Morphometric analysis of sediment conduits on a bathymetric high: Implications for palaeoenvironment and hydrocarbon prospectivity

Canyons and other sediment conduits are important components of the deep‐water environment and are the main pathways for sediment transport from the shelf to the basin floor. Using 3‐D and 2‐D seismic reflection data, seismic facies and statistical morphometric analyses, this study showed the architectural evolution of five canyons, two slide scars and four gullies on the southern part of the Loppa High, Barents Sea. Morphometric parameters such as thalweg depth (lowest point on a conduit's base), wall depth (middle point), height, width and base width, sinuosity, thalweg gradient, aspect ratio (width/height) and cross‐sectional area of the conduits were measured at intervals of 250‐m perpendicular to the conduits’ pathways. Our results show that the canyons and slide scars in the study area widen down slope, whereas the gullies are narrow and short with uniform widths. The sediment conduits in the study area evolved in three stages. The first stage is correlated with a time when erosion and bypass were dominant in the conduits, and sediment transferred to the basin in the south. The second stage occurred when basin subsidence was prevalent, and a widespread fine‐grained sequence was deposited as a drape blanketing the canyons and other conduits. A final stage occurred when uplift and glacial erosion configured the entire southern Loppa High into an area of denudation. Our work demonstrates that the morphometric parameters of the canyons, slide scars and gullies generally have increasing linear trends with down‐slope distance, irrespective of their geometries. The morphometric analysis of the sediment conduits in the study area has wider applications for understanding depositional processes, reservoir distribution and petroleum prospectivity in frontier basins.     

Cretaceous continental margin evolution revealed using quantitative seismic geomorphology,offshore northwest Africa

The application of high‐resolution seismic geomorphology, integrated with lithological data from the continental margin offshore The Gambia, northwest Africa, documents a complex tectono‐stratigraphic history through the Cretaceous. This reveals the spatial‐temporal evolution of submarine canyons by quantifying the related basin depositional elements and providing an estimate of intra‐ versus extra‐basinal sediment budget. The margin developed from the Jurassic to Aptian as a carbonate escarpment. Followed by, an Albian‐aged wave‐dominated delta system that prograded to the palaeo‐shelf edge. This is the first major delivery of siliciclastic sediment into the basin during the evolution of the continental margin, with increased sediment input linked to exhumation events of the hinterland. Subaqueous channel systems (up to 320 m wide) meandered through the pro‐delta region reaching the palaeo‐shelf edge, where it is postulated they initiated early submarine canyonisation of the margin. The canyonisation was long‐lived (ca. 28 Myr) dissecting the inherited seascape topography. Thirteen submarine canyons can be mapped, associated with a Late Cretaceous‐aged regional composite unconformity (RCU), classified as shelf incised or slope confined. Major knickpoints within the canyons and the sharp inflection point along the margin are controlled by the lithological contrast between carbonate and siliciclastic subcrop lithologies. Analysis of the base‐of‐slope deposits at the terminus of the canyons identifies two end‐member lobe styles, debris‐rich and debris‐poor, reflecting the amount of carbonate detritus eroded and redeposited from the escarpment margin (blocks up to ca. 1 km³). The vast majority of canyon‐derived sediment (97%) in the base‐of‐slope is interpreted as locally derived intra‐basinal material. The average volume of sediment bypassed through shelf‐incised canyons is an order of magnitude higher than the slope‐confined systems. These results document a complex mixed‐margin evolution, with seascape evolution, sedimentation style and volume controlled by shelf‐margin collapse, far‐field tectonic activity and the effects of hinterland rejuvenation of the siliciclastic source.     

Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex

Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30′W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed. On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high-permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface.     

