Characteristics of Sturzstrom Deposits

Few direct observations exist of sturzstrom emplacement: Most eyewitnesses have been victims or were fleeing for their lives at the time. Reccurring features are the suddenness of slope failure and the high speed of the landslide. Additional features from emplaced deposits include:

(1) Deposits tend to appear as wide sheets, with peripheries that are often lobate or divided into tongues (Fig. 2); the surface area of a deposit is normally greater than that of the collapse scar, indicating that the landslide has spread during travel [Hsu (1975)].

Fig. 2 The subaerial Blackhawk landslide (black), 150 km from Los Angeles, California. About 300 million m3 collapsed 18 000 years ago to form a lobate tongue 10—30 m thick, 2 km wide and 7 km long that spread over the desert in the Lucerne Valley from mountains of gneiss (dark grey), sandstone and limestone (light grey) and breccia and conglomerate (cross-hatched). Modified from Shreve [1968].

Fig. 2 The subaerial Blackhawk landslide (black), 150 km from Los Angeles, California. About 300 million m3 collapsed 18 000 years ago to form a lobate tongue 10—30 m thick, 2 km wide and 7 km long that spread over the desert in the Lucerne Valley from mountains of gneiss (dark grey), sandstone and limestone (light grey) and breccia and conglomerate (cross-hatched). Modified from Shreve [1968].

(2) Deposits usually preserve their pre-failure stratigraphy, such that a pre-failure sequence of, for example, gneiss over limestone would yield a deposit of broken gneiss on top of disrupted limestone; evidently, mixing is rare between different levels of a sturzstrom [Erismann (1979); Erismann and Abele (2001)].

(3) The surfaces of deposits are fragmented, with debris ranging from fine grains to blocks the size of a house (Fig. 3) [Voight (1978)].

Together, these characteristics suggest that (1) sturzstroms do not behave as rigid blocks, but can effectively flow during emplacement; (2) deformation is concentrated within restricted horizons, against which adjacent layers can move with only a small amount of internal deformation, so inhibiting mixing between layers; and (3) fragmentation may exert an important control on dynamic behaviour. Although these key features have long been recognised, uncertainty has continued on the factors that favour such circumstances [see Melosh (1987); Kilburn (2001)]. Much of this uncertainty has been encouraged by the view that sturzstroms are exotic landslides and so require exotic mechanisms to explain their behaviour. An alternative view, which will be explored here, is that sturzstroms are a natural result of granular fluids being deformed along restricted horizons [Campbell (1989); Iverson (1997); Kilburn and S0rensen (1998); Kilburn (2001)].

Fig. 3 The Kofels deposit exposed (left) along the River Otztaler Ache and (right) near its toe. (Photos: C.R.J. Kilburn.)

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