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1.
Alessandro Batezelli Francisco Sergio Bernardes Ladeira Diego Luciano do Nascimento Mrcio Luiz DA Silva 《Sedimentology》2019,66(2):699-735
The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km2 study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin. 相似文献
2.
GIORGIO BASILICI PATRICK FRANCISCO FÜHR DAL BÓ FRANCISCO SÉRGIO BERNARDES LADEIRA† 《Sedimentology》2009,56(6):1876-1904
Aeolian sand sheets, which are characterized by low relief surfaces that lack dunes, are common in arid and semi‐arid climatic settings. The surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects, or unstable such that it is affected by deflation or sedimentation. The Marília Formation (Late Cretaceous) may be interpreted as an ancient aeolian sand sheet area, where alternating phases of stability and instability of the accumulation surface have been recorded. Detailed field studies were carried out in several sections of the Marília Formation, where cyclic alternations of palaeosols and aeolian deposits were evident, using palaeopedological and facies analysis methods, supported in the laboratory by the analysis of rock samples, cut and polished in slabs, thin sections, scanning electron microscope images and X‐ray diffraction data from the clay minerals. The deposits comprise three lithofacies that, in order of abundance, are characterized by: (i) translatent wind‐ripple strata; (ii) flood deposits; and (iii) ephemeral river channel deposits. Palaeosols constitute, on average, 65% of the vertical succession. Three types of palaeosols (pedotypes) are recognized: (i) Aridisols; (ii) Entisols; and (iii) Vertisols. Erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits. The palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the Marília Formation to be defined as a flat area, dominated by aeolian sedimentation, with subordinate ephemeral river sedimentation, and characterized by a dry climatic setting with occasional rainfall. The climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development. A climate‐controlled model is proposed in which: (i) the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases; and (ii) the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation, thereby increasing the availability of sediment during the drier climatic phases. The ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment. 相似文献
3.
To date, discussion of changes in alluvial style and in the character of palaeosols in relation to changes in accommodation and sediment supply on floodplains has primarily been from a conceptual standpoint: few case studies are available against which to test ideas. One hundred and thirty metres of non-marine strata of the Dunvegan Formation were examined in 14 closely spaced sections in the canyon of the Kiskatinaw River, NE British Columbia, Canada. This site was located about 120 km inland from the transgressive limit of the contemporary marine shoreline and represents almost exclusively freshwater environments. Fluvial channels in the Kiskatinaw River section are of two types. Small, single-storey, very fine- to fine-grained sandstone ribbons with W/T ratios <30, encased in fine-grained floodplain sediments are interpreted as anastomosed channels. Fine- to medium-grained, laterally accreted point-bar deposits forming multistorey sand bodies with individual W/T ratios >30 are interpreted as the deposits of meandering rivers filling incised valleys. Interchannel facies include the deposits of crevasse channels and splays, lakes, floodplains and palaeosols. Floodplain palaeosols consist of laterally heterogeneous, simple palaeosol profiles and pedocomplexes similar to modern Entisols, Inceptisols and hydromorphic soils. Interfluve, sequence-bounding palaeosols adjacent to incised valleys are laterally continuous, up to 3 m thick and can be reliably identified using a combination of (1) stratigraphic position; (2) field observations, such as thickness, structure, colour, degree of rooting; and (3) micromorphological features, such as evidence of bioturbation, clay coatings, ferruginous features and sphaerosiderite. Interfluve palaeosols are similar to modern Alfisols and Ultisols. Correlation of the local stratigraphic succession with the regional sequence stratigraphic framework, based on 2340 well logs and 60 outcrop sections, shows that the vertical changes in coastal plain character (more coals and lakes vs. more pedogenesis) can be related to relatively high-frequency base level cycles (eustatic?) that are expressed as transgressive–regressive marine cycles in downdip areas. Regional isopach maps show that these cycles were progressively overprinted and modified by an increasing rate of tectonic subsidence in the north and west. The character of palaeosols developed on aggrading floodplains primarily reflects local sediment supply and drainage. In contrast, well-developed interfluve palaeosols record pedogenesis during periods of reduced or negative accommodation (base level fall). Vertical changes in floodplain palaeoenvironments and palaeosol types reflect changes in accommodation rate. The detailed micromorphological analysis of interfluve palaeosols represents a powerful application of an under-used technique for the recognition of key surfaces in the geological record. This has broad implications for non-marine sequence stratigraphy. 相似文献
4.
