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1.
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2.
Well-preserved tufa deposits exhibit a diversity of field fabrics along the major structural feature in Gafsa area and extent between Jebel El Mida in the southeast and Sidi Ahmed Zarrouk and Jebel Ben Younes in northwest. The deposition history of the tufa is defined by three major stages; the first occurred in a perched springline or slope system with dominance of the autochthonous facies (stromatolitic tufa facies, i.e. boundstone sheets of micrite and peloids, and phytoherm framestone facies) in the proximal zone and microdetrital tufa in distal sites. The second stage occurred in a mixture of perched springline and poorly drained paludal environments that characterized by the abundance of allochems and chalk and marl. The third deposition stage occurred in a mixture of paludal and braided fluviatile environments with allochthonous tufa facies (oncoidal cyanolith tufa facies, phytoclast tufa facies and lithoclast and intraclast tufa facies). Tufas are of particular interest since their occurrences are linked to tectonic activity and extensional fault systems. Gafsa strike-slip fault, in addition to its tectonic role in creating fluid paths to the surface through flowing springs, it acts as a major regional sill that controlled paleoflow directions, discharge locations, volume, rate and fluctuations of the water supply. Tufa cessation could be explained by increasing aridity during late Holocene and subsequent reduced rainfall, increased evaporation and water table falling, which reduce the amount of recharge and its subsequent dissolution in the recharge area.  相似文献   

3.
Marine Oxygen Isotope Stage 11 (MIS 11, Hoxnian Interglacial) is an important interval for understanding both climate change in an interglacial partially analogous to the Holocene and the response of geomorphic processes, biotic systems, and hominin populations to these changes. In Britain, many sites correlated to MIS 11 have not been studied since the mid-20th century and require reinvestigation, including the Hitchin tufa sequence, where a rich, non-marine molluscan assemblage was originally recovered. Re-excavation of the Hitchin tufa sequence for this study was focussed on combined sedimentological, micromorphological and geochemical analyses of the deposits. These indicate that tufa formation occurred within a perched springline system under temperate climatic conditions. Shifts between paludal and fluvial tufa facies within this system occur concomitantly with changes in carbonate geochemistry, representing increased humidity caused by a change in rainfall amount or seasonality. This research enables a correlation of the sequence to the climatic optimum of MIS 11c, the main warm phase of MIS 11, and permits further insights into temperature and hydrological changes in this interval by generating the first geochemical records of hydroclimatic evolution during the MIS 11 thermal maximum in Britain.  相似文献   

4.
Pedley  Hill  Denton  & Brasington 《Sedimentology》2000,47(3):721-737
Unlithified and partly lithified carbonate sequences are ideally suited to the application of ground‐penetrating radar (GPR), augmented by percussion augering and shallow seismic techniques, all tied to present‐day topography using global positioning system (GPS) methods. This methodology provides the first clear information on the distribution and geometry of lithofacies within buried tufa complexes. The approach has been applied to a thick succession of Holocene tufas filling a gorge site along a 3·5‐km length of the River Lathkill, north Derbyshire. Earlier studies have demonstrated the presence of up to 16 m of tufas and sapropels associated with two transverse tufa dams (barrages). These strata have been accumulating throughout the Holocene, although tufa developments at present are of minor extent. Internal tufa morphologies are recorded by GPR as ‘bright’, laterally continuous reflections for lithified, concretionary and lithoclast‐rich horizons. The ‘brightest’ reflectors occur within well‐cemented barrages and delineate core areas and prograding buttress zones. In contrast, unlithified lime muds and sapropels produce low‐contrast reflections. Lithostratigraphic control and depth calibration of the GPR profiles was provided by percussion augering at selected sites. Six distinct lithofacies and four secondary barrages are identified in the study. Constructional and destructional events can be identified and correlated within the GPR profile network, and the internal growth morphologies of the barrages are apparent. GPR profiles also clearly define the evolution of the facies geometries. Three phases of tufa development can be recognized within the GPR data and greatly extend our understanding of Holocene tufa‐forming processes in valley sites: (a) Early Holocene barrage build‐ups but with limited paludal deposition; (b) Middle Holocene ponding and sapropel accumulation under ‘warm’ conditions; and (c) Late Holocene barrage termination and valley levelling, probably coincidental with anthropogenic activity. This type of multidisciplinary approach should be considered as an essential prerequisite to all biostratigraphic and geochemical studies of Holocene freshwater carbonate sites.  相似文献   

