depositional environments of quaternary lacustrine travertines and stromatolites from highaltitude...

8/11/2019 Depositional environments of Quaternary lacustrine travertines and stromatolites from highaltitude Andean lakes, northwestern Argentina

1/21

Depositional environments of Quaternarylacustrine travertines and stromatolites from high-altitude Andean lakes, northwestern Argentina

Blas L. Valero-Garcs, Concha Arenas, and Antonio Delgado-Huertas

Abstract : Four distinctive depositional subenvironments of fossil travertines and stromatolites are identified in threehigh-altitude (35004000 m above sea level) lacustrine basins: El Peinado, San Francisco (Las Coladas Salar subbasin),and Las Peladas (southern Andean Altiplano, northwestern Argentina). These late Quaternary occurrences are character-ized using geomorphological, sedimentological, petrographic, and stable isotopic data. Stromatolites of cyanobacterialorigin only develop in shallow lacustrine margins of El Peinado basin. In the same basin, macrophytic travertines occurboth near thermal spring seepage areas along the lake margin as in situ facies and in littoral lacustrine environments upto water depths of several metres as phytoclastic travertine facies. The stromatolites and macrophytic travertines haverelatively heavy 18O compositions, suggesting initial 16O-depleted waters and (or) evaporation effects through time.Their high 13C compositions are interpreted as a reflection of intense CO 2 evasion from the thermal groundwaters

feeding the lakes. Similar laminated travertine facies, with no petrographic evidence for biotic origin, occur in both LasColadas and Las Peladas basins. Neither petrographic nor isotopic data alone can differentiate between these two cases.Besides, diagenetic overprint in Las Peladas facies precludes the use of isotopic values as original isotopic signatures.However, the depositional environmental conditions defined by the geomorphological and sedimentological features aredifferent. Laminated aragonitic crusts in Las Coladas basin formed in a shallow, saline lake and are associated withshoreline and terrace deposits cemented by aragonite. These travertine crusts represent periods of spring, 16O-rich dis-charge to the lake, as suggested by the lighter oxygen isotopic compositions. In contrast, travertines from Las Peladasoccur as laminated calcitic and aragonitic units intercalated at the top of fining-upward sequences composed of con-glomerates, sandstones, and intraclastic limestones. Sedimentological data suggest that these travertines originated influvial-influenced lake margins during low lake-level episodes.

1283Rsum : Quatre sous-environnements distincts de dposition de travertins et de stromatolithes fossilifres sont identi-fis dans trois bassins lacustres de haute altitude (35004000 mtres au-dessus du niveau de la mer), soit les bassins ElPeinado, Lad Coladas et Las Peladas (Altiplano andain sud, au nord-ouest de lArgentine). Ces occurrences du Quater-

naire tardif sont caractrises au moyen de donnes gomorphologiques, sdimentologiques, ptrographiques etdisotopes stables. Les stromatolithes dorigine cyanobactrienne ne se dveloppent que dans les marges lacustres peuprofondes du bassin El Peinado. Dans ce mme bassin, les travertins macrophytiques se retrouvent prs de rgions desources dinfiltration thermales le long de la bordure du lac, en tant que facis in situ, et dans des environnements la-custres littoraux jusqu des profondeurs de plusieurs mtres, en tant que facis de travertins phytoclastiques. Les stro -matolithes et les travertins macrophytiques ont des compositions relativement leves en 18O, suggrant des eauxinitialement pauvres en 16O et (ou) des effets de lvaporation dans le temps. Leurs compositions leves en 13C sontinterprtes comme le reflet dun chappement intense de CO 2 des eaux souterraines thermales qui alimentent les lacs.Des facis semblables de travertins lamins, sans vidence ptrographique dune origine biotique, se retrouvent dans lesbassins de Las Coladas et de Las Peladas. Ni les donnes ptrographiques ni les donnes isotopiques ne peuvent seulesfaire la diffrence entre ces deux cas. De plus, la surimpression diagntique dans le facis Las Peladas empchelutilisation de valeurs isotopiques en tant que signatures isotopiques dorigine. Toutefois, les conditions environnemen-tales de dposition dfinies par les caractristiques gomorphologiques et sdimentologiques sont diffrentes. Des cro-tes lamines daragonite dans le bassin de Las Coladas forment un lac sal peu profond et elles sont associes aux

dpts de rivage et de terrasse ciments par laragonite. Ces crotes de travertin reprsentent les priodes de printemps,de dcharge riche en 16O vers le lac, tel que suggr par les compositions isotopiques doxygne plus lger. Par contre,les travertins de Las Peladas se prsentent comme des units calcitiques et aragonitiques lamines intercales au

Can. J. Earth Sci. 38: 1263 1283 (2001) 2001 NRC Canada

1263

DOI: 10.1139/cjes-38-8-1263

Received April 3, 2000. Accepted January 29, 2001. Published on the NRC Research Press Web site at http://cjes.nrc.ca onAugust 20, 2001.

Paper handled by Associate Editor M. Savard.

B.L. Valero-Garcs. Instituto Pirenaico de Ecologa, CSIC, Apartado 202, 50080 Zaragoza, Spain.C. Arenas. 1 Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/ Pedro Cerbuna 12, 50009 Zaragoza, Spain.A. Delgado-Huertas. Estacin Experimental de El Zaidn, CSIC, Prof. Albareda 1, 18008 Granada, Spain.1Corresponding author (e-mail: [email protected]).


2/21

sommet de squences affinement vertical ascendant composes de conglomrats, de grs et de calcaires intraclasti-ques. Les donnes sdimentologiques suggrent que ces travertins proviennent de bordures des lacs influencs par lescours deau au cours dpisodes o le niveau du lac est bas.

