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1.
Vegetation is a major driver of fluvial dynamics in modern rivers, but few facies models incorporate its influence. This article partially fills that gap by documenting the stratigraphy, architecture and palaeobotany of the Lower Pennsylvanian Boss Point Formation of Atlantic Canada, which contains some of the Earth's earliest accumulations of large woody debris. Braided‐fluvial systems occupied channel belts of varied scale within valleys several tens of metres deep and more than 12 km wide, and their deposits predominantly consist of sandy and gravelly bedforms with subordinate accretionary macroforms, high flow‐strength sand sheets and rippled abandonment facies. Discrete accumulations of clastic detritus and woody debris are up to 6 m thick and constitute at least 18% of the in‐channel deposits; they represent lags at the base of large and small channels, fills of minor channels and sandy macroforms that developed in central positions in the upper parts of channel fills. Sandstones with roots and other remnants of in situ vegetation demonstrate that vegetated islands were present, and the abundance of discrete channel fills suggests that the formation represents an anabranching, island‐braided sandbed river, the earliest example documented to date. Although some sphenopsid and lycopsid remains are present, most woody fragments are derived from cordaitalean trees, and the evolution of this group late in the Mississippian is inferred to have exerted a significant influence on fluvial morphodynamic patterns. The formation records a landscape in which active channel belts alternated with well‐drained floodplains colonized by dense, mature forests and local patches of pioneering, disturbance‐tolerant vegetation. Lakes and poorly drained floodplains dominated by carbonate and organic deposition, respectively, were also present. A large supply of woody debris triggered channel blockage and avulsion, and active channel margins and islands within the channel belts were initially colonized by pioneer vegetation and subsequently stabilized by large trees. A similar alternation of stable and unstable conditions is observed in modern braided rivers actively influenced by vegetation.  相似文献   

2.
A recent human campsite, occupied in 1973 by members of the Dassanetch tribe of northern Kenya, was observed from its creation through its subsequent burial in flood events 4 months later. The site was excavated as an archeological occurrence in the summer of 1974. Analyses of field and laboratory data yield a detailed picture of sedimentary structures, bone transport and burial, and site preservation in a well-documented depositional situation. Trampling by site occupants was apparently instrumental in burying much small bone prior to the flood events which acted on the site. About 30 cm of sediments accumulated on the site during four or five flood events. Individual sedimentary beds can be related to specific observed flood events in the drainage system. The ultimate preservation of the site as part of the region's archeological record would depend upon the interaction of sediment deflation and the varying local water table. Specific conditions which tend to preserve human sites in water-poor environments may consistently select seasonally biased vestiges of human settlement and economy.  相似文献   

3.
《Sedimentology》2018,65(6):2171-2201
In modern siliciclastic environments terrestrial and aquatic vegetation binds substrate, controls weathering and erosion rates, influences run‐off, sediment supply and subsequent depositional architecture. This study assesses the applicability of modern depositional models that are impacted by vascular vegetation, as analogues for ancient pre‐land plant systems. A review of pre‐Devonian published literature demonstrates a paucity of described tidal successions; this is possibly due to the application of modern analogues for interpreting the record when there is a lack of tidal indicators. This paucity suggests a need for revised models of tidal deposition that consider the different environmental conditions prior to land plant evolution. This study examines the Ordovician–Silurian Tumblagooda Sandstone, which is exposed in the gorge of the Murchison River and coastal cliffs near Kalbarri, Western Australia. The Tumblagooda Sandstone comprises stacked sand‐rich facies, with well‐preserved bedforms and trace fossils. Previous interpretations of the depositional setting have proposed from a mixed sheet‐braided fluvial and intertidal flats; to a continental setting dominated by fluvial and aeolian processes. An enigmatic element is the rarity of mud‐rich facies preserved in the succession. Outcrop logging, facies and petrographic analysis record dominantly shallow water conditions with episodes of emergence. Abundant ichnotaxa indicate that marine conditions and bi‐directional flow structures are evidence for an intertidal and subtidal depositional environment. A macrotidal estuary setting is proposed, with evidence for tidal channels and repeated fluvial incursions. Physical and biogenic sedimentary structures are indicative of tidal conditions. The lack of clay and silt resulted in the absence of flaser or lenticular‐bedding. Instead cyclic deposition of thin beds and foreset bioturbation replaced mud drape deposits. Higher energy conditions prevailed in the absence of the binding activity of plants in the terrestrial and marine realm. This is suggestive of different weathering processes and a reduction in the preservation of some sedimentary features.  相似文献   

