Ancient River Beds Research Articles

Relationships between fluvial dune cross-set thickness, planview width, and trough geometry

Abstract Sedimentary structures provide critical information for the reconstruction of ancient environments of Earth and other planets. Fluvial dune cross sets, structures that record dune migration via the filling of leading troughs along ancient riverbeds, are particularly useful. Most quantitative methods for interpreting cross sets require thickness measurements, but the growth of planview imaging of sedimentary rocks on Earth and Mars has introduced a need for methods to interpret planview exposures of cross sets. Here, we measured 359 cross-set widths exposed along ancient channel belts of the Cretaceous Cedar Mountain Formation exposed in Utah, USA, and compared these widths to published thicknesses (n = 350). We found that the mean cross-set width was 1.29 m, 9.3 times the mean thickness. The normalized distributions of widths and thicknesses were statistically similar and thus contained similar paleoenvironmental information. Numerical experiments representing a series of trough cut-and-fill structures show that cross-set reworking is equally important in setting width as thickness and that the observed cross-set widths could be explained by a gamma-distributed range of trough widths with a mean of 2.96 ± 0.25 m, ~2.3 times cross-set width. Given the similarity of dune-field geometries across depositional settings and planetary boundary conditions, we suggest this cross-set width-to-thickness relationship could be useful for interpreting other cross sets exposed in planview.

Conservation opportunities for threatened paleochannel grasslands in the South American Dry Chaco

Subtropical drylands, such as the South American Dry Chaco, harbor azonal grasslands in ancient river beds (hereafter, paleochannels). These grasslands host high biodiversity and provide numerous ecosystem services. However, cultivated systems often replace paleochannels grasslands due to their biophysical characteristics and low investment requirements. Furthermore, there is a lack of basic characterization of paleochannel grasslands due to their complexity, which challenges their conservation. Here, we mapped and described for the first time the paleochannel grasslands of the Argentinean Dry Chaco based on their current land cover, spatial configuration, EVI values (a proxy for primary productivity), fragmentation, and protection. We speculate that natural (i.e., shrub encroachment enhanced by overgrazing and fire management prohibition) and anthropogenic processes (i.e., farming and infrastructure) have transformed the Argentinean Dry Chaco grasslands into woody-dominated or agricultural systems. Therefore, while grasslands in low-transformation matrices were mainly replaced by shrubs, those in high-transformation matrices were integrated into agricultural landscapes (i.e., whole units fragmented by croplands and roads). However, paleochannels in medium-transformation matrices showed the highest grassland cover, EVI values, and intermediate fragmentation level. Located in the NW of the Argentinean Dry Chaco, this group of paleochannels should be prioritized in future conservation and management strategies, as currently is not included in any protection scheme. Our work highlights the need to include non-pristine systems that have received perturbation in the past in conservation actions to mitigate and reduce the degradation of threatened dryland habitats.

Landscape development history of the Zabolotsky peat bog in the context of initial settlement of the Dubna River lowland (Upper Volga basin)

