Pleistocene and Holocene Periglacial Forms in the
Transcription
Pleistocene and Holocene Periglacial Forms in the
Pleistocene and Holocene Periglacial Forms in the Cantabrian Mountains (NW Spain) Darío Trombotto Liaudat * & Victoria Alonso ** Ninth International Conference On Permafrost * IANIGLA, CONICET, 5500 Mendoza, Argentina. E-mail: [email protected] * * D E PA R TA M E N TO D E G E O L O G Í A . U N I V E R S I D A D D E O V I E D O . C / J e s ú s A r i a s d e Ve l a s c o s / n . 3 3 0 0 5 O v i e d o , S p a i n . E - m a i l : v a l o n s o @ g e o l . u n i o v i . e s IAN I GLA Universidad de Oviedo 6° W 1a Teverga 6º W 44° N C a n t a b r i a n 42° N Py re Cantabrian Mountains ne es Puerto de Somiedo Cornón 2188 15 1500 1923 S P A I N PORTUGAL Peña Chana 2068 S 6º 12´ 6º 08´ General features of the relief of the CM, considered as an extension of the Pyrenees to the west, were set during the Alpine Orogeny, when the Variscan basement was uplifted. Ever since, dominating denudation processes have been eroding this mountain chain. The CM show a great variety of glacial erosional forms related to a Pleistocene alpine glaciation with valley glaciers and small ice fields; during its maximum, ice fronts reached altitudes around 900-800 m in many of the valleys. In the Somiedo-Babia ice field, distribution of erratics, till and glacially abraded surfaces indicate a minimum ice thickness up to 400 m in some areas (Figs. 1a and 1b; Alonso & Suárez Rodríguez, 2004). Glacial deposits, as primary or reworked tills, cover most of the cold slopes with N or E aspect, although moraines are usually not well preserved. Double ridges and antiscarps are widespread landforms in deglaciated valleys. Recent instabilities removed glacial and periglacial deposits in the glaciated valleys as mass movements, and paraglacial alluvial fans were formed in some of the valleys. However, the former fluvial landscape was not completely destroyed; erosional preglacial surfaces are irregularly preserved close to the divides (Fig. 11). N Lago del Valle Piedrafita de Babia l u n a 00 15 00 i 6º 16´ 43º N Montihuero 2180 L Villablino Peña Ubiña 2417 Main glacial features of the Babia Region 18 a s I b i 1b 2182 Muxivén 2027 00 15 00 Cueto Arbás 2007 Degaña A detailed geomorphological map of an alpine deglaciated environment, in the Cantabrian Mountains (CM), has revealed a great variety of periglacial forms that have been ascribed to different cryomeres during and following deglaciation. N S e a Asturias León Bárzana 1500 300 Sena de Luna El Miro 1985 00 20 Puerto de Somiedo 1600 160 Cañada 2154 Nevadín 2082 1600 0 1500 00 18 Páramo del Sil Catoute 2111 43º Montihuero 2180 m 00´ 10 km 00 1600 18 220 14 1600 00 0 160 1800 Muxivén 2027 m 400 16 00 260 200 L E G E N D Nivation and Periglacial Processes and Forms (fossil, inactive and active) Glacial Forms and Deposits Main peak Arête Glacial cirque Rockglacier Permanent stream Till (undiferenciated glacial deposit) Protalus Lineal channel Moraine crest Stone stripes Rock threshold Slope change in glacial valley Solifluction Abraded and/or glacially eroded rock surface Striae Nivation hollow Erratic or subglacially transported boulder Roche moutonee Whaleback 260 13 150 42º 00 14 0 140 Fractures and faults Mountain lake Boulder slope Slope break Glacier difluence zone 200 260 Villaseca de Laciana 65 Bed strike and dip River Luna 56´ 100 14 00 Rill runoff in permanent and non-permanent streams Furrows Trough limit Structural Features San Félix de Arce 0 Piedrafita Laguna de Villaseca Fluvial and Lacustrine Forms Ridges 140 Sil River 200 120 140 Gravitational Forms Alluvial cone 150 Avalanche Boulder lobe Mixed landslide 16 00 Slided block Province limit Ice thickness during glacial maximum Landslide scar 0 1 2 3 km 1600 Based on