Pediment Surface Research Articles

Late Silurian event stratigraphy and facies of South Wales and the Welsh Borderland, United Kingdom

AbstractLithofacies and biostratigraphical analysis has enabled the establishment of a stratigraphic event framework for Ludfordian and Pridoli strata in south Wales and the Welsh Borderland. In SW Wales, the Golden Grove Axis acted as a structural hinge separating the shallow marine storm-influenced Cae’r mynach Seaway from a pediment surface above which Ludfordian colluvium (Abercyfor Formation) was deposited. The Axis seeded four NW-derived river-influenced delta progrades of Leintwardinian to early Pridoli age (Tilestones Formation). A NE-sourced early Pridoli wave-influenced delta deposited the Downton Castle Sandstone Formation (DCSF), coeval to the youngest Tilestones prograde, with a lateral interface existing between Mynydd Epynt and the Clun Forest area. Except for the Malverns area, the DCSF is no longer recognized south of the Neath Disturbance. Early Pridoli forced regression promoted widespread subaerial exposure north of the Neath Disturbance, with incision into tracts close to the Welsh Borderland Fault System. The basinward-shift in facies belts resulted in marine erosion and deposition of a phosphatic ravinement pebble lag. The wave-influenced Clifford’s Mesne Sandstone Formation delta subsequently seeded on the Gorsley Axis with tidally influenced Rushall Formation accumulating in a back-barrier setting. The Pwll-Mawr Formation records the easterly advance of coeval coastal deposits on the western side of the remnant Cae’r mynach Seaway. Behind migrating delta coastlines, green muds accumulated on coastal plains (Temeside Mudstone Formation) with better drained red dryland alluvium (Moor Cliffs Formation) charting expansion of Old Red Sandstone lithofacies. Mid-Pridoli incision preserves the Pont ar Llechau Formation estuarine valley fill.

Terrain analysis and hydrogeomorphic investigation of the Sita-Swarna river basin, Udupi, SW India: Insights from remote sensing methods

Hydrogeomorphic characteristics of the Sita-Swarna river basin, Western Ghats, India, are investigated to understand the influence of lithology, structure and geological control on the channel morphology. For preliminary investigation, remote sensing, geomorphic and morphometric analysis was carried out, producing slope, aspect and lineament maps from a 30 m resolution Digital Elevation Model (DEM) with the help of GIS. Remote sensing analysis inferred a steep south-to-southwest slope with dominant lineaments trending NW and ENE to E in the middle reaches of the Sita-Swarna river basin. These lineaments are deduced to have possibly been instigated by faults, shear zones, dyke ridges, or subsurface structures that influence the surficial topography. Upland and pediment geomorphic zone were identified, inferring Western Ghats Escarpment (WGE) and gently coastward W-sloping dissected pediment surface. A comprehensive morphometric analysis is performed using various linear, areal, and relief parameters for basin characterization. The linear parameters infer that the basin is structurally controlled and shows moderate to high permeability and flood susceptible areas downstream. The calculated morphometric parameters for basin characterization reveals that the Sita-Swarna river basin is at the mature stage of development and highly asymmetrical with the westward tilt, which indicates a possible influence of paleo tectonism. Rapid runoff is observed due to the steep escarpment leading to less infiltration and flash flood possibility during heavy rainfall. The present study mainly establishes the potential application of remote sensing methods to quantify the hydrogeological aspects, basin morphometry and landscape evaluation in the tropical climatic zone.

Soil-landscape relationships in Vedavathi river basin, Chitradurga district, Karnataka, India: morphology and textural and chemical properties

