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1.
The stratigraphy and sedimentology of the Q3 middle terrace alluvial sequence in the lower Tagus river valley, Portugal, were studied near the village of Alpiarça, approximately 40 km upstream from the estuarine area. Two main stratigraphic units were recognized, separated by an important uncomformity. The Lower Gravels unit (LG) consists of intercalations of medium to coarse gravel deposits, mainly quartzitic, with coarse sandy matrix, organized in tabular bodies. The overlying Upper Sands unit (US) consists of tabular sandy channel deposits and overbank fines, the latter containing well-developed paleosols and backswamp deposits, showing a general aggrading trend, apparently with varying rates; available data indicate that deposition of the US took place under temperate climatic conditions. Within US deposits are several paleolithic archaeological sites, the lower ones in the alluvial stratigraphy being Middle Acheulian, whereas those embedded in overlying deposits are, from bottom to top, Upper Acheulian and Micoquian. Some of these sites have been recently excavated. The quartzite artifacts were apparently abandoned by early humans on the flood plain surface during deposition of the US unit and were subjected to limited reworking during their incorporation in the alluvium. TL/OSL dating of sandy-silty sediments, though imprecise, support archaeological evidence pointing to an age of 150,000 to 70,000 yr B.P. for the US unit.  相似文献   

2.
Extensive terrace and flood plain deposits occur along the Lower Macleay River. A sequence of terraces from oldest to youngest was named: Madron, Corangula, Mungay, Mooneba, Belgrave and Macleay deposits (contemporary). Basal sediments in the Mooneba terrace were dated by radiocarbon analysis at 3,280 ± 55 years; basal sediments of the Mungay terrace were dated at 6,425 ± 105 years. The Madron and Corangula terraces are considered very much older than the Mungay. The flood plain consists of two early cycles of aggradation buried under 23m of estuarine sediment, which in turn is overlain by up to 6m of alluvium. The estuarine sediments were dated at 8,530 ± 200 years at elevation —4m relative to mean sea level. The base of the overlying Smithtown alluvium was dated at 3,295 ± 95 years. A general chronology is presented for the Lower Macleay valley, and a sequence of terrace soils is discussed.  相似文献   

3.
The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km2 indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.  相似文献   

4.
Four late-Quaternary alluvial fills and terraces are recognized in Wolf Creek basin, a small (163 km2) drainage in the Kansas River system of the central Great Plains. Two terraces were created during the late Pleistocene: the T-4 is a fill-top terrace underlain by sand and gravel fill (Fill I), and the T-3 is a strath terrace cut on the Cretaceous Dakota Sandstone. Both Fill II (early Holocene) and Fill III (late Holocene) are exposed beneath the T-2, a Holocene fill-top terrace. The T-1 complex, consisting of one cut and three fill-top terraces, is underlain by Fills III and IV. A poorly developed floodplain (T-0) has formed within the past 1000 yr. As valleys in Wolf Creek basin filled during the early Holocene, an interval of soil formation occurred about 6800 yr B.P. Early Holocene fill has been found only in the basin's upper reaches, indicating that extensive erosion during the middle Holocene removed most early-Holocene fill from the middle and lower reaches of the basin. Valley filling between 5000 and 1000 yr B.P. was interrupted by soil formation about 1800, 1500, and 1200 yr B.P. As much as 6 m of entrenchment has occurred in the past 1000 yr. Holocene events in Wolf Creek basin correlate well with those in other localities in the central Great Plains, indicating that widespread changes in climate, along with adjustments driven by complex response, influenced fluvial activity.  相似文献   

5.
A synthesis of previous work and new data on the stratigraphy of high terraces of the Ohio and Monongahela Rivers upstream from Parkersburg, West Virginia, indicates a correspondence between terrace histories in the ancient Teays and Pittsburgh drainage basins. Four terraces are identified in each. Sediments of the lower three alluvial and slackwater terraces, correlated with Illinoian, early Wisconsin, and late Wisconsin glacial deposits, have been traced along the modern Ohio River through the former divide between the Teays and Pittsburgh basins. Sediments in the fourth terrace, the highest well-defined terrace in each basin, were deposited in two ice-dammed lakes, separated by a divide near New Martinsville, West Virginia. Some deposits of the highest slackwater terrace in both the Teays and Pittsburgh basins have reversed remanent magnetic polarity. This, and the stratigraphic succession in the two basins, suggests that both were ponded during the same glaciation. Reversed polarity in these terrace sediments restricts the age of the first ice-damming event for which stratigraphic evidence is well-preserved to a pre-Illinoian, early Pleistocene glaciation prior to 788,000 yr ago. In contrast, slackwater sediments in the Monongahela River valley, upstream from an outwash gravel dam at the Allegheny-Monongahela confluence, have normal remanent magnetic polarity, corroborating correlation with an Illinoian ponding event.  相似文献   

