Variations In Sediment Supply Research Articles

Interdecadal patterns of total sediment yield from a montane catchment, southern Coast Mountains, British Columbia, Canada

We reconstruct sediment yield for a mountain watershed of western Canada since the mid-twentieth century from studies of annually laminated lake sediments, delta progradation, and solute transfer. Total yield averaged 320 ± 40 Mg km − 2 a − 1 and comprised ∼ 35% suspended load, 50% bedload, and 15% dissolved load. Ratios between the individual yield components varied approximately threefold at interannual timescales because of significant variability in the suspended and bedload fractions. Asynchronous flux in suspended and bedload fractions through time arise from differences in sediment availability and transitory sediment storage in the channel. Periods of elevated yield coincide with rapid glacier recession, an extreme rainstorm, and a landslide. Our results indicate that in montane environments, extrapolation from even decade-long monitoring programs may lead to biased projections of long-term yield and delivery mode proportions if variations in sediment supply and catchment response to hydroclimatic and geomorphic controls are not considered.

Concurrent tectonic and climatic changes recorded in upper Tortonian sediments from the Eastern Mediterranean

Terra Nova, 22, 52–63, 2010 Abstract The upper Tortonian Metochia marls on the island of Gavdos provide an ideal geological archive to trace variations in Aegean sediment supply as well as changes in the North African monsoon system. A fuzzy-cluster analysis on the multiproxy geochemical and rock magnetic dataset of the astronomically tuned sedimentary succession shows a dramatic shift in the dominance of ‘Aegean tectonic’ clusters to ‘North African climate’ clusters. The tectonic signature, traced by the starvation of the Cretan sediment, now enables to date the late Tortonian basin foundering on Crete, related to the tectonic break-up of the Aegean landmass, at c. 8.2 Ma. The synchronous decrease in the North African climate proxies is interpreted to indicate a change in the depositional conditions of the sink rather than a climatic change in the African source. This illustrates that interpretations of climate proxies require a multiproxy approach which also assesses possible contributions of regional tectonism.

Rapid channel response to variability in sediment supply: Cutting and filling of the Tarndale Fan, Waipaoa catchment, New Zealand

In the headwaters of the Waipaoa catchment, fluvio-mass movement gully complexes are currently significant contributors of sediment, delivery of which is buffered by fans which have developed in the small tributaries that lead from the gully complexes. Biannual channel cross-section surveys made since 1983 show seasonal aggradation / incision cycles; channels generally incise in summer and infill during the winter months. This seasonal pattern of cutting and filling is driven by variations in sediment supply from the gully complexes. In this paper, to better understand patterns and processes of sediment delivery at the top of the sediment cascade, we examine cut and fill cycles on the ~ 11 ha Tarndale Fan over a two and a half year period. Analysis of sequential DEMs compiled from survey data obtained between December 2004 and August 2007 indicates cutting and filling may involve up to 47,000 tonnes of sediment over as little as three months. During discrete severe rainstorm and/or wet weather periods, mass movements in the gully complex are enhanced and contribute large quantities of sediment to the fan, via debris flows and landslides, which infill channels. During drier periods, mass movement activity is inhibited and runoff incises the feeder channels.

Empirical orthogonal function analysis of shoreline changes behind two different designs of detached breakwaters

