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1.
Turbidity currents in sinuous submarine channels are an important mechanism for transporting terrestrial sediments to deep water, and their deposits are of increasing importance as hydrocarbon exploration targets. Despite this, the architecture and dynamics of submarine channel systems are not well understood. Analogies are often drawn with fluvial systems due to similarities between their planform shapes even though differences in channel evolution and hydrodynamics have been noted. A key question is the nature of deposition within submarine channel bends; in particular at inner bends where point bars form in alluvial meandering rivers. Recent experimental and numerical work has demonstrated that the fluid dynamics of submarine channel bend flow are markedly different from rivers. Notably, a reversal in the orientation of secondary (helical) flow at bend apices occurs in submarine channels. The potential influence of these differences in fluid dynamics on deposition within submarine channel bends is investigated herein. We report the results of a series of physical experiments in which solute-driven gravity currents were run through pre-formed sinuous channels containing mobile beds. These experiments reveal sedimentation patterns characterised by accumulation zones downstream of bend apices and erosion zones at outer bends. These patterns are broadly analogous to the point bars and outer-bank pools observed in meandering rivers, demonstrating that the longitudinal flow component dominates over the cross-stream component, as also occurs in rivers. However, the data suggest that the reversal in direction of the cross-stream flow component compared with subaerial flows is important in determining the position and morphology of ‘point bars’ relative to bend apices. From analogy with fluvial compound channels, and fluvial theory, this reversal in secondary flow cell orientation is also expected to influence the spatial variations of grain size in submarine channel ‘point-bar’ deposits.  相似文献   

2.
Here we present results from a suite of laboratory experiments that highlight the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents. We released turbidity currents into three channels in an experimental basin filled with water and monitored current properties and the evolution of topography via sedimentation. The three channels were similar in cross-sectional geometry but varied in sinuosity. Results from these experiments are used to constrain the run-up of channelized turbidity currents on the outer banks of moderate to high curvature channel bends. We find that a current is unlikely to remain contained within a channel when the kinetic energy of a flow exceeds the potential energy associated with an elevation gain equal to the channel relief; setting an effective upper limit for current velocity. Next we show that flow through bends induces a vertical mixing that redistributes suspended sediment back into the interiors of depositional turbidity currents. This mixing counteracts the natural tendency for suspended sediment concentration and grain size to stratify vertically, thereby reducing the rate at which sediment is lost from a current via deposition. Finally, the laboratory experiments suggest that turbidity currents might commonly separate from channel sidewalls along the inner banks of bends. In some cases, sedimentation rates and patterns within the resulting separation zones are sufficient to construct bar forms that are attached to the channel sidewalls and represent an important mechanism of submarine channel filling. These bar forms have inclined strata that might be mistaken for the deposits of point bars and internal levees, even though the formation mechanism and its implications to channel history are different.  相似文献   

3.
Sinuous deep-water channels display a wide range of geometries and internal architectures. Most modern examples have been documented from large passive-margin fans, supplied by major rivers carrying huge volumes of dominantly fine-grained sediments, e.g. Amazon, Mississippi, Zaire, Bengal, Indus, Rhône and Nile Fans. However, similar examples have also been documented from tectonically active margins, e.g. Magdalena Fan. In most cases, modern sinuous channels comprise the core element of laterally extensive channel–levee systems that often aggrade significantly above a low-gradient (0.1–0.5°) fan surface; individual channels may extend downslope for 100s of kilometres. Typically, channels are subject to frequent avulsions, with only one channel active at any given time. Present highstand conditions have ensured that activity in many modern sinuous channels is much reduced due to the disconnect between fluvial feeder system and canyon head, and some have even been heavily modified or destroyed by major mass-transport deposits. Exceptions include Zaire Fan, where recent activity has provided useful insights into flow processes.The majority of detailed studies relating to sinuous channels in the subsurface originate from offshore west Africa, where channels typically occur within incisional (confined) slope–channel complexes and often represent the latter stages of channel complex fill. Both dominantly aggradational and laterally migrating styles are recognised, while modern seafloor channels in this region display a similar incisional character, e.g. Cap Timiris Canyon. Morphologic expression of sinuosity is harder to recognise at outcrop, but there are an increasing number of documented examples of lateral accretion deposits, representing point-bar growth, that are currently thought to be diagnostic of sinuous channel forms. Sinuous channel lateral migration, and point-bar growth, appears to be driven by sustained flow of fluvial-sourced (probably hyperpycnal) low-density turbidity currents, although there does appear to be variation in energy conditions, with some outcrop examples showing switches in erosive and depositional phases of activity.Previous studies have frequently focussed on the obvious gross planform morphologic similarities between fluvial and deep-water sinuous channels, e.g. nature of sinuosity, presence of point bars and cut-off loops. However, we suggest here that the differences between submarine and river channels are of greater significance, in terms of geometry, flow processes, migration style and deposit character. Sinuous deep-water channels typically form initially by a moderate amount of incision followed by rapid initial bend growth (associated with bypass of sediment). Channels that show significant aggradation then reach a point where there is a near cessation of planform movement (ossification), and growth is dominated by vertical aggradation. A new process model is proposed for this developmental sequence that synthesises observations and experiments that were previously paradoxical.  相似文献   