Zabolotsky peat bog is a unique biospheric and cultural-historical archive located in the north of the Moscow Region on the territory of the Dubna River lowland. Despite the advances in studying the Zabolotsky region, the question of reconstruction of the primitive population habitat remains unresolved. Until recently, it has been be-lieved that in the Late Valdai period, the Dubna River lowland was covered by the waters of an extensive glacier-dammed Tver paleolake, drained only at the turn of the Pleistocene and Holocene. It was assumed that the lake's existence prevented the settlement of the territory, whereas after its drainage, the shallow residual water pools were actively exploited in the economic activities of the primitive population. However, paleogeographic and ar-chaeological materials have been accumulated during the last two decades that questioned the existence of large dammed lakes in the Upper Volga basin in the Late Valdai time. This paper presents the results of three years (2018–2020) of research, allowing revision of the ideas about the Quaternary geology and development of the geomorphic conditions of this area. A program of research, comprising topographic and geodetic surveys, drilling using a portable boring rig, lithologic description of the core, radiocarbon (AMS) dating, paleo-soil studies, biologi-cal analysis of organic macrofossils, and ground-penetrating radar, has been carried out aimed at reconstruction of the paleogeographic setting and landscape development. Drilling data were used to build the profile across the left bank of the Dubna River floodplain with extension to the low terrace. The lithofacial analysis of samples and AMS dating allowed identifying three generations of ancient riverbeds, the deepest of which (with the bottom at 12 m below the water edge) is more than 30 thousand years old. The biological residues from the dark-coloured loams directly below the peat bottom belong almost exclusively to higher plants, both arboraceous and wetland, which may have been brought in by the floodwaters. The ground-penetrating radar profiles clearly show the boundaries of three electromagnetically homogeneous sedimental layers — the peat, silted peat, and loam. The paleo-geographic data, in conjunction with the geophysical profiling data, indicate the existence of a copious waterway in the lowland (the ancient Dubna River) no later than 15,000–16,000 years ago which formed a floodplain with large features of fluvial paleorelief available for settlement. These data agree well with the new serial AMS-dates for the resin from the grooves of the bone and horn artifacts, which permit extension of the time of the initial de-velopment of the Zabolotsky peat bog by the bearers of the Resseta Culture to 15,500 years ago. The conclu-sions drawn have major significance for the development of an evidence-based chronology of the events and dynamics of the settlement strategy of the population during the transition from the Pleistocene to the Holocene. The new data not only are consistent with the system of global paleoecologic events and history of the develop-ment of the outwash plain zone in Eastern Europe, but also provide the basis for refinement, and, possibly, revi-sion of a range of current concepts.

Application of seismic attribute analysis in Lower Miocene reservoir characterization, northeast Bach Ho field, Vietnam

The Cuu Long basin is an Early Tertiary rift basin of southern Vietnam, and the most potential basin in the country with high productive for oil and gas. Special interest in the Lower Miocene reservoir in the Cuu Long basin is caused by the gradual depletion of unique oil-bearing in the fractured basement and the possibility of transferring production wells to overlying deposits in the terrigenous rocks of the sedimentary cover and entering new deposits into development. In recent years, seismic attributes analysis has emerged as an effective tool to predict ancient riverbeds where sand bodies may exist in nonstructural traps. Understanding the distribution of these sand bodies will be of great significance in the orientation of oil and gas exploration activities. The paper applied seismic attribute analysis method combined with artificial neural network (ANN) and well data to predict the distribution of sandstones reservoirs of Lower Miocene sediments in the Northeastern Bach Ho oil field. Seismic attributes selected as input for ANN training including Relative Acoustic Impedance, Root Mean Square, Sweetness. The attributes provide the most obvious opportunity to display geological features with varying seismic amplitude characteristics as well as predict lithofacies, petrology and the distribution of sand bodies. The research results have identified the potential reservoirs in the Northeastern area of Bach Ho field, which are deposited in the fluvial, marginal lacustrine and deltaic environments.

Evaluation of mining capacity of mines using the combination weighting approach: A case study in Shenmu Mining Area in Shaanxi Province, China.

Aiming at the low mining rate in mines, Xingelao, Dabianyao, and Dongliang Coal Mines in Shenmu Mining Area, Shaanxi Province, China were taken as research objects. Based on this, this study constructed an evaluation index system for the mining capacity of the mines from the perspectives of geological factors, mechanical equipment, humans, and mining design. Moreover, the factors influencing the mining capacity of the mines were evaluated using a combination weighting approach based on an improved analytic hierarchy process and an entropy weight method. A standard cloud was generated based on the mapping standards of each index and a comprehensive cloud was obtained according to comprehensive weight and a backward cloud algorithm. Finally, by combining the comprehensive cloud with local and overall scores of the mines, the mining capacities of the mines were evaluated. The research results demonstrate that the key factor restricting the mining capacity of the mines is the geological environment and five major third-grade indexes affecting mining capacity are igneous rock intrusion, collapse column, scouring zone of the ancient river bed, mechanization level and coal pillar width. In addition, the corresponding suggestions and measures were put forward according to the main factors influencing the mining rate of the mines. In accordance with the weights and scores of each index, the overall scores of the mines were calculated. Dongliang, Dabianyao, and Xingelao Coal Mines were ranked in order based on scores. The research results provide a theoretical basis for improving the mining capacity of the mines under similar geological conditions.