This paper compares the shoreline responses immediately shoreward of two adjacent schemes of segmented shore parallel rubble mound breakwaters undergoing the same forcing by waves and tides. Scheme one consists of four longer, emergent breakwaters that have produced tidal tombolos in their lee. Scheme two consists of five shorter, lower breakwaters that are submerged at higher tides with salients in the breakwater's lee. Empirical orthogonal function analysis was used to decompose a video derived shoreline dataset into the dominant modes of shoreline change for both schemes. The two schemes showed similar modes of change. The primary mode of change for both schemes was the cross-shore growth and shrinking of the salients/tombolos. The secondary mode of change was the longshore movement of the salients/tombolos. For all modes of change, the dominant length scale was dictated by the breakwater dimensions and locations. A new manifestation of hydrodynamic parameters is introduced: the cumulative integral of the de-meaned parameters. This parameterisation allowed for meaningful correlation of the temporal EOF components with forcing parameters and identification of the important influence of the tide on observed morphodynamic change. Clear differences were observed between the shoreline responses measured in schemes one and two; including differences in bay erosion/accretion; and the longshore translation of salients/tombolos. The beaches in scheme two showed behavioural patterns similar to unprotected beaches which were not observable in scheme one. It is postulated that these differences are caused both by the different breakwater designs and by variation in longshore sediment supply.

Effect of incoming sediment on the transport rate of bed load in mountain streams

Field experiments were conducted on bed load transport in the Diaoga River, a mountain stream in southwest China, to study the variation of bed load transport with varying sediment supply. The rate of bed load transport is greatly affected by incoming sediment (load and size). Under the same flow conditions, bed load transport rates may differ by three orders of magnitude depending on whether measurements were taken before or after the first flood of the year. The relation of the “bed load transport rate versus flow intensity” appears to have similar characteristics as a clockwise looped-rating curve. Experiments also were conducted during the non-flood season to study bed load transport processes with different incoming load from an upstream section. Bed load with different sizes can be grouped into two types: traveling bed load and structural bed load. Traveling bed load is composed of sediment finer than a critical size, D c, and its transport rate depends mainly on the incoming sediment rate. The incoming sediment rate can alter the rate of bed load transport by three orders of magnitude. Structural bed load is composed of coarser sediment and its transport rate closely relies on the flow intensity.

Climate as main factor controlling the sequence development of two Pleistocene alluvial fans in the Vienna Basin (eastern Austria) — A numerical modelling approach

Tectonics, climate, base level and source lithology are the main controlling factors on alluvial fan development and evolution. In this study a numerical model approach was used to investigate climate-induced aggradation and degradation cycles, the influence of tectonic subsidence, and the impact of an axial main river on Austria's largest Pleistocene alluvial fan setting, the Mitterndorf Basin, eastern Austria. A simulation time frame of 25 ka was applied. Climatic variations were mainly modelled through variations in sediment supply, whereas the impact of subsidence was modelled through variations in slip rates along faults. Incision and aggradation tendencies of the axial main river were modelled by rising or lowering of the water level. The sequence development and geometry of Mitterndorf Basin's alluvial fans are mainly controlled by Pleistocene climate oscillations. Cold periods generally led to a decrease in vegetation cover and to an increase of periglacial weathering processes which is associated with abundant sediment release and aggradation on fan surface. In contrast, decreasing sediment supply and increasing precipitation during warmer periods led to fan degradation. Our models indicate that the alluvial fan development is primarily affected by the distinct sediment supply pulses during phases of short climatic perturbations. Subsidence becomes the determining factor again when models approach a steady-state behaviour. Similarly, the impact of the axial main river becomes more important during simulated warm periods when sediment release to the drainage basin is low. The overall basin slope additionally controls the evolution of fan and drainage. Variations in sediment supply and subsidence do also significantly control the long term evolution of the drainage network during simulated warm periods and thus control the sequence preservation potential of fans and therefore its long term sequence evolution.

Testing a cellular modelling approach to simulating late-Holocene sediment and water transfer from catchment to lake in the French Alps since 1826

