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Quaternary International 210 (2009) 6–17

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Quaternary International

journal homepage: www.elsevier .com/locate/quaint

Geological, pedological and paleomagnetic study of the late Cenozoicsedimentary sequence in southwestern Uruguay, South America

Ofelia R. Tofalo a,*, Marıa Julia Orgeira a,b, Hector Morras a,c, Carlos Vasquez b,d, Leda Sanchez e,Ernesto Pecoits f, Natalie Aubet f, Gonzalo Sanchez d, Wolfgang Zech g, Lucas Moretti c

a Universidad de Buenos Aires, FCEN, Departamento Ciencias Geologicas, Ciudad Universitaria, Pabellon II, Buenos Aires, Argentinab Consejo Nacional de Investigaciones Cientıficas y Tecnicas (CONICET), Argentinac INTA-CIRN, Instituto de Suelos, 1712 Castelar, Argentinad Universidad de Buenos Aires, CBC, Argentinae Departamento de Geologıa y Paleontologıa, Universidad de la Republica, Montevideo, Uruguayf Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB, T6G 2E3 Canadag Institute of Soils, University of Bayreuth, Germany

a r t i c l e i n f o

Article history:Available online 2 July 2009

* Corresponding author. Tel./fax: þ51 11 4576 3329E-mail address: rtofalo@gl.fcen.uba.ar (O.R. Tofalo

1040-6182/$ – see front matter � 2009 Elsevier Ltd adoi:10.1016/j.quaint.2009.06.022

a b s t r a c t

The results of interdisciplinary studies carried out on the coastal cliffs of southern Uruguay are presentedhere. The Raigon Formation (late Pliocene–middle Pleistocene) has a cratonic provenance and comprisestwo sections limited by a regional discontinuity. Each section is composed of an association of lithofaciesSs, Sp, Sr, rare St, and Fm. The last one represents floodplain deposits and the others are interpreted aschannel-filled deposits. In the upper part, a well-structured paleosol with clay coatings and abundantrhizoliths is observed in a sharp contact with silty deposits of the Libertad Fm. (early-middle Pleisto-cene); these sediments have a mixed provenance (magmatic arc and cratonic), sheet-like geometry, thickstratification and vertical walls, and are composed of lithofacies Fr. These characteristics suggest a loessicorigin. A paleosol with illuvial B horizons and silty loam texture is recognized at the bottom of this unit.The recent soil (Argiudoll) is developed at the top of the Libertad Fm. The characteristics of the sequenceindicate climatic fluctuations and several cycles of erosion, sedimentation and pedogenesis. According tomagnetostratigraphy, the chronology is roughly compatible with that one based on the fossil contentsand is in agreement with magnetic ages determined in Argentine fossiliferous sequences.

� 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

The Raigon Fm. was defined by Goso and Bossi (1966) and cropsout in the coastal cliffs of the Department of San Jose in Uruguay(Fig. 1). It has a maximum thickness of 30 m, and has been assignedto a ‘‘Chapadmalense’’–‘‘Ensenadense’’ age (late Pliocene–middlePleistocene; Perea and Martınez, 2004). In outcrops, it lies over theCamacho Fm., but it also overlies the Fray Bentos Fm. (Bossi, 1966)and on the crystalline basement (Spoturno and Oyhantçabal, 2004).Unconformities can be observed only in some localities. It iscomposed of green clays, clayish green fine sands, white fine sands,white feldspar medium sands, coarse feldspar sands andconglomerate horizons (Bossi and Navarro, 1988). Following Francisand Mones’s (1965) criteria, Andreis and Mazzoni (1967) named itthe San Jose Fm. and divided it into two sections: a lower one

.).

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formed by clays, silts, sandy silts and gray-greenish subordinatesands, and a higher one composed of middle to very coarse, pink toyellowish sandy sediments.

The Libertad Fm., on the other hand, was defined by Goso andBossi (1966) and crops out in southwestern Uruguay (Fig. 1). It isabout 20 m thick and, as most of the fossils attributed to thisFormation have inadequate chronostratigraphic control, it isconsidered, using essentially stratigraphic criteria, to belong to theearly-middle Pleistocene (Martınez and Ubilla, 2004). It liesunconformably on Tertiary and Cretaceous formations and Paleozoicrocks; its top is also unconformable with formations of middle andlate Quaternary age (Spoturno and Oyhantçabal, 2004). It includesfriable solid mudstones with scattered coarse sand and abundantcalcium carbonate (Bossi et al., 1998). According to Panario andGutierrez (1999), Libertad I Fm. is an early Pleistocene eolian loessicmaterial that has a similar origin as those found in the ArgentinePampas. Recently, Ortız et al. (2006) have indicated that althoughthe Libertad Fm. was considered an eolian deposit, it appears to bethe product of weathering of different types of outcropping rocks,

Fig. 1. Location map. 1: Mauricio cliffs, 2: Kiyu beach, and 3: Arazatı cliffs.

