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.     

Disturbance, stream incision, and channel evolution: The roles of excess transport capacity and boundary materials in controlling channel response

Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non-incised surfaces. Large, anthropogenic disturbances, similar to large or catastrophic “natural” events, greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to antrhopogenic and “natural” disturbances are presented for fluvial systems in the mid continent and Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically-controlled sediment (sand) provided by bank erosion.The non-linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that effect available force, stream power or flow energy, or change erosional resistance such that an excess of flow energy occurs can result in incision. Channel incision, therefore, can be considered a quintessential feature of dis-equilibrated fluvial systems.     

The influence of lee sediment behind large bed elements on bedload transport rates in supply-limited channels

A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic boulder-sized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements.     

What feeds shelf-edge clinoforms over margins deprived of adjacent land sources? An example from southeastern Brazil

In southeastern Brazil, the Serra do Mar coastal mountain range blocks the sediment influx from arriving at a ca. 1,500 km long continental margin comprising Santos and Pelotas basins. Despite this deprivation, the margin accumulated a ca. 1 km thick sedimentary succession since the Mid-Miocene. Examination of seismic reflection and oceanographic data indicates that shelf-margin clinoform formation exhibits a regional variability, with major sigmoidal clinoforms developed in the transitional area between both basins. Laterally, poorly developed oblique clinoforms constitute isolated depocenters along the shelf margin. The continuous clinoform development in the transitional area is attributed to the major influence on sediment transport patterns of several ocean bottom currents flowing along the margin, such as the Brazil Coastal Current, the Brazil Current and the Intermediate Water Brazil Current. These currents erode, transport and distribute sediments across the shelf break and upper slope from distant sediment sources located either north or south of the study area. The progressive southward strengthening of the Brazil Current could be responsible for a major southward sediment redistribution from the northern Campos Basin, and/or for sediment entrainment from northward-induced transport by the Brazil Coastal Current, originally derived from the De la Plata Estuary. In the transition between Santos and Pelotas basins, the Intermediate Water Brazil Current splits forming the Santos Bifurcation, allowing for a continuous depositional process and clinoform generation. We suggest that ocean bottom currents may shape other shelf-edge ‘contouritic clinoforms’ in continental margins mainly constructed by along-strike sediment transport largely driven by long-term geostrophic currents.     

Linking subaerial erosion with submarine geomorphology in the western Ionian Sea (south of the Messina Strait), Italy

Sediment supplied by continental sources is commonly suspected to have exerted a strong influence on the development of canyons and other morphological features on the continental slopes, but rarely is the sediment supply known sufficiently quantitatively to test this link. Here, we outline an area where offshore morphology, in the western Ionian Sea, may be linked to estimated sediment fluxes produced by subaerial erosion in NE Sicily and SW Calabria. Shelves in this area are narrow (<1 km), and the bathymetry shows that rivers and adjacent submarine channels are almost directly connected with each other. Integrated topographic analyses were performed on a merged digital elevation model (DEM) of ASTER data for subaerial topography and multibeam sonar data for submarine bathymetry. Spatial variations in sediment fluxes from onshore erosion were assessed using a variety of methods, namely: long‐term sediment flux from Pleistocene uplift rates, decadal sediment flux from landslide occurrences and published long‐term exhumation rates from ¹⁰Be cosmogenic nuclide concentrations. Submarine channels associated with rivers delivering larger sediment fluxes have broad channels, high relief and smooth concave‐upward longitudinal profiles. Conversely, submarine channels that lie offshore small‐flux rivers have straight longitudinal profiles, low relief and steep gradients. Where river catchments supply a greater sediment flux offshore, shelves tend to be wider (ca. 400 m) and submarine channels have gentler gradients. In contrast, where catchments supply less sediment flux, shelves are narrow (250–300 m) and offshore channel gradients are steeper. The variation of submarine morphology with tectonic uplift rate was also studied, but we find that, unlike onshore terrains where tectonics is commonly an important factor influencing channel morphology, in the submarine landscapes, sediment flux appears to dominate here.     

