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1.
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2.
Abstract Accumulation within the unconformity‐based Hauterivian Avilé Sandstone of the Neuquén Basin, Argentina, was characterized by a close interaction between fluvial and aeolian processes developed after a major relative sea‐level drop that almost completely desiccated the entire basin and juxtaposed these non‐marine deposits on shallow‐ and deep‐marine facies. Aeolian deposits within the Avilé Member include dune (A1) and sand sheet (A2) units that characterize the lower part of the unit. Fluvial deposits comprise distal flood units (F1) interbedded with aeolian dune deposits in the middle part of the succession, and low‐ (F2) and high‐sinuosity (F3) channels associated with floodplain deposits (F4) towards the top. The internal characteristics of the aeolian system indicate that its accumulation was strongly controlled by water‐table dynamics, with the development of multiple horizontal deflation super surfaces that truncate dune deposits and form the basal boundary of flood deposits and sand sheet units. A long‐term wetting‐upward trend is recorded throughout the entire unit, with an increase in fluvial activity towards the top and the development of a more permanent fluvial system overlying a major erosion surface interpreted as a sequence boundary. The upward increase in water‐table influence might be related to relative sea‐level rise, which controlled the position of the water table and allowed the accumulation of tabular aeolian units bounded by horizontal deflation surfaces. This high‐frequency, eustatically driven process acted together with a long‐term climatic change towards wetter conditions.  相似文献   

3.
Sedimentological studies of a 30 m thick coastal cliff section within the Middle Proterozoic Eriksfjord Formation in western South Greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams (Type I), semi-perennial streams (Type II), and ephemeral flash floods (Type III). Perennial river sand sheets are characterised by co-sets of medium-scale trough cross-beds, interbedded with isolated medium- and large-scale, high-angle, tabular cross-beds. Indications of desiccation or subaerial exposure are absent. Semi-perennial fluvial sand sheets consist predominantly of low-angle cross-beds, interbedded with isolated sets of high-angle tabular cross-beds with common reactivation surfaces. Horizontal lamination and climbing ripple lamination form subordinate structures. Associated with the sand sheets are adhesion structures and 0.05–0.4 m thick sets of wind ripple-lamination indicating periods of subaerial exposure and aeolian reworking. High-energy ephemeral flash flood sand sheets consist almost exclusively of planar-parallel lamination and climbing ripple lamination with some isolated sets of low-angle cross-bedding. Scouring and internal truncation surfaces are common. The three types of sand sheets are considered to reflect deposition under changing climatic conditions, varying from humid to arid or semi-arid. Aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation (Type II), while sand sheets showing features typical of arid and or semi-arid flow conditions (Type III) contain no preserved aeolian deposits. This selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime. The deposits show that during pre-vegetational times the preservation of aeolian deposits, under certain conditions, may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi-arid or arid circumstances. The occurrence of aeolian deposits within a Precambrian succession of fluvial deposits therefore, need not be an indication of the most arid environmental conditions.  相似文献   

4.
Sweet 《Sedimentology》1999,46(1):171-187
The Permian Upper Rotliegend Group in offshore UK Quadrants 42, 43, 47 and 48 comprises a sequence of mixed aeolian/fluvial/playa deposits. These deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments. The relative dominance of water vs. wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table/playa level driven by a combination of climatic change and syndepositional tectonics. The Rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water. The origin of these surfaces is mostly climatic, with periods of increased runoff resulting in fluvial incision, especially near active faults. During periods of reduced runoff, wind erosion of fluvial deposits occurred, with fluvially derived sand being reworked into expanding aeolian dune fields. Wind erosion also occurred as a rising water table isolated dunes from their sediment supply, resulting in deflation of dunes down to the water table. These surfaces formed in a basin that was subsiding. Thus, even in a background of overall increasing accommodation space, climatically driven falls in the water table allowed for periods of erosion. The occurrence of significant erosion, especially near syndepositional fault zones, resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations.  相似文献   

5.
An understanding of fluvial-aeolian deposition derived from modern case-examples in a previous study is applied to the Permian Cutler Formation and Cedar Mesa Sandstone on the Colorado Plateau. These formations supply an excellent three-dimensional exposure of intertonguing fluvial and aeolian strata. Four distinct facies associations form the bulk of the Cutler Formation and Cedar Mesa Sandstone: (1) aeolian dune deposits; (2) wet interdune deposits; (3) fluvial channel deposits; and (4) overbank-interdune deposits. In addition, two distinctive types of erosion surfaces are found within the Cutler Formation and Cedar Mesa Sandstone: pebble- to granule-rich erosion surfaces (aeolian deflation surfaces) and flood surfaces. Fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank-interdune deposits. Fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones. Overbank-interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones. The primary characteristics of ancient fluvial-aeolian deposition are overbank-interdune deposits and pronounced extensive erosion surfaces (flood surfaces), which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient.  相似文献   

