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1.
青岛灵山岛下白垩统湖泊风暴沉积与风暴作用   总被引：1，自引：0，他引：1  
灵山岛背来石剖面发育了非常典型的湖泊风暴岩和风暴作用。非常典型的丘、洼状构造发育在火山岩底部的砂页岩中。丘状构造呈典型的丘状,规模不一,小者高数厘米,宽数十厘米;大者高2. 4m,宽12. 48m;具有典型的二元结构：底部为灰色或浅灰色的砂砾岩或砂岩;顶部为深灰色 黑色泥岩、页岩或薄层砂岩互层。砂砾岩分选磨圆均很差,砾石主要为片麻岩,直径多在数毫米到3~4cm,呈悬浮式胶结。砾岩底部为凹凸不平的侵蚀面,砾岩向丘状构造两翼变薄甚至尖灭,与下部侵蚀面呈典型的角度交切。丘状构造的上部的细粒层有四个要素：①上凸形态;②丘状交错层理;③下细上粗的二元结构;④底部具侵蚀面。洼状构造也具有典型的洼状,规模也不一,小者高数厘米,宽十余厘米;大者高2. 4m,宽18. 70m,具有下细上粗的（反)二元结构,但底部多为深灰 灰黑色、甚至黑色的泥岩、页岩或夹薄层砂岩,上部多为砂岩、砂砾岩或砾岩。完整的洼状构造也有四个要素：①下凹形态;②或有洼状交错层理;③下细上粗的（反)二元结构;④底部具侵蚀面。丘状构造与洼状构造在空间上相邻共生,但不是同时形成的,一个连续的完整序列是丘状构造形成在先,洼状构造形成在后,其间由一个粒度较细的薄层分隔开（风暴最高水位时的“静”水沉积)。基于实验和丘洼构造参数计算了风暴浪的相关参数。  相似文献   

2.
The Mesoproterozoic Pandikunta Limestone, a shallow water carbonate platform succession in the Pranhita-Godavari Valley, south India, displays well developed climbing ripple lamination and storm deposited structures, such as HCS, wave ripple-lamination, combined-flow ripple-lamination and low angle trough cross-stratification. Different types of stratification developed in calcisiltite with minor amounts of very fine quartz sand and silt. The climbing ripple structures exhibit a complex pattern of superposition of different types (type A, B and S) within cosets pointing to a fluctuating rate of suspension depositionversus bedform migration, and an unsteady character of the flow. Close association of climbing ripple structures, HCS with anisotropic geometry, wavy lamination and combined-flow ripple-lamination suggest that the structures were formed by storm generated combined-flow in a mid-shelf area above the storm wave base. The combined-flow that deposited the climbing ripple structures had a strong unidirectional flow component of variable magnitude. The climbing ripple structure occurs as a constituent of graded stratified beds with an ordered vertical sequence of different types of lamination, reflecting flow deceleration and increased rate of suspension deposition. It is inferred that the beds were deposited from high-density waning flows in the relatively deeper part of the ancient shelf. The structures indicate that the Pandikunta platform was subjected to open marine circulation and intense storm activities. The storm deposited beds, intercalated with beds of lime-mudstone, consist primarily of fine sand and silt size carbonate particles that were hydrodynamically similar to quartz silt. Detrital carbonate particles are structureless and are of variable roundness. The particles were generated as primary carbonate clasts in coastal areas by mechanical disintegration of rapidly lithified beds, stromatolites or laminites, and the finest grade was transported to the offshore areas by storm-generated currents.  相似文献   

