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1.
Abstract The Devono-Carboniferous Horton Group of Cape Breton Island was mostly deposited in two fault-bounded asymmetric sub-basins which were part of a large intracontinental rift system. This system lay at a palaeolatitude of about 10–15o S–a warm, semi-arid climate. The half-graben sub-basins had opposed polarity, were approximately 100 times 50 km in size and were separated by a narrow zone of elevated Acadian basement. These features are common to the adjacent structural segments of known rifts, and are unlike those of transtensive pull-apart systems. Sedimentation occurred in four successive depositional systems which reflect a tectonic evolution of increased and then decreased extensional subsidence through the 8–12 Myr interval represented. Post-Acadian sedimentation began with System 1 bimodal volcanics and grey distal braided fluvial sediments deposited in a slowly subsiding broad linear sag basin. System 2 consists of reddened braidplain sediments near fault-bounded margins and mudflat/playa sediments in sub-basin centres, deposited in two discrete asymmetric sub-basins with a general upward-fining trend. Gradual expansion of the mudflat setting and confinement of coarse marginal fades is interpreted as a response to increasingly rapid and deep fault-bounded subsidence. Depositional System 3, is a complex of grey lacustrine offshore, shoreline and fan delta facies deposited in two adjacent half-graben segments with opposed polarity of asymmetry. An increased rate of tectonic subsidence allowed a large standing body of water to accumulate lacustrine sediments along the axis of each sub-basin during this phase of maximum subsidence. System 4 consists of reddened proximal alluvial fan, medial fluvial and distal grey meandering fluvial/floodplain sediments which accumulated in sub-basins with fault-bounded margins and asymmetry identical to those of earlier systems, indicating a continuation of tectonic style. However, an overall coarsening-upward trend indicates waning of active fault-related subsidence and consequent progradation of marginal coarse wedges to fill the sub-basins. Rapid marine transgression and deposition of Windsor Group carbonates, evaporites and elastics continued within a more extensive rift basin during renewed fault-bounded subsidence.  相似文献   

2.
In order to better understand the evolution of rift‐related topography and sedimentation, we present the results of a numerical modelling study in which elevation changes generated by extensional fault propagation, interaction and linkage are used to drive a landscape evolution model. Drainage network development, landsliding and sediment accumulation in response to faulting are calculated using CASCADE, a numerical model developed by Braun and Sambridge, and the results are compared with field examples. We first show theoretically how the ‘fluvial length scale’, Lf, in the fluvial incision algorithm can be related to the erodibility of the substrate and can be varied to mimic a range of river behaviour between detachment‐limited (DL) and transport‐limited (TL) end‐member models for river incision. We also present new hydraulic geometry data from an extensional setting which show that channel width does not scale with drainage area where a channel incises through an area of active footwall uplift. We include this information in the coupled model, initially for a single value of Lf, and use it to demonstrate how fault interaction controls the location of the main drainage divide and thus the size of the footwall catchments that develop along an evolving basin‐bounding normal fault. We show how erosion by landsliding and fluvial incision varies as the footwall area grows and quantify the volume, source area, and timing of sediment input to the hanging‐wall basin through time. We also demonstrate how fault growth imposes a geometrical control on the scaling of river discharge with downstream distance within the footwall catchments, thus influencing the incision rate of rivers that drain into the hanging‐wall basin. Whether these rivers continue to flow into the basin after the basin‐bounding fault becomes fully linked strongly depends on the value of Lf. We show that such rivers are more likely to maintain their course if they are close to the TL end member (small Lf); as a river becomes progressively more under supplied, i.e. the DL end member (large Lf), it is more likely to be deflected or dammed by the growing fault. These model results are compared quantitatively with real drainage networks from mainland Greece, the Italian Apennines and eastern California. Finally, we infer the calibre of sediments entering the hanging‐wall basin by integrating measurements of erosion rate across the growing footwall with the variation in surface processes in space and time. Combining this information with the observed structural control of sediment entry points into individual hanging‐wall depocentres we develop a greater understanding of facies changes associated with the rift‐initiation to rift‐climax transition previously recognised in syn‐rift stratigraphy.  相似文献   