4.
Submarine channels are major morphological features of the sea floor and are important in the transport of sediment to the deep ocean. Although much is known concerning the large-scale distribution of sediment within and surrounding submarine channels, there is little understanding of the fluid dynamic processes that control this sedimentation. Direct measurement of flow velocities and concentrations has proved to be extremely difficult within submarine channels, with the resultant paucity of direct observations making physical laboratory modelling a critical technique for examining the processes that operate in, and control, submarine channel development.Recent experimental and numerical studies have proposed a new model of secondary circulation within submarine channel bends, characterised by a reversal in the orientation of the secondary circulation cell relative to that found in meandering rivers. This new paradigm for submarine channels thus predicts basal flow from the inside to the outside of the bend at a bend apex, with an upper return flow directed towards the inner bend. The reversal in orientation of the secondary flow cell has been linked to the vertical distribution of downstream velocity and associated changes in centrifugal and pressure gradient forces. However, previous work has additionally proposed that shearing of the within-channel flow by overbank flow may also generate secondary flow reversal.This study assesses the applicability of the proposed submarine bend flow model against a range of key channel parameters. We demonstrate that the sense of secondary circulation is the same for all experimental conditions, strongly supporting the new model of secondary flow in submarine channels. Furthermore, investigation of overbank shear induced secondary circulation confirms for the first time that this mechanism can occur, and identifies the channel styles most likely to exhibit this effect. Such shear-induced circulation is, however, shown to be a secondary mechanism, with the vertical distribution of downstream velocity the principal mechanism. In certain channel configurations, the two mechanisms may act to augment one another.  相似文献   

5.
Quantifying the characteristics of the turbidity currents that are responsible for the erosion, lateral migration and filling of submarine channels maybe useful for predicting the distribution of lithofacies in channel fill and levee reservoirs. This paper uses data from a well-studied submarine channel in Amazon Fan in an attempt to reconstruct the velocity, thickness, concentration, duration, recurrence rates and vertical structure of turbidity currents in this long sinuous channel. Estimates of flow conditions are derived from the morphology of the channels and the characteristics of the deposits within them. In particular, the availability of information on the sediment distribution with respect to the channel topography at the time of deposition allows for insights into the vertical structure of the flow, a key property that has been so far poorly understood. Integration of flow constraints from well and seismic data or from detailed analysis of outcrop with numerical flow models is a critical step toward a complete understanding of the flow and associated deposits. Turbidity currents in sinuous submarine channels, exemplified by Amazon Channel, are found to last for tens of hours and occur on a regular, quasi-annual basis. Model results suggest that these flows had, on average, velocities ranging from 2 to 4 m/s in the canyon/upper fan which decreased to 0.5–1 m/s in the lower fan, travelling in excess of 800 km. The model turbidity currents were subcritical over most of the channel length, indicating a low degree of water entrainment and low rate of deceleration down the channel. The formation of such long, sinuous channels is intrinsically associated with frequent, long-duration, subcritical turbidity currents carrying a silt-dominated sediment load.  相似文献   