The soils on granitic hilly landscapes of southern Karanataka plateau have displayed varying degrees of pedogenesis with argillic/natric/cambic and slickensided horizons. Seven pedons occurring on five geomorphic surfaces of granitic landscape in a part of Vedavathi river basin (summits, side slopes, pediplains, pediments and valley floors) were studied in order to evaluate the pattern and variability of soils in terms of morphology and chemical and granulometric characteristics. The field morphology showed that these soils have dark brown to red clay-rich subsoils on pediment surfaces, but moving down to the slopes of valley floors, salts and carbonates have accumulated. Texturally, the A horizons have loamy sand to sandy loam but have sandy clay loam to sandy clay in argillic B horizons (P1, P4 to P7) and sandy clay to clay in Bhagirathekere (P2) and Shidahlaihankote soils (P3). The lithological discontinuities in P1/P2/P5 were confirmed with irregular distribution of sand to silt ratio and negative sign of uniformity value (> ± 0.6). The results of ANOVA showed significant difference between soils for total silt (F=6.07), for very fine sand (F=7.2) and for total sand (F=6.41). The sediments are mainly fine sand, well to moderately well sorted, positively skewed and meso- to platykurtic in nature due to low-energy sheltered environment. These soils are slightly to moderately alkaline with significant variation in organic carbon (<0.5%), calcium carbonate and cation exchange capacity of argillic horizons with significant gains of clay. The soils on basin floors P1 and P2 have exchangeable sodium more than 15 in subsoils suggesting development of sodicity. The qualifiers such as cutanic (P4/P5/P6) and lamellic (P5) in some argillic horizons and natric horizon in P1 as solonetz indicating sodic conditions under WRB show an advantage over USDA soil taxonomy. Classifying colluvial soils is important in semiarid regions of India to map these units for future correlation activity. The study revealed the difficulties in assigning soil age using clay index under granitic landscapes having different geomorphic surfaces with limited data set.

Late Miocene - Quaternary forearc uplift in southern Peru: new insights from 10Be dates and rocky coastal sequences

We explore the coastal morphology along an uplifting 500 km-long coastal segment of the Central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field surveys to extract sequences of uplifted shorelines along the study area. In addition, we consider continental pediment surfaces that limit both the geographical and vertical extent of the marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We expand this corpus with new 10Be data on uplifted shore platforms. The last 12 Ma are marked by three periods of coastal stability or subsidence dated ~12-11 Ma, ~8-7 Ma and ~5–2.5 Ma ago. The uplift that accumulated between these stability periods has been ~1000 m since 11 Ma; its rate can reach 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the carving of numerous coastal sequences. Within these sequences, we correlated the marine terraces with the sea level highstands (interglacial stages and sub-stages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this last period of uplift increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward, up to 0.45 mm/a, due to the influence of the Nazca Ridge. In this study, we document an unusual forearc cyclic uplift with ~4 Ma-long cycles. This periodicity corresponds to the predictions made by Menant et al. (2020) based on numerical models, and could be related to episodic tectonic underplating (subducting slab stripping) beneath the coastal forearc area.

80,000 years of geophysical stratigraphic record at the Serra da Capivara National Park, in northeastern Brazil: Uncovering hidden deposits and landforms at a canyon's floor

Acquisition of high-resolution geophysical spatial data at the Serra Branca canyon, in the Serra da Capivara National park, in the semiarid of the Northeastern Brazil, enables depicting deposits and relict landforms that have never been reported before for the area. Ground Penetrating Radar (GPR) and GPS real-time kinematics (RTK) enable uncovering wide cut-and-fill structures, a relict perched river channel, relatively well-stratified alluvial deposits and two relict pediment surfaces that border the current canyon's thalweg. Optically Stimulated Luminescence (OSL) geochronology shows that the stratigraphic archive tends to fit important global climatic markers of the last glacial cycle. The relict river channel was active at least from marine isotopic stage (MIS) 5a to MIS 2, showing low sedimentation rates during about 60,000 years. Between the last glacial maximum (LGM) and the Holocene, the paleochannel was abandoned. The canyon's floor was incised, and the current thalweg was carved out, probably following a local baselevel drop. Well-structured alluvial deposits have accumulated near the outlet of the canyon since at least the Early Holocene. During the Middle Holocene, colluvial and alluvial deposits accumulated in sites near the cliffs that border the canyon. The study's geochronology coincides with the pace of independent paleoclimatic proxies from Northeastern Brazil, suggesting that a humid millennial drift was associated with the incision of local bedrocks and accumulation of alluvial and colluvial deposits during the Early and the Middle Holocene. Abandonment of the Pleistocene river channel, incision of the local baselevel, and creation of the canyon's current thalweg were probably caused by a Late Glacial humid spell that may coincide with Heinrich event 1 (H1), about 16,000 years ago. However, complementary geochronologic and sedimentologic data are crucial for improving knowledge about this major drainage system change at the study site.

