Terrace Sequence Research Articles

Diverse fluvial aggradation and incision response to interglacial—Glacial transitions in the headwaters of the Yangtze River, SE Tibetan Plateau

As a key geomorphic archive, staircase sequences of terraces and sediments record the history of fluvial aggradation and incision, which are related to tectonic activities, climatic fluctuations, and varying base levels. While numerous studies have investigated their influence on the formation of fluvial terraces, the manner in which rivers adapt to diverse climatic transitions (specifically, the shifts from warm to cold and from cold to warm) remains a subject of ongoing debate, particularly when considered within varying tectonic and climatic contexts. Here, we address this issue by reconstructing terrace distributions and identifying sedimentary features with OSL dating in the headwaters of the Yangtze River in the southeastern Tibetan Plateau. We discovered two distinct sets of thick valley-filling sequences at different elevations indicating two phases of aggradation during the penultimate and last glacial periods, when the Yangtze River was in a transport-limited condition. During the warmer interglacial and deglacial periods, increased monsoon precipitation led to higher discharge, causing the river to incise into these valley fills. In addition, superimposed tectonic activities produced diverse river response to the climatic transition from interglacial to glacial between the upstream shallow valley and the downstream deeply incised gorge. In the upper valley, stable tectonic conditions resulted in minimal incision, with younger deposits overlying older ones. Conversely, in the downstream gorge, with more intense tectonic activities, sediments only partially covered the riverbed. These sediments acted as abrasion tools, facilitating rapid incision during the warm-to-cold transition, thereby creating two separated valley fills. This finding enhances our understanding of how tectonic forces influence river responses to interglacial—glacial cycles.

Holocene Palaeoenvironmental change at the mouth of Sabarmati River, Gulf of Khambhat, Western India

The fluvio-marine archives have been widely used to assess the imprints of land-sea interaction during the Holocene period, which has been enigmatic in terms of changes in environmental conditions. To comprehend the fluvial-marine interactions during the Holocene period in response to relative sea level changes, a sedimentary record has been studied from the mouth of the Sabarmati river at the Gulf of Khambhat (western India), which has also hosted mighty Harappan cultural centres during Holocene period. We employed a suite of multiproxy technique (sedimentology, OSL dating, geochemistry and foraminiferal content) from a terrace sequence at the Sabarmati River mouth (i.e. Vadgam). Based on an optical dating, the studied terrace sequence spans from 11 ± 1 ka to 1.3 ± 0.3 ka (11300–1300 year BP) covering almost entire Holocene period. The multiproxy investigation identified three distinct depositional zones at the site, namely zone 1 (11300–5300 year BP), which is characterised by a predominantly fluvial to marginal marine environment, zone 2 (5300–3700 year BP), which indicates a change to a marginal marine to predominantly marine environment, and zone 3 (3700–1300 year BP), which demonstrates a change to an again mixed environment. A present analogue to the palaeoenvironments in the area is provided by the deposition that is still occurring at the mouth of the Sabarmati River under the estuarine tidal environment. The variations in the rate of sedimentation and the source of the sediments are consistent with the environmental change, which will be driven by the changes in climatic circumstances in connection with the rapid sedimentation near the mouth of the Sabarmati River.

Extended-range luminescence and ESR dating of Iberian fluvial terraces (Duero and Guadiana basins) associated with the Lower Palaeolithic sites of La Maya I, II, III, Burganes and Albalá (west-central Spain)