ADVANTAGES OF NIEMFE METHOD IN GEOLOGICAL STATE MONITORING WHEN SOLVING APPLIED ENGINEERING PROBLEMS

Problem Statement and Purpose Finding effective methods for monitoring the state of engineering structures and geological conditions in their localization is an extreme­ly urgent issue today. The method of natural impulse electromagnetic field of the Earth (NIEMFE) is widely used for the solution of applied engineering-ecological, geological, geophysical and hydrogeological problems. The purpose of this work is to study the dynamics and the geological environment forecasting in conditions of increasing man-made load and prove the effectiveness of NIEMFE method in solving problems related to the violation of the engineering facilities operating conditions.Data & Methods Data from field observations conducted by employees of the De­partment of Geotechnogenic Safety and Geoinformatics of Ivano-Frankivsk National University of Oil and Gas were used to build the initial models. In particular, measure­ments by the NIEMFE method were performed on the territory of the school in the vil­lage Yunashkiv in the Rohatyn district and on the territory of the university building.Results The efficiency of using the NIEMFE method for solving geological engineer­ing problems is confirmed by the example of two diverse objects. The observations results obtained during the rocks stress-strain state study in the Yunashkiv village school in the Rogatyn district are considered. The NIEMFE method measurements allowed us to establish the stress-strain zones of the study area, which are related to the karst processes development. The analysis of the anomalies distribution and their intensities show that the territory is not stable and one should expect further karst manifestations and earth's surface and building deformation. The monitoring obser­vation results by the NIEMFE method are presented in order to track the dynamics of the change in the the IFNTUOG building state. The reason for the building damage, namely the design on the ancient river bed site, which led to the rocks strength loss due to the suffusion processes and the decomposition of the clay rocks underlying the foundation was identified. The influence of significant rainfall on the stress-strain state change of the studied territory was confirmed. It is proved that the NIEMFE method is a promising method for studying the rock mass stress-strain state in terms of determining the engineering structures stability and providing the deformation pro­cesses forecast.

New profiling and mooring records help to assess variability of Lake Issyk-Kul and reveal unknown features of its thermohaline structure

Abstract. This article reports the results of three field campaigns conducted in Lake Issyk-Kul in 2015, 2016, and 2017. During the campaigns, CTD profiling and water sampling were performed at 34 locations all over the lake. A total of 75 CTD profiles were obtained. Some biogeochemical and thermohaline parameters at the lake surface were also mapped at high horizontal resolution along the ship's track. In addition, thermistor chains were deployed at three mooring stations in the eastern littoral region of the lake, yielding 147-day-long records of temperature data. The measurements revealed that – while the thermal state of the active layer, as well as some biogeochemical characteristics, were subject to significant interannual variability mediated by atmospheric forcing – the haline structure of the entire lake was remarkably stable at the interannual scale. Our data do not confirm the reports of progressive warming of the deep Issyk-Kul waters as suggested in some previous publications. However, they do indicate a positive trend of salinity in the lake's interior over the last 3 decades. A noteworthy newly found feature is a weak but persistent salinity maximum below the thermocline at a depth of 70–120 m, from where salinity slightly decreased downwards. The data from the moored thermistor chains support the previously published hypothesis about the significant role of the submerged ancient riverbeds on the eastern shelf in advecting littoral waters into the deep portion of the lake during differential cooling period. We hypothesize that the less saline littoral water penetrating into the deep layers due to this mechanism is responsible for the abovementioned features of salinity profile, and we substantiate this hypothesis by estimates based on simple model assumptions.

