Accretionary Wedge #23: That is not dead which can eternal lie

The March 2010 Accretionary Wedge is being hosted by Ed at Geology Happens, and here's the proposed theme:

"This AW is to share your latest discovery with all of us. Please let us in on your thoughts about your current work. What you are finding, what you are looking for. Any problems? Anything working out well?"

Well, considering even only the Holocene of the Alps there is a major hiatus of knowledge, and I can provide only a humble, but maybe interesting piece of considerations on the problem:

"That is not dead which can eternal lie,
And with strange aeons, even death may die"
H.P. Lovecraft (1890-1937)

A major unknown factor in rock glacier research is the formation age of these features. But for the understanding of climatic significance, and possible reaction of permafrost in a warmer climate, these information's are very important. Rock glacier develop mostly well about the tree-line, and in areas with strong debris accumulation, a difficult habitat for plants to growth. So in the rubble of rock glaciers organic matter, which would allow a 14C dating, is mostly missing.
Exceptions are known from the Swiss Alps, in the rock glacier "Murtèl" HAEBERLI et al. (1999) were able to date moss remains, and in an actual paper BOMMER et al. (2010) describe the discovery of wood fragments of larch in the front of an (in)active rock glacier.
Sometimes active rock glaciers override peat deposits, that can be dated, (EVIN & BEAULIEU 1985). Exposure age determinations of boulders on the surface, for example by cosmic rays or OSL are problematic because of the unstable surface that reworks constantly the material.
Evidence for a relative age can be supplied by the weathering of rocks, and lichen vegetation on blocks, but also here the moving surface cause troubles, the conditions of lichen growth and surface alteration change with time, so that the final date will be a mixture of different ages or at beast only an extreme value.

All these methods may provide only a maximum or minimum age, and are connected with a number of methodological problems; also an important question is what reflects these dates? The original rock wall exposure to erosion, the formation and deposition of the rock fragments in the talus, the mobilization of the talus by permafrost creep? And what if rock glaciers experience phases of inactive and active periods?

So it's no wonder that estimated ages of rock glaciers range from recent times, or formation during the last glacier high stands in the 16. and 19. century to interpretations of late glacial relics with ice 1.000 of years old (PALACIOS & VAZQUEZSELEM 1996; HUMLUM 1996; KÄÄB et al. 1997).

The rock glacier that I studied in the last years reaches with his front the Lazaun pasture (Ötztaler Alpen) and it is a possible good candidate for an extensive research on internal structure and dynamics of permafrost in the Alps.

Fig.1. The Lazaun pasture with the bog and in te background the active rock glacier. From the front of the rock glacier a glacial river flow from the left to the right, exposing a sequence of peat deposits and sand/pebble layers.

An extensive survey was carried out, geomorphological mapping, GeoRADAR, GPS and hydrological measurements. Also I tried to get some information's about the age of, so local legends tell, this petrified dragon.

A connection of the rock glacier with the glacier high stand of the alpine Little Ice Age is supported in part by lichenometry and moraine stratigraphy, which suggests an age of several hundred years. If, however, the measured GPS velocities are used to derive an age estimation (considering the length and the velocities of creep) the age ranges from 2.200 to 1.300 years. These observations relative the published ages of rock glaciers inferred simply by creep velocities (HAEBERLI et al. 1997), rock glaciers don't creep always at the same rate; reacting to climate change they display a complex behaviour.

A tentative approach to determinate the long term behaviour of the rock glacier is the interpretation of a stratigraphical column in peat deposits in front of the rock glaciers. The outcrops created by a glacial river expose a peat-sediment sequence in an alpine fen.

Fig.2. Outcrop, GeoRADAR measurements and soundings showed that the peat is pretty deep, up to 3m.

There were several peat layers (at least four) alternating with clastic sediments (sand and pebbles), also, as a small sensation, wood fragments where found and recovered (the actual tree line is lowered by climatic and anthropogenic influence by some hundred meters).
Similar sequences are known in the Austrian part of the Ötztaler Alpen, where they were interpreted as a glacier advance - glacier retread cycle. (BORTENSCHLAGER 1984). I myself observed similar sediments in the Rieserferner mountain group, so such records are not rare, and have much potential for future research on the Holocene history of the Southeast-Alps.

The sediments so maybe represents the varying climatic conditions in front of the rock glacier, during climatic favourable conditions moss and peat plants flourished and build up the peat layer, during cold periods the glacier and rock glacier advanced, providing more erosion and a source of clastic sediments that form the sand and pebble layers.

And because finally the wintertime is over, I hopefully soon will go back to the rock glacier and dare to interfere in his sleep...

Fig.3. Peat layers in transition to grey mud and pebbles.

REFERENCES:
BORTENSCHLAGER, S. (1984): Beiträge zur Vegetationsgeschichte Tirols I. Inneres Ötztal und unteres Inntal. Berichte des Naturwissenschaftlich-Medizinischen Vereins in Innsbruck. 71, 19 - 56.

EVIN, M. & BEAULIEU, J.L. (1985): Nouvelles données sur l´age de la mise en place et les phases d´activite du glacier rocheux de Marinet 1 (Haute-Ubaye, Alpes de sud francaises). Mediteranee. 4: 21-30
HAEBERLI, W.; KÄÄB, A.; WAGNER, S.; VONDERMÜHLL, D.; GEISSLER, P.; HAAS, J.N.; GLATZELT-MATTHEIER, H. & WAGENBACH, D. (1999): Pollen analysis and 14C-age of moss remains recovered from a permafrost core of the active rock glacier Murtèl/Corvatsch (Swiss Alps). Journal of Glaciology 45: 1-8
HUMLUM, O. (1996): Origin of Rock Glaciers: Observations from Mellemfjord, Disko Island, Central West Greenland. Permafrost and Periglacial Processes 7: 361-380
KÄÄB, A.; HAEBERLI, W. & GUDMUNDSSON, G.H. (1997): Analysing the Creep of Mountain Permafrost using High Precision Aerial Photogrammetry: 25 Years of Monitoring Gruben Rock Glacier, Swiss Alps. Permafrost and Periglacial Processes 8(4): 409-426
PALACIOS, D. & VAZQUEZSELEM, L. (1996): Geomorphic effects of the retreat of Jamapa glacier, Pico de Orizaba volcano (Mexico). Geografiska Annaler 78A(1): 19-34
SCAPOZZA, C.; LAMBIEL, C.; REYNARD, E.; FALLOT, J.-M.; ANTOGNINI, M. & SCHOENEICH, P. (2010): Radiocarbon Dating of Fossil Wood Remains Buried by the Piancabella Rock Glacier, Blenio Valley (Ticino, Southern Swiss Alps): Implications for Rock Glacier, Treeline and Climate History. Permafrost and Periglac. Process. 21: 90-96