The relationship between palaeosols and sequence stratigraphy is tested in the Lower Permian Abo Member, south‐central New Mexico, by comparing interfluve and fluvial‐terrace palaeosols with palaeosols that developed within lowstand‐fluvial deposits. Interfluve and fluvial‐terrace palaeosols consist of primary pedogenic features, including vertical root traces, vertic structures, Stage II and III pedogenic calcite and translocated clay (argillans), which are cross‐cut or replaced by low‐aluminium goethite, gley colour mottling, sparry calcite veins and ankerite. The polygenetic character of the palaeosols is consistent with initial development for several thousand to tens of thousands of years on well‐drained interfluves or fluvial terraces, followed by waterlogging due to invasion by a rising water table that locally may have been brackish. In contrast, lowstand‐fluvial sediment that filled incised valleys contains only rooted and vertic palaeosols, whose immaturity resulted from high aggradation rates. Palaeosols similar to those in the Abo Member have been recognized in other ancient strata and, when combined with high‐resolution correlation, provide evidence for interpretation of sequence‐stratigraphic surfaces and systems tracts. 相似文献
5.
The Lower Cretaceous (Albian) upper Blairmore Group is part of a thick clastic wedge that formed adjacent to the rising Cordillera in south-western Alberta. Regional transgressive intervals are superimposed on the overall regressive succession. Alluvial conglomerates, sandstones and mudstones were deposited in east-north-eastward draining fluvial systems, orientated transverse to the basin axis. Five facies associations have been identified: igneous pebble conglomerate, thick sandstone, interbedded lenticular sandstone and mudstone, thick mudstone with thin sandstone interlayers, and fossiliferous sandstone and mudstone. The facies associations are interpreted as gravelly fluvial channels, sandy fluvial channels, sand-dominated floodplains, mud-dominated floodplains, and marine shoreline deposits, respectively. Five types of palaeosols are recognized in the upper Blairmore Group based on lithology, the presence of pedogenic features (clay coatings, root traces, ferruginous nodules, slickensides, carbonate nodules) and degree of horizonization. The regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution. In source-proximal areas, palaeosol development was inhibited by high rates of sedimentation. In source-distal locations, poor drainage resulting from high watertables, low topography and lower rates of sedimentation also inhibited palaeosol development. The best-developed palaeosols (containing Bt horizons) occur in intermediate alluvial plain positions (tectonic hinge zone) where the floodplains were most stable due to a balance between sedimentation, erosion and subsidence rates. Extrapolating from the upper Blairmore Group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis. At the hinge zone, soil development is controlled primarily by climate and tectonics and their effect on sediment supply, whereas closer to the palaeoshoreline, relative sea level fluctuations, resulting in poor drainage, may have a more significant influence. 相似文献
6.
《Quaternary Science Reviews》2002,21(7):811-817
Five lateral sand–loess–palaeosol continua occur within the last glacial sediments of the central Loess Plateau of China along a 500 km north to south climatic gradient. The continua shift southward or northward in concert with desert expansion or contraction, respectively. Lateral lithofacies (desert sand to loess) variations are evident at the north end of the gradient and follow Walther's Law of the correlation of facies. Lateral pedofacies (loess to palaeosol) variations are present near the south end of the gradient, where the climate was warmer and wetter. The lateral stratigraphic changes from sand to loess or loess to soil are driven by variations in the rate of sedimentation along a climatic gradient.Vertical stratigraphic profiles at the north end of the gradient reveal alternating sand and loess beds. In contrast, alternating loess and palaeosols occur within the same stratigraphic interval in the southern Loess Plateau, where dust accretion rates were lower. However, in high resolution studies of climate change vertical profiles of alternating loess and palaeosols (especially weak palaeosols) may not reflect regional or global climate change. Alternating loess and weak palaeosols may reflect local variations in the balance between the rates of dust accretion and pedogenesis. Local fluctuations in either of these rates could result in the presence of time equivalent loess and palaeosols at high resolutions. Thus, some of the high resolution loess-palaeosol alternations may reflect local climatic variation rather than global or hemispherical climate change. 相似文献
7.