3.
Miocene strata of the Shadow Valley Basin rest unconformably on the upper plate of the Kingston Range - Halloran Hills detachment fault system in the eastern Mojave desert, California. Basin development occurred in two broad phases that we interpret as a response to changes in footwall geometry. In southern portions of the basin, south of the Kingston Range, phase one began with near synchronous initiation of detachment faulting, volcanism and basin sedimentation shortly after 13.4 Ma. Between c. 13.4 and c. 10 Ma, concordantly bedded phase one strata were deposited onto the subsiding hangingwall of the detachment fault as it was translated 5–9 km south-westward with only limited internal deformation. Phase two (c. 10 to 8–5 Ma) is marked by extensional dismemberment of the detachment fault's upper plate along predominantly west-dipping normal faults. Phase two sediments were deposited synchronously with upper-plate normal faulting and unconformably overlie phase one deposits, displaying progressive shallowing in dip and intraformational onlap. Northern portions of the basin, in the Kingston Range, experienced a similar two-phase development compressed into a shorter interval of time. Here, phase one occurred between c. 13.4 and 12.8–12.5 (?) Ma, whereas phase two probably lasted for no more than a few 100000 years immediately prior to c. 12.4 Ma. Differences in the duration of basin development in and south of the Kingston Range apparently relate to position with respect to the detachment fault's breakaway; northern basin exposures overlie the upper plate adjacent to the breakaway (0–15 km) whereas southern basin exposures occur far from the breakaway (20–40 km). We interpret the phase one to phase two transition as recording breakup of the detachment fault's hangingwall during footwall uplift. We propose a model for supradetachment basin evolution in which early, concordantly bedded basin strata are deposited on the hangingwall as it translates intact above a weakly deforming footwall. With continuing extension, tectonic denudation along the detachment fault leads to an increasing flexural isostatic footwall response. We suggest that isostatic footwall uplift may drive internal breakup of the upper plate as the detachment fault is rotated to a shallow dip, mechanically unfavourable for simple upper-plate translation. Additionally, we argue that continuing hangingwall thinning during phase two places geometrical constraints on the timing, amount and, thus, rate of footwall uplift. Kinematically determined footwall uplift rates (0.5–4.5 mm/yr) are comparable with rates determined independently by thermochronological and geobarometric methods.  相似文献   

4.
Our study explores the geohydraulic history of the Acre retroarc foreland basin by gathering both spatial and temporal information from the upper 400 m of sediments. We also inquire into controls on sediment accommodation space as well as on stream vs. lacustrine domination. The Acre basin is located in south-west Amazonia, proximal to the Serra do Divisor which demarcates the eastern edge of the Andean fold–thrust belt. Radiocarbon ages from a range of materials indicate that the upper 50–250 m of the Solimôes Formation accumulated during the past 50 000 years. Both surficial and drill-core sediment records show lacustrine–fluvial transitions throughout the Late Quaternary. These shifts in depositional environments are in response to episodic changes in hydrological conditions as well as to geodynamic activity, such as subsidence. Juxtaposition of lacustrine and fluvial systems in the vertical Acre basin record mimics the regional-scale trends in the modern, upper and middle Solimôes–Amazon floodplains. In the Acre basin record lacustrine successions are characterized by increasing calcium contents up-section. This is also manifested, in the upper portions of lacustrine sequences outcropping at the surface, as alternating clastic and calcareous layers. The up-section increase in carbonate content is related to increasing salinities brought about by drier hydrodynamic conditions. Desiccation cracks are typically infilled with gypsum as are cavities of fossils in bone-beds. The latter represent isolated ponds in which the original fauna died as aridity intensified and waters became increasingly saline. Modern trunk river systems in the Acre basin flow from south-west to north-east with tributaries entering from the south-west, suggesting the influence of a domino-style, basement, fault regime. Fault or, at least, fracture control on stream channels is also suggested throughout the greater Amazon basin in the orthogonal dispositions and asymmetric terrace systems of trunk rivers as well as of major tributaries.  相似文献   