5.
The effect of changing palaeoclimate and palaeoenvironment on human evolution during the Pleistocene is debated, but hampered by few East African records directly associated with archaeological sites prior to the Last Glacial Maximum. Middle to Late Pleistocene deposits on the shoreline of eastern Lake Victoria preserve abundant vertebrate fossils and Middle Stone Age arte‐facts associated with riverine tufas at the base of the deposits, which are ideal for palaeoenvironmental reconstructions. New data from tufas identified on Rusinga Island and on the mainland near Karungu, Kenya are provided from outcrop, thin sections, mineralogical, stable isotopic and U‐series dating analyses. Tufa is identified in four sites: Nyamita (94·0 ± 3·3 and 111·4 ± 4·2 ka); Kisaaka, Aringo (455 ± 45 ka); and Obware. The age ranges of these tufa deposits demonstrate that spring‐fed rivers were a recurrent, variably preserved feature on the Pleistocene landscape for ca 360 kyr. Poor sorting of clastic facies from all sites indicates flashy, ephemeral discharge, but these facies are commonly associated with barrage tufas, paludal environments with δ13C values of ca 10‰ indicative of C3 plants and fossil Hippopotamus, all of which indicate a perennial water source. Other tufa deposits from Nyamita, Obware and Aringo have a mixed C3/C4 signature consistent with a semi‐arid C4 grassland surrounding these spring‐fed rivers. The δ18O values of tufa from Nyamita are on average ca 1‰ more negative than calcite precipitated from modern rainfall in the region, suggesting greater contribution of depleted monsoonal input, similar to the Last Glacial Maximum. Microdebitage and surface‐collected artefacts indicate that early modern humans were utilizing these spring‐fed rivers. The presence of spring?fed rivers would have afforded animals a reliable water source, sustaining a diverse plant and animal community in an otherwise arid environment.  相似文献   

6.
The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.  相似文献   

7.
Mounds that have formed around spring vents occur in a variety of environmental settings, many at sites generally difficult or inaccessible for sampling. In contrast, over 500 tufa mounds occur in the dry bed of Searles Lake, California. The mounds range from minor features to 45 m in height; most are 5 to 12 m high. These mounds, composed of calcite and aragonite, formed associated with spring vents in the Pleistocene lake bottom. Thus, analyses of these mounds in Searles Lake provide a model with regard to the origin and architecture of tufa mounds. The mounds consist of four distinctive tufa facies. The initial deposits consist of porous tufa, including the innermost (porous 1) and the outermost (porous 2) deposits, followed by nodular tufa, then columnar tufa, and laminated crusts. There are two simple sequences of tufa deposition. The first sequence is from porous 1 to nodular to laminated crusts and, finally, to porous 2. A second sequence consists of: porous 1 to columnar to laminated crusts and, lastly, to porous 2. Facies changes are a response to changes in environmental conditions from deep water (porous 1 facies) to an essentially dry lake phase (during and after the formation of laminated crusts facies), to deep water (porous 2 facies) and, at the present time, totally dry. The primary constituents that comprise the tufa deposits include thin laminae, pisoids, spherulites, peloids and stromatolite‐like crusts. On the microscopic scale, these constituents dominantly make up nano‐spheres, micro‐rods and rod‐like crystals, as well as other calcified bodies. These constituents are interpreted to be the calcified remains of bacterial bodies. These findings suggest that microbial participation in the construct of other mounds should be a major concern of investigation, both for terrestrial and extraterrestrial spring‐fed mounds.  相似文献   