Alonso & Suárez Rodríguez, 2004 El Miro Peña Baquín 2 12 s l o p e s Warm E n v i r o n m e n t a l - glacial and periglacial environments - 13. The Babia intramountainous plain was covered by a small ice field during the LGM. Moraines formed by stabilizations during glacier retreat, in the Luna valley, are marked by dotted lines. almost continuous permafrost in ice free areas, creeping permafrost - local - periglacial processes without glaciers - considerable creeping permafrost - less - nivation Note the glacially abraded limestones. Location shown in Fig. 1b. glacial freezing processes and - covered - mountain - perennial and patches by glaciers for less time Geomorphological Map of El Miro 500 m 0 temporary snow in areas of thawing gelifluction - more - nivation 6º 32´ and freezing solifluction and thawing cycles cycles - rockglaciers - felsenmeer - embryonal periglacial forms within glacial/periglacial domain - boulder - gelifluction - nivation l a n d f o r m s - giant sorted streams - solifluction - boulder - small stripes and rock lobes slopes of cryosediment slopes lobes nivation hollows hollows 42º 55´ 1700 1908 Alto del Bigardón (1939) 3. Block field with longitudinal and transverse ridges and furrows. 1929 Note partially reworked glacier moraines at north and south ends to the south. 1915 Fana del Coronxo(1836) 1858 17 00 64 18 00 (indicated by arrows) and boulder lobes on the talus slope. View 11 Peña Ubiña 2417 m 1700 4 1884 11 3 1800 65 1700 0 160 Cornón de Busmori (1932) 160 0 75 8 73 4 00 15 1829 58 65 1593 16 00 5 4. Tongue-shaped rockglacier at the bottom of north El Miro valley developed between 1540 and 1655 m. The root is disconnected from the cirque floor. Blocks in the lower zone are up to 8.5 m in length. 11. In the foreground on the left, boulder lobes on talus deposits at 1725 m with a south aspect. Partially eroded pre-quaternary surfaces are frequently preserved in the main water divide. View 64 6 t o t h e e a s t ; P e ñ a U b i ñ a l o c a t i o n i s s h o w n i n F i g . 1 a. 1800 7 00 17 160 42º 58 0 54´ 1500 5 Peña Baquín (1986) 10 1833 2 9 1800 71 El Miro (1985) 1937 80 18 00 00 19 00 vegetation cover indicate a younger age than the tongue-shaped 1800 17 with the front at 1725 m and a northeast aspect. Location and form shown in Fig. 4. 6 8 7 * 6. Well developed protalus at 1775 m and a north aspect. In the 7. Small protalus at the foot of talus slopes with a northwest 8. Boulder deposits and curved fractures at the top of warm 9 and 10. Giant sorted stone stripes on warm slopes to the west of El Miro. Lower zones background on the left, glacially abraded bedrock in the north- aspect. Asterisk marks the same position in Fig. 6. slopes. The lower half shows the beginning of the giant stone are usually affected by mixed landslides. In undisturbed zones, a lower limit for the stripes stripes partially deformed by mixed landslides. is calculated at 1700-1650 m. rockglacier in Fig. 5; asterisk marks the same position in Fig. 7. We propose that conditions during deglaciation were probably similar to those of the Pyrenees, where continuous permafrost and important cryogenic landforms with glacier ice at the same time are represented in the Climex Map of 2002 for the Last Glacial Maximum. The peak of the periglacial environment in the CM, however, must have occurred after the LGM, with predominantly much drier climatic conditions and benefitting from vaster areas uncovered by ice; discontinuous permafrost, expressed by rockglaciers during the Late Pleistocene or Early Holocene is likely to have reached 1540 m a.s.l. 9 * south main valley. White arrow indicates the front of the lobate Altitude, aspect, preservation degree and soil development of cryogenic forms around El Miro indicate more than one cryomere, although the lack