INVITED EDITORIAL Clastic sediment supply to basins

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  • Basin Research (1998) 10, 15

    INVITED EDITORIAL

    Clastic sediment supply to basinsNiels Hovius* and Mike Leeder*Department of Geology, Trinity College, Dublin 2, IrelandDepartment of Earth Sciences, The University of Leeds,Leeds LS2 9JT, UK (email: leeder@earth.leeds.ac.uk)

    The increasing trend in the Earth sciences towardsTHE DOWNSLOPE PERSPECTIVE:

    integration of subdisciplines (Dickinson, 1997) has CONTROLS AND CONSTRAINTS ONresulted in studies of fundamental units of the Earth SEDIMENT SUPPLYsystem. The sedimentary basin and its complementary

    The rate of sediment supply to a depositional basin andcounterpart, the drainage basin or catchment, are perhapsthe physical and chemical characteristics of the materialthe most elementary units at the interface of the solidinvolved are determined by a complex of interactionsEarth and its atmosphere. Sedimentary basins constitutebetween rock uplift, weathering, erosion and downslopeour most tangible record of Earth history, containing thetransport in the feeder catchment. The conventionaldepositional reflection of lithospheric, geographical,scenario is one in which erosional landscape evolutionoceanographic and ecological development through geo-and sediment flux are driven by the incision of riverslogical time. Drainage basins, on the other hand, rep-into uplifting bedrock. River channels occupy only aresent the negative imprint of these same developments.minor part of the resulting terrain. The bulk of theirTheir landscapes are the products of the unroofingsediment load is derived from the interfluves. There,history that gave rise to the formation of sedimentarybedrock is exposed to physical and chemical weatheringsequences elsewhere. In drainage basins, geomorphicprocesses, driven by climate and modulated by vegetation.history is recorded in ephemeral landforms, but eventu-These processes cause disintegration of coherent bedrockally all topographic evidence of the past is milled off byand may selectively remove or modify some mineralerosion, producing an ongoing downslope flux of mass.components. Given sufficient topographic energy, theIt is this transfer of sediment that links catchments andweathering products are eroded from the interfluves bysedimentary basins. Sediment supply is a first-orderhillslope masswasting processes, whose rate is thought tocontrol on the pattern and distribution of sedimentarydepend on the local surface gradient as well as thefacies in depositional basins. Consequently, basin fillsprobability distributions of their triggers. Eventually, thereflect the sediment flux across their boundaries and theeroded material is transferred onto the valley floor, wherehinterland conditions driving that flux. The integrativeits removal is a function of the transport capacity of thestudy of sediment supply to basins therefore offers a dualfluvial system. In this scenario, sediment eux from aperspective on Earth surface dynamics. The downslopecatchment is either limited by the rate of bedrockperspective considers the processes responsible forweathering, or by the transport capacity of the stream.catchment denudation and defines sediment yield as a

    There are many ways in which landscape evolution,boundary condition for the development of sedimentaryand therefore sediment supply, can deviate from thissequences in depositional basins. In this context a drain-simple sequence. Three alternatives deserve mentionage basin could simply be regarded as a clastic factory.here. First consider a situation in which the rate ofThe upslope perspective views the sedimentary recordbedrock uplift is matched by the rate of valley lowering,

    as a repository of information on catchment history whichbut surpasses the rate of weathering. Then interfluves

    has the potential to yield unique insights into long-termgrow until topographic elements become unstable and

    erosional landscape development. This special issue ofcollapse, producing bedrock landslides. Given sufficient

    Basin Research contains a set of papers on sedimenttransport capacity of the rivers, this type of landscape

    supply to basins, with a focus on clastic sediments. It yields sediment at a rate that is solely determined by thepresents a mix of catchment and sedimentary basin rate of rock uplift. It is characteristic of most activestudies, both empirical and model driven, bridging compressional mountain belts, and responsible for thegeomorphology, sedimentary geology and geophysics. production of >80% of all clastic material that is eroded

    from the present-day continents (Milliman & Syvitski,1992; Hovius, 1998). Second, valleys are not always and* Present address: Department of Geosciences, The Pennsyl-everywhere occupied by rivers, but may instead bevania State University, 540 Deike Building, University Park,

