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 basin