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18 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River At this lime. the origin arthe white-layer horizon is undetermined. Deposition as a pedogenic horizon is con- sidered unlikely. because of the unifomlity oflhickness. the consistency of its characteristics in sediments of significantly variable textures and depths, the abrupt character of its lower boundary, and the topography of the bed. Although pedogenic CaC01 can occur in lenticular masses having abrupt upper and lo~'cr contacts. this typically is not found in early-slage pedogenic carbonate accumulation. Likewise, thickness and degree of cementation typically vary over the catena; this horizon does not resemble pedogenic horizons observed by members of this research group in other soils or Holocene deposits from desert environments. Similarly, deposition in an evaporitic environment can likely be eliminated on the basis of the absence of desiccation features, oxidation, soil weathering. and groundwater phenomena, and the relation to topography, The wide distribution, topography, and elevational range of the deposil, and the powdery characler of Ihe (apparently CaCOJ) material indicate that deposition in standing water is unlikely. Likewise, the fine grain size and physical characteristics of the deposit appear to eliminate deposition by running water (other than, perhaps. minor redeposition in a slopewash envi- ronment), Physical attributes of this horizon resemble those of air-fall deposits such as vulcanic ash (including the fact that it is significantly purer at the bottom), though its dominantly car- bonate composition does not. One major ditTerence between the white layer and known volcanic ash layers, however, is the apparent degree of reworking and redeposition near the toes of alluvial slopes and alluvial t~lIlS that is lIsU:llly ohserved in ash deposits and is absent in the white layer, Archaeological Significance Multiple cultural features on river left in the Lower Comanehe region have been dated to Ihe Late PI - Early PII Fonnolive period (Fairley and olhers, 1994; National Park Service. 2004). Additional sites contain artifacts that are likely related to prehistoric and early historic Hopi habitation -(Fa'irley .-1I1d othcrs, 1994). and at-Ieast- o'~e site is of ur~kno~v~ cultural affiliation. This area is significant in that it contains a higha concentration of roasting features than any other area in the ea~tem canyon. The sites are devoid of any Puebloan- type structures. Archaeologists infer that this area could have been occupied by non-Puebloan people; the features present at LO\'.:er Comanche are more representative ofCohonina or prehistoric Puebloan occupation, This area appears to have heen used fur processing of food and not for direct habitation; living areas were perhaps located across the river. upstream. or downstream of this region. Monitoring reports trom NPS that date back 10 Ihe 19~Os have identified aeolian deflation, surface erosion. and gullv incision as the greatest impact agents at archaeological sit~s in the Lower Comanche area (National Park Service, 2004). Because the majority of the sites are located in the large dune field, aeolian erosion by deflation and dune migration are par- ticularly influential. The condition of archaeological sites at Lower Comanche has also been impacted to a lesser degree by visitors; the Beamer hiking trail passes directly through this area. Stratigraphic and Geomorphic Analyses Detailed stratigraphic logs for the Lower Comanche area are described in oppendix 2 (figs. 40 through 46). Stratigraphic sections 1,2.4,5. and 6 are located in vertical exposures of sediment that tonn the walls of the arroyo network upstream of the tributary drainage. Section 3 is a pit dug into a flat land surface -270 m north of the arroyo area where the other five sections were recorded. Geomorphic characteristics in the immediate area surrounding each archaeological site are dis- cussed in appendix 2 (figs. 47 through 49). Stratigraphic data and geomorphic observations are summarized in the section below titled "Lower Comanche Site Summary." Grain-Size Analyses The results of grain-size analyses from sediment samples collected from section 2 at Lower Comanche are shown in figure 17. When plolted superimposed on fields that represent the range of grain.size distributions for fluvial and aeolian samples whose depositional environment could be ascertained by field ohserv,Hion, both snTllples fwm section 2 (units C and F) plol within the Auvial field. Unit C was interpreted in the field as dominated by tributary alluvium, and unit F as the edge ofa channel-fill horizon. The similarity in particle- size characteristics between these samples and known fluvial deposits indicates that thcse units may also include (and may aclually be dominated by) interbedded or reworked Colorado River sediment, a result of the complex interaction between fan and fluvial processes. Two of the three samples from sec. tion 6 at Lower Comanche whose depositional environment was uncertain in the field also t:11l into the 'fluvial' range (fig. I S). A -sample .collected from unit B of section 6. a horiion where faint fluvial climbing ripples appeared to be present. and a sample from unit F of section 6, where faint fluvial climbing ripples appeared to be present in the upper part of the deposit, both plot within the Auvial field. These samples are finer grained than a sample from unit E of section 6, which WilS dctennined in the field to be a flood deposit on the basis of readily apparent Auvial climbing ripples. Driftwood Elevation Survey: Lower Comanche Stage-Discharge Relationship The location and elevation of drithvood logs \"'ere sur- veyed in the Lower Comanche area on both banks of the river. In the 'ow~r elevation areas on river left at Lower Comanche (near sections 5 and 6 described above), dense vegetation