Late quaternary of the middle Caquetá River area (Colombian Amazonia)

The late Quaternary history of the middle Caquetá River area in Colombia, northwestern Amazonia is described, based on observations of river bank sections, radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the Caquetá River. It is shown that the Late Pleistocene and Holocene climatic changes that took place in the Andean Cordilleras, were related to the depositional and erosional history of the Caquetá River in the Colombian Amazonian lowlands. The low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material. From these organic low terrace sediments, seven finite radiocarbon dates were obtained of Middle Pleniglacial age, between 56 000 and 30 000 yr BP. The coarse textured basal deposits of the low terrace apparently stem from the early part of the Middle Pleniglaciai period, during which the effective rainfall in the Andes was relatively high and the Andean glaciers had a considerable extension. Palynological data from silty sediments with organic remains at one site, show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today, but Amazonian forest was still the dominating type of vegetation. This interval might correspond to one of the Middle Pleniglacial savanna intervals from eastern Amazonia (Carajas). No organic sediments from the Upper Pleniglacial period were found and hence radiocarbon dates were not obtained. In the Andes this period had a very cold climate with low effective rainfall and in the east Amazonian Carajas area it is characterised by the relative extension of open savanna vegetation. The river run-off and sediment transport must have been much lower than in the Middle Pleniglacial and the Caquetá River cut itself down in its own sediments. Two Late-glacial radiocarbon datings obtained at one site (ca. 12 500 yr BP) indicate the existence of a Late-glacial sedimentation phase, separated from the Holocene sequence by a minor erosional phase. Organic layers in the Holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr BP. Holocene sedimentation started with the rapid deposition of (sandy) clay possibly in a partly permanently inundated Caquetá valley. During the major part of the Holocene (silty) clays were deposited, with a dominant seasonal inundation cycle.     

渤海湾西岸全新世沉积速率对河流供给的响应

渤海湾西岸由北向南获取了3个钻孔,以全新世海相岩心为研究对象,采用AMS14C(Accelerator Mass Spectrometry14C,加速器质谱14C)测年方法建立年代框架并计算平均沉积速率,结合沉积物粒度组成及沿岸古河流三角洲发育历史,探讨了沉积速率对沿岸河流供给变化的时空响应。结果表明,早全新世—中全新世初期(11~6ka),渤海湾西岸整体沉积速率偏低,仅0.03~0.07cm/a,沉积物粒度较粗;中全新世6.43~4.97ka cal BP期间,NP3孔平均沉积速率为0.60~0.93cm/a,高于同期沿岸南部的CH110孔和BT113孔。沉积物组成以粉砂为主,粘土含量低,向上逐渐变粗,具三角洲反粒序特征。该时段的高沉积速率系渤海湾西北岸对潮白河、永定河及滦河沉积物供给的响应;中全新世晚期3.68~2.67ka cal BP期间,BT113孔沉积速率为0.27~1.4cm/a,高于同期沿岸北部CH110孔和NP3孔,沉积物组成以粉砂为主,粘土含量较NP3孔高,向上逐渐变粗,具三角洲反粒序特征。该时段的高沉积速率系渤海湾西岸南部对黄河沉积物供给的响应;晚全新世2.29~0.24ka cal BP期间,沿岸中部CH110孔沉积速率为0.55~0.91cm/a,高于同期沿岸南部的BT113孔和北部的NP3孔,该高沉积速率为渤海湾西岸中部对黄河和海河供给沉积的先后响应。     

Late‐glacial and Holocene river development in the Teleorman Valley on the southern Romanian Plain

