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1.
Forward modeled, balanced cross sections that account for the flexural response to thrust loading and erosional unloading can verify and refine the kinematic sequence of deformation in fold‐thrust belts as well as help assess the validity of a balanced cross section. Results from flexural‐kinematic reconstructions that indicate either the cross section, the kinematic order or both are invalid include: (a) a predicted final topography that is dramatically different from the actual topography; (b) large normal fault or thrust fault bounded synorogenic basins that are not present in the mapped geology; and/or (c) an exhumation history that is not consistent with provenance records in the basin or measured thermochronometers. Where detailed measured foreland basin sections exist, flexural‐kinematic modeling of fold‐thrust belt deformation, including out‐of‐sequence (OOS) faults can predict a foreland basin evolution that can be compared to measured data. The modeling process creates a “pseudostratigraphy” in the modeled foreland. The pseudostratigraphy and predicted provenance of each modeled stratigraphic increment can be directly compared to measured stratigraphic sections. We present a case study using two cross sections through the Himalaya of far western Nepal (Api and Simikot) to assess the validity of the section geometries and the resulting kinematic histories, displacement rates, flexural wave response and predicted provenance for both sections. Insights from combining the flexural‐kinematic models with existing stratigraphic data include: (a) Changing the order of proposed OOS and normal faults to earlier in the evolution of the fold‐thrust belt was necessary to reproduce the foreland provenance data. We argue that OOS thrust and normal faults in the Api section occurred between 11 and 4 Ma. (b) Published shortening estimates for the Simikot cross section are too high (>50 km), resulting in unrealistic shortening rates up to 80 mm/yr between 25 and 20 Ma. (c) Flexural forward models with and without an additional sediment loading modeling step indicate that while sediment loading does not have a measurable effect on the magnitude and location of erosion within the fold‐thrust belt, it does have a small effect on accumulation rates and thus the predicted age of stratigraphic boundaries when compared to measured stratigraphic thicknesses and age. Thickness difference range from 0.2 to 0.5 km and can result in predicted age differences of ca. 1 Ma. Accounting for both flexural isostacy and erosion can eliminate unviable kinematic sequences and when combined with provenance data from measured stratigraphic sections, can provide insight into the order, age and rate of deformation.  相似文献   

2.
We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas > 10 km2 and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5–10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n > 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case > 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved.  相似文献   

3.
Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia   总被引：3，自引：1，他引：3  
Eastward Andean orogenic growth since the late Oligocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation in the Chaco basin. To understand the interaction between Andean tectonics and contemporaneous foreland development, we analyse stratigraphic, sedimentologic and seismic data from the Subandean Belt and the Chaco Basin. The structural features provide a mechanism for transferring zones of deposition, subsidence and uplift. These can be reconstructed based on regional distribution of clastic sequences. Isopach maps, combined with sedimentary architecture analysis, establish systematic thickness variations, facies changes and depositional styles. The foreland basin consists of five stratigraphic successions controlled by Andean orogenic episodes and climate: (1) the foreland basin sequence commences between ～27 and 14 Ma with the regionally unconformable, thin, easterly sourced fluvial Petaca strata. It represents a significant time interval of low sediment accumulation in a forebulge‐backbulge depocentre. (2) The overlying ～14–7 Ma‐old Yecua Formation, deposited in marine, fluvial and lacustrine settings, represents increased subsidence rates from thrust‐belt loading outpacing sedimentation rates. It marks the onset of active deformation and the underfilled stage of the foreland basin in a distal foredeep. (3) The overlying ～7–6 Ma‐old, westerly sourced Tariquia Formation indicates a relatively high accommodation and sediment supply concomitant with the onset of deposition of Andean‐derived sediment in the medial‐foredeep depocentre on a distal fluvial megafan. Progradation of syntectonic, wedge‐shaped, westerly sourced, thickening‐ and coarsening‐upward clastics of the (4) ～6–2.1 Ma‐old Guandacay and (5) ～2.1 Ma‐to‐Recent Emborozú Formations represent the propagation of the deformation front in the present Subandean Zone, thereby indicating selective trapping of coarse sediments in the proximal foredeep and wedge‐top depocentres, respectively. Overall, the late Cenozoic stratigraphic intervals record the easterly propagation of the deformation front and foreland depocentre in response to loading and flexure by the growing Intra‐ and Subandean fold‐and‐thrust belt.  相似文献   

