Late Quaternary turbidity current pathways to the Madeira Abyssal Plain and some constraints on turbidity current mechanisms

Turbidites deposited in the Madeira Abyssal Plain during the last 200 000 yr originated mainly from the flanks of the Canary Islands and from the Northwest African continental margin north of the Canaries. Derivation of these turbidites from sources to the east of the abyssal plain apparently contradicts flow direction indicators derived from the sediments on the plain, which indicate derivation from the north-east. However, two systems of shallow channels, mapped using side-scan sonar and 3.5-kHz data, link the easterly sediment sources to the north-eastern edge of the abyssal plain, reconciling the apparently contradictory flow direction data. A northern system originates in the area between Madeira and the Canary Islands and follows a westerly and then north-westerly path, in part cutting obliquely across regional bathymetric trends. It carries sediment from the African continental margin north of the Canary Islands, and from the eastern Canaries, to the north-eastern abyssal plain. The southern channel system carries material from the western Canary Islands more directly westward to the central part of the plain. The pathways of individual turbidites can be reconstructed in some detail, by combining channel mapping with published information on turbidite provenance and flow directions on the Madeira Abyssal Plain. Interaction between turbidity currents and channel morphology controls turbidite depositional patterns. Small turbidites are completely contained within channels 20 m deep and 2 km wide. It is proposed that these are relatively high-density flows which have evolved in crossing the almost flat floor of a basin south-east of Madeira before entering the channel system. Larger turbidites show evidence of flow stripping where they interact with channels, with the result that their coarse and fine fractions follow different paths to and across the abyssal plain.     

Inferring denudation variations from the sediment record; an example of the last glacial cycle record of the Golo Basin and watershed,East Corsica,western Mediterranean sea

Geophysical data and sampling of the Golo Basin (East Corsica margin) provide the opportunity to study mass balance in a single drainage system over the last 130 kyr, by comparing deposited sediments in the sink and the maximum eroded volume in the source using total denudation proxies. Evaluation of the solid sediments deposited offshore and careful integration of uncertainties from the age model and physical properties allow us to constrain three periods of sedimentation during the last climatic cycle. The peak of sedimentation initiated during Marine Isotopic Stage (MIS) 3 (ca. 45 ka) and lasted until late in MIS 2 (ca. 18 ka). This correlates with Mediterranean Sea palaeoclimatic records and the glaciation in high altitude Corsica. The yield of solid sediment into the Golo Basin drops in the observed present day Mediterranean basins (gauging stations), whereas the palaeo‐denudation estimate derived from the sediments over the last glacial period is one to ten times higher than that predicted using cosmogenic or thermochronometer estimates of exhumation. The catchment‐wide denudation rate calculated from deposited solid sediment ranges from 47 to 219 mm kyr^?1, which is higher than the estimate from palaeosurface ablation in the proximal part of the source (9–140 mm kyr^?1) and lower than the distal, narrow, incised channel of the Golo River (160–475 mm kyr^?1). This mismatch raises questions about the investigation of denudation at millennial‐time scale (kyr) and at higher integrating times (Myr) as a reliable tool for determining the effect of climate change on mountain building and on sedimentary basin models.     

Late Quaternary climates and limnology of the Lake Winnebago basin, Wisconsin, based on ostracodes

Ostracodes document a series of late Quaternary climatic and limnologic changes within the Lake Winnebago basin of east-central Wisconsin. Using a ¹⁴ C, ¹³⁷Cs, and ²¹⁰Pb-based geochronology, Lake Winnebago ostracode abundances were compared to regional patterns of ostracode biogeography and the paleontological, sedimentological, and geochemical records of Elk Lake (Clearwater County), Minnesota, in order to interpret past temperature and hydrochemical changes in Lake Winnebago. Lake Winnebago sediments contain five major ostracode species, Candona ohioensis, Candona rawsoni, Cytherissa lacustris, Limnocythere verrucosa, and Physocypria pustulosa. In combination with sedimentology and geochronology, variations in the abundances of these species allow the late Quaternary record of the Lake Winnebago basin to be subdivided into five major climatic intervals: (1) glacial to post-glacial (15.5–11.0 ka), (2) cold and variable immediate post-glacial (11.0–10.4 ka), (3) warmer and wet early Holocene (10.4–7.8 ka), (4) warm but not particularly dry middle Holocene (7.8–4.2 ka), and (5) warm and moist late Holocene (4.2 ka-present).     