The major river basins of the Iberian Peninsula that drain towards the Atlantic coast contain extensive Lower Palaeolithic (Acheulean) records associated with well-preserved fluvial terrace sequences. These open-air archaeological records are often difficult to constrain chronologically due to a lack of suitable material for dating (e.g., faunal remains) or because their Middle Pleistocene antiquity precludes the use of otherwise routinely applicable geochronometric techniques such as conventional optically stimulated luminescence (OSL) dating. As a result, many important Iberian Acheulean sites lack precise and accurate chronologies, and their existing temporal frameworks are based solely on morphostratigraphic correlations with regional fluvial terrace systems. Here we present the first application of extended-range luminescence dating – namely multi-grain post-infrared infrared stimulated luminescence (pIR-IRSL) dating of K-feldspars and single-grain thermally transferred-OSL (TT-OSL) dating of quartz – to a series of terrace sequences in the Duero basin (central sector) and Guadiana basin, as well as associated Lower Palaeolithic sites of La Maya I, II and III along the Tormes River, Burganes (and related sites) along the Tera River, and Albalá along the Guadiana River. Additionally, we present results of a parallel electron spin resonance (ESR) quartz dating study of the Tormes River terrace (Duero basin), which has been undertaken independently on the same samples to enable stratigraphically paired dating comparisons. Our study indicates that sedimentary quartz from this sector of the Duero basin produces very weak TT-OSL signals, variable Al signal repeatability and below-average Ti signal fitting uncertainties, though it is still possible to determine consistent comparative ages using the measurements protocols employed here. For samples where both pIR-IRSL and TT-OSL signals were measured, the paired luminescence dating ages are in agreement at 1σ. Agreement between the quartz ESR and TT-OSL ages is also observed for one sample from the Tormes River. However, the ESR signals (both Al and Ti) of a second sample showed insufficient bleaching and age overestimation in comparison to the paired TT-OSL dataset. Comparison of our latest dating results with published chronological datasets point to some fluvial terraces of the Duero basin potentially having diachronous formation histories. Despite this, our combined chronological results show that the terrace systems are chronologically ordered and suggest that, regionally, the +8 to +34 m terraces likely post-date marine isotope stage (MIS) 10, whilst the +50 m terraces formed prior to, or during, MIS 10. The related Acheulean sites that contain lithic material in stratigraphic position are dated to MIS 9–7 (193–314 ka at 2σ age range), and temporally overlap with the early Middle Palaeolithic records in the Duero basin, also dated here to MIS 8–7 (or 159–259 ka at 2σ). Our results are consistent with previous observations regarding the chronological overlap of these two technologies in the region, and support the possibility of a non-linear pattern of replacement for the Acheulean in the Iberian Peninsula during the Lower to Middle Palaeolithic transition.

The coupling between tectonic uplift and climate change recorded by the Yellow River terraces during the Zoige basin excavation in the northeastern Tibetan Plateau

Understanding the processes of fluvial geomorphic evolution is based on thorough evaluation of how rivers respond to tectonic activity and climatic change, with river terraces representing a key archive of alternating deposition and incision in response to such forcing, enabling an excellent means for correlation with tectonic and climatic records. An example presented here points to a difference in fluvial response to climate fluctuations along a trunk river and its tributary: the Yellow River and its tributary the Xike River, which has incised the Lajia Gorge to pirate the Zoige Basin, in the eastern Tibetan Plateau. Based on Accelerator Mass Spectrometry 14C dating, the chronological sequence of basin fill top and fluvial terraces below has been established, defining the drainage reorganization of the Yellow River excavation through the Zoige Basin since the Last Glacial Maximum. Spatial and stratigraphic comparison between the two fluvial sequences, combined with correlation with high-resolution climatic records, indicates that the tributary Xike River formed more terraces in response to trunk incision (base-level variation), climatic fluctuations, and underlying weak bedrock than the Yellow River. Thus, our results not only demonstrate a combined archive of climatic fluctuation and tectonic uplift recorded in the fluvial terrace sequences, but also suggest that the generation of fluvial terraces may be influenced by the underlying bedrock within the same tectonic and climatic settings.

