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32 . ......~.... ".<<' " ~~....~ ..........,.;.........,a........._~~ .' .. C.F. WaYlhomas. R.D. }arr~rr I Geomorphology J J (1994) J 5-40 FBO deposits is lacking in the foothills west of TO along the north and south sides of Arthurs Rock Gulch (Fig. 2) and no granodiorite boulders were observed pr0- truding from Ihe colluvium upslope from FBO deposits, and (3) clast-fabric measurements made on FBO deposits indicate a strong preferred orientation, with clast B-axes oriented normal to the channel and a modal imbrication angle of about 50" suggesting fluvial trans- pon. Coarse bouldery flood debris on the TItetrace (FB I. Fig. 5c) records at least one flood. Boulders in FB 1 deposits exhibit considerable weathering (Table I). Natural exposures into TI reveal up to 60 cm of clast- supported gravel and boulders, resting on pebbly, matrix-supported colluvicrn. A thick soil is developed on the terrace gravel and colluvial deposits. This soil is characterized by a well-developed red-brown (7.5YR 3/4) Bt horizon (Table 3). The 1'2 terrace is about 2 m above the active channel, but is not continuous between Sites 3 and 4. FB2 depos- its consist of concentrations of granodiorite and peg- matite boulders that form linear bar-like features along the margin of the 1'2 terrace (Fig. 5c). The largest boulders (up to 1.5 m, B-axis diameter) are present only along the break in slope between the terrace tread and riser. Smaller boulders are present along the middle and back edge of the terrace; however. some of these boulders may be reworked from FB 1 deposits. Weath- ering data were not collected from FB2 deposits at Site 4 because equivalent deposits were bener preserved at Site 3. Paired FB 3 boulder bars are recognized at both the upstream and downstream ends of Site 4 (Fig. 5c). Small wood fragments (twigs, stems) collected from the contact zone between two large FB3 boulders yielded radiocarbon ages of 289 yr BP and 302 yr BP (Table 2). These dateS provide maximum limiting ages for the FB3 flood; however. the dated wood samples were small twigs and siems that were probably < 10 years old before being incorporated into the flood debris. Thus. these dates may actually closely date the FB3 event to about 300 yr BP (1655 AD). Several large granodiorite and pegmatite boulders are present within and immediately adjacent to the active channel. These deposits are mapped as FB4 deposits but are not associated with a terrace. FB4 deposits form imbricate pockets of coarse gravel and cobbles, and lack weathering or lichen cover (Table 2). Some of the FB4 sediments were probably reworked from older flood deposits. Table 4 Clast f:It Location 5.5. Site 5 Fluvial geomorphic features and flood deposits at Site 5 are the only deposits in the foothills zone of Arthurs Rock Gulch that were studied in detail. Most deposits are similar to those observed at Sites 3 and 4. The oldest flood deposits are the granodiorite boulders on the pegmatite weir (FBw) about 10 m above the active channel (Figs. 5d, 6). Some FBw boulders are imbricate. whereas others are present along the margins of a paleochannel that extends over the top of the peg_ matite weir. The paleochannel forms a shallow, rectan- gular notch in the pegmatite (Fig. 3) and exhibits a smooth, polished surface. FBw boulders have B-axis diameters of 0.4 to 1.5 m, are well rounded, and are considerably weathered (Table I). Other granodiorite flood boulders arefound on the downstream (east) side of the pegmatite weir and are additional evidence for paleoflood transport of boulders over the pegmatite weir, The age of the flood(s) that deposited the FBw boulders is unknown. FB I deposits are preserved above the margins of the present charinel at the base of a colluvial apron that slopes toward the channel axis (Fig. 5d). The boulders are likely reworked by colluvial processes but retain a crude linear alignment that is generally parallel to the active channel (Fig. 5d). FB2 and FB3 deposits are within 2 m of the active channel and are inset within the colluvial slopes that flank Site 5 (Fig. 5d). FB2 deposits are recognized at only one locality and are about I mn above FB3 depos- its (Figs. 5d, 6). FB3 deposits are longitudinal bars composed of cobbles and boulders, with strong particle fabric, but relatively low imbrication angles. SomeFB3 deposits are weathered as much as FB2 and FB 1 depos- its in the lower part of the basin, and are differentiated from FBI and FB2 deposits at Site 5 because they are found at a lower elevation. The fabric and weathering characteristics of FB3 deposits indicate that these deposits may have formed by debris-flow processes and consislofreworked flood sediments (Figs. 7.10). Two wood samples collected from FB3 deposits gave disparate radiocarbon ages. One of the samples yielded a radiocarbon age of ca. 509 yr B P (GX-I6694. Table 2) providing a maximum limiting age for the Arthun: ! FBI. ~ FB:!. ~ FB3. ~ FB4. ~ FB2. ~ FB3. ~ MLHe:"1 WescC::-: WestGr. Long Gu Turkey ( Bar; Bar: Bar 3 Tucker ( Site 2 Site 1. Nigel Pa Rocky Iv'. AlIagan. aMinUffil bReferen eAverae-t dOnes~ deposit (GX.j dard 14( in the] provid, Larg considf derswC:. deposit 5.6. C;, Eig: water-f (indue Fabric " '" -i -=-;::;:./. ~~t';~~_~..:..~-::~~ ~'.