Rock Glacier Inventory of the Valles Calchaquíes Region (~ 25°S), Salta, Argentina, Derived from ALOS Data

PERMAFROST AND PERIGLACIAL PROCESSESPermafrost and Periglac. Process. 25: 69–75 (2014)Published online 3 February 2014 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/ppp.1801

Short CommunicationRock Glacier Inventory of the Valles Calchaquíes Region (~ 25°S), Salta, Argentina,Derived from ALOS Data

Daniel Falaschi,1* Mariano Castro,1 Mariano Masiokas,1 Takeo Tadono2 and Ana Lía Ahumada3

1 Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CCT-CONICET Mendoza, Mendoza, Argentina2 Earth Observation Research Center (EORC), Japan Aerospace Exploration Agency (JAXA), Sengen, Tsukuba, Ibaraki, Japan3 Fundación Miguel Lillo-CONICET, Miguel Lillo, Tucumán, Argentina

* CoNivoCONE-ma

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ABSTRACT

Rock glaciers near the Andean mountains of central and northwestern Argentina provide an important supply ofwater for agriculture, but data on their number, size, geographic distribution and altitudinal range are poorly known.Here, we provide the first detailed rock glacier inventory of the Valles Calchaquíes region of Salta, Argentina, basedon 2.5 m x 2.5 m resolution Advanced Land Observing Satellite PRISM (Panchromatic Remote-sensing Instrumentsfor Stereo Mapping) satellite images. Rock glacier outlines were manually digitised and a total of 488 were identified,which cover a total area of c. 59 km2. The lower limits of intact and fossil rock glaciers are at c. 4300 m asl and 4100m asl, respectively. Because rock glaciers are protected by law in Argentina, the inventory provides an importantbasis for political decision-making about water resources and for further periglacial and environmental studies in thisregion. Copyright © 2014 John Wiley & Sons, Ltd.

KEY WORDS: rock glacier inventory; ALOS; Valles Calchaquíes; Cordillera Oriental; Argentina

INTRODUCTION

In the vast, semi-arid areas adjacent to the Andes in centraland northwestern Argentina, agriculture relies on meltwatercarried by mountain rivers. Glaciers and rock glaciers playan important, regulatory role in these hydrologic systems bystoring ice and snow during snowy periods and providing abase flow during dry years when little snowfall accumulatesat high elevations (Brenning, 2005; Azócar and Brenning,2010; Gascoin et al., 2010). Surprisingly, despite consider-able efforts to understand Andean rock glaciers in Argentina(Corte, 1976, 1983; Trombotto et al., 1999; Brenning andTrombotto, 2006), scientific publications on rock glacierinventories there remain scarce (Perucca and Esper Angillieri,2008; Esper Angillieri, 2009; Trombotto Liaudat et al., 2012).Thus, data about the number, size, geographic distributionand altitudinal range of rock glaciers in large portions of theArgentinean Andes are poorly known.In this study, we present a recent inventory of intact and

fossil rock glaciers (Barsch, 1996) in the Valles Calchaquíes

rrespondence to: D. Falaschi, Instituto Argentino delogía, Glaciología y Ciencias Ambientales (IANIGLA), CCT-ICET Mendoza C.C. 330 - (5500) Mendoza, Argentina.il: [email protected]

right © 2014 John Wiley & Sons, Ltd.

- Cordillera Oriental region of Salta, Argentina (Figure 1).In 2010, all natural water resources in a frozen state inArgentina were protected by a federal law, which alsomandated the creation of a national inventory of glaciersand rock glaciers. Our study was developed within theframework of this initiative, which intended to identify,inventory and monitor all glaciers and rock glaciers alongthe Argentinean Andes. The results presented here consti-tute the first detailed assessment of existing periglacialfeatures in the Valles Calchaquíes region (Igarzábal, 1983;Schellenberger et al., 1998; Lazar et al., 2007) and maybe used as a reference for additional hydrological, meteoro-logical, geocryologic and geomorphologic analyses in thislittle known area.