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–17 7

among which are the sedimentary rocks of the Raigon Fm. Accordingto its characteristics and its stratigraphic position, it appears tocorrespond to the Libertad I Fm. (Goso Aguilar, 2006).

Several investigations have been carried out in South Americawith the aim of determining the magnetic age of late Cenozoic

Fig. 2. Magnetic ages of late Cenozoic continental

continental sequences. The sequences composed of loessic sedi-ments bearing mammal fossils are particularly interesting. Fig. 2(taken from Zarate and Orgeira, in press) summarizes the currentstate of the knowledge regarding this subject. The thick lines thatconnect the magnetostratigraphy obtained by the different authors

sequences (from Zarate and Orgeira, in press).

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–178

represent chronostratigraphic levels defined conceptually withcriteria identical to those proposed by Valencio and Orgeira (1983).In particular, they agree partially and represent an expansion ofcorrelations 3 and 4 suggested by Orgeira (1991).

The outcrops at the Atlantic coast of the Province of BuenosAires, Argentina, are particularly interesting since they are rich infossil deposits that represent a complete succession of the tradi-tional South American late Cenozoic mammal ages (sensu Pascualet al., 1965), during and after the ‘‘Great American Land–MammalInterchange’’. This succession was paleomagnetically surveyed insuccessive works by Valencio and Orgeira (1983) and Orgeira (1987,1988, 1990), among others.

As observed in Fig. 2, the upper chronostratigraphic level (whichseparates sediments with Brunhes magnetic age from those ofMatuyama magnetic age) is not always located in the same lithos-tratigraphic unit or mammal age. On the other hand, the subsoil ofthe City of Buenos Aires appears to belong to the Buenos AiresFormation (‘‘Lujanense’’ land–mammal age). In the vicinity of LaPlata is the Ensenada Formation (underlying the former one),corresponding to the ‘‘Ensenadense’’ land–mammal age. In theAtlantic coast of the Province of Buenos Aires, the mentionedchronostratigraphic level (Brunhes–Matuyama) is found in sedi-ments assigned to the Arroyo Seco Formation, which belongs to the‘‘Lujanense’’ land–mammal age, and which would be consistent,from the paleontological point of view, with the position of this levelin the subsoil of the City of Buenos Aires. This could be due to severalfactors that have been discussed by Zarate and Orgeira (in press).

It has been noted that the traditional ‘‘land–mammal ages’’ havebeen replaced by the formal stage-age units. Its use is stronglyrecommended. The magnetic ages for the South American lateCenozoic stages/ages have been established on the basis of theirmagnetostratigraphic correlation (sensu Cione and Tonni, 1995)(Table 1). The chronostratigraphic levels can be associated withsedimentary hiatuses, such as erosive surfaces, which in someoccasions are not clearly identified in the field, as well as with thefact that it is difficult to evaluate the temporal representation thatthey have. As a consequence, the validity of the magnetic ages fora certain unit should be considered as a function of the clearpredominance of a specific polarity of the sediments belonging tothat unit. Therefore, the temporal correlation of the diverse SouthAmerican regions based on the magnetostratigraphy obtained fromthe sedimentary sequences exposes several problems. They shouldtherefore be prudently stated in the light of new findings.

The aim of this study is to present the results of interdisciplinarystudies carried out in the coastal cliffs of the Department ofSan Jose, Uruguay (Fig. 1), including lithofacial, pedological andmagnetostratigraphic analysis of the units involved.

Table 1Magnetic ages for the South American late Cenozoic stages/ages on the basis of themagnetostratigraphic correlation.

Stage/age Magnetic age

Lujanian Brunhes sensu lato (less than 0.7 Ma, Late Pleistocene)At the base, probably Late Matuyama age (>0.7 Ma)

Ensenadan Top Section: probably early Brunhes ageMainly Matuyama age (>0.7 Ma and <2.48 Ma)Bottom Section: probably Late Gauss age (>2.48 Ma,Late Pliocene)

Marplatan Early to Middle Gauss age (Late Pliocene) to Middleto Late Gauss age (>2.48 Ma, Late Pliocene)Top Section: (alternative interpretation) probably EarlyMatuyama age (<2.48 Ma, Late Pliocene)

Chapadmalalan Late Gilbert to Early Gauss age (Middle to Late Pliocene)Montehermosan (Corral Quemado, NW Argentina): Chron 6 (Late Miocene)

to Gilbert chron (Early to Middle Pliocene); boundarywith Huayquerian fauna: ca. 6 Ma.

