Terrace Treads Research Articles

Fluvial terrace formation in the eastern Fenwei Basin, China, during the past 1.2 Ma as a combined archive of tectonics and climate change

The key to understanding the fluvial response to climate change and surface uplift is a thorough distinguishing between their roles in terrace formation. Previous studies have tended to attribute the fluvial behavior of deposit–incision alternation to climate cyclicity. A preliminary explanation is proposed here for terrace formation in the eastern part of the Fenwei Basin, China. The observed fluvial terrace sequence was developed by the Yellow River deeply downcutting into the Emei Platform, which was uplifted within the eastern Fenwei Basin, and therefore it probably records the tectonic history of this platform. On the basis of the magnetostratigraphy and optically stimulated luminescence (OSL) dating, a ca. 1.2Ma chronology was established for this terrace sequence. Pedostratigraphic analysis of the loess deposits accumulated on each tread reveals that the terrace deposits are overlain immediately by a paleosol bed, suggesting that the abandonment of these terraces due to fluvial incision occurred at the transitions from glacial to interglacial climates. The glacial–interglacial climate cycle probably has a temporal control on the fluvial behavior of deposit–incision alternation, even though the Yellow River develops in the subsiding Fenwei Basin. The terrace generation may be sporadic and in the form of unusual stacked pattern before the Emei Platform was uplifted within the eastern Fenwei Basin. The terrace staircases however, formed in synchrony with glacial–interglacial climate cycles since this platform began to uplift in the late Middle Pleistocene. This result indicates that uplift may be necessary in large terrace staircase genesis. It can force the river system to downcut deeply enough during climatic transitions to separate terrace levels adequately, favoring the generation and subsequent preservation of the terrace treads. The terrace sequence of the Yellow River in the eastern Fenwei Basin therefore can be considered as a combined archive of climate change and local tectonic activity.

Using displaced river terraces to determine Late Quaternary slip rate for the central Wairarapa Fault at Waiohine River, New Zealand

We analyse a flight of post-Last Glacial Maximum terraces at Waiohine River, New Zealand that are progressively displaced by the dextral-slip Wairarapa Fault. The Waiohine River is shown to have smoothed its faulted river banks after each earthquake so that riser displacements are only recorded after abandonment of their lower bounding terrace tread. Digital elevation models produced from newly collected high-precision topographic data allowed us to precisely measure the cumulative dextral displacement of five risers and two palaeochannels, and the vertical displacement of six treads. The ratio of horizontal to vertical slip at the terraces ranges from 5.0 to 10.2 (average of 6.9±3.5 (2σ)). Combining our new displacements for the ‘Waiohine’ aggradation terrace and next younger terrace with new and previous optically stimulated luminescence (OSL) ages for Waiohine terrace silts, we calculate Late Quaternary dextral slip rates for this central part of the Wairarapa Fault of 11.9±2.9 mm a−1 (2σ) and >9.2±1.3 mm a−1 (2σ). Based on the smallest and next smallest observed offsets, the magnitudes of the inferred 1855 (smallest) and penultimate (next-smallest) single-event displacements are 12.4±0.8 m (2σ) and 9.7±1.7 m (2σ), respectively.

The relation of Holocene fluvial terraces to changes in climate and sediment supply, South Fork Payette River, Idaho

Well-preserved Holocene terraces along the South Fork Payette River in central Idaho provide a record of fluvial system behavior in a steep mountain watershed characterized by weathered and erodible Idaho Batholith granitic rocks. Terrace deposit ages were provided by 14C dating of charcoal fragments and optically stimulated luminescence (OSL) dating of sandy sediments. Along with pairing of many terrace tread heights, these data indicate episodic downcutting during the Holocene, with a mean incision rate of ∼0.9 m/ka from ∼7 ka to present. Prior to 7 ka, the river incised to within∼3 m of current bankfull, but then aggraded by ∼5 m over at least a ∼10 km-long reach in an episode centered ∼7–6 ka. Aggradation may relate to (1) increased hillslope sediment input from landslides and debris flows in steep tributary basins with abundant grussified granitic bedrock, (2) possible local landslide-damming of the channel, (3) decreased peak discharge, or (4) a combination of these factors. Middle Holocene channel aggradation ca. 7–6 ka corresponds with a period of prolonged and widespread aridity in the northern Rocky Mountains. Between ∼5 and 1.3 ka, the river aggraded slightly and then remained stable, forming a prominent terrace tread at ∼3 m above current bankfull. Modest aggradation to vertical stability of the South Fork Payette River at the 1.5 m terrace level ∼1.0–0.7 ka corresponds with large fire-related debris flows in tributaries during Medieval droughts. Three intervals of incision (∼5.5–5 ka, 1.3–1.0 ka and 0.5 ka) correspond with frequent but small fire-related sedimentation events and generally cooler, wetter conditions suggesting increased snowmelt runoff discharges. Other possible drivers of channel incision include an increase in stochastic or climate-modulated large storms and floods and a reduction in delivery of hillslope sediment to the channel. Aggradation is more confidently tied to climate through increases in hillslope sediment delivery and (or) decreased stream power, both likely related to warmer, drier conditions (including high-severity fires) that reduce snowmelt and decrease vegetation cover on steep slopes. Thus, the Holocene terraces of the South Fork Payette River do not reflect simple stepwise incision with periods of vertical stability and lateral migration, but record substantial episodes of aggradation as well. We infer that increases in hillslope erosion and mass movements combined with reduced discharges during prolonged droughts episodically reverse the post-glacial trend of downcutting, in particular during the middle Holocene. The present bedrock-dominated channel implies a strong tendency toward incision in the late Holocene.