Luminescence chronology of incision and channel pattern changes in the River Ganga, India

The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T₂). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T₁) and the present-day flood plain (terrace-T₀). Terrace-T₁ shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T₀ and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T₂ show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.     

Holocene Brahmaputra River path selection and variable sediment bypass as indicators of fluctuating hydrologic and climate conditions in Sylhet Basin,Bangladesh

The Holocene stratigraphy of Sylhet basin, a tectonically influenced sub‐basin within the Ganges‐Brahmaputra‐Meghna delta (GMBD), provides evidence for autogenic and allogenic controls on fluvial system behaviour. Using Holocene lithology and stratigraphic architecture from a dense borehole network, patterns of bypass‐dominated and extraction‐enhanced modes of sediment transport and deposition have been reconstructed. During a ~3‐kyr mid‐Holocene occupation of Sylhet basin by the Brahmaputra River, water and sediment were initially (~7.5–6.0 ka) routed along the basin's western margin, where limited downstream facies changes reflect minimal mass extraction and bypass‐dominated transport to the basin outlet. Sediment‐dispersal patterns became increasingly depositional ~6.0–5.5 ka with the activation of a large (~2250 km²) splay that prograded towards the basin centre while maintaining continued bypass along the western pathway. Beginning ~5.0 ka, a second splay system constructed an even larger (~3800 km²) lobe into the most distal portions of the basin along the Shillong foredeep. This evolution from a bypass‐dominated system to one of enhanced mass extraction is well reflected in (i) the rapid downstream fining of deposited sand and (ii) a change in facies from amalgamated channel deposits to mixed sands and muds within discrete depositional lobes. The persistence of sediment bypass suggests that seasonal flooding of the basin by local runoff exerts a hydrologic barrier to overbank flow and is thus a principal control on river path selection. This control is evidenced by (i) repeated, long‐term preference for occupying a course along the basin margin rather than a steeper path to the basin centre and (ii) the persistence of an under‐filled, topographically low basin despite sediment load sufficient to fill the basin within a few hundred years. The progradation of two 10–20‐m thick, sandy mega‐splays into the basin interior reflects an alternative mode of sediment dispersal that appears to have operated only in the mid‐Holocene (~6.0–4.0 ka) during a regional weakening of the summer monsoon. The reduced water budget at that time would have lowered seasonal water levels in the basin, temporarily lessening the hydrologic barrier effect and facilitating splay development into the basin interior. Overall, these results place basin hydrology as a first‐order control on fluvial system behaviour, strongly modifying the perceived dominance of tectonic subsidence. Such coupling of subsidence, fluvial dynamics and local hydrology have been explored through tank experiments and modelling; this field study demonstrates that complex, emergent behaviours can also scale to the world's largest fluvial system.     

Event sedimentation in low‐latitude deep‐water carbonate basins,Anegada passage,northeast Caribbean

The Virgin Islands and Whiting basins in the Northeast Caribbean are deep, structurally controlled depocentres partially bound by shallow‐water carbonate platforms. Closed basins such as these are thought to document earthquake and hurricane events through the accumulation of event layers such as debris flow and turbidity current deposits and the internal deformation of deposited material. Event layers in the Virgin Islands and Whiting basins are predominantly thin and discontinuous, containing varying amounts of reef‐ and slope‐derived material. Three turbidites/sandy intervals in the upper 2 m of sediment in the eastern Virgin Islands Basin were deposited between ca. 2000 and 13 600 years ago, but do not extend across the basin. In the central and western Virgin Islands Basin, a structureless clay‐rich interval is interpreted to be a unifite. Within the Whiting Basin, several discontinuous turbidites and other sand‐rich intervals are primarily deposited in base of slope fans. The youngest of these turbidites is ca. 2600 years old. Sediment accumulation in these basins is low (<0.1 mm year^?1) for basin adjacent to carbonate platform, possibly due to limited sediment input during highstand sea‐level conditions, sediment trapping and/or cohesive basin walls. We find no evidence of recent sediment transport (turbidites or debris flows) or sediment deformation that can be attributed to the ca. M7.2 1867 Virgin Islands earthquake whose epicentre was located on the north wall of the Virgin Islands Basin or to recent hurricanes that have impacted the region. The lack of significant appreciable pebble or greater size carbonate material in any of the available cores suggests that submarine landslide and basin‐wide blocky debris flows have not been a significant mechanism of basin margin modification in the last several thousand years. Thus, basins such as those described here may be poor recorders of past natural hazards, but may provide a long‐term record of past oceanographic conditions in ocean passages.     