This paper reports on a radiocarbon‐dated sequence of alluvial terraces from the Teleorman Valley in the southern Romanian Plain and represents the first Late‐glacial and well‐constrained Holocene alluvial sequence from the lower Danube Valley of southeast Europe. The two earliest and most extensive terraces (T1 and T2) are dissected by large, high‐amplitude palaeochannels, which are dated to ca. 12 800 yr BP and are comparable to large meandering palaeochannels identified from other Late glacial contexts across northern and central Europe. The remaining sequence of alluvial deposits show changes in river activity and accelerated sedimentation around 4900–4800 yr BP, 4000–3800 yr BP, 3300–2800 yr BP, 1000 yr BP and within the past 200 yr. A phase of tributary stream alluvial fan deposition is dated to ca. 2400 yr BP. All these periods of alluvial sedimentation correlate well with episodes of climatic cooling, higher rainfall and enhanced river activity, both in terms of incision and greater lateral mobility as well as increased flood frequency and magnitude identified elsewhere in central, western and northern Europe. Human activity appears to have had little effect on this river environment and significant fine‐grained sedimentation is not noted until ca. 2400 yr BP, approximately 5000 yr after the first neolithic farmers settled the area. Whether this record of river activity truly reflects the impact of prehistoric societies on this catchment will only be elucidated through further, ongoing detailed archaeological research. Copyright © 2003 John Wiley & Sons, Ltd.     

The impact of climate change on the bulk and clay geochemistry of fluvial residual channel infillings: the late Weichselian and Early Holocene River Meuse sediments (The Netherlands)

Fine-grained fluvial residual channel infillings are likely to reflect systematic compositional changes in response to climate change, owing to changing weathering and geomorphological conditions in the upstream drainage basin. Our research focuses on the bulk sediment and clay geochemistry, laser granulometry and clay mineralogy of Late-glacial and Early Holocene River Meuse (Maas) unexposed residual channel infillings in northern Limburg (The Netherlands). We demonstrate that residual channel infillings register a systematic bulk and clay compositional change related to climate change on a 1–10 k-yr time-scale. Late-glacial and Holocene climatic amelioration stabilised the landscape and facilitated prolonged and intense chemical weathering of phyllosilicates and clay minerals due to soil formation. Clay translocation and subsequent erosion of topsoils on Palaeozoic bedrock and loess deposits increased the supply of smectite and vermiculite within River Meuse sediments. Smectite plus vermiculite contents rose from 30–40% in the Pleniglacial to 60% in the Late Allerød and to 70–80% in the Holocene. Younger Dryas cooling and landscape instability caused almost immediate return to low smectite and vermiculite contents. Following an Early Holocene rise, within about 5000 yr, a steady state supply is reached before 5 ka (Mid-Holocene). Holocene sediments therefore contain higher amounts of clay that are richer in high-Al, low-K and low-Mg vermiculites and smectites compared with Late (Pleni-)glacial sediments. The importance of clay mineral provenance and loess admixture in the River Meuse fluvial sediments is discussed. © 1998 John Wiley & Sons, Ltd.     

Glacier readvances in the Andes at 12 500–10 000 YR BP: Implications for mechanism of Late-glacial climatic change

Radiocarbon dates from two sites in the Andes (Ecuador and Peru) confirm that glaciers culminated a readvance after 11 000 yr BP. A moraine stage, equivalent in altitude and position relative to existing glaciers, is present in most glacierized ranges, but its age is equivocal. Broadly limiting dates from Colombia and Peru suggest that the stage may be Late-glacial, as it is younger than 12100 yr BP, but formed before the early Holocene; in southern Chile a comparable moraine stage is older than ca. 9100 yr BP. Andean glaciers appear to have advanced at least twice during the Late-glacial interval. Glacier reconstruction from these moraine limits suggests depression of the equilibrium line altitude by at least 300–400 m in the northern and north-central Andes, and possibly less than this farther south. Late-glacial climatic change occurred globally and possibly reflects North Atlantic temperature and circulation changes forced by deglaciation of the northern ice sheets, migrations north and south of the Atlantic Polar Front, and the switching off and on of a ‘dust pump’ in low midlatitudes.     