Evaluating the role of climate and tectonics during non-steady incision of the Yellow River: evidence from a 1.24 Ma terrace record near Lanzhou, China

The competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain. Most of recent studies have attributed terrace formation to climatic changes and held that, even in tectonically active settings, climate variations control cycles of terrace planation and abandonment. Based on field investigations of loess-paleosol sequences, magnetostratigraphy and optically stimulated luminescence (OSL) dating, we develop a new chronology for a spectacular flight of terraces along the Yellow River near Lanzhou, China over past 1.24 Ma. All the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates. However, the ages of terraces cluster in two relatively short time periods (1.24–0.86 Ma and 0.13 Ma – present). During the intervening time between 0.86 Ma and 0.13 Ma, terraces either did not form or were not preserved. We suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation/preservation. Thus, our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision.

Late Quaternary loess landscape evolution on an active tectonic margin, Charwell Basin, South Island, New Zealand

Loess deposits constitute an important archive of aeolian deposition reflecting wider patterns of glacial atmospheric circulation, and more localised interactions between riverine source areas, loess trapping efficiency and geomorphic controls on erosion rate. Conceptual models have been formulated to explain the coeval evolution of loess mantles and associated landscapes (loess landscape models) but none apply to areas of tectonically induced base-level lowering. This study uses an age sequence of alluvial fill terraces in the Charwell Basin, north-eastern South Island New Zealand, which straddles the transpressive Hope Fault, to investigate geomorphic controls on loess landscape evolution in an active tectonic region. We hypothesize that the more evolved drainage networks on older terraces will more effectively propagate base-level lowering by way of a greater areal proportion of steep and convex hillslopes and a smaller proportion of non-eroding interfluves. Eventually, as the proportion of interfluves diminishes and hillslope convexity increases, terraces shift from being net loess accumulators to areas of net loess erosion. We investigate the nature of erosion and the geomorphic thresholds associated with this transition. Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. The number of loess sheets and the thickness of loess increased across the three youngest terraces. The next oldest (ca. 150 ka) terrace remnant had the greatest maximum number of loess sheets (3) and loess thickness (8 m) but the loess mantle was highly variable. A detailed loess stratigraphic analysis and the morphometric analysis place this terrace in a transition between dominantly planar, uniformly loess-mantled landforms and loess-free ridge and valley terrain exemplified by the oldest terrace remnant. Variations in thickness and preservation of loess sheets demonstrated spatially and temporally variable erosion during loess accumulation. To test our hypothesis of loess persistence we calculated critical steady-state hillslope curvatures from a soil transport model, calibrated for the study area, above which the uppermost loess sheet (L1, max. thickness 1.8 m) should be completely eroded. We compared loess distribution mapped in the field to values of slope curvature calculated from topographic surveys and found two of three critical curvature values had acceptable predictive ability. Where predictions failed this is probably due to transient responses related to active steam incision. Because all critical curvature values predicted presence of loess on the oldest terrace remnant where there is none we conclude that important factors other than morphometric ones are important in determining loess distribution in loess landscapes in active tectonic regions. These may include internal changes to regolith affecting erodibility, or vegetation or topographic interaction with wind patterns affecting loess trapping.