Climatic versus halokinetic control on sedimentation in a dryland fluvial succession 总被引：1，自引：1，他引：0
Fluvial systems and their preserved stratigraphic expression as the fill of evolving basins are controlled by multiple factors, which can vary both spatially and temporally, including prevailing climate, sediment provenance, localized changes in the rates of creation and infill of accommodation in response to subsidence, and diversion by surface topographic features. In basins that develop in response to halokinesis, mobilized salt tends to be displaced by sediment loading to create a series of rapidly subsiding mini‐basins, each separated by growing salt walls. The style and pattern of fluvial sedimentation governs the rate at which accommodation becomes filled, whereas the rate of growth of basin‐bounding salt walls governs whether an emergent surface topography will develop that has the potential to divert and modify fluvial drainage pathways and thereby dictate the resultant fluvial stratigraphic architecture. Discerning the relative roles played by halokinesis and other factors, such as climate‐driven variations in the rate and style of sediment supply, is far from straightforward. Diverse stratigraphic architectures present in temporally equivalent, neighbouring salt‐walled mini‐basins demonstrate the effectiveness of topographically elevated salt walls as agents that partition and guide fluvial pathways, and thereby control the loci of accumulation of fluvial successions in evolving mini‐basins: drainage pathways can be focused into a single mini‐basin to preserve a sand‐prone fill style, whilst leaving adjoining basins relatively sand‐starved. By contrast, over the evolutionary history of a suite of salt‐walled mini‐basins, region‐wide changes in fluvial style can be shown to have been driven by changes in palaeoclimate and sediment‐delivery style. The Triassic Moenkopi Formation of the south‐western USA represents the preserved expression of a dryland fluvial system that accumulated across a broad, low‐relief alluvial plain, in a regressive continental to paralic setting. Within south‐eastern Utah, the Moenkopi Formation accumulated in a series of actively subsiding salt‐walled mini‐basins, ongoing evolution of which exerted a significant control on the style of drainage and resultant pattern of stratigraphic accumulation. Drainage pathways developed axial (parallel) to salt walls, resulting in compartmentalized accumulation of strata whereby neighbouring mini‐basins record significant variations in sedimentary style at the same stratigraphic level. Despite the complexities created by halokinetic controls, the signature of climate‐driven sediment delivery can be deciphered from the preserved succession by comparison with the stratigraphic expression of part of the system that accumulated beyond the influence of halokinesis, and this approach can be used to demonstrate regional variations in climate‐controlled styles of sediment delivery. 相似文献
8.
GREGORY J. RETALLACK 《Sedimentology》2012,59(4):1208-1236
The Late Neoproterozoic Ediacara Member of the Rawnsley Quartzite in South Australia has been considered aeolian, fluvial, intertidal and deep marine by various authors. Palaeosols would not be expected for the deep marine interpretation, but some palaeosols should be evident for the aeolian–fluvial–intertidal interpretations, and this is the first study to examine the Ediacara Member at a petrographic and geochemical scale appropriate to recognize potential palaeosols. Recognition of palaeosols and floodplain facies in Neoproterozoic rocks is a challenge because such rocks are too ancient for diagnostic non‐marine fossils such as root traces. The varied thickness of Ediacara Member red siltstones and white sandstones is distinct from laterally persistent overlying and underlying grey shales and limestones with acritarchs, stromatolites and other marine fossils. The sandstones are trough cross‐bedded and fill palaeovalleys. The red siltstones have poorly sorted, highly angular, silt‐size grains characteristic of loess. Particular sandy and silty beds were sampled for detailed petrographic and geochemical studies, because they include desiccation cracks, sand crystals, ice cracks, carbonate nodules and soft‐sediment deformation like those of palaeosols. Chemical and grain‐size variations within these beds reveal surficial clay formation and oxidation from feldspar as in soils. Petrographic studies also revealed surficial disruption of these palaeosols by filamentous structures comparable with microbial ropes of biological soil crusts. This array of palaeosol features may be of use for recognizing palaeosols in other Neoproterozoic siliciclastic sequences. 相似文献
9.
