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1.
ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( > 0)] and the rate of sediment supply [S] function during the growth and evolution of deltaic shorelines. The theory of shoreline autoretreat, recently corroborated in flume experiments, claims that (1) A( > 0) and S can never be in equilibrium, and (2) shoreline or shelf‐edge progradation inevitably turns to retrogradation, when relative sea level is rising even modestly and even if A/S = const (> 0). Autoretreat arises because the area of the clinoform surface of the delta (or shelf edge) per kilometer of shoreline must increase as the relative sea level rises, and the delta (or shelf edge) progrades into deeper water. A finite sediment supply rate is thus liable to become inadequate to sustain progradation. The problem increases further as a rising sea level also greatly increases the delta‐plain volume that needs to be filled, further limiting the progradation of the system. The fundamental trajectory of shoreline migration is thus one characterized by a concave‐landward shape, even under the steady forcing of the basin. The magnitudes of A (> 0) and S, or A/S do not determine whether the landward turnaround of the shoreline is realized or not, but affect merely the length and height of the fundamental trajectory curve. Thus, any attempt to detect and interpret temporal changes in A and S from the observed stratigraphic record of shoreline trajectory needs first to take full account of the inbuilt autoretreat mechanism. We develop here a simple, semi‐quantitative method of reconstructing the basin conditions (A and S) from the stratigraphic record of prograding deltaic shorelines (or prograding shelf‐margin clinoforms) on the basis of the theory of shoreline autoretreat. The deterministic nature of the autoretreat theory is advantageous in managing this latter issue, because any expected or unexpected change emerges as some discrepancy from a trajectory that was predicted for the initial conditions. The autoretreat theory also provides a convenient graphical method of dealing with the uncertainty of the field data, and with evaluating the accuracy of any reconstruction. Our methodology has been developed to deal with the behaviour of deltaic shorelines, but is basically applicable to any clinoform system, the development of which is affected by relative sea level. The suggested method is applied to an Early Eocene (Ypresian) regressive shoreline succession in the Central Tertiary Basin on Spitsbergen. The studied regressive wedge developed as a delta‐driven, progradational shelf‐margin system under a regime of overall (i.e. long‐term) rise of relative sea level, but also suffered short‐term sea‐level falls associated with valley incisions on the coastal plain and shelf. On the assumption that S was constant or was steadily decreasing, the analysis of field data obtained from three sites within the basin suggests that the initial water depth in the basin was around 0.45 km, and that the overall relative sea‐level rise (c. 0.80 km) happened largely during an early time period and was followed by a longer period of much lower rate of rise. This pattern of relative sea‐level rise is consistent with the Palaeogene tectonic subsidence trend of the basin which was determined independently through a geohistory analysis. The uncertainty of the field data does not negate our reconstruction. The combined effects of autoretreat and A/S changes on a deltaic shoreline trajectory are confirmed through the development of an autoretreat‐based methodology. Conventional sequence stratigraphic models that assume a possible equilibrium condition between A and S are both conceptually misleading and insufficient to analyse basin conditions quantitatively. Sequence stratigraphic analyses of shorelines need to incorporate the autoretreat concept.  相似文献   

2.
Late‐middle Miocene to Pliocene siliciclastics in the Northern Carnarvon Basin, Northwest Shelf of Australia, are interpreted as having been deposited by deltas. Some delta lobes deposited sediments near and at the shelf break (shelf‐edge deltas), whereas other lobes did not reach the coeval shelf break before retreating landward or being abandoned. Shelf‐margin mapview morphology changes from linear to convex‐outward in the northern part of the study area where shelf‐edge deltas were focused. Location and character of shelf‐edge deltas also had significant impact on along‐strike variability of margin progradation and shelf‐edge trajectory. Total late‐middle and late Miocene margin progradation is ca. 13 km in the south, where there were no shelf‐edge deltas, vs. ca. 34 km in the north where shelf‐edge deltas were concentrated. In the central area, the deltas were arrested and accumulated a few kilometres landward of the shelf break, resulting in an aggradational shelf‐edge trajectory, in contrast to the more progradational trajectory farther north. This illustrates a potential limitation of shelf‐edge trajectory analysis: only where shelf‐edge deltas occur, there is sufficient sediment available for the shelf‐edge trajectory to record relative sea‐level fluctuations reliably. Small‐scale (ca. 400 m wide) incisions were already conspicuous on the coeval slope even before deltas reached the shelf break. However, slope gullies immediately downdip from active shelf‐edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ca. 100 m deep) than coeval incisions that are laterally offset from the deltaic depocenter (ca. 0.7 km wide, ca. 25 m deep). We interpret this change in slope‐gully dimensions as the result of greater erosion by sediment gravity flows sourced from the immediately adjacent shelf‐edge deltas. Similarly, gullies also incised further (up to 6 km) into the outer shelf in the region of active shelf‐edge deltas.  相似文献   