2. Methods

Two detailed profiles were collected: one located in the Mauriciocliffs (Fig. 3A) and the other one in the Arazatı cliffs (Fig. 3B), on theeastern coast of Rıo de la Plata. Some samples for paleomagneticstudies were also collected near Kiyu beach. For the sedimentary units,the thickness, geometry, bed contacts, sedimentary and soil struc-tures, fossils, composition and texture characteristics of the sedimentsand sedimentary rocks were recorded. The different sedimentaryunits were also carefully sampled. The chart of chromatic patternsfrom the Rock Color Chart Committee (1951) was used for colordetermination. The code of lithofacies corresponds to that proposedby Miall (1978), later modified by the same author (Miall, 1996).

The sedimentary rocks were studied taking into account theirtexture and compositional features. The sandstones were classifiedaccording to Folk et al. (1970) and, in order to minimize the effect ofgrain-size, the method of Gazzy-Dickinson was used. Briefly, thismethod consists in counting as a monomineral fragment every grainsmaller than 62 m even if it forms part of a greater lithic fragment.For carbonate rocks, the Dunham classification (1962) was used.

Soils and paleosols were described and sampled, and differentanalytical determinations were performed: grain-size analysis (bythe pipette method for the fine fractions and by sieving for thesand), calcium carbonate (volumetric method), pH (pH-H2O, ina 1:2.5 soil–water suspension), cation exchange capacity andexchangeable cations (with ammonium acetate), and organiccarbon (the Walkey and Black method). Stable carbon isotopes oforganic material (d13C, by dry combustion with an elementalanalyzer coupled to a Deltaplus IRMS) and bulk magnetic suscepti-bility (c, with a Bartington susceptibility meter) were measured insamples from the Arazatı site. Clay mineralogy was analyzed by X-ray diffractometry in samples from the Mauricio cliffs.

For paleomagnetic studies, in the area of Mauricio cliffs, 15 siteswere collected in a vertical profile at 10–30 cm interval; in the areaof Kiyu and in the area of Arazatı, 7 sites and 17 sites were collectedrespectively in vertical profiles at 10–20 cm interval. The strati-graphic location of the outcrops and the sites in the sedimentarysequences are shown in Fig. 3. Three to seven samples werecollected at each site.

Natural remanent magnetization (NRM) was measured initiallyin all samples. The samples were then analyzed to determine theirstability. For this purpose, the demagnetization treatment wasapplied using decreasing alternating magnetic fields (AF), whenpossible, and high temperatures.

AF demagnetization was performed using a 2G magnetometer,with maximum field ranging 10–100 mT, increasing the maximumfield value by 0.5 mT at each step. Thermal demagnetization wascarried out in successive 50 �C stages between 100 and 550 �C.

On the basis of magnetic response of pilot samples for each site,optimal demagnetization steps were selected. The AF demagneti-zation was optimal for all sites. Thermal demagnetization behaviorabove 400 �C showed signs of mineralogical transformation asso-ciated with anomalous changes in the remanence. Consequently,characterized remanences for this type of treatments were selectedbelow this temperature.

For the analysis of the magnetic data, the Super IAPD program(Torsvik et al., 1996) was used following manual selection of themagnetic components. After selection of the characteristic rema-nence magnetization (CRM) of each sample, the main direction ofeach site was calculated.

3. Results

The Raigon Fm. is 5.30 m thick in the Mauricio cliffs and4.80 m deep in the Arazatı cliffs (Fig. 4A). In both sites, it lies

Fig. 3. A: Profile in Mauricio cliffs; B: profile in Arazatı cliffs.

Fig. 4. A. Raigon and Libertad Fms. in Arazatı cliffs, separated by an unconformity (higher arrow). The Raigon Fm. can be divided in two sections separated by a regionaldiscontinuity (lower arrow). B. Discontinuity (d) between the two sections of Raigon Fm., Mauricio cliffs. C. Lithofacies Sp and Sr in Raigon Fm., Mauricio cliffs. D. Clast-supportedpsephites with tabular crossed stratification, Mauricio cliffs. E. Paleosol developed at the top of the Raigon Fm. with abundant rhizoliths, Mauricio cliffs. F. Tabular geometry ofLibertad Fm. which includes a truncated paleosol at the bottom (arrows), Mauricio cliffs. G. Paleosol at the bottom of the Libertad Fm., Arazatı cliffs. H. Present soil developed in theLibertad Fm., Arazatı cliffs.

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–1710

in unconformity over the Camacho Fm. (Bossi, 1966), whichhas a marine origin and where two separate sectors can beclearly differentiated by a discontinuity of regional extension(Fig. 4B).