6.
A complex late Quaternary alluvial history was documented along Henson Creek, a low order tributary on the Fort Hood Military Reservation in central Texas. Three Quaternary alluvial landforms were recognized: terrace 2 (T2), terrace 1 (T1), and the modern floodplain (T0). The late Pleistocene T2 terrace may contain an array of sites spanning the entire known cultural record, while T1 may have sites spanning the last 5000 years only. Five fluvial units, three colluvial facies, two alluvial fan facies, and two buried paleosols were also recognized. Fluvial deposition was occurring approximately 15,000 yr B.P., 10,000-8000 yr B.P., 7000–4800 yr B.P., 1650-600 yr B.P., and during the last 400 years. Colluvial deposition was ongoing mainly in the early and middle Holocene, while alluvial fan aggradation was proceeding primarily in the middle Holocene. Because of erosional unconformities, there is minimal potential for recovering buried sites dating to intervals between depositional eposides for most of the drainage basin. Preservation potentials for buried sites are greatest in fine-grained fluvial deposits dating to the late Pleistocene, early Holocene, and parts of the late Holocene, and in fine-grained colluvial deposits dating to the early and middle Holocene. This investigation demonstrates that within the study area, and perhaps throughout much of central Texas, a greater continuum of sediments and preservation potentials exists in late Quaternary alluvial deposits of rivers than in low-order tributaries.  相似文献   

7.
This paper deals with new discovery of cosmic spherules and microdiamonds from Middle Pleistocene Selenge river basin on the Teel aggradation terrace. The Teel aggradation terrace is located at the left edge of the Selenge River, at the pedestal of mountain Namnan uul (Dorjnamjaa et al., 2008). The Teel ter-race is those whose cusp and bench entirely composed of alluvial deposits. This indicates that the Selenge River has a long history of development, and had time to develop a flood plain and to deposit alluvium, through which it cut subsequently and left behind a Teel terrace. Alluvium studies may be of great practi-cal interest because in some areas, river terraces are veritable treasure-troves of economic minerals. With river terraces are associated most placer deposits of such important economic minerals as gold, platinum, diamond, etc. Numerous engineering projects, such as bridges(Kherlen, Tuul, Selenge, Orkhon, Baidrag, Zavkhan, etc.), dams (Ulaan Boom), hydropower plants(Durgun Nuur, Ulaan Boom) are built on alluvial deposits. Hence we need to know all the essential fea-tures of the geostructure of river terraces. Our investi-gation of the Selenge river terraces in 2006-2008 gave a possibility to discover the placer diamonds within the Teel aggradation terrace for the first time in Mongolia.  相似文献   

8.
Analysis of a flight of alluvial terraces in the Sierra Nevada National Park near Pico Mucuñuque in the Eastern Mérida Andes has yielded information on geomorphic, pedogenic, and vegetational changes from Late Glacial time to the present. The terraces formed in large part due to stream incision/migration triggered by neotectonic uplift (>7000 yr BP) of a Late Glacial/Early Holocene glaciolacustrine lithosequence and, with the exception of the oldest/highest terrace, exhibit near-uniform lithology/parent materials. Soils developed in the terrace materials range from thin, weakly developed profiles (O/C/Cu horizons) to Entisols with O/Ah/Cox/Cu horizons and similar buried counterparts representing former short periods of floodplain stability or slow aggradation. The buried soils provide organic-rich material that yields radiocarbon ages, which provide time constraints on individual pedons and the geomorphic development of the site. Iron and aluminum extracts of soil matrix material provide information on the formation and accumulation of goethite and hematite, the relative accumulation of ferrihydrite (gain/loss), and the downward translocation of organically complexed Al as a function of soil development and age. SEM analysis of heavy mineral grains indicates varying material sources and degrees of weathering in the soil chronosequence. A qualitative study of plant functional types across the terrace sequence shows that older surfaces support greater plant diversity. The study also suggests ways in which the plant communities influence soil development at the site through varying organic matter inputs and varying soil moisture use by specific species (e.g., ferns on the oldest terrace), which may explain the absence of B horizons in the Late Pleistocene/Early Holocene soils.  相似文献   

