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<br />JARRETI AND TOMLINSON: REGIONAL INTERDISCIPLINARY PALEOFLOOD METHOD
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<br />Table 1. Description of Relative Dating-Methods Used in Northwestern Coiorado
<br />Numerical Rating and Description-
<br />
<br />Type of Relative Dating
<br />Method 0,3 4,6 7-10
<br />Soil horizons C (increasing O/A) OINC OIAlBtj/C
<br />Rock weathering fresh ,partly weathered very weathered
<br />Pitting, % <10. rare/incipient 30-70 >75, common
<br />Grain relief, mm <0.5 0.5-1 >1
<br />Boulder burial, % 0-25 25-75 >75
<br />Surface morphology
<br />Terrace scarp angular moderately rounded well rounded
<br />Slnpe steep moderately muted extremely muted
<br />Terrace tread fresh longitudinal moderate transverse extensive transverse
<br /> flood evidence rills and gullies rills and gullies
<br />Lichenometry .
<br />Largest thalli, mm 0-100 >150. >150
<br />Rock coverage, % <75 >75 >75
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<br />Abbreviation tj indicates incipient accumulation of silicate" clay that has either fanned in situ or is
<br />alluvial [Birkeland, 1984).
<br /><J.A rating of 0 is modern or 0 years; 10 is early Holocene or about 10,000 years or older. The rating
<br />values arc approximate, nonlinear. and applicable for northwestern Colorado river valleys.
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<br />the occurrence of large floods separated by a short time span
<br />where RD techniques (and even absolute techniques) may not
<br />be able to differentiate between several individual oider de-
<br />posits.
<br />The most important factor for using RD techniques for
<br />fluvial deposits is to compare the deposits with flood deposits
<br />and other surfaces immediately adjacent (upslope and down-
<br />slope) in a short reach (site). Within-stream factorswouid have
<br />zero age, whereas deposits and surfaces with increasing height
<br />above the channel have increasing age. Thus state factors (e.g.,
<br />iithology, microclimate, climate, vegetation, and topography)
<br />are assumed to be held constant; therefore differences in
<br />weathering and soil-profile development are directly related to
<br />time at any individual site, which is the justification of RD
<br />methods [Burke and Birkeland, 1979; Harden, 1982; Waytlzomas
<br />and Jan-et!, 1994]. Also, it is important to obtain ages using
<br />these different RD techniques for several s;3.mples within a
<br />reach of channel. Simiiar ages derived from different tech-
<br />niques result in increased confidence in the estimate [Burke
<br />and Birkeland, 1979; Way/Izomas and Jamtt, 1994]. Because the
<br />most limiting factor in age-dating studies is small sample size
<br />[Harden, 1990], dating numerous deposits aiong rivers helps
<br />increase the reliability of age determinations.
<br />The focus of this study was to identify gross changes in RD
<br />features [Burke and Birkeland, 1979], which were then used to
<br />estimate maximum age of flood or noninundation surfaces.
<br />Age estimates for paleoflood deposits are based on relative-
<br />age criteria as proposed by Burke and Birkeland [1979], Colman
<br />and Pierce [1983J, Harden [1982, 1986, 1990], and Waythomas
<br />and Jarrett [1994]. RD techniques used for this study were
<br />degree of soil development (S), surface-rock weathering (W),
<br />surface morphoiogy (M), lichenometry (L), and boulder
<br />burial (B), although not all methods could be used at each site.
<br />For each of these criteria a numerical value from 1 to 10 was
<br />assigned, 1 representing modern channel deposits and 10 ex-
<br />hibiting greatest age corresponding to early Holocene or older
<br />(Table 1). For all RD methods as discussed for each except
<br />lichenometry, the rating scale is 1 for -0 to 1000 years to a
<br />value of 10 for - 10,000 years. For lichenometry, 1 is modern
<br />and 10 is probably 3000 years or less. Finally, an average age
<br />and range of age uncertainty was determined.
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<br />4.2.1. Soils. There is a strong relation between degree of
<br />soil development and time, although rates of soil development
<br />vary widely [Birkeland, 1984; Harden, 1986, 1990]. Correiation
<br />of soil-proflle development with soil chronosequences, dated
<br />with numerical techniques, is crucial when determining the
<br />relative age of surfaces [Birkeland, 1984; Bull, 1990]. Soiis show
<br />a systematic and generally slow progressive soil-profile devel-
<br />opment with age. Readily avaiiabie soil surveys provide useful
<br />data used in conjunction with field checking. The degree of
<br />development of the local soil profile was determined in the
<br />fleld by trenches and cut-bank exposures of flood deposits and
<br />terrace deposits. Age diagnostic parameters include the fol-
<br />lowing: thickening of the total soils and development of the B
<br />horizon; increasing enrichment of the B horizons in secondary
<br />clay (an argillic horizon); presence, abundance, and thickness
<br />of clay films; increasingly diffuse horizon boundaries; abun-
<br />dance of calcium carbonate; oxid8tion depth; pan deveiop-
<br />ments; and rubification of the Band C horizons [Bilzi and
<br />Ciolkosz, 1977; Harden, 1982; Colman and Pierce, 1983; Birke-
<br />land, 1984]. Conditions that increase the rate of soil develop-
<br />ment include the following: warm, humid climate; forest veg-
<br />etation; high permeabiiity; and flat topography. Conditions
<br />that tend to retard soil deveiopment are cold, dry climates,
<br />grass vegetation, low permeability, and steep slopes.
<br />Alluvial soils are often thought of as being young or unde-
<br />veloped, but this is not always true [Gerrard, 1981]. Soils on
<br />river terraces are often interpreted as alluvial and considered
<br />young, but since the majority of river terraces are of Pleisto-
<br />cene age, many such soiis are well deveioped [Gerrard, 1981].
<br />Fot example, Madole [1991a] identified numerous alluvial ter-
<br />races up to 183 m above the present channel, but they have
<br />been dated to about 650 ka. Thus incision rates and time since
<br />inundation are important in developing flood chronology.
<br />4.2.2. Surface-rock weathering. Ratios of fresh to weath-
<br />ered material, the abundance of pitting, and pit depth on rock
<br />clasts are useful for subdividing Holocene deposits by an age of
<br />1000 years or more [Benedict, 1968].lt is assumed that abrasion
<br />of granite, rhyolite, and basalt clasts during flood transport
<br />removes most previous effects of weathering. The degree of
<br />surface-rock weathering of 25-50 of the largest flood-
<br />deposited clasts of the same lithology provides an indication of
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