STUDY AREA

The study area is located in the southernmost reaches of theCordillera Oriental, a high-mountain range located to theeast of the main Andean axis in northwestern Argentina(Figure 1). Several peaks exceed 6000 m asl in the area,including El Libertador (6380 m asl) and Hoygaard (6185m asl) (Vitry, 2008). This study focuses on the Valles

Received 4 July 2013Revised 17 December 2013Accepted 5 January 2014

Figure 1 Location of the study area in the southern tip of South America (inset) and a general view of the inventoried intact and fossil rock.The highlighted A and B sectors are shown in detail in Figures 2 and 3.This figure is available in colour online at wileyonlinelibrary.com/

journal/ppp.

70 D. Falaschi et al.

Copyright © 2014 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 25: 69–75 (2014)

Rock Glacier Inventory of the Valles Calchaquíes, Argentina 71

Calchaquíes region in the Argentinean province of Salta(24°10’S–25°20’S, 66°W–66°30’W, Figure 1).Located at the latitude of the desert Andes, the region is

arid to semi-arid, which precludes the development of largeglaciers. As a result, only a few small perennial snowfieldsand ice patches exist close to the highest summits, generallyabove 6000 m asl. However, the mountains in the VallesCalchaquíes support numerous rock glaciers and otherminor periglacial features. The Valles Calchaquíes exhibitan accentuated seasonal precipitation pattern characterisedby a wet season during the austral summer (November toMarch) and a dry season during the remaining coldermonths. According to Lazar et al. (2007), annual precipita-tion totals can reach c. 400–450 mm in the mountains andresult from convective storms, when tropical moist airmasses from the tropics reach the region (Garreaud, 2009).The bedrock associated with rock glaciers in the study

area includes low- to medium-grade meta-sedimentaryrocks of the Puncoviscana and La Paya Formations,Neoproterozoic to Eocambrian granites of the Cachi Forma-tion and effusive rocks of basaltic to andesitic compositionof Neogene age that crop out in the northernmost sector ofthe study area (Rumibola and Abra del Gallo Formations).

Figure 2 ALOS PRISM detail of sector A of Figure 1 showing large,composite rock glacier structures commonly found in the Valles

Calchaquíes region. See text for abbreviations.

DATA AND METHODS

Four orthorectified, 2.5 m x 2.5 m resolution ALOS PRISM(Panchromatic Remote-sensing Instruments for Stereo Map-ping) images from June 2009 and November 2010 were usedto identify, map and inventory the intact and fossil rock glaciersof theValles Calchaquíes region (Figure 1). The landformsweremanually digitised on screen and the areas were calculatedusing open-source Kosmo GIS software (www.opengis.es).According to their shape, rock glaciers were divided intotongue, lobate and coalescent shaped. Perennial snowfields,glacial moraines and cirques were also identified and used tocreate a geocryological map of the study area. The projec-tion used was UTM zone 19 south, WGS84 datum.Intact rock glaciers (Barsch, 1996) include both active

and inactive landforms and contain subsurface permafrost,whereas all of the ice originally within fossil (relict) rockglaciers has melted. The ice content of rock glaciers isdifficult to determine without direct methods such asboreholes (Haeberli et al., 2006), and few have been drilledin the Andes (Trombotto and Borzotta, 2009). Geophysicalsoundings and geodetic measurements may help to infer theice content and assess the movement of rock glaciers(Haeberli et al., 2006), but none such measurements wereavailable in our study area.We classified the rock glaciers as intact or fossil, based on

indirect, morphologic criteria identified in the satelliteimages. A key indicator used to distinguish between intactand fossil rock glaciers was the steepness of the front talusslopes, which often exceed 35° on intact rock glaciers(Ikeda and Matsuoka, 2002). Intact rock glaciers alsoshowed morphological differences that were tentatively

Copyright © 2014 John Wiley & Sons, Ltd.

interpreted as indicating the degree of activity of the rockglaciers. Based on this evidence, and noting the preliminarynature of this classification, we divided the intact featuresinto active and inactive rock glaciers. The former generallydisplay: (1) steep front slopes (> 45° in some cases);(2) little if any sign of degradation of the surface topography;(3) little if any accumulation of large boulders at the talusfoot; and (4) exposure of fine debris at the talus front(indicating a front slope angle greater than the angle ofrepose of the loose material, which in turn suggests that thefront is advancing).