3.
Fluvio‐deltaic stratigraphy develops under continuous morphodynamic interactions of allogenic and autogenic processes, but the role and relative contribution of these processes to the stratigraphic record are poorly understood. We analysed synthetic fluvio‐deltaic deposits of several accommodation‐to‐supply cycles (sequences) with the aim to relate stratigraphic variability to autogenic and allogenic controls. The synthetic stratigraphy was produced in a series of long time‐scale (105 years) numerical experiments with an aggregated process‐based model using a typical passive‐margin topography with constant rates of liquid and solid river discharge subjected to sinusoidal sea‐level fluctuation. Post‐processing of synthetic stratigraphy allowed us to quantify stratigraphic variability by means of local and regional net sediment accumulation over equally spaced time intervals (1–10 kyr). The regional signal was subjected to different methods of time‐series analysis. In addition, major avulsion sites (>5 km from the coastline) were extracted from the synthetic stratigraphy to confirm the interpretations of our analyses. Regional stratigraphic variability as defined in this study is modulated by a long‐term allogenic signal, which reflects the rate of sea‐level fluctuation, and it preserves two autogenic frequency bands: the intermediate and high‐frequency components. The intermediate autogenic component corresponds to major avulsions with a median inter‐avulsion period of ca. 3 kyr. This component peaks during time intervals in which aggradation occurs on the delta plain, because super‐elevation of channel belts is a prerequisite for large‐scale avulsions. Major avulsions occur occasionally during early stages of relative sea‐level fall, but they are fully absent once the coast line reaches the shelf edge and incision takes place. These results are consistent with a number of field studies of falling‐stage deposition in fluvial systems. The high‐frequency autogenic component (decadal to centennial time scales) represents mouthbar‐induced bifurcations occurring at the terminal parts of the system, and to a lesser extent, partial or small‐scale avulsions (<5 km from the coastline). Bifurcation intensity correlates strongly with the rate of progradation, and thus reaches its maximum during forced regression. However, its contribution to overall stratigraphic variability is much less than that of the large‐scale avulsions, which affect the entire area downstream of avulsion nodes. The results of this study provide guidelines for predicting fluvio‐deltaic stratigraphy in the context of co‐existing autogenic and allogenic processes and underscore the fact that the relative importance and the type of autogenic processes occurring in fluvio‐deltaic systems are governed by allogenic forcing.  相似文献   

4.
《Basin Research》2018,30(4):650-670
The Palaeogene Isparta Basin of southwestern Anatolia formed between two convergent arms of the Isparta Bend orocline of the Tauride orogen. The origin of this tightening orocline is hypothetically explained in plate‐tectonic terms. Basin sedimentation commenced on a down‐warped Mesozoic carbonate platform of a crustal block accreted at the end of Cretaceous to the southern margin of the Anatolian plate. The basin earliest deposits are Palaeocene reddish mudstones with a fossil‐barren condensed basal part and increasingly interspersed with thin calcarenitic turbidites towards the top. The supply of turbiditic sediment to the basin plain subsequently increased, as the upper‐bathyal basin plain became surrounded from both sides by a narrow littoral shelf with an advancing turbiditic slope ramp. A major forced regression occurred at the end of Bartonian, causing incision of subaerial to submarine valleys up 600 m deep, filled in with gravelly to sandy turbidites and debrisflow deposits during the subsequent rise of relative sea level. The half‐filled valleys were re‐incised due to a Rupelian forced regression and were fully filled with fluvio‐deltaic bayhead deposits during a final marine transgression that re‐established the basin‐margin biocalcarenitic shelf. The littoral environment then expanded across the shallowing basin, as the basin axial zone was up‐domed and eroded to bedrock level at the end of Oligocene and the basin was tectonically inverted in Miocene. The pattern of intra‐orocline foreland sedimentation documented by this case study provides tentative criteria for the recognition of synorogenic oroclines and for their distinction from post‐orogenic oroclines.  相似文献   

