Pre-Meeting Field trips28th IAS Meeting, Zaragoza

Geo-Guías

Geo

-Gu

ías

Zar

agoza

2011

July 2011 • Zaragoza (Spain)

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Pre-Meeting Field trips28th IAS Meeting, Zaragoza

Edited by

Concha ArenasLuis Pomar

Ferrán Colombo

2011

International Association of SedimentologistsPublished by the Sociedad Geológica de España

© The authors

ISBN: 978-84-930160-8-1

Edition copyright number: Z-1.258-2011

Picture on the cover: The upper Danian Lizarraga reef complex atLizarraga pass (northem Urbasa-Andia plateau, Navarra province)Photo by: Juan Ignacio Baceta.

It is recommended that reference to parts of this book be made asfollows:

Álvaro, J.J., Subías, I. and López Ciriano, A. (2011): Late Ordoviciancarbonate productivity, iron ore precipitation and glaciomarinedeposits in the Iberian Ranges. In: Pree-Meeting Field tripsGuidebook, 28th IAS Meeting. Zaragoza (C. Arenas, L. Pomar andF. Colombo, Eds.). Sociedad Geológica de España, Geo-Guías, 7:11-26.

Layout and printing:Sdad. Coop. de Artes GráficasLibrería GeneralPedro Cerbuna, 2350009 Zaragoza (Spain)imprentalg@efor.es

Quaternary karstic lakes in the WesternEbro Basin and the Central Pyrenees: TheArreo, Estanya and Montcortès sequences

and their depositional evolutionand paleoclimate implications

B. Valero Garcés1, J. P. Corella1, M. Morellón2,A. Moreno1, P. González Sampériz1, A. Pérez1,

F. Barreiro1, Mayte Rico1 and A. Plata-Montero3

1 Pyrenean Institute of Ecology-Spanish Research Council(IPE-CSIC). Department of Geoenvironmental Processes and Global

Change. Avda. Montañana, 1005. Zaragoza (Spain)blas@ipe.csic.es, pablocorella@ipe.csic.es, amoreno@ipe.csic.es,pgonzal@ipe.csic.es, anaperez@ipe.csic.es, ferbalos@ipe.csic.es

2 Department of Surface Waters. Eawag (Das Wasserforschungs-Institut des ETH-Bereichs). Überlandstrasse 133. P.O.

Box 611. 8600 Dübendorf (Switzerland)mario.morellon-marteles@eawag.ch.

3 Valle Salado de Añana Foundation,Miguel Díaz de Tuesta plaza, 1. 01426 Salinas de Añana

alberto@vallesalado.com

CONTENTS

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229229232232232

235237241243

243245251252253255258258

Geo-Guías, 7

Geological setting ......................................................................................Introduction................................................................................................Geological itinerary ..................................................................................First day: Western Ebro Basin ..................................................................

Stop 1. “Salinas de Añana” salt mine..................................................Stop 2. General view of the Lake Arreo watershed from Arreo vi-

llage ..............................................................................................Stop 3. “A view from the scarp”: Depositional evolution of the lakeStop 4. Lake Arreo shoreline: Limnology and recent sedimentology

Second day: Central Pyrenees, the Catalan-Aragonese transitional area..Stop 5. General view of the Balsas de Estanya karstic lake system

from the Estanya village ..............................................................Stop 6. Estanque Grande de Abajo - Sill ............................................Stop 7. The ‘Font Redona’ spring Holocene paleolake outcrop ........Stop 8. The ‘Camporrells’ fluvial tufa outcrops ................................Stop 9. General view of Lake Montcortès: Geology of the area ........Stop10. Lake Montcortès littoral: “The annually laminated record”..

Acknowledgements....................................................................................References..................................................................................................

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Geological setting: Quaternary Karstic lakes in the Pre-Pyrenees and the Western Ebro Basin

Geographically, the Pyrenees comprises the region from the BiscayGulf to the Creus Cape, about 440 km long. The highest peaks arelocated in the central areas of the range (Aneto Peak, 3404 ml; Posets,3375 m asl; Monte Perdido, 3355 m asl), and the altitude decreasestowards the west and the east, as well as towards the Aquitania andEbro Depressions (north and south, respectively). The oldest materialsappear in the axis of the range, and they are composed of Paleozoicrocks, affected by regional and contact metamorphism, and folded dur-ing the Variscan orogeny (Carboniferous) (Barnolas et al., 1996).Several pre-Stephanian granodioritic intrusions occur within the AxialPyrenees and volcanic rocks emplaced at the end of the Variscanorogeny and during post-Variscan phases in the Upper Stephanian –Lower Triassic continental basins. Variscan materials are overlain bycontinental (Upper Stephanian Permian and Triassic) and marine(Jurassic, Cretaceous, Eocene) sedimentary formations. The Alpineorogeny (Late Eocene-Oligocene) is responsible for the uplifting andpresent tectonic structure of the Pyrenees, characterized by large thrust-ing structures displaced towards the south more than 50 km in somecases. (Teixell, 1998, 2004).

To the south of the Axial Pyrenees, the Southern Pyrenean Range(Sierras Exteriores and Marginales) also follows a WNW-ESE trend.The Sierras are composed of Mesozoic and Tertiary materials, dis-placed to the south by Alpine thrusting structures (i.e., Monte Perdido-Gavarnie and Cotiella-Montsec). The Ebro Basin represents theyoungest, southern foreland basin of the Pyrenees. This basin is a largedepression surrounded by the Pyrenees to the north, the Iberian Rangeto the south, and the Catalan Coastal Ranges to the east, and it is mostlyfilled with Tertiary marine and continental deposits. In this field trip wewill visit a small lake and salt mines located in the westernmost mar-gin of the Ebro Basin (Lake Arreo) and two lakes in the transitionbetween the central-eastern External (Lakes Estanya) and InternalSierras (Montcortès) (Fig. 1).