[Traduit par la Rdaction]

Valero-G arcs et al.Introduction

Travertines are common deposits in many recent andmodern carbonate systems, specifically springs, but alsorivers and lakes (e.g., Chafetz and Folk 1984; Pentecost1995; Ford and Pedley 1996; Sancho et al. 1997; Andrews etal. 1994, 1997). A great variety of deposits are produceddepending on the many variables involved. These includephysical (water temperature, CO 2 degassing rates, andgeomorphology and topography of the depositional areas),chemical (water properties, such as Ca/Mg ratio and Sr content),and biological (presence of macroorganisms and (or) micro-organisms that in some way influence carbonate precipita-tion) processes and factors. Travertines are nonlaminated orcoarsely laminated deposits containing plant and (or) animalremains, frequently with carbonate coatings or encrustations(macrophytic travertines or tufas). On the other hand, manytravertines lack in situ macrophytes (Ford and Pedley 1996),and the resulting deposits are commonly laminated, inde-pendent of their biotic or abiotic origin. In the geological re-cord, laminated travertines have been described in a variety of continental environments of deposition (lakes, rivers,springs, soils, caves; Love and Chafetz 1990; Guo andRiding 1994; Renaut and Jones 2000). In this paper the termtravertine is used as a general name to designate continentalcarbonate rocks that are the result of precipitation of calciumcarbonate from cool and thermal surface waters, either bybiotic or abiotic processes, but excluding those precipitatedin caves. In this sensu latu, travertines include a wide rangeof surface deposits, from massive facies with carbonate-encrusted plant remains to laminated facies without any evi-dence of biological activity. The term stromatolite is onlyused for deposits characterized by fine lamination and withclear evidence of carbonate precipitation related to biologi-cal activity. Stromatolites can form in travertine depositionalenvironments (Ordez and Garca del Cura 1983; Casanova1984; Chafetz et al. 1990), but can also be present in lacus-trine or fluvial systems that lack travertines (Anadn andZamarreo 1981; Freytet and Verrecchia 1989; Arenas et al.1993; Casanova 1994).

Due to the variety of surface environments where travertineand stromatolite facies occur, their ascription to particularenvironmental conditions of formation is not alwaysstraightforward. In this paper, we describe several travertineand stromatolite occurrences in three Quaternary lacustrinebasins from the Andean Altiplano. Geomorphologic,sedimentological, petrographic, mineralogical, and isotopicdata are used to characterize these facies. The proposeddepositional models provide sedimentological criteria toidentify environments of formation of stromatolites and of biotic and abiotic travertines in the geological record.

Geological setting

The three Quaternary basins described in this paper (ElPeinado, San Francisco, and Las Peladas) are located in the

southernmost Andean Altiplano (Catamarca Province,northwestern Argentina) and were formed by tectonic andvolcanic activity during the Plio-Pleistocene (Fig. 1A). Thearea lies in the Ojos del Salado volcanic region, in theCentral Andean Volcanic Province, straddling the borderbetween Chile and Argentina at 2705 S, and coincideswith a major morphological, seismic, and volcanic disconti-nuity (Baker et al. 1987). The Ojos del Salado area is char-acterized by major volcanic structures, including calderas,stratovolcanoes, ignimbrite sheets, compound volcanoes, andcalc-alkaline volcanic rocks. The Central Andes consists of several northsouth mountain ranges and intermontane basinsthat resulted from the subduction of the Pacific Nazca platefrom the Permian to the present (Ramos 1994). The CentralAndes include a high fore-arc region, an active magmatic arc(the Western Cordillera and the Altiplano), and a retro-arcbelt (Eastern Cordillera, and the Chaco Foreland Basin;Brgel Olivares 1983). The Altiplano is a high ignimbriteplateau of 100 000 km 2 in area that extends from 15S to28S at an average altitude of 3800 m, with many activevolcanoes and intermontane lacustrine basins. The threeQuaternary basins belong to a chain of tectonic depressionsbounded by northsouth to NNESSW faults (Fig. 1B).North of the San Buenaventura Cordillera, the largeAntofalla Salar extends for hundreds of kilometres(Martnez 1995). The El Peinado Lake basin constitutes thesouthern end of the Antofalla Salar. South of the SanBuenaventura Cordillera, a large northsouth tectonic valleybounded by Carboniferous to Permian continental andmarine siliciclastic rocks and Ordovician dacites, is filledmainly with Quaternary alluvial and aeolian sediments. Severaltopographic depressions in this tectonic basin containQuaternary lacustrine sediments. They include the SanFrancisco, Las Peladas, Las Lozas, Cazadero Grande, andChaschuil basins.

Methodology

Sediment cores were retrieved in several playa lakes in theSan Francisco and El Peinado basins. Samples from modernsurface sediments and peripheral lacustrine terraces werealso collected. Former lacustrine terraces and a stratigraphicsection were measured and sampled in Quaternary outcropsin Las Peladas. Thin sections were studied by petrographicand cathodoluminiscence microscopy. Scanning electronmicroscope (SEM) observations were carried out with aJEOL JSM 6400 scanning electron microscope at theUniversity of Zaragoza, Zaragoza, Spain. Textures of allochthonous carbonate rocks are described following theclassification of Dunham (1962), with modifications byEmbry and Klovan (1971). The classification of Folk (1962)was used to describe the components and their relative pro-portions in thin section. Mineralogy was determined using aSiemens X-ray diffractometer. The 18O and 2H isotopicvalues for lake waters were determined for El Peinado andthe largest playa lake in the San Francisco basin. Oxygen

2001 NRC Canada

1264 C an. J. E arth S ci. Vol. 38, 2001


3/21

2 0 0 1 NR C C a n a d a

Fig. 1. (A) Geographical location of the Quaternary basins in northwestern Argentina. (B) Geological map of the study area. The basins studiedSan Francisco, and Las Peladas. (C) Location of Las Coladas Salar within the San Francisco basin. (D) Geomorphological map of the Las Peladmeasured Lampallita stratigraphic section.


