels whose hydraulic geometry is consistent with that of near‐threshold alluvial channels. We combine theory for five interrelated processes: bedload sediment transport in equilibrium gravel‐bed channels, channel width adjustment to flow and sediment characteristics, abrasion of bedrock by mobile sediment, plucking of bedrock and progressive loss of gravel‐sized sediment due to grain attrition. This model contributes to a growing class of models that seek to capture the dynamics of both bedrock incision and alluvial sediment transport. We demonstrate the model's ability to reproduce expected fluvial features such as inverse power law scaling between slope and area, and width and depth consistent with near‐threshold channel theory, and we discuss the role of sediment characteristics in influencing the mode of channel behaviour, erosional mechanism, channel steepness and profile concavity.

Reconciling Rapid Glacial Erosion and Steady Basin Accumulation Rates in the Late Cenozoic Through the Effect of Glacial Sediment on Fluvial Erosion

AbstractThe onset of glaciation in the late Cenozoic caused rapid bedrock erosion above the snowline; however, whether the influx of eroded sediment is recorded in continental weathering and basin accumulation rates is an ongoing debate. We propose that the transport of glacially eroded bedrock through the fluvial system damps the signal of rapid headwater erosion and results in steady basin‐integrated sediment flux. Using a numerical model with integrated glacial and fluvial erosion, we find that headwater bedrock erosion rates increase rapidly at the onset of glaciation and continue to fluctuate with climatic oscillation. However, bedrock erosion rates decrease in the downstream fluvial system because larger grain sizes from glaciers result in an increase in sediment cover effect. When erosion and sediment flux rates are averaged, long‐term sediment flux is similar to nonglacial flux values, while localized bedrock erosion rates in the glaciated landscape are elevated 2–4 times compared to nonglacial values. Our simulated values are consistent with field measurements of headwater bedrock erosion, and the pattern of sediment flux and fluvial erosion matches paraglacial theory and terrace aggradation records. Thus, we emphasize that the bedload produced from glacial erosion provides a missing link to reconcile late Cenozoic erosion records.

The Fluvial Battering Ram: Collisional Experiments Reveal the Importance of Particle Impact Energies on Bedrock Erosional Efficiency

AbstractThe battering of bedrock by bedload collisions is the primary mechanism by which bedrock rivers erode and landscapes evolve. The energy imparted via impacts acts to detach bedrock via the growth and intersection of surface fractures. We present impact experiments designed to test the influence of particle impact energy on bedrock erosion rates. We found that erosional efficiency increased with increasing impact energy. Notably, these increases in efficiency are not captured by a widely‐used mechanistic bedrock erosion model. Observed increases in erosional efficiency were linked with enhanced elastic energy dissipation captured by differences in the coefficient of restitution. We suggest that this increase in energy dissipation is indicative of enhanced crack extension for high velocity impacts. Our experiments indicate a clear energy‐dependence for bedrock detachment processes that is not yet captured by bedrock incision models but may be integrated into long‐term erosion rates and landscape evolution.

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Abstract Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39–2.28 Ga magmatism and 1.93–1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452–439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

Controls on Sediment Transport From a Glacierized Catchment in the Swiss Alps Established Through Inverse Modeling of Geomorphic Processes

AbstractAs climate warms, hydrology and geomorphology in glacierized catchments are evolving, changing sediment export from these catchments, thus impacting downstream ecosystems and communities. Currently, much uncertainty persists regarding interactions among geomorphic processes that evacuate sediment from glacierized catchments. Here, we present a catchment‐scale numerical model of subglacial and proglacial sediment transport with debris meltout processes. We apply the model in a Monte Carlo framework to suspended sediment data collected over 2014–2020 from the Fieschertal catchment in the Swiss Alps, assessing possible combinations of geomorphic processes responsible for suspended sediment discharge. The ensemble of model outputs quantifies the interaction of different geomorphic processes, including the trade‐off between bedrock erosion and sediment previously stored in the catchment. Model runs suggest that, at some periods, up to 20% of the sediment leaving the glacier is deposited in the proglacial area relative to the catchment's total sediment discharge. This shows that the model captures the proglacial area change from sediment source to sediment sink in different hydrological and glaciological conditions. Furthermore, the findings highlight the impact of glacier retreat on the catchment's sediment dynamics, which both reduces the proglacial area's slope and introduces additional sediment to the proglacial area through debris meltout. The model outputs and the parameters quantify the interaction among geomorphic processes, yielding key insights into the drivers of sediment transport in alpine regions. The implications of these interactions are discussed in the context of interpreting processes responsible for controlling erosion rates from glacierized regions and modeling sediment transport in glacierized catchments.

History of organic pollution in montane lake Issyk-Kul, Kyrgyzstan, Central Asia, inferred from a sediment core

In arid regions, montane lakes are valuable water sources and play important ecological roles. However, recent human-induced inputs of organic pollutants are threatening lake ecology in such regions and becoming a matter of great concern. To investigate pollutant histories and sources, we measured polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in a dated sediment core that spans the last ∼350 years, from montane Lake Issyk-Kul (Kyrgyzstan, Central Asia). Results showed that organic pollutants were delivered to Lake Issyk-Kul in four stages and that their concentrations increased from Stage I (∼1670-1800 CE) to Stage IV (∼2000-2010 CE). Furthermore, we tracked the sources of sedimented PAHs using their ratios combined with n-alkanes data. Ratios of PAHs Ant/(Ant+Phe), Flt/(Flt+Pyr) and Bap/BghiP indicated that inputs during Stage II (∼1800-1970 CE) and Stage III (∼1970-2000 CE) came mainly from high-temperature combustion of coal and vehicle emissions. PAHs in Stage I and Stage IV, however, were mainly derived from low-temperature combustion and petrogenic sources. Diagnostic PAH ratios, combined with the natural n-alkane ratio (NAR＜0) and unresolved complex mixtures (UCM), showed that the sources of PAHs in Stage I were mainly from erosion of bedrock and partly influenced by forest wildfires, different from the source during Stage IV, which was mainly from refined petroleum caused by accidental spills. Our assessment of the contamination history of the lake indicates that toxicity risk to the waterbody from sediment PAHs is low, but recent discharges arising from traffic deserve attention.

The depositional characteristics of turbidity current in Manila Trench: sediment provenance, geochemical elements and organic carbon burial

The depositional characteristics of turbidity current in Manila Trench: sediment provenance, geochemical elements and organic carbon burial

A salty snapshot: extreme variations in basal erosion patterns preserved in a submarine channel

The bases of active submarine channels are marked by large erosional features, such as knickpoints and plunge pools. However, their presence in ancient channel-fills has rarely been documented, so their importance in submarine channel morphodynamics requires investigation. Using seismic reflection data calibrated by wells from a buried submarine channel-fill, we document erosional features hundreds of metres long and tens of metres deep, here interpreted as knickpoints and a plunge pool, and provide a mechanistic process for their transfer into the stratigraphic record. Channel incision patterns are interpreted to record a transient uplift in an otherwise subsiding depocentre. Local structural complexities in the channel slope formed zones of preferential scouring. A switch to a depositional regime preserved the irregular channel base, inhibiting both the upstream migration of scours and smoothing of the channel base. The formation and preservation of these scours record the responses to salt tectonics and provide a unique snapshot of the formative processes of an ancient submarine channel. The presence of these exceptional basal scours indicates that headward erosion processes did not operate rapidly, challenging the paradigm that knickpoint migration controls channel evolution. Our results show that the primary erosion of the main channel surface and long-term channel evolution are both dominated by far more gradual processes.