Accumulation Terraces Research Articles

Morphostructural evolution of the Labe and Jizera rivers confluence area (the Bohemian massif, Czechia)

This article investigates the morphostructural evolution of the Labe and Jizera rivers confluence area in the Bohemian Massif of Czechia. The main stages of morphostructural evolution are identified and related landform patterns are described. The lithological and tectonic character of this contact area between the Barrandian and the Bohemian Cretaceous Basin has been gradually evolving since the Early Palaeozoic. Tectonic subsidence of the northeastern part of the Bohemian Massif and the Cretaceous transgression of the sea caused extensive marine sedimentation, which covered the pre-Cenomanian relief. The uplift of the Bohemian Massif during the Santonian initiated a widespread erosion and denudation in the Tertiary. The Labe River valley constitutes a remarkable boundary between the morphostructural plateaus in the south and the system of river accumulation terraces of the Labe and Jizera in the north. The originally extensive and currently considerably eroded III. river terrace of the Labe was formed in the Elsterian glacial period (Cromerian complex c). The conspicuous Jizera alluvial fan developed during the aggradation phase of the VII. river terrace in the Upper Pleistocene. Subsidence along the NW-SE-trending Labe Fault zone, deciphered from the vertical throw of the VII. terrace rock bases of Labe as well as related paleochannels of its local tributaries, which caused the current asymmetry of the Labe valley, reached up to 14 meters prior to the Holocene. The different rock resistance to weathering, arrangement of fault structures and neotectonic movements that took place even during the late Quaternary significantly influenced the intensity of varied climate-morphogenetic processes.

Anthropogenic impact on the shores and the bottom of the Jebriyan bay in the Northwestern part of the Black Sea


 
 
 
 The Jebriyan Bay is located in the northern part of the Kiliya Danube Delta, at a junction of the delta cone and the indigenous coast. This is a zone of very high anthropogenic impact on the Danube Biosphere Reserve. The two opposite shores of this bay are fundamentally different. Along the northern shore, the Northwest coastal sand sediment flows discharge from the Cape of the Great Fontanne to the Jebriyan Bay. That is why the northern coast of the bay is made up of sandy forms of coastal topography (marine accumulative terrace and spit). The southern coast is deltaic; composed of a mixture of muddy, siltstone and sandy sediments. The area of the bay is limited to isobaths –11 m and is about 80 km2. The bottom of the bay has a gentle relief, made up of smooth outlines, with an average depth of 6.2 m. The shape of the transverse profile of the underwater slope is mostly convex. The natural system of the bay was affected by fishing, recreation, shipping and industrial sand production on coastal accumulative landforms. Coastal fishing uses a system of fixed bottom seines and small motorized floating equipment. Recreational facilities are designed to serve about 350 thousand people during the warm period each year. The impact of shipping was expressed in the construction and operation of the seaport of Ust-Dunaysk, together with suitable canal and the technical canal between the sea and the branch of the delta breakthrough the system of the large Ochakov branch. The ladle port had an area of about 1.5 km2, a maximal depth of 16 m, and an average depth of 13.7 m. The trough was connected to the Ochakov branch of the Danube Delta by a technical canal with a depth of 4 m. Vessels could enter the harbor of Ust-Danube through an access navigation channel with a depth of 11–12 m and a bottom width of 125 m. The port was used for the transshipment of large containers, general forest cargo from ocean vessels (displacement of 60–100 thousand tons) on regular sea lines from the countries of Southeast Asia to the Black Sea, to the Danube and further to the countries of Central Europe and to the ports of the North and the Baltic Seas. But it was unfortunate that the port construction site did not last as expected. Between 1980–2010 the harbor and approach canal of Ust-Dunaysk were filled with Danube river sediments. The example of Jebriyan Bay has shown that when executing any type of sustainable nature management project, it is very important to take into account the natural milieu.
 