Fossil rock glaciers result from permafrost degradationand tend to have gentler slopes than intact rock glaciers.The following features were considered to differentiate fos-sil from intact rock glaciers: (1) a degraded topography andpoor preservation of transverse or longitudinal furrows andridges; (2) a notable amount of herbaceous vegetationgrowing on the landform’s frontal and lateral slopes; and(3) large block accumulations at the talus bottom(Wahrhaftig and Cox, 1959; Haeberli, 1985; Roer andNyenhuis, 2007). Composite landforms, where newerlandform generations override older ones, were mapped asmultiple units (Figure 2).

As no detailed geologic map is available for the studyarea, the lithology of each rock glacier was obtained fromgeological maps at a 1:250 000 scale (Blasco et al., 1996;Hongn and Seggiaro, 2001), coupled with high-resolutionGoogle EarthTM images. In order to check the remotesensing interpretations, we visited the Las Pailas valley inthe more accessible Nevados de Cachi mountain range inOctober 2012. During this visit, a few landforms missingfrom the preliminary map were included, equivocal rockglacier limits were tracked with GPS, lithology and grainsize of landforms were inspected (the latter to identifylandforms as pebbly or bouldery rock glaciers), and anglesof frontal slopes were measured with an inclinometer todistinguish between intact and fossil landforms.

A 10 m x 10 m resolution digital surface model (DSM)was generated with ALOS PRISM stereopairs using DSMand Ortho-image Generation Software, specially developedfor PRISM data by the Japan Aerospace ExplorationAgency (JAXA). The geolocation accuracy of ALOS

Permafrost and Periglac. Process., 25: 69–75 (2014)

http://www.opengis.es


PRISM nadir-looking, forward- and backward-looking im-ages was reported as 8.1 m, 7.4 m and 9.3 m, respectively,by Tadono et al. (2009). The DSM was used to extractmorphometric parameters of landforms (minimum, meanand maximum elevation) and their talus shed (mean aspect).Since the mean aspect of a certain slope or landform is acircular parameter, it cannot be calculated by simplyaveraging the orientations of individual grid cells of aDSM. Here, this parameter was computed by decomposingthe sine and cosine values of the orientation grid, asrecommended by Paul et al. (2010).No ground control points (GCPs) were used for the DSM

generation, but Takaku and Tadono (2009) indicate that ingeneral, the horizontal accuracy of ALOS PRISM DSMsis less than 6 m and the vertical error is between 2 m and18 m even without the benefit of GCPs. In order to validatethe precision of our DSM, during the field trip to the area wecollected Global Navigation Satellite System (GNSS) dataat ten selected points using a Trimble GEOXH6000(Trimble Navigation Limited935 Stewart DriveSunnyvale,California 94085) receiver. The coordinates and elevationof these points were differentially corrected using data fromthe University of Salta permanent GPS base station. Themean horizontal and vertical precision of these GCPs is0.13 m and 0.20 m, respectively. Comparison with theircoordinates in the DSM showed minor differences that wereconsidered acceptable given the purposes of this study. Thehorizontal shift was negligible and the mean vertical biaswas only 4 m with a standard deviation and root meansquared error of 2.8 m and 4.8 m, respectively.

RESULTS

A total of 488 rock glaciers (422 intact and 66 fossil land-forms) covering an area of c. 59 km2 and with approximatelower limits at c. 4300 m asl and 4100 m asl, respectively,were identified and inventoried in the Valles Calchaquíesregion (Figure 1; Table 1). The largest mapped intact rockglacier in the area is c. 1.17 km2, whereas the area of thesmallest landforms identified and inventoried is of the orderof 0.001 km2. The ability to identify and characterise thesesmall features (which are not readily identifiable in other

Table 1 Summary of the rock glacier inventory in the Valles Calch

Rock glaciers NumberSurface area of rock

glaciers (km2)

Intact 422 38.6Fossil 66 19.9Total 488 58.5

GraniteNumber of rock glaciers 65Surface area of rock glaciers (km2) 9.2


coarser resolution images such as the Advanced SpaceborneThermal Emission and Reflection Radiometer (ASTER) orLandsat) highlights the importance of using high-resolutionoptical imagery for properly mapping small-scale periglaciallandforms in any remote, mountainous region.