The lower part, pink gray (5YR 8/1) to grayish orange (10YR 7/4)has a thickness of 2.4 m in Mauricio (Fig. 3A) and of 2.0 m in Arazatı(Fig. 3B); the upper part, grayish orange (10YR 7/4) to moderatereddish orange (10R 6/6), has a vertical development of 2.9 m in

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–17 11

Mauricio and 3.30 m in Arazatı. Both sectors are formed by a fining-upward succession of tabular and lenticular banks, with fine tocoarse stratification. Internally, the banks are fining-upward andhave a concave erosive base, on which fine gravel or coarse heavysand (lithofacies Ss) lie. These deposits grade to coarse and mediumquartz–feldspar sandstones, with tabular intercrossed stratification(lithofacies Sp), which is the most characteristic sedimentarystructure (Fig. 4C); very rarely, there are sandstones with throughcross-bedding stratification observed (lithofacies St). The associa-tion culminates with medium to fine sandstones with ripples(lithofacies Sr) and towards the top, massive silty–clayey beds areoccasionally intercalated.

The gravelly beds (Fig. 4D) are generally friable to poorlyconsolidated, although in some levels they are well-consolidateddue to carbonate cement. They are clast supported, moderatelyselected, and the clasts are subangular to subrounded. Composi-tionally, metamorphic rock fragments are the predominant ones(mainly polycrystalline cataclastic quartz, together with schists andamphibolites) and acid plutonic rocks, chalcedony and chert, fine-grained intraclasts and quartz and feldspar clasts.

The friable to moderately consolidated very fine gravels, witha moderate selection and a sandy, occasionally muddy, matrix, arecommon. Due to the matrix (up to 40%), the fabric varies from clast-supported to matrix-supported. The clasts are subrounded toangular, decreasing the degree of rounding with the grain-size. Thecomposition is similar to that of the gravel fraction, although thepercentage of quartz and feldspar is increased and that of the lithicfragments is decreased. The clayish material is more abundanttowards the top of the unit, where it appears as grain coatings;where it is abundant, a speckled and parallel striated fabric isobserved.

In general, the psammites are friable to not very consolidatedand have a clast-supported fabric, with tangential contacts betweengrains. The selection is moderate and the clasts are subangular andrarely subrounded. The very coarse and coarse sandstones arefeldspathic lithoarenites, where lithic fragments predominate (40–45%), with almost equal amounts of feldspar (35–43%) and loweramounts of monomineral quartz (15–20%). The medium and finesandstones are classified as lithic feldarenites. Feldspars andmonocrystalline quartz predominate (30–45%) together with lithicfragments (15–20%).

Among the latter, polycrystalline quartz of cataclastic origin isthe one that predominates, together with the granitic lithoclastsand chalcedony clasts. The feldspars are represented by orthosegrains with clayish alteration, by sericitized plagioclase and bygenerally fresh microcline. The quartz grains are individuals withnormal and ondulose extinction. Clasts with secondary growth,worn by erosion, are also observed.

The sands do not contain cement, but some levels have calcitewith bladed texture and in mosaic, whose percentage can occa-sionally be elevated, forming almost 50% of the rock. In these cases,it forms continuous covers around the clasts, thus determiningfloating fabric.

The top of the formation (lithofacies Fr) is pedogenized (Fig. 4E);soil horizons have a silt loam texture and differ clearly from thematerials in the Libertad Fm. by their grayish colors. They presentpolyhedric structures, clay coatings and manganese coatings, andabundant calcium carbonate features of varied morphologyincluding rhizoconcretions (Fig. 3A and B). The diffractometricanalysis in the samples from Mauricio (Fig. 7) as well as the CEC/%clay ratio in both studied profiles (Fig. 5) showed that, in materialsfrom Raigon, smectite is the dominant clay mineral and is accom-panied by small proportions of illite and kaolinite. The magneticsusceptibility measured in pedogenized horizons gave low values,oscillating around 0.3�10�6 m3/kg (Fig. 6).

The Libertad Fm. has a tabular geometry, coarse stratification,forms vertical walls and is approximately 4.5 m thick in bothprofiles (Fig. 3A and B and 4F and G). It is friable to moderatelyconsolidated. The color varies from grayish orange pink (10R 8/2) tomoderate red (5R 5/4). It does not show internal sedimentarystructures and is composed of silt-clayish material that containsvariable amounts of fine and very fine sand and a small proportionof the coarser sandy fractions.