Architecture,formation and implication of active structure-controlled intraslope channel system southeast offshore Sendai,Tohoku, Japan

Located off the Pacific coast of central Tohoku (NE Japan), the Ishinomaki slope channel (ISC) provides an excellent opportunity to study a structure-controlled intraslope channel and downslope sedimentation along the active margin. The seismic reflection data across ISC show an extensive basal surface and overlying channel complexes between the basement structures of the Abukuma ridge to the south and Kitakami massif to the north, indicating that the formation of the intraslope basin, channelization of ISC and sedimentation of the downstream channel-lobe transition zone (CLTZ) are very likely to be structure-controlled. The oblique channel stacking pattern, faulting of the seafloor and subsurface Abukuma ridge in the upper and lower domains of ISC, collectively suggest that ISC has migrated northward and is currently under the influence of active compression. Differences in styles of accommodation space between the upper and lower domains of ISC suggest that differential subsidence occurred along the strike-slip tectonic line. Based on the regional strike-slip tectonic line, we propose that a Kitakami-Abukuma ridge existed before the formation of ISC. The strike-slip faulting divided the Kitakami-Abukuma ridge into the Kitakami massif to the north and the Abukuma ridge to the south, and an intervening fault trough as the precursor of the intraslope basin and ISC. As the subduction of the Pacific Plate and associated compressional events continued, the Abukuma ridge was reactivated to narrow the intraslope basin into a confined channel. Located near the epicentre of the devastating 2011 Tohoku earthquake event, the ISC, downstream CLTZ and underlying intraslope basin provide information on active basement structure and the evolving sediment routing system on the tectonically active margin.     

The Balearic Promontory geomorphology (western Mediterranean): morphostructure and active processes

In this paper, a detailed study of the submarine geomorphology surrounding the Balearic Promontory (western Mediterranean), a northeast prolongation of the Neogene Betic Range in southern Spain, is presented from a series of high-resolution tools including swath bathymetry and seismic reflection profiling. The study identifies the main features of the continental shelf, slope and basins surrounding the Balearic Islands. We show a variety of seafloor relief that owes its origin to several geologic processes, which ultimately control the transport of sediment from the shallower areas to the deep basin. The most important processes are erosion of the shelf and upper slope (terraces associated with different Quaternary sea-level stands and canyons), transport and sediment deposition in the lower slope and base-of-slope by turbidity currents, volcanism and instability processes (landslides scarps and debris lobes). The swath data show that tectonics plays an important role in shaping the submarine slopes of Eivissa and Formentera, the two southernmost islands, as well as its interplay with sedimentary processes, especially mass wasting. Finally, several areas show evidence of pockmarks, which indicate that fluid migration take place in the sediments, probably conditioning several other processes such as mass wasting.     

Clinoform growth in a Miocene,Para‐tethyan deep lake basin: thin topsets,irregular foresets and thick bottomsets

Late Miocene lacustrine clinoforms of up to 400 m high are mapped using a 1700 km² 3‐D seismic data set in the Dacian foreland basin, Romania. Eight Meotian clinoforms, constructed by sediment from the South Carpathians, prograded around 25 km towards southwest. The individual clinothems show thin (10–60 m thick), if any, topsets, disrupted foresets and highly aggradational bottomsets. Basin‐margin accretion occurred in three stages with changing of clinoform heights and foreset gradients. The deltaic system prograded into an early‐stage deep depocenter and contributed to high gradient clinoforms whose foresets were dominated by closely (100–200 m) spaced 1.5–2 km wide V‐shaped sub‐lacustrine canyons. During intermediate‐stage growth, 2–4 km wide canyons were dominant on the clinoform foresets. From the early to intermediate stages, the lacustrine shelf edges were consistently indented. The late‐stage outbuilding was characterised by smaller clinoforms with smoother foresets and less indentation along the shelf edge. Truncated and thin topsets persisted through all three stages of clinoform evolution. Nevertheless, the resulting long‐term flat trajectory shows alternating segments of forced and low‐amplitude normal regressions. The relatively flat trajectory implies a constant base level over time and was due to the presence of the Dacian–Black Sea barrier that limited water level rise by spilling to the Black Sea. Besides the characteristic shelf‐edge incision of the thin clinoform topsets and the resultant sediment bypass at the shelf edge, the prolonged regressions of the shelf margin promoted steady sediment supply to the basin. The high sediment supply at the shelf edges generated long‐lived slope sediment conduits that provided sustained sediment transport to the basin floor. Clinothem isochore maps show that large volumes of sediment were partitioned into the clinoform foresets, and especially the bottomsets. Sediment predominantly derived from frequent hyperpycnal flows contributed to very thick, ca. 300–400 m in total, bottomsets. Decreasing subsidence over time from the foredeep resulted in diminishing accommodation and clinoform height, reduced slope channelization and smoother slope morphology.     