Introduction

During recent years the Late Quaternary sedimentary sequences ofkarstic lakes from the Iberian Range (Barreiro et al., 2010; Moreno etal., 2008; Valero Garcés et al., 2008), the Pyrenees (Montserrat Martí,1992; Pérez-Sanz, 2009), the Pre-Pyrenees (Morellón et al., 2009a, b;Corella et al., in press, a, b) and the Central Ebro Basin (González-Sampériz et al., 2008) have been studied in detail. These records haveallowed the reconstruction of the main environmental changesoccurred in NE Spain during the late glacial period and the Holocene.The External Ranges are composed of E-W trending folds and thrustsinvolving carbonate Mesozoic formations. The outcropping of Triassiccarbonate and saline evaporite formations along tectonic structures hasfavoured the karstification processes and the development of poljes,sinkholes and dolines where the lakes are located (Fig. 1).

These karstic depressions are generated by dissolution processes,often involving subsidence and/or collapse, thus leading to the genera-tion of funnel-shaped dolines with steep margins, which are generallyvery deep for their size (Gutiérrez et al., 2008). This particular mor-phology, together with the frequent interception of aquifers, providinggroundwater input, leads to the development of relatively deep, peren-nial and frequently seasonally or annually stratified lake systems, evenin semi-arid regions with negative hydrological balances like the LakeEstanya catchment area ((Gutiérrez et al., 2008). Their development onevaporite and carbonate substrates favours sulphate, carbonate and cal-cium-rich waters. The relatively small size of these topographically-closed basins and their connection to aquifers make these systems verysensitive to regional hydrological balances, experiencing considerablelake level, water chemistry and biological fluctuations in response tochanges in effective moisture. In addition, the combination of greatdepth with multiple episodes of karstification in these endorheic basinscan lead to thick deposits with high sedimentation rates providing long,continuous sedimentary sequences with high temporal resolution, suit-able for palaeohydrologic and palaeoclimate reconstructions (Corellaet al., in press a; Corella et al., in press b).These small karstic lakesshow abrupt and complex lateral and vertical facies changes becauseinternal thresholds of some key factors (e.g., water chemistry and par-ticularly, salinity; temperature; light penetration; oxygenation levels)are often modified by extreme events, such as floods (Moreno et al.,2008) and mass wasting processes (Morellón et al., 2009b). In order todecipher the high-resolution palaeoenvironmental informationarchived in these lake sequences, detailed depositional models provid-ing a dynamic framework for integrating all these data are needed(Valero-Garcés and Kelts, 1995; Morellón et al., 2009b).

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Figure 1. Geological map of NE Iberian Peninsula. The city of Zaragoza (IAS2011 conference venue) is marked by a black dot and the three karstic lakes to

be visited in this field trip are marked by stars.

The high sedimentation rate in Lake Arreo has allowed the recon-struction of a high-resolution sequence of environmental changes dur-ing the last 2000 years related to climate, geomorphologic and land-usechanges in the westernmost area of the Ebro Basin (Fig. 1). Changesduring the last decades have been reconstructed using a correlation oflimnological monitoring data, regional climate data series and sedimentcores supported by radiometric dating and varve counting (Corella etal., 2010a). The multiproxy analysis in Arreo sediment cores illustratesthe synergy between climate and human activities during the LateHolocene.

Lake Estanya hosts the longest available sequence in the Pre-Pyrenees, comprising the last 21,000 cal years. In this basin, the use ofacoustic, seismic stratigraphy has provided a quasi three-dimensionalimage of the sedimentary basin and direct evidence of major phases oflake-level changes and mass-wasting processes (Morellón et al.,2009b) (Fig. 1). A facies model has been defined combining sedimen-tological features with their mineralogical and organic composition,and blended with the results of a study of present-day depositionalenvironments in this lake (Morellón et al., 2009b). The reconstructedhydrological fluctuations during the Late glacial (Morellón et al.,2009a) and the Holocene period (Morellón et al., 2008b) are in phasewith the main climate changes occurred in the W Mediterranean basin.As in the case of Arreo, during the last millennium the lake evolutionshows the complex interplay between human and climate changes(Morellón et al., in press).

Lake Montcortès (Fig. 1) is the only one with current permanentmeromictic conditions. The dominance of finely laminated facies in therecord has allowed the establishment of an annually resolved sequenceextending back for the last 6000 years. This is the longest varvedsequence ever recovered in the Iberian Peninsula and thus, constitutesan outstanding paleoclimatic and paleoenvironmental record for themid and late Holocene (Corella et al. in press b). A detailed microfaciesstudy of the sedimentary record reflects variable patterns of landscapeoccupation since Medieval Ages through industrialization and recenttimes.

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GEOLOGICAL ITINERARY

FIRST DAY: WESTERN EBRO BASIN:LAKE ARREO

Stop 1

“Salinas de Añana” inland saltwork

Location: From Zaragoza take AP-68 highway towards Bilbao (236km). Take exit 6 to A-2622 road and drive west towards Salinas deAñana (10 km). Parking in the main square of the village (Duration: 2h20 min) (Fig. 2A).

Observations: Visit to “Salinas de Añana” salt exploitation to eval-uate its impact in the landscape since the Roman Period

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Figure 2. Road logs of the first (A) and second (B) days. Detailed maps of thedifferent areas to be visited with the location of the different stops (S1-S10) are

included.

Description: The salt exploitation at “Salinas de Añana”, about 1.5km north of the lake (Fig. 3), resulted in an intense human occupationof the area since Roman times with the subsequent impact in Arreowatershed and thus in the evolution of the lake.