6.
Isaac Channel 3 is a rare outcrop example of a perpendicular cut through a sinuous deep-water channel, and also where levee deposits formed on opposite sides of the channel are well exposed. Strata flanking the outer- and inner-bend margin of the channel show important differences in lithofacies, architecture and association with channel-fill strata. Proximal outer-bend levee deposits are sand-rich (N:G up to 0.68) and comprise medium- to thick-bedded, Ta-d turbidites interstratified with thinly-bedded, Tcd turbidites. The thicker-bedded deposits show lateral variation in grain size and thickness over hundreds of meters whereas thin-bedded strata thin and fine negligibly over similar distances. The distal outer-bend levee (up to 700 m laterally away from the channel) consists predominantly of thin-bedded turbidites interstratified with up to 5 m thick coarse-grained splay deposits. In contrast to the outer-bend, the inner-bend levee deposits are significantly more mud-rich (N:G as low as 0.15) and consist mostly of thin-bedded, Tcd turbidites with less common thicker-bedded, Ta-d turbidites. Lateral thinning and fining trends associated with these less common thicker-bedded deposits occur more rapidly than their outer-bend counterparts.Erosion associated with lateral migration of the channel axis produced a sharp contact along the outer-bend channel margin causing coarse-grained channel-fill deposits to be in erosional contact with levee deposits. This suggests that the crest of the outer-bend levee was elevated above the channel floor and produced a channel margin upon which channel-fill strata onlapped. Positive topography is interpreted to have developed by overspilling processes that deposited abundant sand on the outer-bend levee while the majority of the flow continued through the channel bend and bypassed to areas further downslope. In contrast, some thick-bedded, amalgamated channel-fill deposits in the axial channel area grade laterally over 140 m into thinly-bedded turbidites on the inner-bend levee. The lack of channel-fill on lap relationships implies that topography along the inner bend was sufficiently subtle that at least some flows were able to expand laterally and over the overbank area without becoming separated from the main throughgoing channel flow.Stratal relationships observed in Isaac Channel Complex 3 suggests three main episodes of channel-levee growth that were each initiated by a period of increased levee relief followed by channel filling and distal levee deposition. This consistent depositional history points to the regular variations, in both time and space, of sediment transport and deposition in a deep-marine sinuous channel-levee system.  相似文献   

7.
Several laterally offset and aggradational sinuous submarine channels are contained within a 54 km long segment of the Benin-major Canyon. Axial channel deposits produce high amplitude reflections on three-dimensional (3-D) seismic profiles. Some seismic reflections have U- or V-shaped cross-sectional motifs that were correlated with confidence along linear to meandering paths for distances up to 70 km. They are referred to here as channel-forms (CFs), and are believed to be the axial parts of submarine channels preserved during overall channel floor aggradation. A total of 15 separate CFs were mapped allowing thalweg-gradients, dimensions, and morphology to be studied spatially and through time, providing insight into how submarine canyons fill. Their planform geometry evolved predominantly in a stepwise fashion through alternating periods of cut-and-fill, but more gradual channel migrations are also observed. The largest offsets in successive channel floor position occur after periods of significant vertical CF fill (‘thalweg plugging’—with deposits commonly consisting of lower amplitude, transparent to chaotic seismic reflections). The passage of erosive flows after such periods of fill caused abrupt shifts in channel position, particularly at meander bends, with increased potential for the formation of pseudo meander loop cut-offs. Significant spatial differences in the stacking architecture of CFs are attributed to local slope deformation and perhaps also to a recent channel avulsion just west of the study area. Abrupt channel straightening in the western study area coincides with a period of increased valley-gradient associated with amplification of an underlying anticlinal fold. The youngest CFs in this area show limited aggradation and are characterized by repeated episodes of headward erosion causing knickpoint migration as the recent channel floor tried, unsuccessfully, to establish a smooth graded depth profile. This is in stark contrast to the time-equivalent predominantly aggradational CFs in the eastern study area that show a progressive increase in sinuosity through time.  相似文献   

