Marc Calvet

Chapter 11 - Recent Advances in Research on Quaternary Glaciations in the Pyrenees

In the Pyrenees, new optically stimulated luminescence, 14C and 10Be ages, allows a more detailed image of the Pleistocene glaciations. MIS 6 moraines are dated on Ariege and Gallego. For the last glacial cycle, maximum ice extent occurred early, during MIS 4. A powerful advance during the global last glacial maximum is known on the Mediterranean side only. Deglaciation was very rapid after 20 ka.

Small, isolated glacial catchments as priority targets for cosmogenic surface exposure dating of Pleistocene climate fluctuations, southeastern Pyrenees

A new 10 Be data set from a small glacial catchment in the Pyrenees confirms almost full glacier extent during Marine Isotope Stage (MIS) 2, extends the oldest cosmogenic exposure ages of moraines in this mountain range to MIS 5, and constrains paleoclimate reconstructions in the Mediterranean region. It also illustrates the advantageous conditions for the long-term preservation of terminal moraines provided by small, low-gradient glacier catchments that did not merge with compound ice masses even during glacial maxima. These small, gently sloping, and isolated glacier catchments are proposed as priority targets for terrestrial cosmogenic nuclide dating of peak Pleistocene glacier advances, providing scope for greater accuracy in interregional paleoclimatic correlations.

The Quaternary glaciation of the Pyrenees

Abstract The Pyrenean mountains, extending between the 42–43° lati-tude, today only retain very small relic glaciers in the areas above 3000 m a.s.l. on their western part. However, the Quaternary glaciation of the chain was considerable, but very unequal. On the northern slope the equilibrium line altitude increases from 1200 m – 1600 m W to E and more steeply from N to S where it stands between 2100 to 2300 m. The north slope represent three fourths of the formerly glaciated region. Thick and broad glaciers there reached down to 400 m at the front range, whilst in the south the glacial limit was at 800 m in the interior valleys. The Wurmian glaciation removed most traces of older glaciations, but some evidence of a major glaciation, probably the Rissian, and rare fragments of much older events are known.

Planar landforms as markers of denudation chronology: an inversion of East Pyrenean tectonics based on landscape and sedimentary basin analysis

Abstract For over half a century the Pyrenees were considered to be a mountain range in which compressional structures were ancient (pre-Oligocene) but topography was young due to late Neogene tectonic uplift. Sufficient time had been afforded for a ‘peneplain’ to form at low elevations, undergo vertical uplift and remain partially preserved at high elevations until present times. This model of topographic growth has since been challenged by alternative theories. One of these postulates that topography in active orogens is in a steady-state, hence mountain ranges must be monocyclic and their ‘peneplains’ must have formed at high altitudes during continental convergence as a result of raised foreland base levels. Here we investigate Pyrenean denudation chronology using a range of evidence including provenance stratigraphy, the cross-cutting relations between topographic and tectonic features, and the age of regolith based on fossil faunas and floras. We find that the Eastern Pyrenees underwent a punctuated topographic evolution until recent times driven primarily by tectonic forcing, including kilometre-scale rock and surface uplift after 12 Ma. Climatic and eustatic inputs were subsidiary driving mechanisms.

Large-scale rock slope failures in the eastern Pyrenees: identifying a sparse but significant population in paraglacial and parafluvial contexts