Fluvial terrace riser degradation and determination of slip rates on strike‐slip faults: An example from the Kunlun fault, China

The use of displaced fluvial terrace risers to determine slip rates along strike‐slip faults depends on knowing when risers become passive markers. Typically, chronologic constraints from terrace deposits only place bounds on this age; consequent slip rates may be highly uncertain. Along the Kunlun fault, in northeastern Tibet, we demonstrate that consideration of riser degradation can augment terrace chronology and improve the precision of slip‐rate estimates. Multiple offset risers and 14C age determinations from terrace treads yield a narrow range of slip rates of 4.5 ± 0.5 m/kyr at one site, but permit a wide range of allowable rates (3–10 m/kyr) in an adjacent drainage. Using a locally calibrated sediment transport rate of 3.2–5.4 m2/ka, we show that riser morphology at this latter site is consistent with degradation since abandonment of the upper terrace tread. Our analysis indicates that slip rates at this site are ∼5 ± 2 m/kyr.

Rates of active faulting during late Quaternary fluvial terrace formation at Saxton River, Awatere fault, New Zealand

A flight of faulted fluvial terraces at Saxton River on the Awatere fault, northeast South Island, New Zealand, preserves the incremental slip history and detailed paleoearthquake chronology of this major strike-slip fault. Here, six fluvial terraces have been progressively displaced across the inland Molesworth section of the fault, with horizontal displacements ranging from -6 m for an ephemeral channel on the youngest terrace to 81 m for the riser above the oldest terrace. New optically stimulated luminescence ages for abandonment of the two oldest terrace treads are 14.5 ± 1.5 and 6.7 ± 0.7 ka. When combined with new measurements of incremental horizontal displacements and previous age data, these new ages indicate that strike-slip on this part of the Awatere fault has been occurring at a near-constant rate of 5.6 ± 0.8 mm/yr since ca. 15 ka. This rate is similar to recent slip-rate estimates for an adjoining section of the same fault to the east, which suggests that | there is near-complete slip transfer across the junction between the two fault strands. Comparison of the magnitudes and ages of the terrace riser displacements to the timing of paleoearthquakes on the Molesworth section allows the mean per event horizontal displacement over the eight most recent surface-rupture events to be estimated at 4.4 ± 0.8 m. Between ca. 5 ka and ca. 2 ka, surface-rupturing earthquakes increased in frequency and decreased in their mean coseismic displacements to <2.6 m. During this time, the sense of local dip slip also shifted from north-side-up to south-side-up. Rapid incision of Saxton River in the mid-Holocene may have caused a perturbation in the near-surface stress directions acting on the fault plane beneath the Saxton River valley, forcing a change in the near-surface fault geometry that resulted in the shift in sense of dip slip.

Relations between postglacial fluvial incision rates and uplift rates in the North Island, New Zealand

Fluvial incision rates are increasingly being used as a proxy for rock uplift rates for inland settings. We test the validity of this approach by comparing incision rates and rock uplift rates along the Hikurangi Margin, New Zealand. Postglacial fluvial incision rates were calculated by measuring the difference in altitude between the Last Glacial Maximum (LGM) terrace tread (18 ± 2 calendar ka) and the present river bed for 10 major rivers (catchment areas ∼1400–5900 km2). Incision rates have thus been measured over a half glacial cycle. They also represent average rates over the last 18 kyr, during which time variations are likely due to processes such as knickpoint retreat. Rock uplift rates have been estimated from (1) altitude differences between the treads of pairs of fluvial fill terraces (∼55 and ∼18 calendar ka) assumed to have formed at the same altitude (i.e., rock uplift relative to the geoid), (2) present‐day altitudes of Late Neogene paleoshoreline marker horizons (1.6–5.7 Ma) (i.e., rock uplift relative to sea level), and (3) present‐day altitudes of Neogene mudstones for which maximum depth of burial has been calculated from porosity (i.e., rock uplift relative to sea level). These rock uplift rates overestimate tectonic uplift rates because they do not take into account isostatic compensation due to erosion. Mean incision rates range up to 11.7 mm yr−1, and mean rock uplift rates range up to 4, 1, and 0.9 mm yr−1 for the three methods, respectively. We have made some qualitative and semiquantitative observations about nontectonic controls on postglacial incision rates and find that localized influence only is exerted by rainfall, base level, and bedrock lithology, while negative or no correlations are shown with LGM fluvial fill thickness and stream power. Incision rates and uplift rates display a rough correlation, with incision rates typically 1.5–5 times uplift rates. This relation suggests that tectonic uplift is likely to exert an important control on fluvial incision in the Hikurangi Margin.