The presence of Salionca mentioned by Ptolomeo in the 2nd centuryclose to Salinas de Añana suggests a permanent exploitation of thesaltwork during the Roman Period. The transition between the Romanperiod and the Early Middle Age (V-VIIth Century) resulted in substan-tial changes in the landscape, and although the salt exploitation contin-ued there was a significant production decrease (Plata Montero, 2008).The establishment of the first medieval settlements was a long processdeveloped between the VIIIth and the first half of the Xth century as aresult of different factors as political instability after the Arabic inva-sion and feudalism consolidation. The ‘salineros’ -salt makers- lived indifferent villages surrounding Añana Salt Valley, which triggered a cat-tle raising and agricultural expansion and a more intense salt exploita-tion. Two of these villages, Arreo and Los Lagos (Fig. 3), were locateddirectly on the Lake Arreo watershed. A massive emigration from thesesmall villages to Salinas de Añana took place in the second half of XIIth

century. Thus, in 1156 AD this village held 60% of the total populationof the area after the abandonment of the fields located in the catchment(Plata Montero, 2008). During the Modern period (XV-XVIIIth cen-turies) the salt mines experienced a progressive increase from 2500 saltevaporation pans in 1401 to 3367 at the beginning of the XIXth century(Plata Montero, 2008) because the Spanish Crown monopolized andcentralized production between the XVI and the XIXth centuries,enhancing salt production and sales. During the Contemporary Age(1787 – 1960 AD) the productive surface in the valley was doubledbecause of an increase in production, which forced utilization of theupper part of the valleys for salt exploitation (Fig. 4). However, after1960 the loss of economic competitiveness against coastal salt minesresulted in abrupt exploitation abandonment from 5648 salt evapora-tion pans in 1960 to 40 in 2000, and the subsequent depopulation of thearea (Plata Montero, 2008). Since 2000, Salt Valley is the subject of anambitious recovery plan, which seeks to restore its original landscapeand operation.

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Figure 3. Salinas de Añana diapir and the location of the salt exploitation andthe Lake Arreo catchment.

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Figure 4. Historical photographs in Salinas de Añana. Left: Structure of the saltmines in the early XIXth century Right: Mine workers collecting the salt from

the evaporation pans (Source: Plata Montero, 2008).

Stop 2

General view of the Lake Arreo watershed from Arreovillage

Location: From Salinas de Añana take A-262 road to the west for1,5 km. Then take A-4318 road southward (1.5 km). Parking in themain street of the village of Arreo (Duration: 15 min).

Observations: The Lake Arreo catchment: climatic, vegetational,geologic and hydrogeological setting.

Description: The regional climate is transitional Atlantic-Mediterranean. The mean annual precipitation is 670 mm while themean monthly temperatures range from 20ºC in summer to 5ºC in win-ter. The site is on a transitional vegetation zone between Eurosiberianand Mediterranean bioclimatic regimes (Fig. 5A). Apart from the cul-tivated areas that occupy the lowest part of the valleys, most of the lakewatershed surface is forested (Fig. 5B). The most abundant trees aresubhumid mountain evergreen oaks, located on north-facing slopes,and sub-mediterranean semi-deciduous oaks on drier soils. The lakeshorelines are colonized by hygrophytic vegetation (mainly Cladiummariscus and Phragmites australis)

Lake Arreo (42º 46´ N, 2º 59´ W; 655 m a.s.l.) lies on the Salinasde Añana diapir (see Fig. 3), a 5.5 x 3.2 km halokinetic structure affect-ing Upper Triassic evaporite formations (Keuper facies) (NE Spain).Gypsiferous materials and clays are dominant in the watershed,although volcanic rocks (ophytes) and intrusions are common (Fig.5C). An ENE-WSW fault bounds the lake to the north. The geomor-phological expression of the fault is a steep, active cliff composed ofophytes, responsible for numerous mass-wasting deposits. The lake,24 m water deep, is the deepest body of water with evaporite substratein the Iberian Peninsula (Fig. 5D). The lake-basin has two differentsubenvironments: i) a southern shallow palustrine area that occupies2/3 of the lake total surface and with well-mixed waters throughout theannual cycle, and ii) a deeper funnel - shaped lacustrine central areawith seasonal anoxic conditions, bound to the north by the steep scarp.The lake is a hydrologically open system, with a small eastern inlet andan ephemeral stream flowing outwards from the western side (Rico etal. 1995) (Fig. 5D). The lake is monomictic, with thermal stratificationand anoxic hypolimnetic conditions extending from spring to the endof the summer, and a thermocline located between 4.20 and 3.56 mdeep (Chicote, 2004). Water chemistry is sulphate and calcium-rich:[SO4

2-] > [Ca2+] > [Mg2+] > [Na+], with a high electrical conductivity(EC = 703-1727 µS/cm). Several saline springs (EC = 8265 μS/cm)(Chicote et al., 2004) occur in the watershed and the saline groundwa-ters have a decisive influence on the hydrological and chemical behav-iour of the lake, although it has not been quantified.

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Figure 5. Location map of Lake Arreo: (A) Map of mean annual rainfall in theIberian Peninsula and geographical location of the study area. (B) Vegetationmap of the area. (C) Detailed geological map of the Lake Arreo watershed. (D)

Bathymetric map of the lake and location of the four cores retrieved.

Stop 3

“A view from the scarp”: Depositional evolution of the lake

Location: From Arreo village continue in A-262 road to the south-west for 500 m, take the first road to the right before the lake. Continuefor 500 m and take the first road to the left. Continue 200 m and parkalong the road. (Duration: 15 min).

Observations: The depositional evolution of Lake Arreo during thelast 2500 years.

Description: Three sediment cores were retrieved with a platformfrom the Limnological Research Centre (University of Minnesota).Two cores (1A and 1B) were obtained from the deepest area of the lake(24 m water depth) and the third one (2A) was recovered in the littoralbench (7 m water depth) (see Fig. 5D). Additional gravity cores werecollected containing the water-sediment interphase. The stratigraphiccorrelation between all the cores was based on sedimentary facies andpalinological and magnetic susceptibility profiles (Fig. 6). A multi-proxy analysis of the record allows us to understand the depositionalevolution of the Lake Arreo during the last 2500 cal years (Corella,2011). The sedimentary sequence reflects the abrupt climatic shifts dur-ing the late Holocene but it is also intimately linked to the evolution theenvironmental impact of Salinas de Añana salt mine along its exploita-tion since the Roman Period.