4/21

2001 NRC Canada


Facies a Depth (cm) Sample No. 13C ( PDB) 18O ( PDB) Mineralogy b

Las PeladasIntraclastic limestone LP-1-1 2.4 2.2 Cal, Qtz, PlIntraclastic limestone LP-1-2 2.4 2.8 Cal, Qtz, Pl

Intraclastic limestone LP-3-1 2.4 2.1 Cal (


5/21

2001 NRC Canada

Valero-G arcs et al. 1267

and carbon isotope analyses were performed on bulk sedi-ment and rock samples using standard techniques (McCrea1950). Powder for isotope analyses was obtained from rock slabs with a microdrilling tool to collect homogeneous samples.The isotopic ratios were measured with a Finnigan MAT251 mass spectrometer at the Estacin Experimental de ElZaidn, Granada, Spain. Analytical precision was better than0.1 for 18O and 13C in carbonates and waters and betterthan 2 for 2H in water. Results are expressed in nota-tion against the Pee Dee Belemnite (PDB) standard for car-bonates and the Vienna standard mean ocean water (V-SMOW)standard for waters (Table 1).

Travertine and stromatolite depositionalsubenvironments

Las Peladas basin

Las Peladas paleolakeSouth of the Buenaventura Range, a northsouth-elongated

tectonic valley, 120 km long and


6/21


7/21


8/21


9/21

2001 NRC Canada


Fig. 5. Photomicrographs of laminated travertines of units 2 and 4 of Las Peladas section. (A, B) Calcite travertines (unit 4) consistingof light-coloured laminae of long columnar and fanlike crystals with intercalated thinner dark laminae of anhedral crystals. Note thepresence in B of long fanlike crystals that pass through lamination. (C, D) Aragonite travertines (unit 2) made of light-coloured laminaeof acicular, columnar and fanlike crystals, alternating with dark, generally micritic, irregular laminae. (E, F) SEM photographs of aragonitictravertines showing acicular and columnar crystal laminae, with intercalated irregular laminae of smaller, anhedral crystals. Note thepresence in F of small anhedral crystals among long crystals.


10/21

2001 NRC Canada


among and within laminae of long crystals of aragonite andcalcite and the existence of acicular crystals encased inprismatic carbonates indicate that the original compositioncould be microspar or anhedral spar calcite, acicular aragoniteand calcite, and micrite calcite laminae.

Aragonite crystals in the laminated travertines were primaryprecipitates from a surface solution, but most of them wereenlarged during the early diagenesis and, as a result, acicularcarbonates were mostly replaced by columnar crystals, withouta change in mineralogy (i.e., aragonite to aragonite). Similarly,calcite to calcite recrystallization could have occurred in thecalcite travertines of unit 4, although there is nopetrographic evidence to rule out inversion of aragonite tocalcite processes. Thus it seems that the large aragonite andcalcite crystals occurring in these travertines resulted fromaggradational neomorphism (aragonite to aragonite andcalcitearagonite to calcite recrystallization). Replacementof micrite or anhedral calcite spar with acicular aragonitealso occurred, but was probably a later process, as suggestedby the presence of some micrite and spar calcite patches.

Although laminated travertines of Las Peladas resemblespeleothem deposits because of the fine and regular lamination,their stratigraphic position, areal distribution, and generallyhorizontal morphology indicate that they formed in flatlacustrine areas with a shallow water depth. Both aragoniticand calcitic travertines seem to be mostly inorganic in origin,as no biological evidence is present (SEM observations),although possible biological remains might have been oblit-erated by aggrading neomorphism. Abiotic travertines havebeen described in freshwater environments that includesprings and streams (Love and Chafetz 1988, 1990; Jones

and Renaut 1996). In many hot springs, the most commoncrystal morphology assumed by aragonite seems to beneedle-shaped crystals (Guo and Riding 1992; Jones andRenaut 1996), whereas calcite has a wider range of morphologies (Love and Chafetz 1990; Jones and Kahle1993; Jones and Renaut 1995). There is no sedimentologicalor geomorphological evidence for thermal springs feedingthe Las Peladas paleolake, and at present thermal springs arenot present in the basin, although they exist in the two otherbasins studied (San Francisco and El Peinado).

San Francisco basin

Las Coladas Salar San Francisco basin can be subdivided into two subbasins

(Fig. 6A). To the north, small hills and shallow depressionsforms a landscape dotted with shallow, small playa lakes. Tothe south lies a larger playa lake (Las Coladas Salar). TheLas Coladas Salar (2657.789 S, 6810.260 W, 4000 m asl)is bounded to the west by lava flows from the Incahuasi andSan Francisco volcanoes. The salar has a large surface, flatbottom, and shallow water depth (up to 25 cm). Severalsmall creeks, groundwater seepage areas, and thermalsprings provide the main water input to the lake. The watersof Las Coladas are saline (electric conductivity 41 900 S/cm),alkaline (pH 8.5), Ca poor (73 ppm), and have a high Mg/Caratio (11; Valero-Garcs et al. 1999 a , 2000) and high waterisotopic values ( 18O = 7.3 SMOW, D = 29 SMOW).

Sedimentary facies

Description: Several short cores (up to 30 cm) were re-

Fig. 6. (A) Stratigraphy of the western bay of Las Coladas Salar. (B) Geomorphological sketch of the carbonate-cemented lacustrineterraces, the travertine crusts, and the modern aragonite mud deposits in Las Coladas Salar.