Late Ordovician,deep‐water gravity‐flow deposits,palaeogeography and tectonic setting,Tarim Basin,Northwest China

The Upper Ordovician in the Tarim Basin contains 5000–7000 m of siliciclastic and calciclastic deep‐water, gravity‐flow deposits. Their depositional architecture and palaeogeographical setting are documented in this investigation based on an integrated analysis of seismic, borehole and outcrop data. Six gravity‐flow depositional–palaeogeomorphological elements have been identified as follows: submarine canyon or deeply incised channels, broad and shallow erosional channels, erosional–depositional channel and levee–overbank complexes, frontal splays‐lobes and nonchannelized sheets, calciclastic lower slope fans and channel lobes or sheets, and debris‐flow complexes. Gravity‐flow deposits of the Sangtamu and Tierekeawati formations comprise a regional transgressive‐regressive megacycle, which can be further classified into six sequences bounded by unconformities and their correlative conformities. A series of incised valleys or canyons and erosional–depositional channels are identifiable along the major sequence boundaries which might have been formed as the result of global sea‐level falls. The depositional architecture of sequences varies from the upper slope to abyssal basin plain. Palaeogeographical patterns and distribution of the gravity‐flow deposits in the basin can be related to the change in tectonic setting from a passive continental margin in the Cambrian and Early to Middle Ordovician to a retroarc foreland setting in the Late Ordovician. More than 3000 m of siliciclastic submarine‐fan deposits accumulated in south‐eastern Tangguzibasi and north‐eastern Manjiaer depressions. Sedimentary units thin onto intrabasinal palaeotopographical highs of forebulge origin and thicken into backbulge depocentres. Sediments were sourced predominantly from arc terranes in the south‐east and the north‐east. Slide and mass‐transport complexes and a series of debris‐flow and turbidite deposits developed along the toes of unstable slopes on the margins of the deep‐water basins. Turbidite sandstones of channel‐fill and frontal‐splay origin and turbidite lobes comprise potential stratigraphic hydrocarbon reservoirs in the basin.     

Impact of structural and autocyclic basin-floor topography on the depositional evolution of the deep-water Valparaiso forearc basin, central Chile

The Valparaiso Basin constitutes a unique and prominent deep‐water forearc basin underlying a 40‐km by 60‐km mid‐slope terrace at 2.5‐km water depth on the central Chile margin. Seismic‐reflection data, collected as part of the CONDOR investigation, image a 3–3.5‐km thick sediment succession that fills a smoothly sagged, margin‐parallel, elongated trough at the base of the upper slope. In response to underthrusting of the Juan Fernández Ridge on the Nazca plate, the basin fill is increasingly deformed in the seaward direction above seaward‐vergent outer forearc compressional highs. Syn‐depositional growth of a large margin‐parallel monoclinal high in conjunction with sagging of the inner trough of the basin created stratal geometries similar to those observed in forearc basins bordered by large accretionary prisms. Margin‐parallel compressional ridges diverted turbidity currents along the basin axis and exerted a direct control on sediment depositional processes. As structural depressions became buried, transverse input from point sources on the adjacent upper slope formed complex fan systems with sediment waves characterising the overbank environment, common on many Pleistocene turbidite systems. Mass failure as a result of local topographic inversion formed a prominent mass‐flow deposit, and ultimately resulted in canyon formation and hence a new focused point source feeding the basin. The Valparaiso Basin is presently filled to the spill point of the outer forearc highs, causing headward erosion of incipient canyons into the basin fill and allowing bypass of sediment to the Chile Trench. Age estimates that are constrained by subduction‐related syn‐depositional deformation of the upper 700–800 m of the basin fill suggest that glacio‐eustatic sea‐level lowstands, in conjunction with accelerated denudation rates, within the past 350 ka may have contributed to the increase in simultaneously active point sources along the upper slope as well as an increased complexity of proximal depositional facies.     