4.
The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes.  相似文献   

5.
The last glacial shows large variations in climate, which are reflected in the fluvial record in the Niederlausitz, eastern Germany. The entire sequence resembles the fluvial development in other river basins in northwestern Europe, which show contemporaneous changes in depositional style at the onset of a climatic change. During the Middle and the Late Pleniglacial, permafrost conditions resulted in an episodic river discharge. The presence or absence of vegetation, in combination with such ephemeral stream conditions, determined the type of river during each period. A relatively well-developed vegetation cover on the flood plains during the Middle Pleniglacial resulted in a low sediment yield. In combination with the intermittent discharge, this caused the development of an ephemeral anastomosing river system, a river with stable channels and extensive sandy overbank areas. The decline in vegetation cover at ca. 28 ka BP caused an increase in sediment yield and peak discharges, which resulted in the development of a sandy braided river in adjustment to these new conditions. Following the coldest period at around 20 ka, precipitation was so low that fluvial activity was limited and aeolian deposition took place in the valley. © 1997 by John Wiley & Sons, Ltd.  相似文献   

6.
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   

7.
Shallow marine sediments of the Broughton Formation are dominated by immature volcanic debris of intermediate to basic composition, generated in an adjacent subaerial environment by volcanism responsible for the nine shoshonite units intercalated within sediments of the Kiama region. Sediment was supplied to the offshore environment via periodic storm‐generated, expanded high density turbidity currents. Initial deposition, represented by the Westley Park Sandstone Member, was below storm wave base, during which time the depositional surface was subjected to post‐depositional tractional reworking by northerly directed, tidally influenced bottom currents. The resulting positive‐relief sand bodies on the seafloor contain tractional sedimentary structures (the ‘tractional facies association'). Areas of the substrate between these sand bodies retained their turbidite bedding structure (the ‘rhythmically bedded facies association') but were extensively bioturbated by a diverse deposit‐feeding biomass.

Upon emplacement of the lowest of the nine shoshonite units as a tri‐composite, locally intrusive lava flow, the depositional surface was elevated, transgressing storm wave base. The body of the shoshonite flow also shielded the substrate from the northerly directed tractional currents, allowing the development and preservation of the hummocky cross‐stratified sandstone facies in the Kiama Sandstone Member. Following burial of the shoshonite flow by continued deposition, this local shielding effect was overcome and tractional currents again reworked the entire depositional surface.  相似文献   

8.
Studies on the genesis of subaerial debris flows and associated deposits are relatively rare in the literature, especially in an ice-marginal context of moraine formation. The present contribution reports results from both the macro- and micro-scales of a subaerial depositional setting in order to contribute to closing this gap. At the macroscale, alternating loose, stratified, clast- and matrix-supported diamicts and finely laminated sand units indicate deposition of debris flows and fluvial units in a subaerial, ice-marginal setting that were stacked up to form a terrestrial ice-contact fan. Macroscale and micromorphological analyses show that this fan displays evidence of a three-phased formation: (a) overriding and glaciotectonisation of pre-existing sediments followed by retreat and burial of this core by (b) ice-contact fan deposition dominated by water-rich fluvial deposition with relatively little debris flow activity and (c) a switch to a gravitational sedimentation style with dominantly debris flow deposition and fewer and thinner fluvial units. Thin sections of both the diamict and laminated sand units show evidence of deposition of a mud and fine sand-rich slurry being expelled from the tops of advancing mass flows. Water-rich fine-grained slurries appear to have been progressively overridden and deformed in response to ductile shear occurring at the base of individual flows. Liquefaction and remobilisation of sand within laminated deposits occurred during such basal shear events, resulting in the injection of liquefied sediments into variably deformed laminated sands and clays. Deformation is more likely to have taken place through internal movement of the sediment due to changing porewater conditions and loading upon emplacement. Our approach confirms previous results that highlight the possibilities of increasing the accuracy of sedimentological investigations through combined sedimentological analyses at varying scales.  相似文献   