5.
Landward retreat (marine transgression) is a common response of coastal systems to rising relative sea level. However, given sufficient sediment supply, the coast may advance seaward. The latter response of gravel barriers has been recorded in parts of southeastern and northwestern Canada, where seaward‐rising sets of beach ridges are observed in areas of Holocene RSL rise. Cape Charles Yorke, northern Baffin Island, is a 5 km long gravel foreland characterized by seaward‐rising beach‐ridge crest elevations. The prograded morphology of the Cape Charles Yorke foreland is a prime example of coastal response to a combination of rising RSL and abundant sediment supply, an unusual and little‐documented pattern in the Canadian Arctic. The main gravel supply to Cape Charles Yorke is likely from eroding bedrock and raised marine deposits southwest of the foreland. Although not the dominant sediment source, the Cape Charles Yorke delta contributed to the formation of the foreland by sheltering it from easterly storm waves and providing an anchor point for the prograding ridges. The truncation of relict ridges by the modern shoreline suggests a recent regime shift from continuous deposition to predominant erosion. The cause and timing of this shift are unknown but could result from a recent dwindling in sediment supply, increased accommodation space, increased wave energy, and/or an accelerated rise of relative sea level.  相似文献   

6.
珠江三角洲生物埋葬群与环境变迁   总被引：6，自引：0，他引：6  
黄镇国  张伟强 《地理学报》1995,50(4):310-323
本文将珠江三角洲第四纪环境与现代环境的生物埋葬群进行对照，重建晚更新世中期和全新世中期的古沉积环境，划分出两期的海侵区、海侵波及区、海侵影响区。并对中更新世海侵期表示存疑。  相似文献   

7.
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8.
Gilbert deltas are now recognised as an important stratigraphic component of many extensional basins. They are remarkable due to their coarse‐grained nature, large size and steep foresets (up to 30–35°) and may exhibit a variety of slope instability features (faulting, slump scars, avalanching, etc.). They are also often closely related to major, basin‐margin normal faults. There has been considerable research interest in Gilbert deltas, partly due to their economic significance as stratigraphic traps for hydrocarbons but also due to their sensitivity to relative base level changes, giving them an important role in basin analysis. In addition to field studies, numerical modelling has also been used to simulate such deltas, with some success. However, until now, such studies have typically employed continuum numerical techniques where the basic data elements created by simulations are stratigraphic volumes or timelines and the sediments themselves have no internal properties per se and merely represent areas/volumes of introduced coarse‐grained, clastic and sedimentary material. Faulting or folding (if present) are imposed externally and do not develop (naturally) within the modelled delta body itself. Here, I present first results from a novel 2D numerical model which simulates coarse‐grained (Gilbert‐type) deltaic sedimentation in an active extensional tectonic setting undergoing a relative base level rise. Sediment is introduced as packages of discrete elements which are deposited beneath sea level, from the shoreline, upon a pre‐existing basin or delta. These elements are placed carefully and then allowed to settle onto the system. The elements representing the coarse‐grained, deltaic sediments can have an intrinsic coefficient of friction, cohesion or other material properties appropriate to the system being considered. The spatial resolution of the modelling is of the order of 15 m and topsets, foresets, bottomsets, faults, slumps and collapse structures all form naturally in the modelled system. Examples of deltas developing as a result of sediment supply from both the footwall and hanging‐wall of a normal fault, and subject to changes in fault slip rate are presented. Implications of the modelling approach, and its application and utility in basin research, are discussed.  相似文献   

