Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵–10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10⁴-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and ¹⁴C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic ¹⁰Be and ²⁶Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 10⁴-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.     

Tectonic control of the Tejo river fluvial incision during the late Cenozoic, in Ródão—central Portugal (Atlantic Iberian border)

Staircases of strath terraces and strongly incised valleys are the most typical landscape features of Portuguese rivers. This paper examines the incision achieved during the late Cenozoic in an area crossed by the Tejo river between the border with Spain and the small town of Gavião. In the more upstream reach of this area, the Tejo crosses the Ródão tectonic depression, where four levels of terraces are distinguished. During the late Cenozoic fluvial incision stage, the Ródão depression underwent less uplift than the adjacent areas along the river. This is reflected by the greater thicknesses and spatial extent of the terraces; terrace genesis was promoted by impoundment of alluvium behind a quartzitic ridge and the local presence of a soft substratum. Outside this tectonic depression, the Tejo has a narrow valley incised in the Hercynian basement, with some straight reaches that probably correspond to NE–SW and NNW–SSE faults, the terraces being nearly absent. Geomorphological evidence of tectonic displacements affecting the Ródão dissected terrace remnants is described. Geochronological dating of the two younger and lower terrace levels of this depression suggests a time-averaged incision rate for the Tejo in the Ródão area, of ca. 1.0 m/ka over the last 60 thousand years. A clear discrepancy exists between this rate and the 0.1 m/ka estimated for the longer period since the end of the Pliocene. Although episodes of valley incision may be conditioned by climate and base-level changes, they may also have been controlled by local factors such as movement of small fault-bounded blocks, lithology and structure. Regional crustal uplift is considered to be the main control of the episodes of valley incision identified for this large, long-lived river. A model is proposed in which successive regional uplift events—tectonic phases—essentially determined the long periods of rapid river downcutting that were punctuated by short periods of lateral erosion and later by some aggradation, producing strath terraces.     

河流阶地形成过程及其驱动机制再研究

河流阶地的形成是在内因(河流内部动力变化)和外因(低频和高频气候变化、构造运动、基准面变化)共同作用下的结果。受单一气候变化制约的河流阶地发育模式可以解释由于沉积物通量和径流量变化引起的河流堆积-侵蚀过程,但它难以解释形成多级阶地的逐步(或间歇性)下切过程。多级阶地的形成可能同时受到构造抬升和周期性气候变化的制约。由于下切过程的滞后效应,侵蚀和冰川均衡抬升、河谷的侧向侵蚀过程等影响,山地的构造抬升与河谷的下切之间并非一种简单的线性关系,应当慎用河谷的下切速率来代表山地的抬升速率。     

Exhumation and incision history of the Lahul Himalaya, northern India, based on (U–Th)/He thermochronometry and terrestrial cosmogenic nuclide methods

Low-temperature apatite (U–Th)/He (AHe) thermochronology on vertical transects of leucogranite stocks and ¹⁰Be terrestrial cosmogenic nuclide (TCN) surface exposure dating on strath terraces in the Lahul Himalaya provide a first approximation of long-term (10⁴–10⁶ years) exhumation rates for the High Himalayan Crystalline Series (HHCS) for northern India. The AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to the surface was ~ 1–2 mm/a and occurred during the past ~ 2.5 Ma. Bedrock exhumation in Lahul fits into a regional pattern in the HHCS of low-temperature thermochronometers yielding Plio-Pleistocene ages. Surface exposure ages of strath terraces along the Chandra River range from ~ 3.5 to 0.2 ka. Two sites along the Chandra River show a correlation between TCN age and height above the river level yielding maximum incision rates of 12 and 5.5 mm/a. Comparison of our AHe and surface exposure ages from Lahul with thermochronometry data from the fastest uplifting region at the western end of the Himalaya, the Nanga Parbat syntaxis, illustrates that there are contrasting regions in the High Himalaya where longer term (10⁵–10⁷ years) erosion and exhumation of bedrock substantially differ even though Holocene rates of fluvial incision are comparable. These data imply that the orogen's indenting corners are regions where focused denudation has been stable since the mid-Pliocene. However, away from these localized areas where there is a potent coupling of tectonic and surface processes that produce rapid uplift and denudation, Plio-Pleistocene erosion and exhumation can be characterized by disequilibrium, where longer term rates are relatively slower and shorter term fluvial erosion is highly variable over time and distance. The surface exposure age data reflect differential incision along the length of the Chandra River over millennial time frames, illustrate the variances that are possible in Himalayan river incision, and highlight the complexity of Himalayan environments.     