9.
Distinct styles of fluvial deposition in a Cambrian rift basin   总被引：1，自引：0，他引：1  
Process‐based and facies models to account for the origin of pre‐vegetation (i.e. pre‐Silurian) preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation. In pre‐vegetation fluvial successions, the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad, tabular, stacked sandstone‐sheets has been previously regarded as the dominant sedimentary mechanism. This situation is closely analogous to modern‐day poorly vegetated systems developed in arid climatic settings. However, this study demonstrates the widespread presence of substantially more complex stratigraphic architectures. The Guarda Velha Formation of Southern Brazil is a >500 m‐thick synrift fluvial succession of Cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions: (i) an early rift‐stage system characterized by coarse‐grained channel elements indicative of a distributive pattern with flow transverse to the basin axis; and two coeval systems from the early‐ to climax‐rift stages that represent (ii) an axially directed, trunk fluvial system characterized by large‐scale amalgamated sandy braid‐bar elements, and (iii) a distributive fluvial system characterized by multi‐storey, sandy braided‐channel elements that flowed transverse to the basin axis. Integration of facies and architectural‐element analysis with regional stratigraphic basin analysis, palaeocurrent and pebble‐provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre‐vegetation, rift‐basin setting. Identified major controls that influenced pre‐vegetation fluvial sedimentary style include: (i) spatial and temporal variation in discharge regime; (ii) the varying sedimentological characteristics of distinct catchment areas; (iii) the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style, whereby both the axial and transverse fluvial systems undertook a distinctive response to syn‐depositional movement on basin‐bounding faults. Detailed architectural analyses of these deposits reveal significant variations in geometry, with characteristics considerably more complex than that of simple, laterally extensive, stacked sandstone‐sheets predicted by most existing depositional models for pre‐vegetation fluvial systems. These results suggest that the sheet‐braided style actually encompasses a varied number of different pre‐vegetation fluvial styles. Moreover, this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting.  相似文献   

10.
David J. Went 《Sedimentology》2013,60(4):1036-1058
Quartzites are especially characteristic of Proterozoic and Cambro‐Ordovician shallow marine strata, whereas equivalent age fluvial deposits are commonly arkosic. The absence of land vegetation in the pre‐Silurian influenced weathering processes and styles of fluvial deposition. It may also have had an impact on shallow marine sedimentation. Two field studies from the English Channel region are presented to investigate the processes leading to quartzite formation. On Alderney, nearshore marine and fluvial facies occur interbedded on a metre scale and are interpreted to represent deposition on the lower reaches of an alluvial plain, and in beach and upper shoreface environments. The marine and fluvial sandstones display marked differences in textural and mineralogical maturity, pointing to a process of sediment maturation by the destruction of feldspar and labile grains at the shoreline. At Erquy, fully mature, marine quartzites occur bounded above and below by alluvial deposits via sharp or erosional surfaces, and are interpreted to represent high energy, storm and tidally influenced lower shoreface and inner shelf deposits. A model for quartzite development is proposed where, under a cool climate, frequent storms in un‐vegetated, tectonically rejuvenated uplands provided an abundance of arkosic sand to fluvial basins and clastic shorelines. The model proposes that the marine basins were subject to high wave energies, frequent storm events and tidal currents. These were conditions conducive to transforming arkosic sand to quartz‐rich sand by the attrition of feldspar at the shoreline and in the shallow marine environment. On sediment burial, further feldspar destruction occurred during diagenesis. The proposed model highlights the potential for a step change in sediment maturity to occur at the shoreline in early Palaeozoic depositional systems tracts.  相似文献   