Drainage, sediment transport, and denudation rates on the Nanga Parbat Himalaya, Pakistan

The Nanga Parbat Himalaya presents some of the greatest relief on Earth, yet sediment production and denudation rates have only been sporadically addressed. We utilized field measurements and computer models to estimate bank full discharge, sediment transport, and denudation rates for the Raikot and Buldar drainage basins (north slope of Nanga Parbat) and the upper reach of the Rupal drainage basin (south slope).The overall tasks of determining stream flow conditions in such a dynamic geomorphic setting is challenging. No gage data exist for these drainage basins, and the overall character of the drainage basins (high relief, steep flow gradients, and turbulent flow conditions) does not lend itself to either ready access or complete profiling.Cross-sectional profiles were surveyed through selected reaches of these drainage basins. These data were then incorporated into software (WinXSPRO) that aids in the characterization (stage, discharge, velocity, and shear stress) of high altitude, steep mountain stream conditions.Complete field measurements of channel depths were rarely possible (except at several bridges where the middle of the channel could actually be straddled and probed) and, when coupled with velocity measurements, provided discrete points of field-measured discharge calculations. These points were then used to calibrate WinXSPRO results for the same reach and provided a confidence level for computer-generated results.Flow calculations suggest that under near bank full conditions, the upper Raikot drainage basin produces discharges of 61 cm and moves about 11,000 tons day⁻¹ (9980 tons day⁻¹) of sediment through its channel. Bank full conditions on the upper portion of the Rupal drainage basin generate discharges of 84 cm and moves only about 3800 tons day⁻¹ (3450 tons day⁻¹) of sediment. Although the upper Rupal drainage basin moves more water, the lower slope of the drainage basin (0.03) generates a much smaller shear stress (461 Pa) than does the higher slope (0.12) of the upper Raikot drainage basin (1925 Pa).Dissolved and suspended sediment loads were measured from water/sediment samples collected throughout the day and night over a period of 10 days at the height of the summer melt season but proved to be a minor variable in transport flux. Channel bed loads were measured using a pebble count method of bank material and then used to generate ratings curves of bed loads relative to discharge volumes. When coupled with discharge data and basin area, mean annual sediment yield and denudation rates for Nanga Parbat are produced. Denudation rates calculated in this fashion range from 0.2 mm year⁻¹ in the slower, more sluggish Rupal drainage basin to almost 6 mm year⁻¹ in the steeper, faster flowing Raikot and Buldar drainage basins.     

基于TETIS模型的黑土区乌裕尔河流域径流与侵蚀产沙模拟研究

为科学地认识中国东北黑土区流域土壤侵蚀特征,探讨TETIS模型在该区的适用性,本文以乌裕尔河流域为例,利用1971-1987年日径流与泥沙实测数据对TETIS模型进行了校正与验证,进而分析了流域土壤侵蚀强度特征及其与坡度、土地利用方式的关系。研究结果表明：TETIS模型在乌裕尔河流域适用性好,日径流与日输沙量的纳什效率系数在0.52~0.70之间,决定系数在0.60~0.71之间,体积误差均不超过15%。流域平均侵蚀模数为397.2 t/(km²·a),流域以微度和轻度侵蚀为主,约90%的产沙来自于坡面。平均土壤侵蚀模数随坡度的增大而增大,流域侵蚀量主要来自于0°~5°坡面。不同土地利用方式具不同的土壤侵蚀模数,耕地土壤侵蚀模数最大,达556.3 t/(km²·a)。坡度较大的耕地和植被覆盖度较低的区域是水土流失治理的重点。研究表明,TETIS模型在黑土区模拟土壤侵蚀产沙应用前景好,可为研究区制定水土保持措施提供科学依据。     

Source‐to‐sink analysis of ancient sedimentary systems using a subsurface case study from the Møre‐Trøndelag area of southern Norway: Part 1 – depositional setting and fan evolution