Lacustrine turbidites as indicators of Holocene storminess and climate: Lake Tahoe,California and Nevada

Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic, and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ¹³C_org and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites. Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms between ~3000 and ~900 cal yr B.P. in the Tahoe watershed.     

Provenance of sediments deposited at paleolake San Felipe,western Sonora Desert: Implications to regimes of summer and winter precipitation during last 50 cal kyr BP

Rare earth element, major and trace element and mineralogy in the sediments representing last 50 cal kyr BP from the summer precipitation fed paleolake San Felipe identify the different association of minerals and selective transportation of different grain size fractions and relate them to the variation in pluvial discharge into the basin as well as aeolian activities in the western Sonora Desert. Period of lower pluvial discharge during 14–48 cal kyr BP is contemporary to the regime of dominant winter storms in the region. Transportation of coarser quartz and plagioclase during 40–48 cal kyr BP and dominant finer fractions during 14–40 cal kyr BP possibly mirror the variation in the frequency of winter storms. During 3–14 cal kyr BP, higher catchment erosion (sedimentation increased 4–12 times) and transportation of REE bearing heavy minerals into the basin indicate higher pluvial discharge. We relate this period to a regime of dominant summer precipitation associated with the North American Monsoon and tropical cyclones. Geochemical and mineralogical signatures of the sediments deposited during ca. 8, 12–13 and >48 cal kyr BP suggest selective mobilization of quartz and plagioclase from the surrounding sand dunes by the aeolian processes.     

Tectonic and sedimentary history of a late Cenozoic intramontane basin (The Pitalito Basin, Colombia)

Abstract The Pitaiito Basin is an intramontane basin (15 × 20 km²) situated at the junction of the Central and Eastern Cordillera in the southern part of the Colombian Andes. Tectonic structures, evolution of the basin and distribution of the sediments suggest that the basin was formed adjacent to an active dextral strike-slip fault. Based on sedimentation rates it is estimated that subsidence started around 4.5 Ma. The basin can be divided into a relatively shallow western part (c. 300 m deep) and a deep eastern part (c. 1200 m deep). The transition between both areas is sharp and is delineated by a NW/SE-oriented fault. The position of this fault is reflected by the areal distribution of the deep non-exposed sediments as well as sediments at the surface: west of this fault the basin infill consists of coarse to medium elastics (conglomerates and sand) whereas in the eastern part fine elastics (clay and peat) are present. The lateral transition between both types of sediment is abrupt and its position is stable in time. The surface and near surface sediments in the Pitalito Basin reflect the last stage of sedimentary infill which came to a halt between 17,000 and 7500 years bp . These sediments were deposited by an eastward prograding fluvial system. The western upstream part of this system differs significantly from that of the eastern part which forms the downstream continuation. The western part exhibits unstable, shallow fluvial channels that wandered freely over the surface which predominantly consists of clayey overbank sediments. The alluvial architecture in the eastern half is characterized by stable channels and thick accumulations of organic-rich flood basin sediments and resembles an anastomosing river. The transition between both alluvial systems also coincides with the N/S-oriented normal fault. Palaeoclimatic conditions over the last c. 61,500 years were determined by means of a pollen record. From c. 61,500 to 20,000 years BP the mean annual temperature fluctuated considerably and decreased by 2–3^oC during the relatively warm periods (interstadials) and by 6–8^oC during the cold periods (stadials) in comparison with modern temperatures. These changes led to a displacement of the zonal vegetation belts from c. 200 m during the stadials to c. 1500 m in interstadial times without significant effects on the fluvial system present in the Pitaiito Basin until c. 20,000 years BP. Around this period the organic-rich eastern flood basins were choked with sediments and peat growth came to an end. Palynological and sedimentological data suggest that around that period the climate was cold (Δ 6–8^oC) and very dry and that a sparse vegetation cover was present around the basin. In these semi-arid climatic conditions the river system changed from an anastomosing pattern to one with ephemeral stream characteristics. This may have lasted until at least 17,000 years BP. Somewhere between 17,000 and 7500 years BP the eastward-flowing infilling river system changed into a NW-flowing erosive river system due to climatic or tectonic control and the present state was reached.     