Flood frequencies reveal Holocene rapid climate changes (Lower Moulouya River,northeastern Morocco)

Current high‐resolution palaeoenvironmental records reveal short‐term Holocene coolings. One of these major Holocene rapid climate changes occurred between 3.2 and 2.7 cal. ka BP. The sensitivity of river systems vis‐à‐vis slight and short‐term Holocene climatic variations is a subject of controversy in the scientific community. In this paper, we present a 4.0 to 1.4 cal ka BP palaeoflood record from the Lower Moulouya River (northeastern Morocco) to demonstrate the high sensitivity of semiarid rivers in the southwestern Mediterranean towards Holocene environmental changes. The Lower Moulouya flood deposits are characterised by thick, well‐stratified, predominantly clayey to silty overbank fine sediments. These cohesive sediments show evidence of excellent preservation conditions against fluvial erosion and contain a continuous record of mid to late Holocene flood sequences. The Moulouya palaeoflood record can be interpreted in the context of regional and global high‐resolution proxy data, revealing a strong coupling with Holocene rapid climate changes. The centennial‐scale Moulouya palaeohydrological history will be discussed with palaeoenvironmental data from the same record (palaeomagnetics, sedimentary charcoal record, anthracological analyses, snail analyses) to generate new ideas about the mid to late Holocene hydrological cycle in the southwestern Mediterranean. The deduced features of pronounced Lower Moulouya flooding and the decreased fire recurrence during Holocene cooling remain somewhat inconsistent with the interpretation of other palaeohydrological and paleaoecological records from the southwestern Mediterranean. However, enhanced Lower Moulouya flood frequencies between 3.2 and 2.7 cal. ka BP agree with increased floodplain aggradation in other major river systems of Mediterranean North Africa. Copyright © 2009 John Wiley & Sons, Ltd.     

Deglaciation of the southeastern Norwegian Sea towards the end of the last glacial age

In a region of generally thin Holocene sediment cover along the outer Norwegian continental margin, a 565 cm long piston core was taken, which contained more than 4 m of Holocene clayey silty sediments. A several decimetres thick sandy horizon separated the glacial marine clays with ice-dropped components and the fine-grained Holocene sediments which have bulk sedimentation rates of more than 40 cm/1000 years. The scarcity of biogenous sediment components in the glacial sediments and the increasing frequency of benthonic as well as planktonic fossils in the Holocene deposits points to important changes in the Norwegian Sea hydrography during the time of the Scandinavian deglaciation.     

Geochemistry of Harappan potteries from Kalibangan and sediments in the Ghaggar River: Clues for a dying river

The ephemeral Ghaggar-Hakra River of northwestern India has always been considered to be the remnant of an ancient perennial glacier-fed river(Vedic Saraswati). The exact reason and timing of major hydrological change of this river remains speculative. The river's purported association with the zenith of the Harappan civilisation remains a conjecture because the timings of its fluvial past are still being debated. In this study we have made an attempt to resolve this issue using geochemical provenance of sediments from some dated horizons in the Ghaggar flood plain and that of the material used in the potteries from the Mature Harappan period(4600-3900 yr BP) at Kalibangan. Sampled sedimentary horizons were dated by radiocarbon and optically stimulated luminescence(OSL) methods. Results of our study from the Ghaggar alluvium indicate that the river did have glacial sources during the early Holocene. However, the data from the potteries suggest that during the Mature Harappan period, the sediments in the Ghaggar as used by the potters did not have a higher Himalayan provenance and hence, were not derived from glaciated Himalayas.These findings imply that during the time of the Mature Harappans the Ghaggar had already become a foothill-fed river.     

Holocene river development and environmental change in Upper Wharfedale,Yorkshire Dales,England