4.
Sediment supplied by continental sources is commonly suspected to have exerted a strong influence on the development of canyons and other morphological features on the continental slopes, but rarely is the sediment supply known sufficiently quantitatively to test this link. Here, we outline an area where offshore morphology, in the western Ionian Sea, may be linked to estimated sediment fluxes produced by subaerial erosion in NE Sicily and SW Calabria. Shelves in this area are narrow (<1 km), and the bathymetry shows that rivers and adjacent submarine channels are almost directly connected with each other. Integrated topographic analyses were performed on a merged digital elevation model (DEM) of ASTER data for subaerial topography and multibeam sonar data for submarine bathymetry. Spatial variations in sediment fluxes from onshore erosion were assessed using a variety of methods, namely: long‐term sediment flux from Pleistocene uplift rates, decadal sediment flux from landslide occurrences and published long‐term exhumation rates from 10Be cosmogenic nuclide concentrations. Submarine channels associated with rivers delivering larger sediment fluxes have broad channels, high relief and smooth concave‐upward longitudinal profiles. Conversely, submarine channels that lie offshore small‐flux rivers have straight longitudinal profiles, low relief and steep gradients. Where river catchments supply a greater sediment flux offshore, shelves tend to be wider (ca. 400 m) and submarine channels have gentler gradients. In contrast, where catchments supply less sediment flux, shelves are narrow (250–300 m) and offshore channel gradients are steeper. The variation of submarine morphology with tectonic uplift rate was also studied, but we find that, unlike onshore terrains where tectonics is commonly an important factor influencing channel morphology, in the submarine landscapes, sediment flux appears to dominate here.  相似文献   

5.
The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.  相似文献   

6.
Drainage networks link erosional landscapes and sedimentary basins in a source‐to‐sink system, controlling the spatial and temporal distribution of sediment flux at the outlets. Variations of accumulation rates in a sedimentary basin have been classically interpreted as changes in erosion rates driven by tectonics and/or climate. We studied the interactions between deformation, rainfall rate and the intrinsic dynamics of drainage basins in an experimental fold‐and‐thrust belt subjected to erosion and sedimentation under constant rainfall and shortening rates. The emergence of thrust sheets at the front of a prism may divert antecedent transverse channels (perpendicular to the structural grain) leading to the formation of longitudinal reaches, later uplifted and incorporated in the prism by the ongoing deformation. In the experiments, transverse incisions appear in the external slopes of the emerging thrust sheets. Headward erosion in these transverse channels results in divide migration and capture of the uplifted longitudinal channels located in the inner parts of the prism, leading to drainage network reorganization and modification of the sediment routing system. We show that the rate of drainage reorganization increases with the rainfall rate. It also increases in a nonlinear way with the rate of uplift. We explain this behaviour by an exponent > 1 on the slope variable in the framework of the stream power erosion model. Our results confirm the view that early longitudinal‐dominated networks are progressively replaced by transverse‐dominated rivers during mountain building. We show that drainage network dynamics modulate the distribution of sedimentary fluxes at the outlets of experimental wedges. We propose that under constant shortening and rainfall rates the drainage network reorganization can also modulate the composition and the spatial distribution of clastic fluxes in foreland basins.  相似文献   