Role of transverse structures in paleoseismicity and drainage rearrangement in rift systems: the case of the Valdecebro fault zone (Teruel graben, eastern Spain)

The E–W trending, nearly pure extensional Valdecebro fault zone is a transverse structure at the central sector of the N–S Teruel graben. It was activated by the Late Ruscinian (Early Pliocene, ca. 3.7 Ma), giving rise to structural rearrangement of the graben margin. Until the Late Pleistocene, it has accommodated a net slip ca. 205 m, with slip rate of 0.055 mm/a. Paleoseismicity has been analysed in a 29-m-long, 5-m-deep trench excavated through a fault branch that offsets a Pleistocene pediment surface. The paleoseismic succession includes a minimum of 6–7 events occurred since ca. 142 ka BP, although a model with 12 events could be more realistic. The following paleoseismic parameters have been inferred, assuming a minimum of 6 and a maximum of 12 events: average coseismic slip = 58–117 cm; recurrence period = 8.4–28.4 ka; potential moment magnitude Mw = 5.8–5.9. The recorded displacement since ca. 142 ka BP totalizes 7.0 m, with slip rate of 0.05–0.07 mm/a. Slip on the transverse Valdecebro fault zone has critically contributed to bulk deformation under a prevailing ‘multidirectional’ extensional regime. Drainage patterns have been rearranged, recurrently switching between westward and southward directions as a consequence of diverse slip episodes at the Valdecebro fault zone (E–W) and the neighbouring La Hita (N–S) and Concud (NW–SE) faults. The ultimate westward drainage of the Valdecebro depression incised and dismantled a southward-sloping Pleistocene pediment sourced at the Valdecebro mountain front, representing a capture by the Alfambra river occurred between 124 and 22 ka BP.

Catchment inversion during the Silurian of SW Wales: Pediment plains, embryonic drainage networks and incised valley fills in a dryland range front alluvial system

Ludlow to Přídolí age Old Red Sandstone alluvial to marginal marine deposits of the Anglo-Welsh Basin record a punctuated molasse shed from an inverted, transient catchment area. An erosional low-gradient sequence-bounding pediment surface was created basinward of a structurally controlled, transpressive piedmont junction. A thin mantle of debris flow dominated colluvium comprising the Abercyfor Formation was deposited proximal to a lowstand delta shoreline 90 km to the east and south. Temeside Mudstone Formation coastal plain deposits developed behind the delta passing westward into range front, dryland red bed alluvium of the Moor Cliffs Formation. This embryonic mudstone-dominated alluvial plain aggraded during an interval of enhanced accommodation space development. Muds were liberated from their Lower Palaeozoic bedrock provenance within and to the north of the piedmont junction. Dryland alluvial sandstone channels possibly represent stream capture events associated with hinterland drainage reorganisation. The latter promoted continued headward erosion and enlargement of the catchment area.Basinwide incision (to a depth of approximately 60 m) of the alluvial plain occurred in the mid Přídolí, with valley locations being structurally controlled. Subsequent fills preserve the middle reaches of a simple incised valley system. Lowstand to early transgressive streamflow conglomerates of the Freshwater East Formation were deposited, with sediment transport to the SE. Valley drainage was defeated by neotectonic uplift of the Orielton Anticlinorium in downstream valley reaches leading to drainage reorganisation. Accommodation space was developed at structural tip points where maximum rates of marine transgression ensued preserving backstepping parasequences of a wave dominated estuary. Localised debritic alluvial fans were shed below the interval of maximum flooding. Continued dryland sedimentation led to highstand filling and subsequent basinwide alluvial regression which continued into the late Přídolí.

Evaluating Quaternary activity versus inactivity on faults and folds using geomorphological mapping and trenching: Seismic hazard implications

