Strath Terrace Formation Research Articles

Lithological controls on the timing of strath terrace staircase formation in a collisional mountain belt

AbstractIn mountain belts, strath terrace staircases serve as markers for deriving river incision rates and erosional patterns. Distinguishing between terrace patterns influenced by external perturbations like changes in climate and tectonics and those driven by internal dynamics including feedbacks between topography, erosion and sediment transport remains challenging. We demonstrate that in a collisional mountain belt, lithology can act as a first‐order control on the spatial and temporal scales of strath terrace formation. Here, we investigate the role of lithology in modulating internal dynamics and the formation of strath terraces in the Mgoun River catchment of the High Atlas in Morocco, a region characterised by constant low‐rate rock uplift, a cyclical cool‐warm/arid‐humid Quaternary climate history and contrasting bedrock lithologies. By collecting (1) modern river and terrace clast data, (2) bedrock strath and strath‐top sediment elevations of four terrace levels, (3) terrace sedimentology and (4) integration with published terrace chronology, we found a dominance of local sediment input from hillslopes, mostly from recycled bedrock conglomerates. Additionally, we found valley width, controlled by the stratigraphic and structural configuration of lithological erodibility, significantly impacts sediment connectivity. The isolation between valleys with varying widths results in varied timescales of river channel response to hillslope coupling, with hillslope‐derived stochastic sediment gravity flows preserved in fluvial terraces in some river reaches and not in others. Furthermore, asynchronous terrace formation and abandonment ages result from the low longitudinal river connectivity between multiple valleys formed in erodible rock separated by gorges in high‐strength rock. These gorges limit knickpoint migration rates, inhibiting the ability of terraces formed in one valley to spread through the catchment. These findings can inform future research distinguishing between autogenic and external signals in erosional landscapes and help carefully derive river incision rates and climate insights from terraces.

Inherited anthropogenic disturbance and decadal sediment dynamics in a mountain fluvial system: The case of the Marecchia River canyon, Northern Apennines

We evaluate decadal coarse sediment dynamics along the Marecchia River of the Northern Apennines, a fluvial system with a history of gravel mining that led to the incision of a 6-km-long canyon. To this purpose, we subdivided the river into 21 reaches, seen as sediment reservoirs, to examine (1) historical variations in active channel width (1955−2019) in conjunction with (2) change in alluvial sediment storage (2009−2019), by differencing two sequential LiDAR digital elevation models (DEMs) within the active channel footprint. Combined examination of lateral (widening or narrowing) and vertical (aggradation or degradation) channel changes allowed the identification of composite styles of reservoir adjustment, as well as the refinement of geomorphic inference solely based on changes in active channel width. In particular, we find that different styles of decadal adjustment (1) are compatible with supply- and transport-limited conditions, as constrained by degree of confinement, stream channel slope, and active channel width; and (2) indicate different stages of evolution at reservoirs located upstream and downstream of the canyon head (dynamic equilibrium vs. transient response). The persistence of this geomorphic divide is supported over historical time scales by distinctive trends in planform channel changes, suggesting that sedimentary signal propagation downstream becomes abruptly interrupted at the canyon head. Over this 10-year natural experiment, the spatial pattern of erosion along the canyon exemplifies a striking case of transient response to anthropogenic forcing, where decadal topographic change, modulated by varying styles of hillslope-channel coupling, declines nonlinearly downstream. Depth of incision along the canyon increases progressively upstream, suggesting that the canyon head has been evolving toward a more unstable configuration with no significant change in sediment supply. This tendency, which points to a possible runaway style of development as bedload wearing on weak pelitic side walls continues, may hold basic implications for our understanding of channel incision into bedrock and strath terrace formation.

Constraining a model of punctuated river incision for Quaternary strath terrace formation

In the small fraction of Earth's surface with the highest erosion rates such as the Alps and Himalayas quantifying rates of incision, rock uplift and inferring climatic controls on the landscape can be relatively straightforward once the ages of river terraces cut in bedrock (strath terraces) are constrained. However, in many mid to lower relief settings that are more typical of mountain belts worldwide, periods of net river incision and riverbed lowering are relatively short (punctuated), interrupted by long periods of sediment aggradation or stasis. We define a conceptual model of punctuated river incision and strath terrace formation for the calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An approach using OSL dating of terrace gravels allows us to constrain a detailed ~150 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top aggradation and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and aggradation can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted: this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision.

