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1.
Channel‐scale sedimentary units associated with bedrock‐controlled riffle‐pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south‐eastern central Australia. Pool‐fills comprise cut‐and‐fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse‐grained bar platform; (2) fine‐grained bar supraplatform; (3) fine‐grained pool‐fill; (4) fine‐grained bench; and (5) modern pool‐fill. The last coarse‐grained unit currently lining the pools suggests an altered sedimentation style over the post‐settlement period (post‐ad 1860s). Situated at bedrock valley constrictions, pool‐fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine‐grained bench units reflect eddy recirculation; the pool‐fills are principally forced‐bars associated with bedrock‐controlled or ‘forced’ riffle‐pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool‐fills: multiple phases of cut‐and‐fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool‐fills to bedrock. The resilience of the pool‐fills was illustrated by the passage of a 1‐in‐100‐year flood in 1992, which caused only minor erosion. The presence of pool‐fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high‐magnitude floods exerts the dominant control.  相似文献   

2.
The early Pleistocene clastic succession of the Peri‐Adriatic basin, eastern central Italy, records the filling of a series of piggyback sub‐basins that formed in response to the development of the eastward‐verging Apennine fold‐thrust belt. During the Gelasian (2·588 to 1·806 Ma), large volumes of Apennine‐derived sediments were routed to these basins through a number of slope turbidite systems. Using a comprehensive outcrop‐based dataset, the current study documents the depositional processes, stratigraphic organization, foraminiferal age and palaeodepth, and stratigraphic evolution of one of these systems exposed in the surroundings of the Castignano village. Analysis of foraminiferal assemblages consistently indicates Gelasian deposition in upper bathyal water depths. Sediments exposed in the study area can be broken into seven main lithofacies, reflecting specific gravity‐induced depositional elements and slope background deposition: (i) clast‐supported conglomerates (conglomerate channel‐fill); (ii) amalgamated sandstones (late stage sandstone channel‐fill); (iii) medium to thick‐bedded tabular sandstones (frontal splay sandstones); (iv) thin to thick‐bedded channelized sandstones (sandy channel‐fill); (v) medium to very thin‐bedded sandstones and mudstones (levée‐overbank deposits); (vi) pebbly mudstones and chaotic beds (mudstone‐rich mass‐transport deposits); and (vii) massive mudstones (hemipelagic deposits). Individual lithofacies combine vertically and laterally to form decametre‐scale, disconformably bounded, fining‐upward lithofacies successions that, in turn, stack to form slope valley fills bounded by deeply incised erosion surfaces. A hierarchical approach to the physical stratigraphy of the slope system indicates that it has evolved through multiple cycles of waxing then waning flow energy at multiple scales and that its packaging can be described in terms of a six‐fold hierarchy of architectural elements and bounding surfaces. In this scheme, the whole system (sixth‐order element) is comprised of three distinct fifth‐order stratigraphic cycles (valley fills), which define sixth‐order initiation, growth and retreat phases of slope deposition, respectively; they are separated by discrete periods of entrenchment that generated erosional valleys interpreted to record fifth‐order initiation phases. Backfilling of individual valleys progressed through deposition of two vertically stacked lithofacies successions (fourth‐order elements), which record fifth‐order growth and retreat phases. Fourth‐order initiation phases are represented by erosional surfaces bounding lithofacies successions. The component lithofacies (third‐order element) record fourth‐order growth and retreat phases. Map trends of erosional valleys and palaeocurrent indicators converge to indicate that the sea floor bathymetric expression of a developing thrust‐related anticline markedly influenced the downslope transport direction of gravity currents and was sufficient to cause a major diversion of the turbidite system around the growing structure. This field‐based study permits the development of a sedimentological model that predicts the evolutionary style of mixed coarse‐grained and fine‐grained turbidite slope systems, the internal distribution of reservoir and non‐reservoir lithofacies within them, and has the potential to serve as an analogue for seismic or outcrop‐based studies of slope valley fills developed in actively deforming structural settings and under severe icehouse regimes.  相似文献   