Uplifted Pleistocene Marine Terraces at Active Margins: Modeling Reveals the Effects of Sea Reoccupation and Coseismic Uplift on Uplift Rate Calculation

AbstractUplifted Pleistocene marine terrace sequences are used to quantify uplift rates along active margins by knowing terrace age and elevation, and sea level (SL) position at the time of terrace formation. When terraces are undated, ages are assigned by correlating terraces at progressively higher elevations with progressively older highstands. Uplift at convergent margins can be constant over time or occur coseismically during upper plate earthquakes. We explore the formation of terrace sequences under conditions of constant and earthquake‐driven uplift by using a forward numerical model. The modeling reveals that terraces are generally abandoned at SL highstands but they are carved during all stands, depending on the time spent within the sea erosional‐depth‐range. Therefore sea reoccupation of a same platform after formation is a common occurrence that decreases with increasing uplift rates, suggesting that most platforms in nature may be in fact polygenetic. Furthermore, the model run time influences the terrace sequences: terraces formed at the beginning of longer runs constitute an ‘inherited morphology’ affecting subsequent sequences. When coseismic uplift is applied, the formation and preservation of terraces for a given average uplift rate depend stochastically on the coseismic displacement ‐ recurrence interval combination in relation to the SL position at the time of the earthquake. These factors significantly contribute to a higher likelihood of non‐preserved terraces along a terrace sequence, which may affect age correlation and, consequently, the resulting uplift rates. Further research is needed to explore the effect of the full seismic cycle in shaping a terrace sequence.

The contribution of Rob Westaway to the study of fluvial archives

Robert Westaway was a structural and hard-rock geologist who turned his attention to the study of Late Cenozoic fluvial archives, believing that the preservation of staircases of river terraces, particularly representing the Middle and Late Pleistocene, could only be explained in terms of crustal activity in response to surface processes, the latter affected by climatic change. His entry into this research area coincided with the realisation that such terrace sequences required surface or crustal uplift to have taken place over the time interval represented. Workers were unable to explain the observed amounts of uplift in terms of erosional isostasy, a process that could potentially have explained such one-way crustal movement. Westaway envisaged a mechanism by which mobile lower crust migrated to beneath uplifting areas, maintaining and reinforcing their uplift. The mechanism requires complex mathematics to explain it, as well as lending itself to mathematical modelling of the process, based on varying crustal properties and changes in the rates of surface processes in response to climatic fluctuation. Essentially the lower-crustal effect can be envisaged as a positive-feedback enhancement of erosional isostasy.It became apparent that Westaway's theories could explain geomorphological and sedimentary fluvial archives that were otherwise difficult to elucidate. Mantle-based erosional isostasy could not explain terrace staircases, for example. Many of these occur in regions that are tectonically inactive, and so cannot be attributed to neotectonic activity. A game-changer in terms of persuading the wider community came from the recognition of crustally ultrastable regions in which progressive long-timescale uplift has not occurred: Archaean cratons. Westaway's envisaged lower-crustal flow would not be expected in such regions, which have cold, brittle and immobile crust to its full depth. Ancient fluvial deposits are found close to modern valley-floor levels in such areas. Regions of younger Precambrian crust (Proterozoic) showing intermediate situations were subsequently identified. Other dilemmas could be resolved, such as the ‘back-tilting’ of the early-Middle Pleistocene Bytham River in the English Midlands, caused by its drainage crossing crustal blocks with different properties that have accordingly experienced differential uplift. Although glacio-isostasy, mostly seen in the effects of rebound since the Last Glacial Maximum (LGM), is largely accommodated in the mantle, and thus is reversed as a response to glacial loading and unloading, in areas of suitable crustal type there is evidently a small lower-crustal component that is less readily reversible.Westaway's important contribution has yet to be fully integrated into received wisdom in geomorphological and Quaternary circles, although much of it is now widely accepted and more will be explored and published in due course.