11/21

2001 NRC Canada


trieved in the western bay of Las Coladas Salar. All showedthe same stratigraphy from bottom to top (Fig. 6A): ( i) grav-els composed mostly of volcanic clasts, ( ii) volcanic lapilli ,and (iii ) carbonate lacustrine muds . Current carbonate pre-cipitation in Las Coladas Salar is dominated by aragonite,due to the high Mg/Ca ratio of the waters. In the westernmargin, where a small creek enters the lake, the alluvialgravel unit and the carbonate units are overlain by silts andsands deposited by the creek prograding into the salar(Fig. 6B). The laminated travertine facies occur as dense,tabular white crusts with small domes (Figs. 7A, 7B), up to5 cm thick and decametres long, laterally associated with ce-mented shoreline and terrace clastic deposits (carbonate-cemented gravels). The carbonate-cemented gravels crop outaround the western edge of the salar at less than 1 m abovecurrent lake level. In the south margin of the western bay,large blocks of the cemented gravels have been removed andpiled up due to wave action. Some areas of the western bayshow a patchy distribution of a travertine crust. All thesefeatures suggest a period of intense carbonate crust genera-

tion. Although U/Th series are not commonly used to daterecent materials, the high U-238 content (55 ppm) of thecarbonate allowed the use of this methodology, which pro-vided an age of 1660 82 BP for the travertine limestonedevelopment at Las Coladas Lake (Valero-Garcs et al.2000).

The laminated travertines are composed entirely of aragonite and microscopically show a very regular and fineparallel lamination marked by alternating black and light-coloured (white and grey) laminae, each 550 m thick,with the light-coloured laminae being the thickest (Figs. 7C,7D). These thick, light-coloured laminae are composed of two or three individual laminae. Black and light-colouredlaminae can be grouped into sets, 0.31 mm thick, in whichone type is dominant. Most crystals are acicular and theirlength defines the thickness of the individual laminae. Dark laminae show no distinct crystals. Back-scattered electronimages did not show any differences in composition betweenthe different coloured laminae. No microbial-shaped remainsare present. Curved cracks affecting several laminae are

Fig. 7. Aragonite laminated travertines of Las Coladas. (A, B) Hand specimen in plan and cross sections. Note the fine laminations inB. Squares in the grid pattern are 1 cm 1 cm. (C, D) Photomicrographs showing alternating light-coloured and dark laminae. Notethe presence in D of very fine acicular crystals perpendicular to laminations.


12/21

common. No diagenetic modifications seem to be present inthe studied samples, so they represent the original aragoniteprecipitates.

Interpretation: The core sediment sequence illustrates recentchanges in the salar hydrology and depositionalsubenvironments. Deposition of aragonite-rich muds overlyingthe gravel unit represents a raise in groundwater level andmore frequent flooded episodes in the salar. On the other

hand, the cemented gravels and the laminated crusts in theshoreline indicate very different hydrology and environmentalconditions in the salar in the past (around 1600 BP). In thisand other Andean salars the laminated travertine faciesresulted from aragonite precipitation from saline waters witha high Mg/Ca ratio and (or) high temperature. The abundantthermal springs and seepage areas around the salar suggestthat they represent a large fraction of the water input to thelake. Acicular aragonite is a very common precipitate inmany hot springs that give rise to travertine deposits (Friedman1970; Pentecost 1990; Guo and Riding 1992). The differ-ently coloured laminae could correspond to variable contentin organic matter and (or) could be related to cyclic changesin chemical, physical, or environmental conditions. There

are no discrete organic remains. As shown by severalauthors, aragonite precipitation from hot springs seems to beepisodic and rapid, with growth dependent on changes in thephysicochemical characteristics of the spring waters (Jonesand Renaut 1996). As pointed out by Pentecost (1995), rapiddegassing and cooling of waters at active thermogene sitesleads to a high precipitation rate.

El Peinado basin

El Peinado LakeEl Peinado Lake (2629 59 S, 6805 32 W, 3820 m asl)

lies on an northsouth-elongated, topographically closed

basin north of El Peinado volcano (Fig. 1B). Waters aresaline (electric conductivity 55 500 S/cm), alkaline(pH 7.6), and dominated by SO 42, Cl, Ca2+, and Na +, andhave a relatively high content of strontium (58 ppm) andboron (135 ppm; Valero-Garcs et al. 1999 a ). The isotopicratios of waters ( 18O = 4.3, D = 6.8) are also rela-tively high. There is no surface outflow to the Salar deAntofalla, although the lake could have overflowed to thenorth during former higher lake level periods. Four lacustrine

terraces occur in the northern edge of the basin (Fig. 8).Sedimentary facies

Description: The lacustrine terraces form tabular sedimentarypackages of about 3040 cm thick and with 100 m of lateralcontinuity. Contrary to other lakes in the Altiplano, the olderlacustrine terraces do not show any travertine facies. Thehighest terrace, T1 (up to 8 m high), is composed of conglomerates and sandstones with volcanic rock clasts andparallel stratification. Calcite cementation and coated grainsof very different sizes (


13/21

2001 NRC Canada


Fig. 9. Photomicrographs of different facies in El Peinado basin. (A) Intraclastic limestones of the middle terraces (T2 and T3).(B) Biomicritic facies of the middle terraces made of pennate diatoms (SEM photograph). (C) General view of a stromatolite sampleshowing gently domed to columnar growths from base to top. Note the existence of filamentous micritic bodies perpendicular tosubperpendicular to lamination. (D) Detail of C. Isolated or loosely clustered filaments attributed to cyanobacteria similar to present-dayCalothrix or Dichothrix . (E) SEM photograph of El Peinado stromatolites showing subvertical calcite filaments and their molds.


14/21

buildup. A section about 3 cm thick shows from the base tothe top (Fig. 9C): ( i) at the base, horizontal lamination,gentle undulating lamination, or discrete domes made of alternating laminae of calcitemicrite and microspar;microbial-shaped bodies (mostly micrite filaments) liesubperpendicular to lamination; ( ii) light-coloured, porousmicrite and dark, dense micrite laminae form small domesthat contain dispersed, dark, clotted micrite grains; and(iii ) at the top of the buildups are alternating light-colouredand dark micrite laminae, or dark micrite and light-colouredmicrosparspar calcite laminae, that constitute fanlikegrowths that are grouped into columns up to 0.5 cm high.Abundant micrite filamentous bodies, 1020 m wide and300500 m long, are isolated or loosely clustered. Thefilaments lie subperpendicular to lamination and passthrough several laminae (Figs. 9D, 9E). They resemblepresent-day Calothrix and Dichothrix growths. Under SEM,abundant pennate diatoms are observed among the generallyanhedral calcite crystals, but particularly in association withthe filamentous cyanobacterial bodies.