7.
In the northwestern sector of the Zagros foreland basin, axial fluvial systems initially delivered fine-grained sediments from northwestern source regions into a contiguous basin, and later transverse fluvial systems delivered coarse-grained sediments from northeastern sources into a structurally partitioned basin by fold-thrust deformation. Here we integrate sedimentologic, stratigraphic, palaeomagnetic and geochronologic data from the northwestern Zagros foreland basin to define the Neogene history of deposition and sediment routing in response to progressive advance of the Zagros fold-thrust belt. This study constrains the depositional environments, timing of deposition and provenance of nonmarine clastic deposits of the Injana (Upper Fars), Mukdadiya (Lower Bakhtiari) and Bai-Hasan (Upper Bakhtiari) Formations in the Kurdistan region of Iraq. Sediments of the Injana Formation (~12.4–7.75 Ma) were transported axially (orogen-parallel) from northwest to southeast by meandering and low-sinuosity channel belt system. In contrast, during deposition of the Mukdadiya Formation (~7.75–5 Ma), sediments were delivered transversely (orogen-perpendicular) from northeast to southwest by braided and low-sinuosity channel belt system in distributive fluvial megafans. By ~5 Ma, the northwestern Zagros foreland basin became partitioned by growth of the Mountain Front Flexure and considerable gravel was introduced in localized alluvial fans derived from growing topographic highs. Foredeep accumulation rates during deposition of the Injana, Mukdadiya and Bai-Hasan Formations averaged 350, 400 and 600 m/Myr respectively, suggesting accelerated accommodation generation in a rapidly subsiding basin governed by flexural subsidence. Detrital zircon U-Pb age spectra show that in addition to sources of Mesozoic-Cenozoic cover strata, the Injana Formation was derived chiefly from Palaeozoic-Precambrian (including Carboniferous and latest Neoproterozoic) strata in an axial position to the northwest, likely from the Bitlis-Puturge Massif and broader Eastern Anatolia. In contrast, the Mukdadiya and Bai-Hasan Formations yield distinctive Palaeogene U-Pb age peaks, particularly in the southeastern sector of the study region, consistent with transverse delivery from the arc-related terranes of the Walash and Naopurdan volcano-sedimentary groups (Gaveh-Rud domain?) and Urumieh-Dokhtar magmatic arc to the northeast. These temporal and spatial variations in stratigraphic framework, depositional environments, sediment routing and compositional provenance reveal a major drainage reorganization during Neogene shortening in the Zagros fold-thrust belt. Whereas axial fluvial systems initially dominated the foreland basin during early orogenesis in the Kurdistan region of Iraq, transverse fluvial systems were subsequently established and delivered major sediment volumes to the foreland as a consequence of the abrupt deformation advance and associated topographic growth in the Zagros.  相似文献   

8.
Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km2) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., < 1 t km− 2 yr− 1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors.  相似文献   

9.
We analysed modern mass‐accumulation patterns on the western Adriatic mud wedge (Italy), an elongated belt of shelf mud formed by coalesced prodeltas of the Adige, Po, and Apennine rivers, as part of an integrated strategy aimed at producing a quantitative sediment budget model for muddy continental shelves sourced by multiple compositionally distinct fluvial systems. Sediment provenance and source‐specific accumulation rates of surface sediments were quantified by combining results of grain‐size analysis and geochemical analysis of specific size fractions with bulk mass accumulation rates. Statistical classification algorithms adapted to compositional data were used to partition the total (geochemical) variation of sediment properties into size‐related and provenance‐specific factors. We identified geochemically distinct fluvial end‐member sediment types in two different grain‐size fractions, which were grouped into sediments derived from the Apennine rivers, and sediments derived from the Po and Adige rivers. Compositional fingerprints (end‐member compositions) of each source area were estimated by taking into account relative rates of fluvial sediment supply from rivers as predicted by numerical modelling. The end members allow us to explain geochemical compositional variation of mud‐wedge surface sediments in terms of provenance and size‐selective dispersal, and map mass accumulation rates of sediments from individual source areas (grain size<63 μm), as well as bulk sand accumulation rates (grain size>63 μm) across the western Adriatic mud wedge. The source‐specific rates of fine‐grained sediment supply derived from geostatistical estimates of mass‐accumulation rates were used to calibrate the numerical model of sediment supply to present‐day conditions.  相似文献   