11.
A remarkable suite of shallow-water, gravity-flow deposits are found within very thinly-bedded siltstones and storm-generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non-graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine-grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore-fluid pressure dissipation support the feasibility of such transitions. Raft-bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm-1 and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm-2 and a depositional slope estimate of 0.77°. The lithologies and sedimentary structures in member 2 indicate an overall grain-size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure included in situ liquefaction (‘disturbed bedding’), sliding and slumping. Raft-bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross-stratified sandstone directly overlies slide deposits and raft-bearing beds, linking sediment failure to the cyclical wave loading associated with large storms. The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud-rich, marine deltas. Details of deltaic gravity-flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies primarily carried out with side-scan sonar.  相似文献   

12.
The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south‐east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross‐bedded dune sandstone dominated by grain flow and translatent wind‐ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind‐ripple, wavy and horizontal planar‐laminated strata resulting from accumulation on a range of dry, damp or wet substrate‐types in the flats and hollows between migrating dunes; and (iii) extensive, near‐flat lying wind‐rippled sandsheet strata. Fluvial facies comprise channel‐fill sandstones, lag conglomerates and finer‐grained overbank sheet‐flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south‐east, whereas south‐westerly flowing fluvial systems periodically punctuated the dune fields from the north‐east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo‐water‐table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid‐plains that fed sediment to a coastline that lay to the south‐west and which ran approximately north‐west to south‐east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low‐relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non‐marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north‐eastward directed marine transgression into the basin prior to the onset of exclusively non‐marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small‐scale autocyclic behaviour of competing, coeval depositional systems and larger‐scale allocyclic changes that record system response to longer‐term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea‐level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio‐eustatic driving mechanism.  相似文献   

13.
《Comptes Rendus Geoscience》2014,346(7-8):159-168
The Pennsylvanian portion of the Late Paleozoic Ice Age was characterized by stratigraphic repetition of chemical and siliciclastic rocks in the equatorial regions of the Pangean interior. Known as “cyclothems”, these stratigraphic successions are a 105 yr-record of glacial waxing and waning, superimposed on longer term, 106 yr intervals of global warming and cooling and a still longer term trend of increasing equatorial aridity. During periods of maximum ice–minimum sea level, the interior craton was widely exposed. Epicontinental landscapes were initially subjected to dry subhumid climate when first exposed, as sea level fell, but transitioned to humid climates and widespread wetlands during maximum lowstands. During interglacials (ice-minima) seasonally dry vegetation predominated. The wetland and seasonally dry biomes were compositionally distinct and had different ecological and evolutionary dynamics.  相似文献   

14.
Dinosaur footprints and tracks in the Sousa Basin (Lower Cretaceous, Brazil) occur in at least 37 localities, in distinct stratigraphic positions. Footprints are rare in the Antenor Navarro (lower) and Rio Piranhas (upper) formations, where lithofacies analyses point to sedimentation in ancient alluvial fan to fluvial braided palaeoenvironments. In the Sousa Formation, the generally finer grain sized sediments rendered them more suitable for footprint preservation, where lithofacies analyses point to sedimentation in warm, small/shallow and temporary lakes, swamps and meandering fluvial palaeoenvironments. Microbially induced sedimentary structures are observed in many of the fine-grained lithofacies where dinosaur tracks are also found, and the large number of these tracks in the Sousa Basin (particularly in the Sousa Formation, Lower Cretaceous) may be related to the role of the mats in their preservation. Observations on recent microbial mats show that footprint morphology is related to the mat thickness and to the water content of the mat and the underlying sediment. In dry mats, generally poorly defined or no footprints are produced, while in saturated ones the imprints are well-defined, sometimes with well-defined displacement rims. The formation of well-defined displacement rims around the prints of large dinosaurs occurs in thick, plastic, moist to water-unsaturated microbial mats on top of moist to water-unsaturated sediment. These aspects are commonly observed in the tracks of the Passagem das Pedras site in the Sousa Basin. The footprint consolidation and its early lithification probably occurred due the existence of microbial mats that allowed a more cohesive substrate, preventing the footprints from erosion. The sediments were initially stabilized by early cementation and by the mat fabric over the tracks. Successive flooding, and subsequent sediment influx allowed the large number of layers with dinosaur tracks and sedimentary structures.  相似文献   