Source‐to‐sink analysis of ancient sedimentary systems using a subsurface case study from the Møre‐Trøndelag area of southern Norway: Part 2 – sediment dispersal and forcing mechanisms

The composition, volume and stratigraphic organisation of submarine fan systems deposited along continental margins are expected to reflect the landscape from which the sediment was derived. During the Late Cretaceous, the Møre‐Trøndelag margin, Norwegian North Sea was dominated by the deposition of deep‐marine fines; the emplacement of 11 sand‐rich submarine fan systems occurred only during a c. 3 Myr period in the Turonian‐Coniacian. The systems were fed by sediment that was routed through submarine canyons incised into the basin margin; the canyons are underlain by angular unconformities and are interpreted to have resulted from tectonically induced changes in slope physiography and erosion by gravity flows. The areal extent of the onshore drainage catchments that supplied sediment to the fans has been estimated based on scaling relationships derived from modern source‐to‐sink systems. The results of our study suggest that the Turonian fans were sourced by drainage catchments that were up to ca.3600 km², extending more than ca.100 km inland from the palaeo‐shoreline. The estimated inboard catchment extent correlates with the innermost structures of a large, long‐lived, basement‐involved, normal fault complex. On the basis of our analysis, we conclude that increased sediment supply to the Turonian fan systems reflects tectonic rejuvenation of the landscape, rather than eustatic sea‐level or climate fluctuations. The duration of fan deposition is thus interpreted to reflect the ‘relaxation time’ of the landscape following tectonic perturbation, and fan system retrogradation and abandonment is interpreted to reflect the eventual depletion of the onshore sediment source. We demonstrate that a better understanding of the stratigraphic variability in deepwater depositional systems can be gained by taking a complete source‐to‐sink view of ancient sediment dispersal systems.     

Late Quaternary turbidity current pathways to the Madeira Abyssal Plain and some constraints on turbidity current mechanisms

Turbidites deposited in the Madeira Abyssal Plain during the last 200 000 yr originated mainly from the flanks of the Canary Islands and from the Northwest African continental margin north of the Canaries. Derivation of these turbidites from sources to the east of the abyssal plain apparently contradicts flow direction indicators derived from the sediments on the plain, which indicate derivation from the north-east. However, two systems of shallow channels, mapped using side-scan sonar and 3.5-kHz data, link the easterly sediment sources to the north-eastern edge of the abyssal plain, reconciling the apparently contradictory flow direction data. A northern system originates in the area between Madeira and the Canary Islands and follows a westerly and then north-westerly path, in part cutting obliquely across regional bathymetric trends. It carries sediment from the African continental margin north of the Canary Islands, and from the eastern Canaries, to the north-eastern abyssal plain. The southern channel system carries material from the western Canary Islands more directly westward to the central part of the plain. The pathways of individual turbidites can be reconstructed in some detail, by combining channel mapping with published information on turbidite provenance and flow directions on the Madeira Abyssal Plain. Interaction between turbidity currents and channel morphology controls turbidite depositional patterns. Small turbidites are completely contained within channels 20 m deep and 2 km wide. It is proposed that these are relatively high-density flows which have evolved in crossing the almost flat floor of a basin south-east of Madeira before entering the channel system. Larger turbidites show evidence of flow stripping where they interact with channels, with the result that their coarse and fine fractions follow different paths to and across the abyssal plain.     

Sequence architecture and depositional evolution of the northern continental slope of the South China Sea: responses to tectonic processes and changes in sea level