ANDREW J. NEWELL MICHAEL J. BENTON TIMOTHY KEARSEY GRAEME TAYLOR RICHARD J. TWITCHETT VALENTIN P. TVERDOKHLEBOV 《Sedimentology》2012,59(5):1659-1676
The south Uralian foreland basin forms part of the giant, yet sparsely documented, PreCaspian salt tectonic province. The basin can potentially add much to the understanding of fluviolacustrine sedimentation within salt‐walled minibasins, where the literature has been highly reliant on only a few examples (such as the Paradox Basin of Utah). This paper describes the Late Permian terrestrial fill of the Kul’chumovo salt minibasin near Orenburg in the south Urals in which sediments were deposited in a range of channel, overbank and lacustrine environments. Palaeomagnetic stratigraphy shows that, during the Late Permian, the basin had a relatively slow and uniform subsidence pattern with widespread pedogenesis and calcrete development. Angular unconformities or halokinetic sequence boundaries cannot be recognized within the relatively fine‐grained fill, and stratigraphic and spatial variations in facies are therefore critical to understanding the subsidence history of the salt minibasin. Coarse‐grained channel belts show evidence for lateral relocation within the minibasin while the development of a thick stack of calcrete hardpans indicates that opposing parts of the minibasin became largely inactive for prolonged periods (possibly in the order of one million years). The regular vertical stacking of calcrete hardpans within floodplain mudstones provides further evidence that halokinetic minibasin growth is inherently episodic and cyclical. 相似文献
10.
Numerical, experimental and theoretical models of fluvial architecture and palaeosol development are tested with outcrops of Upper Pliocene-Lower Pleistocene sediment in the southern Rio Grande rift, New Mexico. The sediment was deposited and subsequently exhumed in the Jornada del Muerto basin, a westward-tilted half graben whose footwall corresponds to the Rincon Hills and San Diego Mountain fault blocks. The axial river, the ancestral Rio Grande, shared time between the Jornada del Muerto basin and the adjacent Corralitos basin. The ancestral Rio Grande entered the Jornada del Muerto basin via a gap between the footwall blocks, periodically flowing southward towards San Diego Mountain, or making a broad northward sweep into the northern fluvial salient towards the Rincon Hills fault block and unfaulted northern edge of the basin. Ten logged sections up to 35 m thick are correlated using the top of the formation (La Mesa surface), a 1·59 Ma pumice conglomerate, and a ground-water carbonate/opal bed. Additionally, one of the sections is dated by reversal magnetostratigraphy. Consistent with the model of Bridge & Leeder (1979 ) and Bridge & Mackey (1993a ), differential tilting of the Jornada del Muerto half graben resulted in sections directly adjacent to the faults that consist almost exclusively of multistorey channel sands/sandstones, whereas more distal sections contain a greater proportion of crevasse-splay fine sand and overbank mudstone and calcic palaeosols. Along the axis of the northern fluvial salient, a northward decrease in channel/floodplain ratio, a decrease in channel recurrence interval from 171 kyr to 685 kyr, and an increase in the maturity of calcic palaeosols are consistent with southward tilt of the unfaulted northern edge of the basin. An upsection decrease in sediment accumulation rate in the northern fluvial salient from 0·036 mm/ yr to 0·017 mm/ yr corresponds to an increase in the ratio of channel/floodplain facies and in the number of multistorey channel sands/sandstones, and is consistent with the model of Bridge & Leeder (1979 ) in which avulsion frequency is independent of sediment accumulation rate. Stage II and III calcic palaeosols indicate 103−105 year of landscape stability and soil formation between periods of floodplain deposition in response not only to basin tilting but also because the ancestral Rio Grande had multiple paths within the Jornada del Muerto basin and shared time between the Corralitos and Jornada del Muerto basins. 相似文献
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《Sedimentology》2018,65(4):1277-1300
Fluvially derived tuffaceous Chinle sandstones from Petrified Forest National Park provide a well‐preserved Late Triassic archive of climate information. Petrographic analysis of 38 Chinle sandstones provides new insight into the depositional history and evolution of palaeoclimate during Chinle deposition. This study focuses on the relationship between climate and meteoric diagenesis as a guide for constraining climate change in western equatorial Pangea during the Late Triassic. Petrographic analysis of Chinle sandstones reveals their wide range of textural attributes, as well as pedogenic and shallow burial diagenetic features that occurred during the Late Triassic. These diverse petrological characteristics are indicative of the evolving Late Triassic climate, when placed into a well‐constrained stratigraphic and geochronological framework. The stratigraphic succession is characterized by variations in the abundance of framework grains, detrital matrix, weathering intensity of feldspar and volcanic rock fragments, and the mineralogy of clay cements. Climate records from Chinle palaeosol geochemistry indicate a progression from wet to dry conditions. This trend is also reflected in the meteoric diagenetic features of Chinle sandstones. During deposition of the lower Chinle, elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite, whereas feldspars (especially plagioclase) were partially or completely dissolved. In the upper Chinle, a trend towards drier conditions favoured the formation of smectite and less feldspar dissolution resulting in a higher abundance of well‐preserved plagioclase grains. Shallow burial meteoric weathering reactions in Chinle sandstones reflect the evolving climate during the Late Triassic. 相似文献
13.