This paper describes the application of a hydrogeomorphological numerical model (CAESAR) to simulate, at hourly time steps, changes in the hydrological and sediment regime of the Petit lac d'Annecy catchment. The outputs of the model were validated in three ways. In the short term (~5 years), water discharge outputs were compared against observed instrumental data. Over the longer term, modelled sediment discharge (AD 1825—2005) was compared with proxies for detrital sediment influx (environmental magnetism) and accumulation rates discerned from a 210Pb chronology for the lake sediments. Finally, spatial validation of the modelled erosion and deposition of sediment was undertaken by comparison with a field and remotely sensed survey of catchment geomorphology. The results suggest that while minor perturbations in forest cover during the last 180 years have partially conditioned the response of the sediment system, the bulk of modelled sediment discharge and particularly the peaks in sediment discharge were controlled by flood duration and magnitude, which in turn is driven by precipitation (storms/floods) and snowmelt. Basin geometry and geomorphology of each sub-catchment (Ire and Tamie) were also important in producing differences in the modelled sediment discharge. In essence, these differences were a function of sediment accommodation space and the ability of each system to store and release sediments. Modelled sediment discharge and χpara (lake sediments) display similar histories, and thus are both interpreted as reflecting variations in detrital sediment supply. Intriguingly the style of modelled sediment discharge from the Ire, a confined mountain torrent, displays a greater similarity to and perhaps dominates the lake sediment record. These results provide partial validation of the CAESAR model and indicate that perhaps in the future it may be used as an exploratory and predictive tool in determining the impact of changes in climate, meteorology and land use on lake-catchment systems.

Depositional development of an isolated mound and adjacent area in the southern Yellow Sea during the last postglacial sea-level rise

A detailed analysis of the Chirp (2–7 kHz) profiles, core sediments and 14C ages from an isolated mound and adjacent area in the southern Yellow Sea off the Jiangsu coastal area reveals a highly complex development of transgressive and highstand deposits during the last postglacial sea-level rise. In water depths of 50–60 m, the terrace consists of thin transgressive sediments directly above transgressive surface. The isolated mound (40.7 km long and 29.6 km wide) occurs above 40–50 m in water depth, and is detached from the Jiangsu and East China Sea tidal sand ridges. It comprises the SSE-ward prograding sandy unit (PU) and the overlying muddy flank unit (FU) at the outer margin. The terrace with thin transgressive sediments was most likely formed when sea level rose rapidly from −58 to −45 m around about 11.6 ka. As rate of sea-level rise was attenuated from −45 to −36 m between ca. 11.6 and 9.6 ka, the nearly flat lower boundary of the PU was produced at water depths of 37–42 m by a ravinement process. Subsequent rapid sea-level rise from −36 to −16 m between ca. 9.6 and 9.1 ka created accommodation for deposition of the PU. Sea-level rise stagnated from −16 to −10 m between ca. 9.1 and 7.5 ka during which the Huanghe River apparently shifted south of the Shandong Peninsula and discharged directly into the western Yellow Sea. The combination of slow sea-level rise and sediment supply from the early Holocene Huanghe delta by the southward-flowing coastal currents most likely favoured the formation of the SSE-ward prograding transgressive sandy PU. The highstand muddy FU overlying the PU was accumulated after ca. 7.5 when sea level approached its present position and the Huanghe River shifted back to the north. Development of transgressive and highstand deposits in the study area during the last postglacial sea-level rise was highly variable in time and space in response to changes in rate of sea-level rise together with variations in sediment supply caused by the shifting of the Huanghe River.

The spatial and temporal patterns of aggradation in a temperate, upland, gravel‐bed river

AbstractIntensive field monitoring of a reach of upland gravel‐bed river illustrates the temporal and spatial variability of in‐channel sedimentation. Over the six‐year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land‐use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. Copyright © 2009 John Wiley & Sons, Ltd.