The continental slopes of the South China Sea (SCS), the largest marginal sea on the continental shelf of Southeast Asia, are among the most significant shelf‐margin basins in the world because of their abundant petroleum resources and a developmental history related to sea floor spreading since Late Oligocene time. Based on integrated analyses of seismic, well‐logging and core data, we systematically document the sequence architecture and depositional evolution of the northern continental slope of the SCS and reveal its responses to tectonism, sea‐level change and sediment supply. The infill of this shelf‐margin basin can be divided into seven composite sequences (CS1–CS7) that are bounded by regional unconformities. Composite sequences CS3 to CS7 have formed since Late Oligocene time, and each of them generally reflects a regional transgressive–regressive cycle. These large cycles can be further divided into 20 sequences that are defined by local unconformities or transgressive–regressive boundaries. Depositional–geomorphological systems represented on the continental slope mainly include shelf‐edge deltas, prodelta‐slope fans, clinoforms of the shelf‐margin slope, unidirectionally migrating slope channels, incised slope valleys, muddy slope fans, slope slump‐debris‐flow complexes and large‐scale soft‐sediment deformation of bedding. Changing sea levels, reflected by evidence from sequence architecture in the study area, are generally comparable with those of the Haq (1987) global sea level curve, whereas the regional transgressions and regressions were apparently controlled by tectonic uplift and subsidence. Composite sequences CS3 and CS4 formed from Late Oligocene to Middle Miocene time and represent continental‐slope deposition during a time of northwest‐northeast seafloor spreading and subsequent development of sub‐basins in the southwest‐central SCS. The development of composite sequences CS5 to CS7 after Middle Miocene time was obviously influenced by the Dongsha Movement during convergence between the SCS and Philippine Sea plates. Climatic variations and monsoon intensification may have enhanced sediment supply during Late Oligocene‒Early Miocene (25–21 Ma) and Late Pliocene‒Pleistocene (3–0.8 Ma) times. This study indicates that shelf‐edge delta and associated slope fan systems are the most important oil/gas‐bearing reservoirs in the SCS continental‐slope area.     

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.     

VARIABILITY OF IN-CHANNEL SEDIMENT STORAGE WITHIN A SMALL SEMIARID DRAINAGE BASIN

This study determines the spatial and temporal variability of in-channel storage within a small semiarid drainage basin in equatorial East Africa, and establishes a tentative sediment budget for coarse (>200 μm) in-channel sediments. Detailed measurements of in-channel sediment storage (mass) within third and fourth-order ephemeral channels were obtained using channel-pit excavations and probing with metal rods. Eighty-seven monumented cross-sections were established in February 1986 and resurveyed in December 1986, following the last runoff event of the year. These provided data on change in sediment storage on a 30-m channel reach basis. In addition, measurements of bankfull channel width, mean depth, cross-sectional area, wetted perimeter, hydraulic radius, channel slope and distance from the basin outlet were measured at each cross section. Total in-channel sediment storage was approximately 8640 t with 83% of this total stored within the Main (fourth-order) Channel. Stepwise multiple regression of In-transformed data indicated that bankfull channel width and distance from the outlet (which is strongly related with slope) were significantly related to in-channel storage. The variation in the ratio of stream power:critical power along the Main Channel may explain the distribution of in-channel sediments. Net aggradation of 50 to 60 t during 1986 was minor in relation to the total in-channel storage reservoir, but indicates that a static equilibrium condition cannot be assumed. Bedload output during 1986 was approximately 125 t, and the computed input of coarse sediments to the major channels within the basin was approximately 185 t. The sediment delivery ratio for the coarse material was approximately 68%, which indicates a relatively efficient transport system. [Key words: geomorphology, sediment budget, in-channel sediment storage, semiarid, drainage basin.]     

The Western Irish Namurian Basin reassessed

ABSTRACT Current basin models for the Western Irish Namurian Basin (WINB) envisage an elongate trough along the line of the present‐day Shannon Estuary that was infilled with clastic sediments derived from a hinterland that lay to the W or NW. This paper argues for an alternative basin configuration with source areas to the SW supplying sediment to a basin where deepest water conditions were in northern County Clare. Rapid subsidence along the present‐day Shannon Estuary ponded sediment in this area throughout the early Namurian and, only with the rapid increase of sedimentation rates within the mid‐Namurian (Kinderscoutian Stage), were substantial amounts of sediment able to prograde to the NE of the basin. This alternative model better explains the overwhelming predominance of NE‐directed palaeocurrents in the Namurian infill, but requires fundamental revisions to most aspects of current depositional models. Deep‐water black shales (Clare Shale Formation) initially accumulated throughout the region and were progressively downlapped by an unconfined turbidite system (Ross Formation) prograding to the NE. This in turn was succeeded by an unstable, siltstone‐dominated slope system (Gull Island Formation) characterized by large‐scale soft‐sediment deformation, which also prograded to the NE. In the northern‐most basin outcrops, in northern County Clare, this early phase of basin infill was developed as a condensed succession of radiolarian‐rich black shales, minor turbiditic sandstones and undisturbed siltstones. The new basin model envisages the northern exposures of County Clare to be a distal, basin floor succession whereas the traditional model considers it a relatively shallow, winnowed, basin margin succession. Later stages of basin infill consist of a series of deltaic cycles that culminate in major, erosive‐based sandstone bodies (e.g. Tullig Sandstone) interpreted either as axial, deltaic feeder channels or incised valley fills genetically unrelated to the underlying deltaic facies. Within the context of the new basin model the former alternative is most likely and estimated channel depths within the Tullig Sandstone indicate that the basal erosive surface could have been generated by intrinsic fluvial scour without recourse to base‐level fall. The northerly flowing Tullig channels pass down‐dip into isolated channel sandbodies interbedded with wave‐dominated strata that suggest the deltas of the WINB were considerably more wave‐influenced than hitherto proposed. The retreat of the Tullig delta during sea‐level rise saw the rapid southerly retrogradation of parasequences, as may be expected if the basin margin lay to the SW of the present‐day outcrops.     