3.
Pachham Island is the westernmost island in a chain of four rocky islands within the Great Rann of Kachchh that are bounded in the north by the Island Belt Fault (IBF). The island is characterized by two parallel and structurally controlled Kaladungar hill range and Goradungar hill range separated by a synclinal central valley. The central valley has acted as a sink for the Quaternary sediments. The island is transversely traversed by a N-S trending basement high termed as Median high that shows pronounced geomorphic and drainage anomaly. The Quaternary sediments comprise coarse-grained colluvio-fluvial deposits, miliolite, and sandy alluvium. The coarse-grained Quaternary deposits were primarily derived from the colluvium generated due to the uplift of the Kaladungar hill range and Goradungar hill range along the IBF and Goradunagar Fault, respectively. The colluvial debris were subsequently reworked, transported down slope and deposited in the central valley. The presence of the deeply incised valleys in Quaternary sediments, two levels of terrace surfaces over the Median high, and the raised Rann floor surrounding the island suggest a dominant role of neotectonics in the landscape evolution and Quaternary sedimentation.  相似文献   

4.
Sedimentological outcrop analysis and sub‐surface ground‐penetrating radar (GPR) surveys are combined to characterize the three‐dimensional sedimentary architecture of Quaternary coarse‐grained fluvial deposits in the Neckar Valley (SW Germany). Two units characterized by different architectural styles are distinguished within the upper part of the gravel body, separated by an erosional unconformity: (i) a lower unit dominated by trough‐shaped depositional elements with erosional, concave‐up bounding surfaces that are filled by cross‐bedded sets of mainly openwork and filled framework gravel; and (ii) an upper unit characterized by gently inclined sheets of massive and openwork gravels with thin, sandy interlayers that show lateral accretion on a lower erosional unconformity. The former is interpreted as confluence scour pool elements formed in a multi‐channel, possibly braided river system, the latter as extensive point bar deposits formed by the lateral migration of a meandering river channel. The lateral accretion elements are locally cut by chute channels mainly filled by gravels rich in fines, and by fine‐grained abandoned channel fills. The lateral accretion elements are associated with gravel dune deposits characterized by steeply inclined cross‐beds of alternating open and filled framework gravel. Floodplain fines with a cutbank and point bar morphology cover the gravel deposits. The GPR images, revealing the three‐dimensional geometries of the depositional elements and their stacking patterns, confirm a change in sedimentary style between the two stratigraphic units. The change occurred at the onset of the Holocene, as indicated by 14C‐dating of wood fragments, and is related to a re‐organization of the fluvial system that probably was driven by climatic changes. The integration of sedimentological and GPR results highlights the heterogeneity of the fluvial deposits, a factor that is important for modelling groundwater flow in valley‐fill aquifers.  相似文献   

5.
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6.
Abstract The Red River, Manitoba, is a mud‐dominated, meandering stream that occupies a shallow valley eroded into a clay plain. The valley‐bottom alluvium is the product of incision and lateral migration of river meanders. As revealed by a transect of five boreholes located across the floodplain at each of two successive river meanders, the alluvial deposits range from about 15 to 22 m thick and are composed primarily of silt. Sedimentary structures in the cores are weakly defined and consist mostly of beds of massive silt, thick (>0·4 m) massive silt and disturbed silt. Interlaminated sand and silt, and sand beds form relatively minor deposits, principally within the lower half of the alluvium, and thin beds of medium‐coarse sand and pea gravel can be present locally within the lower metre of the alluvium. The alluvium is interpreted to consist of overbank deposits from 0 to 2–3 m depth, oblique accretion deposits from 2–3 to 8–12 m depth and oblique accretion and/or channel deposits from 8–12 m to the base of the sequence. The massive bedding within the oblique accretion deposits is interpreted to represent the remnants of couplet deposits that were initially composed of interbedded, muddy silt and sand‐sized silt aggregates, as is consistent with the contemporary bank sedimentation. The post‐depositional disintegration and/or compaction of the aggregates has caused the loss of the sand‐sized texture. The disturbed silt beds are interpreted as slump structures caused by large‐scale rotational failures along the convex banks. Overall, the Red River represents a portion of a continuum of muddy, fine‐grained streams; where the alluvium lacks a distinct coarse unit, oblique accretion deposits form a majority of the floodplain, and large‐scale slump features are present.  相似文献   