Influence of Patagonian glaciers on Antarctic dust deposition during the last glacial period

Dust deposition over Antarctica was much higher during the last glacial period, but the causes of this flux remain unclear. An analysis of the composition and age of glacial outwash sediments indicates that growth and decay of glaciers in Patagonia may have influenced dust transport to Antarctica at that time. Ice cores provide a record of changes in dust flux to Antarctica, which is thought to reflect changes in atmospheric circulation and environmental conditions in dust source areas1,2,3,4,5,6,7,8,9. Isotopic tracers suggest that South America is the dominant source of the dust10,11,12, but it is unclear what led to the variable deposition of dust at concentrations 20–50 times higher than present in glacial-aged ice8,9. Here we characterize the age and composition of Patagonian glacial outwash sediments, to assess the relationship between the Antarctic dust record from Dome C (refs 9, 13) and Patagonian glacial fluctuations14,15,16 for the past 80,000 years. We show that dust peaks in Antarctica coincide with periods in Patagonia when rivers of glacial meltwater deposited sediment directly onto easily mobilized outwash plains. No dust peaks were noted when the glaciers instead terminated directly into pro-glacial lakes. We thus propose that the variable sediment supply resulting from Patagonian glacial fluctuations may have acted as an on/off switch for Antarctic dust deposition. At the last glacial termination, Patagonian glaciers quickly retreated into lakes, which may help explain why the deglacial decline in Antarctic dust concentrations preceded the main phase of warming, sea-level rise and reduction in Southern Hemisphere sea-ice extent13.

Thirty-year changes (1970 to 2000) in bathymetry and sediment texture recorded in the Lagoon of Venice sub-basins, Italy

A detailed comparison was made of two bathymetric charts of the Lagoon of Venice (LV) from 1970 and 2000, in tandem with a comparison of sediment grain size data. Analysis of the data revealed marked changes in both morphology and sedimentation, with more than 50% of the 400 km 2 assessed in this study 15–20 cm shallower in 1970 than in 2000. The four sub-basins into which the LV is subdivided saw different patterns of change. The Northern basin A was identified as “ pristine”, i.e. still in quasi-natural condition, with slight clay enrichment and a small degree of deepening (4–5 cm), essentially due to sea level rise (SLR). The bathymetry and sedimentology of the northern-central sub-basin B (identified as “ urban”) and the southern-central basin C (identified as “ open”) were affected by infill activities and excavation of industrial navigation channels in the 1970s, causing the loss of ~ 60 km 2 of mudflats, and creating an “open” lagoon. The southernmost basin D (identified as “ exploited-subsiding”) of the LV was found to be relatively unchanged and still in semi-natural condition. Comparison of sediment types showed depletion of fine-grained fractions (< 22 μm) in all sub-basins except the northernmost one. Consequent enrichment in sand (> 63 μm) was recorded, except in the southern-central sub-basin C where medium and coarse sand fractions declined. The results suggest that climate-related SLR accounts for a small fraction of bathymetric change, whilst variations in hydrodynamics (currents and wind patterns) and sediment supply are likely causal factors for the different evolution of the four sub-basins. Definition of the attributes of each sub-basin provided data that was essential not only for the LV but also for predicting the fate of transitional environments facing both anthropic (fishing, navigation, land use changes) as well as natural (SLR, eustatism) forcing factors.

Fluvial development and the sediment regime of the lower Calder, Ribble catchment, northwest England

In the lower reaches of the Calder (Lancashire) tributary of the Ribble catchment (northwest England), five river terraces set into the surrounding glacial terrain have been mapped and their underlying sediments radiocarbon dated. The earliest terrace (T1) reflects sandur-style deposition during deglaciation and it aggraded in a reach cut into glacial diamict and glaciolacustrine muds. Incision below T1 spanned the start of the Holocene, driven by a combination of reduced sediment supply under a stabilising landscape and lower base-levels. T2 had aggraded and was being abandoned by 4000 BC; its formation latterly involving a period of comparative stability in terms of channel migration, perhaps a function of extensive forest cover and sea levels reaching above present day levels. The last 3000 years were marked by substantial changes in the sedimentary regime, with increased and varying sediment supply driving cycles of cut-and-fill and greater lateral channel migration. These changes coincided with the first widespread and substantial reductions in forest cover and a progression towards an agricultural landscape. Between the aggradation of terraces T3 and T4 there was a substantial increase in sediment supply, which is broadly in keeping with the erosion history in the surrounding uplands. This sediment slug appears to have induced a switch from highly sinuous meandering to lower sinuosity channels characterised by lateral scroll-style migration. Late Holocene vegetation changes, particularly woodland removal, driven by anthropogenic land-pressure appear to have rendered the floodplain prone to channel migration and the landscape more susceptible to erosion.