The incorporation of active faults in seismic hazard analyses may have a significant impact on the feasibility, design and cost of major engineering projects (e.g., nuclear facilities, dams), especially when located in the site vicinity. The regulatory definition of active versus inactive fault is generally based on whether the fault has ruptured or not after a specific chronological bound (i.e. fault recency). This work presents a methodology, mainly based on geomorphological mapping and trenching, for determining whether specific faults can be considered as active or inactive. The approach is illustrated through the analysis of several faults located in the Spanish Pyrenees (Loiti, Leyre, La Trinidad, Ruesta faults). The 29 km long Loiti Thrust was included in the Neotectonic Map of Spain as a probable neotectonic structure. Previous works, based on geomorphological investigations, incorporated the 28 km long Leyre Thrust as a significant seismic source in a probabilistic seismic hazard analysis, which challenged the seismic design of nearby large dams. The production of detailed geomorphological strip maps along the faults allowed the recognition of specific sites where the faults are covered by Quaternary deposits. The establishment of chronosequences (pediments-terrace sequences) and the available geochronological data helped identifying the most adequate morpho-stratigraphic units for satisfactorily evaluating fault activity vs. inactivity. The excavation of trenches at the selected sites provided unambiguous information on the presence or lack of deformation in the Quaternary cover overlying the fault, and the origin of scarps (tectonic versus erosional). Trenches were also useful for collecting samples and reliably measuring the relative height of terraces overlain by thick colluvium. The evidence gathered by these methods were complemented with the numerical dating of non-deformed slope deposits covering a fault, the analysis of the longitudinal profiles of old pediment surfaces located in the proximity of a fault, the examination of a cave situated next to a fault searching for speleoseismological evidence, and regional geodetic and seismotectonic data (GPS measurements, earthquake focal mechanisms). The integration of all the data, and especially the trenches dug in non-deformed old terrace deposits (>100 ka) truncating the faults, indicates that the analysed faults can be considered inactive and that previous neotectonic postulations were based on non-valid geomorphological interpretations.

Impacts of Material Engineering Properties on Slope Wash and Stability in Fine-Grained Bedrock Slopes at Fossil-Bearing Sites, Badlands National Park, South Dakota, USA

Engineering properties of bedrock materials at Badlands National Park were used to develop models for Park managers to assess slope erosion and stability for fossil resource protection. Six fully instrumented sites were used to document slope conditions. Bedrock consisted of Oligocene White River Group rocks. Bulk erosion rates correlated to grain size with silty-sandy materials producing higher mass erosion rates as a function of the silt-to-clay ratio and plastic index. Data indicated that as grain size decreased, plastic index increased leading to a decrease in erodibility. These parameters were used to construct a grain-size proxy, ψ, that was substituted for grain size, D, in Bagnold’s entrainment equation and provided significant improvement in calculation of critical entrainment velocities for fine-grained materials. Hydraulic analyses of slope and pediment surface processes indicated surface roughness was a controlling factor and materials washed from rough steep slopes were effectively transported across smooth low-angle pediments with slope-to-pediment angle ratios of nearly 6:1. Slope stability modeling of ten slopes produced high factors of safety for all slopes, even under saturated conditions and was attributable to clay cohesion. All results were used to construct models that predicted years until net slope erosion equaled 2.5 cm (1 inch). Using these results, Park managers were advised to visit erosion-prone sites on a 1- to 6-year schedule, based on site geology and slope aspect, to adequately protect critical fossil resources from destruction.

Late Miocene-Pliocene geomorphological evolution of the Xiaoshuizi peneplain in the Maxian Mountains and its tectonic significance for the northeastern Tibetan Plateau

As a remnant elevated peneplain, the Tangxian planation surface is widely distributed in North China and the Ordos Plateau. Further into the Tibetan Plateau (TP), the Main planation surface can also be traced across this, the highest plateau on the Earth's surface. What the relation is between these two planation surfaces and whether they represent the remnants of a unique planation surface remains open to question. As the intermediate region between the low-altitude North China and the high-altitude TP, the Maxian Mountains and such bilateral basins located in the northeastern TP margin retain a series of well-preserved geomorphic features such as planation surfaces, pediment surface, and the fluvial terraces of the Yellow River. When and how the Xiaoshuizi peneplain was formed are crucial questions to elucidate the geomorphic evolution of the northeastern TP and infer the tectonic history of the TP. Here we present two parallel red clay-loess drill cores from the uplifted Xiaoshuizi bedrock peneplain in the Maxian Mountains, which document both the upper formation age of the planation surface and the evolution of Asian aridification. Based on the integration of high-resolution magnetostratigraphy analysis and Pliocene mammal fossils, we demonstrate that the red clay-loess succession deposited on the Xiaoshuizi peneplain probably began to accumulate at ~6.9Ma, implying that the Xiaoshuizi peneplain had begun to form before this. These results support the hypothesis that the Xiaoshuizi planation surface is equivalent to Tangxian planation surface of North China and Main planation surface of the TP. Furthermore, the onset of red clay sequence may indicate that the Xiaoshuizi peneplain was ultimately destroyed by the intensive tectonic uplift of the northeastern TP before 6.9Ma. Taking into account the synchronous Late Miocene red clay deposition between the Xiaoshuizi peneplain and central Chinese Loess Plateau (CLP), we propose that the onset of interior Asian aridification occurred in the Late Miocene and was principally triggered by the rapid uplift of TP.