Evolution of the Júcar-Cabriel fluvial system on the Mediterranean watershed of the Iberian Peninsula (Valencia, eastern Spain)

This study presents the characterization and chronology of the Quaternary terrace sequence developed in the confluence zone of the Júcar and Cabriel river valleys. The study area covers a radius of 10 km from the confluence of the two valleys near the locality of Cofrentes (Valencia). It is located in the northern zone of the Ayora-Cofrentes Graben in the northeastern sector of the External Prebetic Zone adjacent to southern Iberian Chain. This N-S graben is an inherited tectonic feature with an axial salt wall subject to different periods of fluvial dissection and refilling since at least the end of the Messinian Salinity Crisis (MSC). Regional isostatic uplift and local uplift and subsidence processes related to salt flow and dissolution during the Quaternary period, favored by fluvial entrenchment and terrace development. The terrace system and the previous pre-incision erosional and depositional surfaces are analyzed from a geomorphological point of view. The terrace system only develops within the soft Mio-Pliocene sedimentary fill of the graben, whilst upstream and downstream this zone the studied valleys develop important gorges (canyons) carved in the Cretaceous tablelands (Caroch Massif) and Mio-Pliocene deposits (Llanura Manchega). The geochronological control is established from 20 numerical ages obtained by different dating methods, such us Electron Spin Resonance (ESR), Optically Stimulated Luminescence (OSL) in detrital sequences and Th/U series in calcareous tufa deposits. Additionally, four K/Ar dates available for volcanic materials disrupting the upper and oldest terrace have been considered. A total of 14 terrace levels were identified between +130–135 m (T1) and +3–4 m (T14) of relative height above the river thalwegs. The T1 has an approximate age of c. 1.6–2.0 M.a. as indicate the age of the volcanic materials from the Agras Volcano intruded in the terrace, marking the onset of Quaternary fluvial dissection in the zone. The obtained geochronological data indicate that the Lower-Middle Pleistocene boundary is slightly above T6 (+60–65 m), which has an ESR age of 577 ± 43 ka. This terrace also documents the onset of the most important period of rise of the salt within the graben interfering drainage development. The geochronological data indicate the occurrence of a second eruptive event during the second half of the Middle Pleistocene related to the terraces T7 (+50 m) and T8 (+35–40 m), updating the volcanic activity in the zone. The beginning of the Upper Pleistocene is recorded by the top sequences of the T9 terrace (+25–30 m) where numerous OSL and Th/U data have been obtained with ages between ca. 105 and 81 ka (MIS 5). However, these young terraces (T8 to T13) are thickened (8–10 m) recording Middle Pleistocene sediments in their basal sequences with ages between ca. 193 and 137 ka (MIS 6). The study provides important data on valley evolution under the interference of volcanic activity, salt –related deformation (diapirism, solution subsidence) and fluvial incision fostered by the rise of the axial diapir (salt wall) protruding the graben since at least the Brunhes-Matuyama boundary. The analyzed fluvial evolution documents interesting cases of river capture, drainage antecedence and tectonic adaptation of the drainage throughout the Quaternary Period. The paper also poses interesting geochronological data on the long-term Quaternary evolution of Mediterranean valleys in the Iberian Peninsula, poorly explored to the date.

Uplift rate calculation based on the comparison between marine terrace data and river profile analysis: A morphotectonic insight form the Ionian coastal belt of Basilicata, Italy

Geomorphological analyses of a wide coastal belt between northern Calabria and Puglia, where a well-exposed staircase of marine terraces extensively crops out, were performed in order to unravel the uplift history of this key-area. As a matter of fact, the Ionian coast of southern Italy represents a strategic sector for investigating the middle-late Quaternary morphotectonic evolution of the orogenic system of southern Italy.The main goal of our research was the reconstruction of a detailed uplift history using spatial distribution of marine terraces and river profile inversion. This was achieved by producing a new, detailed and complete map of the entire sequence of marine terraces and associated palaeoshorelines along the entire coastal sector. A critical review of the literature related to geochronological data from the study area, integrated with a modeling of the interaction between eustatic changes and tectonic uplift allowed demonstrating that the different terraces are correlated to many highstand peaks, dating the highest/oldest terrace to mid-Pleistocene times. Additional constraints on the spatio-temporal variation of tectonic uplift (or base-level fall) have been derived from the inversion of longitudinal river profiles. Such an approach adopts an independent geomorphic marker than the marine terraces and allowed us to obtain an additional estimation the uplift history of the area. The uplift values from the two methods differ in some cases by an order of magnitude, which could be attributed to the rigorous assumption of the river profile inversion or a limitation related to the chronological scheme of marine terraces.Our results highlight a spatial and temporal variation in the uplift rates, which can be only explained with a morphotectonic framework and likely the whole chain-foredeep-foreland system, remained under a prevailingly compressional tectonic regime during the Quaternary, with significant consequences for the seismic hazard of the region.

