Annals of Glaciology 2 1981 © International Glaciological Society MORAINE IN-TRANSIT AS PARENT MATERIAL FOR SOIL DEVELOPMENT AND THE GROWTH OF VALDIVIAN RAIN FOREST ON MOVING ICE: CASA PANGUE GLACIER, MOUNTTRONADOR (LAT. 41 010'5), CHILE by Jorge Rabassa, (Department of Geography, Universidad Nacional del Coma hue, Av. Argentina 1400, 8300 Neuquen, Argentina) Sigfrido Rubulis, (Hidronor S. A" C.C. 1207,8400 San Carlos de Bariloche, Argentina) and Jorge Suarez (Argentine Institute of Nivology and Glaciology, C.C. 330, 5500 Mendoza, Argentina) ABSTRACT polated over the last 45 a, the trees have Casa Pangue glacier is a regenerated moved an average distance of at least 1 000 m valley glacier, covered by supraglacial moraine in that time. of variable thickness (1 to 3 m). Seven The discovery of established communities moraine in-transit arcs have originated from (also observed at Rfo Blanco glacier, south- avalanches falling on the ice from the steep west of Mt Tronador) on moving ice constitutes walls at the head of the valley. The debris has an event not previously recorded in glaciologi- been relocated by glacier movement, and vegeta- cal literature. Some significant aspects are tion has become established on areas with discussed: time lapse for soil development on sediment cover. Primitive soil has developed moraine deposits, primary plant succession on very rapidly, probably in less than 100 a. moraine deposits, the need of stable ground for Chemical analysis of soil samples show that it plant colonization, the absolute validity of is acid (pH 4.6 to 4.7), nitrogen and organic radiocarbon dating of glacier advances using matter are absent, calcium content is low, and fossil wood found within terminal moraines, and potassium content is high. Pedogenic processes the validity of lichenometric techniques in de- are mostly related to intense chemical weather- glaciated terrain. ing, under conditions of more than 4 000 mm a-l of total precipitation. 1. INTRODUCTION The plant communities established on the In only a few places in the world, trees moraine are essentially similar to the Va1divian have been observed growing on supraglacial till rain forest that characterizes the region. The and, in most of them, the ice has been consider- most frequent tree is "coihue" (Nothofagus ed as dead or stagnant (Russell 1893, Tarr and dombeyi), followed by "teniu" (Weinmannia Martin 1914, Odell 1948, Miller 1957, Sharp trichospeY'ma) and "notro" (Embothrium coccineum). 1958, Clayton 1964, Post 1967, Porter and Carson One specimen of N.dombeyi was found to be 18 m 1971, Post and Streveler 1976). In these tall and 0.62 m in diameter; many others were examples, glacier ice has lost its mobility and taller than 12 m and reproduction was abundant. it provides a rather stable substratum for Dendrochronological analysis indicates a minimum pedogenic development and vegetation growth. age of 45 a for the oldest tree. Other plants This stability isdisrupted only by melting of found here are shrubs, some hemiparasitic and the ice below the til I. However, there is a creeping plants, mosses, lichens, ferns, and reference to the Agassiz Glacier, Alaska (Seton fungi. Karr 1887: 85) as being an advancing glacier Using air photographs of 1942, 1953, 1970, with a thick forest growing along the extremity and our own oblique photographs of 1978, taken of its ice-cored, moving, terminal moraine. from the summit of a neighbouring peak 1 200 m (We thank Dr Austin Post for this reference. above the glacier tongue, average surface veloc- Dr Post also mentions that this moraine is now ity of the glacier tongue was estimated at 22.5 a stagnant portion of the Malaspina Glacier m a-l for the lower and intermediate sectors (written communication).) Reid 1969[bJ, 1970[a], where the trees are growing. If the total 19701p.l descri bes trees growi ng on the supra- length of the regenerated valley glacier is glacial drift of Martin River Glacier, Alaska, taken into account, the mean annual movement on a partially active intermediate zone. rate is 35.0 m a-l. If this value is extra- Dr Stephen C. Porter also informed us that he 97 Downloaded from https://www.cambridge.org/core. 29 Sep 2021 at 09:31:48, subject to the Cambridge Core terms of use. Rabassa, Rubulis, Suarez: Moraine in-transit, Casa pangue glacier, Chile and Dr Giuseppe Orombelli found 1.5 m tall larch trees growing on an active glacier in the GLACIAR Italian Alps (written communication). Matthews CAS A PANGUE (1973) reports the finding of lichens growing on medial moraines or active supraglacial debri~ Heusser (1972) describes mosses growing on a thin film of dust or stones covering the bare ice of Gilkey Glacier, Alaska. During field work for the glacier inventory of the northern Patagonian Andes (Rabassa and others 1980)we had the opportunity