9.
The geomorphology of the river terraces in the lower Vistula River valley of North PÖland is briefly described. They were earlier regarded as Late Wiirmian (terraces IX-IV) and Holocene (terraces III–I). Litho- and biostratigraphical studies of terrace depressions together with radiocarbon datings of their bottom layers indicate that even terrace II was formed during the Allerød Chronozone (11,800 to 11,000 B.P.) or earlier. In addition, radiocarbon datings have shown that the surface sediments of the flood plain were deposited in middle Holocene. These datings are of importance to the chronology of other large river valleys in the southern Baltic region. The interrelationship between erosion/accumulation in the Vistula valley and the shore level of the Baltic is also discussed.  相似文献   

10.
Recent research on the Somme Valley terrace deposits, that began in 1987, has resulted in an integrated sequence of geological and geomorphological evidence for this area that is of particular significance for prehistoric investigations. This has been achieved using previous evidence and new results from archaeological and geological excavations. These include longitudinal profiles, cartography and detailed synthesis of sections of alluvial deposits, stratigraphy of river deposits and overlying loess sequences. The work, based on a study of the geometry of the terrace system, has resulted in the identification of ninestepped alluvial formations that occur along 70 km of the valley. Detailed study permits the integration of the various alluvial sequences. This suggests that each alluvial sequence results from sedimentation through a single glacial-interglacial climatic cycle. This hypothesis is confirmed by investigation of soil and loess sequences of the deposits overlying the fluvial sediments. The synthesis presented provides a basis for understanding the deposition of the terrace system and an initial chronostratigraphic interpretation for the deposits.  相似文献   

11.
14C ages supplemented by 230Th234U determinations have been obtained for calcareous deposits on Koror, Babelthuap, Auluptagel, and Adorius Islands in the Palau Group, West Caroline Islands. Test borings for a bridge between Koror and Babelthuap reveal shoreface terraces consisting largely of bioclastic sand, but resembling fringing reefs. The base of the shoreface terraces dates to between 7000 and 8000 14C yr B.P. and the upper surfaces are slightly younger than 4000 yr B.P. Spacing of the subsurface isochrons indicates that the rate of sedimentation increased up-section on Koror terrace and decreased up-section on Babelthuap terrace. The average rate of deposition in the terraces was 0.5 cm/yr. Isochrons in the Babelthuap shoreface terrace are 7 m higher than those in the Koror terrace. If deposition was sufficient to keep the surfaces of the terraces at low tide level, then the Babelthuap side was essentially stable and the Koror side was uplifted between about 8000 and 6300 yr ago, and then subsided until 4000 yr ago; since then there has been about 2 m of uplift. “Top hat” microatolls on Koror terrace indicate that relative sea level has dropped about 30 cm in the past 75 yr. In the absence of good evidence for changes of level in water ponded in a moat, it is likely that the microatolls indicate uplift of the terrace. On the other hand, if tectonic activity was minimal, then differences in the two terraces are due to differences in sedimentation with the Koror side of the channel being substantially subtidal between about 7000 and 5000 yr B.P. Taking the composite eustatic sea-level curve of Hawaii and elsewhere as a reference standard, it is deduced that Auluptagel Island has risen 0.8 m in the last 2900 yr, and Adorius Island has risen approximately 8 m in the last 5000–6000 yr.  相似文献   