 
 

Study and modelling of rapid shoreline displacement due to coseismic subsidence along the Kamchatka subduction zone

The first studies of geological evidences of coastal coseismic subsidence (associated with subduction-zone earthquakes) were carried out in Russia at the Institute of Volcanology and Seismology, in Kamchatka. We developed a special method based on tephrostratigraphy and tephrochronology, descriptions and dating of the soil-pyroclastic sequence (SPS) overlying the coastal wave-build beach ridges. Three seismic events accompanied by coastal coseismic subsidence were detected in the northern part of Avachinsky Bay during the past ~5 thousand years. We found subsidence from one of the greatest historical tsunamigenic earthquake (1952 AD) south of Petropavlovsk-Kamchatsky. We identified, that 5 events of coastal coseismic subsidence had occurred during the past ~6 thousand years at the coast of Kronotsky Bay and Shipunsky Peninsula. Amplitudes of subsidence were estimated by geological data using three different methods. Erosion of the active beach and marine accumulative terrace becomes active after coastal subsidence. We calculated the shoreline retreat process and the amount of horizontal erosion by numerical simulation using Bruun rule. In some areas, shoreline retreat was about 300 m according to the model results.

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.

Research Method of Silting the Mountain Reservoirs Under the Current Climate Change

The mountain reservoirs are an accumulator of the processing products of river banks and sediments. These elements form the silting prism and its bench, which are located in the reservoir and the riverbeds of the tributaries. As a result, there is increased riverbeds and there are a number of catastrophic flooding risk and deficit of the beach-forming sediment on the sea coast. The aim of our research is the estimation of accumulation of solid material in the reservoirs and tributaries, as well as a change of channel processes. In order to study the sedimentation prism-formation and equilibrium bad processes, the method of field experiment is proposed, which was used in the mountain reservoirs of Georgia. For analysis of obtained results were used the methods of mathematical statistics - least square method and differential calculus. The experiment showed that the final result of the reservoirs' silting is an accumulative terrace, which produced an equilibrium alluvial bed. Fractional distribution of sediments in silting prism is determined by reservoir control type and intra-annual distribution of fluvial sediments. The network of sediment extraction quarries has to be located along the reservoir taking into account this principle.

Differential tectonic movements in the confluence area of the Huang Shui and Huang He rivers (Yellow River), NE Tibetan Plateau, as inferred from fluvial terrace positions

In the Northeastern Tibetan Plateau (NETP), the courses of the Huang Shui and Huang He near their confluence are characterized by alternating gorges and wide depressions, segmenting the fluvial systems. The river valleys have developed terrace staircases, which are used to infer relative tectonic motions between the segments. The terrace staircases are correlated by means of relative height and optically stimulated luminescence (OSL) dating. At least eight terraces are present, two of which have been dated by OSL (the sixth and the third ones; c. 70 and c. 24 ka, respectively). The correlated longitudinal terrace profiles show no distinct relative tectonic movements within the confluence area, demonstrating that this area behaved as one tectonic block. The correlation of the terrace staircase of this block with areas upstream (Xining area) and downstream (eastern Lanzhou area) indicates relative tectonic movements, which therefore represent different tectonic blocks. The fluvial incision rate since c. 70 ka was much higher in the confluence area than in the blocks upstream and downstream, possibly indicating relative uplift. This relatively strong uplift provided more space for differentiation within the terrace staircase as a result of climatic changes, leading to six terraces formed as a response to minor climatic fluctuations (103–104 year timescale) since the last interglacial. This may indicate that the stronger the tectonic movement the better the climatic imprint as expressed in the form of terrace development. Over a shorter timescale, two accumulation terraces with thick stacked deposits (>18 m) may indicate relative subsidence in the confluence, occurring sometime between 20 and 70 ka. This indicates changes in relative vertical crustal motions at timescales of tens of thousands of years. We speculate that the inferred tectonic motions are related to transpression movements in the NETP as a result of the collision of the Indian and Asian plates.