03/02032 An optimisation model of geothermal-energy conversion: Drozdz, M. Applied Energy, 2003, 74, (1—2), 75–84

Sequences of alluvial fill-terraces record the long-term variation of fluvial transport regimes in response to Quaternary climatic changes. Through a production of increasingly precise chronologies, the broadening range of sediment dating methods in recent years has improved our understanding of correlations between alluvial dynamics and external forcing mechanisms. However, results in this field have mostly focused on periglacial and other mid-latitude settings. These may not readily apply to glaciofluvial environments, where alluvial regimes are narrowly dependent on the behavior of the adjacent ice-field. Here we constrain the ages of three glaciofluvial fill terraces based on five vertical 10Be age profiles in the Ariège River catchment, and produce a chronofunction of related contrasts in terrace tread topsoil characteristics. Each profile consists of 9 to 11 gravel samples collected over depths of 4.6 to 15 m. Terrace T1 (+ 15 m above the current riverbed) was sampled at ~ 4, 22 and 53 km downstream from the Last Glacial Maximum terminal moraine system of the Ariège outlet glacier. The profiles of T2 (+ 30 m) and T3 (+ 50 m) complete the data set. Beryllium-10 age profiles date the age of terrace tread abandonment as a result of fluvial incision. Results show that the incision of T1 occurred over a protracted period during the Last Termination, beginning ca. 17.5 –3.5+2.0 ka. Prior to this, aggradation occurred from MIS 4 to the global Last Glacial Maximum (LGM), i.e.,under maximum ice extent conditions, and lasted a few thousand years thereafter. The protracted influx of debris after the LGM was sustained by paraglacial storage release in the catchment and promoted by strong coupling between slope and channel processes. Fluvial incision of T1 occurred during the post-LGMcold-to-warm transition after a time (supported by paleobotanical evidence) when the vegetation cover was capable of ensuring catchment-wide slope stability. The wave of incision propagated downstream between 17.5 –3.5+2.0 ka (proximal profile) and 13.0 –0.5+3.5 ka (distal profiles). The exposure-age confidence intervals for T2 and T3 are much larger than for T1 (60–145 ka and 204–226 ka, respectively) but the data nonetheless allow the aggradation of T2 and T3 to be attributed to MIS 6 and MIS 8, respectively. The soil chronosequence permitted by the fill-terrace age sequence indicates that the Luvic Cambisols capping T1 formed after ~ 13–16 ka, i.e.,during the Holocene; (ii) the Haplic Luvisols capping T2 required at least ~ 130 ka, i.e.,two interglacials; and (iii) the deeply rubified, highly eluviated Luvisols capping T3 required at least ~ 250 ka, i.e.,at least three interglacials.

The effectiveness of bench-terracing and afforestation for erosion control on Raña sediments in central Spain

Bench-terracing and Pinus afforestation have been extensively adopted as a means of soil conservation in Spain. This paper questions the appropriateness of this technology to extensively gullied soils and sediments of the Rana formation of central Spain. Bench-terracing has markedly altered the characteristics of the surface soil horizon as compared with nearby undisturbed matorral (natural scrub) vegetation. The construction process and resultant bench-terrace morphology have imposed a new, more spatially structured pattern in soil properties giving enhanced opportunities for overland flow generation and erosion. The terracing has exposed subsurface Bt horizons on inner treads and lower risers, producing a very compact clay rich surface horizon, low in organic carbon and of poor structural stability. Despite subsoiling hydraulic conductivity is very low. Excavated soil material is ploughed sideways to partially bury the original surface horizon and form part of the next riser downslope. Downslope drainage on the terraces gives rise to soil moisture contents approaching saturation on the lower risers and treads. Consequently only small rainfall inputs are required to initiate ponding and overland flow generation on sloping treads. Rainfall simulation demonstrates the importance of antecedent soil moisture to runoff generation, and the role of vegetation and organic matter in retaining water. Erosion bridge measurements show that erosion rates on sloping treads are as high as those recorded on gully sidewalls. Gully sidewall erosion is also accelerated by runoff waters shed from terrace treads. Management of native matorral vegetation may provide a more sustainable approach to soil conservation than bench terracing with afforestation.