12.
 Land subsidence in the form of sinks has occurred on and near farmlands near Tucson, Pima County, Arizona, USA. The sinks occur in alluvial deposits along the flood plain of the Santa Cruz River, and have made farmlands dangerous and unsuitable for farming. More than 1700 sinks are confined to the flood plain of the Santa Cruz River and are grouped along two north-northwestward-trending bands that are approximately parallel to the river and other flood-plain drainages. An estimated 17,000 m3 of sediment have been removed in the formation of the sinks. Thirteen trenches were dug to depths of 4–6 m to characterize near-surface sediments in sink and nonsink areas. Sediments below about 2 m included a large percentage of dispersive clays in sink areas. Sediments in nonsink areas contain a large component of medium- to coarse-grained, moderately to well sorted sand that probably fills a paleochannel. Electromagnetic surveys support the association of silts and clays in sink areas that are highly electrically conductive relative to sand in nonsink areas. Sinks probably are caused by the near-surface process of subsurface erosion of dispersive sediments along pre-existing cracks in predominantly silt and clay sediments. The pre-existing cracks probably result from desiccation or tension that developed during periods of water-table decline and channel incision during the past 100 years or in earlier periods. Submitted, April 1997 · Revised, November 1997 · Accepted, April 1998  相似文献   

13.
Early diagenesis of organic matter in sediments from two sites in the Peruvian Upwelling Zone (12°05′S, 77°39′W; 15°17′S, 75°24′W) has been studied by observing changes in the total organic carbon and lipid and humin fractions with depth. Transformations of the total carbon and humin fraction have been characterized by conventional and time-dependent solid state NMR techniques, while lipid diagenesis was monitored by measuring the concentration of sterols in the same sedimentary horizons. Both the quantity of total sterols and the relative abundances of individual sterols vary with sampling location, suggesting a difference in the input of biomass to the sediments at the two sites. Total sterol concentrations decrease with depth at both sites, but the loss of sterols occurs much more slowly at the more anoxic northern site, where sedimentation rates and organic carbon contents are approximately twice those at the southern site. 13C-NMR spectra of the total organic carbon and the humin fraction suggest that humin-like compounds are an original component of the sedimentary biomass, and dipolar-dephased spectra of the humin residue indicate that diagenetic alterations of the humin fraction are occurring even in these very young sediments. Conventional and time-dependent spectroscopic data support the hypothesis that humin formation results from selective preservation of microbially-resistant biopolymers which are an original component of the sedimentary biomass combined with loss of certain labile compounds.  相似文献   

14.
Investigation of deeply stratified archaeological sites on the Piscataquis River in central Maine has revealed a continuous sequence of human occupation from ca. 10,000 yr B.P. to the near present. As an integrated aspect of the archaeological excavations, geomorphic features and surficial deposits were mapped from aerial photographs and field checked. Stratigraphic profiles were recorded from various locations and sediment column samples were collected and analyzed for grain size distribution in 1-cm vertical intervals from a 3-m deep excavation at the Sharrow site (ME 90-2D) to examine the flood deposits. Results include a general chronology of fluvial processes of the Piscataquis River since deglaciation and evidence of dominant vertical accretion of flood sediments on floodplain terraces throughout the Holocene epoch. This article defines an archaeologically significant but rarely recognized depositional regime characterized by a laterally controlled river channel and the exclusive vertical accretion of flood sediments. © 1994 John Wiley & Sons, Inc.  相似文献   

15.
A soil survey around the archaeological site of Harappa, Pakistan revealed alluvial deposits of five distinct ages based on relative position in the landscape and degree of soil profile development. the youngest deposit (age 1) is in the lowest landscape position and has received flood waters as recently as 1988. Soils there are in an incipient stage of development: only organic carbon and soluble salts have accumulated at the surface of the profile. the age 2 deposit has not undergone significant pedogenic change, but is in a slightly higher landscape position than the youngest deposit. Elevated concentrations of P, and the presence of sand-sized pottery and brick fragments, indicate that this deposit was derived at least partially from archaeological material. the presence of small, soft calcite nodules (Stage II) and some soluble salt translocation are the primary pedogenic changes observed in the age 3 deposit. the age 4 deposit shows evidence of both carbonate and gypsum accumulation. Presence of large gypsum nodules in deep By horizons suggests that a high groundwater table has altered these soils. the oldest deposit, age 5, forms a late Pleistocene stream terrace of the Ravi River. the soil formed in this deposit exhibits considerable carbonate accumulation, with large, dense nodules (Stage II + ) and an argillic horizon. A 14C date from pedogenic calcite gives an age of 7080 ± 90 years B.P., indicating a minimum age of early Holocene. the soil survey suggests that the ancient city of Harappa was built on an age 5 stream terrace remnant, surrounded by Holocene floodplains and a meandering channel of the Ravi River.  相似文献   