长江中游江汉—洞庭盆地全新世以来水文环境演变与人类活动

本文基于长江中游江汉—洞庭盆地18个钻孔岩性、沉积特征及年代数据,结合研究区考古遗址点时空分布特征,恢复研究区全新世以来水文环境演变过程,并探讨了区域水文环境演变的成因机制及其与人类活动的关系。结果表明,受东海海面上升和泥沙淤积等因素影响,距今11.5—5.5 ka,长江中游地区河湖水位呈上升趋势,随着新石器文化的发展及稻作农业活动的增加,人类文化聚落自山前平原地带逐渐向盆地平原中部扩展;距今5.5—4.0 ka,长江中游河湖水位有所下降,新石器晚期的屈家岭—石家河文化迅速发展,聚落数量增多,平原腹地聚落比例增加;距今4.0 ka前后,河湖水位再次有所上升,洪泛过程加剧,可能是石家河文化快速衰落的主要原因。     

Quaternary features beneath Lake Simcoe,Ontario, Canada: drumlins,tunnel channels,and records of proglacial to postglacial closed and overflowing lakes

Lake Simcoe is a large lake 45 km across and in places over 30 m deep, located between Lake Huron and Lake Ontario, in the glaciated terrain of southern Ontario, Canada. Seismostratigraphic analysis of high-resolution seismic reflection profiles, together with lakebed sediment sampling and pollen study, revealed distinctive sequences in the sediments beneath Lake Simcoe, Ontario. A surface unit (Blue Sequence) of soft Holocene mud (low-amplitude surface reflection, discontinuous parallel internal reflections) lies in the deeper basins of the lake. The underlying unit (Green Sequence) is characterized by high-amplitude parallel internal reflections; basal sediments of this sequence consist of clay rhythmites with dropstones. The Green Sequence was deposited by lacustrine sedimentation in proglacial Lake Algonquin; sedimentation persisted until the basin was isolated from other glacial lakes at about 10 ¹⁴C ka at the Penetang post-Algonquin phase. Subsequent erosion of the uppermost portion of the Green Sequence is attributed to wave action in a low-level early Holocene lake, possibly closed hydrologically and coeval with closed lowstands in the Huron and Georgian Bay basins. Two sequences with high-amplitude surface reflections and chaotic internal reflections (Purple and Red Sequences) lie below the Green Sequence. Northeast-southwest trending ridges, tens of metres in height, on the Red Sequence (the lowermost of these two units) are interpreted to be drumlins. An erosion surface descends into narrow valleys 50–80 m deep beneath the lake in bays to the west and south of the main lake basin. These depressions are interpreted as subglacial tunnel channels cut by rapid flows of meltwater. The sediments of Purple Sequence are interpreted as channel-fill sediments rapidly deposited during waning stages of the meltwater drainage. The Red Sequence is correlated with the Newmarket Till of the last glacial maximum identified beneath the Oak Ridges Moraine to the south.     

Late Quaternary stratigraphy,sedimentology and geochemistry of an underfilled lake basin in the Puna plateau (northwest Argentina)

Depositional models of ancient lakes in thin‐skinned retroarc foreland basins rarely benefit from appropriate Quaternary analogues. To address this, we present new stratigraphic, sedimentological and geochemical analyses of four radiocarbon‐dated sediment cores from the Pozuelos Basin (PB; northwest Argentina) that capture the evolution of this low‐accommodation Puna basin over the past ca. 43 cal kyr. Strata from the PB are interpreted as accumulations of a highly variable, underfilled lake system represented by lake‐plain/littoral, profundal, palustrine, saline lake and playa facies associations. The vertical stacking of facies is asymmetric, with transgressive and thin organic‐rich highstand deposits underlying thicker, organic‐poor regressive deposits. The major controls on depositional architecture and basin palaeogeography are tectonics and climate. Accommodation space was derived from piggyback basin‐forming flexural subsidence and Miocene‐Quaternary normal faulting associated with incorporation of the basin into the Andean hinterland. Sediment and water supply was modulated by variability in the South American summer monsoon, and perennial lake deposits correlate in time with several well‐known late Pleistocene wet periods on the Altiplano/Puna plateau. Our results shed new light on lake expansion–contraction dynamics in the PB in particular and provide a deeper understanding of Puna basin lakes in general.     