The compositional analysis of the sand and coarse silt fractionsindicated a predominance of feldspars (55–65%), with higheramounts of fresh or scarcely altered plagioclase over the orthosewith clayish alteration; fresh microcline is very scarce. Thepercentage of quartz is not higher than 15%, in general is in thecoarser fractions and usually has a ferruginous coating. Vestiges ofpolycrystalline quartz were observed. The pumiceous fragments(10–15%) are fresh and have tube-like vesicles filled by clayishmaterial. The organogenic material is abundant (10–15%) and isconstituted by sponge spicules, very well preserved silicophytolithsand fragments of little bones. The heavy mineral fraction is scarce(1–2%) and is mainly opaque minerals, epidote, augite, hornblende,biotite and titanite.

The Libertad Fm. includes a truncated paleosol at the bottomand the present soil developed at the top. In Mauricio (Figs. 3A and4F), the paleosol has two thick B subhorizons, structured in blocks,with dark clayey cutans, some manganese concentrations and animportant proportion of calcium carbonate mainly as rhizocon-cretions. There is no C horizon and the thick B horizon in Libertad iswelded with the B horizon in Raigon Fm. In Arazatı, the paleosol isnot as thick, having a thin illuvial B horizon and showing the Bt–BC–C sequence (Fig. 3B and 4G). In the sampled site, the profile hasno carbonates and the lower level, described as a 2Cb horizon,preserves the features of the loessic sedimentary material: massivestructure, porous and friable. Nevertheless, some characteristics ofthis paleosol display a marked lateral variability, particularly thecalcareous features that are abundant in some sectors of the cliff.

The paleosol in the Libertad Fm. is silty loam in texture and inboth studied sites the maximum clay content is around 25% (Fig. 5).In Arazatı, where the paleosol shows a higher number of horizons,the sand fraction presents irregular content along the profile,oscillating between 8 and 22%. The predominant sand fractions arethose from very fine sand (50–100 mm) to medium sand (250–500 mm). The pH oscillates between 6.5 and 7. The soil cationexchange capacity (CEC) is high in all the horizons. Mineralogicalanalyses of the fine fraction in samples from Mauricio indicate thatsmectite is the predominant clay in this paleosol. Its proportion islower than in the Raigon Fm. but higher than in the present soil inthe Libertad Fm (Fig. 7).

The surface soil developed in the Libertad Fm. (Fig. 4H), showssome morphological and analytical differences between Mauricioand Arazatı cliffs (Fig. 3A and B). The Bt horizon in Arazatı, althoughclearly differentiated by its polyhedric structure, is weakly illuvialand reaches up to 30% clay in the 2Bt2 (Fig. 5): soil texture is siltyloam, with the exception of the 2Bt2, where it is silty clay loam. Theclay content in Mauricio is higher than in Arazatı all along the soildepth, reaching a maximum of 45% in the 2Bt2 horizon and givinga silty clay texture.

Mineralogical analysis of the soil clay fraction from Mauricioshows an association of illite and irregular interstratified illite–smectite together with a small content of kaolinite (Fig. 7). The CEC/% clay ratio oscillates between 60 and 90 meq/100 g clay in both soilprofiles, which coincides with the mineralogy determined by XRD.

In both sites, the BC and BC/C horizons show a slight increase inclay content in comparison to the content at the base of the Bthorizon, together with an increase of the CEC (Fig. 5). At the sametime, and according to XR analysis, both BC horizons in Mauricio

RA

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Org. C (%)0.0 0.5 1.0 1.5 2.0

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Fig. 5. Analytical data along soils and paleosols profiles in Mauricio and Arazatı sites: clay content, cation exchange capacity, pH and total organic carbon.

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–1712

show a higher content of smectite than the Bt horizons (Fig. 7).These results suggest that the sedimentary material on which theBt horizons have developed is in discontinuity in regard to thesediment in the BC horizons.

In Mauricio, the 2BCk2 horizon shows an important proportionof calcium carbonate crystallized as nodules and rhizoconcretions,which are not observed in Arazatı. In the latter, the pH is 5.5 in the Ahorizons and around 6.5–7 in the lower horizons, while in Mauricioit reaches 8.5 in the Bt and BC horizons, in relation with carbonateaccumulation.

The present soil in Arazatı is covered by a thin sedimentary bedaround 20 cm thick, coarse-textured (loam texture, with 40% sand)and with 1.74% organic C, which constitutes the more surficial A

horizon; from there, organic C content decreases progressivelydown to a minimum of 0.03% in the 2BC1/C horizon (Fig. 5). InMauricio, both A1 and A2 horizons are developed on that surficialdeposit, which is thicker (about 40 cm thick) but less sandy(between 25 and 30% sand) than in Arazatı.