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

11.
The role, function, and importance of large woody debris (LWD) in rivers depend strongly on environmental context and land use history. The coastal watersheds of central and northern Maine, northeastern U.S., are characterized by low gradients, moderate topography, and minimal influence of mass wasting processes, along with a history of intensive commercial timber harvest. In spite of the ecological importance of these rivers, which contain the last wild populations of Atlantic salmon (Salmo salar) in the U.S., we know little about LWD distribution, dynamics, and function in these systems. We conducted a cross-basin analysis in seven coastal Maine watersheds, documenting the size, frequency, volume, position, and orientation of LWD, as well as the association between LWD, pool formation, and sediment storage. In conjunction with these LWD surveys, we conducted extensive riparian vegetation surveys. We observed very low LWD frequencies and volumes across the 60 km of rivers surveyed. Frequency of LWD ≥ 20 cm diameter ranged from 15–50 pieces km− 1 and wood volumes were commonly < 10–20 m3 km− 1. Moreover, most of this wood was located in the immediate low-flow channel zone, was oriented parallel to flow, and failed to span the stream channel. As a result, pool formation associated with LWD is generally lacking and < 20% of the wood was associated with sediment storage. Low LWD volumes are consistent with the relatively young riparian stands we observed, with the large majority of trees < 20 cm DBH. These results strongly reflect the legacy of intensive timber harvest and land clearing and suggest that the frequency and distribution of LWD may be considerably less than presettlement and/or future desired conditions.  相似文献   

12.
Megafan conglomerates of foreland basins chronicle the combined effect of palaeoclimate conditions, tectonic processes and the flux and granulometric composition of the supplied sediment. However, the architecture of these deposits is seldom uniquely compatible with a single driving force. This problem is illustrated here with a field‐based analysis of the ca. 30–20 Ma‐old Napf deposits in the north Alpine foreland basin which are coeval with a substantial global warming of ca. 6°C during the Late Oligocene. The observed larger grain sizes and a change in fluvial style from wandering to braided could be explained climatically by a shift to drier conditions with sparse vegetation, but would have resulted in less than 400 m of additional accommodation space during the 1 Ma duration of change. Accordingly, a climate scenario alone is also not compatible with rapid sediment accumulation rates of >1000 m Ma?1 recorded at Napf, or with a lack of any remarkable shifts in the Froude number, which would be expected if water discharge patterns changed substantially. Alternatively, flexural downwarping in response to a tectonic pulse could account for the increase in grain size and the change in fluvial style from wandering (more distal facies) to braided (proximal equivalent). However, a third driving force is required to explain the contemporaneous backstepping of the distal gravel front and progradation of the proximal braided facies. We suggest that the erosional hinterland steepened in response to an inferred tectonic pulse, resulting in a more widespread exposure of lithologies with higher erosional resistance, as inferred from an increasing contribution of crystalline constituents in the clast suites. Such a change would result in a larger D50 and a higher clast size variability in the supplied sediment, which in turn would contribute to the observed change from wandering to braided and the related shift in depositional systems. This study highlights the importance of tectonic processes and the role of changing surface lithologies in the source area for explaining variations in megafan construction even in the light of substantial palaeoclimate shift.  相似文献   

13.
Knowledge of fluvial sediment supply to the coastal sediment budget is important for the assessment of the impacts on coastal stability. Such knowledge is valuable for designing coastal engineering schemes and the development of shoreline management planning policies. It also facilitates understanding of the connection between rivers in the hinterland and adjoining coastal systems. Ghana's coast has many fluvial sediment sources and this paper provides the first quantitative assessments of their contributions to the coastal sediment budget. The methods use largely existing data and attempt to cover all of Ghana's significant coastal rivers. Initially work was hindered by insufficient direct measured data. However, the problem was overcome by the application of a regression approach, which provides an estimated sediment yield for non-gauged rivers based on data from gauged rivers with similar characteristics. The regression approach was effective because a regional coherence in behaviour was determined between those rivers, where direct measured data were available. The results of the assessment revealed that Ghana's coast is dissected by many south-draining rivers, stream and lagoons. These rivers, streams and lagoons supply significant amounts of sediment to coastal lowlands and therefore contribute importantly to beaches. Anthropogenic impoundment of fluvial sediment, especially the Akosombo dam on the Volta River, has reduced the total fluvial sediment input to the coast from about 71 × 106 m3/a before 1964 (pre-Akosombo dam) to about 7 × 106 m3/a at present (post-Akosombo dam). This sharp reduction threatened the stability of the east coast and prompted an expensive ($83 million) defence scheme to be implemented to protect 8.4 km-long coastline at Keta. Sections of Ghana's coast are closely connected to the hinterland through the fluvial sediment input from local rivers. Therefore, development in the hinterland that alters the fluvial sediment input from those local rivers could have significant effects on the coast. There is the need, therefore, to ensure that catchment management plans and coastal management plans are integrated or interconnected.  相似文献   