9.
《Basin Research》2018,30(4):671-687
The Mesozoic shelf margin in the Mahajanga Basin, northwest Madagascar, provides an example where inherited palaeobathymetry, coupled with sea‐level changes, high sediment supply and fluctuations in accommodation influenced the stacking patterns and geometry of clinoforms that accreted onto a passive rifted margin. Two‐dimensional (2D) seismic profiles are integrated with existing field data and geological maps to study the evolution of the margin. The basin contains complete records of transgression, highstand, regression and lowstand phases that took place from Jurassic to Cretaceous. Of particular interest is the Cretaceous, Albian to Turonian (ca. 113‐93 Ma), siliciclastic shelf margin that prograded above a drowned Middle Jurassic carbonate platform. The siliciclastic phase of the shelf margin advanced ca. 70 km within ca. 20 My, and contains 10 distinct clinoforms mapped along a 2D seismic reflection data set. The clinoforms show a progressive decrease in height and slope length, and a fairly constant slope gradient through time. The successive shelf edges begin with a persistent flat to slightly downward‐directed shelf‐edge trajectory that changes to an ascending trajectory at the end of clinoform progradation. The progressive decrease in clinoform height and slope length is attributed to a decrease in accommodation. The prograding margin is interpreted to have formed when siliciclastic input increased as eastern Madagascar was uplifted. This work highlights the importance of sediment supply and inherited palaeobathymetry as controls on the evolution of shelf margins and it provides a new understanding of the evolution of the Mahajanga Basin during the Mesozoic.  相似文献   

10.
Middle Miocene to Pliocene siliciclastics of the Bare Formation represent a long‐lived (ca. 11 Myr) break in the otherwise carbonate‐dominated shelf of the Northern Carnarvon Basin, Northwest Shelf of Australia. The quartz‐sandstone interval is correlated with the appearance of spectacular clinoform sets mapped on 3D and dense 2D seismic data. Twenty‐seven clinoform sets are interpreted as delta lobes primarily based on their plan‐view morphology (strike‐elongate to lobate features) and their 40–100‐m‐high clinoform amplitudes. The delta lobes were deposited on outer‐shelf to shelf‐edge positions, and the older deltas show evidence of a higher degree wave reworking than the younger deltas. Measurements of the along‐strike (migration) and down‐dip (progradation) movement of these deltas are compared with relative sea‐level behaviour inferred from shelf‐edge trajectory analysis. Delta lobes exhibit greater lateral shifting during relative sea‐level rise, whereas delta lobes are more restricted to dip‐oriented fairways during sea‐level fall, although no major incised valleys have been identified. Long‐term (cumulative) progradation of this delta system and subsequent backstepping correlates with long‐term sea‐level fall and rise during the late middle and late Miocene. In addition, a long‐term northeastward migration trend for these delta lobes was likely a result of localized uplift of an inversion anticline in the Rosemary–Legendre Trend; the growth of this anticline probably steered the fluvial source for the delta system towards the northeast. The Bare Formation siliciclastic influx correlates with other middle Miocene increases in siliciclastic sediment supply worldwide. Global cooling and a shift to more arid conditions, negatively influencing vegetation cover, may have combined with more seasonally variable rainfall to generate the high sediment supply that built the deltas. Retreat of the siliciclastics could correlate with ice‐sheet growth in the Northern Hemisphere and/or increase in the Indonesian Throughflow and Leeuwin Current (ca. 1.6 Ma), which might have modified climate regionally.  相似文献   

11.
Peat horizons are characteristic features of delta plains worldwide. In this study, we tested the use of peat‐based correlations to assess the deformation of Holocene strata in the Po coastal plain (Northern Italy). The Holocene stratigraphy, about 30 km inland from the modern coastline consists of a peat‐bearing, estuarine and deltaic succession, up to 23 m thick. Through the analysis of 31 core data and 100 piezocone penetration tests, we identified and mapped three 10–40 cm‐thick peat layers (T1–T3) dated to 6.6–5.8, 5.5–5.0 and 3.3–2.7 cal kyr BP respectively. These peat horizons were found to be suitable stratigraphic markers within the Holocene succession over an area of about 200 km2. The mid‐late Holocene palaeogeography, reconstructed through high‐resolution peat correlation, supported by 72 radiocarbon dates, highlights a typical upper delta plain environment, with ribbon‐shaped distributary channels and swamp interdistributary areas. Peat layers are inclined towards E‐NE with gradients that increase downsection from ~0.016% (T3) to 0.021% (T1). The gradient of the oldest peat horizon is one order of magnitude larger than the slope of the modern delta plain (~0.0025%). We infer that peat horizons accumulated during periods of low sediment supply mainly controlled by autogenic processes and were deformed after deposition. Differential compaction of underlying sedimentary strata and recent tectonic activity of the buried Apenninic thrust systems are the most likely drivers of strata deformation. Based on isochore maps, we document that higher sedimentation rates in topographically depressed areas compensated, in part at least, the ongoing deformation, keeping unaltered the topographic gradient and the depositional environment. This study demonstrates that peat‐based correlation and mapping can shed lights on the mechanisms of strata accumulation and deformation in deltaic settings, constituting a robust basis for reconstructing delta evolution.  相似文献   