8.
等深流影响的水道沉积体系的沉积特征及其沉积过程是当前深水沉积学研究的热点、难点和前沿科学问题,但研究程度较为薄弱。该文以北礁凹陷上新统（地震反射T20?T30）为研究对象,利用覆盖北礁凹陷局部的三维地震资料,采用均方根属性、相干属性、时间域构造,再结合地震切片等方法,研究北礁凹陷深水区上新统斜交斜坡（走向）的特殊水道沉积体系特征及其沉积过程。研究发现,该水道沉积体系分为早、晚两期,早期发育水道和片状、扇状溢堤沉积,晚期仅发育水道和片状溢堤沉积,其中扇状溢堤沉积仅发育在水道右侧弯曲处,片状溢堤沉积仅分布在水道左侧,水道始终与区域斜坡斜交,水道对称分布且无明显迁移现象。结合该时期北礁凸起发育等深流相关的丘状漂积体和环槽,认为该水道沉积体系特殊的形态主要受控于等深流与浊流交互作用的沉积结果:浊流流经水道,其上覆浊流溢出水道,形成溢岸浊流,在水道左侧,该溢岸浊流与等深流发生相向运动,被等深流“吹拂”到单侧,大面积分布,延伸千米,形成片状溢堤沉积;而在水道弯曲处（右侧）,溢岸浊流与等深流发生相对运动,抑制溢岸浊流进一步扩展,形成相对小范围扇状溢堤沉积,该沉积结果与前人水槽实验结果相一致。  相似文献   

9.
Several knickpoints have been identified along the present-day thalweg of a sinuous submarine channel–levee system (CLS) on the slope of the western Niger Delta using 3D seismic data. The knickpoints form as a result of gradient changes caused by the uplift of a thrust and fold belt orthogonal to the CLS. The channel gradient is lower locally upstream of folds causing turbidity currents within the channel to decelerate and deposit the coarsest sediment load. The basinward dipping fold limb causes local steepening of the gradient, which leads to increased flow velocity and turbulence within the turbidity currents. This enhances erosion at the base of the channel and leads to the formation of a knickpoint. If preserved, e.g., as a result of channel avulsion or abandonment, the deposits upstream of the knickpoints could constitute an important hydrocarbon reservoir element. They can, however, also be partially eroded by headward-migrating knickpoints, as the channel strives to regain its equilibrium profile, leaving remnant sand pockets preserved on channel margins. Although knickpoints are difficult to recognise from subsurface seismic or outcrop data, it is anticipated that they can form at any stage of the evolution of a channel–levee system and may be particularly important in controlling 3D channel architecture where channels intersect dynamically changing seabed bathymetry.  相似文献   

10.
Several types of sediment failures in the Gulf of Cadiz were observed using multibeam bathymetry, acoustic imagery and high-resolution seismic. These instabilities are mainly sediment failures and flows. Their width and length vary from 1 to more than 10 km. The failures are mainly related to high sedimentation rates, particularly in places where the Mediterranean Outflow Water (MOW) spills over, such as channel bends and the outer side of the giant contourite levee. Steep slopes are also a trigger for failure at the continental shelf-slope transition, on valley sides, on canyon flanks, and on the sides of bathymetric highs. Other mass movements are related to fluid escape (mud volcanoes) and earthquakes. In areas where the MOW flows along the seafloor, the constant shearing and related erosion can add to the overall stresses. The frequency of failures can be estimated using the deposits resulting of their distal transformations into turbidites.  相似文献   

11.
High-resolution 3D seismic data of several subsurface examples reveal significant differences in internal architecture and evolution of fluvial and deep-water sinuous channel systems, although there are many similarities in external morphologies of both systems. Channel migrations or shifts in fluvial systems, with point-bar scrolls, are relatively continuous laterally and show a downstream component; they are commonly a single seismic phase thick, with flat tops. In deep-water systems, channel migrations or shifts, with or without point-bar scroll-like features, may be lateral, either continuous or discrete, and laterally to vertically aggrading, again either continuous or discrete; they are single to multiple seismic phases thick, with or without a downstream component. Even the most laterally migrated channel complex commonly aggrades, to varying degrees, from the inside to the outside of sinuous loops. Similarities between fluvial and deep-water sinuous channel systems discussed here imply that sinuosity enhancements in both cases are the result of gradual processes, involving interaction of flows, sediments and alluvial plain or seafloor in attempts to build equilibrium profiles. Flat gradients, high width to depth ratios of valleys/channel belts, fine sediment grain sizes, a certain degree of bank cohesiveness, and presence of secondary circulations in flows were pre-requisites in both systems. However, a number of factors appear to have caused major differences in the internal architecture and modes of evolution of fluvial and deep-water channels. These include differences in (1) density contrasts of flows relative to ambient fluids, (2) entrainments of ambient fluids into flows, (3) effects of centrifugal and Coriolis forces on flows, (4) frequency, volume and duration of steady vs. catastrophic flows, (5) modes of sediment transport, and (6) effects of sea level changes on deposition. Furthermore, within deep-water systems, changes in flow parameters and sediment grain size can cause erosion, bypassing or deposition in space and time and result, through cuts and fills, in sinuous channels with lateral migrations, vertical aggradations and combinations thereof.  相似文献   