Role of flood discharge in shaping stream geometry: Analysis of a small modern stream in the Uinta Basin, USA

This small modern river system is located on a relatively flat (about 1°–2°), unconsolidated sandy pediment surface in the Uinta Basin of Utah, USA, and it is with a scale of about 30 m long and 0.4–0.8 m wide, similar as a natural flume experiment model. The small stream is informally divided into upstream, midstream and downstream. The analysis shows that flood discharge influences channel sinuosity and morphology to produce an initial meandering pattern which is later changed to a braided and then a straight pattern in the downflow direction.The upstream segment has a high sinuous geometry dominated by both erosion (cutbanks) and deposition (point bars). In the resistance of sporadic vegetation rooting in banks, the upstream flood deviates its original direction, which results in the powerful flood intensively eroding the cutbank and accreting clastics to build point bars, and thus producing a high sinuous channel.The midstream is dominated by deposits (many small bars) with a moderate to low sinuosity. Due to the bad drainage of the high sinuous channel in the upstream, the strong flood can cut off the point bar completely or even surmount the levee in the last meandering upstream, which widens the channel suddenly with a quick decreasing current power. Then, the clastics from the upstream are unloaded in the midstream and form many small bars. Unloaded sediments protect the bank, and the low-power current brings a moderate erosion to the bank, which forms a moderate to low sinuous channel in the midstream.The downstream shows multistage erosional terraces in its relatively straight channels. After the midstream water drops its load, it becomes “clear” and reaches downstream, the lower current power is helpless to reform channel geometry. Thus, the downstream channel segment keeps a lower sinuous geometry, even straight partially. Small amounts of fine clastics are deposited, and simultaneously multistage terraces are formed due to regressive flood erosion.This stream example demonstrates the subtleties of stream flow and the importance of flood discharge in shaping the channel geometry. Although it is difficult to scale up this example to a large river system that carves geomorphic landscape, this case shows how river geometries vary from the traditional patterns due to different gradient.

Chronology of Chilean Frontal Cordillera building from geochronological, stratigraphic and geomorphological data insights from Miocene intramontane‐basin deposits

AbstractThe Chilean Frontal Cordillera, near 28°45′S, provides a remarkable example to explore the evolution of the Central Andes; this area provides conspicuous pediment surfaces and continental deposits, which allowed us to analyse the timing and propagation of deformation which controlled the Andes building during the Cenozoic using structural, geomorphological, sedimentological, stratigraphic and geochronological data. The study area is characterized by outcrops of the Cerro del Burro Gravels, a continental deposit which is surrounded by four morphostructural mountain systems. Based on a 46 Ma tuff affected by a syncline, which is sealed by a 44 Ma tuff, we recognized an Eocene fault activity that contributed to the uplift of the western and northern systems, which have remained inactive during the last 44 Ma. The deformed lithologies during the last pulse of activity of the western fault and the youngest lithology carved by pediment processes (21 Ma) indicate a pediment surface developed during the Late Eocene and Oligocene. This pediment extended below the Cerro del Burro Gravels associated to a base level which drained to the east. We also recognized Miocene fault activity that played a main role in the uplift of the eastern and southern systems. Geochronological, stratigraphic and geomorphological data suggest a first pulse of fault activity between 19 and 13 Ma, which interrupted the pedimentation processes, developed an intramontane depocenter, and forced the accumulation of the Laguna Grande Succession in an alluvial‐braided fluvial environment. After 13 Ma, an erosive event evidenced by the incision of valleys, resulted after the change in the extension and configuration of the hydric network.

Long-term denudation rates from the Central Andes (Chile) estimated from a Digital Elevation Model using the Black Top Hat function and Inverse Distance Weighting: implications for the Neogene climate of the Atacama Desert