Analysis of stable carbon isotopes in the profile from Arazatıshows a value of d13C of �23& in the A1 surface horizon, in accor-dance with the present humid climate and C3 vegetation (Fig. 6). Onthe contrary, in the 2A2 and Bt horizons, the isotopic signatureshows an enrichment in 13C, giving d13C values of around �19 &.These values indicate that the organic matter is derived from a mixof C3 and C4 plants, which would be adapted to more contrastedclimatic conditions, with dry and humid periods. At the base of the

A1

2A2

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2Bt2

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13C (‰)-20 -150.0 0.5 1.0 1.5 2.0

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LFHF

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Fig. 6. Analytical data of soil and paleosols in Arazatı site. Magnetic susceptibility values are expressed as mass specific susceptibility, (10�6 m3 kg�1) at high (HF) and low frequency(LF). Stable carbon isotope composition of organic matter as 13C (in &).

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–17 13

present soil and in the paleosols in Libertad and Raigon, the isotopicsignal changes again, and the d13C values recorded are around �23and �24&, which correspond to C3 plants.

The magnetic susceptibility in the soil and the paleosol devel-oped in the Libertad Fm. in Arazatı displays a progressive depletionfrom the deeper C and BC horizons of each edaphic body up to therespective upper horizons (Fig. 6). In the present soil, bulk MSvalues decrease from a maximum of 1.76�10�6 m3/kg in the 2BC1to a minimum of 0.41�10�6 m3/kg in the A1 horizon. In the pale-osol, the bulk MS values decrease from 1.52�10�6 m3/kg in the2BC2b1 and 2Cb1 horizons down to 1.23�10�6 m3/Kg in the2Bt1b1 horizon. In the soil and in the paleosol, the frequency-dependent magnetic susceptibility (cfd) gave values between 3 and5; this result indicates that there were not superparamagneticparticles developed during the pedogenic process.

Fig. 8 shows some representative behaviors of thermal and AFdemagnetization for different outcrops of the studied sequence.Fig. 9 shows the mean direction of the characteristic magnetizationof the sampled sites for the complete sequence.

A virtual geomagnetic pole (VGP) was then calculated for eachsite, using the CRM direction and geographic coordinates of thesampling sites. These VGPs were used to calculate the paleo-magnetic pole (PP) for the area. For this purpose, Fisherianstatistics (Fisher, 1953) were used. Using these data, departuresranging from 0� to 90� were considered normal polarity, andthose between 90� and 180� were considered to indicate reversedpolarity. The obtained magnetostratigraphy is represented inFig. 8. The Obtained PP is located at lat. 88.2� S Long 189.7� Dp5�

Dm 7.2� N¼ 39.

4. Discussion

Both in Mauricio and Arazatı, an unconformity separating theCamacho and the Raigon Fms. can be observed (Fig. 3A and B). In

addition, the latter can be subdivided in two separate sections bya regional discontinuity (Fig. 4B), which marks an episode ofsedimentary reactivation. Both sections are characterized bya group of lithofacies with erosive base on which gravel or granulescorresponding to the lithofacies Ss is deposited, which indicatesa fast deposit of coarse bed load. This is followed by lithofacies Spcorresponding to deposits generated by the migration of bidi-mensional dunes. During waning energy flows, lithofacies Sr isoriginated by ripple migration. Occasionally, lithofacies St formedby migration of three-dimensional dunes was observed. The lith-ofacies Fm correspond to sedimentation from suspensions.

The association of lithofacies Ss, Sp and Sr that lies over concaveand erosive base indicates deposits of channel fill. The lithofaciesFm, of tabular geometry, is interpreted as flood plain deposits. Thepresence of a paleosol at the top of the unit appears to be related toa period when no deposits took place, probably developed underrelatively humid conditions with illuviation of clays. The predom-inant clay mineral is smectite. Both sections include the highersubsequence determined in the preliminary analysis of theMiocene–Pliocene sequence of southwest Uruguay (De Santa Anaet al., 1988) and do not correspond with the sections analyzed byAndreis and Mazzoni (1967), as both are predominantly sandy.

The composition of the clasts both of the psephites and thepsammites of the Raigon Fm. indicates that the generating rocksare plutonic–metamorphic (fragments of acid plutonites, poly-crystalline quartz, schists, and amphibolites). Therefore, theirprovenance is from the Precambric crystalline basement, eitherdirectly by erosion or from sedimentary rocks to which it gave rise,as indicated by the quartz individuals with secondary growth wornby erosion. The chalcedony and chert clasts appear to come fromamygdale infillings from the basalt of the Arapey Fm. Obviously, theintraclasts are local. The abundance of metastable species withimportant alteration and the scarce rounding of clasts, mainlysubangular, suggest proximity to the provenance area.