3.
Climbing‐ripple cross‐lamination is most commonly deposited by turbidity currents when suspended load fallout and bedload transport occur contemporaneously. The angle of ripple climb reflects the ratio of suspended load fallout and bedload sedimentation rates, allowing for the calculation of the flow properties and durations of turbidity currents. Three areas exhibiting thick (>50 m) sections of deep‐water climbing‐ripple cross‐lamination deposits are the focus of this study: (i) the Miocene upper Mount Messenger Formation in the Taranaki Basin, New Zealand; (ii) the Permian Skoorsteenberg Formation in the Tanqua depocentre of the Karoo Basin, South Africa; and (iii) the lower Pleistocene Magnolia Field in the Titan Basin, Gulf of Mexico. Facies distributions and local contextual information indicate that climbing‐ripple cross‐lamination in each area was deposited in an ‘off‐axis’ setting where flows were expanding due to loss of confinement or a decrease in slope gradient. The resultant reduction in flow thickness, Reynolds number, shear stress and capacity promoted suspension fallout and thus climbing‐ripple cross‐lamination formation. Climbing‐ripple cross‐lamination in the New Zealand study area was deposited both outside of and within channels at an inferred break in slope, where flows were decelerating and expanding. In the South Africa study area, climbing‐ripple cross‐lamination was deposited due to a loss of flow confinement. In the Magnolia study area, an abrupt decrease in gradient near a basin sill caused flow deceleration and climbing‐ripple cross‐lamination deposition in off‐axis settings. Sedimentation rate and accumulation time were calculated for 44 climbing‐ripple cross‐lamination sedimentation units from the three areas using TDURE, a mathematical model developed by Baas et al. (2000) . For Tc divisions and Tbc beds averaging 26 cm and 37 cm thick, respectively, average climbing‐ripple cross‐lamination and whole bed sedimentation rates were 0·15 mm sec?1 and 0·26 mm sec?1 and average accumulation times were 27 min and 35 min, respectively. In some instances, distinct stratigraphic trends of sedimentation rate give insight into the evolution of the depositional environment. Climbing‐ripple cross‐lamination in the three study areas is developed in very fine‐grained to fine‐grained sand, suggesting a grain size dependence on turbidite climbing‐ripple cross‐lamination formation. Indeed, the calculated sedimentation rates correlate well with the rate of sedimentation due to hindered settling of very fine‐grained and fine‐grained sand–water suspensions at concentrations of up to 20% and 2·5%, respectively. For coarser grains, hindered settling rates at all concentrations are much too high to form climbing‐ripple cross‐lamination, resulting in the formation of massive/structureless S3 or Ta divisions.  相似文献   

4.
Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s?1 (equivalent to pure wind velocity of 6 to 8 m.s?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s?1.  相似文献   

5.
The hydrodynamic mechanisms responsible for the genesis and facies variability of shallow-marine sandstone storm deposits (tempestites) have been intensely debated, with particular focus on hummocky cross-stratification. Despite being ubiquitously utilized as diagnostic elements of high-energy storm events, the full formative process spectrum of tempestites and hummocky cross-stratification is still to be determined. In this study, detailed sedimentological investigations of more than 950 discrete tempestites within the Lower Cretaceous Rurikfjellet Formation on Spitsbergen, Svalbard, shed new light on the formation and environmental significance of hummocky cross-stratification, and provide a reference for evaluation of tempestite facies models. Three generic types of tempestites are recognized, representing deposition from: (i) relatively steady and (ii) highly unsteady storm-wave-generated oscillatory flows or oscillatory-dominated combined-flows; and (iii) various storm-wave-modified hyperpycnal flows (including waxing–waning flows) generated directly from plunging rivers. A low-gradient ramp physiography enhanced both distally progressive deceleration of the hyperpycnal flows and the spatial extent and relative magnitude of wave-added turbulence. Sandstone beds display a wide range of simple and complex configurations of hummocky cross-stratification. Features include ripple cross-lamination and ‘compound’ stratification, soft-sediment deformation structures, local shifts to quasi-planar lamination, double draping, metre-scale channelized bed architectures, gravel-rich intervals, inverse-to-normal grading, and vertical alternation of sedimentary structures. A polygenetic model is presented to account for the various configurations of hummocky cross-stratification that may commonly be produced during storms by wave oscillations, hyperpycnal flows and downwelling flows. Inherent storm-wave unsteadiness probably facilitates the generation of a wide range of hummocky cross-stratification configurations due to: (i) changes in near-bed oscillatory shear stresses related to passing wave groups or tidal water-level variations; (ii) multidirectional combined-flows related to polymodal and time-varying orientations of wave oscillations; and (iii) syndepositional liquefaction related to cyclic wave stress. Previous proximal–distal tempestite facies models may only be applicable to relatively high-gradient shelves, and new models are necessary for low-gradient settings.  相似文献   