Terrace aggradation during the 1978 flood on Powder River, Montana, USA

Flood processes no longer actively increase the planform area of terraces. Instead, lateral erosion decreases the area. However, infrequent extreme floods continue episodic aggradation of terraces surfaces. We quantify this type of evolution of terraces by an extreme flood in May 1978 on Powder River in southeastern Montana. Within an 89-km study reach of the river, we (1) determine a sediment budget for each geomorphic feature, (2) interpret the stratigraphy of the newly deposited sediment, and (3) discuss the essential role of vegetation in the depositional processes.Peak flood discharge was about 930 m³ s^− 1, which lasted about eight days. During this time, the flood transported 8.2 million tons of sediment into and 4.5 million tons out of the study reach. The masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach. The flood inundated the floodplain and some of the remnants of two terraces along the river. Lateral erosion decreased the planform area of the lower of the two terraces (~ 2.7 m above the riverbed) by 3.2% and that of the higher terrace (~ 3.5 m above the riverbed) by 4.1%. However, overbank aggradation, on average, raised the lower terrace by 0.16 m and the higher terrace by 0.063 m.Vegetation controlled the type, thickness, and stratigraphy of the aggradation on terrace surfaces. Two characteristic overbank deposits were common: coarsening-upward sequences and lee dunes. Grass caused the deposition of the coarsening-upward sequences, which had 0.02 to 0.07 m of mud at the base, and in some cases, the deposits coarsened upwards to coarse sand on the top. Lee dunes, composed of fine and very fine sand, were deposited in the wake zone downstream from the trees. The characteristic morphology of the dunes can be used to estimate some flood variables such as suspended-sediment particle size, minimum depth, and critical shear velocity. Information about depositional processes during extreme floods is rare, and therefore, the results from this study aid in interpreting the record of terrace stratigraphy along other rivers.     

Climatic and tectonic controls on the fluvial morphology of the Northeastern Tibetan Plateau (China)

The geomorphological evolution of the Northeastern Tibetan Plateau (NETP) could provide valuable information for reconstructing the tectonic movements of the region. And the considerable uplift and climatic changes at here, provide an opportunity for studying the impact of tectonic and monsoon climate on fluvial morphological development and sedimentary architecture of fluvial deposits. The development of peneplain-like surface and related landscape transition from basin filling to incision indicate an intense uplift event with morphological significance at around 10–17 Ma in the NETP. After that, incision into the peneplain was not continuous but a staircase of terraces, developed as a result of climatic influences. In spite of the generally persisting uplift of the whole region, the neighbouring tectonic blocks had different uplift rates, leading to a complicated fluvial response with accumulation terraces alternating with erosion terraces at a small spatial and temporal scale. The change in fluvial activity as a response to climatic impact is reflected in the general sedimentary sequence on the terraces from high-energy (braided) channel deposits (at full glacial) to lower-energy deposits of small channels (towards the end of the glacial), mostly separated by a rather sharp boundary from overlying flood-loams (at the glacial-interglacial transition) and overall soil formation (interglacial). Pronounced incision took place at the subsequent warm-cold transitions. In addition, it is hypothesized that in some strongly uplifted blocks energy thresholds could be crossed to allow terrace formation as a response to small climatic fluctuations (10³–10⁴ year timescale). Although studies of morpho-tectonic and geomorphological evolution of the NETP, improve understanding on the impacts of tectonic motions and monsoonal climate on fluvial processes, a number of aspects, such as the distribution and correlation of peneplain and the related morphological features, the extent and intensity of tectonic movements influencing the crossing of climatic thresholds, leading to terrace development, need to be studied further.     