Controls on the sedimentary architecture of a single event englacial esker: Skeiðarárjökull, Iceland

Eskers have been used to infer the dynamics and palaeo-hydrology of large ice sheets yet there are few suitable contemporary analogues for esker sedimentation during Quaternary glaciations. Consequently, models of esker sedimentation have been derived from morpho-sedimentary analyses of Quaternary eskers, but these lack rigorous constraints on their depositional timescales. We identify the controls on the large-scale sedimentary architecture of a large (>700 m long, <30 m high) proto-esker deposited during a single, high-magnitude jökulhlaup at Skeiðarárjökull, Iceland. Over 3.8 km of ground-penetrating radar (GPR) lines were collected (in 2006 and 2007) as grids on all workable areas of the proto-esker and associated supraglacial ice-walled canyon-fill sediment. Six main radar facies were identified: (1) discontinuous, undular reflections, interpreted as trough-cross strata, associated with the development of antidune or dune sequences; (2) sub-horizontal reflections (dip <10°), associated with deposition of upper-stage gravel plane beds; (3) up-flow dipping moderate to high-angle (>10°) inclined reflections, interpreted as backset accretion associated with large-scale bedform development; (4) down-flow dipping high-angle (>15°) inclined reflections, interpreted as foreset accretion associated with macroform progradation; (5) quasi-hyperbolic reflections, interpreted as zones of boulder clustering; and (6) zones of enhanced noise and EM signal attenuation, bounded on the upper surface by a continuous reflection sub-parallel to the ground surface, interpreted as buried ice associated with the conduit/channel base. These data suggest that deposition took place within a non-uniform englacial conduit, which has resulted in major up-flow to down-flow variations in sedimentary architecture. The complexities observed are a direct consequence of variations in sediment supply and the within-event evolution of conduit geometry. This study provides the first detailed insight into the large-scale sedimentary architecture of a single event jökulhlaup proto-esker.

Reconstructing transport pathways for late Quaternary dust from eastern Australia using the composition of trace elements of long traveled dusts

The southeast Australian dust transport corridor is the principal pathway through which continental emissions of dust from central and eastern Australia are carried to the oceans by the prevailing mid-latitude westerly circulation. The analysis of trace elements of aeolian dust, preserved in lake sediment on North Stradbroke Island, southeast Queensland, is used to reconstruct variation in the intensity and position of dust transport to the island over the past 25,000 yrs. Separation of local and long traveled dust content of lake sediments is achieved using a unique, four-element (Ga, Ni, Tl and Sc) separation method. The local and continental chronologies of aeolian deposition developed by this study show markedly different records, and indicate varied responses to climate variability on North Stradbroke Island (local aeolian sediment component) and in eastern and central Australia (long traveled dust component). The provenance of the continental component of the record to sub-geologic catchment scales was accomplished using a ternary mixing model in which the chemical identification of dusts extracted, from the lake sediments, was compared to potential chemical characteristics of surface dust from the source areas using 16 trace elements. The results indicate that the position and intensity of dust transport pathways during the late Quaternary varied considerably in response to changing atmospheric circulation patterns as well as to variations in sediment supply to dust source areas, which include the large anabranching river systems of the Lake Eyre and Murray–Darling Basins.