Facies with encrusted plant remains (macrophytic travertineor tufa) occur only in the lower terrace and the littoral areasof El Peinado Lake. The macrophytic travertine faciesappear ( i) along the vegetated shorelines close to thermal

seepage areas that feed the lake, and ( ii) in the submergedlittoral zones. The emergent travertines show an open meshwork of calcite-coated stems. Coated stems are very thin (around12 mm in diameter) and lie with vertical orientation(Fig. 10B). A 2.5 m long core collected at 2 m water depthcontained different facies: indurated calcitic crusts, lami-nated muds rich in travertine debris, and in situ and

reworked travertine facies (phytoclastic travertines; Fig. 8;Valero-Garcs et al. 1999 a , 2000). Subrecent travertinedeposits at 2 m water depth (Fig. 8A; facies 1a) display anopen vertical fabric of interlocking stems,


15/21

Las Peladas (unit 2) contain both calcite and aragonite. Theinitial water isotopic composition and the subsequent evapo-rative effects in these saline lakes are more influent factorsthan those due to the isotopic fractionation related tomineralogy; in general, aragonite is only about 0.6 morepositive than calcite precipitated from the same waters(Anderson and Arthur 1983). There is no indication of

diagenetic alteration in El Peinado and Las Coladasoccurrences, so the mineral phases are considered to retain origi-nal isotopic signatures. Diagenetic alterations may havechanged the isotopic compositions of Las Peladas samples tosome degree. In this sense, the relatively large range of vari-ability (3 for 18O) does not preclude a diagenetic imprint.On the other hand, all the stromatolites and travertines studiedhere are fossil occurrences, and consequently we lack thetemperature and isotopic composition data for the formationwaters.

The high 18O values in these high-altitude lake depositsmost likely reflect the large evaporation rates in this aridenvironment. (Table 1; Fig. 11). In the case of Las Peladas,the diagenetic imprint prevents us from drawing conclusionson the environment of formation based on the isotopic signa-tures. In general, the oxygen isotopic composition of thelacustrine carbonates cannot be interpreted exclusively assalinity or evaporation ratio indicators. Valero Garcs et al.(1999 a , 2000) found in the El Peinado core sediments a gen-eral positive correlation between 18O and salinity proxies(Na, Li, and B), but they concluded that the large data dis-persion indicates that other factors besides evaporativeeffects control both chemical and isotope water concentra-tion (e.g., water inputs with different 18O, redissolution of previously precipitated salts). The modern emergentmacrophytic travertines developed on the shoreline of ElPeinado Lake show lighter oxygen and carbon compositionsthan the sediment core samples and the stromatolites(Fig. 11). The higher isotope values of carbonates precipi-tated in the lake waters as submerged travertines andstromatolites suggest enrichment processes due to evapora-tion and a longer residence time of the waters.

The 18O compositions for modern aragonite sediments inLas Coladas display increasing values from the margin(1.0 to +1.0 PDB; site closer to the creek) to thenorthern areas (+2.0 to +3.5 PDB) and to the center of the lake (+5.0 PDB). They also show a clear covarianttrend between 13C and 18O (Fig. 11A). These patterns indi-cate increasing isotopic enrichment of the lake waters due toevaporative effects farther away from the surface inflow anda typical hydrologically closed behaviour (Talbot 1990;Talbot and Kelts 1990; Li and Ku 1997). Samples from thetravertine crusts in Las Coladas show the lowest compositions(6.0 to 1.0 PDB), suggesting less evolved waters.Geomorphological evidence indicates that the formation of the travertine crusts and the cementation of the terracesoccurred during the same period of relative increase in lakelevel. The lower isotopic values of the travertines are inagreement with rapid precipitation of aragonite related toperiods of higher discharge into the lake.

The Pleistocene Las Peladas samples show an isotopicrange (4.0 to 1.0 PDB; Fig. 11A) similar to that of the travertine crust in Las Coladas Salar. The Upper PleistoceneLas Peladas Lake was large and relatively deep (up to several

tens of metres), in accordance with the altitude of theterraces and paleoshorelines. A scenario of large river inflowduring these humid periods, and likely lower evaporation,agrees with the lower isotopic values of the primary carbon-ates precipitated in Upper Pleistocene Las Peladas whencompared with the late Holocene El Peinado and LasColadas lacustrine systems. For comparison, we also have

plotted data from the associated carbonate facies. The LasPeladas biomicrites and intraclastic limestones lie in anarrower, lower 18O field than the travertines (Fig. 11B).Unlike Las Coladas Salar, where travertines show the light-est oxygen compositions, Las Peladas travertines show alarge range and generally higher isotopic ratios than faciesfrom the same sequences. The isotopically heaviest samplesare from the aragonite travertines of unit 2, whereasintraclastic limestones from the same unit show lower val-ues. These differences in isotopic compositions agree withthe following sedimentological interpretation: relativelyhigher lake levels related to fluvial inputs during depositionof the intraclastic facies, and shallower, more concentratedwaters during the formation of the travertines, although less

concentrated in the case of calcitic travertines (unit 4). Thebiomicrites from the northern Las Peladas basin terraces thatcorrespond to the highest lake level also show generallylower oxygen isotope ratios.

Although these environmental interpretations are coherent,caution should be used when considering diagenetic facies.The isotopic composition of Las Peladas travertines mayhave been modified during diagenesis, and thus such recon-structions cannot be considered as conclusive. In these lakebasins, diagenetic waters that caused aragonite to aragoniterecrystallization could have been more evaporated waters,with higher 18O values than the lake waters. This processcan be a consequence of evaporative effects and the resultingaragonite travertines record 18O enrichment with respect totheir original isotopic composition. In the case of calcite tocalcite or aragonite to calcite recrystallization, the isotopiccomposition of the diagenetic waters could have beenlighter, heavier, or even the same as that of the primaryprecipitating lake waters. Thus conclusions on environmen-tal conditions based exclusively on isotopic composition of travertines in Las Peladas basin cannot be established.