This paper provides the first radiometrically dated evidence of Holocene alluvial landform development in Upper Wharfedale, Yorkshire Dales. Four river terraces are identified. Terraces 1 and 2 are closely linked to Late Devensian and early Holocene environmental change, with gravel reworked from local glacial and periglacial sources prior to cementation by carbonate‐rich waters. U‐series dating of cement provides age estimates for cementation of between ca. 5.1–7.4 kyr BP for Terrace 1 and ca. 3.6–>8.0 kyr BP for Terrace 2. U‐series dating of tufas overlying Terraces 1 and 2 produced ages of ca. 4.2–4.5 kyr BP and ca. 2.1–2.2 kyr BP respectively, and provide upper age limits for terrace formation. Terrace 3 marks a change in sediment calibre, supply and sedimentation style, and ¹⁴C dating suggests that the principal source of fine‐grained material may be agricultural expansion in the Yorkshire Dales from ca. ad 600 (1350 cal. yr BP). Radiocarbon dates indicate that Terrace 4 was deposited from the eleventh century, with initiation of the contemporary floodplain between the fifteenth and seventeenth centuries ad. Both these lowest units contain sediments contaminated with heavy metals as a result of mining activities within the catchment. The evidence presented in this study is comparable to that of research undertaken in upland environments elsewhere in northern and western Britain, thereby adding to the corpus of information currently available for evaluating the fluvial geomorphological response to climate and vegetation change during the Holocene. Copyright © 2000 John Wiley & Sons, Ltd.     

Fluvial system response to external forcing and human impact – Late Pleistocene and Holocene fluvial dynamics of the lower Guadalete River in western Andalucía (Spain)

The 170 km long river course of the Guadalete River (western Andalucía) provides an excellent record of Late Pleistocene and Holocene fluvial sedimentation dynamics. Furthermore, its floodplain sediments are very well suited to describe geomorphic changes forced by climate fluctuations, sea‐level changes, tectonic influences and human activity. Multiproxy investigations were based on field mapping and the study of 18 profile sections, mainly including sedimentological characterisation, soil‐chemical analyses and radiocarbon dating of 34 samples. Findings were complemented by drillings and electrical resistivity tomography. The lowermost 50 km of the river section are divided into an upper and lower part (each 25 km long), based on different sediment preservation conditions. The boundary corresponds to the disappearance of the Late Pleistocene river terrace. Significant floodplain aggradation occurred at around 10 000 cal. years BP, while dynamics were strongly affected by sea‐level fluctuations until the early Holocene. Furthermore, sedimentation starting at 8000, 6100, 4600, 2200, 900 and 400 cal. years BP is related to enhanced fluvial dynamics due to environmental stress that presumably was triggered by climate fluctuations, that is, aridification. However, the strongest intensity of sedimentation at 400 cal. years BP points to climate anomalies in the course of the Little Ice Age. In contrast, several periods of stability associated with alluvial soil formation took place during the Bølling and Allerød interstadials, prior to 8000, 6100 and 5100, and after 4300 and 2000 cal. years BP. The anthropogenic signal in floodplain evolution is not clearly distinguishable from that of climate. However, human land use had the potential to amplify geomorphic processes, especially during periods of climate deteriorations that caused increasing stress on the environment.     

Late Weichselian and Holocene sediment flux and sedimentation rates in Andfjord and Vågsfjord,North Norway

Late Weichselian and Holocene sediment flux and sedimentation rates in a continental‐shelf trough, Andfjord, and its inshore continuation, Vågsfjord, North Norway, have been analysed. The study is based on sediment cores and high‐resolution acoustic data. Andfjord was deglaciated between 14.6 and 13 ¹⁴C kyr BP (17.5 and 15.6 calibrated (cal.) kyr BP), the Vågsfjord basin before 12.5 ¹⁴C kyr BP (14.7 cal. kyr BP), and the heads of the inner tributary fjords about 9.7 ¹⁴C kyr BP (11.2 cal. kyr BP). In Andfjord, five seismostratigraphical units are correlated to a radiocarbon dated lithostratigraphy. Three seismostratigraphical units are recognised in Vågsfjord. A total volume of 23 km³ post‐glacial glacimarine and marine sediments was mapped in the study area, of which 80% are of Late Weichselian origin. Sedimentation rates in outer Andfjord indicate reduced sediment accumulation with increasing distance from the ice margin. The Late Weichselian sediment flux and sedimentation rates are significantly higher in Vågsfjord than Andfjord. Basin morphology, the position of the ice front and the timing of deglaciation are assumed to be the reasons for this. Late Weichselian sedimentation rates in Andfjord and Vågsfjord are comparable to modern subpolar glacimarine environments of Greenland, Baffin Island and Spitsbergen. Downwasting of the Fennoscandian Ice Sheet, and winnowing of the banks owing to the full introduction of the Norwegian Current, caused very high sedimentation rates in parts of the Andfjord trough at the Late Weichselian–Holocene boundary. Holocene sediment flux and sedimentation rates in Andfjord are about half the amount found in Vågsfjord, and about one‐tenth the amount of Late Weichselian values. A strong bottom current system, established at the Late Weichselian–Holocene boundary, caused erosion of the Late Weichselian sediments and an asymmetric Holocene sediment distribution. Copyright © 2002 John Wiley & Sons, Ltd.     