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.     

晚更新世以来贵州清水江阶地发育及地貌意义

对贵州清水江上游马寨、翁东、三江、施洞沿江4个剖面的阶地特征、年代学结果进行了综合分析。发现以凯里断层为界,上游地区的马寨和翁东2个剖面的T2阶地形成时代约为51~57 ka B.P.,T1阶地的形成时代约为25 ka B.P.,下游地区的三江和施洞2个剖面的T2阶地形成时代约为122~102 ka B.P.,T1阶地的形成时代约为78 ka B.P.。选取各剖面的T2阶地的基座高度来计算了河流下切速率,发现上游地区2个剖面（马寨、翁东）的河流下切速率较接近,约为0.41~0.34 m/ka,明显高于下游地区的2个剖面（三江、施洞）的0.16~0.20 m/ka,表现为上游下切速率高,越往下游方向下切速率逐渐降低。这表明自晚更新世以来,清水江上游区域受到构造作用的影响而发生差异抬升,具体表现为西部构造抬升幅度大,阶地下切速率快;东部构造抬升幅度小,阶地下切速率慢。     

河流对0.8 Ma B.P.环境突变事件的地貌响应研究

以兰州盆地0.8 Ma B.P.阶地为例证,运用古地磁测年方法,通过收集相关文献,分析讨论了0.8 Ma B.P.阶地与0.8 Ma B.P.环境突变事件的联系。结果表明：① 0.8 Ma B.P.环境突变事件主要表现在气候转型、构造运动等方面,具有群发性和全球性特点;② 兰州盆地以及其他区域0.8 Ma B.P.阶地存在的证据,表明河流在0.8 Ma B.P. 前后普遍发生过一次下切事件;③ 0.8 Ma B.P.阶地是河流对0.8 Ma B.P.环境突变事件的地貌响应,构造运动为提供了下切驱动力,而气候变化则控制了下切时间。     

Controls on erosion patterns and sediment transport in a monsoonal,tectonically quiescent drainage,Song Gianh,central Vietnam

The Song Gianh is a small‐sized (~3500 km²), monsoon‐dominated river in northern central Vietnam that can be used to understand how topography and climate control continental erosion. We present major element concentrations, together with Sr and Nd isotopic compositions, of siliciclastic bulk sediments to define sediment provenance and chemical weathering intensity. These data indicate preferential sediment generation in the steep, wetter upper reaches of the Song Gianh. In contrast, detrital zircon U‐Pb ages argue for significant flux from the drier, northern Rao Tro tributary. We propose that this mismatch represents disequilibrium in basin erosion patterns driven by changing monsoon strength and the onset of agriculture across the region. Detrital apatite fission track and ¹⁰Be data from modern sediment support slowing of regional bedrock exhumation rates through the Cenozoic. If the Song Gianh is representative of coastal Vietnam then the coastal mountains may have produced around 132 000–158 000 km³ of the sediment now preserved in the Song Hong‐Yinggehai Basin (17–21% of the total), the primary depocenter of the Red River. This flux does not negate the need for drainage capture in the Red River to explain the large Cenozoic sediment volumes in that basin but does partly account for the discrepancy between preserved and eroded sediment volumes. OSL ages from terraces cluster in the Early Holocene (7.4–8.5 ka), Pre‐Industrial (550–320 year BP) and in the recent past (ca. 150 year BP). The older terraces reflect high sediment production driven by a strong monsoon, whereas the younger are the product of anthropogenic impact on the landscape caused by farming. Modern river sediment is consistently more weathered than terrace sediment consistent with reworking of old weathered soils by agricultural disruption.     

