INVESTIGADORES
MILANA Juan Pablo
congresos y reuniones científicas
Título:
Interpretation of debris cones or molards on rock-avalanches as degraded permafrost boulders
Autor/es:
MILANA, J.P.
Lugar:
Leipzig
Reunión:
Congreso; Sediment 2011; 2011
Institución organizadora:
Universität Leipzig
Resumen:
Molards, or
debris cones on top of rock-avalanche have been observed in a large number of
landslides since more than a century. As a result of they are conspicuous they
were termed molards in the Alps, and there are no less than six different
origins proposed. Molards are observed in many rock avalanches all over the
world, however they tend to be more common in glacial-periglacial settings. A recent
rock slide and avalanche at the Chita creek created a large rock-avalanche
deposit characterized by lobes completely covered of molards. The study of characteristics
suggests molards originate from the fragmentation of frozen ground like debris-rich
permafrost, during the transport process of a rock-slide, and subsequent
ice-melting and degradation of the original boulder into a colluvial full-cone.
The first evidence comes from the protolith: Aerial photographs of 1966 shows
this tributary valle had an active Rock Glacier (RG), whose talus base was
located near 4430m of altitude. The valley was like that until 1996, while it shows
the landslide since 2003. The most likely explanation for the timing was the
excessive melting of the permafrost top of the rock glacier due to two
consecutive years without snow and hence water-ice input to, followed by a
heavy snowy year that placed extra weight on the RG. As a result, 6.4 Hm3 of
rocks slid down from the RG front, and that block broke up in boulders during the
1420 m
of transport. The rock avalanche was deposited over an alluvial fan, and it is
possible to see 4 main lobes, of progressive less relief, probably by an
increase of flow lubrication to the end of the flow. Lobe tops are full of
molards, and many of them are also lying isolated on the vegetated ground. Clearly
they were boulders that rolled down from the lobe front and stopped at
different distances. Distant ones are 100 m far from lobe edge at the slope direction,
while molards aside lobes are no more than 30 m far.
Many
molards were excavated, and the best information comes form one molard 7 m tall and almost 10 m wide, that was eroded by
the creek. In the internal structure was possible to observe that the core was
formed by a quite heterogeneous sedimentary fill, quite inclined, and two
bedding types were recognized: well stratified colluvial gravel and debris-flow
deposits, both typical of cryogenic RG. The colluvial sediment was quite clean,
open and with no possibilties to keep the boulder cohesion while the other
sediment type could have kept the cohesion. On top of the boulder core and
draping all margins, there was a colluvial
microtalus in which the boulders fell until the base of the cone, while
the fine-grained sediment was staying at the draping slope, creating the idea
the sediment was finer grained (as described) than boulder and hence not coming
from it. This was a colluvial sorting process well known in sedimentology.
Therefore, all cones are the orthogonal projection of c. equidimensional
boulders on the ground. The different slopes of the cones are due to (a)
different shapes of original boulders something we could control at this
relatively young rock avalanche and (b) time ellepsed for microtalus colluvial
transport, As there is not successive material added to these talus cones, the
tendency with time is to be a flatter cone, a fact we controlled with another
RG slide involving permafrost but looking much older.
This landslide provides very strong evidence for
the interpretation of molards in periglacial environments as degraded
permafrost boulders. However, they could also be formed by cohesive soil
fragments transported semi passively atop slides that weather afterwards. Molards
could therefore serve to understand the transport modes of rock-slides to avalanches.