6.
Aeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on Skei?arársandur, Southern Iceland. The largest aeolian dune complex on the sandur covers an area of 80 km2 and is characterized by four distinct landform types: (i) spatially isolated aeolian dunes; (ii) extensive areas of damp and wet (flooded) interdune flat with small fluvial channels; (iii) small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small, predominantly damp, interdune corridors; and (iv) larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain. The morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune, interdune and fluvial processes that operate coevally on the sandur surface. The geometry, scale, orientation and facies composition of sets of strata in the cores of the aeolian dunes, and their relationship to adjoining interdune strata, have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. Seasonal and longer‐term flooding‐induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds. Most of the dunes are currently undergoing active construction and migration and, although sediment availability is limited because of the high water table, substantial aeolian transport must occur, especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture. Bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing. Despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century, sediment compaction‐induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long‐term preservation. Wet, dry and stabilizing aeolian system types all co‐exist on Skei?arársandur and the dunes are variously undergoing coeval construction, accumulation, bypass, stabilization and destruction as a result of interactions between localized factors.  相似文献   

7.
Meltwater flows emanating from the Pyrenees during the Pleistocene constructed a braided outwash plain in the Ebro Basin and led to the karstification of the Neogene gypsum bedrock. Synsedimentary evaporite dissolution locally increased subsidence rates and generated dolines and collapses that enabled the accumulation and preservation of outwash gravels and associated windblown deposits that were protected from erosion by later meltwater flows. In these localized depocentres, maximum rates of wind deceleration resulted from airflow expansion, enabling the accumulation of cross‐stratified sets of aeolian strata climbing at steep angles and thereby preserving up to 5 m thick sets. The outwash plain was characterized by longitudinal and transverse fluvial gravel bars, channels and windblown facies organized into aeolian sand sheets, transverse and complex aeolian dunes, and loess accumulations. Flat‐lying aeolian deposits merge laterally to partly deformed aeolian deposits encased in dolines and collapses. Synsedimentary evaporite dissolution caused gravels and aeolian sand deposits to subside, such that formerly near‐horizontal strata became inclined and generated multiple internal angular unconformities. During episodes when the wind was undersaturated with respect to its potential sand transporting capacity, deflation occurred over the outwash plain and coarse‐grained lags with ventifacts developed. Subsequent high‐energy flows episodically reached the aeolian dune field, leading to dune destruction and the generation of hyperconcentrated flow deposits composed in part of reworked aeolian sands. Lacustrine deposits in the distal part of the outwash plain preserve rhythmically laminated lutites and associated Gilbert‐type gravel deltas, which developed when fluvial streams reached proglacial lakes. This study documents the first evidence of an extensive Pleistocene proglacial aeolian dune field located in the Ebro Basin (41˙50° N), south of what has hitherto been considered to be the southern boundary of Pleistocene aeolian deposits in Europe. A non‐conventional mechanism (evaporite karst‐related subsidence) for the preservation of aeolian sands in the stratigraphic record is proposed.  相似文献   

8.
M. L. PORTER 《Sedimentology》1987,34(4):661-680
The Lower Jurassic Aztec Sandstone is an aeolian-deposited quartzose sandstone that represents the western margin of the southerly-migrating Navajo-Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments. In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue-shaped aeolian sand bodies interstratified with fluvially-deposited coarse sandstone and mudstone. Above, evaporite-rich fine sandstone and mudstone are overlain by thick, cross-stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west. Thin aeolian sequences are interbedded with volcanic flow rocks, ash-flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian-deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc-derived sediment. A two-part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross-strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore-erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian-deposited sediment encased in fluvial and sabkha deposits. Continued sand-sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central-erg sediments buried the fore-erg deposits. The trailing, upwind margin of the erg is represented by back-erg deposits in northern Utah and Wyoming.  相似文献   

9.
Based on a detailed sedimentological analysis of Lower Triassic continental deposits in the western Germanic sag Basin (i.e. the eastern part of the present‐day Paris Basin: the ‘Conglomérat basal’, ‘Grès vosgien’ and ‘Conglomérat principal’ Formations), three main depositional environments were identified: (i) braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits; (ii) marginal erg (i.e. braided rivers, aeolian dunes and aeolian sand‐sheets); and (iii) playa lake (an ephemeral lake environment with fluvial and aeolian sediments). Most of the time, aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity, such as wind‐worn pebbles (ventifacts), sand‐drift surfaces and aeolian sand‐sheets. In such arid continental environments, stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension. Elementary stratigraphic cycles, i.e. the genetic units, have been identified for the three main continental environments: the fluvial type, fluvial–aeolian type and fluvial/playa lake type. At the time scale of tens to hundreds of thousands of years, these high‐frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland. Lower Triassic deposits from the Germanic Basin are preserved mostly in endoreic basins. The central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the Hercynian (Variscan)–Appalachian Mountains. Consequently, a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition, whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments.  相似文献   

10.
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   

11.
The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuquén Basin in Argentina represents a succession up to 40 m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial–aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level.  相似文献   

12.
The Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3-D outcrops allow a detailed study of structures and sequenses in the erg body. The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well-developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low-angle dipping toesets, and upper steeply dipping foresets. The foresets reveal significant across-crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross-bedding that documents strong along-crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along-crest transport of sand. The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along-crest transport of sand. The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.  相似文献   