Middle Turonian trace fossils from the Bystrzyca and Długopole sandstones

The Middle Turonian sediments of the Nysa Klodzka Graben (Bystrzyca Sandstone in the Stara Bystrzyca outcrop and the Dlugopole Sandstone in Dlugopole Gorne Quarry) contain trace fossils, which include Curvolithus simplex, ?Macaronichnus isp., Ophiomorpha nodosa, Ophiomorpha isp., Palaeophycus tubularis, Thalassinoides cf. paradoxicus, T. suevicus and Thalassinoides isp. The assemblage of trace fossils points to the proximal Cruziana ichnofacies, that characterizes the distal lower shoreface and the archetypal Cruziana ichnofacies, typical of upper offshore settings. The trace fossils evidence implies that sedimentation took place in a shallow basin with periods of a sudden sediment input, good oxygenation and normal salinity. The Bystrzyca and Dlugopole sandstones are deposits of the shallow epicontinental sea that were deposited between the fair-weather and storm-wave base, in the distal lower shoreface–upper offshore setting. The Bystrzyca Sandstone is recognized as storm-originated deposits, whereas the Dlugopole Sandstone is probably the part of prograding “accumulation terrace”. The source of material for the sandstone was the East Sudetic Island and probably also the Orlica–Bystrzyca Uplift. The studied sandstones are related to a regression that started in the early/middle Middle Turonian and caused a relative uplift of the surrounding land. The Middle Turonian sediments of the Nysa Klodzka Graben (Bystrzyca Sandstone in the Stara Bystrzyca outcrop and the Dlugopole Sandstone in Dlugopole Gorne quarry) contain trace fossils, which include Curvolithus simplex , ? Macaronichnus isp., Ophiomorpha nodosa , Ophiomorpha isp., Palaeophycus tubularis , Thalassinoides cf. paradoxicus , T . suevicus and Thalassinoides isp. The assemblage of trace fossils points to the proximal Cruziana ichnofacies, that characterizes distal lower shoreface and the archetypal Cruziana ichnofacies, typical of upper offshore settings. The trace fossils evidence implies that sedimentation took place in a shallow basin with periods of a sudden sediment input, good oxygenation and normal salinity. The Bystrzyca and Dlugopole sandstones are deposits of the shallow epicontinental sea that were deposited between fair-weather and storm-wave base, in the distal lower shoreface - upper offshore setting. The Bystrzyca Sandstone is recognized as storm-originated deposits, whereas the Dlugopole Sandstone is probably the part of prograding “accumulation terrace”. The source of material for the sandstone was the East Sudetic Island and probably also the Orlica-Bystrzyca Uplift. The studied sandstones are related to a regression that started in the early/middle Middle Turonian and caused a relative uplift of the surrounding land.

Morphostratigraphy of accumulation terraces of the Sázava and Vltava Rivers in the Bohemian Massif and its correlation with the North European stratigraphical classification of the Quaternary

Morphostratigraphy of accumulation terraces of the Sázava and Vltava Rivers in the Bohemian Massif and its correlation with the North European stratigraphical classification of the Quaternary

Differential impact of small-scaled tectonic movements on fluvial morphology and sedimentology (the Huang Shui catchment, NE Tibet Plateau)

The aim of this paper is to investigate the morphological implications of the interplay between tectonic movements at different rates, timescales, and spatial extent in a catchment of intermediate size, the Huang Shui River (a main tributary of the Yellow River in the NE Tibet Plateau). River incision started from a peneplain-like surface that developed in post-Miocene–Pliocene times. At the end of the Tertiary, a general tectonic uplift of the Tibetan Plateau initiated river incision. This general uplift also caused local fragmentation of the Huang Shui catchment into blocks (kilometers or maximally, a few tens of kilometers) of local extent that have subsided and/or uplifted relative to one another. Fluvial deposition of > 30 m in the subsiding blocks contrasts with erosion and formation of gorges in the uplifted blocks. Incision into the former peneplain was not continuous but a staircase of terraces developed under climatic influences. Terrace deposits are sometimes capped by interglacial soils or soil-derived material. Apparently, terrace incision occurred at the transition to the next cold period. The different rates of uplift and subsidence of the individual blocks resulted in the simultaneous development of erosion and accumulation terraces of different sizes within the same catchment, even within the same tectonic block. This makes it impossible to connect the terraces of the different blocks, except for the three youngest terraces (representing the last 300,000 years), thus illustrating the uniform tectonic history of the catchment since that time.