A methodology for determining long-term denudation rates from morphologic markers in a Digital Eleva- tion Model (DEM) is checked by a comparative study of two drainage basins in the Precordillera of the Central Andes. In both cases the initial confi guration of an incised pediment surface has been restored by using two different methods: the Black Top Hat (BTH) function and the Inverse Distance Weighting (IDW) interpolation. Where vertical incision and hillslope erosion are recorded, the IDW appears to be the most adequate to reconstitute the pediment surfaces. Conversely, where only vertical incision is observed, the BTH describes more precisely the former pediment surfaces and it is easier to solve. By subtracting the DEM from the reconstructed marker we calculated an eroded volume, and estimated its uncertainty by considering Root Mean Square Error (RMSE) and DEM grid error. For the last ~10 Myr we obtained long-term denudation rates of 7.33±1.6 m/Myr in the San Andres drainage basin and 13.59±1.9 m/Myr in the El Salado drainage basin. These estimations are largely in agreement with other reported estimates of long-term denudation rates in the Atacama Desert. Comparison with long-term denudation rates reported in a wide range of climatic regimes suggests that our estimates cannot be explained by the current rainfall in the Precordillera. However they could be explained by a rainfall similar to that reported 40 km to the east in the Puna. This suggests that during the time span concerned the geomorphologic evolution of the study area, this evolution is dominated by an orographically controlled rainfall pattern. The preserved pediment surface and the small long term denudation rates determined in this study also indicate that the Precordillera was never reached by humid tropical air masses and precipitation as currently observed in the Altiplano during the summer months.

Pace of Landscape Change and Pediment Development in the Northeastern Sonoran Desert, United States

Pediments of the Sonoran Desert in the United States have intrigued physical geographers and geomorphologists for nearly a century. These gently sloping bedrock landforms are a staple of the desert landscape that millions visit each year. Despite the long-lived scientific curiosity, an understanding of the processes operating on the pediment has remained elusive. In this study we revisit the extensive history of pediment research. We then apply geospatial, field, and laboratory cosmogenic 10Be nuclide dating and back-scattered electron microscopy methods to assess the pace and processes of landscape change on pediment systems abutting the Salt River in Arizona. Our study focuses on the Usery pediments linked to base-level fluctuations (river terraces) of the Salt River. Relict pediment surfaces were reconstructed with dGPS data and kriging methodologies utilized in ArcGIS—based on preserved evidence of ancient pediment surfaces. 10Be ages of Salt River terraces established a chronology of incision events, where calculating the volume between the reconstructed relict pediment and modern surface topography established minimum erosion rates (∼41 mm/ka to ∼415 mm/ka). Pediment area and length appear to have a positive correlation to erosion rate and development of planar pediment surfaces. Field and laboratory observations reveal that pediment systems adjust and stabilize at each Salt River terrace. Relief reduction across the pediment begins with pediment channel incision via headward erosion. Next, tributary drainage capture begins and collapses interfluves. Lateral stream erosion promotes planation where the porosity of decayed granite along channel banks exceeds the bedrock underneath ephemeral channels.

Earliest Dates and Implications of Microlithic Industries of Late Pleistocene from Mahadebbera and Kana, Purulia District, West Bengal

Microlithic industries, a technology associated with modern humans, as defined by the production of microblades have been found in different parts of the Indian subcontinent with the earliest date being 48 ka. The present communication reports on recent archaeological excavations of these industries from a colluvial context located in the pediment surface of Precambrian hills in Purulia, West Bengal. These are dated to 34-25 ka by optically stimulated luminescence dating and are the earliest dates for microlithic industries in eastern India. To our knowledge such dating does not exist for any prehistoric site in Bengal. The context of the sites - hill-slope colluvium - is also unique and a rarity in the subcontinent. These findings add additional inputs to the knowledge of these industries, providing supporting evidence to their antiquity.

A cosmogenic view of erosion, relief generation, and the age of faulting in southern Africa

Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards. To infer regional rates of denudation, we measured 10Be in river sediment samples and found that south-central South Africa is eroding ~5 m m.y.−1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured 10Be and 26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of 10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone. Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys deepen, isolating stable upland surfaces and creating the spectacular relief for which the region is known.

Tectonic implications of fluvial incision and pediment deformation at the northern margin of the Central Anatolian Plateau based on multiple cosmogenic nuclides

AbstractWe document Quaternary fluvial incision driven by fault‐controlled surface deformation in the inverted intermontane Gök𝚤rmak Basin in the Central Pontide mountains along the northern margin of the Central Anatolian Plateau. In‐situ‐produced 10Be, 21Ne, and 36Cl concentrations from gravel‐covered fluvial terraces and pediment surfaces along the trunk stream of the basin (the Gök𝚤rmak River) yield model exposure ages ranging from 7 ± 1 ka to 346 ± 45 ka and average fluvial incision rates over the past ~350 ka of 0.28 ± 0.01 mm a‐1. Similarities between river incision rates and coastal uplift rates at the Black Sea coast suggest that regional uplift is responsible for the river incision. Model exposure ages of deformed pediment surfaces along tributaries of the trunk stream range from 60 ± 5 ka to 110 ± 10 ka, demonstrating that the thrust faults responsible for pediment deformation were active after those times and were likely active earlier as well as explaining the topographic relief of the region. Together, our data demonstrate cumulative incision that is linked to active internal shortening and uplift of ~0.3 mm a‐1 in the Central Pontide orogenic wedge, which may ultimately contribute to the lateral growth of the northern Anatolian Plateau.