14.
《Basin Research》2018,30(3):544-563
Previous research demonstrates that large basins on the periphery of the northern edge of the Tibetan Plateau were partitioned during development of intrabasin mountain ranges. These topographic barriers segregated basins with respect to surface flow and atmospheric circulation, ponded sediments, and formed rain shadows. However, complex mixing between airmasses and nonsystematic isotope‐elevation lapse rates have hampered application of quantitative paleoaltimetry to determine the timing of development of critical topographic barriers. We address the timing and drivers for changes in surface connectivity and atmospheric circulation in the Linxia and Xunhua basins using a multidisciplinary approach incorporating detrital zircon geochronology, Monte Carlo inverse flexural modelling, and published stable isotope data. Disruption of surface flow between the two basins during exhumation of the Jishi Shan preceded development of topography sufficient to intercept moisture‐bearing airmasses. Detrital zircon data point to disruption of an eastward‐flowing axial fluvial network between 14.7 and 13.1 Ma, coincident with the onset of exhumation in the Jishi Shan. Flexural modelling suggests that by 13 Ma, the Jishi Shan had developed 0.3 ± 0.1 km of relief; sufficient to disrupt eastward‐flowing drainage networks but insufficient to intercept moisture‐bearing airmasses. Stable isotope data indicate that, although surface connections between the Xunhua and Linxia basins were broken, the two basins continued to be dominated by a common climate regime until 9.3 Ma. Subsequent reintegration of surface flow between the basins occurred between 9.3 and 7.6 Ma. Divergence in the stable isotope and depositional environment records between the two basins suggests that at 9.3 Ma the paleo‐Yellow River breached the growing Jishi Shan dam, and may have reintegrated surface flow between the two basins via erosion of the modern Yellow River gorge, which transects the Jishi Shan. The reintegration of the Xunhua and Linxia basins’ surface connection is confirmed by reintroduction of a Songpan‐Ganzi flysch sediment source by 7.6 Ma. Continued exhumation and uplift of the Jishi Shan developed 0.8 ± 0.2 km of relief by ca. 8 Ma capable of intercepting moisture‐bearing airmasses; isolating and increasing aridity in the Xunhua Basin while decreasing it in the Linxia Basin. Our findings point to protracted development of the modern ca. 1 km of relief in the Jishi Shan between 14 and ca. 4.5 Ma followed by attainment of a topographic equilibrium which persists into modern times.  相似文献   

15.
Basin‐wide correlation of Messinian units and Plio‐Quaternary chronostratigraphic markers (5.3 Ma, 2.6 Ma, 0.9 Ma and 0.45 Ma), the mapping of total sediment thickness and the determination of overall sedimentary volumes enabled us to provide a high‐resolution quantitative history of sediment volumes for the last 6 Ma along the Gulf of Lions margin. The results point to (i) a dramatic increase in terrigenous sediment input during the Messinian Salinity Crisis. This increased sedimentation reflects enhanced regional fluvial erosion related to the dramatic fall of Mediterranean base‐level. Stronger weathering due to a regional wetter climate probably also increased erosional fluxes. (ii) A sediment input three times higher during the Plio‐Quaternary compared to the Miocene seems in agreement with published measurements from World's ocean. However, the timing of this increase being uncertain, it implies that the trigger(s) also remain(s) uncertain. (iii) A decrease in detrital volume around 2.6 Ma is attributed to a regional change in the drainage pattern of rivers in the northwestern Alps. (iv) This study also highlights the Mid‐Pleistocene Revolution around 0.9 Ma, which resulted in an almost doubling of sediment input in the Provencal Basin.  相似文献   