Quaternary evolution of a hyperarid drainage under climatic fluctuations and rift-margin base-level fall, NE Negev, Israel

The long-term bedrock incision of streams in the northeastern Negev has been dictated, since the Early Pliocene, by the subsidence of the Dead Sea basin. However, this incision is accompanied by very long intervals of sediment aggradation. Here, we document the Quaternary-scale fluvial response to changing base-level, lithology, structure, and climate along the hyperarid Nahal Zafit, NE Negev. The oldest (~1–2 Ma) alluvial remnants point to a widespread beveled bedrock pediment at the base of the Hazera anticline. This pediment, like other stream-pediment-anticline relationships in the region, provides the baseline framework into which Nahal Zafit incises. During the Early to Middle Pleistocene, this pediment was the base-level for all streams emerging out of the anticlines. The tectonic subsidence of the regional base-level at the rift margin farther downstream, has caused incision and upstream migration of bedrock knickpoints, triggering the development of a wide incised valley in the Mazar syncline, adjacent to the Hazera anticline. Since ~330 ka, fluvial terraces, landslides and taluses record intervals of basin-wide fluvial deposition under prevailing hyperarid to arid climate. Despite the general arid climate, prolonged fluvial deposition intervals occurred at ca. 330–265, 230–125, and 110–20 ka. Rapid and quite short-duration bedrock incision occurred in-between these, much longer, intervals of aggradation. In addition to the proposed climatic imprint on fluvial aggradation-incision cycles and the first-order base-level control of the rift margin fault, the formation of strath terraces and distribution of stored sediments within the drainage basin point to the fundamental role that lithology and structure play in the Quaternary evolution of the NE Negev watersheds.

Anthropogenic strath terrace formation caused by reduced sediment retention

Across North America, human activities have been shown to cause river incision into unconsolidated alluvium. Human-caused erosion through bedrock, however, has only been observed in local and isolated outcrops. Here, we test whether splash-dam logging, which decreased in-stream alluvial cover by removing much of the alluvium-trapping wood, caused basin-wide bedrock river incision in a forested mountain catchment in Washington State. We date incision of the youngest of four strath terraces, using dendrochronology and radiocarbon, to between 1893 CE and 1937 CE in the Middle Fork Teanaway River and 1900 CE and 1970 CE in the West Fork Teanaway River, coincident with timber harvesting and splash damming in the basins. Other potential drivers of river incision lack a recognized mechanism to cause T1 incision or are not synchronous with T1 incision. Hence, the close temporal correspondence suggests that reduced sediment retention triggered by splash damming led to the observed 1.1 mm⋅y-1 to 23 mm⋅y-1 of bedrock river incision and reduction of the active floodplain to 20% and 53% of its preincision extent on the Middle and West Forks, respectively. The development of such anthropogenic bedrock terraces may be an emerging, globally widespread physiographic signature of the Anthropocene.

Controls on strath terrace formation and evolution: The lower Guadiana River, Pulo do Lobo, Portugal