6.
内波、内潮汐沉积和复合流沉积是二十余年来在古代深水环境中新发现的一种具牵引流性质的沉积相类型,由于地层沉积记录十分有限,制约了深水底流沉积的沉积学研究。大比例尺实测地质剖面和精细露头测量表明,底流沉积发育于凭祥盆地深水沉积之中,通过对其沉积构造精细剖析和古水流测量,识别出内波、内潮汐沉积和复合流沉积,其中内波、内潮汐沉积以双向交错层理、单向交错层理、透镜状层理为特征,复合流沉积以复合流层理、丘状交错层理和较陡的爬升波纹层理为特征,进一步表明存在浊流和底流的交互作用,为古代地层中的深水底流沉积提供了又一研究实例。  相似文献   

7.
Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.  相似文献   

8.
Hummocky cross-stratification is a sedimentary structure which is widely interpreted as the sedimentary record of an oscillatory current generated by energetic storm waves remobilizing surface sediment on the continental shelf. Sedimentary structures named hummocky cross-stratification-like structures, similar to true hummocky cross-stratification, have been observed in the Turonian–Senonian Basque Flysch Basin (south-west France). The bathymetry (1000 to 1500 m) suggests that the observed sedimentary structures do not result from a hydrodynamic process similar to those acting on a continental shelf. The morphology of these three-dimensional structures shares similarities with the morphology of hummocky cross-stratification despite a smaller size. The lateral extent of these structures ranges from a few decimetres to many decimetres; they consist of convex-up domes (hummock) and concave-up swales with a non-erosive base. Four types of hummocky cross-stratification-like geometries are described; they occur in association with structures such as climbing current ripple lamination and synsedimentary deformations. In the Basque Flysch, hummocky cross-stratification-like structures are only found in the Tc interval of the Bouma sequence. Hummocky cross-stratification-like structures are sporadic in the stratigraphic series and observed only in few turbidite beds or bed packages. This observation suggests that hummocky cross-stratification-like structures are linked genetically to the turbidity current but form under a very restricted range of parameters. These structures sometimes show an up-current (upslope) migration trend (antidunes). In the described examples, they could result from standing waves forming at the upper flow interface because of Kelvin–Helmholtz instability.  相似文献   

9.
Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.  相似文献   

10.
湘中涟源下石炭统的陆源碎屑风暴岩   总被引：4，自引：1，他引：3  
描述了测水组具丘状层理的石英砂岩风暴岩和具丘状层理的粉砂岩风暴岩以及邵东组的具粒序层理的粉砂质风暴岩和泥质介壳层风暴岩,四种风暴岩的环境分布依次为前滨——上临滨、下临滨——过渡带、远滨或滨外泥质陆棚。   相似文献   

11.
The Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine-grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low-angle to horizontally laminated, fine-grained sandstone with laminae and lenses of granules and pebbles represents upper flow-regime, overbank deposition within a braided stream system that was close to a coastline. Medium-scale, planar-tabular cross-stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large-scale, planar-tabular cross-stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage. The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium- to very coarse-grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large-scale planar-tabular and trough cross-stratification reflects deposition within the deepest areas of subtidal channels, whereas medium-scale cross-stratification represents deposition in shallower water on shoals separating channels. Fining- and thinning-upward sequences most likely resulted from the longshore migration of channels and shoals. The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine-grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave-base by combined-flow storm currents. The quartz arenite facies is characterized by an absence of fine-grained units and lithologically consists of a super-mature, medium- to coarse-grained quartz arenite. Large-scale planar-tabular cross-stratification and abundant low-angle cross-stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge-and-runnel system or a system of nearshore bars. Large-scale, planar-tabular cross-stratification (upper quartz arenite facies), which forms the top of two 40 m-thick coarsening-upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand-starved shelf as transgression caused the detachment and reworking of shoreface channel-shoal couplets. Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non-marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.  相似文献   

