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02064 . Appendix 2: Stratigraphic Sections and Geomorphology, Lower Comanche Area 45 pinches out laterally into gravelly alluvium. This surface locally has 20-30 em of topographic variation in this exposure. The lo\\<'er boundary appears to be confonnabJe. and there is no evidence of erosional activity (lnmcation oflaminae). The upper surface is. however. eroded. which probably accounts for the absence of the upper 1\'..'0 phases of this white-layer unit. Irs thickness varies between 0.5 and 1.5 em locally. 79 - 83 em (Unit E): This is one of several westward (riven\t"ard)-migraling channels representing paleoarroyo feafllres in this locality. Thinly bedded silt laminae are present. primarily horizontal. though inclined at the edge of the channel. Silly nne sand occurs where this stratigraphic section was measured, hut this interbeds laterally with channel gravels locally d~rived from the Dox Fonnation. Th~ lower contact of unit E is abl1lpt. representing one stage in channel filling, but does not appear to be significantly erosive. This unit, \\ihich occurs as a horizon as much as 50 crn thick in a nearby channel-till exposure. is the edge of channel-fill deposits. 83 - 87 em (Unit F): The facies within unil F strongly resembles unit E above (channel-fill material). with the exception thar cross-stratification is ob.sen'ed in the channeJ- fill deposits. This stratum also grades locally into channel deposits derived from the Dox Fonllation and locally reaches 50 em in thickness. One sample was collected from unit F for grain-size analysis: Com Strat 2. Unit F (samplc depth not recorded). 87 - 132 em (Unil C): The texture of unit G is primarily fine sand but with occasional small pebble-gravel lenses. The lowermost deposits ofrhis stratum (\....hich arc generally not observed) are gravel. Sediments at the top and bottom of unit G appear to be slightly cemented and are redder in color than the lower deposits of this unit. This horizon represents channel fill and in places is signific~H1tly thinner than at the recorded section location. 132 - 135+ em (Unit H. 10 base of section): This 10\Vennost stratum contains vcry fine sandy channel-margin deposits. These are thinly bedded, with laminae in some places and indistinct bedding in others. These sediments are interpreted as tributary alluvium and appear to have been modified by wetting and drying related to ground water, producing contorted liesegang-like oxidation bands in some areas. This IVould imply a relati,ely high water lable, which is difficull to explain in this particular location. Section 2 sllmmary.- This profile represents various facies of tributary alluvium deposited in and adjacent to a series of eastward (shoreward)-migrating channels, which may interbed with sediment originally derived from fluvial and/or aeolian deposit~. The white-layer horizon \....as deposited at a time when the deepest channels had largely filled but a gravel-bottomed channel still persisted adjacent to the bedrock valley wall. Additional tributary alluvium was deposited above the white layer, although associated channels are not as apparent or as deep as those from earlier times (at least until the modem arroyo-cutting episode). Sediments contained within this alluvial stack are unusually fine. and. \'v'ith the exc.:cption of gravel-bottomed channels, contain very limited amounts ofbedrock-d~rived sediment. The source of alluvial sediments is believed to be reworked aeolian deposits from upstream and valley-slope areas. The relationship of these sediments to modem coppice accumulations is not clear. Lower Comanche, Section 3 The pit from which section 3 was described is shown in figure 42. Figure 43 shows the stratigraphic profile for section 3. Measurements are in ern below the grollnd surface. Surraee: This profile is a pit dug into a ftal land surface wilh sparse vegetation. The surface shows evidence of recently ponded water (abundant mudcracks) over:J large area, and crust formation 2-2.5 mm thick. Minor mud curls are pres~nt; cryptogamic development is present but weak, and short grasses (chem grass) have colonized the area. 0-5 em (Unit A): lIpperolOS! sediment in this section contains soft. very friable (to single grains) coarse, weakly subangular, blocky nne sandy silt. Sand content is estimated to be approximately 25-30 percent. with the remainder silt. Considerable bioturbation, primarily by rootlet growth. has obscured most sedimentary structures. Minor cross- lamination is visible, but this unit is generally massive. The lower contact is clear to sharp, and wavy. An aeolian origin is infen'ed; the llnit ::lppears to be a loose aeoli.an cover on the surt:1ce. 5 - 20 em (Unit B): Unit B is a silty fine sand that contains subhorizontal. paper-thin laminae throughout that may be related to wetting episodes. Sand content is estimated to be -75 percent. with the remainder silt. Sediment is friable, and is slightly lllore consolidated than unit A. No cross- bedding is visible. The lower contact is somewhat arbitrary, and has been placed af 20 em. in il zone where silt lamill::le become more prominent and frequent. These laminae may be remnants of a wetted surface, weathering out to appear more consolidated than the sediment above or below. Unit B is interpreted as a likely aeolian deposit on the basis of its texture and lamination. This may represent aeolian deposition with periodic wetting. 20 - 40 em (Unit C): Lamination in unit C shows some disturbancc attributed fo biowrbation in the upper 3 em of this horizon. There is a broad channel in unit C with a maximum depth of 8 cm that has lamination in the channel- fill sediment (cross-lamination is not present). Scattered pebbles and granules occur in the channel-nil material, which may represent a deflation or lag deposit. Silt laminae are present at the base of the channel fill, but they become indistinct toward its edges. Sediment in the channel is dominantly soft, fine sand and silt with occasional pebbles and granules. Bedding both inside and oulside the channel