Laserfiche WebLink
<br />w <br /> <br />f <br /> <br />C.F. WayrhomaJ'. R.D. Jarrett/Geomorphology I J (/994) /5-/.0 <br /> <br />(c) <br /> <br />37 <br /> <br />T2-FB2 Phase <br /> <br /> <br />F~ g. 13. Schem:uic ftuvilll-geomorphic reconstnlctions of sites 3 and 4. showing the disuiburion of terrace and Bood-boulder deposits. (a) TO. <br />F30 phose. (b) T1-FB 1 phase. (eJ T2-FB2 phose. <br /> <br />were deposited by a flash flood(s) sometime after for- <br />mation of the TO terrace (Fig. 13a). ' <br />Afte, the formation of TO and deposition ofFEO. the <br />c":umel was incised and the Tl terrace was formed <br />(Fig. 13b). FE I deposits are found only along the outer <br />margins ofTI (Fig_ 13b) and are not part of the thin <br />gravel wedge that forms the terrace deposit. Because <br />FE I ::.,ulders are on top of TI terrace gravels at Sites <br />3 and~. we believe the deposition of FE I postdates the <br />formation of Tl. I <br />Cvntinued incision and erosion of T1 terrace depos- <br />its ai'Or the FE I flood and subsequent alluvial deposi- <br />tion resulted in the formation of the T2 terrace (Fig. <br />13c). FE2 flood boulders were deposited on the T2 <br />terrace as coarse overbank deposits. Overbank (slack- <br />w.te(~) sediments consisting of silty fine sand cap the <br />T2 terrace and indicate episodic vertical accretion, <br />probably by smaller floods. These deposits are tenta- <br />tively correlated with the organic-rich silty alluvial till <br />underlying FE3 deposits at Site I (Unit B, Fig. 8) <br />because they occupy similar stratigraphic positions <br />within the alluvial sequence. <br /> <br /> <br />...,. <br /> <br />Further incision and lateral erosion after deposition <br />ofFE2 was limited by the annoring effect ofFB2 flood <br />boulders which were probably too large for transport <br />by post-FE2 floods. Thus, the T3 terrace and FE3 sed- . <br />iments probably formed only where FE2 deposits could <br />be removed by fluvial erosion. <br />Stratigraphic relations at Site I indicate that a flood <br />(FE]?) occurred sometime after deposition of unit B <br />which is at least 297 years old (Fig. 8). We mapped <br />the flood deposits (Unit C, Fig. 8) as FE3 deposits <br />(Fig. Sa) on the basis of their position above units A <br />and B which, in turn. we think overlie FE2 deposits at <br />Site 1. FE3 deposits at Site 4 are ca. 295 years old, <br />indicating that FE3 deposits at Site I must have formed <br />soon after unit B was deposited. Radiocarbon dates on <br />unit B at Site 1 are stratigraphically inverted; however, <br />the dates are statistically equivalent since they overlap <br />at I standard deviation. Because we interpret unit B as <br />reworked colluvium formed by gully erosion in lhe <br />upper part of the basin. probably upstream from Site 5. <br />the dates may reflect the sequential erosion and rede- <br />position of colluvial sediment. <br />