Analysis of stacked Permo‐Pennsylvanian palaeosols from north‐central Texas documents the influence of palaeolandscape position on pedogenesis in aggradational depositional settings. Palaeosols of the Eastern shelf of the Midland basin exhibit stratigraphic trends in the distribution of soil horizons, structure, rooting density, clay mineralogy and colour that record long‐term changes in soil‐forming conditions driven by both local processes and regional climate. Palaeosols similar to modern histosols, ultisols, vertisols, inceptisols and entisols, all bearing morphological, mineralogical and chemical characteristics consistent with a tropical, humid climate, represent the Late Pennsylvanian suite of palaeosol orders. Palaeosols similar to modern alfisols, vertisols, inceptisols, aridisols and entisols preserve characteristics indicative of a drier and seasonal tropical climate throughout the Lower Permian strata. The changes in palaeosol morphology are interpreted as being a result of an overall climatic trend from relatively humid and tropical, moist conditions characterized by high rainfall in the Late Pennsylvanian to progressively drier, semi‐arid to arid tropical climate characterized by seasonal rainfall in Early Permian time. Based on known Late Palaeozoic palaeogeography and current hypotheses for atmospheric circulation over western equatorial Pangea, the Pennsylvanian palaeosols in this study may be recording a climate that is the result of an orographic control over regional‐scale atmospheric circulation. The trend towards a drier climate interpreted from the Permian palaeosols may be recording the breakdown of this pre‐existing orographic effect and the onset of a monsoonal atmospheric circulation system over this region. 相似文献
14.
Little is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change. 相似文献
15.
Columnar structured horizons have been recognized in ancient coastal palaeosols of several Lower Permian (Asselian) stratigraphic units of north-central Kansas. These strongly developed columnar, polygonal-shaped peds are characteristic of sodium-influenced (natric) argillic horizons, and are commonly indicative of semi-arid to arid environments. Evaporite features above and below these palaeosols support the conclusion for a dry palaeoclimate. The columnar peds are typically 3–15 cm in diameter and exhibit domed tops. Fine clay fills the cracks between the columnar peds, and is generally of a darker colour than the peds. Each natric horizon has a low value and chroma colour, apparently the result of carbonate accumulation. The natric horizons in these Permian palaeosols appear to have been partially influenced by sodium-rich groundwaters. Root traces and root moulds are found between peds in all natric horizons, indicating plant succession after columnar ped formation. These sodium-influenced palaeosol profiles occur as part of a spectrum of palaeosol types that indicate cyclical climate change associated with glacioeustatic sea-level fluctuations. 相似文献
16.
Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes 
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下载免费PDF全文 Carys E. Bennett Timothy I. Kearsey Sarah J. Davies David Millward Jennifer A. Clack Timothy R. Smithson John E. A. Marshall 《Sedimentology》2016,63(6):1677-1700
Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems. 相似文献
17.
