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020" , '.',1\ , .1 'I-" and/or discontinuous) that has' what appears to b~ bioturbated sands. apparently occurring in voids resembling insect bur- ro\\'S; and an uppemlost. mixed and impure layer not as white as the lowermost phase. In the general area of the 1l1oulh of the unnamed tributary south of Comanche Creek, this layer appears to be restricted to low places in the topography. slIch as interdune areas and valley bottoms within the arroyo net- work. In the valley bottoms. beds pinch out toward the- edges of the arroyo branl:hes. Where present, the white layer COIll- manly undulates with the topography of the paleosurface on which it occurs; if depositional. this would indicate that it had been deposited at a time when gullies that had previously dis- sected the valley floors \"'ere essentially completely healed, The texture and composition of underlying scdiments does not seem to significantly influence the character or thickness of the layer. Typically, the layer unconfonnably overlies fine-grained alluvial and/or slope-wash deposits and is overlain by similar sediments, but locally it is overlain and underlain by relatively coarse, high-energy alluvial gravels. Its lower contact is con- cordant with underlying topography and is micro-irregular; the upper contact is slightly marc gradational and less abrupt, but is still quite dis.tinct. This upper contact appears to have undergone minor degrees ofmi.\ing and bioturbation. The white layer lacks desiL'cation cracks, oxidation. salt cT)'stals or pseudomorphs, indications of salt-induced turbation of sedi- ment, or root stmctures of obviously halophytic plants that are commonly associated with evaporite deposits. The material is significantly \vhiter in color than nearby relatively pure silt deposits. It is found over a wide enough range of elevation and variety of topographic seuings to imply that a ground\.\.':1ter- related origin is unlikely. In at least one locality (interdune area). a number of discontinuous, very white laminae, presum- ably of the samc material, are found within several cm above the primary layer. The clevation of the top of the white layer at selected outcrop points was mapped by R. E. Hunter during this snldy by hand-leveling relative to thc closest survey data points (at stratigraphic sections 2 and 4). The locations of the~c outcrop poinfs (shown as: s:mall black circles on figure 14) were deter- mined by tape-and-compass mapping relative to the surveyed points. The distribution of the white layer is considered to represent the topography of the surface at the time the layer was emplaced" The gully system at Lower Comanche is not sufficiently three-dimensional for a contour map ofthC' original topography to be made, but it is apparent from this mapping exercise that the upper elevation of the white layer descends southwestward Ii-om 795.52 m to 794.13 m (points A and B on figure 14, respectivcly) ova a distance of 37 III (a slope of 2.15U). This southwestward slope is the general direction in which the modem gullied land surface slopes and is probably the approximate slope direction of the original surt:1ce at the time of while-layer emplacement. In addition to the overall slope of the white layer. the exposures revealed several smaller features of its lopography. In the smallest notable topographic irregularity (point C on figure 14). the white layer was observed to be present in a Lower Comanche Area 17 preexisting scour deposit thatllleasurcd 5 Clll deep and 40 cm wide. The layer maintains its unifonn thickness throughout the depression. as well as on both rims. The white layer is dirt:ctly overlain by locally derived grm.e' that hils the scour. Allhough the white layer in this depression is directly underlain by silt or nne sand instead of gra\"el, gravel-filled scour depressions separated from one another by silt or sand occur at several horizons in the half-meter of section vertically above and bclow this depression. The location of point C was apparently a site for episodically mnning tributary flow, localized chan- nel cutting. and gravel transport. while at intervening times the channel was covered by finer sediment. The white layer at this location was emplaced at a time after a channel had been cut but before the channel had been hlled by gravel. Belween points DI and D2 (fig. 14). the while layer descends eastward into a large dt'pression filled partly by locally derived gravel. The depression, like the one (J! point C, was apparently a stream channel, although the opposite side of the channel is no longer visibh:: because it has been eroded by the present gully system. The channel was> I m deep and several meters wide. In contrast to the slllall channel at point C, IhlS larger channel was not complelely filled by gravel but instead migrated eastward as sand and silt beds prograded east- ward into the channel at dips Ihallocally approach the angle of repose. The white layer is both overlain and underlain by silt or tine sand on the flal west side of the channel margin (point DI J and on the upper slope of the channel. Near the channel center (at point D2 and northwest) the white layer is both overlain and underlain by gravel. Between points E I and E2 (and for some distance north and south of those points) the white layer dips \....estward as steeply as 25'" descending nearly 0.5 n1 in a distance of 1.0 m. If the exposures had extended fanher in an east-west direction. evcn greater elevation change might have been documented, as the dip angles were not observed to flatten out in either direc- tion. In this area. the white layer is both overlain and underlain by silt or fine sand. II coult! not be detennined whether the topo- graphic irregularity in this area was due to a channel to the west or a dune fonn to the e.1Sl. This site is located m the castem edge of the present arroyo, which may have existed for a long time. In aliaI' these topographic irregularities (points C, D I -D2, and EI-E2), the unifonll thickness of the white-layer horizon is suggestive of deposition of suspended sediment in a rebtively quiescent fluid (water or air) whose vertical extent was great compared to the vertical extent of the topographic irregulari- ties. Although recent studies have not shown major post-Pleis- tocene activity of the Tanner debris fan, 2. km downstream of LO\ver Comanche (Pederson and others, 2004), it is possible that a relatively minor Tanner debris flow caused short-lived impoundment of water in this area; othenvise. an airt111l origin of the white layer seems more likely. One problem with an airfall explanJtion is that the relations at point C would require that the small channel was first cut by water, then dried out and coated by Ihe airfalllayer and finally filled with Iributary gravel without disruption of the fine-grained white-layer material below.