5.
Paleolimnological investigations of a marginal lake in the Lake Michigan basin revealed signals of long-term lake-level changes primarily controlled by climatic forces. Multiple analyses identified concurrent signals in sediment chemistry, grain size, and the microfossil record. Coarse-grained sediments, benthic diatoms, and nutrient response species increased as lake levels rose or fell. Finer sediments and higher percentages of taxa associated with stable thermocline conditions occurred during high-lake periods. Sedimentary evidence revealed corresponding strong high-lake signals c. 2500–2200, 1800–1500, 1170–730, and 500–280 BP. Low-lake periods occurred c. 1500–1170 and 700–500 B.P. An additional signal of lake-level decline was apparent beginning c. 280 BP but was interrupted by anthropogenic effects. Evidence of extreme low-lake levels (c. 1400–1300 BP), and signals for a medieval warming period (1030–910 BP) and the Maunder minimum (370–325 BP) indicate occurrence of short-lived dry climatic conditions.  相似文献   

6.
Swath bathymetry, single‐channel seismic profiling, gravity and box coring, 210Pb down‐core radiochemical analyses and sequence stratigraphic analysis in the Gulf of Alkyonides yielded new data on the evolution of the easternmost part of the Gulf of Corinth. Three fault segments, the South Strava, West Alkyonides and East Alkyonides faults, dipping 45, 30 and 45°, respectively, northwards, form the southern tectonic boundary of the Alkyonides Basin. Two 45° southwards dipping segments, the Domvrena and Germeno Faults, form the northern tectonic margin. The Alkyonides Basin architecture is the result of a complex interaction between fault dynamics and the effects of changes in climate and sea/lake level. Chrono‐stratigraphic interpretation of the seismic stratigraphy through correlation of the successive seismic packages with lowstands and highstands of the Late Quaternary indicates that the evolution of the basin started 0.40–0.45 Ma BP and can be divided in two stages. Subsidence of the basin floor during the early stage was uniform across the basin and the mean sedimentation rate was 1.0 m kyear?1. Vertical slip acceleration on the southern tectonic margin since 0.13 Ma BP resulted in the present asymmetric character of the basin. Subsidence concentrated close to the southern margin and sedimentation rate increased to 1.4 m kyear?1 in the newly formed depocentre of the basin. Actual (last 100 year) sedimentation rates were calculated to >2 mm year?1, but are significantly influenced by the presence of episodic gravity flow deposits. Total vertical displacement of 1.1 km is estimated between the subsiding Alkyonides Basin floor and the uplifting Megara Basin since the onset of basin subsidence at a mean rate of 2.4–2.75 m kyear?1, recorded on the East Alkyonides Fault. Gravity coring in the Strava Graben and in the lower northern margin of Alkyonides Basin proved the presence of whitish to olive grey laminated mud below thin marine sediments. Aragonite crystals and absence of the marine coccolithophora Emiliania huxleyi indicate sedimentation in lacustrine environment during the last lowstand glacial interval.  相似文献   

7.
Located at approximately 150 m above the present base level, the caves of Niaux, Lombrives and Sabart are an old drainage system, which worked between the Vicdessos and Ariège Valleys. In these caves, three successive sedimentary units were studied in detail. Each unit consists of two parts: the lower deposit is detrital, of fluvial origin, and testifies to a hydrological working of the conduit system; the upper deposit of each unit is mostly speleothems, corresponding to a draining of the conduits as the palcokarst evolved above base level. This is similar to the present environment of the caves.

Only the speleothems from the upper part of each unit were dated by the 230Th/234 U method. The dates, from 27 samples, are in good agreement with Europe and North America data. The data from the middle (250 to 200 ka BP) and upper (90 to 20 ka BP) fluvial sediments correspond to major glacial events, which re-activated the karst system.

Two further events resulted in a lack of carbonated sedimentation, the first between 350 and 290 ka BP, the second between 175 and 130 ka BP. Comparing the data from other places, these two events can be related to glacial periods of minor importance in the study area, because of the lack of erosion and of detrital sediments.

