Interpretation of debris cones or molards on rock-avalanches as degraded permafrost boulders

MILANA, J.P.

Leipzig

Congreso; Sediment 2011; 2011

Universität Leipzig

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.