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4 ,'., ....._......~....v>.w___~ CF. Waythomas. R.D. Jarrell I Geomcrphology 11 (199-1) 15-40 'ell-exposed. However, with the passage of time, both ebris-flow and flash-flood deposits may be modified y the combined effects of weathering, colluviation, nd removal. of interstitial sedimen~ As a resul~ the hysical characteristics of the deposits become more like. Criteria especially applicable to older deposits re needed. In this study we auempt to differentiate ash-/lood from debris-/low and other mass-wasting eposits using the clast fabric of the deposi~ Clast fabric data from deposits of known origin are )mpared with clast fabric data from Anhurs Rock 'ulch flash-flood deposits. Data on clast fabric were )tained by measuring the azimuthal orientation and p of the A-D plane (A-axis = long axis, D- :is = intermediate axis) of disc-shaped cobbles and )ulders. Twenty-five random measurements were ade On each deposit during a traverse along the :posit axis. Clast-orientation data were analyzed by otting the poles normal to A-B planes on a lower :misphere equal-area projection (Schmidt projec- )0) from which eigenvalues were derived by com. lter (Woodcock, 1977). Three normalized genvalues (SI. .52. S3) provide a measure of the ,gree of clustering of the poles to A-B planes about eir associated eigenvectors (VI, V2, V3). The eigen- ctors.ndicate the predominant orientation of the .Ies to A-B planes. The eigenvalue SI is a measure of , degree to which poles to planes cluster about the ~envector Vi and is equivalent to the mean vector 1ark. 1973; Woodcock. 1977). Eigenvector V3:is ..... rmal to VI. and S3 is inversely proponional to SI iloodcock. 1977; Lawson, 1979). If the deposit hibits a high degree of preferred orientation the poles planes will, be parallel or tightly clustered and Sl II approach 1.0, whereas S3 will approach O. Fur- ,"nore, the natural logarithm of the ratio 51 I S3 also Iicates the degree of preferred orientation and values 1.0 indicate random or uniform distributions and ,ater values (> 4) indicate increasingly stronger grees of preferred orientation (Woodcock, 1977). Methods for graphical representation of eigenvalues , discussed by Woodcock (1977). We use a two- .s plot of the eigenvalue data to illustrate fabric dif- ences for debris-/low deposits, /lash-/lood deposits, j deposits from Arthurs Rock Gulch. The plot pro- ies a method for investigating the origin of deposits ose mode of formation is unknown or uncemin. This differentiation is needed for subsequent assess- ment of discharge estimates from Coarse particles. 5. Flood geomorphology and chronology 5.1. Sire 1 Site I is located near the mouth of Arthurs Rock Gulch at Horsetooth Reservoir, in the valley between two north-south trending hogbacks (Fig. 2). Grano- diorite and pegmatite boulders record at least four paleo/loods (Fig. Sa). Flood deposits are differentiated by their maximum height above the active channel, radiocarbon ages. and by their weathering characteris- tics. The oldest flood. deposits (FBI. Fig. Sa) are about 6 m above the active channel (Fig. 6). FBI deposits are c1ast-supponed and lack a fine-grained ( < 3 mm) matrix. All FB 1 granodiorite boulders are weathered and show considerable surface pitting and grain relief (Fig. 7; Table I). FB2 deposits are limited to one locality near the base of the sandstone hogback on the east side of Site I (Figs. 4.. Sa) and form a crescent-shaped boulder bar about 1.7 m above,the active channel. FB2 deposits are little weathered in comparison to older FB I deposits (Fig. 7; Table 2). However. the weathering character- istics of FB2 boulders probably do nOt reflect the true relative age of the deposit because most of Site 1 is beneath the waters of Horsetooth Reservoir at least a month or more each year (Fig. Sa). Episodic submer- gence and wave action on FB2 deposits has removed overlying sediment and weathering products, and FB2 granodiorite boulders appear fresher than other FB2 granodiorite boulders in Arthurs Rock Gulch. It appears that FB2 deposits were buried by fine-grained alluvium after a brief(?) period of subaerial exposure and weath- ering and were recently exhumed by wave action at the margin of Horse tooth Reservoir. FB2 deposits are strat- igraphically and topographically beneath radiocarbon- dated alluvium and FB3 deposits (Fig. 8). FB3 /lood sediments (small boulders, cobbles and gravel) are preserved on the upstream end of an alluvial terrace (T3, Fig. Sa) along the right (south) bank of Site I. These sediments were examined, but no weath- ering data were collected because granodiorite cobbles are absen~ The stratigraphy of the T3 terrace deposits was examined in a hand dug trench that exposed about _. r - . - , , , " ~z..:.;~' - ~~, , , z o I- <t Z <t ...J 0.. X UJ \~,).-; .~~::;:. ;::::~'.".;~ , 'J < ,..., J:'=::: ~. ~ UJ ~ co - ~-;'~.,