12.
The oolitic ironstones of the Minette were deposited during Toarcian/Aalenian times in a nearshore environment of the Paris Basin. The sedimentary sequence comprises up to 13 coarsening upward depositional cycles. The development of the cycles and different facies types are described. The iron ooids accumulated in a subtidal environment under the effects of tidal currents. Sand waves, which form an important part of the Minette sediments, were deposited under both time-velocity symmetrical and time-velocity asymmetrical tides. From the study of their internal structures the time-velocity patterns of the currents, current speeds (up to 0±9m s?1) and net transport rates have been estimated (29 g m?1 s?1 in the case of large-scale sand waves controlled by time-velocity symmetrical tides). Sand wave heights and calculated near-bed current speeds suggest a mesotidal regime. Sedimentary facies include large-scale and waves, subtidal shoals with channels and an offshore muddy shelf. Conclusions as to bathymetry are drawn from ichnofaunal associations and from the study of wave ripple marks. Finally a depositional model of the Minette iron formation is proposed which may be applicable to other oolitic ironstones.  相似文献   

13.
Interpreting the physical dynamics of ancient environments requires an understanding of how current‐generated sedimentary structures, such as ripples and dunes, are created. Traditional interpretations of these structures are based on experimental flume studies of unconsolidated quartz sand, in which stepwise increases in flow velocity yield a suite of sedimentary structures analogous to those found in the rock record. Yet cyanobacteria, which were excluded from these studies, are pervasive in wet sandy environments and secrete sufficient extracellular polysaccharides to inhibit grain movement and markedly change the conditions under which sedimentary structures form. Here, the results of flume experiments using cyanobacteria‐inoculated quartz sand are reported which demonstrate that microbes strongly influence the behaviour of unconsolidated sand. In medium sand, thin (ca 0·1 to 0·5 mm thick) microbial communities growing at the sediment–water interface can nearly double the flow velocity required to produce the traditional sequence of ripple→dune→plane‐bed lamination bedforms. In some cases, these thin film‐like microbial communities can inhibit the growth of ripples or dunes entirely, and instead bed shear stresses result in flip‐over and rip‐up structures. Thicker (ca≥1 mm thick) microbial mats mediate terracing of erosional edges; they also, foster transport of multi‐grain aggregates and yield a bedform progression consisting of flip‐overs→roll‐ups→rip‐ups of bound sand.  相似文献   

14.
On the transition between 2D and 3D dunes   总被引：3，自引：0，他引：3  
Sediment transport in sand-bedded alluvial channels is strongly conditioned by bedforms, the planimetric morphology of which can be either two- or three-dimensional. Experiments were undertaken to examine the processes that transform the bed configuration from two-dimensional (2D) dunes to three-dimensional (3D) dunes. A narrowly graded, 500 μm size sand was subjected to a 0·15 m deep, non-varying mean flow ranging from 0·30 to 0·55 m sec−1 in a 1 m wide flume. Changes in the planimetric configuration of the bed were monitored using a high-resolution video camera that produced a series of 10 sec time-lapsed digital images. Image analysis was used to define a critical value of the non-dimensional span (sinuosity) of the bedform crestlines that divides 2D forms from 3D forms. Significant variation in the non-dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random. Images also reveal that, once 2D bedforms are established, minor, transient excesses or deficiencies of sand are passed from one bedform to another. The bedform field appears capable of absorbing a small number of such defects but, as the number grows with time, the resulting morphological perturbations produce a transition in bed state to 3D forms that continue to evolve, but are pattern-stable. The 3D pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment, which leads to bifurcation. The experiments demonstrate that 2D bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator.  相似文献   