15.
Integrated fluvial sequence stratigraphic and palaeosol analysis can be used to better reconstruct depositional systems, but these approaches have not been combined to examine halokinetic minibasins. This study characterizes the temporal and spatial patterns of lithofacies and palaeosols in a sequence stratigraphic framework to reconstruct a model of minibasin evolution and identify halokinetic influences on fluvial deposition. This research documents fluvial cycles and stratigraphic hierarchy, palaeosol maturity and apparent sediment accumulation rates in the Chinle Formation within the Big Bend minibasin. This study also uses palaeosols to help identify fluvial aggradational cycle (FAC) sets. The Chinle is divided into two hectometre‐scale (102 m) fluvial sequences, six decametre‐scale (101 m) FAC sets, and variable numbers of metre‐scale FACs depending on proximity to the minibasin. Ten pedotypes representing 225 palaeosol profiles are recognized. The pedotypes include palaeosols similar to modern Entisols, Inceptisols, Aridisols, Vertisols and Alfisols. A maturity index (1–5) is assigned to each pedotype to assess its variability in palaeosol development. Estimated palaeosol development time is used to approximate apparent sediment accumulation rates. Increased subsidence resulted in a greater number and thicker FACs, thicker FAC sets and fluvial sequence sections, and lithofacies associations reflecting more rapid sedimentation along the minibasin axis. Palaeocurrent indicators converge towards the minibasin axis and indicate that it formed and drifted through time. Relative palaeosol maturity is inversely related to stratal thickness, and decreases towards the minibasin where episodic burial by fluvial sediment was more frequent. Metre‐scale FACs are most abundant towards the minibasin axis, and locally have Entisols and Inceptisols developed upon their upper boundaries reflecting increased sediment accumulation rates. Areas outside the minibasin are characterized by fewer FACs that are associated with more mature palaeosols. Palaeosol‐derived apparent sediment accumulation rates are as much as two orders of magnitude greater within the minibasin than in marginal areas. The combined stratigraphic, palaeocurrent and palaeosol evidence is used to develop a model for the evolution of the Big Bend minibasin that illustrates the halokinetic affect on fluvial and landscape processes.  相似文献   

16.
尽管还存在一些不同的认识和争论,陆生植被对河流沉积作用的影响,得到了越来越多的地质事实的支持,主要表现在以煤或泥炭的形式保存下来的碳质物质对河堤的保护,从而促进了曲流河与网状河的发育。四川盆地上三叠统须家河组为一套含有煤系地层的冲积序列为主的地层,属于具有前陆盆地性质的磨拉石序列,成为研究植被发育条件下河流序列的典型代表。重庆永川普安剖面的须家河组是以河流相沉积为主的须家河组的典型代表,包括6个正式的岩性段,在其中一段、三段和五段为含煤细粒碎屑岩组成的低能河流相地层,二段、四段和六段由砂质底荷载高能河流相沉积构成,岩石地层划分及其所反映出的旋回性成为层序地层的识别和划分的基础,从而将须家河组划分成3个河流相沉积层序。两个主要的特征赋予了普安剖面须家河组河流相层序序列重要意义：（1）总体向上变粗的而且与现行的河流相层序模式形成巨大差异的河流相沉积序列组成的冲积构架;（2）明显受到植被发育影响的高能底荷载河道相砂岩。基于植被发育对河流沉积作用的影响,对普安剖面须家河组的河流相沉积序列的观察与研究,对长期以来须家河组沉积相分析存在的较大争议将提供重要的线索和思考途径。  相似文献   