The chronology of the Arreo sequence is based on 15 AMS 14Cdates (Corella, 2011) (Fig. 6). Littoral core 2A represents littoral sedi-mentation between 620 yr BC-860 yr AD, while core 1A reflects sedi-mentation in the distal, deepest part of the lake since 585 AD. TheArreo sedimentary sequence displays 10 different facies (Table1, Figs.6 and Fig. 7).

Ten lithostratigraphic units have been distinguished in thesequence, units I to VI in the deep-water environment and units A to Din the littoral core (Figs. 6 and 7).

Unit D (620 BC- 150 AD) is composed of coarse, chaotic, hetero-metric, clastic facies 8. The abundant limestone and sandstone clasts atthe base evidence strong alluvial influence, while transitional allu-vial/wetland conditions are interpreted for deposition of the upper partof the unit.

Unit C (150-465 AD) is composed of organic-rich facies 7. The pro-gressive decrease of carbonate clastic facies and the increase in theorganic carbon content (Fig. 7B) reflects the transition from shallow,carbonate-producing lake littoral to a wetland with high organic pro-ductivity and accumulation.

Units A (830-860 AD) and B (465-830 AD) are littoral, banded, car-bonate-rich units. Both units were deposited in littoral carbonate-pro-ducing environments, but the presence of clastic facies 6 in unit Areflect higher clastic input to the lake, while the coarse bioclast-richfacies 10 in unit B suggest shallower conditions.

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Units II (1964-1994 AD), IV (1180-1605 AD) and VI (585-895 AD)are variegated, finely laminated units rich in carbonate and gypsum(facies 4 and 5), with intercalated clastic facies 1, 2 and 3. The preser-vation of fine laminations suggests meromictic conditions during dep-osition of these units.

Units I (1994 AD-Present-day), III (1605- 1964 AD) and V (895-1180 AD) are banded to laminated, black and grey units composed ofalternating clastic facies 1, 2 and 3 with intercalations of laminatedfacies 4. These units reflect an increase in the catchment run-off andhigher clastic inputs to the lake.

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Figure 6. Lithostratigraphic correlation of the 3 cores retrieved in Lake Arreo.Each core image is accompanied by its lithostratigraphic column and magneticsusceptibility values. Continuous and non continuous lines represent the corre-lation between the limits of the main sedimentary units and subunits respec-tively. Red dashed line shows the correlation between the littoral and distal

cores. The AMS 14C dates in the cores are also indicated.

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Facies Depositional environments1. Greyish, massive, sand Mass wasting processes

Massive 2. Massive, blackish silty mud Distal deposits fron turbidites or out ofsuspension

Banded to 3. Graded sand-silt layers Proximal deposits from turbidite processeslaminated

4. Biogenic varves Distal, anoxic hypolimnion, high biopro-ductivity, and production of carbonate in

Laminated the epilimnion

5. Gypsum-rich laminated Distal, anoxic hypolimnion, high concen-trated waters

6. Graded, carbonated silt Littoral deposits from storm events andhigher run-off

Massive 7. Brownish, peaty mud Wetland

8. Massive, conglomerate Alluvia/wetland

9. Banded, brownish and Carbonate-producing littoralBanded to greyish carbonate siltlaminated 10. Banded silts with Shallow, carbonate-producing littoral with

Chara and gastropods Chara

Dist

al c

ore

Litto

ral c

ore

Table 1. Lake Arreo facies and inferred depositional environments.

Figure 7a. Sedimentological, geochemical and mineralogical profile from thedifferent lithostratigraphic units and subunits defined from core 1A. AMS 14Cdates are indicated in the left hand-side of the figure. From left to right: MS:Magnetic susceptibility; TIC: Total Inorganic Carbon; TOC: Total OrganicCarbon; Qz+Plag: Quartz+plagioclase; Gy: Gypsum; Cc: Calcite. Legend is

shown in Figure 6.

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Figure 7b. Sedimentological, geochemical and mineralogical profile from thedifferent lithostratigraphic units and subunits defined from core 2A. AMS 14Cdates are indicated in the left hand-side of the figure. From left to right: MS:Magnetic susceptibility; TIC: Total Inorganic Carbon; TOC: Total OrganicCarbon; Qz+Plag: Quartz+plagioclase; Py: Pyrite; Cc: Calcite. Legend is

shown in Figure 6.

Stop 4

Lake Arreo shoreline: Limnology and recentsedimentology

Location: From the scarp follow the path down to the shore of thelake (Duration: 5 minutes walking)

Observations: Recent sedimentation in the lake and comparisonwith monitoring and instrumental data.

Description: Based on aerial photograph analyses, the land use ofthe Lake Arreo catchment during the last 60 years comprised four mainperiods (Fig. 8): i) Prior to 1963, forest dominated the watershed andflooded areas and wetlands had the maximum surface area of the lastdecades; ii) Between 1963 and 1979, cultivated areas increased as aresult of the regional smallholding concentration processes, cultivatedareas increased; iii) between 1979 and 1992 the creation of larger fieldcrops replaced small, traditional farming lots and cultivated areasreached the maximum surface; and iv) between 1992 and 2009, wet-land decreased to a minimum, while shrubland increased considerably,replacing cultivated areas.

The 15 years long monitoring of physical and chemical dataobtained from Arreo Lake (Chicote 2004; Rico, unpublished data) con-stitutes an exceptional limnological record. These monthly surveyshave documented a decrease in the number of anoxic weeks per yearsince 1994 and large level fluctuations during the last 15 years, mainlyrelated to illegal water extractions. Thus, average September lake waterlevel dropped during 9 years, since 1993 to 2000, and 2002 due toincreased water extraction for irrigation purposes (Fig. 9). During theperiod with a maximum drop recorded in 2002 (4.7 m) the south andwest littoral platform areas emerged and the palustrine depositionalenvironment disappeared with the subsequent erosion of the littoralsediments.