The oldest detrital sediments are covered by speleothems older than 350 ka BP and, partly, older than 720 ka BP (from paleomagnctic data to be confirmed); from sedimentary data, they may not be related to a glacial event. They are perhaps contemporary with the area's initial cave formation.  相似文献   

8.
Abstract In the Latnjavagge drainage basin (68°21′N, 18°29′E), an arctic‐oceanic periglacial environment in northernmost Swedish Lapland, the fluvial sediment transport and the characteristics and importance of high‐magnitude/low‐frequency fluvial events generated by intense snowmelt or heavy rainfall have been investigated and compared with snowmelt‐ and rainfall‐induced discharge peaks in the Levinson‐Lessing Lake basin (Krasnaya river system) on the Taimyr Peninsula, an arctic periglacial environment in northern Siberia (74°32′N, 98°35′E). In Latnjavagge (9 km2) the intensity of fluvial sediment transport is very low. Most of the total annual sediment load is transported in a few days during snowmelt generated runoff peaks. Due to the continuous and very stable vegetation covering most areas below 1300 m a.s.l. in the Latnjavagge catchment, larger rainfall events are of limited importance for sediment transport in this environment. Compared to that, in the c. 40 times larger Krasnaya riversystem rainfall‐generated runoff peaks cause significant sediment transport. The main sediment sources in the Latnjavagge drainage basin are permanent ice patches, channel debris pavements mobilized during peak discharges and exposing fines, and material mobilized by slush‐flows. In the Krasnaya river system river bank erosion is the main sediment source. In both periglacial environments more than 90% of the annual sediment yield is transported during runoff peaks. The results from both arctic periglacial environments underline the high importance of high‐magnitude/low‐frequency fluvial events for the total fluvial sediment budgets of periglacial fluvial systems. Restricted sediment availability is in both arctic environments the major controlling factor for this behaviour.  相似文献   

9.
The valley‐fill sequence of Nowlands Creek, a 5.5 km2 basin in the Central Lowlands of the Hunter Valley, is characterised by three inset river terraces whose sediments contain either Aboriginal or European artefacts. The highest and oldest terrace is characterised by a well‐developed yellow duplex soil with Aboriginal artefacts in the A horizon. Deposition of the bulk of the terrace sediments occurred before 11 400 yrs BP when Nowlands Creek was a low‐sinuosity, high‐energy, gravel‐bed stream. Texture contrast of the terrace soil is due partly to the superpositioning of Holocene sandy colluvium over Pleistocene fluviatile clay. The middle terrace is characterised by a deep minimal prairie soil containing Aboriginal artefacts. Deposition of the middle‐terrace sediments occurred mainly after 11 400 yrs BP when Nowlands Creek was a small‐capacity, mud‐bed channel with chains of ponds and well‐vegetated banks. The lowest terrace occupies a discontinuous trench incised into the middle terrace and was abandoned by incision between 1902 and 1938 due to open‐cut coal mining. All Aboriginal artefacts found so far are confined to Holocene sediments.  相似文献   

10.
A geological feature in the Qaidam Basin known as the “Shell Bar” contains millions of freshwater clam shells buried in situ. Since the 1980s, this feature in the now hyper-arid basin has been interpreted to be lake deposits that provide evidence for a warmer and more humid climate than present during late marine isotope stage 3 (MIS 3). Global climate during late MIS 3 and the last glacial maximum, however, was cold and dry, with much lower sea levels. We re-investigated the feature geomorphologically and sedimentologically, and employed optically stimulated luminescence (OSL) dating to verify the chronology of the sediments. We interpret the Shell Bar to be a remnant of a river channel formed by a stream that ran across an exposed lake bed during a regressive lake phase. Deflation of the surrounding older, fine-grained lacustrine deposits has left the fluvial channel sediments topographically inverted, indicating the erosive nature of the landscape. Luminescence ages place the formation of the Shell Bar in MIS 5 (~113–99 ka), much older than previous radiocarbon ages of <40 ka BP, but place the paleoclimatic inferences more in accord with other regional and global climate proxy records. We present a brief review of the age differences derived from 14C and OSL dating of some critical sections that were thought to represent a warmer and more humid climate than present during late MIS 3. We attribute the differences to underestimation of 14C ages. We suggest that 14C ages older than ~25 ka BP may require re-investigation, especially dates on samples from arid regions.  相似文献   