In this study, we use seismic reflection, well and core data to investigate the role that basin physiography and sediment routing systems played on the distribution, geometry and stratigraphic architecture of Upper Cretaceous submarine fans (SF) offshore Norway. The Late Cretaceous Møre‐Trøndelag margin of western Norway was characterised by steep submarine slopes (gradient of ~0.3°–3°). Mudstones dominate the Upper Cretaceous slope succession, although a few regionally extensive, sandstone‐dominated units are developed. We focus on the most regionally extensive sandstone unit, which is of Late Turonian‐to‐Early Coniacian age. Mapping and visualisation of 2D and 3D seismic reflection data and analysis of well data indicates that the sandstone unit comprises a total of 11 SF, which were fed by sand‐rich sediment gravity flows routed through multiple upper slope canyons. Based on the internal organisation of seismic facies, four fan types have been identified: (i) Type Ia fans, which are characterised by <10 erosional channel complexes at their bases and aggradational to landward‐stepping lobes in their upper parts; (ii) Type Ib fans, which are characterised by >10 erosional channel complexes at their bases and aggradational to landward‐stepping lobe and mass‐transport deposits near the fan apex in their upper parts; (iii) Type II fans, which are dominated by aggradational lobe deposits; and (iv) Type III fans, which are dominated by a single channel complex that passes downdip into a small terminal lobe. The different fan types are interpreted to reflect variable stratigraphic responses to source proximity and basin physiography, which is principally related to the degree of local fault reactivation and differential compaction. This variability highlights the diversity of fan types that may occur over short distances along continental margins, and demonstrates the importance of local controls in understanding the internal stratigraphic variability that may be present in deep‐marine successions.     

Active foreland deformation evidenced by shallow folds and faults affecting late Quaternary shelf–slope deposits (Adriatic Sea, Italy)

A succession of depositional sequences, recording middle-late Pleistocene and Holocene glacial–interglacial cycles, documents the impact of short-term tectonic deformation on the western Adriatic margin. The western Adriatic margin is part of the Apennine foreland which was intensely, though variably, deformed during the Meso-Cenozoic evolution of the Adriatic region from a passive margin to a foreland basin. The study area extends offshore Gargano Promontory, an uplifted sector of the Adriatic foreland, and includes three major deformation belts located along or cross-strike to the margin: (1) the NW-SE Gallignani-Pelagosa ridge, (2) the WSW-ENE Tremiti-Pianosa high (both located north of Gargano) and (3) the W-E to NW-SE Gondola fault deformation belt (in the south Adriatic). Long-term deformation along these tectonic lineaments is documented on conventional low-frequency seismic profiles by regional folds and faults affecting Eocene–Miocene units overlain by dominantly draping Plio-Quaternary deposits. At this scale of observation, only north of Gargano Promontory there is some evidence of Plio-Quaternary units thinning against structural highs, thus suggesting that tectonic deformation was protracted through this interval. Based on new high-resolution seismic data, we show that deformation along these pre-existing tectonic structures continued during the Quaternary, affecting middle-late Pleistocene and even Holocene units on the shelf and upper slope north and south of Gargano Promontory. These recent deformations consist of gentle folds and high-angle faults, locally producing topographic relief that affects the stratigraphy and thickness of syn-tectonic deposits. We interpret the small-scale, shallow faults and gentle folds affecting middle-late Pleistocene and Holocene deposits, north and south Gargano Promontory, as the evidence of ongoing foreland deformation along inherited regional fold and fault systems.     

Holocene mass‐wasting events in Lago Fagnano,Tierra del Fuego (54°S): implications for paleoseismicity of the Magallanes‐Fagnano transform fault

High‐resolution seismic imaging and coring in Lago Fagnano, located along a plate boundary in Tierra del Fuego, have revealed a dated sequence of Holocene mass‐wasting events. These structures are interpreted as sediment mobilizations resulting from loading of the slope‐adjacent lake floor during mass‐flow deposition. More than 19 mass‐flow deposits have been identified, combining results from 800 km of gridded seismic profiles used to site sediment cores. Successions of up to 6‐m thick mass‐flow deposits, pond atop the basin floor and spread eastward and westward following the main axis of the eastern sub‐basin of Lago Fagnano. We developed an age model, on the basis of information from previous studies and from new AMS‐¹⁴C ages on cored sediments, which allows us to establish a well‐constrained chronologic mass‐wasting event‐catalogue covering the last ～12 000 years. Simultaneously triggered, basin‐wide lateral slope failure and the formation of multiple debris flow and postulated megaturbidite deposits are interpreted as the fingerprint of paleo‐seismic activity along the Magallanes‐Fagnano transform fault that runs along the entire lake basin. The slope failures and megaturbidites are interpreted as recording large earthquakes occurring along the transform fault since the early Holocene. The results from this study provide new data about the frequency and possible magnitude of Holocene earthquakes in Tierra del Fuego, which can be applied in the context of seismic hazard assessment in southernmost Patagonia.     