Rapid terrace incision and Quaternary landscape evolution in central Patagonia

AbstractThe debate over isostatic uplift versus discharge as drivers of Quaternary river incision is explored here through geospatial analysis of a ~250‐m‐relief terrace sequence from the Río Pinturas (Argentine Patagonia). The geomorphic setting of the Cañadon Caracoles reach allows evaluation of discharge as a driver of terrace incision because advancing ice during Pleistocene glaciations blocked Pacific drainage and routed meltwater from an expanded ~104‐km2 catchment to the Atlantic through the dryland steppe of the study area. Twenty‐two terrace levels, some assigned to previously dated ice margins [Marine Isotope Stage (MIS) 32–36, MIS 16, MIS 8 and MIS 2], were identified. Average net incision since 800 ka (~0.34 m ka−1) exceeded regional uplift rates. The MIS 2 terraces, with ante quem and post quem age constraint on the timing of terrace formation, show that terrace incision was episodic and faster still during a transitional warming climate. Glacier recession and proglacial lake formation at ~18 ka led to rapid incision of ~11.7 m ka−1 over a few millennia. River capture and negligible flow from ~15.0 ka caused fan‐dammed lake formation on the valley floor and vertical stability during MIS 1. The Pinturas terraces demonstrate rapid incision can be driven by discharge and sediment dynamics.

Provenance Analysis of Yellow River Terraces Highlights Causes of Siltation and Natural Hazards in the North China Plain

AbstractThe avulsion and overflow of river courses during floods, a result of channel siltation above the alluvial plain, is a cause of major natural disasters, including huge property damage and destruction of farmlands. The most sensitive to such risks is the very densely populated area of the Yellow River lower reaches, one of China's most important grain‐producing regions. Problems caused by excessive siltation are a critical issue that should be addressed with full knowledge of sediment‐generation processes, and provenance studies are essential to provide fundamental information for effective environmental management. The Sanmen Gorge is the key region where the great sediment mass produced upstream is conveyed from the middle reaches of the Yellow River to its lower reaches. This study of the terrace sequence exposed along the Sanmen Gorge combines geomorphological observations and provenance analysis based on U‐Pb zircon geochronology and heavy minerals to provide a history of sediment transport through time. The combination of previous research and the outcome of the present study demonstrate that before the Yellow River was formed as an integrated sediment‐routing system, sediments fed into the North China Plain were largely derived from local sources. Instead, after the formation of the Sanmen Gorge, huge quantities of sediment, also derived from the Qinling mountains and stored in the Fenwei basin, were flushed into the North China lowlands by the Yellow River. Such an event took place at 1.2 Ma and has since then impacted severely on floodplain landscape and eventually profoundly affected human activities in the region.

Holocene vertical velocity fields in the Calabrian Arc (southern Italy): New insights from uplifted sea-level markers in the Crotone Peninsula