Lag and mixing during sediment transfer across the Tian Shan piedmont caused by climate‐driven aggradation–incision cycles

Transient sediment storage and mixing of deposits of various ages during transport across alluvial piedmonts alter the clastic sedimentary record. We quantify buffering and mixing during cycles of aggradation–incision in the north piedmont of the Eastern Tian Shan. We complement existing chronologic data with 20 new luminescence ages and one cosmogenic radionuclide age of terrace abandonment and alluvial aggradation. Over the last 0.5 Myr, the piedmont deeply incised and aggraded many times per 100 kyr. Aggradation is driven by an increased flux of glacial sediment accumulated in the high range and flushed onto the piedmont by greater water discharge at stadial–interstadial transitions. After this sediment is evacuated from the high range, the reduced input sediment flux results in fluvial incision of the piedmont as fast as 9 cm year⁻¹ and to depths up to 330 m. The timing of incision onset is different in each river and does not directly reflect climate forcing but the necessary time for the evacuation of glacial sediment from the high range. A significant fraction of sediments evacuated from the high range is temporarily stored on the piedmont before a later incision phase delivers it to the basin. Coarse sediments arrive in the basin with a lag of at least 7–14 kyrs between the first evacuation from the mountain and later basinward transport. The modern output flux of coarse sediments from the piedmont contains a significant amount of recycled material that was deposited on the piedmont as early as the Middle Pleistocene. Variations in temperature and moisture delivered by the Westerlies are the likely cause of repeated aggradation–incision cycles in the north piedmont instead of monsoonal precipitation. The arrival of the gravel front into the proximal basin is delayed relative to the fine‐grained load and both are separated by a hiatus. This work shows, based on field observations and data, how sedimentary systems respond to climatic perturbations, and how sediment recycling and mixing can ensue.     

RADIOCARBON AGES ON ORGANICS FROM PIEDMONT ALLUVIUM,AJO MOUNTAINS,ARIZONA

Alluvium in dry lands is considered difficult to date by radiocarbon methods because of the paucity of organic matter. Although organic materials of sufficient size for conventional ¹⁴C dating are rare, wet sieving of alluvium in the Sonoran Desert yields sufficient organics for ¹⁴C measurements by accelerator mass spectrometry (AMS). Detrital charcoal from two Quaternary fluvial fill terraces on the western side of the Ajo Mountains yielded ¹⁴C ages of 14,880 ± 70 yr B.P. (CAMS-12408) for the Qt 1 terrace and 2490 ± 60 yr B.P. (CAMS-12414) and 2510 ± 60 yr B.P. (CAMS-12415) for the smaller inset Qt 2 terrace. These ¹⁴C ages are consistent with what is known about rates of soil development in the region. The earlier aggradation event appears to be supported by regional and possibly global climate change at about 14,000 ¹⁴C yr B.P. The more recent aggradation event does not appear to be synchronous with periods of frequent paleofloods in the southwest. The offset between ¹⁴C and ³⁶Cl ages for the same terraces provides a general indication of the time taken for the clasts to be transported to their current positions on the terraces. [Key words: soils, organic matter, 14C, 36Cl, Quaternary dating methods, piedmont, geomorphology, Sonoran Desert.]     

Rates of fluvial bedrock incision within an actively uplifting orogen: Central Karakoram Mountains, northern Pakistan

Terrestrial cosmogenic nuclide (TCN) ¹⁰Be surface exposure ages for strath terraces along the Braldu River in the Central Karakoram Mountains range from 0.8 to 11 ka. This indicates that strath terrace formation began to occur rapidly upon deglaciation of the Braldu valley at  11 ka. Fluvial incision rates for the Braldu River based on the TCN ages for strath terraces range from 2 to 29 mm/a. The fluvial incision rates for the central gorged section of the Braldu River are an order of magnitude greater than those for the upper and lower reaches. This difference is reflected in the modern stream gradient and valley morphology. The higher incision rates in the gorged central reach of the Braldu River likely reflect differential uplift above the Main Karakoram Thrust that has resulted in the presence of a knickpoint and more rapid fluvial incision. The postglacial fluvial incision rate (2–3 mm/a) for the upper and lower reaches are of the same order of magnitude as the exhumation rates estimated from previously published thermochronological data for the Baltoro granite in the upper catchment region and for the adjacent Himalayan regions.     