7.
Buried palaeo‐valley systems have been identified widely beneath lowland parts of the UK including eastern England, central England, south Wales and the North Sea. In the Midland Valley of Scotland palaeo‐valleys have been identified yet the age and genesis of these enigmatic features remain poorly understood. This study utilizes a digital data set of over 100 000 boreholes that penetrate the full thickness of deposits in the Midland Valley of Scotland. It identified 18 buried palaeo‐valleys, which range from 4 to 36 km in length and 24 to 162 m in depth. Geometric analysis has revealed four distinct valley morphologies, which were formed by different subglacial and subaerial processes. Some palaeo‐valleys cross‐cut each other with the deepest features aligning east–west. These east–west features align with the reconstructed ice‐flow direction under maximum conditions of the Main Late Devensian glaciation. The shallower features appear more aligned to ice‐flow direction during ice‐sheet retreat, and were therefore probably incised under more restricted ice‐sheet configurations. The bedrock lithology influences and enhances the position and depth of palaeo‐valleys in this lowland glacial terrain. Faults have juxtaposed Palaeozoic sedimentary and igneous rocks and the deepest palaeo‐valleys occur immediately down‐ice of knick‐points in the more resistant igneous bedrock. The features are regularly reused and the fills are dominated by glacial fluvial and glacial marine deposits. This suggests that the majority of infilling of the features happened during deglaciation and may be unrelated to the processes that cut them.  相似文献   

8.
The Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine-grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills. Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th-order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle. In the lowstand sequence set, incision associated with successive 4th-order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th-order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine-grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack discretely.  相似文献   

9.
Sea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent planform timeslices of distributive deep‐water systems but commonly only limited information on cross‐sectional architecture. Extensive outcrops in the Tanqua depocentre, south‐west Karoo Basin, provide these three‐dimensional constraints on lithofacies distributions, stacking patterns, depositional geometries and the stratigraphic evolution of submarine lobe deposits at a scale comparable with modern lobe systems. Detailed study (bed‐scale) of a single‐lobe complex (Fan 3) over a 15 km by 8 km area has helped to define a four‐fold hierarchy of depositional elements from bed through to lobe element, lobe and lobe complex. The Fan 3 lobe complex comprises six distinct fine‐grained sandstone packages, interpreted as lobes, which display compensational stacking patterns on a 5 km scale. Between successive lobes are thin‐bedded, very fine‐grained sandstones and siltstones that do not change lithofacies over several kilometres and therefore are identified as a different architectural element. Each lobe is built by many lobe elements, which also display compensational stacking patterns over a kilometre scale. Thickness variations of lobe elements can be extremely abrupt without erosion, particularly in distal areas where isopach maps reveal a finger‐like distal fringe to lobes. Lobe deposits, therefore, are not simple radial sheet‐dominated systems as commonly envisaged.  相似文献   

10.
The River Supersequence represents a 2nd‐order accommodation cycle of approximately 15 million years duration in the Isa Superbasin. The River Supersequence comprises eight 3rd‐order sequences that are well exposed on the central Lawn Hill Platform. They are intersected in drillholes and imaged by reflection seismic on the northern Lawn Hill Platform and crop out in the McArthur Basin of the Northern Territory. South of the Murphy Inlier the supersequence forms two south‐thickening depositional wedges on the Lawn Hill Platform. The northern wedge extends from the Murphy Inlier to the Elizabeth Creek Fault Zone and the southern wedge extends from Mt Caroline to the area south of Riversleigh Station. On the central Lawn Hill Platform the River Supersequence attains a maximum thickness of 3300 m. Facies are dominantly fine‐grained siliciclastics, but the lower part comprises a mixed carbonate‐siliciclastic succession. Interspersed within fine‐grained facies are sharp‐based sandstone and conglomeratic intervals interpreted as lowstand deposits. Such lowstand deposits represent a wide range of depositional systems and palaeoenvironments including fluvial channels, shallow‐marine shoreface settings, and deeper marine turbidites and sand‐rich submarine fans. Associated transgressive and highstand deposits comprise siltstone and shale deposited below storm wave‐base in relatively quiet, deep‐water settings similar to those found in a mid‐ to outer‐shelf setting. Seismic analysis shows significant fault offsets and thickness changes within the overall wedge geometry. Abrupt thickness changes across faults over small horizontal distances are documented at both the seismic‐ and outcrop‐scales. Synsedimentary fault movement, particularly along steeply north‐dipping, largely northeast‐trending normal faults, partitioned the depositional system into local sub‐basins. On the central Lawn Hill Platform, the nature of facies and their thickness change markedly within small fault blocks. Tilting and uplift of fault blocks affected accommodation cycles in these areas. Erosion and growth of fine‐grained parts of the section is localised within fault‐bounded depocentres. There are at least three stratigraphic levels within the River Supersequence associated with base‐metal mineralisation. Of the seven supersequences in the Isa Superbasin, the River Supersequence encompasses arguably the most dynamic period of basin partitioning, syndepositional faulting, facies change and associated Zn–Pb–Ag mineralisation.  相似文献   