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.     

The influence of geological fabric and scale on drainage pattern analysis in a catchment of metamorphic terrain: Laceys Creek, southeast Queensland, Australia

The relationship between geological fabric and drainage patterns in the 81.8 km² Laceys Creek sub-catchment of the North Pine River catchment, southeast Queensland, Australia, is analysed using a new channel–ordination system. The Laceys Creek catchment is situated on the South D'Aguilar Block, which underwent metamorphism, faulting and uplift from the Late Carboniferous to Late Triassic. The catchment drains exposures of two main rock units, the Neranleigh–Fernvale Beds and the Bunya Phyllite. Both units are composed of metamorphosed deep-sea sediments that accumulated as an accretionary wedge during late Palaeozoic subduction of the palaeo-Pacific plate under the eastern margin of the Australian craton. The new channel ordination system used in this study allows improved classification of stream segments of equal prominence or rank in comparison to previous schemes. A 10 m contour digital elevation model (DEM) was produced within which drainage channels were digitised. Planar geological features, including bedding, faults, joints and cleavage, were mapped in the field and collated with data from previous geological mapping programs.Regional and local trends of geological fabric are reflected in the variable orientation of channels of different rank in the catchment. Cleavage and fractures are the dominant planar features of the Bunya Phyllite and these correlate most closely with the orientation of middle-order incised stream segments. In contrast, middle-order channels on the Neranleigh–Fernvale Beds most closely correlate with bedding, which dominates the fabric of this unit. Although anthropogenic factors exert local influence and climatic processes exert broad influence on the catchment, this study focuses on structural and lithological fabrics, which are the apparent dominant controls on middle-order channel orientations. Identification of congruent patterns between bedrock fabric and channel ranks is variable, depending on the scale and number of channels included in the analysis. Many low-rank channels correspond closely to the orientation of fine-scale bedding and foliation and these influences may not be detected by coarse-scale mapping. Understanding the extent of geological controls on the morphology of a catchment may assist geo-hazard identification, land-use planning and civil-engineering projects.     

Timing of depth‐dependent lithosphere stretching on the S. Lofoten rifted margin offshore mid‐Norway: pre‐breakup or post‐breakup?

Depth‐dependent stretching, in which whole‐crustal and whole‐lithosphere extension is significantly greater than upper‐crustal extension, has been observed at both non‐volcanic and volcanic rifted continental margins. A key question is whether depth‐dependent stretching occurs during pre‐breakup rifting or during sea‐floor spreading initiation and early sea‐floor spreading. Analysis of post‐breakup thermal subsidence and upper‐crustal faulting show that depth‐dependent lithosphere stretching occurs on the outer part of the Norwegian volcanic rifted margin. For the southern Lofoten margin, large breakup lithosphere β stretching factors approaching infinity are required within 100 km of the continent–ocean boundary to restore Lower Eocene sediments and flood basalt surfaces (～54 Ma) to interpreted sub‐aerial depositional environments at sea level as indicated by well data. For the same region, the upper crust shows no significant Palaeocene and Late Cretaceous faulting preceding breakup with upper‐crustal β stretching factors <1.05. Further north on the Lofoten margin, reverse modelling of post‐breakup subsidence with a β stretching factor of infinity predicts palaeo‐bathymetries of ～1500 m to the west of the Utrøst Ridge and fails to restore Lower Eocene sediments and flood basalt tops to sea level at ～54 Ma. If these horizons were deposited in a sub‐aerial depositional environment, as indicated by well data to the south, an additional subsidence event younger than 54 Ma is required compatible with lower‐crustal thinning during sea‐floor spreading initiation. For the northern Vøring margin, breakup lithosphere β stretching factors of ～2.5 are required to restore Lower Eocene sediments and basalts to sea level at deposition, while Palaeocene and Late Cretaceous upper‐crustal β stretching factors for the same region are < 1.1. The absence of significant Palaeocene and late Cretaceous extension on the southern Lofoten and northern Vøring margins prior to continental breakup supports the hypothesis that depth‐dependent stretching of rifted margin lithosphere occurs during sea‐floor spreading initiation or early sea‐floor spreading rather than during pre‐breakup rifting.     