12.
The Bulgheria canyon-fan system in the eastern Tyrrhenian Sea displays well-developed, small-scale, fluvial-like features and has formed alongside the northern slope of the Sapri peri-Tyrrhenian basin. This study reveals, for the first time, the morphology and course of the present-day system as well as the buried elements based on a Digital Terrain Model and high-resolution seismic profiles interpretation. Two adjacent canyons (Infreschi and Luna) originate in the Cilento outer shelf at a short distance from each other and feed an intraslope basin fan through two main sub-parallel channels that run about 12 and 8 km, respectively. Channel and levee development seems to be controlled primarily by the local slope gradient and by Coriolis forces that induce a faster vertical growth of the right-side features, as is often observed in the Northern Hemisphere. Centrifugal forces, on the other hand, have induced episodic flow-stripping at the meander loops and bends, causing local destruction of the main channel levees rather than new levee growth at the outer bends. Overbank deposits are associated with overspill turbidite deposition in the mid fan where a topographic constraint occurs, whereas large-sediment, low-angle wave fields are mainly developed on the outer fan. Buried features and relict morphologies suggest that the Infreschi channel experienced at least two phases of re-incision since the final stages of the middle Pleistocene. Local re-adjustment of outer lobe growth due to channel avulsion and meander abandonment is possibly a consequence of relative base-level fluctuations. The sedimentary record of the mid and outer fan includes outrun mass wasting deposits from extensive failures of the Sapri slope. Indeed, a marked scar is present on the eastern side of the modern outer lobe that indicates the persistency of mass flow passages up to recent times. In addition to the environmental factors that are currently considered to cause canyon formation on the shelf margin, this study proposes the possibility that the head canyon branch close to the mainland was incised by massive and persistent underground freshwater flow from the adjacent aquifer when the sea-level was lower than at present.  相似文献   

13.
High-resolution multichannel 2-D and 3-D seismic data, primarily from upper fan reaches of near-seafloor channel-levee systems on the Niger Delta slope and in the Arabian Sea, reveal a high level of detail and architectural complexity. Several architectural elements are common to each system examined in this study. They include inner levees, outer levees, erosional fairways, channel-axis deposits, rotational slumps blocks, and mass transport deposits. Although the scale of individual systems varies significantly, similarities in first-order architectural elements and their configurations suggest that common depositional processes are involved regardless of scale differences.Most of the channel-levee systems examined in this study are characterized by a basal erosional fairway that is bordered by outer levees of varying thickness. Together these elements define the base and margins of the channel-belt, where channel-axis deposits and inner levees are the dominant architectural elements. Vertical, sub-vertical, and lateral stacking patterns of sinuous and/or meandering channels create seismic facies that range from narrow to wide zones of high amplitude reflections (HARs) with chaotic to continuous and shingled to horizontal reflections. Some HARs appear as isolated or stacked asymmetric to symmetric u- and v-shaped reflections, referred to here as channel-forms. Channel-belts evolve within the confines of the scalloped erosional fairway walls (flanked by outer levee), and are similar in morphology to meander-belts in fluvial systems, but commonly have a greater component of vertical aggradation. Detailed study of one particular channel-levee system on the Niger Delta slope shows a period of incision followed by three distinct phases of channel development during its aggradational history. Each fill phase corresponds to a different channel stacking architecture, planform geometry, and nature of terrace development, with important implications for reservoir architecture. In some cases, multiple phases of inner levee growth are observed, each intimately linked to the channel migration and aggradation history. Channel sinuosity evolves dynamically, with some meander loops undergoing periods of accelerated meander growth at the same time that others show little lateral migration.  相似文献   

