Sequence Stratigraphy: A key tool to study palaeolake sediments (Presentació Sevilla Octubre...

ESTUDIO GEOQUÍMICO DE LOS SEDIMENTOS GLACIOLACUSTRES DE LA MASSANA Y ORDINO (ANDORRA,

PIRINEOS ORIENTALES): INFLUJO SEDIMENTARIO ENTRE LAGOS DE OBTURACIÓN YUXTAGLACIAR E INTERPRETACIÓN

PALEOAMBIENTAL

Valentí Turu i Michels

Fundació Marcel Chevalier

Av. Príncep Benlloch 66. D-0500 Andorra la Vella

Jaume Bordonau i Ibern

RISKNAT. Dpt. Geodinàmica i Geofísica

Universitat de Barcelona. C/ Martí i Franquès (S/N). 08028-Barcelona

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Palaeogeography of the glacial dammed lakeThree glacial tongues converged on the La Massana site during the local

glacial maximum prior to the LGM. A post maximum glacial retreat occured followed by a glacial advance and stabilisation (post maximum stabilisation).

At that moment the glacial tongues converged several times at La Massana damming intermittently the lake. A final disjunction phase gave way to an ice-

free lake until its final drainage.


Sketch map of the palaeolake at the post-maximum phase Outcrop at the Hortals proglacial delta

Idealized palaeoreconstruction of the dammed lake by Marta

Vilaplana (1981)

Sequence Stratigraphy: A key tool to study palaeolake sediments

Mercy to a sequence stratigraphy study of the Delta d’Erts proglacial complex, a roboust correlation across the basin has been obtained.

Erts delta sequence stratigraphy (Arinsal valley). Relative position

of the glaciolacustrine samples for geochemical analysis and AMS

available data (from all valleys). More than 150 outcrops were described in the Arinsal valley

AMS data and geochemical samples taken at different places

in the basin. Because a robust correlation is avaiable is now

possible to locate such data in a chronostratigraphic section


Accumulation-related mechanical denudation for the paleolake

In this slice the denudation rate is shown evaluating the sediments deposited in the paleolake that acted as a a sediment trap. The age model model is based on bulk

sediment 14C-dating and cosmogenic-datingversus glacial extention

Glacial extent curve obtained from the volume of the stored sediments in the lake. More than 250 outcrops were used. Some analogies can be observed if it is compared with the results obtained in the Southern Pyrenees glaciolacustrine deposits.

Bordonau (1992)


Geochemistry: A key tool to study palaeoclimate changes

Geochemistry studies give all its potential if a good chronostratigraphic modelhas been previously stablished. Data about weathering, chemical conditions in paleolake

waters and its relationship with paleoclimate are available with this kind of studies.

Basement weathering seems to follow climatic favourable conditions that promote Al leaching.

Attention should be placed at GS-2b stadial, were Al/Ti ratio overflow the basement mean value.


Chemical denudation in mountains is an important process in humid conditions, particularly when water pH low values appears by sulfide alteration from metamorphic rocks, or/and humid soils (histosols) and stagnant waters cover extended areas of the basin.

Titanium is an immobile element and is used to illustrate the relative input of terrestrial materials.

Aluminium may be partly transported in soil waters with a pH value below four.

We try to estimate the weathering patterns comparing the Al/Ti ratio of sediments and the unaltered bedrock. In that sense we think that aluminium comes form the hand of weathered pyrite rich metamorphic rocks..

Isotope analysis: A key tool to reconstruct palaeoenvironments

In proglacial palaeoenvironments, pollen can be distroyed by the energy of the glacial meltwaters. In such cases it is possible to reconstruct the palaeovegetation from theδ13C ratio. In continental environments less depletion in δ13C values are correlatedwith an increasing presence of aquatic plants and also arid zone plants (C4), on theother hand high depleted values are related to terrestrial (C3 plants).


Positive correlation between both variables are observed. During the LGM, δ13C shows a very low depletion value. At the same time the weathering Al/Ti ratio is low. How should we interpret this evidences?, cooling?, high amount of metwaters?, increasing of aridity?... We interpret that the paleolake receive highs amounts of meltwater and drive to desestabilize the sulfurs of the metamorfic terrains.

C3 plants

C4 plants

Decoding the sedimentological record of the palaeolake

A similar glacial evolution was previously proposed (Bordonau, 1992) from sedimentological studies of glacial paleolakes from Southern Pyrenees.However a more complete chronological reconstruction can be stablished here.


Thank you


Sequence Stratigraphy: A key tool to study palaeolake sediments (Presentació Sevilla Octubre...

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