Relationships Between Flow Hydraulics, Sediment Supply, Bedload Transport and Channel Stability in the Proglacial Virkisa River, Iceland

We present data from a proglacial river in Iceland that exhibits very different sedimentological characteristics when compared to its alpine counterparts. The braidplain is characterised by coarse outburst gravels that inhibit sediment transport and channel change. Bedload transport is restricted to the movement of fine-grained gravels that pass through the channel system without promoting significant changes in channel geometry. Bar forms are erosional features, inherited from the last major peak flow, rather than depositional in nature. On the basis of our observations we conclude that braidplain morphology is controlled by low frequency, high magnitude flow events, possibly associated with glacial outburst floods. This is in marked contrast to process-form relationships in more dynamic alpine proglacial channels that are characterised by high rates of sediment transport and channel change.     

Thesis: Ideology and Power Relations in Cultural Landscape Evaluations

Factors of importance to the generation of complex sediment rating curves involving hysteresis effects are discussed. Temporary storage of sediments in channels, and interaction between fluvial processes and some other sediment producing process, may be recognized as two fundamentally different situations associated with the phenomenon. An examination of sediment transport in glacierized river basins shows that variable discharge in broad shallow channels may cause temporary storage of sediments within the channel system. Implications for the sediment budget in river systems are discussed.     

The kinematics and pattern of escarpment retreat across the rifted continental margin of SE Australia

Rifted continental margins generally display an interior, low-relief, highly weathered upland area and a deeply incised, high-relief coastal area. The boundary between the two zones is commonly demarcated by an abrupt, seaward-facing escarpment. We investigate the rate and pattern of escarpment erosion and landscape evolution along the passive margin of south-east Australia, in the region of the New England Tableland. The process of rifting is shown to initiate an escarpment across which rivers flow, resulting in an escarpment that takes the form of dramatic, elongated gorges. Using a mass balance approach, we estimate the volume/unit length of continental material eroded seaward of the escarpment to be between 41 and 68 km², approximately an order of magnitude less than the 339 km² of terrigenous sediments calculated to have been deposited offshore, but consistent with earlier denudation estimates based on apatite fission track data. On the bedrock rivers draining the New England Tableland region, the escarpment is manifested as a series of sharp knickpoints punctuating the river longitudinal profiles. The knickpoints are situated the same distance upstream along the different channels and uniform escarpment retreat rates on the order of 2 km Myr⁻¹ are estimated, despite some differences in bedrock lithologies. Gorge head migration appears to be very important as a bedrock incision mechanism. Field observations indicate a coupling between escarpment retreat and knickpoint propagation, bedrock channel incision, and hillslope development.     

Relationships between shelf‐edge trajectories and sediment dispersal along depositional dip and strike: a different approach to sequence stratigraphy

Analysis of shelf‐edge trajectories in prograding successions from offshore Norway, Brazil, Venezuela and West Africa reveals systematic changes in facies associations along the depositional dip. These changes occur in conjunction with the relative sea‐level change, sediment supply, inclination of the substratum and the relief of the margin. Flat and ascending trajectories generally result in an accumulation of fluvial and shallow marine sediments in the topset segment. Descending trajectories will generally result in erosion and bypass of the topset segment and deposition of basin floor fans. An investigation of incised valley fills reveals multiple stages of filling that can be linked to distinct phases of deepwater fan deposition and to the overall evolution of the margin. In the case of high sediment supply, like the Neogene Niger and Orinoco deltas, basin floor fans may develop systematically even under ascending trajectory styles. In traditional sequence stratigraphic thinking, this would imply the deposition of basin floor fans during a period of relative sea‐level highstand. Facies associations and sequence development also vary along the depositional strike. The width and gradient of the shelf and slope show considerable variations from south to north along the Brazilian continental margin during the Cenozoic. During the same time interval, the continental shelf may display high or low accommodation conditions, and the resulting stacking patterns and facies associations may be utilized to reconstruct palaeogeography and for prediction of lithology. Application of the trajectory concept thus reveals nuances in the rock record that would be lost by the application of traditional sequence stratigraphic work procedures. At the same time, the methodology simplifies the interpretation in that less importance is placed on interpretation and labelling of surface boundaries and systems tracts.     