Late Quaternary tephrochronology of Sweden: a review

Studies of Late Quaternary sediments in south and central Sweden have yielded a detailed tephrochronology for the Last Glacial–Interglacial transition (LGIT; ca. 15,000–10,000 cal. yr BP) and the Holocene. More than ten tephra layers have been detected and geochemically characterised. The most widespread tephra from the LGIT is the rhyolitic phase of the Vedde Ash (ca. 12,000 cal. yr BP) which has been found in lacustrine sediments and marine clays south of the Younger Dryas moraines in south Sweden. Other horizons from the LGIT identified to date include the Borrobol tephra (ca. 14,400 cal. yr BP), the Hässeldalen tephra (ca. 11,500 cal. yr BP), the 10-ka Askja tephra (ca. 11,300 cal. yr BP) and the Högstorpsmossen tephra (ca. 10,200 cal. yr BP). The most significant Holocene isochrones are Hekla-4 (ca. 4260 cal. yr BP), Hekla-Selsund/Kebister (ca. 3750 cal. yr BP), Hekla-3 (ca. 3000 cal. yr BP) and Askja-1875. Two new Late Holocene tephra horizons (the Stömyren tephra, ca. 2100 cal. yr BP and the Gullbergby tephra; ca. 2700 cal. yr BP) were identified in single sites and are so far less valuable as marker horizons, but are potentially important for the future.     

Sedimentology and stratigraphy in the cave Hamnsundhelleren,western Norway

The stratigraphy in Hamnsundhelleren is as follows. A basal weathered rock bed of unknown age is followed by laminated clay deposited under stadial conditions and correlated with palaeomagnetism to the Laschamp excursion (43–47 000 yr BP). Angular blocks, bones and clay above this are ¹⁴C dated to the Ålesund Interstadial (28–38 000 yr BP). Another stadial laminated clay following the Ålesund Interstadial includes a palaeomagnetic excursion correlated with Lake Mungo (28 000 yr BP). The newly discovered Hamnsund Interstadial above this consists of frost-weathered clay and scattered angular blocks. It is ¹⁴C dated to 24 500 yr BP on bones mixed into the Ålesund Interstadial. The Hamnsund Interstadial is succeeded by another stadial laminated clay and then a Late-glacial–Holocene mixture of bones and blocks. In Hamnsundhelleren and other similar caves four successive phases of sedimentary environments for each ice-free–ice-covered cycle have been identified: (i) ice-free phase (deposition of bones and frost-weathered blocks); (ii) subaerial ice-dammed lake phase (sand or silt deposited in a lateral glacial lake); (iii) subglacial ice-dammed lake phase (cave closed by ice, deposition of till, debris flows and laminated clay); (d) ice-plugged phase (cave is plugged by frozen lake water and/or glacial ice, no deposition).     