14.
In the westernmost Ionian Sea lies a steep, tectonically active marine basin influenced by turbidity currents generated by terrigenous river input from the adjacent mountains and strong tidal currents propagating through the Strait of Messina. Like many young marine rifts, the basin is lined by steep streams draining the uplifting coasts and supplying sediment across narrow shelves. However, unlike many rifts, this basin is semi-enclosed. The present study explores the seabed morphology and sediment structures in this complex environmental setting, based on multibeam sonar, chirp profiler and seismic reflection data collected in 2006. Offshore channels include many that can be directly linked to onshore streams, suggesting that hyperpycnal flows are important for their formation. Near the Strait of Messina in depths shallower than 400 m, the channels are subdued, plausibly explained as an effect of strong tidal currents. The Messina Channel is characterised by abundant mass-wasting features along its outer bends, particularly on the Calabrian side. Coincidence of the channel course with faults suggests that the channel is structurally controlled in places. The chirp profiles generally show only shallow penetration, the evidence for coarse texture being consistent with the steep gradient of the basin that inhibits deposition from turbidity currents. By contrast, some locally discontinuous mounds exhibiting layered sub-bottom reflectors in the chirp profiles are interpreted as modern levee deposits formed from channelised turbidity current overspill. Overall, this semi-enclosed basin shows little evidence of substantial accumulations associated with modern turbidity current activity, any contemporaneous sediment supply evidently bypassing the area to be deposited in the Ionian Trench; as a consequence, this trench should be an archive of local slope failure and flood events.  相似文献   

15.
This study integrates newly acquired stratigraphic data, geologic mapping, and paleocurrent data to constrain the stratigraphic evolution of the oldest channel-lobe complex in the Upper Cretaceous Cerro Toro Formation in the Silla Syncline area of the Magallanes Basin, termed the Pehoe member. The Pehoe member ranges in thickness from 60 m in the north to at least 410 m farther down system and comprises three separate divisions (A, B, and C). A lower conglomerate unit and an upper one, termed Pehoe A and C divisions respectively, represent the fill of major incised submarine channels or channel complexes. These are separated by stratified sandstone of the Pehoe B division, representing a weakly confined lobe complex, either transient or terminal.The integration of new data with observations from previous studies reveal that the three main coarse-grained conglomerate and sandstone members in the Cerro Toro Formation in the Silla Syncline include at least seven distinct submarine channels or channel complexes and two major lobe complexes. The thinning and disappearance of these units along the eastern limb of the syncline reflect confinement of the flows to a narrow trough or mini-basin bounded to the east by a topographic high. This confinement resulted in unidirectional paleocurrents to the south and southeast in all deposits. Changes in depositional geometries are interpreted as reflecting changes in sediment supply and relative confinement. Submarine channels were from 700 m to 3.5 km wide and occupied a fairway that was 4-5 km wide. Flows moving south and southeast in this mini-basin probably crossed the eastern topographic high south of the present exposures and joined those moving southward along the axis of the foreland basin at least 16 km to the east.  相似文献   

16.
Channel-levee systems are frequently interpreted as having a long history of cut-and-fill by channel-shaped features of different scales. Results from a simple geometric model based on a centerline migration algorithm combined with a vertical channel trajectory show that an incising-to-aggrading trajectory of a single channel can produce realistic morphologies similar to systems observed on the seafloor and subsurface, including features such as a basal erosional surface, coeval inner and outer levees, internal erosional boundaries, and terraces draped by inner levee deposits. Channel migration results in composite erosional surfaces that are distinct from topographic surfaces, and their formation does not require larger than usual erosional flows. Many submarine channels interpreted as underfit were probably carved by flows similar to the ones that eroded and deposited the entire channel system. We suggest that the features of most submarine channel-levee systems do not require large temporal variations in flow magnitude but can be explained by a simpler model whereby incision, migration and aggradation of a single channel form over time results in an apparently complex system.  相似文献   

17.
Submarine channel levee systems form important hydrocarbon reservoirs in many deep marine settings and are often deposited within a structurally active setting. This study focuses on recent submarine channels that developed within a deepwater fold and thrust belt setting from the Levant Basin, eastern Mediterranean Sea. Compressional deformation within the study area is driven by the up-dip collapse of the Nile cone above the ductile Messinian Evaporites. Structures such as folds and strike slip faults exert a strong control on channel location and development over time. From this study four end-member submarine channel–structure interactions can be defined: Confinement, diversion, deflection and blocking. Each of these channel–structure interactions results in a distinct submarine channel morphology and pattern of development compared to unconfined channel levee systems. Each interaction can also be used to assess timing relationships between submarine channel development and deformation.  相似文献   