Influence of the post-Miocene tectonic activity on the geomorphology between Andes and Pampa Deprimida in the area of Provincia de La Pampa, Argentina

The genesis of the relief between the Andes and the Pampa Deprimida plain between 36° and 39°S has never been considered. The region is intermediate between two contrasting geomorphic styles, the meridian-oriented highs and depressions of the Sierras Pampeanas to the north and the eastwards sloping northern Patagonian mesetas to the south. From geophysical data, it coincides with an intermediate zone between a flat-slab subduction zone to the north and a normal subduction zone to the south. From west to east (68° to 64°W), four units follow each other: the easternmost portion of the Sub-Andean piedmont, the depression of the Río Chadileuvú, a Plateau, and a high scarp separating it from the Pampa Deprimida lowland. The Plateau is the southernmost portion of the Brazilian shield. Geomorphological and sedimentological analyses led us to the following conclusions: 1. the Andes uplift created a large piedmont reaching the Pampa Deprimida and including the Plateau which between the Pliocene and the Middle Pleistocene was shaped in a series of stepped levels covered by Andean fluvial sediments; 2. the meridian-oriented Rio Chadileuvú depression is of tectonic origin, younger than the Middle Pleistocene, and breaks the continuity between the piedmont and the Plateau: this depression could be an incipient foreland basin; 3. the eastern scarp is a fault scarp, probably Upper Pleistocene in age, due to a faster activity of the fault zone between the craton and the Macachín Trough. This young morphotectonic activity coincides with the change from a west–east Patagonian pattern to a north–south orientation of the relief typical of the Sierras Pampeanas, but younger than them. The river network was affected by this evolution. During the Upper Miocene, a palaeo-Río Negro flowed to the north–east, then shifted southwards. The Río Colorado entered the Pampa region during the Upper Pliocene creating a set of stepped fluvial accumulation terraces, while the piedmont was drained by eastward streams. Following the formation of the Chadileuvú depression, a north–south drainage, largely endoreic, replaced the Sub-Andean flows. A capture of the Río Chadileuvú by the Río Colorado is going on due to the incision of the Río Colorado in the south. The regional landscape is therefore directly related to Late Pleistocene tectonic activity, whereas climate changes explain the different sedimentary characteristics of the surficial deposits.

Distribution and Forming Model of Fluvial Terrace in the Huangshui Catchment and its Tectonic Indication

Abstract:The Huang Shui River, a main tributary of the Yellow River, crosses a series of tectonically subsided and uplifted areas that show different patterns of terrace formation. The distribution of fluvial terrace of the Huang Shui River is studied through topographic and sedimentologic terrace mapping. Three terraces in the Haiyan Basin, four terraces in the Huangyuan Basin, 19 terraces in the Xi'ning Basin (the four high terraces may belong to another river), nine terraces in the Ping'an Basin, five terraces in the Ledu Basin and 12 terraces in the Minhe Basin are recognized. Sedimentology research shows that the geomorphologic and sedimentological pattern of the Huang Shui River, which is located at the margin of Tibet, are different from that of the rivers at other regions. The formation process of the terrace is more complicated at the Huang Shui catchment: both accumulation terrace and erosion terrace were formed in each basin and accumulation terraces were developed in some basins when erosion terraces were formed in other basins, indicating fluvial aggradation may occur in some basins simultaneously with river incision in other basins. A conceptual model of the formation process of these two kinds of fluvial terraces at Huang Shui catchment is brought forward in this paper. First, the equilibrium state of the river is broken because of climatic change and/or tectonic movement, and the river incises in all basins in the whole catchment until reaching a new equilibrium state. Then, the downstream basin subsides quickly and the equilibrium state is broken again, and the river incises at upstream basins while the river accumulates at the subsidence basin quickly until approaching a new equilibrium state again. Finally, the river incises in the whole catchment because of climatic change and/or tectonic movement and the accumulation terrace is formed at the subsidence basin while the erosion terrace is formed at other basins. The existence of the accumulation terrace implied the tectonic subsidence in the sub‐basins in Huang Shui catchment. These tectonic subsidence movements gradually developed from the downstream Minhe Basin to the upstream Huangyuan Basin. Dating the terrace sequence has potential to uncover the relationship between the subsidence in the catchment and the regional tectonic at the northeastern Tibetan Plateau.

Late Quaternary landscape evolution in a small catchment on the Chinese Loess Plateau

Late Quaternary landscape evolution was analysed with reference to a small (0.08 km 2) drainage basin system in southeastern Gansu Province, southwestern Chinese Loess Plateau. In this region, V-shaped valleys with broad valley floors are prominent landscape features whose basic shape already existed prior to MIS 2. During MIS 2 the valley bottoms were infilled with fluvially reworked loess, today forming accumulation terraces. Infrared optically stimulated luminescence (IRSL) dating of these loess-like deposits assigned them to the time span ranging from 22.5 to 17.7 ka BP. During the late Holocene, valley fills were partly eroded, and the receiving stream channel was deepened by at least 25 m close to the confluence of the study site's channels. These alternating erosion and accumulation phases were triggered by climatic factors or human impact. In the most recent past, the high landscape sensitivity of the Chinese Loess Plateau has led to increasing soil conservation measures that have effectively confined surface erosion. However, the lack of drainage on the artificial terraces induces piping processes.