The study of fossil coastlines is one of the most widely used methods to unravel the vertical movements of the Earth's solid surface. All along the coast of the Calabrian Subduction Arc, well preserved Pleistocene marine terrace sequences document a protracted uplift history. Although Pleistocene vertical movements have been extensively explored, the Holocene history is not well understood. Numerous uplifted coastlines of the Holocene period have been studied along the Tyrrhenian side of Calabria, but the Ionian coastal sector is considerably under-studied. Our new data from the coast of the Crotone Peninsula fill this crucial data-gap. AMS 14C dating of seventeen sea level markers, elevated above the current mean sea level, show ages between 2300 and 7000 Cal. Yr BP. The elevation of the markers was corrected for the paleo sea level using the most updated GIA models ICE-6G (VM5a) and ICE-7G (VM7). We found that a coastline dated to 7 ka shows uplift rates of 0.8 mm/yr, consistent with late Pleistocene uplift rates (≈0.8 mm/yr). In contrast, a 2.3 ka coastline was uplifted by only 1 m, resulting in lower uplift rates of ≈0.5 mm/yr. However, there are clues that the abandonment of this younger coastline may have occurred with a rapid uplift pulse. Reconstructing the past vertical movements of the Crotone Peninsula reveals an unsteady uplift history. At some times fast uplift rates are required, while for other periods, lasting as long as 3–4 kyr, the uplift rate is slower or zero. An uplift history consisting of periods of accelerated uplift, or even coseismic pulses, would also explain the difference in uplift rates between short term (<5 ka) and long term (>5 ka) markers through a metric we have defined as uplift rate sensitivity. Understanding the details of this uplift history has social relevance. The co-seismic uplift hypothesis requires further investigation but if confirmed would likely affect the seismic and tsunami hazard of the Crotone Peninsula. Similarly, the unsteady uplift rate would cause a risk given the current rapid rate of global sea level rise. Periods of sustained vertical stability would increase the risk of flooding even in areas characterized by long-term uplift and thus considered safe.

Unravelling the morphogenesis of coastal terraces at Cape Laundi (Sumba Island, Indonesia): Insights from numerical models

AbstractThe morphology of coastal sequences provides fundamental observations to unravel past sea level (SL) variations. For that purpose, converting morphometric observations into a SL datum requires understanding their morphogenesis. The long‐lasting sequence of coral reef terraces (CRTs) at Cape Laundi (Sumba Island, Indonesia) could serve as a benchmark. Yet, it epitomizes a pitfall that challenges the ultimate goal: the overall chronology of its development remains poorly constrained. The polycyclic nature of the terraces, involving marine erosion and reoccupation of old coral colonies by more recent ones hinders any clear assignment of Marine Isotope Stages (MIS) to specific terraces, in particular the reference datum corresponding to the last Interglacial maximum (i.e., MIS 5e). Thus, to overcome these obstacles, we numerically model the genesis of the sequence, testing a range of eustatic SL (ESL) reconstructions and uplift rates, as well as exploring the parameter space to address reef growth, erosion and sedimentation. A total of 625 model runs allowed us to improve the morpho‐chronological constraints of the coastal sequence and, more particularly, to explain the morphogenesis of the several CRTs associated with MIS 5e. Our results suggest that the lowermost main terrace was first constructed during the marine transgression of MIS 5e and was later reshaped during the marine regression of MIS 5e, as well as during the MIS 5c and MIS 5a highstands. Finally, we discuss the general morphology of the sequence and the implications it may have on SL reconstructions. At Cape Laundi, as elsewhere, we emphasize the necessity of addressing the development of CRT sequences with a dynamic approach, that is, considering that a CRT is a landform built continuously throughout the history of SL oscillations, and not simply during a singular SL maximum.

Oblique Collision of the Bahamas Platform at the Northern Boundary of the Caribbean Plate Recorded by the Late Cenozoic Coastal Terraces of SE Cuba

AbstractThe southeastern tip of Cuba Island is limited to the south by the N‐Caribbean boundary. By revisiting the impressive sequences of coastal terraces of this region, we decipher the Quaternary deformation pattern of this plate boundary. We present a detailed mapping of coastal terraces uplifted over a hundred kilometers of coastline, and U/Th dating. At Punta de Maisí, the deformation pattern shows (a) a faster uplift close to the transform boundary and (b) a northward propagation of folding produced by the convergence of the Bahamas platform toward the Caribbean plate. Along the southern coast of Punta de Maisí, the sequence displays 29 coastal terraces up to 520 m in elevation and a upper Pleistocene uplift rate of 0.23 ± 0.07 mm yr−1. We interpret this deformation as resulting from an offshore north‐dipping reverse fault near the coast. This uplift rate corresponds to 3% to 1.6% of the short‐term horizontal slip rate of Septentrional Oriente Fault Zone (10 ± 0.1 mm yr−1). Along the northern coast of Punta de Maisí, the sequence displays height coastal terraces up to 220 m in elevation and the uplift rates amount to 0.1 ± 0.05 mm yr−1 and likely result from the reverse faulting and folding associated with the offshore North Hispaniola Fault Zone. Uplift rates quickly decrease to the West, in agreement with the westward decrease in the activity of the North Hispaniola Fault Zone due to the docking of the Bahamas Platform against Cuba, while the platform more gently underthrusts Cuba to the East.