Quaternary soil chronosequences on terraces of the Susquehanna river, Pennsylvania

Susquehanna River terraces are used to establish time lines along a 150 km reach of the river, from the Lower Piedmont to the edge of the Appalachian Plateau. This is achieved by generating soil chronosequences at two locations — Marietta, PA, in the Lower Piedmont, and Muncy, PA, near the glacial border on the boundary between the Valley and Ridge province and the Appalachian Plateau. These sites preserve the most complete record of fluvial incision on the Susquehanna River with flights of seven Quaternary terraces ranging in elevation from 3 m to 51 m above the modern river.Soil characteristics used to develop the soil chronosequences include complexity of horizonization, thickness of B horizon, clay content of B horizon, soil color, CBD extractable Fe, Al, and Mn, total extractable Fe, and clay mineralogy. Terrace age constraints are based on soil development, correlation to regional glacial stratigraphy, correlation to dated fluvial and glaciofluvial deposits, and by paleomagnetic analysis of sediments. Terrace ages at the Muncy site range from modern (< 150 ybp) to Middle Middle through Early Middle Pleistocene (∼ 300 ka to ∼ 770 ka). Marietta has terrace ages ranging from modern (< 150 ybp) to Early Pleistocene through Late Pliocene (∼ 770 ka to ∼ 2400 ka).     

Luminescence chronology of incision and channel pattern changes in the River Ganga, India

The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T₂). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T₁) and the present-day flood plain (terrace-T₀). Terrace-T₁ shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T₀ and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T₂ show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.     

Numerical modelling of Quaternary terrace staircase formation in the Ebro foreland basin,southern Pyrenees,NE Iberia

The southern foreland basin of the Pyrenees (Ebro basin) is an exorheic drainage basin since Late Miocene times. Remnants of an early exorheic Ebro drainage system are not preserved, but morphology provides evidence for the Pliocene–Quaternary drainage development. The incision history of the Ebro system is denoted by (i) extensive, low gradient pedimentation surfaces which are associated with the denudation of the southern Pyrenean piedmont around the Pliocene–Quaternary transition and (ii) deeply entrenched Quaternary river valleys. Presumably since the Middle Pleistocene fluvial incision intensified involving the formation of extensive terrace staircase in the Ebro basin. Terrace exposure dating in major Ebro tributary rivers indicates climate‐triggered terrace formation in response to glacial–interglacial climate and glacier fluctuations in the Pyrenean headwaters. The overall (semi)parallel longitudinal terrace profiles argue for progressive base level lowering for the whole Ebro drainage network. The landscape evolution model, TISC, is used to evaluate climatic, tectonic and base level scenarios for terrace staircase formation in the Ebro drainage system. Model simulations are compared with morpho‐climatic, tectonic and chronologic data. Results show that climatic fluctuations cause terrace formation, but the incision magnitudes and convergent terrace profiles predicted by this climate model scenario are not consistent with the (semi)parallel terraces in the Ebro basin. A model including previous (late Pliocene) uplift of the lower Ebro basin results in rapid base‐level lowering and erosion along the drainage network, small late stage incision magnitudes and terrace convergence, which are not in agreement with observations. Instead, continuous Quaternary uplift of both the Pyrenees and the Ebro foreland basin triggers (semi)parallel terrace staircase formation in southern Pyrenean tributary rivers in consistency with the observed longitudinal terrace profiles and Middle–Late Pleistocene incision magnitudes. Forward model simulations indicate that the present Ebro drainage system is actively incising, providing further evidence for uplift.     