16.
Radiocarbon and thermoluminescence (TL) age-determinations have been obtained for a large Pleistocene alluvial terrace on the Nepean River near Sydney, New South Wales, Australia. The deposit was laid down by a braided river system prior to the last glacial maximum. Six thoroughly pretreated samples of charcoal and degraded wood buried within gravels at the base of the terrace yielded apparent 14C ages of 37,000–42,000 yr B.P. These compare favorably with four TL determinations that gave apparent ages of 41,000–47,000 yr B.P. for the same deposit. Adjustment of the 14C ages to take geomagnetic effects into account further improves the correlation between these two independent dating techniques. In addition, 14C and TL correctly identified a reworked portion of the fine-grained alluvial overburden as being substantially younger than the main body of the terrace. These results augur well for the usefulness of TL age determinations of certain alluvial deposits.  相似文献   

17.
孙健  吴良芳  杨献忠 《江苏地质》2006,30(3):226-230
根据地形地貌和岩土组合特征,该区可分为低山丘陵—山麓基岩区、山前平原—阶地中硬土区和长江漫滩软土沉积区,覆盖层主要是长江冲、淤积形成的新近系沉积软弱土。主要讨论漫滩软土及中硬土的特征,并对其工程地质适宜性及建筑适宜性进行了评价。  相似文献   

18.
Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km2, with a maximum depth of 680 m and contained an estimated 832 km3 of water; the smaller lake contained an estimated 80 km3 of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 14C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 14C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 × 106 m3 s−1. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.  相似文献   

19.
In 1998, geoarchaeological research was carried out at the Upper Paleolithic site of Studenoe in the Transbaikal Region of Russia. The site is divided into three loci situated on two terraces overlooking the Chikoi River. Alluvial sediments beneath the oldest terrace (T2) consist of two depositional units. Radiocarbon ages indicate that aggradation of the T2 fill began before 18,000 yr B.P. Alluvial sediments beneath the lower terrace (T1) range in age from 13,000 to 10,000 yr B.P. and are divided into three depositional units. Both terraces are overlain by Holocene colluvium. Archaeological materials at Studenoe 1/1, 1/2, and 2 include dwellings, hearths, and thousands of bone and stone artifacts assigned to the late Upper Paleolithic through the Bronze Age. Evidence of microblade technology is present in all components of the site. Material from recent excavations of Paleolithic levels in the T2 fill at Studenoe includes mobiliary art, bone needles, and a large dwelling with four hearth features. © 2003 Wiley Periodicals, Inc.  相似文献   

20.
Gao, C. & Boreham, S. 2010: Ipswichian (Eemian) floodplain deposits and terrace stratigraphy in the lower Great Ouse and Cam valleys, southern England, UK. Boreas, 10.1111/j.1502‐3885.2010.00191.x. ISSN 0300‐9483. Thick argillaceous deposits named the Mannings Farm Beds recently uncovered in the third terrace at Mannings Farm near Willingham, Cambridgeshire contain a pollen sequence covering the transitions from Ipswichian/Eemian substages I to II and II to III, when oak and hornbeam expanded, respectively. This is the longest record hitherto obtained in Britain, providing important insight into the major forest successions in this temperate stage. The frequent occurrence of Ipswichian deposits in the third terrace suggests the development of an extensive floodplain on the valley bottom, similar to the case for the present‐day lower Great Ouse and Cam. The Mannings Farm Beds testify to a complete interglacial sequence emplaced between cold‐climate gravels that was directly associated with the terrace development. The third terrace developed during the Ipswichian and the preceding and succeeding cold stages. Major river downcutting, which shaped the third terrace, occurred during the Early Devensian/Weichselian. Previously reported interglacial fossils from this terrace that are inconsistent with an Ipswichian affinity are probably reworked material derived from pre‐Ipswichian interglacial deposits, or their significance as biostratigraphical indicators needs to be confirmed. The second and first terraces developed from the late Early Devensian onwards. Ipswichian deposits filling flood‐scoured deep channels in bedrock are preserved locally below these low terraces.  相似文献   

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