Early Miocene sequence development across the New Jersey margin

ABSTRACTSequence stratigraphy provides an understanding of the interplay between eustasy, sediment supply and accommodation in the sedimentary construction of passive margins. We used this approach to follow the early to middle Miocene growth of the New Jersey margin and analyse the connection between relative changes of sea level and variable sediment supply. Eleven candidate sequence boundaries were traced in high‐resolution multi‐channel seismic profiles across the inner margin and matched to geophysical log signatures and lithologic changes in ODP Leg 150X onshore coreholes. Chronologies at these drill sites were then used to assign ages to the intervening seismic sequences. We conclude that the regional and global correlation of early Miocene sequences suggests a dominant role of global sea‐level change but margin progradation was controlled by localized sediment contribution and that local conditions played a large role in sequence formation and preservation. Lowstand deposits were regionally restricted and their locations point to both single and multiple sediment sources. The distribution of highstand deposits, by contrast, documents redistribution by along shelf currents. We find no evidence that sea level fell below the elevation of the clinoform rollover, and the existence of extensive lowstand deposits seaward of this inflection point indicates efficient cross‐shelf sediment transport mechanisms despite the apparent lack of well‐developed fluvial drainage.

Significant contribution of the 18.6 year tidal cycle to regional coastal changes

While sea-level rise will generate major reshaping of coasts in the next decades , severe or catastrophic coastal erosion is commonly generated by local to regional factors among which are variations in sediment supply , natural or human-induced subsidence, especially in deltaic areas subject to cyclones , and tsunami . Here, we confirm the hypothesis of and show from satellite imagery that the fluctuations of the 1500 km-long muddy coast of South America between the Amazon and the Orinoco rivers have been governed primarily by the lunar 18.6 year nodal cycle over the last twenty years, with sea-level fluctuations from global warming or Nino Nina events being of secondary importance. From now to 2015, the predictable 18.6 cycle will lead to an approximate mean high water sea-level rise of 6 cm on the Amazon-Orinoco coast, compared to a more than 2 cm rise due to global warming, and will generate sixty percent of a projected 150 m shoreline retreat in French Guiana. Many of the world’s coasts will experience a tide constituent-induced rise in sea-level exceeding ten centimetres over the next decade.

Assessing the significance of along‐strike variations of middle to late Miocene prograding clinoformal sequence geometries beneath the New Jersey continental shelf

ABSTRACTSeismic mapping of high‐resolution multichannel seismic profiles along the New Jersey margin illustrates how characteristics of middle–late Miocene clinoformal sequence boundaries (SBs) change markedly along strike in presumed response to local depositional and erosional processes. Most SBs converge from SW to NE, in part as a result of the influence of underlying basin morphology on accommodation space, but also in response to differential subsidence and presumed along‐strike variations in sediment supply from the adjacent margin. The curvature of clinoform breaks, historically viewed as marking palaeo‐shelf edges, is variable and such breaks are rarely sharp. Gently curved palaeo‐shelf/slope transitions cannot be assigned precise palaeobathymetric significance and probably instead reflect post‐depositional sediment reworking. The amount of erosional truncation landward of clinoform breaks varies significantly. Documented along‐strike variability in SB morphology occurs, even though middle–late Miocene palaeo‐shelf edges are nearly linear in plan view. Therefore, such linearity cannot be a product of uniform sedimentary processes and/or accumulation along strike, but instead reflects elongation of depocentres of originally variable cross‐sectional geometry, possibly with the assistance of along‐strike currents. The observed lateral geometric heterogeneity of Neogene sequences can exert profound and unwanted influences on the outcome of scientific drilling intended to calibrate seismic stratigraphic interpretations in the absence of sufficient three‐dimensional (3D) seismic control.

The Cosmonaut Sea Wedge

A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.

