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6 Sedimentology and Stratigraphy 01 the Palisades, Lower Comanche, and Arroyo Grande Areas 01 the Colorado River Grain-Size Analyses Grain-size analyses ofs~diment samples provide infomla- tion about sedimentary dynamics thalcanno! necessarily be dctennined in the field. Grain-size data call. in some cases. also facilitate identification of the depositional environment of certain stratigraphic units. Although variation may occur between the grain-size distributions of sedimentary units deposiled by Ihe same lype of el'ent. and these data are Ihere- fore not necessarily representative of every deposit with a similar event history, 36 samples with known depositional environments that were analyzed during this sludy yielded dislributions with little enough overlap between. for example. fluvial and aeolian fields that this technique may be considered lIseful in identifying a likely depositional environment for samples without diagnostic sedimentary stmctures. The grJin-sizC' distribution of ftood deposits is detemlined by the concentration and grain size of sediment available in the flow at various stages of the flood event~ concentration and grain size are in tum detennined hy the nature and intensity of the event that inillated the ftood (main-stem river snow-melt Aood vs. tribut:lry flood, for example) and the source area from which the sediment is derived. The latter is influenced in part by channel morphology; ill areas where a river flo\\'s over an alluvial floodplain at high discharge, the flow may entrain fine- grained floodplain sediment that is not available as a sediment source when the river is confined to its main ch;mneJ at lower discharge. Within aeolian deposits, grain-siz~ distribution depends upon such factors as the intensity and duration of the wind, which dt:tennine the degree of winnowing and sorting that occur, as \....ell as the composition and gri'lin size of sand in the source region that provides sediment to the aeolian deposits. Slope-wash deposits contain probably the greatest range of possible sedimentary characteristics: their local derivation leads to substantially less "maturity' orthe resulting deposit compared to flll\';.J1 or aeolian strali'l, in which grains have typically undergone extensive transport before deposition. The grain-size distribution and sediment composition of slopc- wash deposits depend on the intensity. location; and duration of the precipitation event Ihm generated thelll. as well as the lithology of source rocks that produce the sediment Local precipitation often transfomls the landscape in ways that alter characteristics of the stratigraphic record where sllch deposits are found; rill and gully fonnation and subsequent filling can result in channel-fill stmctures and lens.shaped slope~wash deposits. Sediment samples were analyzed for grain-size distribu- tion using a Beckman Coult~r LS 100Q laser particle~size analyzer linked to a LS variable-speed fluid module. The Beckman Coulter LS IOUQ computes particle-size distribu- (ions based on the Fratlflhofer laser diffraction theory. which statl:s thal small particles in the path ora laser \vill ditTract the light in a known, symmetrical pattern. The angle of ditTraction iO\'crsely corresponds to the size of tht., particle. This instm- ment uses 126 detector rings to measure the pattern of dif- fraCled light intensities as a function of the angle from the axis of the original laser beam in order to compute the particle-size distributi~n. par'ticles from 0.375 11m 1O 948.2 /-llll in diameter can be measured in this v...'ay (http://www.coulter.com/prod- uClslinstrument/partCharlpc _ls230.asp). Sample preparation was minimal because of the over- all lack of organic material and relative coarseness of the samples. negating the need for hydrogen pero.xide (H,O) and sodium hexametaphosphate ((NaPO,) ) treatments. Deion- ized water was added to the samples' i~l a small beaker. and tht., beaker placed on a stir plate for agitation. A representative subsample was then extracted fr0111 suspension in the beaker using a pip~fte. The sediment and WJter mixture was added into the Coulter until the optimum obscuration range (8-12 percent) was reached. This process was repeated up to four rimes for each sample; fhe average of Ihe 4 nllls was used in subsequent Jata analy~is to minimize potential subsampling errors. Grain-size distribution is reported as percent by \'olumc and assumes spherical particles. Palisades Area Geomorphic Setting The Palisades (Palisades oflhe Desert) area (fig. 2) lies within Ihe fluvial geomorphic reach described by Schmidt and Graf ( 1987) as the Furnace Flats reach. Bedrock at river level in this area is the Do.\. Fomwtion, a series of interbed- ded fin~-grained sandstones and shah~s of Precambrian age that commonly display a characteristic red to nearly purple color (originally named the Do\ Sandstone' by Noble, 1914. this unit was redescribed by Stevenson and Beus, 1982. as the Dox Fonnation 011 the basis of its varied lithology). Sandstone beds of the Do\ Formation are locally olTset by major fauli displacement along the Palisades segment of (he Butte Faulf, a northwest-trenJing structure related to the east Kaibab Mono- cline that crosses the river in this area (Huntoon and others, 1996). .Various surficial deposits are present on the wide, rela- tively flat expanse downstream of Palisades Creek. At least three episodes of Holocene debris-fan activity have been inferred for the Palisades Creek debris fan on the basis of stratigraphic relationships and repeat photography (Her- eford. 1993; Hereford and others, 1996). the youngest having occurred between 1965 and 1984. Additional. undated recent debris-flow activity is apparent in leveed, unvarnished debris- fan deposits on the tributary north of Palisades Creek. Terrace morphology in the l:enlral Palisades area is attributed to fluvial deposition. Hereford (1993) mapped four alluvial terrace ? 00.\ Sam.btone remalllS the official USGS name of this unit. fhlS report 10lJou's the use of. Dox Fonm:lIion' J/JlrodlJcl'd by Stevenson and Beus (1982\. ba:,.ed on the shale-rich lithology of tills Untt in the study area.