11.
The continuous Cenozoic strata in the Xining Basin record the growth and evolution of the northeastern Qinghai–Tibetan Plateau. Here, the mechanisms and evolution of the Xining Basin during the Cenozoic were investigated by studying the sedimentary facies of 22 Cenozoic sections across the basin and detrital zircon U‐Pb ages of three Cenozoic sections located in the eastern, central and western basin, respectively. In the Eocene (ca. 50–44 Ma), the India‐Eurasia Collision affected the northeastern Qinghai–Tibetan Plateau. The Central Qilian Block rotated clockwise by ca. 24° to form the Xining Basin. The Triassic flysch sediments surrounding the basin were the primary sources of sediment. Between ca. 44–40 Ma, the basin enlarged and deepened, and sedimentation was dominated by saline lake sediments. Between ca. 40–25.5 Ma, the Xining Basin began to shrink and dry, resulting in the deposition of saline pan and saline mudflat sediments in the basin. After ca. 20 Ma, the Laji Shan to the south of the Xining Basin was uplifted due to the northward compression of the Guide Basin to the south. Clasts that eroded from this range dominated the sediments as the basin evolved from a lacustrine environment into a fluvial system. The Xining Basin was an extensional basin in the Early Cenozoic, but changed into a compressive one during the Late Cenozoic, it was not a foreland basin either to the Kunlun Shan or to the western Qinling Shan in the whole Cenozoic. The formation and deformation of the Xining Basin are the direct responses of the India‐Eurasia Collision and the growth of the Qinghai‐Tibetan Plateau.  相似文献   

12.
In arid and semiarid regions of Central West Argentina, paleoenvironmental conditions and history of the Holocene vegetation is still scanty. This paper presents the analysis of palynological records of two Mid- to Late Holocene alluvial sequences (La Escala -LES- and Brazo Abandonado -BA-) in the fluvial basin of the Arroyo La Estacada (33°27′S and 69°03′W), placed in the Cordillera Frontal central piedmont of Mendoza province. Variations in pollen assemblages of the two sequences related to the environments developed in the floodplains of the arroyo allowed us to infer the evolution of local plant communities. Between the ca. 4000 and 3000 14C years BP and between ca. 500 and 400 14C years BP, a larger representation of hydrophytic communities related to environments with local water availability was inferred; and between ca. 3000 and 500 14C years BP an increase in shrubby xerophytic communities related to environments with lower water availability was detected. These results led us to propose that the Holocene pollen spectra fluctuations observed in the alluvial sedimentary sequences prove mainly changes in plant communities of the floodplain environment in response to the fluvial dynamics of the arroyo.  相似文献   

13.
The Late Glacial and Holocene geomorphology of the Manx uplands has received scant attention in previous researches. Solifluction deposits and terraces provide the earliest evidence for geomorphic activity after deglaciation. Fluvial incision into drift-choked valleys is correlated with the formation of the large mountain front alluvial fans that flank the Manx uplands. Formation of these alluvial fans is constrained to 15,000–10,500 cal. years BP by 14C dates on organic deposits beneath and above the alluvial fan gravels. Alluvial fan and river terraces along four valleys postdate this incision. Optically Stimulated Luminescence (OSL) and 14C dating provide a tentative chronology for these landforms. The higher terraces are Late Glacial fluvial surfaces that were probably occupied by rivers into the Holocene. Incision during the Late Holocene led to the abandonment of the higher surfaces, producing a suite of younger river terraces and alluvial fan surfaces. Independent dating constrains this fluvial activity to post-Bronze Age (3500–2800 cal. years BP). Increased human activity and climatic change during the Late Holocene are possible causes for this increased geomorphic activity.  相似文献   

14.
Stratigraphic pollen analysis done on sediment cores from two sites in the upper North Saskatchewan drainage basin of the eastern slopes foothills of the Rocky Mountains in west central Alberta, Canada combined with sedimentological data provide a local vegetational and environmental history. Radiocarbon AMS dates provide a chronology back to 17960 BP. Reconstruction and interpretation of the local pollen zones includes reevaluation of steppe and grassland as analogs for full- and late-glacial vegetation. Regional vegetation from c. 17960 to 16 100 BP is interpreted as an extremely cold semi-arid Artemisia steppe, the vegetation c. 16 100 to 11 900 BP as an Artemisia-Betula shrubland, and the vegetation c. 11 900–10 200 BP as a Picea woodland, in an environment characterized by consistently arid and windy conditions. This reconstruction emphasizes the significance of aridity, as opposed to simply low temperatures, as the critical factor in determining the late Quaternary vegetation of Alberta.This is the 18th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.  相似文献   