Amazon diversity in light of the paleoecological record

Disjunct distributions of Amazonian species have been explained previously by a refugial theory which postulates that Amazonian rain forest was preserved in large highland regions throughout the Pleistocene. No direct, radiocarbon dated evidence exists for the last glacial maximum with which to test this theory. The only radiocarbon dates of Pleistocene age from the Amazon basin are of fossiliferous deposits in the proposed Napo refugium of the West, where both pollen assemblages and wood samples indicate that forest with cool Andean elements existed there at two intervals in the last cycle of northern hemisphere glaciation, implying a temperature depression of at least 4°C in the Amazon lowlands.Under modern climatic conditions, lateral erosion by river meander, together with surface erosion, serves as a rejuvenating mechanism for the rain forests of Peru and Ecuador. The instability of late Holocene Amazonian climates is demonstrated by documenting a precipitation event in the eastern Andean cordillera that caused widespread flooding of western Amazonian forests 800–1300 BP. Late Holocene pollen histories from widely dispersed parts of central Amazonia distinguish between vegetation histories in the drainage of northern, south-western and western watersheds, but all show histories of fluctuating intensities of dry seasons. Radiocarbon dating of charcoal layers in soils of Venezuelan Amazonia demonstrates the apparently random incidence of wild fires at wide intervals over at least the last 6 ka.The high species richness of Amazonia is a result of numerous opportunities for vicariance because of a very large total area, wide variety of habitats and intermediate levels of disturbance, particularly by hydrological processes, that has varied on timescales from years to millennia. Amazonian disjunct distributions probably reflect regional environmental discontinuities in both interglacial and glacial times.     

Holocene Environmental Change and River-Mouth Sedimentation in the Baie des Anges, French Riviera

River mouths on the steep, high-relief coast of the French Riviera exhibit thick sequences of Holocene marine, estuarine, deltaic, and river channel-floodplain sediments that overlie basal fluvial Pleistocene gravel. Gravel is uncommon in most of the early to middle Holocene aggradational-progradational marine, estuarine, deltaic sediments, despite an ample supply from rock units in the steep adjoining uplands. River-mouth gravel is common only in late Holocene river channels and in barrier beaches perched on finer-grained nearshore sediments. Neither downslope grain-size fining on alluvial fans nor sediment stacking patterns during sea-level (base-level) rise readily account for the lack of early to middle Holocene gravel in the river-mouth sediment wedges. Holocene sea-level rise led to the storage of fine-grained sediments in shallow marine, estuarine, and deltaic environments in the present coastal zone. We infer that humid temperate conditions, a dense forest cover, landscape stabilization, and a regular quiescent river flow regime associated with the Atlantic climatic optimum limited gravel supply in the adjoining catchments and gravel entrainment downstream during the early Holocene. Sea-level stabilization in the middle and late Holocene coincided with a marked change in bioclimatic conditions toward the present Mediterranean-type regime, which is characterized by a less dense forest cover, soil erosion, and episodic catastrophic floods. The late Holocene was thus a time of downstream bedload channel aggradation, fine-grained floodplain and paludal sedimentation, and seaward flushing of clasts leading to the formation and consolidation of the gravel barrier beaches that bound the rivermouths and embayments.     

A preliminary history of Holocene colluvial (debris-flow) activity,Leirdalen, Jotunheimen,Norway

A stratigraphic succession of alternating peat and minerogenic sediments at the foot of a steep mountain slope provides the basis for the reconstruction of a preliminary colluvial history from the alpine zone of Jotunheimen, southern Norway. Layers of silty sand and sandy silt, typically 5–10 cm thick and interpreted as distal debris-flow facies, are separated by layers of peat that have been radiocarbon dated. Deposition from at least 7500 to about 3800 ¹⁴C yr BP of predominantly minerogenic material suggests relatively infrequent but large-magnitude debris-flow events in an environment warmer and/or drier than today. Particularly low colluvial activity between about 6500 and 3900 ¹⁴C yr BP was terminated by a succession of major debris-flow events between about 3800 and 3400 ¹⁴C yr BP. Unhumified peats indicative of higher water tables, separate six debris-flows that occurred between about 3300 and 2300 ¹⁴C yr BP and signify a continuing high frequency of colluvial activity. Uninterrupted peat accumulation between about 2400 and 1600 ¹⁴C yr BP indicates reduced debris-flow activity; subsequent renewed activity appears to have culuminated in the ‘Little Ice Age’ after about 600 ¹⁴C yr BP. This pattern of colluvial deposition demonstrates a long history of natural Holocene low-alpine landscape instability, suggests an increase first in the magnitude and then in the frequency of debris-flow activity coincident with late Holocene climatic deterioration, and points to the potential of debris-flow records as a unique source of palaeoclimatic information related to extreme rainfall events. © 1997 John Wiley & Sons, Ltd.     