15.
Cyclic characteristics of bed forms (sedimentary structures) of early Permian fluvial Barakar rocks are studied statistically, using quasi-independence Markov chain and entropy. The hierarchy of sedimentary structures confirms that the corresponding bed forms do not represent random depositional event and follow a definite pattern of Markovian mechanism in a predictable cyclic arrangement. The preferential upward transition of sedimentary structures that can be derived for the Barakar sandstone is scoured surface (Ss) → horizontal bedding (Sh) → planar cross-bedded sandstone (Sp) → trough cross-bedded sandstone (St) → ripple cross-lamination (Sr) → parallel lamination (Fl) → scoured surface (Ss). The sequence is an asymmetrical cycle and characteristic of the lateral accretion and aggradations of fluvial channels. Entropy analysis corroborate the above inference, and E (pre) vs E(post) plots for each sedimentary structure correspond to the type B category, suggesting lower and upper truncated asymmetrical cycles. Hydro-dynamically, the Barakar cycles represent a vertical sequence of bedding types which implies a steady upward decrease in the intensity of flow—from the upper flow regime in the lower part, the lower flow regime of moderate to high intensity in the middle part, and the lower flow regime of low to very low intensity in the upper part.  相似文献   

16.
Late Albian to Cenomanian upper shoreface deposits from the Grajaú Basin, northern Brazil, consist of well‐sorted, very fine‐ to fine‐grained sandstones with swaley, trough, tabular and minor hummocky cross‐stratification. A striking feature of these deposits is the abundance of large‐scale scour‐and‐fill structures, which consist of regularly spaced, repetitive, very shallow swales with either symmetrical or asymmetrical profiles, arranged along an undulose surface or as a succession of superimposed troughs. The sediment filling these scours is characterized by very fine‐grained sandstone with gently undulose, near‐parallel lamination to very low‐angle dipping cross‐stratification intergraded with swaley and hummocky cross‐stratification. The nature of the scours and the sedimentary structures of their fills reveal the action of combined flows, which are hydrodynamically similar to those developed during storms. However, it is speculated that the combined flows responsible for the genesis of these structures were formed by tsunami waves enhanced by tsunami‐induced ebb currents and/or tidal currents. This interpretation is proposed on the basis of several lines of reasoning: (1) palaeogeographic reconstructions of the study area during the late Cretaceous show that it was outside the belt favourable for the development of storms; (2) comparison of the scour‐and‐fill structures with stratigraphically correlatable deposits exposed north of the study area, where similar features occur in association with abundant seismically induced, soft‐sediment deformation structures; and (3) the presence of several styles of soft‐sediment deformation features (i.e. convolute lamination, bed collapse, large‐scale folds, massive bedding, sand‐filled fractures and diastasis cracks) are suggestive of synsedimentary seismic activity in Cretaceous deposits located in and near to the study area. This study proposes that episodic, high‐amplitude tsunami waves, enhanced by tsunami‐induced ebb currents, develop powerful flows capable of producing complex patterns of erosion and sedimentation, which may be represented by scour‐and‐fill structures similar to those described here.  相似文献   

17.
Gravel antidunes in the tropical Burdekin River, Queensland, Australia   总被引：4，自引：0，他引：4  
The geological record is punctuated by the deposits of extreme event phenomena, the identification and interpretation of which are hindered by a lack of data on contemporary examples. It is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle-transporting, high-velocity currents, and therefore, their characteristics are poorly documented. The deposits of such flows are exposed however, in the dry bed of the Burdekin River, Queensland, Australia following tropical cyclone-induced floods. Long wave-length (up to 19 m) gravel antidunes develop during short (days) high-discharge flows in the upper Burdekin River (maximum recorded discharge near the study reach over 25 600 m3 s?1 in February 1927). Flood water levels fall quickly (metres in a day) and flow is diverted away from raised areas of the river bed into subchannels, exposing many of the high-stage bedforms with little reworking by falling-stage currents. Gravel bedforms were observed on the dry river bed after the moderate flows of February 1994 (max. 7700 m3 s?1) and January 1996 (max. 3200 m3 s?1). The bedforms had wave-lengths in the range 8–19 m, amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak-discharge flow direction. The gravel fabric and size sorting change systematically up the stoss and down the lee faces. The antidune deposits form erosive based lenses of sandy gravel with low-angle downstream dipping lamination and generally steep upstream dipping a-b planes. The internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses. The erosive based lenses of low-angle cross-bedded gravel with steep upstream dipping a-b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes. The antidune gravel lenses are associated with thick (to 1 m) high-angle cross bed sets. Ancient antidune gravel lenses may be diagnostic of episodic high-discharge conditions and particularly when they are associated with high-angle cross-bedded gravelly sand they may be useful for palaeoenvironmental interpretation.  相似文献   