Formation of river terraces in conditions of high seismicity

We have investigated the river terraces on the left bank of the Irkut river in the Torskaya depression a short distance from the village of Guzhiry. On the basis of lithological-geological findings and radiocarbon datings of buried soils from two sections of the second terrace (12–14 m), we identified nine formation stages of the terrace in the latter half of the Late Neopleistocene and in the Holocene. They reflect multiple changes of the leading exogenous process, implying a variety of the genetic types of deposits (soils, and aeolian and alluvial sediments) during the Early and Mid-Holocene. The formation stages of alluvium are correlated with periods of high water. It is found that the final transition of alluvial to cover deposits is associated with incision of the river to 2–4 m and is dated to 5.2–4.5 cal. ka. We examine the alternation of the natural factors for the formation of deposits of the second terrace of the Irkut river in the Late Neopleistocene and Holocene. One (hydroclimatic) factor implies accumulation of deposits of the alluvial and cover complex depending on climate and water runoff fluctuations, landscape changes, and on variation in the base level of erosion. The other (seismic) factor is correlated with data on high activity of the Tory paleoseismogenic structure, which seems to have caused the lowering as well as the rise of the bottom of the depression at the time of strong earthquakes and, as a consequence, erosion or accumulation of deposits of the channel facies of alluvium. It is established that the chief causes for the change of the terrace’s deposit types were the natural-climatic changes, the character and directedness of tectonic movements, the variations in the base levels of erosion, and the height of floo ds.     

贵州高原北部河流阶地发育与喀斯特地貌演化

贵州高原北部发育平缓丘丛和深切峰丛2种喀斯特地貌组合,保存于喀斯特山间盆地的河流阶地对区域地貌演化具有指示意义.本文根据阶地发育特征和光释光(OSL)测年,分析阶地形成的时代和动力,结合区域地质背景,探讨构造抬升和河流侵蚀对黔北喀斯特地貌演化的驱动作用.结果显示,绥阳盆地T1阶地时代18.8?8.2 ka,T2时代14...     

Influence of periglacial cover beds on in situ-produced cosmogenic Be in soil sections

Cover beds, widespread on hillslopes of temperate climate zones, represent layers of allochthonous material laterally transported by periglacial processes during the Late Pleistocene. Two soil sections comprised of cover beds from the Bavarian Forest, SE Germany, have been analysed for in situ-produced cosmogenic ¹⁰Be. Major changes in the nuclide concentration agree well with soil section boundaries defined by field observations and grain size analyses. Numeric modeling of these cosmogenic nuclide sections demonstrates that simple continuous erosion and regolith mixing models fail to explain the measured nuclide concentrations. Instead, the measured data can be best described by modeling an admixture of material such as loess or reworked allochthonous material, which have different nuclide concentrations.A comparison of cosmogenic nuclide concentrations from the two cover bed sections with concentrations from river bedload sediments of the Regen catchment reveals that cover bed formation might affect the result of basin-wide erosion rate determinations based on cosmogenic nuclides. Nuclide concentration of river bedload is potentially a binary mixture produced by (1) spatial erosion of the soil surfaces; and (2) spatially nonuniform incision into deep cover bed layers that contributes sediment low in nuclide concentration.     

黄土高原0.8Ma以来地面抬升的时空特征研究

根据黄土高原地区黄河阶地的形态特征和成因分析,认为其形成主要是地面抬升所致并且在黄河达到均衡状态下形成,可以推断黄土高原的地面抬升。根据对黄土高原地区黄河0.8 Ma阶地的研究并结合相关文献资料,选取兰州段、黑山峡段、晋陕峡谷段和三门峡段作为典型研究区域,得出黄土高原0.8 Ma以来的地面抬升存在显著的时空特征,即空间特征表现为地面抬升量有西大东小的规律,时间特征表现为地面抬升速率有后期加速趋势、特别是晚更新世以来。并认为黄土高原0.8 Ma以来的地面抬升与青藏高原的构造抬升有成因上的联系。