江苏省里下河(兴化-泰州)地区浅表沉积物特征及古地理环境演变

里下河地区浅部沉积物多为全新世中、晚期堆积,多数地区缺失全新世早期沉积。沉积物以淤泥质亚粘土、亚粘土、亚砂土,粉砂质淤泥、淤泥质粉砂、淤泥及泥炭为特征。沉积前的古地貌为四周高、中间低的碟形盆地。古气候经历了温湿—暖湿—温湿—温干的变化过程。区内曾有两次海侵,第一次海侵发生在全新世早中期,范围几乎遍及全区;第二次海侵发生在全新世晚期,规模小、范围窄、时间短,为海水沿江河倒灌、回溯形起。该区古环境自全新世以来经历了浅水海湾—古泻湖—湖沼地及平原的演变过程,最终形成现今河湖稠密的低凹平原。     

山东菏泽古沉船地区沉积地层和地貌研究

赵王河古沉船河道沉积层具有明显的阶段性.早期沉积以中细砂、粉砂为主,河水游荡性沉积韵律明显;晚期沉积物以粉细沙、粉砂为主.自船底部垂向上,自下而上分为两大段,即黄泛期沉积和河道沉积,其粒径呈愈向上愈细的趋势.剖面分析表明,河道沉积有横向摆动迹象.沉船撞击边滩沉没后,辫状河道逐渐消失,沉船处接受较稳定的河道沉积.     

Late- and postglacial history of the Great Belt, Denmark

On the basis of shallow seismic records, vibrocoring, macrofossil analyses and AMS radiocarbon-dating, five stratigraphical units have been distinguished from the deepest parts of the central Great Belt (Storebælt) in southern Scandinavia. Widespread glacial deposits are followed by two lateglacial units confined to deeply incised channels and separated by an erosional boundary. Lateglacial Unit I dates from the time interval from the last deglaciation to the Allerød; lateglacial Unit II is of Younger Dryas age. Early Holocene deposits show a development from river deposits and lake-shore deposits to large lake deposits, corresponding to a rising shore level. Lake deposits are found up to 20 m below the sea floor, and the lake extended over some 200–300 km2. The early Holocene freshwater deposits are dated to the time interval c. 10900 to c. 8800 cal. yr BP and the oldest shells of marine molluscs from the Great Belt are dated to c. 8100 cal. yr BP.     

Late Quaternary landscape history and geoarchaeology of two drainages on Black Mesa,northeastern Arizona,USA

Two approximately 5‐ to 6‐km drainage segments on Black Mesa preserve unusually complete sequences of late Quaternary alluvium and soils. Radiocarbon‐ and tree‐ring‐dated alluvial and soil stratigraphy suggests entrenched paleoarroyos were beginning to aggrade at about >24,260, 11,070, 9660, 8800, 7060, 3500, 2140, and 1870 ¹⁴C yr B.P. Using the quantity of sediment removal from post‐A.D. 1900 arroyos as analogue, at least 77–200% of total valley alluvium has been removed and replaced by younger sediments during an estimated 11 late Pleistocene and Holocene erosion epicycles. Given that most (59%) of the 150 recorded prehistoric sites in the two study areas occur on valley floors where only about 3% of surface alluvium predates Lolomai phase Basketmaker II occupation (˜1900–1600 yr B.P.), it may be inferred that pre‐Lolomai phase Basketmaker II sites which may have been located along washes have been removed or buried by fluvial erosion. Identification of five buried hearths in alluvial sections, including White Dog and Lolomai phase Basketmaker II sites (dating about 3500 and 1870 ¹⁴C yr B.P., respectively) and one possible Early Archaic site, supports this conclusion. © 2005 Wiley Periodicals, Inc.