Evolution of Quaternary river terraces related to the uplift of the central part of the Bohemian Massif

Fluvial sediments in the Vltava, Berounka, Sázava and Labe valleys are preserved as extensive river terrace sequences. These accumulation terraces originated from an interaction of climate-morphogenetic and neotectonic processes in the late Cenozoic. The palaeogeographical history of the central part of the Bohemian Massif is described. Geomorphological analysis of late Cenozoic fluvial sediments preserved in the Bohemian Massif confirm that in total 7 main terrace accumulations with several secondary levels can be differentiated. A chronostratigraphical scheme of erosion and accumulation periods and their relations to variable uplift rates in the late Cenozoic is suggested. The relative height of the oldest fluvial terraces above the present-day bottoms of river valleys is more than 100 m which indicates the approximate depth of erosion in the Quaternary.

Ground ice and slope sediments archiving late Quaternary paleoenvironment and paleoclimate signals at the margins of El'gygytgyn Impact Crater, NE Siberia

An accumulation terrace close to the El'gygytgyn Impact Crater in northeastern Siberia contains stratigraphic and periglacial evidence of the paleoenvironmental and paleoclimatic history and permafrost dynamics during late Quaternary time. A succession of paleo active-layer deposits that mirror environmental changes records periods favorable for the establishment and growth of ice-wedge polygonal networks and sediment variations. These two elements of the periglacial landscape serve as complementary paleoenvironmental archives that can be traced back to ∼ 14,000 cal yr BP. The slope sediments and the ground ice contained therein have prominent relative maxima and minima in properties (grain size, total organic content, oxygen isotopes). They document a regional early Holocene thermal maximum at about 9000 cal yr BP, followed by a transition to slightly cooler conditions, and a subsequent transition to slightly warmer conditions after about 4000 cal yr BP. Results from sedimentary analysis resemble morphological and geochemical (oxygen and hydrogen isotopes) results from ice wedge studies, in which successive generations of ice-wedge polygonal networks record warmer winters in late Holocene time. Moreover, peaks of light soluble cation contents and quartz-grain surface textures reveal distinct traces of cryogenic weathering. We propose a conclusive sedimentation model illustrating terrace formation in a permafrost terrain.

中部山岳地域における最終氷期極相期以降の山岳永久凍土環境の変動とその地形変化への影響

The relationships between the formation of accumulation terraces and climatic changes, especially changes in the mountain permafrost environment, were investigated in the upper reaches of the Gamata River on the western side of the Yari-Hotaka Mountains, northern Japanese Alps. Optically stimulated luminescence (OSL) dating of these river terrace deposits is now in progress. The present distributions of mountain permafrost and permafrost-related landforms (e.g. rock glaciers) were investigated around Mt. Shirouma, located in the northern part of the northern Japanese Alps, and Mt. Suisyo, located in the central part of the northern Japanese Alps. The low bottom temperature of the winter snow cover (BTS <- 3 °C) and the negative mean annual ground surface temperature (MAST) on the blocky landform (protalus lobe) in the Suisyo C cirque indicate the presence of permafrost.

Geomorphologic evidence of phased uplift of the northeastern Qing-hai-Tibet Plateau since 14 million years ago