Assessment of earthquake-induced ground liquefaction susceptibility for Hanoi city using geological and geomorphologic characteristics

Assessment of earthquake-induced ground liquefaction susceptibility for Hanoi city using geological and geomorphologic characteristics

Characteristics of the Yangtze River incised valley in the last glacial maximum in Nanjing–Haimen reach, China

The Yangtze River is the largest river in the Asian monsoon region, research on the Yangtze River’s paleochannel in the last glacial maximum (LGM) will provide an important basis for an in-depth understanding of the hydrological characteristics and processes of large rivers in the Asian monsoon area during the last glacial period. The Nanjing–Haimen reach of the Yangtze River in Jiangsu Province was selected as the research subject. Five cross sections of incised valley depositional sequences were established on a section of the Yangtze River in the Nanjing–Haimen reach, with 173 bore data and eight documents from the references. The cross sections came from the following Yangtze River Bridges: Nanjing No. 4, Runyang, Yangzhong, Jiangyin, and Sutong. In the analysis of seven drilling core samples near the Nanjing No. 4 and the Yangzhong Bridges, carbon-14(14C) dating was used for eight age results and electron spin resonance for one age result. The results indicate that in the LGM the incised valley was cut down into bedrock at the upper reach of the Zhenjiang, and the paleochannel of the Yangtze River flowed through old riverbed sedimentary facies at the lower reach of the Zhenjiang. The Nanjing reach during the LGM had a deep incised valley at a depth of −63 to −94 m. The deep incised valley was relatively narrow and steep with a smaller width–depth ratio (B 1/2/H). At the Zhenjiang reach, the ratio of width to depth increased. At the lower reach of the Yangzhong, the incised valley became wider and a bifurcated incised valley system of the Yangtze River was formed. From the Nanjing reach to Haimen near eastern China region, the sediment of the ancient riverbed of the Yangtze River is mainly coarse sand and gravel that gradually tapers from upstream to downstream. Sediment of the incised valley displays a clear pattern, from pebble gravel and coarse sand to fine sand, from bottom to top.

Satellite traces ancient riverbeds buried in the Sahara

Satellite traces ancient riverbeds buried in the Sahara

The everyday-life in neanderthal times: a full-immersive Pleistocene reconstruction for the Casal De' Pazzi Museum (Rome)

<p>The Museum of Casal De' Pazzi (Rome) is built since about ten years on a Pleistocene floor consisting in the ancient riverbed of the Aniene river, at about 200.000 bp. Since its opening, it has been characterized by a strong narrative approach, and communication effort. The digital project realized by the Italian CNR (Institut for Technology applied to Cultural Heritage) implies three applications: a flood simulation, to show the water refilling of the ancient riverbed; a<br />movie in computer-graphic, to be shown to the public, explaining neanderthal man and ancient elephant's life, and an edu-game, for children's learning, the Plei-stostation, implemented by touch-screen interaction dynamics.</p>

Middle to Late Pleistocene hominin occupation in the Three Gorges region, South China

The contributions of the Chinese Paleolithic record to broader ranging paleoanthropological debates – have long been difficult to decipher. The primary problem that hinders many contributions that include or focus on the Chinese record is that relatively few regions outside of the main flagship sites/basins (e.g., Zhoukoudian, Nihewan Basin, Bose Basin) have been intensively researched. Fortunately, systematic archaeological survey and excavations in the Three Gorges region, South China over the past two decades has led to the discovery of a number of important hominin fossils and Paleolithic stone artifact assemblages that have contributed to rethinking of ideas about hominin adaptations in Pleistocene China. This paper provides a detailed review of the results of recent paleoanthropological, particularly Paleolithic archaeological, research from this region.The Three Gorges region is located in the transitional zone between the upper and middle reaches of the Yangtze River (Changjiang River). Vertebrate paleontological studies indicate that the faunas from this region belong primarily to the Ailuropoda–Stegodon faunal complex, a group of taxa representative of a subtropical forest environment. Systematic field surveys identified sixteen Paleolithic sites in caves and along the fluvial terraces of the Yangtze River. Based on geomorphology, biostratigraphy, and geochronology studies, these sites were formed during the Middle to Late Pleistocene. Follow up excavations at these sites led to the discovery of a large number of Paleolithic stone artifacts, Pleistocene mammal fossils, as well as some hominin fossils. Analysis of these materials has provided the opportunity to reconstruct hominin technological and mobility patterning in a restricted spatial point. The Paleolithic technology from the Three Gorges region is essentially an Oldowan-like industry (i.e., Mode 1 core and flake technologies) comprised of casual cores, whole flakes, fragments, and chunks as well as a low percentage of retouched pieces. The utilized stone raw material is primarily high sphericity cobbles and limestone, which were locally available along the ancient river bed and surrounding terraces. Most of the artifacts are fairly large in size. All flaking is by direct hard hammer in a single direction without core preparation. Unifacial choppers are the predominant core category, with fewer bifacial choppers, sporadic discoids, polyhedrons, and bifaces. The flake types demonstrate that the first stage of core reduction is represented by a low percentage of Type III and VI flakes. Some flakes are retouched unifacially by direct hard hammer percussion on the dorsal surface of the blanks. Archaic Homo sapiens and modern H. sapiens identified from some of the cave deposits are likely the hominins responsible for the production of the stone artifacts. Implications for Oldowan-like technological patterning in South China are discussed.