The lower Guadiana River carves a wide canyon in the Variscan metamorphic rocks of the SW Iberian Peninsula and presents an exceptionally homogeneous and well-preserved case of a strath terrace (ST) some 40 km long and 400 m wide. The rocky surface or strath shows different characteristics and development stages. Upstream, the lateral erosion level forms a flooded riverbed, or submerged strath, that gradually rises above the thalweg and develops into an active floodplain. Downstream, below the 15 m high Pulo do Lobo waterfall, the strath is incised by an inner canyon that forms a terrace on both sides. The inner channel increasingly widens and the ST loses its longitudinal continuity, yet remains prone to flooding during extraordinary magnitude floods. To assess the factors controlling the formation of this ST, we examined the roles of key drivers of ST formation: lithological hardness, tectonic structures and climate change, using cosmogenic data to create a chronologic context. The ST developed favoured by a monotonous succession of phyllites, as shown by Schmidt hammer resistance data, and interbedded more resistant quartz veins do not significantly alter its flat configuration. Variscan structures and measured joints determine the main orientation of the river and control the drainage network pattern. The intersection of the NS and NE-SW fractures promotes a wavy surface due to three-dimensional minor fold trains that act as discontinuities and play an important role in ST erosion through quarrying and plucking. However, we detected no evidence of ST deformation attributable to recent or Late Cenozoic tectonic activity.The upstream active riverbed progressively changes downstream into an ST. This implies that Be radiometric ages obtained at Pulo do Lobo may not represent the ST's genetic age but rather the time of dismantling of its alluvial or sedimentary cover. This means the strath surface formed prior to its cosmogenic age and was therefore controlled by a higher base level, indicating a higher sea level than the current one, given the downstream area studied. The Quaternary sea level fluctuations show records of only a few meters above the current level. Instead, the sea level low stand maintains an average depth of −60 m, reaching −140 m during the last glacial period. This low base level context could justify the strong incision of the inner channel within the ST, but not the age of strath surface genesis and expansion and thus this surface should have evolved previously. The highest possible sea level stand corresponds to the Pliocene transgression, which reached an elevation of 120 m in the region, covering Miocene paleorelief. The mouth of the Guadiana River at that time resembled a fluvio-deltaic embayment between the Algarve domain and the Guadalquivir depression, which was infilled with fluvial deposits at the end of the transgression. Collectively, these data suggest that cosmogenic dating would define the time of exhumation of this Pliocene sedimentary cover.Despite great variation of the ST ages, estimated incision rates at the knickpoint area are consistent with the total entrenchment of the Pulo do Lobo fall, especially if we consider that data correspond to an active erosion site. Highest erosion rates were found for the pothole area compared to those obtained for the higher ST surface. Radiometric ages correspond to the period of lowest Quaternary sea level and during which headward erosion would have been very active in the inner channel. The waterfall area shows more resistant lithology in its concentrated quartz veins, but this effect is inconspicuous at the ST surface. The regular gradient of 0.02 m/m and vast extension of this perched strath, linked upstream to the Guadiana riverbed, indicates a relic exhumed surface beneath its sedimentary cover. Notwithstanding the subsiding activity of the Gulf of Cádiz, the continental margin behaves as a tectonically stable block, which allows for the persistence of former morphologies in these ancient landscapes reshaped by bedrock river dissection.

The role of climate and tectonics in aggradation and incision of the Indus River in the Ladakh Himalaya during the late Quaternary

AbstractThe geomorphic evolution of the upper Indus River that traverses across the southwest (SW) edge of Tibet, and the Ladakh and Zanskar ranges, was examined along a ~350-km-long stretch of its reaches. Based on the longitudinal river profile, stream length gradient index, and river/strath terraces, this stretch of the river is divided into four segments. Valley fill river terraces are ubiquitous, and strath terraces occur in the lower reaches where the Indus River cuts through deformed Indus Molasse. Optically stimulated luminescence ages of river/strath terraces suggest that valley aggradation occurred in three pulses, at ~52, ~28, and ~16 ka, and that these broadly coincide with periods of stronger SW Indian summer monsoon. Reconstructed longitudinal river profiles using strath terraces provide an upper limit on the bedrock and provide incision rates ranging from 1.0±0.3 to 2.2±0.9 mm/a. These results suggested that rapid uplift of the western syntaxes aided by uplift along the local faults led to the formation of strath terraces and increased fluvial incision rates along this stretch of the river.

Climatic and tectonic controls on strath terraces along the upper Weihe River in central China

The Weihe River in central China is the largest tributary of the Yellow River and contains a well-developed strath terrace system. A new chronology for the past 1.11 Ma for a spectacular flight of strath terraces along the upper Weihe River near Longxi is defined based on field investigations of loess–paleosol sequences and magnetostratigraphy. All the strath terraces are strikingly similar, having several meters of paleosols that have developed directly on top of fluvial deposits located on the terrace treads. This suggests that the abandonment of each strath terrace by river incision occurred during the transition from glacial to interglacial climates. The average fluvial incision rates during 1.11–0.71 Ma and since 0.13 Ma are 0.35 and 0.32 m/ka, respectively. These incision rates are considerably higher than the average incision rate of 0.16 m/km for the intervening period between 0.71 and 0.13 Ma. Over all our results suggest that cyclic Quaternary climate change has been the main driving factor for strath terrace formation with enhanced episodic uplift.