A typical sequence of fluvial terraces and aeolian deposits overlying these ter- races were multidisciplinary investigated. New evidences for uplift process of the northeastern Qinghai-Tibetan Plateau in the past 14 million years were obtained. At least 11 river terraces along Huangshui, the first-class tributary of Yellow River, at the Xining-Huzhu region are identi- fied. While the first one (T1) is classified as an accumulation terrace, the others are all basement river terraces, which consist of the Tertiary sandstone and siltstone bedrock, fluvial gravel and pebbles and the overlying aeolian loess-Red Clay deposit. Samples from the aeolian deposits were examined for paleomagnetic stratigraphic reconstruction (1030 samples), luminescence dating (16 samples), magnetic susceptibility and grain-size distribution (more than 4000 sam- ples). The luminescence dating and stratigraphic correlation suggest that terraces of Tll, T10, T8, T7, T3, T2, T1 were formed at 14, 11.3, 1.55, 1.2, 0.15, 0.07 and 0.01 million years ago, re- spectively. Sedimentological analysis and geomorphological observation indicate that formation and evolution of these terraces were mainly driven by tectonic uplift. Therefore, the terrace se- quence provides an ideal geological record, of the uplift process of the northeastern during the past 14 million years, and the timings of the terraces formation are regarded as the timings of tectonic uplift. The significant uplifting events took place at 14, 11.3, 1.2 and 0.15 mil- lion years ago, respectively. The fluvial incision at the Xining-Huzhu region is less than 100 m during a period of ~12 million years in the Miocene era (between the Tll and T9), while the Huangshui River had incised 432 m during the past 1.2 million years (from T7 to the present floodplain). The river incision process clearly demonstrates that accelerated rising of the north- eastern Qinghai-Tibet Plateau during the late Cenozoic, and provides new evidence of previous thoughts. There was a significant readjustment of the fluvial catchment during 1.55--1.2 million years ago: before this time, the paleoriver flowed to southwest. After this time the Huangshui River flows to southeast. A tectonic movement dominates reorganization of this fluvial system.

Depositional systems and sequence stratigraphy of coarse‐grained deltas in a shallow‐marine, strike‐slip setting: the Bohemian Cretaceous Basin, Czech Republic

Deposits of coarse‐grained, Gilbert‐type deltas showing varying degrees of reworking of foresets by basinal currents were identified in Middle Turonian to Early Coniacian sandstones of the Bohemian Cretaceous Basin. The progradation of the deltaic packages, earlier interpreted as large‐scale subaqueous dunes, shelf ridges or subaqueous fault‐scarp ‘accumulation terraces’, was controlled by high‐ and low‐frequency, relative sea‐level changes in a relatively slowly subsiding, intracontinental strike‐slip basin. End‐member types of the Bohemian Cretaceous coarse‐grained deltas are deep‐water deltas, characterized by thick (50–80 m) foreset packages with steep (10–30°) foresets, and shallow‐water deltas, which deposited thin (&lt;15 m) packages with foresets typically between 4° and 10°. The differences in thickness and foreset slope angle were controlled predominantly by the accommodation available during progradation. The depositional regime of the deltas was governed by (i) the fluvial input of abundant sand bedload, with a minor proportion of gravel; (ii) gravity flows, most probably caused by liquefaction of the upper part of the unstable foreset slope; and (iii) migration of sandy bedforms on the foreset slopes. The bedform migration was driven by unidirectional currents of possible tidal origin. Individual foreset packages represent systems tracts, or parts of systems tracts, of depositional sequences. A variety of stacking patterns of high‐frequency sequences exists in the basin, caused by low‐frequency relative sea‐level changes as well as by local changes in sediment input. Because of generally low subsidence rates, fluvial or beach topset strata were not preserved in the cases studied. The absence of preserved fluvial facies, which has been one of the main arguments against the fluvio‐deltaic origin of the sandstone bodies, is explained by erosion of the topsets during transgression and their reworking into coarse‐grained lags of regional extent covering ravinement surfaces.