Deposits of a rapidly eroding meandering river: Terrace cut and fill in the Taupo Volcanic Zone

Abstract River terraces in a section of the upper Rangitaiki River have formed in the last 1850 years as the river rapidly regrades following a major volcanic eruption (Taupo Tephra Formation). Terrace treads are cut at the migrating outside edge of meander loops and are subsequently filled by scroll and point bar deposits associated with the same loop. Entrenchment rates as high as 5 mm/yr cause the meanders to be permanently abandoned following cutoff. This results in the widespread preservation of fluvial cover sequences up to 6 m thick in an otherwise strongly erosive regime.

Weathering trends in terrace deposits up to 350,000 years old in northeast Southland, New Zealand

Gravelly floodplain deposits and six terrace treads in the Waimea Plains, northeast Southland, New Zealand, range in age from ca. 1000 years old to 350,000 yr old. Deposits are progressively more weathered with increasing parent material age. Soils developed in them are classified as Recent Soils, Brown Soils and Ultic Soils in the New Zealand Soil Classification and Udifluvents, Dystrochrepts, Haplohumults, Hapludults and Paleudults in the USDA Soil Classification (Soil Taxonomy). The youngest soils are slightly acid. Those developed in deposits 35,000–70,000 yr old are strongly acid. In soils in deposits 160,000 yr old and older, that have weathered in two or three interglacial periods, including the present one, pH and total exchangeable bases values are higher than in soils in younger deposits. The higher pH of the soils in older deposits is attributed to release of cations by stone weathering. Soils developed in deposits up to 70,000 yr old have clays dominated by mica and chlorite. Stones in these soils are at most weakly weathered, and the < 2 mm fraction contains 2–13% clay and 63–95% sand. Weathering of older deposits has destroyed the fabric of most stones. The < 2 mm fraction of soils developed in deposits 250,000–350,000 yr old contains 34–38% clay and 20–24% sand; clays are dominated by kaolin-group and mica-smectite minerals and 60% of the total Fe in the < 2 mm fraction is in secondary forms. Two phases of weathering are distinguished: a phase of approximate duration 100,000 yr in which most weathering and leaching occurs in the matrix, which becomes increasingly acidified; and a phase from 100,000 to 350,000 yr which is dominated by cation release from stones, reversal of the acidification trend, and breakdown of stone fabric.

Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes

We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process analogs to interpret the Holocene history of forest fire–related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire-related sedimentation within valley-side alluvial fans of Soda Butte Creek. Fire-related deposits make up approximately 30% of the late Holocene fan alluvium. Fifty 14 C ages on fire-related events cluster within the intervals of 3300–2900, 2600–2400, 2200–1800, and 1400–800 yr B.P. and suggest earlier episodes of large fires and fan aggradation around 7500, 5500, and 4600–4000 yr B.P. A major pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. a.d. 1050–1200). Instrumental climate records over the last ∼100 yr in Yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods, enhancing the potential for severe short-term drought, major forest fires, and storm-generated fan deposition. Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca. 8000 yr B.P. (terrace level T1a), 7000–5600 (T1b), 3100–2600 (T2), 2000–1300 (T3), and post–800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of fire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions during terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater resistance to downcutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale climatic cycles, wherein alluvial fans aggrade and prograde over flood plains in drier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. “Small-scale” climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.

Stream-terrace genesis: implications for soil development

Genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting, base level of erosion, complex response, threshold of critical power, diachronous and synchronous response times, and static and dynamic equilibrium. Climatic and tectonic stream terraces are major terraces below which flights of minor complex-response degradation terraces can form. These three types of terraces can be summarized by describing a downcutting-aggradation-renewed downcutting sequence for streams with gravell bedload. By tectonically induced downcutting, streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion. Lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream-terrace landform. Aggradation events record brief reversals of long-term tectonically induced downcutting because they raise active channels. They may be considered as major (the result of climatic perturbations) or minor (the result of complex-response model types of perturbations). Climatically controlled aggradation followed by degradation leaves an aggradation surface; this type of fill-terrace tread is the fundamental climatic stream-terrace landform. Aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces. Raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form; first in alluvial fill and then in underlying bedrock along tectonically active streams. These are complex-response terraces because they result from interactions of dependent variables within a given fluvial system. Pauses in degradation to a new base level of erosion, and/or minor episodes of backfilling, lead to formation of complex-response fill-cut and strath, or of fill terraces. Fill-cut terraces are formed in alluvium; they are complex-response terraces because they are higher than the base level of erosion. Good exposures and dating are needed to distinguish static equilibrium complex-response minor strath terraces from dynamic equilibrium tectonic (major) straths. Strath terraces may be regarded as complex-response terraces where degradation rates between times terrace-tread formation exceed the long-term uplift rate for the reach based on ages and positions of tectonic terraces. Late Quaternary global climatic changes control aggradation events and even the times of cutting of major (tectonic) straths, because the base level of erosion can not be attained during times of climatically driven aggradation-degradation events. Most terrace soils form on treads of climatic and complex-response terraces. Aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence. Each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads. Multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces. Pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors. Climatic terraces of adjacent watersheds also should be roughly synchronous (correlatable) - variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds. Such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times, and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations. Soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full-glacial times and interglacial times. Soils on some complex-response terraces may be diachronous within a given fluvial system, and typically are diachronous between watersheds.