of datable material has not allowed us to determine a precise time for these cryomeres. A tongue-shaped rockglacier in a cold and low position (Fig. 4) and giant sorted stripes on warm slopes (Figs. 9 and 10), both proposed to be coeval with cirque glaciers, suggest a continuous mountain permafrost in ice free areas during deglaciation, when the most important indicators were cryogenic periglacial and not glacigenic. At higher altitudes, sparsely vegetated small forms with scarce or none development of soil –lobate rockglaciers, protalus and boulder lobes- would correspond to more recent times; some of these landforms were probably active during the LIA, when cirque glaciers developed in Picos de Europa. The rockglacier fronts end at 1730 m a.s.l. approximately. 68 5. Lobate rockglacier at the bottom of the north El Miro valley Most of these forms are fossil features; at present, meteorological data indicate important cryogenic activity approximately above 2000 m. Solifluction in relation with nival processes is still active. Deglaciation in this zone of the CM was dated to have occurred before 34 000±1400 C -14 y r B P i n L a g u n a d e V i l l a s e c a , a t 1 3 0 5 m a.s.l. (Jalut et al., 2004; see Fig. 1b for location). But terminal moraine complexes at lower altitudes, formed when glaciers still were 11 km in length in areas with calculated previous ice thicknesses up to 260 m (Figs. 1b and 13), suggest a long evolution between the beginning of deglaciation and stabilization phases during retreat. Other data worth mentioning (Pallàs et al., 2006) about the close region of the Pyrenees, where an early maximum glacier extent during the last glacial cycle is not excluded, indicate an extensive glaciation at ca 18-20 ka (MIS 2), coinciding with the global LGM. permafrost - A c t i ve c r y o g e n i c 3 6º 33´ This zone, as many others in the CM, shows a strong asymmetry in processes and forms (Figs. 1c and 2). Rockglaciers, felsenmeer, protalus, boulder lobes, solifluction forms and talus deposits, formed by glacially derived material or by cryosediment, are frequent on cold slopes (Figs. 2 to 7). Warm slopes, slighter or even non glaciated and with a more regular gradient, developed giant sorted stone stripes, boulder lobes and stone-banked lobes from cryosediment (Figs. 8 to 11). C o n d i t i o n s P r o c e s s e s 1c Analysis of the map, made in a regional context, indicates that, during and after deglaciation, permafrost and periglacial processes affected to a great extent the highest parts of the deglaciated areas. slopes 2. View to the southeast of El Miro showing asymmetry between facing slopes with a more regular gradient to the top. The geomorphological map, at a 1:5000 scale, was made for a sector around El Miro peak (1895 m), located to the south of the main divide and characterized by a homogeneous bedrock formed mainly by cambro-ordovician quartzites (Figs. 1a and 1c). Hypothetical Conditions, Processes and Landforms during Recent Cryomeres (Greenland stadial 1/ Lower Holocene, Neoglacial phases, Little Ice Age) C o l d east facing slopes, with glacier cirques, and non-glaciated west Today, Picos de Europa, to the east, contain small ice bodies from the LIA in glaciokarst depressions (González Suárez & Alonso, 1994). Conditions, processes and landforms distribution during recent cryomeres have been summarized in a table (Fig. 12). 13 Cornón 2188 m CONICET REFERENCES: V. Alonso & A. Suárez Rodríguez (2004) Revista de la Sociedad Geológica de España, 17: 61-70; CLIMEX World Maps (2002) N. Petit-Maire, Ph. Bouysse (Scientific editors), CCGM, CGMW & ANDRA, France; J.J. González Suárez & V. Alonso (1994) J. Glaciology, 40: 198-199; G. Jalut, et al. (2004) Geo-Temas, 6: 105108; Pallàs et al. (2006) Quaternary Science Reviews, 25: 2937-2963.