Decoding the Late Pleistocene-Holocene Terrace Evolution along the Main Frontal Thrust Zone, Northeastern Himalayas, India: Insights from Tectonic Geomorphology, Sedimentology, and Optical Dating

Abstract Fluvial archives are the best preserved and provide valuable means of obtaining information pertaining to climate and tectonic deformation of an area. The present study is an attempt to reconstruct the landform evolution history in the northeastern Himalayan region of India. Based on field investigation, tectonogeomorphic mapping, and OSL dating, strath and various levels of cut-and-fill terraces were investigated along the mountain front. Optically stimulated luminescence ages of the terrace sequences suggest that the valley incision occurred in three phases, at ~23 ka, ~14 ka and ~9 – 3 ka and one phase of aggradation was also identified at ~ 7 ka. The strath terraces suggest two major episodes of tectonic uplift in the region around ~ 8 ka and ~ 6 ka. This yields an uplift rate of 0.4 and 0.9 mm/yr. The result from the study suggested that the rapid rejuvenation along the mountain front led to the formation of the strath terraces with enhanced incision of the fluvial terrace and influenced the sedimentary architecture of valley fill sequences.

SEQUENCE OF VALLEY DEVELOPMENT OF ALAKNANDA AND ITS TRIBUTATIES &amp; QUATERNARY SEDIMENTATION, GARHWAL HIMALAYA, PARTS OF CHAMOLI TEHRI UTTAKASHI &amp; PAURI UTTAR PRADESH (UTTRARKHAND STATE ) INDIA

SEQUENCE OF VALLEY DEVELOPMENT OF ALAKNANDA AND ITS TRIBUTATIES &amp; QUATERNARY SEDIMENTATION, GARHWAL HIMALAYA, PARTS OF CHAMOLI TEHRI UTTAKASHI &amp; PAURI UTTAR PRADESH (UTTRARKHAND STATE ) INDIA

Using a post-glacial terrace sequence to better understand landscape evolution and paleohydrology in the eastern San Juan Mountains, USA

Terraces provide a unique record of environmental change as they form due to changes in sediment supply and hydrology. In the upper Rio Grande drainage, terraces are post-glacial in age and have not been impacted by tectonics during that period. As such, these terraces are the result of changes to the fluvial system since glaciers retreated. We mapped four terrace units across two sections of the upper Rio Grande. We then examined soil profiles on terrace units, described the sedimentology, and dated the terraces where possible. Our findings indicate that the upper terrace (FT1) formed as an outwash terrace during glaciation and that incision began immediately after glaciers had melted around 12.1 ka. Two intermediate terraces likely formed during brief pauses in post-glacial incision as aggradation of the lowest terrace (FT4, 7–14 m below FT1) had already begun by 9.6 ka. Cumulic soils and fine-grained sediment on FT4, combined with radiocarbon ages, indicate that FT4 aggraded consistently in both rivers throughout the Holocene. Specifically, overbank deposits emplaced during decadal-scale flooding have allowed FT4 to aggrade consistently since 9.6 ka. The main channel appears to have been stable throughout the entire Holocene given that it is pinched between fine-grained deposits. Previous work indicates that other authors have noted similar landforms although they seem to be underreported in the literature. The timing of terrace formation differs from that in nearby headwater streams, indicating that headwater systems are more responsive to hillslope processes while larger trunk streams appear to receive sediment from upstream. Apparent differences in flooding between the two basins may indicate that basins with water managed by reservoirs may flood the FT4 surface less often, thereby highlighting the impact of water management on both fluvial hydrology and sediment storage.