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Figure 8. Land use changes in Arreo Lake catchment during the last 50 yearsbased on aerial photographs.

Multidisciplinary analyses of gravity cores (43 cm long) retrievedfrom the deepest part of the lake (Fig. 5) reflects four main stages in thelake since the 1950s (Fig. 8): i) During the period 1952-1963 (Unit III),with low agricultural pressure and higher rainfall, lake level was stableat the maximum determined by the outlet, and anoxic conditions dom-inated at the lake bottom. ii) After the first smallholding concentrationprocesses in 1963 (Unit II.b), farming activities, increased, but lakelevel remained high. iii) Only after the second, local small holding con-centration process in 1979 (Unit II.a), sediment delivery to the lakegreatly increased and formation of varved facies stopped. iv) Thelargest limnological change occurred during the last two decades(1995-2008, unit I) characterized by decreasing lake levels caused bylake water extraction for irrigation purposes, accompanied by adecrease in rainfall and higher increase in clastic delivery from scarpsfailures (facies 1). As a consequence, the lake became holomictic.

Comparison of the monitoring, instrumental and sediment coresshows a clear correlation among the increase in precipitation, the num-ber of anoxic weeks per year and the subsequent formation of lami-nated/varved facies. Thus, the study of these short sediment coresreveals how the interplay between land use changes and climate fluc-tuations has controlled the sediment deposition and the mixing-stratifi-cation dynamics of Lake Arreo during the last decades.

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Figure 9. The evolution of Lake Arreo Lake during the last decades: compari-son of the meteorological (since 1952) and monitoring (since 1986) data with

the biological and geochemical data from short cores.

SECOND DAY: CENTRAL PYRENEES, THECATALAN-ARAGONESE TRANSITIONAL AREA

Stop 5

General view of the Balsas de Estanya karstic lake systemfrom the Estanya village

Location: From Stop 4, take road A-4318 northwards, then continueby A-4318 eastwards and then, take AP-68 towards Zaragoza. Fromthis city, take A-23 to Huesca and then N-240 until Barbastro. Finally,take N-123 to Benabarre (Huesca) (4h30’). From this town, go southby N-230 till km 54 and then take the road to Estopiñán del Castillo.Finally, take the local road to the village of Estanya. Parking in thenorth end of the village, close to the spring that provides water to thevillage.

Observations: The Balsas de Estanya karstic system, geomorpho-logic and hydrogeological setting.

Description: The Estanya lakes are located at 670 m. a.s.l., in aregion with a Mediterranean continental climate with a mean annualtemperature of 14 ºC, ranging from 4 ºC (January) to 24 ºC (July) anda mean annual rainfall of 470 mm. The dissolution and collapse ofTriassic limestones and dolostones (Muschelkalk Facies) and evapor-ites (Keuper) has favoured the development of these karstic lakes (Fig.10).

The lake basin is endorheic and thus, hydrological balance is mostlycontrolled by evaporation output and groundwater input from the sur-rounding local dolostone aquifer, related to the hydrogeological systemof the Estopiñán Syn clinal (Pérez and Lambán, 2009) (Fig. 11). A per-manent spring from the main aquifer is located at the North end of thecatchment, feeding the small Estanque Grande de Arriba (Fig. 10A andFig. 11). Today, an artificial canal brings water periodically from thislake to the main lake, Estanque Grande de Abajo. This canal was in useduring the last centuries, but archaeological evidences of water man-agement in the area date from the XIIth century (Riera et al., 2006;Riera et al., 2004), just after the conquest of this region by theChristians and the subsequent development of agriculture in the area.Several ephemeral small springs occur in the watershed and subaque-ous springs also discharge in the lake (Fig. 10). The substrate, com-posed of non-permeable Keuper facies, limits groundwater losses (Fig.10A and Fig. 11).

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Figure 10. A) Geomorphologic map of the ‘Estanya lakes’ karstic system. B)Land use map of the watershed (modified from López-Vicente et al., 2009). C)

Location of the study area marked by a star in the Ebro River watershed.

Figure 11. Conceptual model of the hydrological balance of the Estanya Lakes(modified from Villa and Gracia, 2004).

Stop 6

Estanque Grande de Abajo – Sill

Location: From the Estanya village take a small road to the souththat borders the larger lake. Parking along the road.

Observations: The main lake (Estanque Grande de Abajo): deposi-tional subenvironments, seismic stratigraphy and sedimentarysequence for the last 21,000 cal years.

Description: The main lake basin, ‘Estanque Grande de Abajo’ is‘eight-shaped’, composed of two sub-basins with maximum waterdepths of 12 and 20 m (Fig. 10A) and very steep margins. These twosub-basins are separated by a sill, 2-3 m below present-day lake level,only emerged during long dry periods, e.g. in 1994-1996 drought(Morellón et al., 2008b). Thus, the response of the system to precipita-tion is relatively rapid, as indicated by this lake level drop and prelim-inary data obtained by hydrological modelling of the basin (Pérez andLambán, 2009). The lake is monomictic, with thermal stratification andanoxic hypolimnetic conditions extending from March to September,and a thermocline located between 5 and 10 m deep. Water chemistryis sulphate and calcium-rich, with a relatively high electrical conduc-tivity (EC = 3.4 mS/cm), leading to the precipitation of calcite in theepilimnion.

Sedimentary facies in modern Lake Estanya were described andinterpreted based on 34 superficial sampling points (Morellón et al.,2009b). According to sedimentological and compositional features, 3main depositional sub-environments can be identified in the modernlake basin (Fig. 13):

(i) The ‘littoral platform’ comprises:

The ‘internal platform’ extends from the inner limit of the littoralvegetation belt to the lake shoreline. This area is only occasionallyflooded and sediment is composed mainly of light grey, massive, bio-turbated coarse silts with abundant plant remains.