16.
Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene. Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate) and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year) from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years, hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP.  相似文献   

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

18.
ABSTRACT Foreland basins form by lithospheric flexure under orogenic loading and are filled by surface transport of sediment. This work readdresses the interplay between these processes by integrating in a 3D numerical model: the mechanisms of thrust stacking, elastic flexural subsidence and sediment transport along the drainage network. The experiments show that both crustal tectonic deformation and vertical movements related to lithospheric flexure control and organise the basin-scale drainage pattern, competing with the nonlinear, unpredictable intrinsic nature of river network evolution. Drainage pattern characteristics are predicted that match those observed in many foreland basins, such as the axial drainage, the distal location of the main river within the basin, and the formation of large, long-lasting lacustrine systems. In areas where the river network is not well developed before the formation of the basin, these lithospheric flexural effects on drainage patterns may be enhanced by the role of the forebulge uplift as drainage divide. Inversely, fluvial transport modifies the flexural vertical movements differently than simpler transport models (e.g. diffusion): Rivers can drive erosion products far from a filled basin, amplifying the erosional rebound of both orogen and basin. The evolution of the sediment budget between orogen and basin is strongly dependent on this coupling between flexure and fluvial transport: Maximum sediment accumulations on the foreland are predicted for a narrow range of lithospheric elastic thickness between 15 and 40 km, coinciding with the T e values most commonly reported for foreland basins.  相似文献   

19.
Sediment is fractionated by size during its cascade from source to sink in sediment routing systems. It is anticipated, therefore, that the grain size distribution of sediment will undergo down‐system changes as a result of fluvial sorting processes and selective deposition. We assess this hypothesis by comparing grain size statistical properties of samples from within the erosional source region with those that have undergone different amounts of transport. A truncated Pareto distribution describes well the coarser half of the clast size distribution of regolith, coarse channel bed sediment and proximal debris flows (particularly their levees), as well as the coarser half of the clast size distribution of gravels that have undergone considerable amounts of transport in rivers. The Pareto shape parameter a evolves in response to mobilization, sediment transport and, importantly, the selective extraction of particles from the surface flow to build underlying stratigraphy. A goodness of fit statistic, the Kolmogorov–Smirnov vertical difference, illustrates the closeness of the observed clast size distributions to the Pareto, Weibull and log‐normal models as a function of distance from the depositional apex. The goodness of fit of the particle size distribution of regolith varies with bedrock geology. Bedload sediment at catchment outlets is fitted well by the log‐normal and truncated Pareto models, whereas the exponential Weibull model provides a less good fit. In the Eocene Escanilla palaeo‐sediment routing system of the south‐central Pyrenees, the log‐normal and truncated Pareto models provide excellent fits for distances of up to 80 km from the depositional apex, whereas the Weibull fit progressively worsens with increasing transport distance. A similar trend is found in the Miocene–Pliocene gravels of the Nebraskan Great Plains over a distance of >300 km. Despite the large fractionation in mean grain size and gravel percentage from source region to depositional sink, particle size distributions therefore appear to maintain log‐normality over a wide range of transport distance. Use of statistical models enables down‐system fractionation of sediment released from source regions to be better understood and predicted and is a potentially valuable tool in source‐to‐sink approaches to basin analysis.  相似文献   

20.
ABSTRACT Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland‐basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800–22 600 km2), fan‐shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low‐relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid‐channel and bank‐attached sand bars. Overbank areas are characterized by crevasse‐splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long‐term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening‐upward, > 2‐km‐thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold‐thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid‐Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse‐splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold‐thrust belt. In addition, the present‐day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid‐Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold‐thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 104 km2– a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold‐thrust belt.  相似文献   

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