18.
The research of deep-water combined-flow deposits is still in its infancy at present, which has great significance for discovering new case study and discussing the indication effects of combined-flow sedimentary structures on depositional environment. The Upper Ordovician Lashenzhong Formation in Zhuozishan area, Inner Mongolia, in the north of western Ordos Basin, is mainly deposited by turbidity currents concomitant with contour currents as well as internal-waves and internal-tides in deep-water slope and basin floor environment which display typical complex hydrodynamic conditions. Here we studied the deep-water combined-flow sedimentary structures of Lashenzhong Formation in detail based on careful field work, combined with the results of flume experiments and other related case studies in literatures. Several combined-flow sedimentary structures were found in deep-water environment, which include combined-flow-ripple lamination, quasi-planar lamination and small-scale hummocky cross-stratification-like structures. These sedimentary structures can be summarized vertically as 6 sedimentary structure successions: normal graded layer through wave-ripple lamination to combined-flow-ripple lamination (a), quasi-planar lamination through normal graded layer to combined- flow-ripple lamination (b), amphibolous normal graded layer overlaying by quasi-planar lamination (c), quasi-planar lamination through bi-directional cross-lamination to small-scale hummocky cross-stratification-like structures (d), amalgamated small-scale hummocky cross-stratification-like structures enveloped claystone (e) and combined- flow-ripple lamination enveloped claystone (f). The origin of deep-water combined-flow deposits perhaps is the interactions of turbidity currents, contour currents and internal-wave currents according to the sedimentary types and environment in Lashenzhong Formation. Based on the interactions between sedimentary currents and sea floor topography, four sedimentation zones were suggested: turbidity currents restrained zone (succession a and b), strong interaction zone (succession c), internal-wave action zone (succession d) and weak interaction zone (succession e and f). This study will be helpful for the study of interactions of internal-waves and internal-tides with sea floor topography, and related sedimentation zone division in geological records.  相似文献   

19.
The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.  相似文献   

20.
波—流相互作用是复杂水动力条件下流体相互作用的主要方式之一,由波—流相互作用形成的复合流沉积是目前沉积学在流体相互作用这一领域研究较多的一种沉积类型。以已有的文献为基础,对波—流相互作用下细砂级颗粒的运动机制进行了综述,预测了波—流相互作用的沉积特征,总结了复合流的沉积构造鉴别标志。取得的主要认识有: (1)波—流相互作用总体上属于衰弱流(waning flow)悬浮沉积,其微观的沉积机制可分为5种: 越过崩落点的喷射沉积(S1)、残余涡动沉积(S2)、未到崩落点的喷射沉积(S3)、背流面的崩落沉积(S4)、垂直降落沉积(S5);(2)波—流相互作用的沉积过程总体上受悬砂量和沉积时间的控制,5种微观沉积机制在不同的悬砂量和沉积时间条件下可形成不同的沉积机制组合,从而导致不同的底床形态;(3)复合流沉积构造鉴别标志主要有: 复合流波痕、复合流层理、爬升型复合流层理、不对称丘状交错层理、准平行层理和频繁交替的不能充分发育的浪成波纹层理与流水层理等6类。上述认识对于复杂水动力条件下的沉积学研究及对深水、浅水沉积环境的识别均具有重要的意义。  相似文献   

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