Talus fabric, clast morphology, and botanical Indicators of Slope Processes on the Chaos Crags (California Cascades), U.S.A.

Abstract

<jats:title>Abstract</jats:title><jats:p>The Chaos Crags, a group of dacite domes in the Cascades Mtns (California), were affected by volcanic debris avalanches ca. 1675 A.D.; these left a sizable deposit and a scar on the north mountain flank, now covered by talus. This report examines the fabric and morphology of talus debris, their spatial variation, and the geomorphic processes presently affecting the slope. The talus presents a bi-segmented profile with a steep upper rectilinear segment and a shorter concave, basal zone. Debris are sorted by size both along (larger clasts downslope) and across the talus (larger particles below the cliffs). Shape sorting is weaker, but clast sphericity increases, and elongation decreases, toward the footslope. Upper-talus fabrics (long axes parallel to talus plane and slope) show that clasts there move by sliding, while basal blocks are deposited by rockfall, which causes more iso- tropic fabrics. Field observations and botanical evidence indicate the overall significance of grain flows, which are prevalent duetoanabundantsupplyofrubbleonthe upper talus. Comparison with similar recent slopes and repeat photography suggest the Chaos talus formed swiftly following dome collapse, when much debris may have collected below the unstable rockwalls. The talus has experienced only modest sedimentation during the 20<jats:sup>th</jats:sup>century, and is currently affected by 'normal' mass-wasting processes, which also include snow avalanches and debris flows.</jats:p>

Sediment movement rates and processes on cinder cones in the maritime Subantarctic (Marion Island)

“…Firstly, the mean size of the gravels transported (mean a-axis 44 mm ± 3 mm) exceeds the competence of unconcentrated slopewash. Observations of talus movement in the California Cascades by Pérez (1998), and experimental studies by French and Lewkowicz (1981) and Lewkowicz (1988) suggest that slopewash is likely to move only fine material in periglacial regions. Secondly, needle ice occurs frequently on Macquarie Island (Taylor, 1955a,b;Löffler et al, 1983;Löffler, 1983;Peterson et al, 1983;Selkirk-Bell, 2000), and Selkirk (1998) notes that painted clasts at the experimental site were turned over by freeze-thaw processes.…”

Depositional processes and particle characteristics on fans in the Southern Alps, New Zealand

“…This can probably be attributed to the lack of streamflow on the Hooker Corner fan. Another possible contributor to the down-fan increase in particle size on the Hells Gate and Hooker Corner fans is rockfall (Kotarba and Strömquist, 1984;Blikra and Nemec, 1998;Pérez, 1998). The clear down-fan decreases in particle size and angularity on the Mueller Hut fan (Figs.…”

Landform–vegetation units for investigating the dynamics and geomorphologic evolution of alpine composite debris cones (Valle dell'Avio, Adamello Group, Italy)

“…Furthermore, avalanches and enduring heavy snow cover exert strong mechanical stresses on plants that are often bent. This is particularly true in the case of shrubby and arboreal species such as Alnus viridis, Betula pendula, Larix decidua, Picea excelsa, Prunus padus and Salix appendiculata (Pérez, 1998).…”

“…These deposits are also characterized by isolated specimens of shrub and tree species (Alnus viridis, Betula pendula, Larix decidua, Picea excelsa, Salix appendiculata, etc. ), with trunks and branches leaning or broken, or with the typical sabre-shape due to the mechanical stresses caused by recent large-scale avalanches (Pérez, 1998).…”