The quantifiable clinothem – types,shapes and geometric relationships in the Plio‐Pleistocene Giant Foresets Formation,Taranaki Basin,New Zealand

The understanding of how clinoforms develop is approached based on shape and dimensions, correlation between geometric parameters, and internal characteristics of clinothems bounded by clinoform surfaces in high‐resolution 2D seismic data from the Giant Foresets Formation, Taranaki Basin, offshore New Zealand. The study subdivides the observed clinothems to identify nine types: 1. Oblique 2. Tangential oblique 3. Tangential oblique chaotic 4. Sigmoidal symmetrical 5. Sigmoidal divergent 6. Sigmoidal chaotic 7. Asymmetrical top‐heavy 8. Asymmetrical bottom‐heavy 9. Complex. Accommodation is a dominant control on the type of clinothem that develops, whereby limited accommodation promotes clinothems with significant shelf‐edge advance and low trajectory angles, while increasing accommodation promotes higher trajectory angles and increased deposition on the shelf. Further variations in shape, slope and deposition are influenced by many factors of which sediment influx appears be a fundamental driver. Sigmoidal clinothems tend to show a strong relationship between their maximum thickness and average thickness, their overall slope and maximum foreset angle, along with a high correlation between average thickness and toe advance. This suggests that they distribute sediment in a manner that may be possible to predict and quantify. The increasing steepness of the foreset slope from bottom‐heavy to symmetrical to top‐heavy clinothems, respectively, is dominantly the result of decreasing sediment influx. The clinothems with the steepest slopes, along with chaotic clinothems, are associated with comparatively large toe advance suggesting a strong link between over‐steepened slopes and/or collapse, and processes promoting sediment deposition along the basin floor. Apart from toe advance, the two types of chaotic clinothems develop differently from each other, and from their assumed parent‐clinothem. Tangential oblique chaotic forms steepen, and shelf‐edge advance is limited, suggesting upper slope collapse. Sigmoidal chaotic clinothems have comparatively higher shelf‐edge advance, lower shelf‐edge trajectories and gentler slopes and profiles, suggesting different processes are responsible for their development and resulting shape.     

Shelf-margin clinoforms and prediction of deepwater sands

Early Eocene successions from Spitsbergen and offshore Ireland, showing well‐developed shelf‐margin clinoforms and a variety of deepwater sands, are used to develop models to predict the presence or absence of turbidite sands in clinoform strata without significant slope disturbance/ponding by salt or mud diapers. The studied clinoforms formed in front of narrow to moderate width (10–60 km) shelves and have slopes, 2–4°, that are typical of accreting shelf margins. The clinoforms are evaluated in terms of both shelf‐transiting sediment‐delivery systems and the resultant partitioning of the sand and mud budget along their different segments. Although this sediment‐budget partitioning is controlled by sediment type and flux, shelf width and gradient, process regime on the shelf and relative sea‐level behaviour, the most tell‐tale or predictive signs in the stratigraphic record appear to be (1) sediment‐delivery system type, (2) degree of shelf‐edge channelling and (3) character of shelf‐edge trajectory through time. The clinoform data sets from the Porcupine Basin (wells and 3‐D seismic) and from the Central Basin on Spitsbergen (outcrops) suggest that river‐dominated deltas are the most efficient delivery systems for dispersing sand into deep water beyond the shelf‐slope break. In addition, low‐angle or flat, channelled shelf‐edge trajectories associate with co‐eval deepwater slope and basin‐floor sands, whereas rising trajectories tend to associate with muddy slopes and basin floors. Characteristic features of the shelf‐edge, slope and basin‐floor segments of clinoforms for these trajectory types are documented. Seismic lines along the slope to basin‐floor transects tend to show apparent up‐dip sandstone pinchouts, but most of these are likely to be simply sidelap features. Dip lines aligned along the axes of sandy fairways show that stratigraphic traps are unlikely, unless slope channels become mud‐filled or are structurally partitioned. Another feature that is prominent in the data sets examined is the lack of slope onlap. During the relative rise of sea level back up to the shelf, the clinoform slopes are generally mud‐prone and they are characteristically aggradational.     