15.
《Basin Research》2018,30(1):59-74
It is crucial to understand lateral differences in paleoclimate and weathering in order to fully understand the evolution of the Himalayan mountain belt. While many studies have focused on the western and central Himalaya, the eastern Himalaya remains poorly studied with regard to paleoclimate and past weathering history. Here, we present a multi‐proxy study on the Mio‐Pliocene sedimentary foreland‐basin section along the Kameng River in Arunachal Pradesh, northeast India, in order to obtain better insight in the weathering history of the eastern Himalaya. We analysed a continuous sedimentary record over the last 13 Ma. Heavy‐mineral and petrography data give insight into diagenesis and provenance, showing that the older part of the section is influenced by diagenesis and that sediments were not only deposited by a large Trans‐Himalayan river and the palaeo‐Kameng river, but also by smaller local tributaries. By taking into account changes in diagenesis and provenance, results of clay mineralogy and major element analysis show an overall increase in weathering intensity over time, with a remarkable change between ca. 10 and ca. 8 Ma.  相似文献   

16.
Bowser Lake, a fiord lake in the northern Coast Mountains of British Columbia, contains a thick Holocene fill consisting mainly of silt and clay varves. These sediments were carried into the lake by proglacial Bowser River which drains a high-energy, heavily glacierized basin. Sedimentation in the lake is controlled by seasonal snow and ice melt, by autumn rainstorms, and by rare, but very large jökulhlaups from glacier-dammed lakes in the upper Bowser River basin which complicate environmental inferences from the sedimentary record. Sediment is dispersed through the deep western part of the lake by energetic turbidity currents. The turbidity currents apparently do not overtop a sill that separates the western basin from much shallower areas to the east. Large amounts of silt and clay are deposited from suspension in the eastern part of the lake, but sediment accumulation rates there are much lower than to the west. Several strong acoustic reflectors punctuate the varved fill in the western basin; these may be thick or relatively coarse beds deposited during jökulhlaups or exceptionally large storms. The contemporary sediment yield to Bowser Lake, estimated from sediments in the lake, is about 360 t km-2a-1. This is a relatively high value, but it is less than yields insome other, similar montane basins with extensive snow and ice cover.The most likely explanation for the difference is that large amounts of sediment have been, and continue to be, stored on the Bowser delta andin small proglacial lakes.  相似文献   

17.
The Southern Alps of New Zealand are the topographic expression of active oblique continental convergence of the Australian and Pacific plates. Despite inferred high rates of tectonic and climatic forcing, the pattern of differential uplift and erosion remains uncertain. We use a 25-m DEM to conduct a regional-scale relief analysis of a 250-km long strip of the western Southern Alps (WSA). We present a preliminary map of regional erosion and denudation by overlaying mean basin relief, a modelled stream-power erosion index, river incision rates, historic landslide denudation rates, and landslide density. The interplay between strong tectonic and climatic forcing has led to relief production that locally attains 2 km in major catchments, with mean values of 0.65–0.68 km. Interpolation between elevations of major catchment divides indicates potential removal of l01–103 km3, or a mean basin relief of 0.51–0.85 km in the larger catchments. Local relief and inferred river incision rates into bedrock are highest about 50–67% of the distance between the Alpine fault and the main divide. The mean regional relief variability is ± 0.5 km.Local relief, valley cross-sectional area, and catchment width correlate moderately with catchment area, and also reach maximum values between the range front and the divide. Hypsometric integrals show scale dependence, and together with hypsometric curves, are insufficient to clearly differentiate between glacial and fluvial dominated basins. Mean slope angle in the WSA (ψ = 30°) is lower where major longitudinal valleys and extensive ice cover occur, and may be an insensitive measure of regional relief. Modal slope angle is strikingly uniform throughout the WSA (φ = 38–40°), and may record adjustment to runoff and landsliding. Both ψ and φ show non-linear relationships with elevation, which we attribute to dominant geomorphic process domains, such as fluvial processes in low-altitude valley trains, surface runoff and frequent landsliding on montane hillslopes, “relief dampening” by glaciers, and rock fall/avalanching on steep main-divide slopes.  相似文献   