Carbon isotopesCarbon isotopic ratios of authigenic lacustrine carbonates

reflect isotopic variations in the dissolved inorganic carbon(DIC), controlled by input, biological processes (mainlyrespiration and photosynthesis), and physical processes(evaporation, residence time, CO

2 degassing; Talbot and

Kelts 1990). Most samples show 13C > 1 PDB (Fig. 11)and are clearly distributed into two groups: ( i) a group withvalues higher than 7.0 PDB (El Peinado and Las Coladas),and (ii) a group with values lower than 6.0 PDB (LasPeladas). Lacustrine carbonates with very high 13C valueshave been reported from very concentrated evaporatingbrines (Stiller et al. 1985; Mees et al. 1998), anoxic sedi-ments (Talbot and Kelts 1990), fresh waters with methaneinput (Nissenbaum and Magaritz 1988), and travertines (Turi1986). High 13C values for carbonates in the AndeanAltiplano have been found in other saline lakes (Grosjean1994; Grosjean et al. 1995; Schwalb et al. 1999) and fluvial

2001 NRC Canada



16/21

travertine deposits (Aravena and Suzuki 1990). Theseauthors have suggested that there is a significant contribu-tion of volcanic-hydrothermal CO 2 in the Altiplano area.

The mechanisms that can generate 13C enrichment in

Andean lakes over values in equilibrium with atmosphericCO2, including evaporation processes and CO 2 degassing,have been discussed elsewhere (Valero-Garcs et al. 1999 a ).The similar, heavy 13C values for the modern aragonite mudsamples at Las Coladas and other facies in the El Peinadoterraces (biomicrites, intramicrites) show that heavy carbonisotopic compositions are not restricted to travertine facies.The large reservoir effect indicated by the accelerator massspectrometry (AMS) 14C dates from El Peinado core sedi-ments (Valero-Garcs et al. 1999 a , 2000) indicates a signifi-cant input of 14C-free volcanic and geothermal CO 2 into ElPeinado. In a volcanic region devoid of carbonate rocks,contamination of the sediments by older carbonates can be

rejected; consequently, the most likely reason for thereservoir effect is the large input of 14C-free volcanic andgeothermal CO 2 from the numerous thermal springs in thearea. An intense CO 2 evasion from the volcanic and geother-

mal springs would preferentially enrich the waters in 13

C.This mechanism provides a coherent explanation for theheavy carbon isotope compositions of lacustrine carbonates inactive volcanic areas. Large enrichments may result from thenonequilibrium gas-transfer isotope fractionation during CO 2degassing from thermal springs and groundwater discharge.Further degassing of carbon dioxide during lake water evap-oration also contributes to increasing 13C and 18O trends.These data also indicate that physical rather than biologicalprocesses are controlling the 13C enrichment in El Peinadoand Las Coladas lakes.

The Las Peladas samples have 13C values between +1and +5.5 PDB (Table 1; Figs. 11A, 11B). Intramicrite and

2001 NRC Canada


Fig. 11. Isotopic composition. (A) Cross-plot including carbonate facies from the three basins. (B) Isotopic composition of the carbonatefacies from Las Peladas basin.


17/21

biomicrite facies display lower values (+1 to +2.5PDB), and travertine facies show higher values (+3.5 to+5.5 PDB). High 13C values in the travertine facies canbe explained by a combination of several processes:(i) increased evaporation effects in the Las Peladas

paleolake during periods of lower lake level, when the lakedid not overflow into Las Lozas subbasin, and decreasedinput of biogenic CO 2; (ii) CO2 degassing processes relatedto travertine formation (Turi 1986); and ( iii ) increased inputfrom heavier thermal-spring waters or 13C-enriched

2001 NRC Canada


Fig. 12. Sedimentary facies models for the different basins studied: (A) El Peinado, (B) San Francisco (Las Coladas subbasin), and(C) Las Peladas. In C, the sketch corresponds to a low lake level period (travertine formation).


18/21

groundwaters from the San Francisco basin. The occurrenceof 13C-enriched carbonates in the San Francisco basin indicatesthe presence of 13C-enriched surface and groundwaters thatcould be transported to the Las Peladas basin duringhydrologically open periods. The lower 13C values of theintramicrites and biomicrites could reflect higher lake-levelconditions when there was a shorter residence time, and

increased input of organic-derived CO 2. A higher input of isotopically lighter carbon sources, like soil-derived CO 2, orproportionally lower recharge of 13C-enriched spring waterscould contribute to such lower values.

Sedimentary models for lacustrinetravertines and stromatolites. Discussionand conclusions

Travertines and stromatolites are common deposits inmany high-altitude Andean Quaternary lake basins(Grosjean 1994; Grosjean et al. 1995; Schwalb et al. 1999).Geomorphological and sedimentological studies and micro-scopic (cathodoluminescence, petrographic, and SEM)observations and stable isotopic analyses ( 13C, 18O) haveallowed reconstructions of the paleohydrological conditionsof different depositional subenvironments for lacustrine trav-ertine and stromatolite occurrences in some of the Andeanlate Quaternary lacustrine basins in northwestern Argentina.Such deposits formed under different geomorphological,hydrological, chemical, and biological conditions. They alloccur in high-altitude basins (35004000 m asl) that origi-nated by tectonic and volcanic activity during the late Qua-ternary. Both El Peinado and San Francisco (Las Coladassubasin) basins are located in active volcanic areas, withgeothermal springs discharging in the lakes. Although todaythe region is one of the driest in the world, the three lakebasins show geomorphological evidence (highstand terracesand shorelines) of large fluctuations in the water balanceduring the Holocene and Upper Pleistocene. The sedimentaryfacies models for the different basins are shown in Fig. 12.The geomorphological, hydrological, geological, and isotopiccharacteristics of these Andean travertines and stromatolitesare summarized in Table 2.