Late Pleistocene–Holocene uplift driven terrace formation and climate-tectonic interplay from a seismically active intraplate setting: An example from Kachchh, Western India

Fluvial terrace formation is often regulated by external forcings like climate, tectonic and eustatic changes. These terraces, particularly in a dryland environment, preserves the discrete signatures of these external forcings, thus enabling us to reconstruct the fluvial response to the late Quaternary palaeoenvironmental changes and factors governing them. The present study focuses on reconstructing the aggradation/incision phases in the Lotia River which is located in the eastern segment of the Northern Hill Range (NHR) of the Kachchh Peninsula. The Lotia river drains through Mesozoic rocks before cutting across the Kachchh Mainland Fault (KMF) and finally debouch in the Banni Plains. Reconstruction based on tectonic geomorphology, sedimentology, sediment geochemistry, mineral magnetic, and OSL chronology suggests the fluvial response to monsoon variability archived during the last 15ka. The time frame was also marked by incision enhanced by uplift along the KMF, which led to strath terrace formation. The accommodation space thus created was filled by an aggradational event between 14.8ka and 10.6ka. Sedimentological and geochemical parameters have also suggested that the time period between 12.5ka and 11.5ka showed a decline in the monsoon strength, which coincides with ‘Younger Dryas’. It has been observed that the sediments spanning between 10.6ka and 7.8ka are absent from the archive, which is most likely the manifestation of the early Holocene optimum that led to severe erosional processes. The period between 7.8ka and 3.3ka is marked as another aggradational phase with fluctuating climatic conditions. At 3.3ka, the region has experienced an incision of 4m, which led to the formation of Holocene terrace T1, most likely due to tectonic uplift. During the last 3.3ka, another pulsative uplift has occurred, which led to the formation of unpaired Holocene terrace T2, along with tilting of the Lotia basin. Based on the OSL chronology of bedrock strath terrace, we reported a minimum uplift rate of 1.04mm/a for the eastern KMF during the Late Pleistocene–Holocene period, hinting seismically active nature of the KMF during this period. The terrace formation in the eastern Northern Hill Range is mostly regulated by tectonic uplifts along the KMF.

Lithologic controls on valley width and strath terrace formation

Valley width and the degree of bedrock river terrace development vary with lithology in the Willapa and Nehalem river basins, Pacific Northwest, USA. Here, we present field-based evidence for the mechanisms by which lithology controls floodplain width and bedrock terrace formation in erosion-resistant and easily friable lithologies. We mapped valley surfaces in both basins, dated straths using radiocarbon, compared valley width versus drainage area for basalt and sedimentary bedrock valleys, and constructed slope-area plots. In the friable sedimentary bedrock, valleys are 2 to 3 times wider, host flights of strath terraces, and have concavity values near 1; whereas the erosion-resistant basalt bedrock forms narrow valleys with poorly developed, localized, or no bedrock terraces and a channel steepness index half that of the friable bedrock and an average channel concavity of about 0.5. The oldest dated strath terrace on the Willapa River, T2, was active for nearly 10,000years, from 11,265 to 2862 calibrated years before present (cal YBP), whereas the youngest terrace, T1, is Anthropocene in age and recently abandoned. Incision rates derived from terrace ages average 0.32mmy−1 for T2 and 11.47mmy−1 for T1. Our results indicate bedrock weathering properties influence valley width through the creation of a dense fracture network in the friable bedrock that results in high rates of lateral erosion of exposed bedrock banks. Conversely, the erosion-resistant bedrock has concavity values more typical of detachment-limited streams, exhibits a sparse fracture network, and displays evidence for infrequent episodic block erosion and plucking. Lithology thereby plays a direct role on the rates of lateral erosion, influencing valley width and the potential for strath terrace planation and preservation.

Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington

Measurements of channel bed and bank incision into bedrock coupled with mapping and radiocarbon dating of strath terraces in the West Fork Teanaway River, Washington, provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape. The West Fork drains 102 km 2 of the tectonically quiescent southeastern North Cascade Range, and it is rapidly eroding its bed and creating strath terraces in its lower reach. Minimum vertical incision, measured annually relative to nails embedded in the streambed, was greater in the seasonally exposed, weathering-dominated, high-flow channel (mean = 10.9 mm yr −1 ) than in the perennially wet, abrasion-dominated, low-flow channel (3.8 mm yr −1 ), documenting unsteady lowering of the channel margin. Ages of radiocarbon-dated materials from alluvium on strath terraces, 0.1 m to 5.4 m above the water surface, suggest three episodes of strath abandonment at maximum ages of ca. A.D. 830, A.D. 1560, and A.D. 1890, and average incision rates of 1.3 mm yr −1 , 1.4 mm yr −1 , and 7.4 mm yr −1 for the oldest to youngest surfaces, respectively. Weathering-promoted vertical incision in the high-flow channel provides a mechanism for “top-down” rapid lateral strath planation in which scour of alluvium on incipient strath terraces incorporates the surface into the high-flow channel, allowing rapid removal of bedrock weathered during wetting and drying cycles. Relationships among channel width, channel confinement by bedrock terrace risers, modeled bankfull shear stress, and alluvial bed cover suggest that rapid channel widening could also internally limit vertical incision by slowing incision as shear stresses decline and more alluvium is retained on the bed. The timing of the most recent (ca. A.D. 1890) strath abandonment corresponds with historic anthropogenic removal of fluvial wood, suggesting that the relative abundance of fluvial wood may influence episodes of vertical bedrock incision by affecting the retention of alluvium on streambeds.

Differential uplift and incision of the Yakima River terraces, central Washington State

AbstractThe fault‐related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies ~2 mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. South of Ellensburg, Washington, the Yakima River cuts a ~600 m deep canyon across several Yakima folds, preserving gravel‐mantled strath terraces that record progressive bedrock incision and related rock uplift. Here we integrate cosmogenic isochron burial dating of the strath terrace gravels with lidar analysis and field mapping to quantify rates of Quaternary differential incision and rock uplift across two folds transected by the Yakima River: Manastash and Umtanum Ridge. Isochron burial ages from in situ produced 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. Average bedrock incision rates across the Manastash (~88 m/Myr) and Umtanum Ridge (~46 m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (~14 m/Myr) and outside the canyon (~10 m/Myr). These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to ~0.13 and ~0.06 mm/yr shortening across postulated master faults dipping 30 ± 10°S beneath the Manastash and Umtanum Ridge anticlines, respectively. The reported Quaternary shortening across the anticlines accounts for ~10% of the ~2 mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation.

Asymmetrical river valleys in response to tectonic tilting and strike‐slip faulting, northeast margin of Tibetan Plateau

ABSTRACTThe northeast margin of the Tibetan Plateau, a particularly important area to understand the mechanism of plateau formation, is characterized by large transpressional arcuate faults. There is debate on the amount of Quaternary sinistral displacement on the major Haiyuan Fault. Previously unrecognized systemic asymmetrical valleys have developed between the Haiyuan and Xiangshan faults. Southeast tilting and sinistral displacement on the northeast side of the Haiyuan Fault resulted in southeast migration of large rivers and asymmetrical widening of their valleys, leaving a systematic distribution of tilted strath terraces along their northwest sides. Where asymmetrical widening created by tilting kept pace with sinistral displacement, rivers have not been deflected, and the increase in valley width downstream from the fault should equate to total lateral displacement since river formation (e.g. Yuan River, a 7 km asymmetrical valley with a c. 2.2 Ma paleomagnetic age). Where river deflection and asymmetrical valley growth are coeval, valley width is less than total horizontal displacement (e.g. Hebao River, a c. 2.1 km asymmetrical valley with c. 2 km deflection). All rivers north of the Haiyuan Fault converge to cut across the Xiangshan Mountains as a gorge. Northeast thrusting of the upthrown side of the Xiangshan Fault has resulted in degradation and related strath terrace formation as the valleys asymmetrically widened. A probable earthquake‐induced landslide caused by movement on the Xiangshan Fault in latest Pleistocene blocked the gorge causing aggradation along all rivers and their tributaries. Deposition terraces were formed after the landslide dam was breached. Together with previous research on the Xiangshan Fault, it is concluded that there has been c. 7 km of Quaternary sinistral displacement on the Haiyuan and Xiangshan faults along the northeast margin of the Tibetan Plateau since the formation of rivers that intersect them. Copyright © 2014 John Wiley & Sons, Ltd.