18.
Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope.North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment.  相似文献   

19.
In the Castle Creek study area, a vertically dipping, 2.5 km-thick succession of basin-floor to base-of-slope Neoproterozoic rocks are superbly exposed. In part of that outcrop, inner-bend (point-bar) deposits of sharp-based, laterally accreting sinuous channels are exposed, of which one is described in detail (Isaac Channel unit 2.2—IC2.2). IC2.2 is up to 13 m thick and extends laterally for at least 400 m. Lateral-accretion deposits, or simply lateral accretion deposits (LADs), are inclined at 7–12° toward the channel base and are about 120–140 m long. Grain size changes little obliquely upward along an individual LAD, or vertically upward through the channel-fill. LADs consist of two repeating and interstratified kinds: coarse-grained LADs consisting of strata up to granule conglomerate, and fine-grained LADs composed of thin- to medium-bedded finer-grained turbidites. In the lower part of the channel-fill, strata consist only of amalgamated coarse-grained LADs composed of decimetre-thick beds composed of very coarse sandstone/granule conglomerate that grade upward to medium sandstone. Tractional sedimentary structures are absent and fine-grained strata, specifically mudstone, occur only as isolated patches of intraclast breccia. In the upper part of the channel-fill, however, LADs consist of a rhythmic interfingering of coarse- and fine-grained LADs. Coarse-grained LADs consist of 2–3 bed-thick packages that are separated and then pinch-out rapidly into fine-grained LADs. Close to their up-dip pinch-out these coarse strata consist commonly of poorly sorted, ungraded very coarse sandstone/granule conglomerate overlain abruptly by planar-laminated or medium-scale (dune) cross-stratified, medium-grained sandstone. Fine-grained LADs are composed of mudstone interbedded with thin- and medium-bedded Tbcd and Tcd turbidites that obliquely downward and become truncated as the super- and subjacent coarse-grained LADs amalgamate.The rhythmic intercalation of coarse- and fine-grained LADs is interpreted to be related to temporal changes in the nature of sediment deposition along the point-bar of a deep-marine sinuous channel. Following failure along the cut-bank margin (outer bend), deposition of coarse-grained sediment on the point-bar (inner bend) occurred in order to re-establish an equilibrium channel geometry, and thereby equilibrium sediment transport conditions (i.e. sediment bypass). Once equilibrium was re-established deposition of finer, thinner-bedded strata of the succeeding fine LAD resumed. These strata represent deposition from the dilute tail region of flows that for the most part had already transited that particular channel bend and transported the bulk of its coarse sediment further down-dip. This history of alternating coarse and fine-grained sedimentation was repeated several times in the channel bend as it migrated laterally. Moreover, in coarse LADs, the restricted occurrence of tractional sedimentary structures close to their up-dip pinch-out suggests that although suspension deposition may have dominated over much of the lateral accretion surface, it was succeeded, at least on the upper part of the lateral accretion surface, by sediment reworking and bed-load transport, possibly related to elevated turbulent stresses caused by mixing along the sharp density interface in a strongly stratified turbulent flow.Although seemingly similar to LADs reported from fluvial point-bars, deep-marine LADs of the Windermere exhibit many important differences. Some of these differences are likely related to the differences in the mode of sand (and coarser) sediment transport in deep-marine versus non-marine environments, specifically, suspension versus bed load, respectively. In addition, fundamental differences in the flow structure between subaqueous suspension currents and open-channel flows most probably exert an additional first-order control contributing to these differences.  相似文献   

20.
A high-resolution bathymetric and seismic study of sinuous midfan channels on the Amazon Fan shows that some common elements of seismic profiles across the channel/levee system may be side echoes (sideswipe) from reflective, coarse channel-floor sediments Which lie to the side of the ship track. This includes portions of a dipping zone of high-amplitude reflectors beneath the channel. If these strong echoes are side echoes rather than buried coarse sediments, there may be less coarse material present within the midfan channel/levee systems than predicted, and channel evolution is still poorly resolved. Side echoes may be common in other areas of complex deep-sea morphology.  相似文献   

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