11.
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12.
High resolution seismic lines from the inner and mid-shelf of the Durban Bight reveal an unprecedented view of the seismic stratigraphy of the central KwaZulu-Natal uppermost continental margin. Seven units are recognised from the shelf on the basis of their stratal architecture and bounding unconformities. These comprise four incompletely preserved sequences consisting of deposits of the highstand systems tract (Unit B), falling stage systems tracts (Unit C), the transgressive systems tract (Units A, D and G) and lowstand systems tracts (early fill of the incised valleys and strike diachronous prograding reflectors of Unit A). Seismic facies recognised as incised valley fills correspond to the lowstand and transgressive systems tracts. When integrated with published accounts of onshore and offshore lithostratigraphy and local sea level curves, we recognise an Early Santonian transgression (Unit A to Unit B), superimposed by uplift-induced pulses of forced regression. A Late Campanian relative sea level fall (Unit C) followed. Sediments of the Tertiary period are not evident on the Durban Bight shelf except for isolated incised valley fills of Unit D lying within incised valleys of Late Pliocene age. Overlying these are two stages of Pleistocene shoreline deposits of indeterminate age. Erosion concurrent with relative sea level fall towards the last glacial maximum shoreline carved a third set of incised valleys within which sediments of the Late Pleistocene/Holocene have infilled.  相似文献   

13.
Baffin Bay, Texas is the flooded Last Glacial Maximum incised valley of the Los Olmos, San Fernando and Petronila Creeks along the north‐western Gulf of Mexico. Cores up to 17 m in length and high‐resolution seismic profiles were used to study the history of Baffin Bay over the last 10 kyr and to document the unusual depositional environments within the valley fill. The deposits of the Baffin Bay incised valley record two major and two minor events. Around 8·0 ka, the estuarine environments backstepped more than 15 km in response to an increase in the rate of sea‐level rise. Around 5·5 ka, these estuarine environments changed from environments similar to other estuaries of the northern Gulf of Mexico to the unusual suite of environments found today. Another minor flooding event occurred around 4·8 ka in which several internal spits were flooded. Some time after 4·0 ka, the upper‐bay mud‐flats experienced a progradational event. Because of its semi‐arid climate and isolation from the Gulf of Mexico, five depositional environments not found in the other incised‐valley fills of the northern Gulf of Mexico are found today within Baffin Bay. These deposits include well‐laminated carbonate and siliciclastic open‐bay muds, ooid beaches, shelly internal spits and barrier islands, serpulid worm‐tube reefs and prograding upper‐bay mud‐flats. Based on these unusual deposits, and other characteristics of Baffin Bay, five criteria are suggested to help identify incised valleys that filled in arid and semi‐arid climates. These criteria include the presence of: (i) hypersaline‐tolerant fauna; (ii) aeolian deposits; and (iii) carbonate and/or evaporite deposits; and the absence of: (iv) peat or other organic‐rich deposits in the upper bay and bay‐margin areas; and (v) well‐developed fluvially dominated bayhead deltas.  相似文献   