Implications of diverse sedimentation patterns in Hala Lake, Qinghai Province, China for reconstructing Late Quaternary climate

Hala Lake is located in the Qilian Mountains, Qinghai Province, China, at 4,078?m a.s.l. Its sediments contain an archive of climate and hydrologic changes during the Late Quaternary, as it is located close to the area influenced by the East-Asian summer monsoon and westerly-driven air masses. Sedimentation patterns and depositional conditions within the lake were investigated using eight sediment cores from different water depths, and this information was used to evaluate the feasibility of using a single core to reconstruct past climate and hydrological conditions. Long core H7, from the center of the lake (65?m water depth) and core H8 from a western, near-shore location (20?m water depth), were compared in detail using sediment composition and geochemical data (X-ray fluorescence, loss-on-ignition and CNS analysis). Age models were constructed using 17 AMS radiocarbon dates and indicate negligible reservoir error for sediments from the lake center and?~1,000?year errors for the near-shore sediment core. Cores H1?CH5 and HHLS21-1 revealed a sediment succession from sand and silty clay to laminated clay on the southern side of the lake. Undisturbed, finely laminated sediments were found at water depths???15?m. Core H5 (2.5?m long), from 31?m water depth, yielded abundant green algal mats mixed with clayey lake deposits and was difficult to interpret. Algae occurred between 25 and 32?m water depth and influenced the dissolved oxygen content of the stratified lake. Comparison of cores H7 and H8 yielded prominent mismatches for different time periods, which may, in part, be attributed to internal lacustrine processes, independent of climate influence. We thus conclude that data from a single sediment core may lead to different climate inferences. Common shifts among proxy data, however, showed that major climate shifts, of regional to global significance, can be tracked and allow reconstruction of lake level changes over the last 24,000?years. Results indicate advance of glaciers into the lake basin during the LGM, at which time the lake experienced lowest levels, 25?C50?m below present stage. Stepwise refilling began at ca. 16 kyr BP and reached the ?25?m level during the B?lling/Aller?d warm phase, ca. 13.5 kyr BP. A desiccation episode falls within the Younger Dryas, followed by a substantial lake level rise during the first millennium of the Holocene, a result of climate warming, which promoted glacier melt. By ca. 7.6 kyr BP, the lake reached a stable high stand similar to the present level, which persisted until ca. 6 kyr BP. Disturbed sediments in core H7 indicate a single mass flow that was most likely triggered by a major seismic event?~8.5 kyr BP. Subsequent lake development remains unclear as a consequence of data mismatches, but may indicate a general trend to deteriorating conditions and lake level lowstands at ca. 5.0?C4.2, 2.0 and 0.5 kyr BP.     

Tectono-sedimentary processes along an active marine/lacustrine half-graben margin: Alkyonides Gulf, E. Gulf of Corinth, Greece

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year^?1. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year^?1 of footwall uplift and hence 0.5–0.6 mm year^?1 of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year^?1. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp ) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.     

Dynamics of giant mass transport in deep submarine environments: the Matakaoa Debris Flow,New Zealand

The Matakaoa Debris Flow (MDF) is a 200‐km‐long mass‐transport deposit resulting from the failure of the Matakaoa continental margin, northeast New Zealand, ca. 38–100 ky ago. In this study, high‐quality bathymetric and seismic reflection data are used to identify the morpho‐structural characters that reflect the kinematics of the MDF, as well as its interactions with basin sediments. We demonstrate how the transport energy, together with the local topography led to the present geometry and complex structure of the MDF deposits. The remarkable transport energy of the MDF is demonstrated by its dynamic impact on adjacent sedimentary series, including erosion of the substratum, shearing and compressional deformation. In the proximal zone of transport, momentous substratum erosion, demonstrated by giant tool marks and truncated sediments at the base of the debrite, triggered the excavation of a large volume (>200 km³) of basin sediments. The size of transported blocks (up to 3‐km long) is used to estimate the matrix yield strength in an early stage of transport. In the distal zone of transport, 100 km north of the source, seismic profiles show the propagation of thrust structures from the MDF into adjacent basin sediments. This study highlights that the remarkable volume of 2000 km³ of deposits partly resulted from the propagation of compressive structures within the basin sedimentary series to the front of the debrite.     