8.
Marine Isotope Stage (MIS) 11 palaeoclimate has so far been documented in marine and ice sheet isotopic records. However, excepting some lacustrine pollen records, very little is known about palaeoclimatic conditions in continental areas. This study uses geochemical records in calcareous tufa deposits from rivers as a basis for reconstructing temperate palaeoclimatic conditions. Tufa deposits are now proven to record high‐quality palaeoclimatic information in recent to Holocene deposits. Work on older interglacial tufas is just starting and in this paper we present the first comprehensive results from a MIS 11 tufa. The tufa comes from the Seine Valley (La Celle, northern France). Geochemical data in the tufa calcite are interpreted to record primarily air temperature (δ18O) and humidity (δ13C and Mg/Ca and Sr/Ca). The combined data identify a warm and wet climatic optimum followed by two temperature decreases associated with oscillations in humidity. These marked climatic variations recorded through the La Celle profile are strongly coherent with the palaeoenvironmental reconstructions from malacological data. The abrupt climatic and environmental events recorded could be related to short‐term degradation of vegetation cover in Europe, which is itself controlled by global palaeoclimatic events. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

9.
Continental carbonates of Quaternary age in southern Italy commonly exhibit the facies of calcareous tufa, often reported as related to shallow aquifers fed by meteoric waters and to organic processes. A close spatial relationship exists between the mappable tufa deposits and major Quaternary extensional faults. With respect to the Ca‐Mg‐HCO3 composition of limestone aquifers’ springs, tufa‐depositing springs exhibit higher salinity and alkalinity, are slightly warmer, have lower pH and are enriched in SO4 and CO2. Their δ13C values are systematically positive and compatible with a deep‐seated carbon source. A clear input of soil‐derived organic carbon is indicated only for small, non‐mappable tufas deposited by perched springs. The dataset indicates that the large tufa deposits owe their origin to a supplementary source of CO2 advected by degassing through active faults, as a necessary prerequisite for inducing a rise of total dissolved salts and alkalinity. Meteoric waters that have come from a shallow aquifer are able to precipitate only limited amount of carbonates.  相似文献   

10.
Tufa deposits are potential terrestrial archives of palaeoenvironmental and palaeoclimatic information. This study assesses the potential of stable isotopic archives from two closely juxtaposed Holocene tufa sites in SE Spain. The Ruidera site contains deep‐water lacustrine micrites and tufas, whereas the nearby Alcaraz site represents a shallow barrage tufa. Understanding site characteristics is critical to interpreting the stable isotopic variations. These Holocene lacustrine micrites have isotopic compositions consistent with modern European lake shore microbial carbonates, where the isotopic chemistry is strongly influenced by hydrological and residence time effects. All the lacustrine micrite δ13C values were influenced by microenvironmental microbial effects to some degree. Because of these effects, stable isotope data from lacustrine microbial micrites and tufas will not normally yield precise information on the isotopic composition of palaeoprecipitation, temperature or vegetation composition of an area. In contrast, Holocene tufas that formed in shallow, fast‐flowing riverine settings may contain valuable palaeoclimatic archives. The tufa deposits must be largely autochthonous, as at Alcaraz, where in situ reed stem encrustations are present. Records of relative change in air temperature and changes in the source of airmasses are potentially resolvable in the δ18O data. These interpretations can be verified by other independent climatic data where chronology is constrained. Variations in riverine tufa δ13C values probably record changes in local vegetation and/or soil respiration. Covariation between δ18O and δ13C values may be intrinsically linked to climatic factors such as aridity. Tentative palaeoclimatic interpretations for the middle Holocene at Alcaraz based on the isotope data suggest warming (or increasing influence of Mediterranean‐sourced precipitation) between approximately 5000–3000 radiocarbon years BP, accompanied by increased aridity. These interpretations are consistent with the sparse independent palaeoclimatic data and climate modelling results for the Holocene of SE Spain. This study supports the growing evidence that well‐chosen tufa sites could yield valuable palaeoclimatic information.  相似文献   