Modelling the impact of regional uplift and local tectonics on fluvial terrace preservation

A terrace formation model (TERRACE) combined with a longitudinal river profile model (FLUVER) was used to simulate fluvial terrace formation and preservation in the northwest Iberian lower Miño River basin under the influence of three tectonic conditions; namely regional vertical uplift, local basin subsidence, and localised differential uplift. The simulation results were compared against mapped terrace altitudes and deposit thicknesses. The best results were achieved by combining all three tectonic factors, indicating that specific terrace formation is a complex interplay of regional and local tectonics. The best fit regional uplift rate of 0.10mka−1 over the past 600ka is higher than the 0.08mka−1 previously estimated for a section farther to the west, which can be attributed to an increase in tectonic uplift from the NW Iberian Atlantic margin toward the east. Local relative subsidence causes sediment accumulation in the local basin and less sedimentation in the fluvial terraces on the surrounding uplifting blocks. Different uplift rates on both sides of the valley caused preservation of unpaired terraces, which are fill terraces on one side of the valley and strath terraces on the other side. Usually, the formation of fill or strath terraces is considered to be only climate-dependent. Our results indicate that local tectonics can be important in the terrace formation and preservation. This suggests that terrace correlations in other river systems, based on deposit thicknesses only, might be over-simplified.

Timing and nature of alluvial fan and strath terrace formation in the Eastern Precordillera of Argentina

Sixty-eight 10Be terrestrial cosmogenic nuclide (TCN) surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper-arid San Juan region of the Argentine Precordillera. This region is tectonically active, and numerous fault scarps traverse Quaternary landforms. The three study sites, Marquesado strath complex, Loma Negra alluvial fan and Carpintería strath complex reveal a history of alluvial fan and strath terrace development over the past ∼225 ka. The Marquesado complex Q3m surface dates to ∼17 ± 3 ka, whereas the Loma Negra Q1ln, Q2ln, Q3ln, Q4ln, and Q5ln surfaces date to ∼24 ± 3 ka, ∼48 ± 2 ka, ∼65 ± 13 ka, ∼105 ± 21 ka, and ∼181 ± 29 ka, respectively. The Carpintería complex comprises eight surfaces that have been dated and include the Q1c (∼23 ± 3 ka), Q2c (∼5 ± 5 ka), Q3ac (∼25 ± 12 ka), Q3bc (∼29 ± 15 ka), Q4c (∼61 ± 12 ka), Q5c (∼98 ± 18 ka), Q6c (∼93 ± 18 ka), and Q7c (∼212 ± 37 ka). 10Be TCN depth profile data for the Loma Negra alluvial fan complex and Carpintería strath terrace complex, as well as OSL ages on some Carpintería deposits, aid in refining surface ages for comparison with local and global climate proxies, and additionally offer insights into inheritance and erosion rate values for TCNs (∼10 × 10410Be atoms/g of SiO2 and ∼5 m Ma−1, respectively). Comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the Carpintería and Loma Negra areas with only younger alluvial fan surfaces preserved both to the north and south. These data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages, which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region.