Changes in glacial lakes and glaciers of post-1986 in the Poiqu River basin, Nyalam, Xizang (Tibet)

Glacial lakes and glaciers are sensitive indicators of recent climate change. In the Poiqu River basin of southern Tibet, 60–100 km NW of Mt. Everest, Landsat imagery defines post-1986 changes in the size and distribution of both glacial lakes and glaciers. Total area of glaciers in the 229-km² drainage area has decreased by 20%. The number of glacial lakes with areas in excess of 0.020 km² has increased by 11%, and the total area of glacial lakes has increased by 47%. The areas of typical large glacial lakes of the area (Galongco, Gangxico, and Cirenmaco) have increased by 104, 118, and 156%, respectively, and these increases are confirmed by field investigations.Comparing the 1986 data, the area of glaciers in the basin headwaters has decreased by 46.18 km² to a present total area of 183.12 km², an annual rate of change of 3.30 km²/year. Trends indicate that the total area of glaciers will continue to decrease and that both the numbers and areas of glacial lakes will continue to increase. Accompanying these trends will be an increased risk of debris flows, formed by entrainment of sediment in glacial-outburst floods and in surges from both failure and avalanche- and landslide-induced overtopping of moraine dams. Based on both the local and world-wide history of catastrophes from flows of these origins, disaster mitigation must be planned and appropriate engineering countermeasures put in place as soon as possible.     

河北平原第四纪地层划分研究

通过对以往地层岩性、成因类型、结构、标志层1、4C、古地磁、钻孔等资料的重新分析与对比,依照《中国地层指南》对河北平原第四纪地层进行了重新划分和修订。以2.58 Ma为第四系下限,以0.78 Ma为中更新统底界,以相当深海氧同位素5阶段开始的0.128 Ma为上更新统底界,以大体相当深海氧同位素1阶段开始的0.01 Ma为全新统底界,修订后的河北平原第四系厚度减小40~220 m。该文是在以往研究基础上对河北平原第四纪地层划分的一次探讨,其成果为建立华北平原含水层结构模型搭建了平台。由于一些小区缺乏古地磁测年资料,而标志层、沉积物颜色特征界限又不明显,故影响了第四系下限和各统底界划分精度。     

Late Quaternary Alluviation Along Intermittent Streams in Kakadu National Park,Northern Territory

Sedimentation along small, intermittent streams on Kapalga Research Station in Kakadu National Park may have responded to increased base levels following post-glacial flooding of the valleys of the Alligator Rivers. Alternatively, regional climate changes may have controlled sedimentation. Using thermoluminescence dating, we determined that sediments from two streams at Kapalga date from 21.5 ± 4.0 ka. On a third stream, sediments dated from 7.6 ± 1.1 ka, with younger sediments occurring downstream. We interpreted the pre-Holocene dates and the lack of evidence of upstream progradation to indicate that climate variation was more important to sedimentation than base levels. Predicted increases in rainfall variability and in the frequency of high-intensity rainfall under enhanced greenhouse conditions may cause renewed sediment mobilisation. At the outflow of one stream on to the South Alligator flood plain, we found 15 m of sandy alluvia underlying 3-5 m of estuarine muds deposited as a result of sea-level rise. These sandy alluvia dated from about 77 ka at 4 m to more than 300 ka at 19 m depth. These ages are consistent with those recorded on the Magela Creek system, 50 km to the east.     

Controls on erosion patterns and sediment transport in a monsoonal,tectonically quiescent drainage,Song Gianh,central Vietnam

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.     

Wind control on the accumulation of heavy metals in sediment of Lake Ulubat,Anatolia, Turkey

Freshwater Lake Ulubat (z _mean = 1.5–2.0 m and Area = ~138 km²), NW Anatolia, Turkey was filled in by fine-to-medium-grain silts during the late Holocene. Deposition in Lake Ulubat has been 1.6 cm year⁻¹ for the last 50 years, but the sedimentation rate over the last ~1,600 years was lower (0.37 mm year⁻¹). The organic matter and carbonate contents of the infill show cyclic changes that reflect environmental fluctuations. The silt-dominated lithology and the vertically uniform heavy metal distributions are probably due to wind-controlled sedimentation in the lake. Heterogeneous mud, derived from a large, mountainous drainage basin, is deposited in the lake mostly during summer, June to October, when conditions are hot and calm. Winter months are stormier and sediments are re-suspended due to the shallow water depth and the effect of waves on the lake bottom. It is likely that re-suspended sediments, particularly fine-grained particles, together with the heavy metals, are transported out of the lake via the outlet, especially during periods of high lake level. This resuspension and removal process probably caused the lake sediments to become silt-dominated and depleted in heavy metals. The role of broad shallow lakes in sequestering sediments and heavy metals can be described more accurately when wind data are considered. Such information may also be helpful for land-use planning in downstream areas.