Geometry and Quaternary slip behavior of the San Juan de los Planes and Saltito fault zones, Baja California Sur, Mexico: Characterization of rift-margin normal faults

An array of north-striking, left-stepping, active normal faults cuts the southwest margin of the Gulf of California and across the southern tip of the Baja California peninsula. This is the gulf margin fault system of the oblique-divergent plate boundary within the Gulf of California. Detailed geologic and geomorphic mapping along the onshore San Juan de los Planes and Saltito fault zones allowed us to delineate geometric sections and to infer the tectonic history of the fault zones. To achieve a more complete understanding of these individual normal faults within a larger array, we mapped faults to ∼10 km offshore using seismic CHIRP (compressed high-intensity radar pulse) profiling. Both onshore faults slip at a low rate and have a low total offset. Along the San Juan de los Planes fault zone, which is entirely onshore, the young, scarp-forming fault reactivated older faults to rupture a broad, low-relief pediment surface with thin Quaternary cover, reflecting a two-stage slip history along this fault zone. The offshore study suggests a northward continuation of the onshore Saltito fault, and a complex fault array north of the La Gata fault on the east side of the San Juan de los Planes basin extending northward to the west Cerralvo fault. Our results suggest relatively low rates of active faulting of <1 mm/yr across the San Juan de los Planes system of faults compared to high rates on the active gulf-axis system, and relatively higher rates on earlier Neogene gulf margin faults in other areas along the southwest Gulf of California margin.

Recently active contractile deformation in the forearc of southern Peru

Geomorphic and structural features of southern Peru (14–18°S) provide strong evidence for distributed crustal deformation along range-sub-parallel contractile structures. We use in situ produced cosmogenic radionuclides, in conjunction with field and remote mapping, to determine the ages of geomorphic features and find (1) ancient surfaces (>1Ma) preserved as a result of very low surface erosion rates, (2) young (∼30ka) low-relief pediment surfaces developed during recent landscape modifications, (3) active tectonic structures accommodating compressional stresses, and (4) Pleistocene river incision rates of ∼0.3mm/yr consistent with longer-term rates. In this region of southern Peru, the steep western wedge (trench to arc area) of the Andean margin presently maintains the high topography of the Altiplano through a combination of uplift and contractile deformation along steep east-dipping faults and isostatic responses to the focused removal of large amounts of crustal material through canyon incision.

Mechanisms, timing, and rates of arid region mountain front recharge

Summary Arid region mountain front recharge (MFR) for two catchments (17.5 and 25.6 km 2 ) in the Silver Island Mountains, Utah, USA was evaluated by an analysis of catchment precipitation, mountain front and playa infiltration rates, aquifer parameters, and groundwater level responses in 59 shallow monitoring wells located at the distal end of the catchments. The 3–7 km long mountain front surfaces transition from alluvial fan deposits to a thin alluvial covering on lacustrine pediment surfaces. Precipitation and groundwater level data were collected from October 2005 to October 2010, which represents five recharge cycles. Average annual precipitation for all rain gauges during the 2005–2010 water years was 14.9 cm. Monitoring well water levels increased 91–97% of the time in response to storm events during times of general monitoring well water level increases and 29–38% of the time during times of general monitoring well water level declines. A surprising 81.7% of total catchment precipitation resulted in positive water level responses. Precipitation events as small as 2 mm resulted in positive water level responses and initial responses occurred within hours of some precipitation events. Water level responses that occur within hours of the larger precipitation events are attributed to focused groundwater recharge in near field braided stream channels. The time lag between the end of precipitation events and the maximum water level responses, which may be several days, is attributed to far field recharge. Direct MFR was calculated using a modified version of the water table fluctuation method. For the two catchments the average calculated direct MFR was 2.7–3.2% using PRISM precipitation data, and 3.7–4.5% using rain gauge data.