    PA 16802, USA. E-mail: nhovius@geosc.psu.edu conduits of glaciers. Glaciers are thought to have a

    1998 Blackwell Science Ltd 1

  • N. Hovius and M. Leeder

    greater carrying capacity than rivers for a given discharge no longer be assumed to be constant. Any climatic ortectonic perturbation overcoming the systems thresholdand are therefore able to process more and coarser

    material. They take longer to evacuate an entrained forces it to adapt to a new set of conditions. Such changesin boundary conditions are likely to result in shifts insediment particle from the catchment, and do so with

    different periodic variation of rate (cf. Hallet et al., 1996). the quantity and quality of the sediment yield, withconsequences for landscape evolution, development ofFurthermore, downvalley rounding and comminution

    patterns differ between glaciers and rivers. Third, valley fluvial architecture and basin fill, and, ultimately, tectonicmass budgets. For these reasons it is desirable to considerformation may not occur to any significant extent, causing

    bedrock to be exposed to sheet-type erosion. This may sediment supply to basins on longer time scales. Fissiontrack analyses and cosmogenic isotope studies have theoccur under conditions of extreme aridity, or during full

    glaciation. Sediment is then no longer supplied from a potential to provide key hole views of exhumation histor-ies. Alternatively, catchment erosion and sediment supplypoint source, but may instead be introduced along lat-

    erally extensive segments of the basin margin. Finally, on geological time-scales can be explored by means ofnumerical simulations.and hors categorie, clastic sediments may be sourced from

    submarine topography, fitting none of the available land- Recent studies have considered the coupled tectonicuplift/subsidence and erosion system in partly process-scape evolution models.

    From this elementary discussion it transpires that there based models involving drainage network evolution andhillslope processes (Willgoose et al., 1991; Kooi &are many controls on clastic sediment supply to basins,

    even for the limited but prevalent case of fluvially Beaumont, 1996; Tucker & Slingerland, 1996; Braun &Sambridge, 1997; Densmore et al., 1998). In parallel,dominated catchments. They include rock uplift, lith-

    ology and structure, precipitation, temperature, veg- some more complex and specific surface processes modelshave been developed (de Ploey et al., 1991; Kirkby, 1995;etation, mean elevation, amplitude and gradient of local

    relief, and hydraulic conditions in the channel. Further Kirkby & Cox, 1995). These numerical models facilitatethe evaluation of sediment flux on geological time-scales.complications may arise from spatial inhomogeneities and

    temporal variability in the relative and absolute potency In this issue of Basin Research, Leeder et al. considersediment supply as a function of soil production rate andof these controls within a drainage basin, and from time

    lags in the transfer of sediment from source to ultimate sediment transport rate, and emphasize the role of veg-etation in mediating the precipitation and runoff controlrepository. It should therefore not come as a surprise

    that empirical studies of modern drainage basins (e.g. on both these variables. Using the cumulative seasonalerosion potential model of Kirkby & Cox (1995), theyFournier, 1960; Milliman & Meade, 1983; Jansson, 1988;

    Pinet & Souriau, 1988; Milliman & Syvitski, 1992) have explore the effects of climate change, on temporal scalesof glacial to interglacial cycles, on fan development in anot so far produced a reliable universal relationship

    between sediment yield and catchment characteristics. Mediterranean and a Great Basin (USA) setting. Incontrast with this study, Allen and Hovius argue thatAlthough strong correlations between erosion rates and

    one or more catchment characteristics can be found landslide-derived sediment fluxes from montane catch-ments are controlled mainly by the rate of tectonic rocklocally (e.g. Ruxton & McDougall, 1967; Ahnert, 1970),

    the search for universality, in this form, is futile. The uplift, and should therefore be more or less independentof climate. Paola and Swenson employ a simple land-principal reason for this is that sediment eux of a

    catchment is the integrated effect of a series of tectonic, scape erosion model to investigate the production ofsediment from stratigraphy that is advected throughclimatic and geomorphic processes and not of the catch-

    ment characteristics that are normally considered in steady-state topography. They demonstrate how theshape of the sediment production function is a derivativethese analyses.