The ‘external platform’ (0 to 4.5 m water depth) is a permanentlysubmerged, shallow area, colonized by submerged macrophytes andcharophyte meadows. This environment is the main carbonate factoryin the lake, comprising biogenic and non-biogenic particles. Sedimentis composed of banded to massive yellowish/light grey, bioturbatedcarbonate-rich silts to fine-grained sands with plant remains.

(ii) The ‘transitional talus’ (4.5 - 8 m water depth) is characterizedby a steep morphology and limited presence of carbonates originatingin the platform are transported to the talus and mass-wasting processesremobilize these sediments downslope to distal areas. Sediments aredark grey massive silts with carbonates. (iii) The ‘offshore, distal area’(8- 19 m water depth) comprises the central, deepest and relatively flatareas characterized by black, massive to laminated fine-grained silts.The presence of carbonates is limited because of the large distance tothe producing littoral areas and to the dissolution processes. Seasonalanoxic hypolimnetic conditions lead to reduced or absent bioturbation.

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Lake basin topography, water depth and distance to shore appear tobe the main factors controlling the distribution of present-day surfacesediments in the lake. Grain-size and carbonate content decreasetowards the distal areas, whereas organic matter content is maximum inthe littoral and distal areas. Distal organic matter is characterized bycomparatively low atomic C/N ratios indicating a higher contributionof algal sources.

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Figure 12. (A) Depositional sub-environments identified in Lake Estanya. (B)Changes in sediment properties (Y-axis) in respect of water depth (m) (X-axis).

Modified from Morellón et al. (2009b).

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Figure 13. (A) NW–SE seismic section showing the 3 seismic units (‘A’ to ‘C’)identified in the sedimentary succession and correlation with core images (2A,4A and 5A) and additionally density (g cm−3) profile for core 5A. (B) Aerialimage with the seismic grid and long-core locations. Modified from Morellón

et al., 2009.

Five sediment cores up to 11.5 m thick, recovered at the distal areasof the two sub-basins, allowed the reconstruction of the sedimentarybasin evolution (Morellón et al., 2009b). Seismic stratigraphic analysisrevealed a sediment thickness up to ca 15 m in both sub-basins andallowed the identification of 3 major seismic units (‘A’ to ‘C’; Fig. 14)and several seismic horizons, correlated with the core lithostratigraphy,have been identified:

Seismic unit ‘C’ is characterized by low-amplitude reflections,mostly continuous and intercalated with transparent units and fewmedium-amplitude reflections towards the top. The thickness of thisunit is highly variable and it is nearly absent in proximal areas and onthe sill.

Seismic unit ‘B’ is characterized by closely spaced high-amplitudereflections, increasing upwards as a result of more frequent lithologicalchanges. Alternating parallel-to-chaotic seismic facies occur in proxi-mal areas, probably reflecting truncation surfaces as a result of alter-nating deposition and erosion cycles. Seismic unit ‘A’ is characterizedby varying seismic facies but laterally continuous. The limited lateralchanges in seismic facies can be related to the present-day distributionof depositional sub-environments in the lake basin previouslydescribed (Fig. 12 and Fig. 13). Unit ‘A’ is also of a draping geometrywith a thickness of up to ca 3 m, reaching far into the littoral areas ofthe lake.

The thinning and onlapping of unit ‘C’ towards the slopes and sillareas suggest that the two sub-basins were not permanently connectedduring the early stages of basin evolution. In contrast, the rather con-stant thickness of units ‘A’ and ‘B’ throughout the basin, drapinggeometries and wider lateral extent, indicating that they have been pre-dominantly connected during deposition of those units. Unit ‘A’ is notonly laterally continuous throughout the lake basin but also shows asimilar thickness in the littoral areas, pointing to a regular and stablesedimentation over the entire lake area in recent times.

The multi-proxy analysis of sediment cores provides a high-resolu-tion, continuous sedimentary sequence extending back the last 21000years. Ten facies and seven sedimentary units were defined and corre-lated within the five sediment cores. The integration of sedimentaryfacies, elemental and isotopic geochemistry, biogenic silica (Bio Si)and diatoms valve abundance (VA), together with a robust age modelbased on 17 AMS radiocarbon dates and 137Cs/210Pb dating, enablesprecise reconstruction of the main hydrological changes in the region(Fig. 14):

• Unit VII (21000 – 18000 cal. yrs BP) is mostly composed by car-bonate-rich facies 7 with intercalations of gypsum-rich facies 9and clastic facies 2.2. These sequences are interpreted as a shal-low, brackish, carbonate-producing lake, with saline fluctuationsand increased runoff events. Relatively high δ13Corg, (-20 to -25‰) indicate high alkalinity and low productivity, contrastingwith relatively high Bio Si, reflecting a relatively high abundanceof diatoms. This situation might represent the existence of anopen hydrological system in an early stage of lake developmentor relatively increased water availability at a regional scale.

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• Unit VI (18000 – 14500 cal yrs BP) is characterized by the depo-sition of gypsum-rich facies (8, 9) and decreased Bio Si, reflect-ing closed, saline lake conditions, coinciding with the so-called‘Mystery Interval’ (MI) (17500 – 14500 cal. yrs BP).

• Unit V (13500 – 13300 cal. yrs BP) is defined by the return to thedeposition of carbonate-rich facies 7 and increased δ13Corg, val-ues, indicative of lower salinity and higher organic productivity,with brackish conditions. This period coincides with the moisterand warmer Bölling/Allerød interstadial. Gypsum-rich facies 9and more negative δ13Corg towards the top reflect a saline andshallow (arid) environment, coinciding with the Younger Dryas.

• Unit IV (11600 – 9400 cal years BP) reflects arid conditions,characterized by alternating coarse gypsum-rich facies (10) andmassive clastic deposits (6) with bioturbation, indicatingephemeral, saline lake conditions with flooding and desiccationperiods. The dominance of facies 6 versus facies 10 after 9900 calyrs BP marks an increase in water availability.

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Figure 14. Palaeohydrological evolution of the Lake Basin. Modified from Morellón et al., (2009).