Tongue-shaped rock glaciers are more abundant thanlobate rock glaciers (47.7% vs 9.2% of the total number ofunits, respectively), and a large percentage (43.1%) of rockglaciers are coalescent. Most of the inventoried rockglaciers are of the talus type (Barsch, 1996), and only eightwere classified as debris rock glaciers; the latter originatefrom lateral moraines probably affected by cryogenicprocesses after deglaciation. Compound rock glaciers arecommon in the area. We found several sites showing asequenced arrangement of intact rock glaciers disposed ina continuum where newer lobes override older ones, whichin turn override fossil lobes usually located at the bottom ofthe compound landform. A clear example was observed atthe Nevados de Cachi range, where fossil rock glaciers areattached to a conspicuous lateral moraine along the LasPailas valley up to the enclosing cirque in a continuum offossil and possibly inactive and active landforms (Figure 3).

The GIS analyses, later supported by our field surveys in thisarea, suggest an altitude of c. 4300 m asl as the approximatelower limit of intact landforms and 4100 m asl for fossil onesin the eastern side of the study area. The western, drier flanksof the Cordillera Oriental exhibit far fewer rock glaciers, andexcept for occasional, large granite rock glaciers, the rockglaciers in this sector are consistently smaller than those locatedfurther east. The western neighbours do not display acompound arrangement of landforms, and the intact landformsappear at higher elevations, generally above 4650 m asl.

The minimum elevation of a subset of 284 intact, individualrock glaciers (discarding composite landforms with partly hid-den or overridden lobes) and the orientation of their respectivetalus shed are shown in Figure 4A. The analysis of the averageminimum elevation of these rock glaciers shows that landformswith south-exposed contributing areas generally reach onaverage lower altitudes (c. 4800 m asl) than those with north-oriented ones (c. 5200 m asl for the NW sector).

Granitic rock glaciers were mainly identified in theNevados de Cachi range, whereas volcanic ones aregenerally located on the northernmost sector of the study

aquíes region.

Rock glacier area (km2) Elevation (m asl)

Min. Max. Mean Min. Max. Mean

0.001 1.17 0.091 4183 5908 48730.002 1.875 0.301 4072 5397 4695

LithologySchist Volcanic Mixture Total363 39 21 48843.1 2.7 3 58.5


Figure 3 Middle section of the Kühn amphitheatre above 4700 m asl (sector B in Figure 1) showing a lateral moraine (m) and polymorphic rock glaciers withfossil (f) and intact – possibly inactive (ii) and possibly active (ia) – phases.

Figure 4 (A) Minimum altitude of intact rock glaciers vs aspect of contributing area. The grey line represents the average elevation for each aspect sector; (B)spatial density of presumably active rock glaciers as a function of elevation.


area. At the same elevation and slope aspect, rock glaciersin granitic areas are on average larger than rock glaciers inother geological units. The median size of intact, graniterock glaciers is 0.044 km2, whereas schist, mixed graniteand schist, and volcanic rock glaciers average 0.031 km2,0.095 km2 and 0.02 km2, respectively. According to aKruskal-Wallis H test, these difference in rock glacierlithologies are statistically significant at the 99 per centconfidence level (χ2 = 12.025, df= 3, p = 0.007). Graniticactive layers may have> 2-m diameter boulders and com-monly form ‘bouldery rock glaciers’. On the other hand,the more labile schist and volcanic rocks cannot producesuch large blocks (< 50 cm at most) and normally form‘pebbly rock glaciers’ with matrix-supported active layers(Ikeda and Matsuoka, 2006).


Active and inactive rock glaciers are concentrated mostlyin two altitudinal belts, with the former generally above c.4600 m asl. A few landforms appear at lower elevations,probably relating to their location in narrow cirques with asouthern orientation. The ratio between the area covered byactive rock glaciers (23.8 km2) and the area above 4600 masl results in an average spatial density of c. 2.2 per cent(Figure 4B). The altitudinal belt between 4600 m asl and4100 m asl contains both fossil and intact rock glaciers(including active and inactive landforms), and hence couldbe identified as an area of sporadic permafrost (Brenning,2005). According to this tentative delimitation, discontinuousmountain permafrost, whose lower limit can be estimatedfrom the occurrence of the active rock glacier fronts (Barsch,1977, 1978; Brenning, 2005), may exist above 4600 m asl.