Four travertine and stromatolite environments of formationin Andean lakes have been identified:(1) Fossil stromatolites are only present in El Peinado

basin. Thermal waters constitute a major component of the hydrological input of the lake. Petrographic andSEM observations indicate a cyanobacterial origin. Thestromatolites were formed in the littoral zone during lowlake levels, when conditions were more favourable tocyanobacterial than to macrophyte development.

(2) Macrophytic travertines (El Peinado Lake) occur in veg-etated areas near thermal spring seepage along the lakemargin and in littoral lacustrine environments up towaters depths of several metres. In both cases, in situtravertines consist of an open meshwork fabric of calcite-coated stems ( 12 mm in diameter) that lie in asubvertical position. Most lake sediments in El Peinadoare composed of reworked travertine facies (phytoclastictravertines).

(3) Laminated aragonitic travertines in salars (Las Coladas)lack morphological or microscopic evidence of a

2001 NRC Canada


T y p e

M a i n w a t e r

s o u r c e

H y d r o l o g y

H y d r o c h e m i s t r y

M i n e r a l o g y

B a s i n

m o r p h o l o g y

O r g a n i s m s

C a r b o n a t e

p r e c i p i t a t i o n

D i a g e n e t i c

p r o c e s s e s

1 8 O ( P D B )

1 3 C ( P D B )

M a c r o p h y t i c

t r a v e r t i n e

T h e r m a l

s p r i n g s

N o s u r f a c e

o u t l e t

H i g h s a l i n i t y ; l o w

M g / C a r a t i o s

C a l c i t e

T o p o g r a p h i c a l l y

c l o s e d ; l o w

s d r

E m e r g e d p l a n t s

B i o m e d i a t e d

N o n e

E m e r g e d : 3 . 3

E m e r g e d : 8 . 2

A q u a t i c

m a c r o p h y t e s

S u b m e r g e d : 4 . 5 7 . 8

( n =

3 9 )

S u b m e r g e d : 8 . 2 1 1

. 4

( n =

3 9 )

L a c u s t r i n e

s t r o m a t o l i t e s

T h e r m a l

s p r i n g s

N o s u r f a c e

o u t l e t

H i g h s a l i n i t y ( ? ) ; l o w


C a l c i t e


c l o s e d ; l o w

s d r

C y a n o b a c t e r i a ;

d i a t o m s

B i o m e d i a t e d

N o n e

5 . 3

9 . 2

S a l a r t r a v e r t i n e

T h e r m a l

s p r i n g s

N o s u r f a c e

o u t l e t

H i g h s a l i n i t y ; h i g h


A r a g o n i t e


c l o s e d ; h i g h

s d r

N o n e

P h y s i c o c h e m i c a l l y

i n d u c e d

N o n e

5 . 6

t o 1 . 0

( n = 4

)

8 . 3 1 0

. 9 ( n =

4 )

F l u v i a l -

i n f l u e n c e d

t r a v e r t i n e

R i v e r i n f l o w

( s p r i n g s ? )

S u r f a c e o u t l e t

F l u c t u a t i n g s a l i n i t y a n d


A r a g o n i t e a n d

c a l c i t e


o p e n ; l o w s d r

N o n e

P h y s i c o c h e m i c a l l y

i n d u c e d

R e c r y s t a l l i z a t i o n ;

r e p l a c e m e n t ;

c e m e n t a t i o n

C a l c i t i c 3 . 5

t o 1 . 0

( n =

4 ) ; a r a g o n i t i c

1 . 8

t o 0 . 9 ( n =

3 )

C a l c i t i c 2 5 . 2 ( n =

4 ) ; a r a g o n i t i c 3 . 7

5 . 3 ( n =

3 )

N o t e : s d r , s u r f a c e - t o - d e p t h r a t i o .

T a b l e 2

. C l a s s i f i c a t i o n o f A n d e a n t r a v e r t i n e a n d s t r o m a t o l i t e o c c u r r e n c e s i n n o r t h w e s t e r n A r g e n t i n a a n d s u m m a r y o f t h e

p h y s i c a l , c

h e m i c a l , b

i o l o g i c a l , a n d i s o t o p i c c h a r a c t e r i s t i c s o f

t h e d i f f e r e n t d e p o s i t i o n a l e n v i r o n m e n t s .


19/21

biogenic origin and are interpreted as abiotic carbonateprecipitates. The aragonite mineralogy reflects the highMg/Ca ratio of the lake brine. Their isotopic signaturessuggest lighter initial water compositions and, likely,less important evaporation processes than in El Peinadowaters.

(4) Laminated calcitic and aragonitic travertines in fluvial-

influenced lacustrine margins (Las Peladas) are found atthe top of fining-upward sequences deposited in afluvial-dominated lake margin. During periods of highriver discharge, conglomerates and sandstones weredeposited, whereas during lower river discharge andlower lake level, intraclastic limestones and travertinefacies formed. Like Las Coladas Salar, there is no evidenceof microbial activity in the Las Peladas travertines.Neomorphic processes in these Late Pleistocene travertinesresulted in carbonate cementation, recrystallization, andreplacement textures. Geomorphological, sedimentological,and isotopic data indicate that the main water input tothe Las Peladas paleolake was from rivers, althoughthere may have been some thermal recharge.

Sedimentological, isotopic, and petrographical data fromthese Andean lacustrine facies show how similar laminatedfacies correspond to different depositional environments.Such data provide criteria that may serve to describe andidentify them and better define the paleoenvironmentalsetting of other travertine and stromatolite occurrences.Similar laminated travertine facies with no biotic evidenceoccur in both Las Coladas and Las Peladas, but they havetwo opposite hydrological interpretations. In relatively deeplake basins like Las Peladas, travertine forms in littoral areasduring episodes of low lake level and low river discharge. Incontrast, in shallow and flat basins, like Las Coladas Salar,travertines represent periods of higher spring discharge tothe lake. Neither petrographic nor isotopic data alone candifferentiate between these two cases.