Estimating amount and spatial distribution of groundwater recharge in the Iullemmeden basin (Niger) based on 3H, 3He and CFC‐11 measurements

AbstractA new approach is presented to estimate the amounts and spatial distribution of groundwater recharge based on the groundwater dating tracers 3H, 3He and CFC‐11. The method was applied in a groundwater project in southwest Niger (Africa). The model developed to simulate observed tracer concentrations is composed of a model to account for the water transported through the unsaturated zone followed by a dispersion box model to determine the age distribution of the sampled groundwater volume. For the unsaturated zone transport, a one‐dimensional approach was used considering tracer transport in the water and the gas phase. The calculated tracer concentrations at the groundwater table are then used as input functions for the subsequent dispersion box model. Using a dispersion box model it is possible to outline regions where recharge preferably occurs. To account for the high variance of soil properties in space, a Monte Carlo simulation was performed.The results show estimated recharge rates of 50–300 mm year−1 (±50%). The large error bars are mainly due to high uncertainties of the soil parameters. Therefore, a better estimation of the average soil moisture would lead to more accurate results. The resulting spatial distribution of dispersion parameters indicates that recharge mainly occurs in the ancient riverbeds where surface runoff from the laterite plateaux is gathered and leads to spatially limited high recharge events. Where the soil is not permeable enough, these events lead to the observed temporary or permanent lakes. This new conceptual view of major recharge mechanisms contradicts the former picture of recharge occurring in this region, but it coincides better with other independent measurements, such as the stable isotope ratios of water. Copyright © 2005 John Wiley & Sons, Ltd.

A PRELIMINARY STUDY ON THE DISTRIBUTION PATTERNS AND CHARACERISTICS OF AMMOPIPTANTHUS MONGOLICUS POPULATIONS IN DIFFERENT DESERT ENVIRONMENTS

Ammopiptanthus mongolicus, which is an endemic and endangered species in China listed in the Red Data book, is a type of evergreen broad-leaved shrub in dry deserts of Asia. A. mongolicus is distributed mainly in the Alashan region of Inner Mongolia, but, for unknown reasons, the range of the population has been declining. We studied the distribution patterns and characteristics of A. mongolicus as a preliminary step in understanding the dynamic changes of the population and community and the factors causing the population decline. In Alashan Zuoqi, 36 quadrats (20 m×20 m) and 150 grids (5 m×5 m) were set up at 3 sites (Dongqingliang, Sumutu and Luanjingtan). There were at least 12 quadrats and 50 grids in every plot. The distribution patterns, stand density characteristics, morphological characteristics, and dynamic changes of the A. mongolicus populations at the 3 sites were compared. The results showed that A. mongolicus populations formed different distribution patterns under environmental conditions. The population was distributed evenly on flowing-sand or semi-fixed windy sand but was distributed contagiously on ancient riverbeds or stone slopes caused by seasonal floods. Individuals of A. mongolicus had different morphologies based on the population density and environment characteristics. On flowing-sand, individuals had shorter heights and smaller diameters but the relationship between height and width was not significant (R_1=0.096). There was a negative relationship between population density and the height and diameter of individuals in the population on flowing-sand (R_2=-0.214, R_3=-0.339). Individuals growing on semi-fixed windy sand and stone slopes had significantly greater heights and diameters, but there was no significant relationship between population density, and the diameter and the height. On stone slopes, the relationships between individual diameters and the heights were not significant (R_1 = 0.886), and there were no significant relationships between population density and height and width (R_2 = 0.089, R_3= 0.055). The densities of A. mongolicus populations were the highest at the flowing-sand site and the lowest at the stone slope population where seasonal floods occurred. Populations of A. mongolicus had different age structures at the different sites. At the flowing-sand site, there were even numbers of mid- to old-aged individuals but few young individuals, indicating that the population was senescing. At the semi-fixed windy sand and stone slope sites, there were also very few young individuals, and the number of individuals in the different age classes was distributed very unevenly. All three populations had limited the regeneration questioning the long-term viability of these populations.