Late Quaternary movement on White Creek Fault, South Island, New Zealand

Abstract The faulted Buller River terrace sequence at White Creek Fault is the best locality to document the faulting that occurred during the Murchison earthquake of 17 June 1929. The upper faulted tread at White Creek is correlated with the later Otiran aggradation surface of the Buller valley. Riser height consistency and constant scarp height on all terrace treads, and a planar granite platform, are consistent with one single fault movement since the aggradation surface formed c. 18 000 y. ago. Tree-ring dating of Nothofagus fusca trees that were growing on the fault scarp indicates a significant event in 1929 and is attributed to the faulting during the Murchison earthquake. Periodicity of faulting at two Buller-NW Nelson reverse faults—Inangahua Fault and White Creek Fault—is found to be longer than 6000 y., about one order of magnitude longer than for faults within the shear belt of New Zealand.

Cryoplanation Terraces: Indicators of a Permafrost Environment

Cryoplanation terraces are bedrock steps or terraces on ridge crests and hilltops. The tread or “flat” area is 10 to several hundred meters wide and long and slopes from 1 to 5° parallel to the ridge crests. Terrace scarps may be from 1 to 75 m high. Terraces are cut into all bedrock types and are best developed on closely jointed, fine-grained bedrock. The scarps and treads are covered with frost-rived rubble 1 to 2 m thick. The rubble on treads is perennially frozen at a depth of 1 to 2 m or less on sharp but inactive terraces in Alaska.Cryoplanation terraces exist in many parts of the world in present or past periglacial environments. They occur chiefly in nonglaciated regions and near the general altitude of snowline. Cryoplanation terraces form by scarp retreat as the result of nivation. Surficial debris is removed across the terrace tread by mass-wasting. Terrace morphology depends mainly upon climate, bedrock type, and terrace orientation.No climatic data are available from active terraces. Indirect evidence indicates that climatic requirements include low snowfall and cold summer temperatures. Shallow permafrost is necessary to provide moisture and a base for mass movement as well as a base for nivation.Hundreds of sharp but inactive terraces occur in some areas in Alaska where the summer temperature is colder than 10°C. When these terraces were active, temperatures were colder. Recent work in Alaska indicates that terraces were active in some areas when the mean July temperature was about 4°C. The mean annual air temperature probably was in the neighborhood of -12°C or colder.

Nonsorted Circle Development in a Colorado Alpine Location

The origin, development, and dynamics of nonsorted circles, a variety of patterned ground produced by seasonal frost heaving, are described and analyzed for an alpine location in the Colorado Front Range. Age relations of nonsorted circles and allied features are also discussed.Nonsorted circles favour exposed snow-free locations, particularly the frontal areas of turf-banked lobe and terrace treads. Here, wind scour causes the disruption and removal of the protective mat of the alpine turf. Frost penetration into freshly exposed sites promotes, in the presence of adequate moisture, differential frost heaving with the subsequent doming of circle centres.A buried A horizon observed in a number of pits dug into nonsorted circles occupying a turf-banked lobe, was dated at 10,400 ± 400 radiocarbon years. The material in which the circles subsequently developed became sufficiently stable for soil development by the close of the Pinedale Glaciation (7,500 B.P.). Field evidence suggests they may be a recent phenomena, initiated after the close of the Gannett Peak stade of Neoglaciation.

Transport and rounding of large boulders in mountain streams

On the floors and terrace treads of mountain valleys it is not unusual to find exceptionally large boulders. The size of these boulders is often beyond the ability of the existing stream, even in flood conditions, to have been moved by them. This raises the question of their mode of transportation from their point of origin to their resting place. This article gives four possible explanations: ice transport during the earlier glaciation of the region; mudflow transport; water-traction by rare floods of cataclysmic proportions; and intermittent progress due to alluvium erosion.