of climbing the international boundary range, immediately north of Mt Tronador, and of observing the total extent of Casa Pangue glacier. The surface of this debris-covered glacier was found to be covered by tree communities, which,although scattered allover it, were concentrated on several moraine in- transit arcs. In April 1979 the glacier was visited and the three outermost morainic arcs ~1 11)'S studied. These arcs are composed of supra- glacial till and they display the growth of well-adapted, extremely healthy tree / CHILE communities. Trees and other plants also ",/ ARGENTINA extend over the surface of the glacier between arcs, although the density of vegetation is much lower. Air photographs show that tree communities also grow on Rfo Blanco glacier ~outh-west slope of Mt Tronador) but in this Fig. 1. Casa Pangue glacier. Map compiled from case they extend more~uniformly over the 1970 air photographs. References: surface. Access to R10 Blanco glacier is (1) Granodiorites. (2) Tronador Formation: difficult and it has not been possible for us to volcanics and pyroclastic rocks. (3) Neo- visit it yet. Nevertheless, the general glacial moraines. (4) Ice-cored moraines. environmental conditions and glacial processes (5) Glaciofluvial plains. (6) Moraine in- look very much the same and show that the case transit arcs. (7) Ice and snow avalanches. of Casa Pangue glacier is not unique. (8) Landslides and debris avalanches. (9) Ice This paper presents the finding of well- movement. (10) Local ice shed. (11) Point of developed trees growing on active ice and the known elevation in m a.s.l. (12) International processes leading to the genesis of supra- border. glacial till on the glacier. Some questions concerning radiometric dating and lichenometric techniques are discussed. 2. GEOGRAPHY others. According to Kolliker, it is a re- Casa Pangue glacier is located on the constituted glacier covered with debris on northern slope of Mt Tronador (lat. 410 08' S, which shrubs were growing in 1911, when its long. 710 55' W), Chile (Fig. 1). Its accumu- tongue reached 370 m a.s. 1. lation area starts at the base of the summit The geology of the area is composed of (3 556 m a.s.l.) and its tongue reached 540 m Cretaceous granodiorites, overlain by Plio- a.s .1. on Apri 1 1979. Pleistocene basalts,breccias and tuffs of the The lake district of south-central Chile, Tronador Formation. These volcanic units between lat. 400 Sand 410 S,"has an oceanic correspond to the eruptions of Mt Tronador, a climate with abundant rainfall (> 4 000 mm a-l). basaltic shield volcano superimposed on a late According to Heusser (1974) the climate is Tertiary palaeorelief. In the valley, we controlled by cyclonic storms entering the observed glacigenic deposits corresponding to continent in rapid succession during the winter the Llanquihue glaciation (Heusser 1974, Mercer months (May to October). Polar Pacific air 1976, Heussercand Flint 1977). This is also becomes unstable when it faces the Andean ranges. called the El Salto glaciation by Paskoff (1977) Rain and snow fall heavily on the western slopes; and is Wisconsin i~ age. In Argentina its mean temperatures for summer and winter are 15.0 equivalent is the Nahuel Huapi gl~ciation (Flint and 6.4 0C, respectively. The vegetation of the and Fidalgo 1964). Deposits of the last neo- region belongs to the Valdivian rain forest and glacial readvance are found near the glacier the north Patagonian rain forest (Heusser 1974). snout. The movement of the glacier ice and, The Valdivian rain forest extends from sea- therefore, of the morainic arcs, and the level up to 650 or 700 m a.s.l. The north recession of the ice front have been assessed by Patagonian rain forest covers the landscape the methodology applied by Miller (1957) on above this level up to 1 000 m a.s.l., where it Yakataga Glacier, Alaska. Air photographs of is replaced by the sub-Antarctic deciduous 1942 (approximate scale 1:40 000), 1953 forest and the alpine tundra. The valley of (1:37 000), and 1970 (1:31 000) were used. The Casa Pangue glacier has very strong local relie~ approximate position of the arcs and the ice between 2 000 m a.s.l. at the divide and 450 m front were also recorded on colour slides taken at the valley floor. Thus, all the units in 1978 from a peak 1 100 m above the glacier. described are represented within the valley. A map (Fig. 1) was prepared based on an enlarge- ment of the 1970 air photographs. Elevations 3. CASA PANGUE GLACIER were estimated by means of a hand altimeter. 3.1 Previous work and methodology 3.2 Vegetation Casa Pangue glacier has been mentioned by The vegetation cover extends almost K611iker and others (1917), Reichert (1927), continuously over the glacier surface, Lliboutry (1956), and Mercer (1967), among especially near its terminus.