Human-threatened ecosystem: new signs of groundwater connection between Yacyreta reservoir and Ibera wetland (South America)

Ibera wetland is one of the most pristine and singular ecosystems of South America. Satellite-derived thermal analysis of its lacustrine system supported the existence of a groundwater connection with the Yacyreta reservoir through basaltic fractures located along the ancient river bed of the Parana. The groundwater outflow would be located on the geological discontinuity of the eastern border of Ibera wetland, concretely Laguna Iberá and Laguna Luna–Disparo. This new sign of connection comes on top of the hydrological signs which already exist. A call to prudence recommends not extending the recently projected increase of the water level of Yacyreta reservoir. Deeper hydrogeological studies should analyze the hypothesis emerged from this study before modifying the water level of Yacyreta.

The Application of Steroidal Biomarkers to Track the Abandonment of a Roman Wastewater Course at the Agora (Athens, Greece)*

A subterranean, stone‐built, sediment‐filled culvert discovered during excavations at the Agora (Athens, Greece) was dated to the Roman period on the basis of its characteristic construction and associated finds, including coins. The location of the culvert relative to other adjacent watercourses and an ancient river bed suggested that the structure was a sewer. This was confirmed through a multi‐molecular biomarker approach based on analysis of the bile acids associated with sediment filling the sewer, using gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The acid fraction contained predominantly deoxycholic and lithocholic acids, while the neutral steroid fraction comprised a complex mixture of Δ5 sterol and 5β‐ and 5α‐stanols, dominated by coprostanol, suggesting the presence of faecal matter of predominantly human origin. The concentrations of neutral and acidic faecal biomarkers were observed to vary in tandem, with the highest concentrations being found in the sediment at the base of the fill in the culvert. A reduction in concentration occurred with decreasing depth of the fill, with concentrations in the uppermost samples being little different from control samples of sediment taken beyond the confines of the culvert. The enhanced concentration of bile acids relative to 5β‐stanols compared with fresh human faeces must reflect the enhanced diagenetic stability of the former, thereby making bile acids the possibly preferred biomarker for this type of study. The quantitative data obtained suggest that the culvert fell rapidly out of use, possibly coinciding with the Slavic incursion in AD 582–3.

A study of the surface-emerging palaeochannels on the South Coast Plain of Laizhou Bay

By employing many methods including distinguishing the land-maps, interpreting the aerophotoes and Satellite-photographes, consulting historical documents, investigating ancient cultural relics, making investigations in the field, analysing the names and features of the place, and analysing the characteristics of the deposit, etc., a systematic study of the surface-emerging palaeochannels on the South Coast Plain of Laizhou Bay has been made. Twenty-five surface-emerging palacochannels were discovered and verified. According to the formation age, these palaeochannels were divided into two stages: six palaeochannels were formed in the Early and Middle Holocene, and nineteen palaeochannels were formed in the Late Holocene (the historical period). According to the landform features, the palaeochannels are divided into four categories: upland palaeochannel belts, strip-shaped upland palaeochannels, trough-shaped depression palaeochannels and ancient riverbeds. Among the four categories, the strip-shaped upland palaeochannels are the main types. The swinging and movement of the channels helped the alluvial plains expand and cause the coastal ancient lakes to vanish in succession.