Stream Channel Enlargement Response to Urban Land Cover in Small Coastal Plain Watersheds,North Carolina

Stream channel response to urban land use has not been well documented for southeastern Coastal Plain streams. In this study, urban channel response was evaluated in small Inner Coastal Plain watersheds (<5 km²) in eastern North Carolina. Reaches were selected across a range of watershed total impervious area (0-67% TIA). Channel dimensions and sediment grain size data were collected along 20 urban (>10% TIA) and 20 rural reaches (<10% TIA), and at 10 stormwater outfall sites (180 cross-sections). Urban cross-sectional area, channel incision ratio, and channel grain size (gravel%, D₅₀, and D₈₄) were greater, relative to rural channels. Bankfull cross-sectional areas were approximately 1.78 times greater for urban watersheds than for rural watersheds. Channels in urban watersheds were incised and had median full-channel capacities approximately 3.4 times greater than channels draining rural watersheds. Watershed TIA explained 65-72% of channel capacity enlargement. Urban expansion in the region began in the 1960s, with major urbanization occurring over the last 25 years. Channels draining urban watersheds are still responding to this land use change by downcutting and widening. Urban channel incision has frequently cut off streams from their floodplains, reducing floodplain sediment retention and water quality functions.     

Along‐strike variability in shelf‐margin morphology and accretion pattern: An example from the northern margin of the South China Sea

Interplays among diachronous tectonism, uneven sediment supply, and local marine hydraulic processes make the northern margin of the South China Sea (SCS) an ideal location to investigate the complexity of along‐strike variability in shelf margins. This study examines shelf‐margin morphology, stratigraphy, and sedimentation from the northern SCS using multichannel seismic reflection profiles complemented with the data from commercial and ocean drilling sites. Analysis of seismic reflection profiles reveals three categories of shelf‐margin cross‐sectional profiles, the concave‐up, linear, and sigmoidal, according to which five margin sectors were recognized. Results show that these margin segments differ in relief, shelf‐edge trajectory, submarine canyon development, and long‐term accretion pattern. The westernmost margin sector, or the Yinggehai (YGH)‐western Qiongdongnan (QDN) margin, has appeared to be supply dominated since its commencement at ca. 10.5 Ma, which is characterized by well‐developed prograding clinoforms, low‐angle shelf‐edge trajectories, and an absence of canyons. Presence of concave‐up profiles is also suggestive of high sediment influx. In contrast, the eastern QDN margin was primarily regulated by local subsidence and faulting, leading to a stationary shelf‐edge migrating pattern and linear upper‐slope morphology. Densely distributed slope‐confined gullies indicate the margin’s disequilibrium and erosive nature. Further east, the Pearl River Mouth (PRM) margin formed much earlier (ca. 30 Ma) and experienced a more complicated accretion history, including three phases which were dominated by sequential marginal faulting (before ca. 30 Ma), basement structure (ca. 30–23 Ma), and sediment supply (ca. 23 Ma to the present). The overall sigmoidal morphology and truncated stratigraphy of this margin probably resulted from the sculpting of local marine processes, especially ocean currents and internal waves. The exception of the central PRM margin where concave‐up profiles develop is mainly related to canyon erosion. Overall, this study highlights the vital role of local forcing factors in controlling along‐margin variations and determining the final fates of different margin segments. A comparison between the northern SCS and other well‐established examples reveals that concave‐upward shelf‐margin shapes, which are usually associated with high sediment supply, little influence from hydraulic regimes, or sometimes, high degree of canyon development, may be an indicator of good reservoir potential beyond the shelf edge.