13.
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14.
The existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km2. Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies. A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas. The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand.  相似文献   

15.
Cratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet-like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re-study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non-marine deposition followed by varying degrees of marine reworking during transgressions. Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large-scale cross-stratified facies believed to represent aeolian ergs passes laterally into a planar-and-channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning-upward sets of trough cross-stratification, reactivation surfaces, low-index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse-sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non-marine deposits. In contrast is a burrowed and trough cross-stratified facies characterized by medium-scale cross-bedding alternating with bioturbated intervals and rare brachiopod or trilobite-mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non-marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense. It is suggested that the sheet-like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders-of-magnitude more important on pre-Devonian, vegetation-free landscapes than they have been since, not only helps to explain the sheet-like character but also the exceptional maturity of the older cratonic sandstones.  相似文献   

16.
The Lower Cretaceous geological record of the intracratonic Paraná Basin in southern Brazil comprises a thick succession of aeolian sandstones and volcanic rocks. The intercalation between aeolian sandstone and volcanic floods allowed the preservation of distinct aeolian genetic units. Each genetic unit represents an accumulation episode, bounded by supersurfaces, that coincides with the base of lava flood events. The entire package can be subdivided into a Lower Genetic Unit, which corresponds to aeolian sandstones preserved below the initial lava flows (Botucatu Formation), and an upper set of genetic units, which comprises interlayered aeolian deposits and lava floods (Serra Geral Formation). The Lower Genetic Unit is up to 100 m thick. Its base is composed of ephemeral stream and aeolian sand sheet deposits that are overlain by cross‐bedded sandstones whose origin is ascribed to simple, locally composite, crescentic and complex linear aeolian dunes. Aeolian accumulation of the lower unit was possible as a result of the existence of a wide topographic basin, which caused wind deceleration, and a large sand availability that promoted a positive net sediment flux. The Upper Genetic Units comprise isolated sand bodies that occur in two different styles: (1) thin lenses (<3 m thick) formed by aeolian sand sheets; and (2) thick sand lenses (3–15 m) comprising cross‐bedded cosets generated by migration and climbing of simple to locally composite crescentic aeolian dunes. Accumulation of the aeolian strata was associated with wind deceleration within depressions on the irregular upper surface of the lava floods. The interruption of sedimentation in the Lower and Upper Genetic Units, and related development of supersurfaces, occurred as a result of widespread effusions of basaltic lava. Preservation of both wind‐rippled topset deposits of the aeolian dunes and pahoehoe lava imprints indicates that lava floods covered active aeolian dunes and, hence, protected the aeolian deposits from erosion, thus preserving the genetic units.  相似文献   

17.
Eighteen coastal-plain depositional sequences that can be correlated to shallow- to deep-water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 × 15 km scale mountainside exposures. The overall mud-prone (>300 m thick) coastal-plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7–44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse-grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide-dominated estuarine deposits, with coarse-grained river-dominated facies followed further seawards by high- and low-sinuosity tidal channels, upper-flow-regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal-plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large-scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner-estuarine tidal channels.  相似文献   

18.
Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid‐Cretaceous outer back‐erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain‐size distributions, mud playa, ephemeral floodplain, pebble‐sand and cobble‐sand bedload stream, pebble–cobble‐sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub‐environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind‐driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back‐erg depositional system comprises an outer back‐erg close to the Variscan highlands, and an inner back‐erg close to the central‐erg area. The inner back‐erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back‐erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud‐cracked surfaces, resulting in largely structureless sandstones with boulder‐size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser‐grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back‐erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back‐erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non‐exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti‐clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well‐preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre‐Cretaceous times.  相似文献   

19.
FIKRY KHALAF 《Sedimentology》1989,36(2):253-271
Several types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al-Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand budget.  相似文献   

20.
ABSTRACT Permian aeolian sediments on the island of Arran are divisible into dune (including draa) and interdune deposits. Both types display a distinctive and unusually wide variation in grain size. The dominant features of the dune deposits are grainfall lamination, sandflow lamination, and inverse graded lamination associated with ripple-form lamination and normal graded lamination. The flat-lying aeolian interdune deposits are characterised by granule and sand ripples, horizontal lamination in coarse sand and granules, plane bed lamination and inverse graded lamination. Associated structures include ripple-form lamination and deflation lags. Three types of trace fossil associated with completely bioturbated horizons occur in some low-angle dune and interdune deposits.
The aeolian facies interfinger with alluvial fan deposits giving rise to three recognizable facies belts. Marginal aeolian deposits are associated with fluvial conglomerates and are dominated by interdune deposits and occasionally very thin barchan deposits (set height 3-37 cm). Intermediate aeolian deposits are characterized by interbedded crescentic dune, small draa (dune set height 5 cm-4.5 m) and interdune deposits, and rare fluvial and lake sediments. Basinal aeolian deposits are dominated by draa deposits (dune set height 0.2-28 m) associated with rare interdune sediments. Transverse dunes and draas were moved by north-eastern palaeowinds towards the foot of the alluvial fans. The aeolian sediments were deposited in a fault-bounded desert basin.  相似文献   

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