Seismic stratigraphy and development of Avon canyon in Benin (Dahomey) basin, southwestern Nigeria

Interpretation of a grid of high resolution seismic profiles from the offshore eastern part of the Benin (Dahomey) basin in southwestern Nigeria area permitted the identification of cyclic events of cut and fill associated with the Avon canyon. Seismic stratigraphic analysis was carried out to evaluate the canyon morphology, origin and evolution. At least three generations of ancient submarine canyons and a newly formed submarine canyon have been identified. Seismic reflection parameters of the ancient canyons are characterized by transparent to slightly transparent, continuous to slightly discontinuous, high to moderate amplitude and parallel to sub-parallel reflections. Locally, high amplitude and chaotic reflections were observed. The reflection configurations consist of regular oblique, chaotic oblique, progradational and parallel to sub-parallel types. These seismic reflection characteristics are probably due to variable sedimentation processes within the canyons, which were affected by mass wasting. Canyon morphological features include step-wise and spoon-shaped wall development, deep valley incision, a V-shaped valley, similar orientation in the southeast direction, and simple to complex erosion features in the axial floor. The canyons have a composite origin, caused partly by lowering of the sea level probably associated with the formation of the Antarctic Ice Sheet about 30 Ma ago and partly by complex sedimentary processes. Regional correlation with geological ages using the reflectors show that the canyons cut through the Cretaceous and lower Tertiary sediments while the sedimentary infill of the canyon is predominantly Miocene and younger. Gravity-driven depositional processes, downward excavation by down slope sediment flows, mass wasting from the canyon walls and variation in terrigenous sediment supply have played significant roles in maintaining the canyons. These canyons were probably conduits for sediment transport to deep-waters in the Gulf of Guinea during their period of formation.

RESPONSE OF SHALLOW MARINE BIOTAS TO SEA-LEVEL FLUCTUATIONS: A REVIEW OF FAUNAL REPLACEMENT AND THE PROCESS OF HABITAT TRACKING

Associations of fossil genera and species commonly display repeated and predictable patterns of change in stratigraphic sections. These changes exhibit some analogies with the phenomenon of ecological succession but are longer-term allogenic temporal changes, occurring over time scales of tens to hundreds of thousands of years, that should be referred to as biotic (faunal-floral) replacement. Habitat tracking is one of a suite of possible mechanisms of biotic replacement, but one that may be important in certain marine settings. Evidence of tracking includes (1) recurrence of similar replacement series of differing age; (2) mirroring of vertical replacement of faunas by lateral gradients of species associations along single time planes; (3) the occurrence of similar gradient transects at different time planes; (4) the correlatability of highs and lows of quantitative faunal curves (e.g., variations of detrended correspondence analysis scores) at different localities despite offsets of absolute scores; and (5) the high fidelity recurrence of stenotopic species in particular associations representing narrowly defined environments. The degree of ecological fidelity (e.g., similarity of species richness, guild structure) that is maintained during tracking is variable. Recurrent assemblages in different cycles, separated by as much as several million years, can be very similar in terms of species composition and trophic structure. Common species, however, may show significant differences in rank and relative abundances. This evidence indicates individualistic tracking of preferred habitat by various species. In shallow-shelf and ramp settings, sea-level fluctuations may produce approximately symmetrical patterns of biotic replacement where biofacies are arrayed typically in elongate belts parallel to depositional strike. Asymmetries, however, are common and may result from variations in sediment supply during sea-level fluctuation. Hence, the low siliciclastic input typical of transgressions predictably favors those organisms that require lower sedimentation or turbidity and perhaps firmgrounds to hardgrounds, whereas the regressive half cycle at analogous depths favors more eurytopic organisms that tolerate or prefer higher sedimentation or turbidity. The phenomenon of tracking may be of considerable importance in evolutionary paleoecology. Tracking implies that species commonly respond to long-term physical changes, not by adaptation, but primarily by migration of species to preferred habitats, if the rate or magnitude of environmental change is not too great. Provided that the same basic environment existed through time, despite lateral shifting by up to hundreds of kilometers, most species of benthic invertebrates were capable of surviving with little or no evolutionary change. Tracking may be the primary basis of patterns of morphological stasis, as well as relative stability in biofacies richness, composition, and trophic structure.