11.
The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km2) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ13C and δ18O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.  相似文献   

12.
This review considers the geological and geomorphological context of tufa barrages that occupy buried valley settings in the Wye catchment, Derbyshire. It describes the potential relationship of the tufa with locations of hypothesised river captures and inception horizon-guided groundwater flow paths. Tufa barrage development is associated with steps in the bedrock, which may be related to knick-point recession during river capture. Broad estimates of valley incision have been calculated from previously dated deposits. These support current interpretations of particularly significant effective base-level lowering during the Anglian and Devensian stages of the Quaternary and have the potential to add to the knowledge of regional uplift histories.  相似文献   

13.
《Quaternary Science Reviews》2004,23(7-8):947-958
High-resolution chronologies in continental carbonate deposits such as tufas are required for detailed palaeoclimatic and environmental studies. This work set out to establish if high-resolution U-series dating of detritus-rich Holocene tufas is routinely possible. The study centres on a paludal Holocene tufa from southern England that already has an existing Holocene chronology, based on 14C and supported by biostratigraphy, against which to compare U-series dates. The results show that significant detrital contamination combined with low initial U concentrations, and short time for ingrowth of radiogenic 230Th make high-resolution U-series dating of Holocene tufa very difficult. Moreover, a single (230Th/232Th)initial value to correct for the presence of detrital 230Th is not appropriate at the study site, a finding that may apply to most Holocene tufas. Total sample dissolution of coeval samples demonstrates considerable variability in the isotopic composition of the detritus. The total sample dissolution data are too scattered to constrain chronologies at the required resolution and may indicate the incorporation of a 230Th-rich component in the detritus.  相似文献   

14.
Tufa samples from 16 consecutive barrages along a 13 km section of the groundwater‐fed Krka River (Slovenia) were analysed for their petrographical, mineralogical, elemental and stable carbon (δ13C) and oxygen (δ18O) isotope composition, to establish their relation to current climatic and hydrological conditions. Waters constantly oversaturated with calcite and the steep morphology of the Krka riverbed stimulate rapid CO2 degassing and subsequent tufa precipitation. The carbon isotope fractionation (Δ13C) between dissolved inorganic carbon and tufa in the Krka River evolves towards isotopic equilibrium being controlled by continuous CO2 degassing and tufa precipitation rate downstream. The Δ13C increased from 1·9 to 2·5‰ (VPDB); however, since tufa precipitation rates remain similar downstream, the major controlling factor of carbon isotope exchange is most probably related to the continuous 12CO2 degassing downstream leaving the carbon pool enriched in 13C. In the case of oxygen, the isotope fractionation (Δ18O) was found to be from 1·0 to 2·3‰ (VSMOW) smaller than reported in the literature. The observed discrepancies are due to different precipitation rates of calcite deposits because Krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools. Due to non‐equilibrium oxygen isotope exchange between Krka tufa and water, the δ18O proxy showed from 1·2 to 8·2°C higher calculated water temperatures compared to measured water temperatures, demonstrating that δ18O proxy‐based temperature equations are not reliable for water temperature calculations of fast‐growing tufa on cascades. Because Mg is bound to the terrigenous dolomite fraction in the Krka tufa samples, the Mg/Ca was also found to be an unreliable temperature proxy yielding over up to 20°C higher calculated water temperatures.  相似文献   