Late Quaternary stream piracy and strath terrace formation along the Belle Fourche and lower Cheyenne Rivers, South Dakota and Wyoming

Stream piracy substantially affected the geomorphic evolution of the Missouri River watershed and drainages within, including the Little Missouri, Cheyenne, Belle Fourche, Bad, and White Rivers. The ancestral Cheyenne River eroded headward in an annular pattern around the eastern and southern Black Hills and pirated the headwaters of the ancestral Bad and White Rivers after ~660ka. The headwaters of the ancestral Little Missouri River were pirated by the ancestral Belle Fourche River, a tributary to the Cheyenne River that currently drains much of the northern Black Hills. Optically stimulated luminescence (OSL) dating techniques were used to estimate the timing of this piracy event at ~22–21ka. The geomorphic evolution of the Cheyenne and Belle Fourche Rivers is also expressed by regionally recognized strath terraces that include (from oldest to youngest) the Sturgis, Bear Butte, and Farmingdale terraces. Radiocarbon and OSL dates from fluvial deposits on these terraces indicate incision to the level of the Bear Butte terrace by ~63ka, incision to the level of the Farmingdale terrace at ~40ka, and incision to the level of the modern channel after ~12–9ka. Similar dates of terrace incision have been reported for the Laramie and Wind River Ranges. Hypothesized causes of incision are the onset of colder climate during the middle Wisconsinan and the transition to the full-glacial climate of the late-Wisconsinan/Pinedale glaciation. Incision during the Holocene of the lower Cheyenne River is as much as ~80m and is 3 to 4 times the magnitude of incision at ~63ka and ~40ka. The magnitude of incision during the Holocene might be due to a combined effect of three geomorphic processes acting in concert: glacial isostatic rebound in lower reaches (~40m), a change from glacial to interglacial climate, and adjustments to increased watershed area resulting from piracy of the ancestral headwaters of the Little Missouri River.

Sediment supply, base level, braiding, and bedrock river terrace formation: Arroyo Seco, California, USA

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Here, we focus on Arroyo Seco in the central California Coast Ranges, where we test hypotheses for strath planation and subsequent strath terrace formation. Several lines of evidence indicate that strath planation is triggered by braiding in bedrock channels. In particular, hydraulic modeling reveals that the width of Arroyo Seco’s most recently formed terrace is comparable to the width of currently braided channel reaches. Additionally, a comparison of currently braided reaches to abandoned bedrock meander cutoffs shows that braided channels have valleys that are several times wider than single-thread meandering bedrock channel reaches. Lastly, in locations where the modern channel is currently braided, terraces are poorly preserved, suggesting that evidence for past episodes of braiding, in the form of paired strath terraces, is apparently largely destroyed by subsequent episodes of braiding. Field observations combined with mapping of terrace levels using an objective light detection and ranging (LiDAR)–based terrace identification algorithm reveal that temporal variation in tectonic uplift rate, sea level, and/or alluvial cover along the river cannot explain strath planation and subsequent terrace formation in Arroyo Seco. Rather, our results provide evidence that aggradation and degradation of alluvial sediments downstream of the Reliz Canyon fault result in impulsive base-level forcing of Arroyo Seco’s bedrock channel. Strath abandonment and terrace formation apparently occur as incision into downstream alluvial sediments propagates upstream into bedrock. Braiding and planation of straths, in contrast, occur during intervals of low vertical incision rate associated with downstream aggradation or immediately following pulses of vertical lowering triggered by downstream incision of alluvial sediments.

Climate-controlled landscape evolution in the Western Transverse Ranges, California: Insights from Quaternary geochronology of the Saugus Formation and strath terrace flights