Coastal cliff evolution: Modelling the long-term interplay between marine erosion, initial topography, and uplift in an arid environment

Coastal cliff evolution is modulated by several factors, such as uplift, marine erosion, previous topographical conditions, and changes in global sea level. In this study, a numerical model is used to understand the influence of these processes on the evolution of coastal cliffs. This model is based on erosional and tectonic conditions of the Great Coastal Cliff in the Atacama Desert, the tallest and most uninterrupted coastal cliff on Earth. The results show that a faster uplift rate (> 0.3 mm/yr) or an older onset of uplift (3 Myrs) favors the formation of inactive cliffs, whereas active cliffs preferentially form under conditions of a slow uplift (0.2 mm/yr) or a younger uplift onset (1 Myr). Low erosion rates (< 1 m2/yr) also favor the preservation of sequences of staircase marine terraces, separating the seashore from an inactive coastal cliff. In contrast, high erosion rates (≥ 1 m2/yr) inhibit marine terrace formation, keeping the cliff active. The effect of erosion on cliff activity is enhanced or reduced by the slope of the initial topography. Steep coasts (> 5–10°) may hamper cliffs from reaching an inactive state. Thus, this model gives insights on the relative importance of these variables on the development of active and inactive cliffs, especially in an environment such as the Atacama Desert in northern Chile.

Tectonic and climate controls on river terrace formation on the northeastern Tibetan Plateau: Evidence from a terrace record of the Huangshui River

River terraces are significant geomorphic markers of climate and tectonics. However, the competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain. A series of well-developed terrace sequences of the Yellow River and its tributaries formed in intermontane basins and on mountains on the NE Tibetan Plateau. A flight of 10 river terraces has been preserved along the lower Huangshui River, a major tributary of the Yellow River. Based on electron spin resonance (ESR) analyses we found that the oldest river terrace of the lower Huangshui River on Alagu Mountain formed at ∼1.2 Ma. Moreover, the results of provenance analysis based on detrital zircon U–Pb dating showed that the fluvial system of the modern Huangshui River was established before ∼1.2 Ma. After comparing the ages of the oldest river terraces of major rivers on the NE Tibetan Plateau, we found that they are essentially synchronous. Then, we comprehensively analyzed the climate and tectonic conditions, and suggested that the abrupt climate change dominated the synchronous formation of the oldest terraces of major rivers at approximately 1.2 Ma on the NE Tibetan Plateau. Moreover, to assess the effects of the rock uplift rate on river terrace formation, we integrated 22 terrace sequences with long timescales on the NE Tibetan Plateau, and we found that the preservation rate of river terraces is linearly correlated with the rock uplift rate. Thus, our results suggest that the timing of terrace formation is controlled by abrupt climate change and that the amount of terrace formation/preservation is controlled by the rock uplift rate on the NE Tibetan Plateau.

Hillslope Sediment Supply Limits Alluvial Valley Width

AbstractRiver‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography.

High interstadial sea levels over the past 420ka from the Huon Peninsula, Papua New Guinea

The history of sea level across the Quaternary is essential for assessing past and future climate. Global sea-level reconstructions are typically derived from oxygen isotope curves, but require calibration with geological constraints that are scarce prior to the last glacial cycle (>130 thousand years ago). Here we show that the coral reef terrace sequence at the Huon Peninsula (Papua New Guinea) provides such constraints up to ∼420 thousand years ago, through a geometric analysis of high-resolution topographic data. We derive a northward tectonic tilt as regional deformation pattern, and estimate relative sea level for 31 Quaternary periods, including several periods for which no relative sea level data exists elsewhere. Supported by numerical reef models, these estimates suggest that oxygen isotope-based global mean sea-level curves systematically underestimate interstadial sea-level elevations, by up to ∼20 m. Compared to those curves, our results imply a stronger degree of non-linearity between ice-sheet volumes and global temperatures within Quaternary glacial cycles.