15.
We carried out a geologic survey and a preliminary archaeological survey of four fossil‐spring tufa localities in Kharga Oasis, Egypt, to constrain the timing of pluvial episodes in the Western Desert, and to document prehistoric occupation contemporaneous with times of increased rainfall. Uranium‐series dating of the tufas confirms that at least five episodes of tufa deposition are represented in Kharga, although not every event is represented at each locality. Across the region studied, tufas were most frequently deposited as part of a fluvial barrage system, characterized by terraced, vegetated pools impounded by arcuate tufa dams and separated by small waterfalls. Available water resources during pluvial phases would have included not only spring‐fed streams but also small freshwater lakes. While Earlier Stone Age (ESA) and Middle Stone Age (MSA) lithic artifacts may be found either as surficial lags on tufas, or, less commonly, encased within tufas, Epipalaeolithic and Neolithic artifacts are generally found in or on silts within surface deflation depressions in the tufas, principally at Wadi Midauwara. © 2004 Wiley Periodicals, Inc.  相似文献   

16.
Tufa occurs widely in the Tatar A.S.S.R. and adjacent republics. It is associated with springs escaping from aquifers in deposits of Permian age. No tufa deposits were observed in the region where Jurassic and later deposits are developed. Tufa deposits occur because of a breakdown of calcium bicarbonate to CaCO3, water and CO2. The presence of organic matter and phosphorous inhibits this reaction, accounting in part for the lack of tufa deposition in the other regions. It appears also that tufa deposition is favored by 1) increase of water surface, hence increased evaporation and CaCO3 concentration and 2) slopes where argillaceous material is present.—J.J. Finney  相似文献   

17.
《Geodinamica Acta》2013,26(1-3):129-150
This work analyses a suite of relict tufa mounds generated by artesian karstic springs in Isona area (Spanish Pyrenees). Geological and geophysical data (seismics and vertical electrical resistivity soundings) indicate that the location of the discharge area in which the spring mounds formed was controlled by (1) a bulge in the axial zone of the anticline that affects the Areny-Montsec aquifer, with the consequent thinning of the overlying confining unit and (2) N-S and E-W trending extensional faults. These uncommon meteogene mounds occur in two stepped morphostratigraphic units that constitute the caprock of a mesa 9 km2 in area. The upper tufa complex is 47 m thick and has yielded several U/Th dates of >350 ka. The obtained U/Th ages for the mounds of lower tufa complex, 10 m thick, range from >350 to 214 ± 11 ka. The sedimentological analysis of the scarce exposures and electrical resistivity profiles show the same morphological and depositional components as those described in geothermal springs: (1) Cylindrical vents; (2) Pools fed by the vents and dammed by annular tufa barriers; (3) Tufa barriers (rimstones) constructed by overflowing waters through vertical accretion and progradation. These rimstones may have overhanging upflow sides. (4) Slope tufa facies with terracettes and microgours.  相似文献   

18.
对山西娘子关绵河不同阶地上沉积的泉钙华进行了高精度的230Th定年和碳氧稳定同位素组成测定。结果发现,绵河II级阶地沉积的娘子关泉钙华的最老年龄在407～466ka,远老于早前通过钙华中的石英砂热发光法(TL)获得的年龄,即绵河II级阶地上的娘子关泉钙华是在中更新世MIS12/11阶段形成的,而非原来认为的是晚更新世的产物。由此推测,绵河III级阶地娘子关泉钙华形成的年代更早,可能是中更新世的MIS14/13阶段。230Th测年获得的绵河I级阶地顶部的钙华形成于5ka前,即是在全新世中期以前形成的。结合钙华规模及其碳氧稳定同位素组成分析进一步发现,上述绵河三级阶地上的娘子关泉钙华均主要是湿热气候下的产物。然而,自II级阶地钙华形成至今,气候总体上在向干冷方向发展,泉域植被则呈现退化的趋势。  相似文献   

19.
Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by230Th/234U ages of the cave samples,230Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous14C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported14C ages.  相似文献   

20.
An integrated petrographic and spectroscopic (X‐ray diffraction, phosphor imaging and synchrotron X‐ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X‐ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X‐ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine‐scale inclusions.  相似文献   

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