12.
We analyzed surface-sediment samples collected along transects from three sub-basins of a relatively large (~115 ha), bathymetrically complex lake, in northwest Ontario, Canada, to assess the reproducibility of diatom species habitats and diversity along a water-depth gradient. Transects displayed different orientations with respect to prevailing wind direction and varied in complexity and degree of slope along the lake bottom. Each transect consisted of three replicate samples at a resolution of ~1 m water depth from ~1 to 30 m for the two deep-basin transects and from ~1 to 18 m in the shallower basin. Distinct diatom assemblages were identified in all transects: (1) a near-shore community composed largely of attached life-forms and some motile benthic taxa, (2) a mid-depth community composed largely of motile life-forms and other benthic taxa that are adapted to lower light conditions (e.g. Staurosirella pinnata), and (3) a deep-water community dominated by planktonic taxa. Species richness was highest in the benthic zones (<9 m), with greatest species evenness in the mid-depth zone (~3–9 m). Species richness and evenness were highly correlated across the three transects (r = 0.89–0.93, p < 0.01). Diatom-inferred depth models were developed from the individual transects to assess reproducibility and applicability for down-core analyses using modern analog (MAT) and weighted-averaging (WA-PLS) approaches. Coefficients of determination (r 2) for these models ranged from 0.80 to 0.98, and RMSEP ranged from 1.2 to 4.2 m. The models developed from the transect with the highest resolution sampling, gentlest non-complex slope and shallowest maximum depth were the strongest ( r\textMAT2 = 0.97 r_{\text{MAT}}^{2} = 0.97 ; r\textWA - PLS2 = 0.98 r_{\text{WA - PLS}}^{2} = 0.98 ) and had the lowest RMSEP (MAT = 1.2 m, WA-PLS = 1.3 m). These inference models can be used to infer past fluctuations in the depth of the benthic/planktonic boundary from cores retrieved near this ecotone and provide a sensitive record of the past change in location of the benthic zone. These types of data can be used to assess past variability in droughts and lake levels to better plan for potential future extremes. Such records incorporate more realistic estimates of natural variability than the ~100-year instrumental records currently used by water resource managers.  相似文献   

13.
Based on grain-size analysis of sediment samples from a delta at Altermarka, it has previously been suggested that two tills were deposited on the delta surface as a glacier planed off the delta topsets and created its adverse surface slope. In this reinvestigation of the delta, there was no evidence that a glacier overrode the surface: the topset beds remain intact and no appreciable till was found. Sediments resembling the ‘till’ samples were located, but soil processes and other sedimentary processes easily account for the wide grain-size distributions of these sediments. As a result, this study cautions against using grain-size analysis, on its own, to interpret the genesis of sediments. Other processes potentially responsible for the adverse surface slope include differential isostatic rebound, recent differential slip along ancient thrust faults, and failure of the delta surface to reach sea level. Based on the sedimentology of the topset beds and the local relative sea level history (indicating the delta (9495±70 14C yr BP) formed about the time of the local marine limit (9560–9375 14C yr BP) 17 m higher than the delta surface), the feature is determined to have built up below sea level with an adverse surface slope.  相似文献   

14.
We present an interpolation model that describes Holocene groundwater level rise and the creation of accommodation space in 3D in the Rhine‐Meuse delta – the Netherlands. The model area (ca. 12 400 km2) covers two palaeovalleys of Late Pleistocene age (each 30 km wide) and the overlying Holocene deposits of the Rhine‐Meuse delta, the Holland coastal plain, and the Zuiderzee former lagoon. Water table rise is modelled from 10 800 to 1000 cal. BP, making use of age‐depth relations based on 384 basal peat index points, and producing output in the form of stacked palaeo groundwater surfaces, groundwater age‐depth curves, and voxel sets. These products allow to resolve (i) regional change and variations of inland water table slopes, (ii) spatial differences in the timing and pacing of transgression, and (iii) analysis of interplay of coastal, fluvial and subsidence controls on the provision of accommodation space. The interpolation model is a multi‐parameter trend function, to which a 3D‐kriging procedure of the residuals is added. This split design deploys a generic approach for modelling provision of accommodation space in deltas and coastal lowlands, aiming to work both in areas of intermediate data availability and in the most data‐rich environments. Major provision of accommodation space occurred from 8500 cal BP onwards, but a different evolution occurred in each of the two palaeovalleys. In the northern valley, creation of accommodation space began to stall at 7500 cal BP, while in the southern valley provision of new accommodation space in considerable quantities continued longer. The latter is due to the floodplain gradient that was maintained by the Rhine, which distinguishes the fluvial deltaic environment from the rest of the back‐barrier coastal plain. The interpolation results allow advanced mapping and investigation of apparent spatial differences in Holocene aggradation in larger coastal sedimentary systems. Furthermore, they provide a means to generate first‐order age information with centennial precision for 3D geological subsurface models of Holocene deltas and valley fills. As such, the interpolation is of use in studies into past and present land subsidence and into low land sedimentation.  相似文献   