Abstract This first overview of large‐scale rock slope failure (RSF) in the yrenees addresses the eastern third of the range. Around 30 principal RSFs greater than 0.25 km2 and 20 lesser or uncertain cases have been identified from remote imagery and groundtruthing. Compared with other European mountain ranges, RSF incidence is relatively sparse, displays no obvious regional trend or spatial clustering, and occurs across diverse landscape types, if mainly on metamorphic rocks. A transition is observed from paraglacial RSFs in formerly glaciated valleys to what are here termed ‘parafluvial’ RSFs, within wholly or mainly fluvial valleys but where slope failure is not directly provoked by or linked to river erosion. RSFs are particularly found in three topographic settings: at cirque and trough‐head thresholds (transition zones of elevated instability between cirque and main glaciated trough walls); near the upper or outer periphery of the ice field, where glacial adaptation of fluvial valleys is incomplete; and in fluvial valleys beyond glacial limits where incision is locally intense. RSF is absent from the range divide, from within cirques, and from most main valleys. In the montane areas, RSF is strongly associated with vestiges of preglacial summit surfaces, confirming that plateau ridges are less stable than sharpened crests and horns. RSF is contributing significantly to the progressive destruction of this paleic relief. The overall sparsity of RSF indicates insufficient rock mass stresses, including rebound after concentrated bedrock erosion. This may reflect a relatively weak imprint of glacial erosion, including breaching, in a context of relatively low mean rates of neotectonic uplift, possibly signalling overall that eastern yrenees landscapes are close to dynamic equilibrium.

Présence de Progonomys (Muridae, Rodentia, Mammalia) dans une association de rongeurs de la fin du Miocène moyen (Castelnou 1B; Pyrénées-orientales, France)

The fossil mammal bearing locality of Castelnou 1 B, a filling of a karstic cavity in the Pyrenees-Orientales (France), yields several species of rodents which are considered as not younger than Late Middle Miocene (Megacricetodon and Democricetodon) together with Progonomys cf. hispanicus, which is up to now found only in the Upper Miocene. The hypothesis that the fauna of Castelnou 1 B results from a mixture of at least two faunas of different age is rejected. Its Late Middle Miocene age raises the question of the date of Progonomys immigration into Europe. A discussion of available data about this event allows to conclude that Progonomys cf. hispanicus of Castelnou 1 B may be at least 12 millions years old.

Did Pyrenean glaciers dance to the beat of global climatic events? Evidence from the Würmian sequence stratigraphy of an ice-dammed palaeolake depocentre in Andorra

Abstract The ice-marginal depositional sequence of La Massana provides a chronostratigraphic benchmark for reconstructing the Würmian glacial evolution of the Valira catchment in Andorra, SE Pyrenees. The sedimentary record of Andorra confirms the asynchronous chronology of glacier fluctuations in different parts of the Pyrenean mountain range. A major ice recession occurred at the end of Marine Isotope Stage (MIS) 4. High magnitude valley-glacier fluctuations during MIS 3 constitute another important finding. Major readvances occurred toward the end of MIS 3, whereas MIS 2 (in particular, the global Last Glacial Maximum, or LGM) featured sharp contrasts in ice recession rates between Pyrenean valleys. Substantial distances separated MIS 4 glacier fronts (the Würmian maximum ice extent, or MIE) from those reached during the global LGM, in contrast to situations in the eastern Pyrenees, where Würmian MIE and global LGM ice fronts nearly coincided. Overall, the Valira glaciers reveal patterns that are more similar to those recorded elsewhere in the western and central Pyrenees than in the eastern Pyrenees. The rapid fluctuations recorded by Andorran glaciers during the second half of the Würm also suggest a response to global forcing events such as Heinrich events in the North Atlantic.

The Têt River Valley: A Condensed Record of Long-Term Landscape Evolution in the Pyrenees

While coinciding with a unique biogeographical hotspot where the Mediterranean and Atlantic biomes meet in the Eastern Pyrenees, the Tet river catchment displays a spectacular suite of tectonic, glacial, periglacial, fluvial and hillslope processes. Prominent fragments of smooth, pre-Quaternary Paleic relief survive where glacial erosion has not serrated the interfluves, and well-preserved exposures of both continental and marine basin fill sequences provide a compelling record of post-orogenic landscape evolution and base-level changes since the Early Neogene. The heritage value of the physical landscape is enriched by a palimpsest of agrarian, pastoral, industrial and cultural vestiges from the Neolithic to recent history.