In situ and reworked macrophytic travertines do notprovide reliable indications of water depth during deposition.In El Peinado Lake, similar facies occur in both emergentsettings related to spring seepage areas and in lake floors upto several metres depth. Encrusted charophyte facies occurin up to 9 m water depth in other Altiplano lakes (Valero-Garcs et al. 1996, 1999 b). Phytoclastic travertine facies canreach even deeper areas in the sublittoral zones.

Acknowledgments

Financial support for fieldwork was provided by theUniversidad Nacional de Catamarca, Argentina, and by theDepartamento de Relaciones Internacionales (Consejo Superiorde Investigaciones Cientficas, Spain). This investigation wasalso funded by project PB97-0882-C03-02 of the Spanishgovernment. We are very grateful to the participants of theArcheological Expedition funded by the UniversidadNacional de Catamarca for fieldwork assistance and particularlyto their leader, Norma Ratto (University of Buenos Aires),for her enthusiasm, energy, and constant encouragement. Weappreciate the help of Loren Hoppe as field assistant anddedicated supporter. The logistical support of the ArgentinianGendarmeria Nacional, particularly the Destacamento deGendarmeria Nacional in Las Grutas Paso de San

Francisco, is also greatly appreciated. Ramon Juli kindlyprovided the U/Th date for the Las Coladas travertine sample.We thank Robin Renaut for his inspiring comments on aprevious version of the manuscript. We also thank the re-viewers Henry Chafetz, Hartmut Maecker, and MartineSavard and the editor Brian Jones, whose suggestions andcriticisms improved the manuscript.

ReferencesAnadn, P., and Zamarreo, I. 1981. Paleogene non-marine algal

deposits of the Ebro Basin, northeastern Spain. In Phanerozoicstromatolites. Edited by C. Monty. Springer-Verlag, Berlin,pp. 140154.

Anderson, T.F., and Arthur, M.A. 1983. Stable isotopes of oxygenand carbon and their application to sedimentologic andpaleoenvironmental problems. In Stable isotopes in sedimentarygeology. Chap. 1. M.A. Arthur, organizer. Society of EconomicPaleontologists and Mineralogists (Society for Sedimentary Ge-ology), Short Course 10, pp. 1151.

Andrews, J.E., Pedley, H.M., and Dennis, P.F. 1994. Stable isotope

record of palaeoclimatic change in a British Holocene tufa.Holocene, 4: 349355.

Andrews, J.E., Riding, R., and Dennis, P.F. 1997. The stableisotope record of environmental and climatic signals in modernterrestrial microbial carbonates from Europe. Palaeogeography,Palaeoclimatology, Palaeoecology, 129 : 171189.

Aravena, R., and Suzuki, O. 1990. Isotopic evolution of riverwater in the northern Chile region. Water ResourcesResearch, 26 : 28872895.

Arenas, C., Pardo, G., and Casanova, J. 1993. Bacterialstromatolites in lacustrine Miocene deposits of the Ebro Basin(Aragn, Spain). Bolletino della Societ Paleontologica Italiana,Special Vol. 1, pp. 922.

Arenas, C., Gutirrez, F., Oscar, C., and Sancho, C. 2000.Sedimentology and geochemistry of fluvio-lacustrine tufadeposits controlled by evaporite solution subsidence in thecentral Ebro Depression, NE Spain. Sedimentology, 47 :883910.

Assereto, R., and Folk, R.L. 1980. Diagenetic fabrics of aragonite,calcite and dolomite in an ancient peritidalspelean environ-ment: Triassic calcare rosso, Lombardia, Italy. Journal of Sedi-mentary Petrology, 50: 371394.

Baker, P.E., Gonzlez-Ferrn, O., and Rex, D.C. 1987. Geologyand geochemistry of the Ojos del Salado volcanic region, Chile.Journal of the Geological Society of London, 144 : 8596.

Brgel Olivares, R. 1983. Geomorfologa. Geografa de Chile.Vol. 2. Instituto Geogrfico Militar de Chile, Santiago deChile.

Casanova, J. 1984. Gense des carbonates dun travertin

plistocne: interpretation palocologique du sondage Peyre I(Comprgnac, Aveyron). Geobios, Special Memoir 8, pp. 219225.

Casanova, J. 1994. Stromatolites from the East African Rift: a syn-opsis. In Phanerozoic stromatolites II. Edited by J. Bertrand-Sarfati and C. Monty. Kluwer Academic Publishers, Dordrecht,The Netherlands, pp. 193226.

Chafetz, H.S., and Folk, R.L. 1984. Travertines: depositionalmorphology and the bacterially constructed constituents. Journalof Sedimentary Petrology, 54: 289316.

Chafetz, H.S., Utech, N.M., and Fitzmaurice, S.P. 1990. Differ-ences in the 18O and 13C signatures of seasonal laminaecomprising travertine stromatolites. Journal of SedimentaryPetrology, 61: 10151028.

2001 NRC Canada



20/21


21/21


dean Altiplano lakes, northwest Argentina. Earth and PlanetaryScience Letters, 171 : 236266.

Valero-Garcs, B.L., Grosjean, M., Schreir, H., Kelts, K., andMesserli, B. 1999 b. Holocene lacustrine deposition in theAtacama Altiplano: facies models, climate and tectonic forcing.Palaeogeography, Palaeoclimatology, Palaeoecology, 151 : 101125.

Valero-Garcs, B.L., Delgado-Huertas, A., Ratto, N., Navas, A.,and Edwards, L. 2000. Paleohydrology of Andean saline lakesfrom sedimentological and isotopic records, northwesternArgentina. Journal of Paleolimnology, 24(3): 343359.

depositional environments of quaternary lacustrine travertines and stromatolites from highaltitude...

Documents