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3579111315171921

2

3579111315171921

2

3579111315171921

2

3579111315171921

2

3579111315171921

2

2Bt2 2BC1 2BCk22Btk b1 1 3Btk b2 2 4Btk b3 2

MAURICIO

KK K

K K K

I

I II I

I-S

I-SS S

S

S

A2

K

I

I

S

Fig. 7. X-ray diffraction patterns of the clay fraction; selected samples from soil and paleosols in Mauricio profile. (K¼ kaolinite; I¼ illite; I–S¼ irregular interstratified illite–smectite; S¼ smectite); natural, oriented, Mg-saturated clays.

N

NN

0.4

100mT50mT

1.0

1.0

100 mT50 mT

0.5

2 24 46 68 810 10

1.01.0

x100 C x100 C

Sample: MK3AP corr Sample: ME2PA corr

Sample: Az1b corr Sample: ME2F corr

Fig. 8. Representative behaviors of thermal and AF demagnetization for different outcrops.

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–17 15

The geometry of the deposits, the type of stratification, theproperty of staying in vertical walls, the color, and the grain-sizewith modes in silt and clay fractions, together with the variablepercentages of fine and very fine sand of the Libertad Fm., indicatethat this unit corresponds to loessic deposits accumulated by thewind in semiarid regions with low slope. The composition of the

Fig. 9. Mean direction of the characteristic magnetization of the samples sites for thecomplete sequence.

sand and silt fractions suggests two origins: a volcanic-pyroclasticone (vitroclasts, plagioclase, hornblende, augite, and opaqueminerals) and a smaller one with basement and sedimentaryprovenance (plagioclase, quartz with a ferruginous coating andvestiges of polycrystalline quartz). An organogenic contributiontypical of continental deposits also is observed (spicules,silicophytoliths).

The clay mineralogical analysis and the calculated CEC of clay inthe samples from the Libertad Fm. indicate an increase of smectiticminerals with depth. Nevertheless, the smectite content is lowerthan in the samples from the upper part of the Raigon Fm., whichcoincides with some results obtained by Perez Peirano (1991).

The present soil at the top of the Formation is classified as anArgiudoll. In Arazatı as well as in Mauricio, this soil seems to becomposed of three superposed sedimentary deposits: the mostevident one coincides with the A1 horizon in Arazatı and with theA1 and A2 horizons in Mauricio, and is constituted by a coarsertextured deposit. The second material is the silty loessial sedimenton which the Bt horizons were developed; the argillic Bt horizonhas a higher content of clay in Mauricio than in Arazatı. The thirdmaterial would be the one composing the BC and BC/C horizons,characterized by higher contents of clay and smectite than thoseobserved at the base of the Bt. As this discontinuity is not evident inthe field and should be corroborated with more refined analyticalmethods, it is not indicated in the soil horizon nomenclature. Thepaleosol observed at the base of this formation is also characterizedby illuvial B horizons, which are thicker in Mauricio than in Arazatı.

The pedological information provides some interesting indica-tions concerning the paleoenvironmental evolution in the area. Onthe one hand, the superposition of truncated paleosols in Raigonand Libertad Fms., as well as the existence of different sedimentarylevels in the present soil, are indicative of different cycles oferosion, sedimentation and pedogenesis from the Middle Pleisto-cene. On the other hand, the C isotopic ratio shows depth functionssimilar to those observed in soils and paleosols in Quaternary

Table 2Mammal association.

Formation Stage/Age sensu Cione and Tonni (1995)

Libertad Fm. Marplatan (Late Vorohuean–Sanandresian) to EnsenadanRaigon Fm. Late Chapadmalalan to Marplatan (early Vorohuean)Camacho Fm. Late Huayquerian – Montehermosan to early Chapadmalalan

O.R. Tofalo et al. / Quaternary International 210 (2009) 6–1716

sediments in the Argentinean Pampa (Morras et al., 2007). The lessnegative d13C in the Bt horizon appears to reflect relatively recenthydric stress conditions that would coincide with the Last GlacialMaximum and/or with other dry periods during the Holocene. Theisotopic data from deeper horizons indicate a dominance of C3vegetation in previous periods. Diverse factors play a role in theisotopic fractionation in C3 plants. Nevertheless, as a first approx-imation it can be considered that this type of vegetation appears tobe related with a more humid environment, similar to the presentone, during the periods of stability in the Middle and UpperPleistocene.

The similarity of the MS curves obtained in the soil and thepaleosol developed in the Libertad Fm. is noteworthy. These resultscoincide with results obtained in some profiles in the ArgentineanPampean region, where the MS values in soils and paleosols arelower than those measured in the loessic levels (Orgeira et al., 1998,2003; Nabel et al., 1999, among others).