• Unit III (9400 – 1200 cal. yrs BP) represents an increase inhumidity, associated to the Early Holocene. The deposition oflaminated, gypsum and organic-rich facies (4,5), the markedincrease in Bio Si and the decrease in δ13Corg, represent the estab-lishment of a much deeper saline lake environment, with eventsof increased clastic input around 8600, 6200 and 4800 cal. yrs BP(facies 2).

• Unit II (1200 - 750 cal. yrs BP) represents the transition towardsdeeper conditions, characterized by the deposition of facies 2 andinterrupted by an aridity crisis defined by a gypsum-rich facies 4level around 800 cal yrs BP, probably related with the MedievalClimate Anomaly.

• Unit I (750 cal yrs BP – present) reflects conditions similar to thepresent (fcs. 1, 2 and 3) with an increase in farming practices inthe lake bas in (Riera et al., 2006; Riera et al., 2004), responsiblefor the increase in detrital input. Internal fluctuations within theLittle Ice Age are discussed in Morellón et al. (in press).

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Stop 7

The ‘Font Redona’ spring Holocene paleolake outcrop

Location: Back to the Estanya village take a local road eastwards toEstopiñán del Castillo and take a small path towards the south beforeapproaching the village. Parking along the road.

Observations: Brief geomorphologic setting, sedimentologicaldescriptions and chronology of a Holocene paleolake deposit and asso-ciated fluvio-lacustrine tufa dam.

Description: The contact between the Eocene limestones outcrop-ping along the Estopiñán Syncline (aquifer) and Keuper facies(aquitard) has allowed the development of several springs of HCO3

-

and Ca2+ rich water s. In relation to these springs, during theQuaternary, several tufa systems developed (currently non-active)(Sancho-Marcén, 1988) (Fig. 15).

In the northern margin of the Synclinal, close to the spring ‘FontRedona’ and nearby the village of Estopiñán del Castillo, fluvial-lacus-trine Holocene deposits outcrop. These materials are 3 m thick, grey,banded, carbonate-rich silts with some black, massive, cm-thick interca-lations of peat layers, reflecting a steady, lacustrine deposition. Thisdeposit is associated with a small tufa dam unit downstream. AMS radio-carbon dating of a piece of charcoal revealed an age of ca. 7000 cal yrsBP, consistent with its geomorphologic location, almost at the same alti-tude as the creek draining the spring. The age of these deposits also coin-cide with a humid stage recorded in the nearby Lake Estanya sequenceand other records from the Iberian Peninsula (Morellón et al., 2008a).

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Figure 15. Geological map of the Estopiñán Synclinal (modified from Sancho-Marcén, 1988). Outcrops of tufa (El Molá and Camporrells) and recent fluvio-

lacustrine deposits (Font Redona) are indicated by a star.

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Stop 8

The ‘Camporrells’ fluvial tufa outcrops

Location: Back to the junction between the local road and the mainroad N-230, go southwards and take the next local road towardsCamporrells (eastwards).

Observations: Brief geomorphologic setting, sedimentologicaldescription and chronology of several tufa deposits along the valleyand in the village.

Description: The fluvio-lacustrine tufa formations of Camporrellsare developed as a result of the past damming of the headwaters of‘Molino del Pubill’ creek, a tributary of the Noguera-RibagorzanaRiver.

Outcrops located upstream, in ‘El Molá’ area are mainly composedof yellowish, several cm thick layers of well preserved phytoclasts inliving positions with stromatolitic lamination and coatings, interbeddedwith intercalations of grey, massive, marl levels of up to 20 cm thick-ness. Tufa deposits reach a total maximum thickness of 20 m and theirbase level is located at 5 m above the current altitude of the river. U/Thseries dating of a stromatolitic laminated interval revealed an age of82263 cal yrs BP for this fluvio-lacustrine tufa deposit (Table 2, sam-ple B).

Downstream, in the village of Camporrells, an outstanding severalmeters thick, well-cemented tufa deposit occurs. A gradual lateralchange from waterfall facies (mosses) to fluvio-lacustrine/dam facies,composed of several cm-thick accumulations of phytoclasts with thickcarbonate coatings can be appreciated in the outcrop. These tufadeposits are located at 10 m above the present-day altitude of the riverwith an approximate age of 210926 cal yrs BP obtained by U/Th dat-ing of phytoclasts (Table 2, sample A).

Downstream, several other tufa deposits occur at a similar or upperaltitude respect to the present river channel, probably corresponding tosimilar chronologies. The chronology of these aggradation episodescoincide with the major development stages for tufas in the IberianPeninsula and correlate with interglacial periods (MIS 5a and 7c), char-acterized by mild and humid conditions. The high clay mineral content,as a potential source of 232Th, discouraged further dating of these tufadeposits.

Table 2. U/Th dating tufa deposits (Morellón et al., 2008a).Corrected

Sample Weight 238U 232Th δ234U [230Th/238U] [230Th/232Th] ages(g) ppb ppt ppm (years BP)

1.43310 ± 210926.3 ±A 0.1131 660 ± 1 19292 ± 70 540.3 ± 2.6 0.00743 807.8 ± 4.8 3025.6

0.93761 ± 82263.6 ±B 0.1108 446 ± 3 13232 ± 48 686.7 ± 12.5 0.00868 520.4 ± 4.0 1428.6

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Stop 9

General view of Lake Montcortès: Geology of the area

Location: From Camporrells take N-230 road towards Vielha (37km). Take C-1311 road towards Tremp. In La Pobla de Segur continuethrough N-260 to Senterada, and then take L-503 road towardsMontcortès. Park in a small parking area by the shore of the lake beforeMontcortès village. (Duration: 1h 30 min) (Fig 2B).