17.
A 467-cm-long core from the inner shelf of the eastern Laptev Sea provides a depositional history since 9400 cal yr. B.P. The history involves temporal changes in the fluvial runoff as well as postglacial sea-level rise and southward retreat of the coastline. Although the core contains marine fossils back to 8900 cal yr B.P., abundant plant debris in a sandy facies low in the core shows that a river influenced the study site until 8100 cal yr B.P. As sea level rose and the distance to the coast increased, this riverine influence diminished gradually and the sediment type changed, by 7400 cal yr B.P., from sandy silt to clayey silt. Although total sediment input decreased in a step-like fashion from 7600 to 4000 cal yr B.P., this interval had the highest average sedimentation rates and the greatest fluxes in most sedimentary components. While this maximum probably resulted from middle Holocene climate warming, the low input of sand to the site after 7400 cal yr B.P. probably resulted from further southward retreat of the coastline and river mouth. Since about 4000 cal yr B.P., total sediment flux has remained rather constant in this part of the Laptev Sea shelf due to a gradual stabilization of the depositional regime after completion of the Holocene sea-level rise.  相似文献   

18.
A. G. PLINT 《Sedimentology》1986,33(3):387-399
Pennsylvanian fluvial channel sandstones in New Brunswick and Nova Scotia contain numerous examples of eroded mudstone surfaces, including in situ mudstone beds, boulders and slumped blocks. The eroded surfaces bear a variety of structures including linear scours, flutes, longitudinal furrows and rill marks. A block of interchannel mudstone up to 40 m in extent, displays a basal slip-plane, slump-related deformation and evidence of intense corrasion on a channel floor. Mudstone clasts from small pebbles to boulders over 4 m long are common immediately above channel-base erosion surfaces and represent a lag. Clasts over 20 cm diameter are commonly fluted, occasionally on all sides, suggesting clast rotation. Rill marks occur on large mudclasts and in situ mudstone surfaces and indicate emergence and erosion by surging water or surface runoff. Preservation of the delicate erosional structures depended on a highly cohesive mud substrate and subsequent rapid burial. A previous interpretation of the mud blocks and their surficial features as the result of mud intrusion is inconsistent with the field evidence.  相似文献   

19.
Along the West Greenland continental margin adjoining Baffin Bay, bathymetric data show a series of large submarine fans located at the mouths of cross‐shelf troughs. One of these fans, termed here ‘Uummannaq Fan’, is a trough‐mouth fan built largely by debris delivered from a fast‐flowing outlet of the Greenland Ice Sheet during past glacial maxima. Cores from this fan provide the first information on glacimarine sedimentary facies within a major West Greenland trough‐mouth fan and on the nature of Late Weichselian–Holocene glacigenic sediment delivery to this region of the Baffin Bay margin. Glacigenic debris flows deposited on the upper slope and extending to at least 1800 m water depth in front of the trough‐mouth are related to the remobilization of subglacial debris that was delivered onto the upper slope at times when an ice stream was positioned at the shelf edge. In contrast, sedimentary facies from the northern sector of the fan are characterized by hemipelagic and ice‐rafted sediments and turbidites; glacigenic debris flows are notably absent in cores from this region. Quantitative X‐ray diffraction studies of the <2‐mm sediment fraction indicate that the bulk of the sediment in the fan is derived from Uummannaq Trough but there are distinct intervals when sediment from northern Baffin Bay sources dominates, especially on the northern limit of the fan. These data demonstrate considerable variation in the nature of sediment delivery across the Uummannaq Fan when the Greenland Ice Sheet was at the shelf edge. They highlight the variability of glacimarine depositional processes operating on trough‐mouth fans on high‐latitude continental margins during the last glacial maximum and indicate that glacigenic debris flows are just one of a number of mechanisms by which such large depocentres form. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

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