15.
长江河口地区第四纪古地理古环境变迁   总被引：21，自引：4，他引：17  
陈中原  杨文达 《地理学报》1991,46(4):436-448
通过对钻孔样品的粒度、层理揭片、矿物、微体古生物、孢粉、地球化学元素及古地磁等项目的实验室分析,着重探讨长江河口地区第四纪古地理、古环境变迁特征和长江三角洲的发育过程。  相似文献   

16.
As an approach to understand how diurnal and seasonal plant water potentials (Ψ) are related to soil water-content and evaporative demand components, the responses of six thornscrub shrubs species (Havardia pallens, Acacia rigidula, Eysenhardtia texana, Diospyros texana, Randia rhagocarpa, and Bernardia myricaefolia) of the north-eastern region of Mexico to drought stress were investigated during the course of 1 year. All study species showed the typical diurnal pattern of variation in Ψ. That is, Ψ decreased gradually from predawn (Ψpd) maximal values to reach minima at midday (Ψmd) and began to recover in the late afternoon. On a diurnal basis and with adequate soil water-content (>0.20 kg kg−1), diurnal Ψ values differed among shrub species and were negatively and significantly (p<0.001) correlated with air temperature (r=−0.741 to −0.883) and vapor pressure deficit (r=−0.750 to −0.817); in contrast, a positive and significant (p<0.001) relationship was found to exist with relative humidity (r=0.758–0.842). On a seasonal basis, during the wettest period (soil water-content>0.20 kg kg−1), higher Ψpd (−0.10 MPa) and Ψmd (−1.16 MPa) values were observed in R. rhagocarpa, whereas lower figures (−0.26 and −2.73 MPa, respectively) were detected in A. rigidula. On the other hand, during the driest period (soil water-content<0.15 kg kg−1), Ψpd and Ψmd values were below −7.3 MPa; i.e. when shrubs species faced severe water deficit. Soil water-content at different soil layers, monthly mean relative humidity and monthly precipitation were significantly correlated with both Ψpd (r=0.538–0.953; p<0.01) and Ψmd (r=0.431–0.906; p<0.05). Average soil water-content in the 0–50 cm soil depth profile explained between 70% and 87% of the variation in Ψpd. Results have shown that when gravimetric soil water-content values were above 0.15 kg kg−1, Ψpd values were high and constant; below this threshold value, Ψ declined gradually. Among all shrub species, A. rigidula appeared to be the most drought tolerant of the six species since during dry periods it tends to sustain significantly higher Ψpd in relation to B. myricaefolia. The remaining species showed an intermediate pattern. It is concluded that the ability of shrub species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux.  相似文献   

17.
Downstream variation of hydraulic geometry in rivers, characterized by fine textured banks and low width to depth ratios (7–25), is investigated in Victoria, Australia, with the aim of developing predictive models of channel geometry for large-scale spatial modeling applications. A one-dimensional hydraulic model is used to determine the mean bank-full geometry and discharge (Qbf) for 93 sites which are investigated in relation to discharge of fixed average recurrence interval (ARI). The median bank-full ARI is estimated at 0.8 years with 75% of sites between 0.5 and 2.5 years. Exponents in the downstream hydraulic geometry relations for width, depth and velocity are respectively 0.43, 0.40 and 0.18 (Q = Qbf) and 0.44, 0.38 and 0.03 (Q = Q2, i.e., 2-year ARI), falling near the mode of global values. Q2 and slope explain 66% of variance in Qbf, while Q2 explains 73% and 69% of the variance in width and depth relations, respectively: Q2 provides a reliable substitute for Qbf in spatial modeling applications. Spatial variation in hydraulic geometry relations within and between river basins remains largely unexplained. The W/D ratio characteristically decreases with increasing distance along the lower reaches of most rivers and this has contributed to the lower than expected value for the width exponent.  相似文献   