Observations: Brief introduction to the geology of the area andLake Montcortès climatic, limnological and hydrogeological setting

Description: Lake Montcortès (42° 19’ 50” N, 0°, 59’ 46” E, 1027m a.s.l.) is located in the Southern Pyrenean Range, in the Léridaprovince, near the village of the same name. It is emplaced in the north-ern edge of the South Pyrenean Central Tectonic Unit, mostly com-posed of thrusting sheets of Mesozoic and Paleogene carbonate rocks.Montcortès lies between the Boixols and Les Nogueres thrust sheets.The Boixols thrust sheet limits to the south with the Monsech Thrustunit and towards the north with the Les Nogueres antiformal stack. LesNogueres sheets involved basement (Variscan) formations. TheMontcortès area, close to the northern edge of the unit shows manyexamples of thrust and backthrust tectonic subunits. The detachmentlevel of the thrusting units is commonly the evaporites and fine mud-stones of the Upper Triassic (Keuper facies) (Rosell, 1994). The lakewatershed is situated in Triassic bedrock, mainly comprising carbon-ates, evaporites, claystones and shales (Fig. 16B). The lake morphol-ogy is roughly circular, with a total diameter of 1320 m and a maxi-mum depth of 30 m (Camps et al., 1976). The very steep talus of thelake results from karstification processes in calcareous and gypsiferousmaterials (Corella et al., in press b).

Total mean annual rainfall in this area is 860 mm and the mosthumid seasons are spring and autumn, with mean precipitations of 280and 230 mm, respectively. Mean monthly temperature ranges from 1.9ºC in the coldest month (January) to 20.3 ºC in the warmest (July).

The hydrology of Lake Montcortès is mainly controlled by ground-water and runoff inputs and by evaporation and a small outlet streamoutput. This emissary ravine in the northern shore (Fig. 16C) is a trib-utary of the Flamisell River (3 km W) and controls the maximum waterlevel of the lake. The lake is oligotrophic and meromictic. Thismeromictic nature, relatively high surface/depth ratio (Surface = 0.093km2; Zmax=30 m), and steep margins, have facilitated the depositionand preservation of finely laminated facies.

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Figure 16. Location of Lake Montcortès: (A) Geographical location within theEbro River watershed. (B) Detailed geological map of the Lake Montcortèssurrounding area. Lake catchment area is also indicated. (C) Bathymetric map

of Lake Montcortès and location of the four Kullenberg cores retrieved.

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Stop 10

Lake Montcortès littoral: “The annually laminated record”

Location: Take the first road to the right going south before enter-ing Montcortès village and park 100 m ahead by a small parking areain the southern margin of the lake.

Observations: The depositional evolution since Mid-Holocene andthe varved facies of Lake Montcortès.

Description: Four sediment cores were retrieved in the deepest partof the lake (Fig. 16C) which allowed us to distinguish 6 facies in thelacustrine sedimentary record (Table 3, Fig. 17): i) finely laminated(facies 1 and 2); ii) banded (facies 3); iii) graded, turbidite-like (facies4); and iv) gravitational deposits (facies 5 and 6).

Table 3. Lake Montcortès facies and inferred depositional environments.Facies Depositional context

1. Biogenic varves (Fig. 18) Distal, anoxic hypolimnion, seasonal

Laminated facies calcite production in the epilimnion

2. Finely laminated greenish, organic Shallow lake, high bioproductivity,clay-rich layers and greyish detrital anoxic hypolimnionlayers3. Banded, brownish and greyish LittoralBanded facies carbonate silts

Proximal and distal deposits fromGraded facies 4. Graded detrital layers turbiditic processes

5. Convoluted and disrupted facies Gravitational processes affectinglacustrine sediments

Gravitational 6. Massive, conglomerate Gravitational processes affectingdeposit facies alluvial/colluvial deposits

One of the most conspicuous characteristics of the Montcortès sed-imentary sequence is its finely laminated nature, and the presence ofbiogenic varves (facies 1) in most of the record. They consist on calcite(spring/summer deposition), organic layers (autumn/winter deposi-tion), punctuated by graded, thick detrital layers (Facies 4) (Fig. 18).

The chronology of the Montcortès sequence is based on 17 AMS14Cdates for the whole sequence and varve counting carried out in the last1500 years. The robust age model and the sedimentological, miner-alogical and compositional analyses performed on the sediment coresdemonstrate large depositional and paleohydrological changes duringthe last 6000 cal years (Corella et al., in press b):

- Relatively shallower lake conditions prevailed during the MidHolocene (6000-3500 cal. years BP, unit VI), and deeper envi-ronments with deposition of varves with calcite laminae preser-vation occurred since then (units V to I).

- Lower clastic input occurred during the Iberian-Roman HumidPeriod (unit V), the Little Ice Age (LIA) (unit III) and the XXth

century (unit I). On the contrary, two episodes of high clasticinput to the lake developed: i) during the Middle Ages, coincid-

– 256 –

Figure 17. Lithostratigraphic correlation of the 4 cores retrieved in LakeMontcortès. Each core image is accompanied by its sedimentological descrip-tion and magnetic susceptibility values. Dashed lines represent the correlationbetween the limits of the main sedimentary units. The position of the AMS 14C

dates in the cores is also indicated.

– 257 –

ing with an increase in the farming activity in the area and theMedieval Climate Anomaly (unit IV); and ii) during the XIXth

century (unit II), coinciding with the end of the Little Ice Age andthe maximum human occupancy of the mountain areas in thePyrenees.

- Slope instability has caused large gravitational deposits duringthe Mid Holocene (prior to 6 ka, unit A) and Late Holocene (priorto 1600 and 1010 cal. yr BP, units B and C respectively).

Figure 18. Biogenic varves form Lake Montcortès. (A) 3A core image andstratigraphic section; (B) Thin section photos and SEM images of Montcortès

varved sediments: i) calcite layer; ii) organic layer and iii) detrital layer.

– 258 –

Acknowledgements

The Spanish InterMinistry of Science and Technology (CICYT)provided financial support for this research, through the projects LIM-NOCAL (CGL2006-13327-C04-01), GLOBALKARST (REN2003-09130-C02-02) and GRACCIE (CSD2007-00067).

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