    Notwithstanding these limitations, studies of modern of the source area hypsometry, while the quality of theunroofing sequences is controlled by both the stratal dipdrainage basins add significantly to our understanding of

    sediment supply to basins by contributing observa- angle and the ratio of layer thickness to relief. Mostsurface process models run on spatial and temporal scalestions of sediment loads and process rates. However,

    these observations are almost invariably short-term of greater magnitude than those of the individual geo-morphic processes operating on the landscape. They(101102 years) and therefore do not necessarily resolve

    for the largest possible event given a certain set of employ parameters such as effective diffusivity and fluvialdiffusivity that lump together different processes andboundary conditions. High-magnitude low-frequency

    manifestations often dominate catchment erosion, causing integrate the effects of individual events. Such parametershave no direct significance outside the model context anddistinct and volumetrically important depositional signals.

    Furthermore, the erosion and transport processes need to be calibrated in order to have quantitativepredictive power. Van der Beek and Braun hypothesizeinvolved in such events may be mechanically different

    from those more frequent processes that are commonly that morphometric and fractal characteristics of a land-scape can provide constraints on the relative magnitudesobserved. On the time scales at which high-magnitude

    low-frequency events typically occur (102104 years) of such model parameters. However, they did not succeedin finding any morphometric and fractal observable thatboundary conditions of the catchment erosion system can

    1998 Blackwell Science Ltd, Basin Research, 10, 152

  • Clastic sediment supply to basins

    uniquely characterizes the influence of erosional forcing follows that the spatial distribution of grain-size contoursfor a given input signal is a function of the pattern offunctions in a study of SE Australian topography.

    Estimates of long-term denudation rates, van der Beek accommodation space generation, and therefore carriesthe signature of the dominant mechanism by which thisand Braun say, have the potential to calibrate surface

    process model parameters much better than the observed is achieved. In the Campanian Castlegate sandstone andassociated conglomerates of eastern Utah, which rep-present-day topography alone. Such estimates can be

    derived, for instance, from fission track analysis. Morris resent an almost complete record of Sevier frontalthrust to nearshore marine deposits, Robinson andet al. used an analytical approach to fission track length

    distributions, proposed by Gallagher (1995), to generate Slingerland recognize a trend from proximal subsidencecontrol to distal sea-level control on stratal architecture.detailed timetemperature plots for post-closure cooling

    of rocks from the eastern and central Pyrenees. These Having constrained the hydraulic conditions and subsid-ence regime, they derive the sediment supply rate thatplots have been converted into estimates of erosional

    denudation over the past 40 Myr. Morris et al. have best matches the Castlegate stratigraphy. A similar inver-sion of the stratigraphic record is presented by Alleninterpolated the local data to construct a map of total

    exhumation depths for a part of the orogen, and translated and Hovius for the specific case of fans adjacent torapidly uplifting mountain ranges. Without obtainingthe observed pattern into volumes of material eroded

    within a series of time intervals. unique solutions they reconstruct palaeo-catchment sizes,tectonic uplift rates and average catchment slopes forsource areas of several stacked fan sequences.THE UPSLOPE PERSPECTIVE:

    While stratal and grain-size patterns are controlled byINVERTING THE SEDIMENTARYaccommodation and supply, the compositional character-RECORDistics of sediments reflect their parent lithology, andweathering and erosion history. Weltje et al. haveOnce delivered to the depositional basin, sediment is

    distributed, deposited or transferred out of the basin developed a numerical process response model of sedi-ment production focusing on the properties of sedimentsaccording to the ambient transport and tectonic con-

    ditions. For any given sediment supply rate, the distri- instead of the landscapes produced. This model allowscharacterization of detritus produced in terms of volume,bution of sedimentary facies is controlled, at first order,

    by the pattern and rate of tectonic subsidence and the grain size and residence time in source area. The latteris reflected in the extent of weathering, and should berate of sea-level change, together creating accommodation

    space. In more detail, sedimentary patterns are governed expressed in the relative abundance of the frameworkelements quartz, feldspar and rock fragments, in theby the interplay of the hydraulic conditions in the basin

    and the volume and grain-size properties of the sedi- material leaving the catchment. Where it is reasonable toassume an intimate link between weathering rate andment input.

    Within the fluvial realm, the tendency of streams to sediment supply rate, Weltje et al. argue, it should bepossible to back calculate sediment supply rates from theadjust their long profile and planform to the available

    accommodation space, water discharge, sediment supply framework element composition observed in the stratalrecord.and grain sizes in transport (cf. Mackin, 1948) provides

    a useful tool for evaluation of the boundary conditions In the final article of this set, Jaeger et al. exploit theradio isotope composition of shelf sediments from theon the sedimentary system. Talling and Sowter present

    a study of spatial patterns of erosion and deposition in north-eastern Gulf of Alaska to reconstruct sedimentsupply and deposition patterns over the last centuries.relation to systematic downstream changes in hydraulic

    conditions. They demonstrate that the margin of the Po 210Pb profiles in this region reflect steady and extremelyhigh rates of sediment supply from a line source formeddepositional basin, Italy, as defined by the transition from

    erosion to deposition in feeder streams, coincides with a by a heavily glaciated coastal mountain range, punctuatedby catastrophic events related to seismicity and glacierwell-defined change in channel gradient. Downstream

    changes in bed shear stress and stream power, Talling surging.and Sowter argue, cause the variations in bedloadtransport rates reflected in the observed pattern of erosion OUTLOOKand deposition. Ultimately, they invoke a tectonic controlon the location of the stream profile knicks. More than any other issue in basin research, there is a

    need to explore the consequences of temporal and spatialRobinson and Slingerland introduce the concept ofstream grading to the stratigraphic record, using grain- changes in water and sediment supply, and to intersect

    time series of these variables with other basin-definingsize trends to evaluate the relative importance of bothrate and magnitude of tectonic subsidence, eustacy and variables such as basin subsidence rate, sea- and

    lake-level change, catchment uplift rate and climate.sediment supply on stratal architecture. Given constantboundary conditions, they reason, a graded river in a It is therefore of critical importance to construct

    time-integrated datasets covering periods over whichsubsiding basin will generate stratigraphy in which thegrain-size contours are stationary through time. It then catchment and depositional basin-forming processes

    1998 Blackwell Science Ltd, Basin Research, 10, 15 3

  • N. Hovius and M. Leeder

    operate. This includes both the detailed measurements encompass various end-member scenarios, notably trans-port-limited, weathering-limited and rock uplift-limitedof rates of individual processes that can be made in the

    present-day environment and estimates on time-scales of (landslide-dominated) sediment supply, as well as thestochastic nature of its forcing functions (cf. Benda &103106 years derived from the geological record.

    Some geomorphic and tectonic processes are now Dunn, 1997).All issues mentioned here are encapsulated in one pre-routinely monitored in settings that range from the

    extremely active compressional orogens of the Pacific eminent question: How does the interaction of tectonicsand climate control the long-term sediment flux fromRim to the passive margins and continental interiors of

    Australia and Africa. Amalgamation of these observations source areas to depositional basins? We offer this questionas a koan, knowing that the answer requires completein global data sets would provide an overview of most

    combinations of boundary conditions for sedimentary and direct insight into the stream of manifestations ofthe Earths surface.basins that have expressions in the modern environment.

    The basin research community should undertake to locateand bring together the many high-resolution data sets REFERENCESthat exist in the obscurity of local archives as a matter

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