18.
Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A mix), mean August water temperature (T Aug), dissolved organic carbon (DOC), and relative water depth (Z rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R 2 boot= 0.72, RMSEPboot= 0.131), and a thermistor measurements-based PLS model for A mix (R 2 boot= 0.71, RMSEPboot= 0.006 log10 Julian days). The latter showed a better prediction than T Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A mix. The multi-proxy data suggest that the A mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.  相似文献   

19.
ABSTRACT The Eridanos fluvio‐deltaic system, draining most of north‐western Europe, developed during the Late Cenozoic as a result of simultaneous uplift of the Fennoscandian shield and accelerated subsidence in the North Sea Basin. This seismo‐stratigraphic study aims to reconstruct the large‐scale depositional architecture of the deltaic portion of the basin fill and relate it to external controls. A total of 27 units have been recognized. They comprise over 62×103 km3 in the Southern North Sea Basin alone, and have an average delta surface area of 28×103 km2, which suggests that the size of the drainage area was about 1.1×106 km2. Water depth in the depocentre is seen to decrease systematically over time. This trend is interrupted by a deepening phase between 6.5 and 4.5 Ma that can be correlated with the simultaneous occurrence of increased uplift of the Fennoscandian shield, increased subsidence of the Southern North Sea Basin, and a long‐term eustatic highstand. All these observations point to a tectonic control on long‐term average rates of accommodation and supply. Controls on short‐term variations are inferred from variations in rates of sediment supply and bifurcation of the delta channel network. Both rates were initially low under warm, moist, relatively stable climate conditions. The straight wave‐dominated delta front gradually developed into a lobate fluvial‐dominated delta front. Two high‐amplitude sea‐level falls affected the Pliocene units, which are characterized by widespread delta‐front failures. Changes in relative sea level and climate became more frequent from the late Pliocene onward, as the system experienced the effects of glacial–interglacial transitions. Peaks in sedimentation and bifurcation rates were coeval with cold (glacial) conditions. The positive correlation between rates of supply and bifurcation on the one hand, and climate proxies (pollen and δ18O records) on the other hand is highly significant. The evidence presented in this study convincingly demonstrates the control of climate on time‐averaged sediment supply and channel‐network characteristics, despite the expected nonuniformity and time lags in system response. The presence of a clearly discernible climate signal in time‐averaged sediment supply illustrates the usefulness of integrated seismo‐stratigraphic studies for basin‐wide analysis of delta evolution on geological time scales.  相似文献   

20.
We set up an automatic weather station over a playa (the flat floor of an undrained desert basin that becomes at times a shallow lake), approximately 65 km east–west by 130 km north–south, located at the U.S. Army Dugway Proving Ground (40°08′N, 113°27′W, 1124 m above mean sea level) in north-western Utah, U.S.A., in 1999. This station measured the incoming (Rsi) and outgoing (Rso) solar or shortwave radiation using two CM21 Kipp & Zonen pyranometers (one inverted), the incoming (Rli or atmospheric) and outgoing (Rlo or terrestrial) longwave radiation, using two CG1 KippZonen pyrgeometers (one inverted), and the net (Rn) radiation using a Q*7 net radiometer (Radiation Energy Balance System, REBS). We also measured the 10-m wind speed (U10) and direction (R.M. Young wind monitor) and precipitation (Campbell Sci., Inc.). The measurements were taken every 2 s, averaged into 20-min, continuously, throughout the year. The annual (August 1999 – August 2000) comparisons of global or solar radiation and windiness with two other stations in central (Hunter) and northern (Logan) Utah, indicate higher solar radiation (Rsi,Dugway=7797 MJ m−2 period−1vs. Rsi, Hunter=7021 MJ m−2 period−1 and Rsi, Logan=6865 MJ m−2 period−1) and much higher annual mean windiness (UDugway=387 km day−1vs. UHunter=275 km day−1 and ULogan=174 km day−1) throughout the period over the playa. These data reveal the possibility of simultaneously harvesting these two sources of clean energies at this vast and uniform playa.  相似文献   

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