Late Ordovician carbonate buildups and erosional features northeast of Gotland, northern Baltic Sea

The dimensions and form, structure and distribution of the Upper Ordovician carbonate buildups in the Baltic Sea, northeast of the Island of Gotland, have been determined by means of shallow-marine seismic reflection profiling. They are shown to decrease in size and number downslope, towards the deeper part of the Palaeobaltic Basin. The seismic signature variations, reflecting the structural and compositional changes of the carbonate buildups point to an increase in reef skeleton forming organisms and enhanced rigidity and stability of the buildups upslope. Supported by the tectonic setting, a local shallow-water break was formed on the slightly sloping carbonate ramp of the Late Ordovician sea, closely east of the Isle of Gotska Sandön. This depth development promoted the formation of a major linear reef of fringing or barrier character, separating a patch reef complex and reefs in the offshore belt adjacent to deep shelf muds from a beach or tidal flat. In the offshore area northeast of northern Gotland, the Late Ordovician sedimentary environment was entirely different from that farther east, offshore the Island of Hiiumaa, Estonia. The great diversity of the sediments, and the rapidly changing thicknesses of the layers, laid down on the fore-reef slope, were to a large extent governed by the numerous mud mounds and reef structures. Syn- and post-depositional compaction effects induced relative movements of the masses inside the deep-water carbonate buildups which led to their lateral spalling and collapse. The locally discordant reflectors from these rapidly changeable lithological units and strongly anomalous, chaotic reflector configuration around the collapsed carbonate buildups are intertwined with reflectors of erosional origin. Channel-like depressions, which probably form a unique set of ancient riverbeds, appear in the Late Ordovician sequence about midway between northeastern Gotland and Hiiumaa. They were formed before the formation of the Vormsi-Fjäcka layers, i.e., before the establishment of the Upper Ordovician carbonate buildup environment. The latest Ordovician erosional surface, most likely related to the Gondwana glaciation, is distinctly imprinted in the bedrock of the area.

Stress and Disturbance: Vegetation Dynamics in the Dry Chaco Region of Argentina

This paper analyses two processes acting on The second process is the dynamics of gallery forest vegetation dynamics in the dry Chaco region of Argentina. resulting from intensive river-bed migrations which The major one is the response of herbaceous/woody species characterize the region. A main process model relates the to overgrazing. Extensive cattle breeding reached a peak largest and most frequent floods to the complex structure, shortly before the 1940s, when animal production in the high diversity and Amazonian lineage of the floristic region became saturated. Thus over-exploitation reached a composition of the forests. It also attributes the null water crisis during this decade and there followed a sharp supply of abandoned river-beds to Chaco lineage forests, decrease in the number of cattle and the efficiency of the adjusted to rainfall seasonality. However, owing to the high production system. The number of puestos (stations), on the morphological instability of the region these courses may contrary, remained fairly constant. Thus overgrazing has become active or inactive throughout time by means of led to degradation of the natural systems. Today the most digression or filling in, which leads to well-developed intensely degraded areas form a 25-50 ha fringe around forests growing on ancient river-beds and, conversely, dry each puesto headquarters and represent less than 1 % of the forests growing along the margin of permanent rivers. A total area studied. Here, marked changes in soil model is presented which, by analysing river bank physiochemical properties can be seen together with the community dynamics in terms of the geomorphological elimination of trees and shrubs. Away from this fringe, instability (constant river-bed migration, river-bed filling, degradation basically involves the herbaceous cover, bank formation and disintegration), shows how the changing the original landscape of forests and savannas structure and composition of these forests would be into one of forests and shrub patches. There are not only determined by past floods rather than by the present one. structural alterations but also changes in the system's dynamics due to modifications in the relationships between Key words. Overgrazing, riverine, savanna vegetation its components. The sequence of changes seems to exhibit dynamics, disturbance, gallery forests, stability, Chaco, resilient behaviour showing hysteresis. Argentina.

Utilization of the analysis of ancient river beds for the detection of Holocene crustal movements

On the present-day surface of the continuously subsiding Great Hungarian Plain, bends of ancient rivers can be traced. The axes of the ancient beds are parallel to each other in some zones, which, on the other hand, cross each other. This provides a possibility of detecting the temporal succession of their formation. The study of the ancient river beds permits of drawing conclusions as to the tendencies and intensities of bed wandering. The age of the beds is (Early) Holocene, with differences within this range. The tendency of the movements changed at the end of the Pleistocene, subsidence starting along the NW marginal faults instead of the SE ones. This resulted in a tilting of the Great Plain to the NW.