14.
Modern fluvial meander plains exhibit complex planform transformations in response to meander‐bend expansion, downstream migration and rotation. These transformations exert a fundamental control on lithology and reservoir properties, yet their stratigraphic record has been poorly evaluated in ancient examples due to the lack of extensive three‐dimensional exposures. Here, a unique exhumed meander plain exposed to the north of Scarborough (Yorkshire, UK) is analysed in terms of architecture and morphodynamics, with the aim of developing a comprehensive model of facies distribution. The studied outcrop comprises tidal platforms and adjacent cliffs, where the depositional architecture of un‐tilted deposits was assessed on planform and vertical sections, respectively. In its broader perspective, this study demonstrates the potential of architectural mapping of extensive planform exposures for the reconstruction of ancient fluvial morphodynamics. The studied exhumed meander plain is part of the Scalby Formation of the Ravenscar Group, and originally drained small coastal incised valleys within the Jurassic Cleveland Basin. The meander plain is subdivided into two storeys that contain in‐channel and overbank architectural elements. In‐channel elements comprise expansional and downstream‐migrating point bars, point‐bar tails and channel fills. Overbank elements comprise crevasse complexes, levées, floodplain fines and lake fills. The evolution of the point bars played a significant role in dictating preserved facies distributions, with high flood‐stage nucleation and accretion of meander scrolls later reworked during waning flood‐stages. At a larger scale, meander belt morphodynamics were also a function of valley confinement and contrasts in substrate erodibility. Progressive valley infilling decreased the valley confinement, promoting the upward transition from prevalently downstream migrating to expansional meander belts, a transition associated with enhanced preservation of overbank elements. Strikingly similar relations between valley confinement, meander‐bend transformations and overbank preservation are observed in small modern meandering streams such as the Beaver River of the Canadian prairies and the Powder River of Montana (USA).  相似文献   

15.
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16.
Tree-ring studies carried out on subfossil oak trunk deposits within the Holocene valley fills of the River Main can reconstruct phases of increased fluvial activities. These phases have been dated on the base of two absolutely tree-ring dated chronologies and in addition by 11C-datings of eleven floating tree-ring scries of subfossil oaks.
Geological-pedological investigations reveal an alternation between increased and reduced fluvial activity during the Holocene.
Periods of increased gravel redeposition are dated by dendrochronology, and by 14C and cultural findings. Increased fluvial activity becomes more frequent towards Modern Times with culminations in the Middle Atlantic, the Subboreal, the Iron-Roman Age, the Main Middle Ages till earliest Modern Times, and in the last century.
On the sequence of Holocene river deposits there developed specific soil types as indicators for the age of the river deposits since the Last Glacial.
Among other palaeoecological results an important finding is the correlation between tree-ring width, flood-loam sedimentation, and soil development.  相似文献   

17.
Gregers Dam 《Sedimentology》2002,49(3):505-532
ABSTRACT After a period of early Palaeocene faulting and uplift of the Nuussuaq Basin, West Greenland, two valley systems were incised into the underlying sediments. Incision of the older Tupaasat valley took place during a single drainage event of large water masses, which resulted in catastrophic deposition. The valley was cut along early Palaeocene NW‐ to SE‐trending normal faults, clearly showing that the trend and the relief of the valley were structurally controlled. The valley fill is up to 120 m thick and consists of a lower part of sandstones and conglomerates deposited from catastrophic flows characterized by very high concentrations of suspended coarse‐grained sediment load. Catastrophic deposition was followed by rapid decrease in flow discharge and the establishment of a lacustrine environment within the valley characterized by the deposition of heterolithic sediments. The younger Paatuutkløften valley system was mainly cut into the Tupaasat valley fill, which was completely or nearly completely eroded away in many places. The younger valley is 1–2 km wide and up to 190 m deep. Incision of the Paatuutkløften valley probably reflected renewed tectonic activity and uplift of the basin. This phase was shortly followed by rapid major subsidence. The valley‐fill deposits comprise a uniform succession of fluvial and estuarine sandstones. The valley fill is topped by shoreface sandstones, which are succeeded abruptly by offshore mudstones deposited shortly before and during the initial extrusion of a thick hyaloclastite succession. The Paatuutkløften valley fill is attributed to a very rapid rise in relative sea level contemporary with extensive volcanism. It is suggested that this sequence of events coincided with the arrival of the North Atlantic mantle plume. In several respects, the early Palaeocene valley‐fill deposits of the Nuussuaq Basin are different from idealized facies models for incised valley systems and represent very special cases of incised valleys. Major differences from published examples include the dominance of catastrophic deposits and indications of large changes in relative sea level of several hundreds of metres taking place rapidly in less than 1 Myr. These changes were governed by the rise of the North Atlantic mantle plume.  相似文献   