DISCUSSION

Our findings are consistent with those of Schellenbergeret al. (1998), who located the lower limit of active rockglaciers at 4650 m asl based on field observations, althoughthey identified the lowermost altitudinal limit of fossil land-forms at 4300 m asl, which is higher than in the presentstudy. On the other hand, the lower limit of discontinuouspermafrost in the Nevados de Cachi range was placed at c.4800 m asl by Lazar et al. (2007). These differences mayarise from these authors limiting their surveys to a restrictedportion of the upper Pailas valley rather the entire mountainrange. Our elevation limit of 4600 m asl for active rockglaciers is somewhat higher than those found in other stud-ies at similar latitudes, where the front talus of active rockglaciers occurred at 4000 m asl, 4100 m asl and 4200 masl in the Nevados de Aconquija, Nevados de Chañi andNevados de Catreal, respectively (Ahumada et al., 2009).Differences in rock glacier size depend on the orientation

and lithology of the inventoried rock glaciers. At similarelevations, the largest units were consistently found on theeastern flanks of the mountains, reflecting the dominanteasterly (Atlantic) source of moisture in this region. Inaddition, because schist is the most widespread lithologyin the study area, most rock glaciers are composed of thisrock type and the ‘pebbly rock glacier’ is the most commontypology. Bouldery rock glaciers were associated withgranitic slopes. This agrees with Ikeda and Matsuoka(2006), who indicate that the main difference betweenbouldery and pebbly rock glaciers lies in the inherited char-acteristics of the source rockwall. These differences may berelated to the sensitivity of each feature to a given change inclimatic conditions. Barsch (1996) suggested that activelayers consisting of large, openwork boulders form macro-spaces of air that act as an isolating layer, being moreeffective for preventing thaw of the underlying permafrostthan pebbly active layers. The opposite thermal behaviourcan be expected in well-coupled, matrix-supported blockyactive layers (Brenning et al., 2012).Although we found statistically significant differences in

the size of rock glaciers relating to their lithology, recentstudies of frost weathering (Matsuoka and Murton, 2008)reveal that moisture availability, sustained freezing for longperiods over freeze-thaw cycling and the amount of frostcracking related to existing cracks on the rock are moreimportant for debris production than the rock type itself.


CONCLUSIONS

This inventory of rock glaciers is the most detailed availablein the Valles Calchaquíes region in northwestern Argentina.The use of high-resolution ALOS satellite images allows aclear and detailed identification not only of the limits ofthe rock glaciers but also of important morphological fea-tures used to classify them. Additional analyses could beimplemented to corroborate the classification and activitystate of a representative number and type of the unitsinventoried here. However, these more detailed methodsfor deriving rock glacier flow such as photogrammetric,geodetic, or differential GPS surveys are generally applica-ble to a reduced number of landforms but are of limitedviability at a regional scale. Unfortunately, none of thesemeasurements were available for our study area. AlthoughRoer and Nyenhuis (2007) demonstrated the limitations ofdetermining the activity status of rock glaciers based ongeomorphologic evidence alone, we suggest that geomor-phologic interpretations can still provide a valuable insightinto rock glacier activity at a regional inventory scale.

Because rock glaciers are protected by law as strategicwater resources for Argentina, this inventory builds animportant base for political decision-making and identifiesresearch opportunities for regional geocryologic studies. Ahighly relevant but poorly studied issue in this water-scarceregion relates to rock glacier hydrology and its significanceas a water source for local communities in the lower por-tions of the Valles Calchaquíes. Unfortunately, such analy-ses are hampered by the poor quality of the hydro-climaticdata currently available. The establishment, maintenanceand analysis of high-altitude meteorological and hydrologi-cal records could provide valuable information to improvethe current knowledge of frozen water resources in theValles Calchaquíes region.

ACKNOWLEDGEMENTS

This study was funded by the Agencia Nacional de PromociónCientífica y Técnica (grant PICT 2007–0379), the JapanInternational Cooperation Agency and the JAXA through theproject Developing a glacier inventory in the ArgentineanAndes using high-resolution ALOS (Advanced LandObserving Satellite) data. Santiago Casimiro, Juan PabloScarpa and Ivanna Pecker provided valuable field support.

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Rock Glacier Inventory of the Valles Calchaquíes Region (~ 25°S), Salta, Argentina, Derived from ALOS Data