The ‘Red Clay’ is an important deposit underlying the Quaternary loess–palaeosol sequence in the Chinese Loess Plateau, being regarded as an excellent record of palaeoclimate changes in the late Tertiary. Several properties of the ‘Red Clay’ have been measured previously in order to derive climatic information. However, the sedimentary processes involved and the origin of the materials remain controversial. Here we present results of grain‐size analyses of the ‘Red Clay’ from four representative sites in the Chinese Loess Plateau. In particular their grain‐size distribution is compared with that of typical Quaternary aeolian loess–palaeosol, as well as lacustrine and fluvial sediments. It appears from the sedimentological evidence that the major part of the ‘Red Clay’ is of aeolian origin. It is rather similar in some of its properties to the Quaternary loessic palaeosols. The dust forming the ‘Red Clay’ was transported by a wind system that was weaker than that involved in the accretion of the Quaternary loess. Furthermore, the ‘Red Clay’ sediment has been modified by post‐depositional weathering. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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RICARDO NÉSTOR MELCHOR 《Sedimentology》2007,54(6):1417-1446
Well‐exposed Triassic rift strata from the Ischigualasto–Villa Unión Basin (NW Argentina) include a 80 to ca 515 m thick lacustrine‐dominated package that can be correlated across a half‐graben using key stratigraphic surfaces (sequence boundaries, lacustrine flooding surfaces and forced regressive surfaces). The characteristics of the synrift lacustrine fill in different parts of the half‐graben have been examined and the mechanisms controlling sedimentation inferred. A variety of sedimentary environments are recognized including; volcaniclastic floodplain, mildly saline lake and playa lake, offshore lacustrine, delta front to fluvial‐dominated and wave‐dominated deltas, distributary and fluvial channel, and interdistributary bay. The succession can be divided into four stratigraphic sequences (SS1 to SS4), the oldest of which (SS1) contains volcaniclastic, fluvial and saline lake deposits; it is thickest close to the western border fault zone, reflecting more rapid subsidence here. Accommodation exceeded sediment and water input during SS1. The second and third sequences (SS2 and SS3) mark the onset of widespread lacustrine sedimentation, reflecting a balance between accommodation creation and water and sediment fluxes. Sequences SS2 and SS3 are represented by offshore meromictic lacustrine and deltaic deposits, the latter mostly sourced from the flexural and southern axial margins of the half‐graben. The presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake‐level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone. The youngest sequence (SS4) is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies, palaeocurrents and sandstone composition, suggesting a switch in sediment supply to the footwall margin to the NW. The change in the sediment source is related to reduced footwall uplift, the possible presence of a relay ramp and/or supply from a captured antecedent drainage network. During SS4, the rate of creation of accommodation was exceeded by the sediment and water discharge. The stratigraphic evolution of lacustrine strata in the half‐graben was mainly controlled by tectonic processes, including subsidence rate and the growth and evolution of the border fault zone, but changing climate (inducing changes in water balance and lake level) and autocyclic processes (delta lobe switching) were also important. 相似文献
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JAVIER MARCELO KRAUSE EDUARDO SERGIO BELLOSI MARÍA SOL RAIGEMBORN 《Sedimentology》2010,57(7):1721-1749
Palaeosols of the Koluel‐Kaike Formation, a red colour‐banded, pyroclastic succession from southern Argentina, constitute a proxy for Eocene climate changes. Reticulated and vertically elongated ferric mottles, along with iron and manganese nodules are the most significant climate indicators, which originated by alternating cycles of waterlogging and drying conditions causing Fe‐Mn mobilization and fixation. Clay minerals vary from a kaolinite > smectite suite in the lower and middle sections, to a smectite > kaolinite one in the upper part. High concentrations of iron oxides/hydroxides and kaolinite, lack of exchangeable bases, absence of carbonate cement, presence of ironstone and redness of hue in most of the palaeosols suggest intense chemical weathering related to leaching and lateritization processes. Five pedotypes, ordered in a stratigraphic sense, were identified. Strongly developed, red to orange Chornk (Fragiaquult) and Kápenk (Plinthaquult) pedotypes display argillic horizons, abundant ferric nodules and slickensides; they are dominant in the lower and middle sections, and formed in seasonal humid and megathermic (tropical) conditions with a mean annual precipitation of 1200 to 1300 mm and a mean annual temperature of 15 °C. Weakly developed, less structured Ornek (Vitrand) and Pólnek (Placaquand) pedotypes occur in the middle and upper sections, and originated in sub‐humid and mesic‐megathermic conditions with a mean annual precipitation around 1000 mm and a mean annual temperature around 12 °C. The brownish Soorsh (Argialboll) pedotype exhibits a granular structure and is present at the uppermost part of the unit. It developed in sub‐humid/semi‐arid and mesic conditions, with a mean annual precipitation of 600 to 700 mm and a mean annual temperature around 10 °C. This pedotype succession and clay mineral distribution indicates a decrease in chemical weathering and degree of soil development with time. Koluel‐Kaike palaeosols from Central Patagonia are some of the first continental non‐palaeobiological data linked to the Early Palaeogene global warming in South America; they show an especially close relationship with the Early Eocene Climatic Optimum and the following long‐term cooling and drying initiated by Middle to Late Eocene time. 相似文献