The Las Posas and Ojai Valleys, located in the actively deforming Western Transverse Ranges of California, contain well-preserved flights of strath terraces and Quaternary strata (i.e., Saugus Formation) that when numerically dated elucidate the tectonic, geomorphic, and fluvial histories that sculpted the landscape since ca. 140 ka. This study includes 14 new optically stimulated luminescence and 16 new terrestrial cosmogenic nuclide ages from the late Pleistocene to Holocene that record two regional aggradation events and four intervals of strath terrace formation. Geochronologic data indicate that terrestrial Saugus strata in the Las Posas Valley (Camarillo Member) prograded over marine deposits at ca. 125 and 80 ka and are as young as 60–25 ka, which is an order of magnitude younger than the youngest Saugus strata elsewhere in Southern California. These results highlight the need for precise dating of Saugus strata where identified and utilized to assess rates of tectonic deformation. Based on its compositional character, thickness, stratigraphic relations, and inferred ages, the Camarillo member of the Saugus Formation is correlated with sediments of the Mugu aquifer identified in subcrop throughout the Ventura Basin and thus provides a new regional chronostratigraphic subsurface datum. The aggradation of these sediments and similar deposits in the study area between 13 and 4 ka is subsequent to the transition from humid to semiarid climate correlating to the end of the ultimate and penultimate glacial maximums. Aggradation is inferred to have resulted from increased sediment supply in response transient vegetative conditions and consequent hillslope destabilization. Similar to aggradational events, strath terrace cover sediments ages correlate to dry warm climate intervals, indicating straths in Southern California were cut at ca. 110–100 ka, 50–35 ka, 26–20 ka, and 15–4 ka. These results support recent mathematical and experimental models of strath formation, where increased sediment flux and decreased water discharge enhances lateral erosion rates and inhibits vertical incision. Subsequent incision and strath terrace formation is inferred to occur during intervening wet climate intervals. The correlation of strath terrace ages and aggradational events with environmental changes that are linked to global climate indicates that climate rather than tectonics exhibits first-order control of depositional, denudational, and incisional processes in the Western Transverse Ranges. Moreover, these results provide a chronostratigraphic framework that allows these landforms to be regionally correlated and used to assess rates of active tectonics where geochronologic data are unavailable.

Does decreasing paraglacial sediment supply slow knickpoint retreat?

In four rivers in western Scotland for which there is a well constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantifi ed by measuring the concentration of in situ cosmogenic 10Be on strath terraces downstream of headward retreating knickpoints. Along-channel gradients in 10Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a defi ciency in “tools” for bedrock erosion. Our results imply that episodes of major fl uvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.

Interaction of mantle dynamics, crustal tectonics, and surface processes in the topography of the Romanian Carpathians: A geomorphological approach

Tectonic processes and dynamic mantle flow impart a unique imprint on topography and geomorphic responses over time scales of 104 to 106yr. First-order topographic features in a tectonically active landscape represent ways to quantitatively characterise the interaction between crustal tectonics, mantle dynamics, and geomorphology, providing a basis for modelling landscape evolution. We analysed the topographic features of the Romanian Carpathians, a mountain range characterised by two straight segments connected by a narrow curvature zone. The deformation started in the Late Jurassic and includes two collisional phases during the Cretaceous and Miocene. We examined the tectonic geomorphology of the Romanian Carpathians focusing on regional and local topographic setting, drainage pattern, and river long profiles. Our main database is composed of DEM-based topographic analysis, supplemented with field investigations in the Slănic River basin, located in the Carpathian curvature zone. The longitudinal profiles of rivers draining the southern Carpathians are close to the equilibrium shape, in agreement with the older emersion of the chain. The longitudinal profiles of the rivers draining the eastern and southeastern Carpathians are in a transient state of disequilibrium as a consequence of a more recent emersion of the chain and of the Pliocene–Pleistocene tectonic activity in the Bend Zone. Filtering the topography at different wavelengths, we observe a relative depression in correspondence with the Carpathian Bend, where mantle seismicity and a high-velocity zone in tomography data are located and commonly interpreted as related to an almost inactive and dying subduction zone. Contrastingly, the filtered topography presents a high in the Transylvanian basin, where tomography data show a low-velocity area, interpreted as upwelling of hot asthenospheric materials. We hypothesise that local mantle convection generates positive and negative dynamic topographies. In the Slănic River basin, the foredeep deposits (Middle Miocene–Upper Pliocene) are folded and tilted, suggesting active compression beginning in the Lower Pleistocene. During the Middle–Late Pleistocene and Holocene, four terrace orders developed by the interaction between regional uplift and climate changes. The Carpathian topography results from a diachronous uplift superimposed on crustal tectonics. This diachronous uplift influenced the chain topography, the shape of river longitudinal profiles, and the formation of strath terraces. Crustal tectonics dominated the hydrographic net organisation. In correspondence with the Carpathian Bend and the Transylvanian basin, mantle flow driven by slab pull produced negative and positive dynamic topographies.