18.
An unusually thick and laterally persistent fluvial sand body crops out at the Paleocene–Eocene boundary within the northern part of the Bighorn Basin in northwest Wyoming, USA. The generation of this ‘Boundary Sandstone’ was previously ascribed to a period of reduced subsidence; however, a new carbon isotope record presented herein shows it to be intimately correlated to the Paleocene–Eocene Thermal Maximum (PETM), an extreme global warming event ca. 56 Ma. This study evaluates the impacts of the PETM on fluvial deposition in the basin by integrating sedimentological data with geochemical, palaeoichnological, and palaeobotanical proxy records. Compared to pre‐ and post‐PETM fluvial sand bodies, the Boundary Sandstone is more highly amalgamated, both vertically and laterally, but shows no changes in lithofacies associations, palaeodispersal directions, palaeoflow depths, or palaeochannel widths. At its thickest, the Boundary Sandstone resides entirely within the main body of the PETM, an ca. 113 kyr time interval when global pCO2 levels and temperatures were at their highest, and local mean annual rainfall low, floodplains well drained and vegetation comparatively sparse. The totality of data sets imply that the Boundary Sandstone is related to the preferential removal of fine‐grained floodplain deposits by either: (i) rapid readjustments in river gradients related to documented short‐term precipitation oscillations or (ii) reductions in the cohesiveness of overbank sediments related to decreased rooting density and water table fluctuations. Hence, short‐term climate perturbations may manifest within large‐scale depositional patterns in ways ostensibly like tectonics.  相似文献   

19.
Analyses of lithology, organic-matter content, magnetic susceptibility, and pollen in a sediment core from Okpilak Lake, located in the northeastern Brooks Range, provide new insights into the history of climate, landscape processes, and vegetation in northern Alaska since 14,500?cal?year BP. The late-glacial interval (>11,600?cal?year BP) featured sparse vegetation cover and the erosion of minerogenic sediment into the lake from nearby hillslopes, as evidenced by Cyperaceae-dominated pollen assemblages and high magnetic susceptibility (MS) values. Betula expanded in the early Holocene (11,600?C8,500?cal?year BP), reducing mass wasting on the landscape, as reflected by lower MS. Holocene sediments contain a series of silt- and clay-dominated layers, and given their physical characteristics and the topographic setting of the lake on the braided outwash plain of the Okpilak River, the inorganic layers are interpreted as rapidly deposited fluvial sediments, likely associated with intervals of river aggradation, changes in channel planform, and periodic overbank flow via a channel that connects the river and lake. The episodes of fluvial dynamics and aggradation appear to have been related to regional environmental variability, including a period of glacial retreat during the early Holocene, as well as glacial advances in the middle Holocene (5,500?C5,200?cal?year BP) and during the Little Ice Age (500?C400?cal?year BP). The rapid deposition of multiple inorganic layers during the early Holocene, including thick layers at 10,900?C10,000 and 9,400?C9,200?cal?year BP, suggests that it was a particularly dynamic interval of fluvial activity and landscape change.  相似文献   

20.
Frequent and serious aeolian disasters occur in the upper and middle reaches of the Yarlung Zangbo River, which runs through the high-elevation Tibet Plateau. Sediment geochemical characteristics can be used as a proxy to identify the sediment's provenance. To determine the provenance of aeolian sediments in the river's basin, we analysed major and trace element contents from surface samples and local clastic rocks throughout the basin. We found that the major and trace elements differed between the middle reaches, upper reaches and regions south of the river. Major element contents were similar in the upper and middle reaches, but trace elements differed. Al2O3, MgO and Na2O concentrations increased from the upper reaches to the lower reaches, and in the lower reaches, MgO and Na2O were enriched compared with the crustal average. The similarities between samples in the lower part of the upper reaches and those in the middle reaches indicated that sediment transported by fluvial systems from the upper reaches were first deposited in the wide valleys near Xigaze, where they formed a large area of shifting sand. These deposits were then transported by the wind to the river's middle reaches, where they formed a large area of shifting sand. When we compared aeolian sediment in the middle reaches with the local clastic rocks, they appeared to be unrelated. The difference between sediments south of the river and those in the middle reaches means that the southern sediments were not transported to the middle reaches. Therefore, the aeolian sediment in the middle reaches of the Yarlung Zangbo River mainly came from the lower part of the upper reaches, not from the local clastic rocks.  相似文献   

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