Alluvial-fan slope as a function of sediment transport efficiency

ABSTRACT The nature of slope acquisition by alluvial fans was studied by comparing the slope evolution of 37 tributary fans to the San Juan river in an arid region of western Argentina. Each of these fans is characterized by truncation of the distal end as a result of incision by the San Juan river. The alluvial-fan slope prior to incision and the slope of the active channel for each fan was measured. River accumulation terraces indicate that fan incision was probably synchronous along the valley. Incised channels in each fan evolved over the same interval, but they show different degrees of advance of erosion. Values of old fan slope fit well with known power laws relating them to drainage area (r = 0.865). Slopes of incised channels also fit with power laws, but with a lower regression coefficient (r = 0.741). The regression lines in each case have almost the same slope (0.218, sd = 0.02, and 0.215, sd = 0.03, respectively), which suggests that, although the incised-channel slope is not an equilibrium feature, both the original and the secondary fans follow the same law of variation. An explanation for this phenomenon is that the transport efficiency of the fans may be related to the size of the basin. If mean runoff and peak floods are roughly regulated by drainage area, fan slopes of larger drainage basins will evolve faster than those of smaller basins. Apparently, as a group of fans approach an equilibrated state, their slopes become distributed along successive parallel regression lines, progressively displaced upward in the diagram, as long as climatic conditions remain constant. To see the effects of water availability on fan slope, we also compared slopes and drainage-basin areas of fans located in settings with different specific runoff, which is approximately a function of annual rainfall. The regression lines for each set of alluvial fans are roughly parallel. The smaller intercept value for humid fans indicates that for a given drainage area, fans in arid settings are steeper than those in more humid settings. Fans dominated by mud flows and dilute flash floods were plotted together, but no major differences were observed in this general behavior. This suggests that the steepness of a fan is not primarily related to compositional factors or to the sediment transport process, but to transport efficiency. Episodic-flood fans of Argentina show slopes comparable to the mudflow fans of Death Valley because water supply and therefore transport efficiency are similar. Drainage-basin area combined with mean annual rainfall may largely determine transport efficiency, by controlling mean runoff across the fan. As a result, transport efficiency may define not only alluvial-fan slope but also the rate at which the alluvial-fan slope reestablishes equilibrium after some external change.

Detection of Accumulation-erosional Terrace Association formed in the Last 20, 000 Years

A radiocarbon dating analysis clarified that the latest accumulation terraces, which were found at eight river-basins in the Ou and the Dewa mountains (fault and fold mountains in the northern Japan), were simultaneously completed around 20, 000yr B. P. At the age of the terrace completion, the polar frontal zone began to recede to the present position, and subsequently caused a substantial increase in heavy-rainfall frequency due to a typhoon attack even in the northern Japan.Detailed field survey and tephrochronological analysis for the Kariya river-basin in the upwarped Kitakami mountains, located in almost the same latitude as the Ou and the Dewa mountains, revealed that the latest accumulation terrace was also completed around 20, 000yr B. P. Simultaneity of the terrace formation regardless of their great geomorphological difference among those mountains described above, proved that a great increase in the heavy-rainfall frequency was primarily responsible for the initiater of the accumulation terrace formation. Accordingly, it was safely concluded that the formation of the latest accumulation terraces occurred around 20, 000yr B. P. by the climatic change, not crustal movements in the northern Japan.A sizable volume of previous papers concerning the accumulation and the erosional terraces formed in the last 20, 000 years throughout Japan, was intensively reviewed, and the terrace distribution was mapped out. Consequently, it was confirmed that the formation of the accumulation terraces surely extended to the northern to central Japan, and that it commonly accompanied the erosional terrace formed at latest by 10, 000yr B. P. This accumulation-erosional terrace association characterized the river-landform development during the last 20, 000 years.In the southern Japan, only the erosional terrace without the accumulation terrace was identified at the Miya, the Ishite and the Mimi river-basins. Since fossil periglacial landforms in the last Glacial time, widely distributed in Japan, are known to be closely associated with the accumulation terrace, the accumulation-erosional terrace association was plausible to be formed in the southern Japan, and the erosional terrace, therefore, was considered to be the counterpart of the terrace association.In addition to the terrace association in the last 20, 000 years, if a similar key landform is detected in the landforms before 20, 000yr B. P., there is a great possibility to develop a new topographical analysis method.

The Last Glacial Age Accumulation Terracing around the Iide and the Asani Mountains, Northeastern Japan

River terraces, including accumulation terrace, were investigated in the Iide and Asahi Mountains. This paper especially describes the terraces of the Last Glacial age on the basis of stratigraphical, geomorphological and pedological evidences as well as 14C age determination.The results are summarized as follows.1) In the studied river basins, such as the Miomote, the Arakawa, the Tamagawa, the Tainai and the Kajikawa Rivers, two accumulation stages in the Last Glacial age were detected. The accumulation and the following incision are considered to have been caused by the climatic changes.2) The earlier and the later accumulation stages terminated at least before 35, 000yr B. P. and before 12, 000yr B. P., respectively.3) The earlier accumulation terrace are widely distributed along the middle courses of rivers, although not in the upper valleys. The latter one are formed only in the upper valleys in the Mountains and at the outlets of gorges through high mountain area. These facts indicate that the earlier accumulation is more conspicuous than the later one.