1. VI Congreso Ibérico
International Permafrost Association
Mieres, Spain 21–23 June 2017
Glacial phases and present-day geomorphological processes in the Upper Garonne valley (Central Pyrenees)
Marcelo FERNANDES (1), Marc OLIVA (2), Luís LOPES (1), Jesús RUIZ-FERNÁNDEZ (3)
(1) Centre for Geographical Studies - IGOT, Universidade de Lisboa, Portugal
(2) Departament of Geography, University of Barcelona, Spain
(3) Departament of Geography, University of Oviedo, Spain .
Abstract
The Pyrenees was the most extensively glaciated Iberian mountain range during the Last Glaciation. The Maximum Ice Extent is known to have been asynchronous with respect to the LGM, though the chronology of glacial phases is still under discussion. This study focuses on the environmental evolution in the upper Garonne valley since the Last Glaciation. The distribution of glacial records suggests the existence of four glacial stages, from the maximum expansion to the end of the glaciation, whose extent and configuration have been reconstructed through GIS modelling. We also examine post-glacial environmental dynamics in the upper Garonne basin; today, the highest lands are organized following three elevation belts: subnival ( m), nival ( m) and cryonival/periglacial ( m).
Key words: Central Pyrenees, Upper Garonne, Last Glaciation, Holocene, deglaciation, geomorphology.
Study area
The upper Garonne basin is located in the northern Central Pyrenees, covering an area of 1260 km2 from Montréjeau depression ( m) to the head located on the central axis of the Pyrenees (with peaks >3000 m). The lowest part of the basin is shaped on limestones surrounded by Quaternary deposits. In the highest parts (Aran valley) there are intrusive igneous materials (granites) near the heads and detrital sedimentary rocks in the lowest areas. Climate regime is under Atlantic influence, with annual precipitations of 1200 mm and MAAT of 2.7ºC, at 2200 m.
Methodology
Glacial features were firstly identified based on satellite imagery and then validated in situ to establish the different stages of glacial retreat in the upper Garonne basin. This analysis was complemented by modelling the extent and geography of paleoglaciers carried out using GlaRE tool (Pellitero et al., 2016) and also validated with geomorphological evidences. Besides, this approach was complemented with the geomorphological mapping of present-day landforms and processes in Ruda valley and Beret massif.
Results
Four different glacial complexes were identified in the study area:
Glacial Stage
Glacial
type
Elevation moraines
(m)
Distance from head (km)
Max. ice thickness (m)
Glaciated surface (km2)
Ice volume (km3)
ELA
Possible chronology
Stage 1
Alpine/
piedmont 88
830
852
244
1487
MIS 4 -
MIS 2
Stage 2
Alpine
23-12
385 97 11
2253
Older
Dryas
Stage 3
cirque
173 16 1
2332
Younger Dryas
Stage 4
Cirque/
alpine
2-0.4
120
5.7
0.19
2408
Holocene
Stage 1
Stage 2
Present-day geomorphic processes
Domain
Environment
Processes
Landforms
Subnival
Valley bottoms and
surrounding slopes
( m)
Fluvial activity, peat development, soil formation, avalanches, debris flows
Fluvial plain, peatlands, alluvial fans, talus slopes
Nival
Overdeepened glacial basins and surrounding slopes
( m)
Incipient soil formation, avalanches, debris flows, cryoturbation, mechanical weathering, solifluction
Talus cones, talus slopes, solifluction landforms
Periglacial/ cryonival
Glacial cirques, peaks and rocky ridges ( m)
Talus cones, talus slopes, solifluction landforms, protalus ramparts
Stage 3
Stage 4
Conclusions
We identified four main glacial stages related to the maximum glacier extent and subsequent deglaciation in the Garonne valley since the last Pleistocene glaciation. The comparison with the glacial chronology from other areas in the Central Pyrenees may suggest that these four stages are related to the : Maximum Local Extent occurred prior to the LGM, Oldest Dryas, Younger Dryas and Holocene advances. The deglaciation of the highest parts of Upper Garonne valley was also accompanied by a gradual shift of periglacial processes to higher elevations. Currently, the distribution of geomorphological processes is organized in three elevation belts: subnival, nival and cryonival/periglacial. We will complement this study with absolute and relative dating techniques to reconstruct the chronology of the deglaciation process and subsequent environmental dynamics.
Reference
Fernandes, M.; Oliva, M.; Palma, P.; Ruiz-Fernández, J. & Lopes, L. (2017). Glacial stages and post-glacial environmental evolution in the Upper Garonne valley, Central Pyrenees. Science of the Total Environment, ,