18.
The Fall River Formation is a 45 m thick layer of fluvial-dominated valley-fills and shore-zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, in the south-west corner of South Dakota (USA), expose a cross-section of a 3.5 km wide valley-fill sandstone and laterally adjacent marine deposits. The marine deposits comprise three 10 m thick upward-shoaling sequences; each composed of multiple metres-thick upward-coarsening successions. The lower two of these sequences are laterally cut by the valley-fill sandstone, and are capped by metres-thick muddy palaeosols. The upper sequence spans the top of the valley-fill sandstone, and is overlain by the Skull Creek Shale. The 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contain many metres-thick channel-form bodies. Deposits in the lower parts of these fills are sheet-like, top-truncated channel bodies, whereas deposits in the upper parts of fills are upward-concave, laterally amalgamated channel bodies, more completely preserved heterolithic channel bodies, or wave-deposited sheets. Each valley-fill basal erosion surface records an episode of valley incision and relative sea-level fall, and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea-level rise. All fills are dominantly channel deposits and are capped by marine flooding surfaces. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley-fill tops all suggest that sediment supply initially kept pace with relative sea-level rise and valleys filled during late marine lowstand and transgression, not during subsequent highstands. Recently proposed facies models have focused on variations in the relative strength of tide, wave and river currents as controls on valley-fill deposits. However, relative rates of sediment supply and basin accommodation change, and the shift in this ratio along the depositional profile during multiple-scale cycles in relative sea-level, are equally important controls on the style of valley-fill deposits.  相似文献   

19.
Abstract Analysis of extensive exposures of the Permian Laingsburg Formation, Karoo basin, South Africa, have enabled a detailed reconstruction of the base of slope stratigraphy and palaeoenvironments in a deep-water system characterized by a very narrow grain-size range (fine sandstone). The deposits include an ≈ 4 km wide and 80 m thick channel complex, fringed by sandy sheet deposits that extend laterally for at least 6 km across depositional strike. Within the channel complex, individual channel fills are marked by shallow basal erosion surfaces draped by thin, parallel-stratified beds of very fine sandstone and siltstone, interpreted as flow tails to largely bypassing flows. These thin beds are overlain by 0·4 to 5 m thick beds of structureless, fine-grained sandstone that represent the majority of the channel fills. The basal packages may be partially to completely removed by localized scour in the axial zone of the channel complex but can be mapped laterally into overbank areas where they thicken and are dominated by rippled fine sandstones with intercalated siltstones. Axial confinement resulted from subtle topography on the basin floor, whereby the lower, dense parts of the initially erosive and bypassing flows were partially confined in the lows and the more dilute, slower moving upper parts of the flows deposited sheet-like successions across slightly elevated overbank areas. The narrow grain-size distribution prohibited the formation ofcoarse-grained residual bypass deposits during the initial phases of channel formation. With decreasing magnitude, later flows became more depositional, filling remaining axial depressions with thick-bedded structureless sandstone. The smaller volumes of late-stage sediment were more axially focused, producing local scour-and-fill features and starvation of the overbank areas. Resulting grain-size vertical profiles are complex. The basal flow tail packages and overlying massive deposits form a thickening and slightly coarsening-upward trend in the channel fills. The overbank deposits show a thinning- and fining-upward profile as a result of less bypass plus late-stage starvation of sand. Application of traditional deep-water facies models could therefore potentially lead to erroneous interpretations of the channel complex as a prograding lobe and the overbank sheets as channel-fills.  相似文献   

20.
The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice-rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles. The massive diamictites formed as aprons and fans in front of the ice-grounding line, whereas the stratified diamictites represent more distal debris-flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes. Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter-term fluctuation of the ice margin is present within the major advance-retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea-level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono-eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer-term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.  相似文献   

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