Stratigraphic evolution of the upper slope and shelf edge in the Karoo Basin, South Africa

The Permian Ecca Group of the Karoo Basin, South Africa preserves an extensive well-exposed siliciclastic basin floor, slope and shelf-edge delta succession. The Kookfontein Formation includes multiple sedimentary cycles that display clinoform geometries and are interpreted to represent the deposits of a slope to shelf succession. The succession exhibits progradational followed by aggradational stacking of deltaic cycles that is related to a change in shelf-edge trajectory, and lies within two depositional sequences. Sediment was transferred to the slope via overextension of deltas onto and over the shelf edge, resulting in failure and re-adjustment of local slope gradients. The depositional facies and architecture of the Kookfontein Formation record the change from a bypass- to accretion-dominated margin, which is interpreted to reflect a decrease in sediment transport efficiency as the slope gradient decreased, slope length increased and shelf-edge trajectory rose. During this time the delivery system changed from point-sourced basin-floor fans fed by slope channels to starved basin-floor with sand-rich slope clinoforms. This is an example of a progradational margin in which the younger slope system is interpreted to be of a different style to the older slope system that fed the underlying sand-rich basin floor fans.     

黄土地区梁峁坡的坡地特征与土壤侵蚀

本文指出黄土地区梁峁坡的坡角主要分布在14°—28°之间,特征坡角的上限相当于黄土的内摩擦角,下限相当于内摩擦角的0.45倍。黄土地区的坡地形态可分为直形坡、凸形坡、凹形坡、复合形坡和阶梯形坡等五种类型。土壤侵蚀主要受坡地形态和坡度的控制,侵蚀量随着坡度的增大而增加,坡度大于15°时,侵蚀量突增,坡面冲刷加剧,26°达极大值,此后冲刷作用减弱,重力侵蚀逐渐显著,至45°侵蚀达到最大值,以后又趋减弱。     

The nature and evolution of deep-sea channel systems

Abstract A distinction is drawn between sea-floor canyons, which are incised into bedrock, and fan valleys and deep-sea channels, which are cut in unconsolidated sediment. The formation of continental margin canyons/fans and deep-sea channels is an inevitable consequence of continental margin rifting and sea-floor subsidence. Such submarine sediment transport systems are amongst the longest-lived physiographic features on earth, with the Bounty Channel system being more than 50 Myr old. Many deep-sea channels form the distal part of ocean-margin sediment transport systems, being incised 100–350 m into ocean-floor sediments, traversing great distances over the ocean-basin floor, and generally terminating on an abyssal plain. The course of each deep-sea channel is, however, unique. Channel locations are controlled primarily by inherited basement relief, and, during their evolution, by rates and patterns of lithospheric subsidence and sedimentation. In the early stages of ocean-basin formation, deep-sea channels may issue from the axial parts of marginal rifts, or directly from slope canyon-fan systems. As an ocean basin widens, margin-connected channels may become trapped within the strip of oldest (and therefore deepest) oceanic crust at the continent/ocean interface, and will therefore be margin-parallel features. In some cases, as for the Cascadia Channel, channels may escape from the ocean-margin deep, bypassing the spreading ridge via a fracture zone. Deep-sea channels and their associated sediments are influenced also by global sea-level change, by rate of turbidity current generation from the headward continental margin, by rates of pelagic sediment supply, by differential levee development consequent upon the Coriolis effect, and by the operation of deep-sea current systems with their associated sediment drifts. The survival of deep-sea channels as long-lived features necessitates that rates of long-term subsidence at the channel terminus exceed sediment accumulation.     

黄河中游流域地貌形态对流域产沙量的影响

在黄河中游地区 ,选择了 5 0多个面积约 5 0 0～ 2 5 0 0平方公里的水文测站流域 ,分别代表 6种不同自然地理类型 ,在流域沟壑密度、沟间地坡度小于 15°面积百分比等地貌形态指标量计的基础上 ,进行了流域产沙量与地貌形态指标相关分析。结果表明 ,对于不同类型流域 ,流域产沙量随流域地貌的变化遵循不同的响应规律 ,而且视流域其它下垫面环境条件的均一程度 ,其相关程度和响应速率各不相同。受地面物质、植被、地貌发育阶段等流域其它下垫面环境条件的制约 ,除沟壑密度外 ,流域产沙量与流域地貌形态的关系都没有人们以前所预期的好。