According to the paleontological data suggested by Bertoni-Machado and Farina (personal comm.) the oldest mammal associ-ation is in the late Miocene–Pliocene Camacho Fm., of Huayquerianto Montehermosan affinities (Table 2). A Pliocene–early Pleistoceneage has been assigned to the Raigon Fm, of Chapadmalalan affini-ties. The Libertad Fm., which was at the top of the studied sequence,has been considered early-middle Pleistocene in age with Mar-platan–Ensenadan affinities (Bertoni-Machado and Farina, personalcomm.).

According to the obtained magnetostratigraphy, the followingmagnetic ages are suggested:

The reversed and normal polarities found for the Camacho Fm.sampled in the localities of Mauricio and Arazatı are assignable todifferent subchrons of the Gilbert magnetic chron (>3.32 Ma, Lanzaand Meloni, 2006; Fig. 8).

The obtained normal polarity in the Raigon Fm., both in thesamples from Mauricio and from Arazatı, suggested a Gaussmagnetic age (<3.32 Ma and >2.43 Ma) for this section (Fig. 8).

The Libertad Fm. sampled in the Kiyu section showed a reversemagnetic polarity, which refers to the Matuyama magnetic chron(<2.43 Ma and >0.69 Ma). However, sites of this Formationcollected in the locality of Arazatı showed a normal magneticpolarity. This section could be correlated either with a normalsubchron of the Matuyama chron, possibly Jaramillo (between 0.89and 0.95 Ma, Lanza and Meloni, 2006; Fig. 2), or with the earlyBrunhes chron (less than 0.69 Ma).

The chronology is roughly compatible with that based on thefossil contents of those formations and shows an importantagreement with the established magnetic ages determined previ-ously in Argentinean fossiliferous sequences (Fig. 2).

5. Conclusions

- There is an unconformity that separates the Camacho Fm. (lateMiocene), which has a marine origin, from the Raigon Fm. (latePliocene–early Pleistocene), which has a fluvial genesis.

- The Raigon Fm. can be divided in two predominantly sandysections separated by a discontinuity of regional character,which marks the episode of reactivation in the sedimentation.

These sections differ from the ones determined by Andreis andMazzoni (1967).

- Each of these sections is integrated by associations of lith-ofacies Ss, Sp, Sr and rare St, which lie on the concave anderosive base, and are interpreted as channel fill deposits.

- The occasional lithofacies Fm., of tabular geometry, are inter-preted as floodplain deposits.

- A paleosol is developed at the top of the unit. It is well struc-tured and has abundant coatings and rhizoliths. It appears tohave been formed under relatively humid environmentalconditions as it is suggested by clay illuviation and C isotopesdata, while calcareous features appear to derive from thedecarbonation of overlying sediments and soils.

- The Libertad Fm. corresponds to loessic deposits accumulatedby the wind in semiarid regions with scarce slope. Thisconclusion agrees with the eolian origin suggested by differentauthors and synthesized by Panario and Gutierrez (1999) anddiffers from the one proposed by Ortız et al. (2006).

- A clear compositional difference is observed between the Rai-gon and Libertad Fms.: while the former has mainly a cratonicorigin, the latter has a mixed origin, since it also has a volcanicarc contribution.

- At the top of the Libertad Fm., and forming the A horizon of thepresent soil, a coarse-textured sediment appears in disconti-nuity. Some analytical results also suggest a sedimentarydiscontinuity between the materials of the Bt and BC horizonsof the soil. A typical feature along the surveyed cliffs is a pale-osol at the bottom of this unit.

- The superposition of truncated paleosols in Raigon andLibertad Fms., the existence of different sedimentary beds inthe present soil, and the mineralogical and C isotopes dataindicate fluctuations of climatic conditions and the existence ofseveral cycles of erosion, sedimentation and pedogenesis fromthe Middle Pleistocene.

- From magnetostratigraphy studies, the following magneticages are suggested for the sampled formations: the CamachoFm. is assignable to different subchrons of the Gilbert magneticchron (>3.32 Ma). The Raigon Fm. can be assigned to the Gaussmagnetic age (<3.32 Ma and>2.43 Ma), while the Libertad Fm.could be correlated either with the Jaramillo subchron(between 0.89 and 0.95 Ma) or with the early Brunhes chron(less than 0.69 Ma).

- The magnetic ages obtained are consistent with those obtainedin fossiliferous sequences from the Argentinean Pampeanregion.

Acknowledgments

The authors wish to thank the Universidad de Buenos Aires(Argentina) and the Universidad de la Republica (Uruguay) for theirinstitutional support, and UBACyT grant X236 which providedfinancial support for the research. This work has benefited from theuseful suggestions made by the reviewers Sergio Matheos andCesar Goso Aguilar.

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