lable at ScienceDirect

Journal of South American Earth Sciences 56 (2014) 200e213

Contents lists avai

Journal of South American Earth Sciences

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

New high precision U-Pb ages for the Vinchina Formation:Implications for the stratigraphy of the Bermejo Andean forelandbasin (La Rioja province, western Argentina)

P.L. Ciccioli a, *, C.O. Limarino a, R. Friedman b, S.A. Marenssi a

a Departamento de Ciencias Geol�ogicas- IGeBA, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires e CONICET. Ciudad Universitaria,Pabell�on 2, 1� piso, C1428EHA Buenos Aires, Argentinab UBC-EOS PCIGR, 6339 Stores Road, Vancouver, BC V67 1Z4, Canada

a r t i c l e i n f o

Article history:Received 13 June 2014Accepted 3 September 2014Available online 18 September 2014

Keywords:Vinchina formationU-Pb agesMioceneAndean foreland basin

* Corresponding author. Departamento de CienciasExactas y Naturales, Universidad de Buenos Aires. Pab11 4576 3329.

E-mail addresses: ciccioli@gl.fcen.uba.ar, patriciacihotmail.com (S.A. Marenssi).

http://dx.doi.org/10.1016/j.jsames.2014.09.0050895-9811/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Vinchina Formation is one of the thickest Cenozoic units related to the Andean orogeny in Argentinatotaling more than 5100 m in thickness. Different ages, from Eocene to latest Miocene, have beenpostulated for this red-bed succession based on fission track, magnetostratigraphy and whole rockisotopic analyses. Two new high precision U-Pb zircon ages are reported herein for this unit. A maximumU-Pb age of 15.6 ± 0.4 Ma was obtained from detritic zircons collected from a thick tuffaceous interval ofthe Lower Member of the Vinchina Formation at La Cueva (Precordillera), while a depositional U-Pb ageof 9.24 ± 0.034 Ma was derived from volcanic zircons collected from a thin tuff bed in the Upper Memberat Quebrada de Los Pozuelos (Northwestern Sierras Pampeanas).

At La Cueva, the Vinchina Formation unconformably overlies eolian sandstones of the Vallecito For-mation and was divided into four units representing 1) deposits of high-sinuosity ephemeral riversassociated with 2) a playa-lake passing upwards to 3) low-sinuosity sandy ephemeral rivers and finally,4) a gravelly-sandy braided plain. The tuffaceous level corresponding to unit 1 is located 280 m above thebase of the formation.

At Quebrada de Los Pozuelos, the Vinchina Formation unconformably overlies the Vallecito Formationand is covered by a deeply incised surface at the base of the Toro Negro Formation. We divided theVinchina Formation into four units. Unit 1 represents sedimentation in shallow fluvial channels withsandy to muddy floodplains. Units 2 and 3 record sedimentation in braided, meandering and anasto-mosing rivers. Finally unit 4 represents deposition in braided and wandering fluvial systems. Thesampled tuff is located within unit 4 at ~3470 m above the base of the formation.

The new ages indicate that the bulk of the Vinchina Formation is Miocene in age but they do notpreclude a longer time span for the sedimentation of the whole unit. Ages of the sampled volcaniczircons match an important episode of volcanism recorded in the Cerro Las T�ortolas Formation, located~90 km to the west in the Andean Cordillera, but also the upper tuff could be related to the late MiocenePuna volcanism. Comparison of the new ages with previous chronological data suggests coetaneoussedimentation along different depocenters of the Bermejo basin (e.g., Vinchina and Talampaya depo-centers in Western Sierras Pampeanas and La Troya depocenter and Huaco-Mogna sections in Pre-cordillera) and strenghten the need for correlation among them. In addition the age of 15.6 ± 0.4 Maconstrains the end of the severe arid conditions recorded in the Sierras Pampeanas and Precordilleraregion.

© 2014 Elsevier Ltd. All rights reserved.

Geol�ogicas eInstituto de Geociencias B�asicas, Aplicadas y Ambientales de Buenos Aires (IGeBA), Facultad de Cienciasell�on 2, 1� piso, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina. Tel.: þ54 11 4576 3300/3009int. 317; fax: þ54

ccioli@yahoo.com.ar (P.L. Ciccioli), limar@gl.fcen.uba.ar (C.O. Limarino), rfriedma@eos.ubc.ca (R. Friedman), smarenssi@

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 201

1. Introduction

Our studies were carried out in the northern Sierra de Los Col-orados (Quebrada de Los Pozuelos, Western Sierras Pampeanas,Fig. 1) and to the south of the Sierra de Umango (La Cueva area,Precordillera, Fig. 1) in the La Rioja province, northwesternArgentina.

The age of the Vinchina Formation has become a key questionfor the understanding of the Andean orogeny along the 28�e29� ofSouth Latitude (northern Bermejo foreland basin) for several rea-sons: 1) there seems to be a general agreement that this thicksuccession of red beds (more than 5000 m) records the beginningof the Andean compression at those latitudes; 2) recent paperssuggest a link between the deep erosive surface separating theVinchina Formation and the overlying Toro Negro Formation withone of the main phases of uplift of the Nevados del Famatina(Limarino et al., 2010); 3) as shown by stratigraphic analysis(Ciccioli et al., 2011), the northern part of the Bermejo forelandbasin records a complex evolution. The age of the Vinchina For-mation is critical for separating the transpressive retroarc stage(Vinchina Formation) from the transpressional foreland stage withan increasing participation of the western orogenic front (ToroNegro Formation, Ciccioli et al., 2011, 2013a). Dating the UpperMember of the Vinchina Formation is important for estimating theonset of the broken foreland basin stage in the retroarc area.

Lack of precision in previous dating of the Vinchina Formationalso clouds these issues. Different ages (Tabbutt et al., 1987, 1989;Reynolds et al., 1990; Tedesco, 2007; Ciccioli, 2008; D�avila et al.,2008; Collo et al., 2011), from Eocene to latest Miocene, arepostulated for this unit. The Vinchina Formation was initiallyreferred to the latest Miocene based on a 7.3 ± 1.2 Ma fission trackage reported by Tabbutt et al. (1987, 1989) from a tuff level ofimprecise stratigraphic location in the Quebrada de La Troya sec-tion (Western Sierras Pampeanas). Reynolds et al. (1990) presenteda 12.1 ± 1.4 Ma fission track age from the upper member of theVinchina Formation (nominated informally La Brea member) at LasJuntas (~19 km to the south of La Cueva) in the Precordillera.Magnetostratigraphic data complemented by that dating indicatethat the Vinchina Formationwas deposited between ~18 and 12 Maalthough the top of the Neogene sequence (Zapallar Formation)reach to ~9 Ma (Reynolds et al., 1990). R�e and Barredo (1993a, b)using paleomagnetic data but based on the previous ages sug-gested that the whole of the Neogene succession deposited in theVinchina basin (Vinchina and Toro Negro formations) would recordsedimentation between 9.0 and 1.5 Ma. Later on, unpublished K/Arages obtained by Tedesco (2007) and Ciccioli (2008) suggested thatthe sedimentation of the Vinchina Formation began as early as theearliest Miocene or latest Oligocene (Ciccioli, 2008, unpublished) oreven in the latest Oligocene (Tedesco, 2007 unpublished). Finally,D�avila et al. (2008) and Collo et al. (2011) reported a maximum U-Pb age of 19.1 Ma for detritic zircons collected from a tuffaceoussandstone of the Lower Member of the Vinchina Formation in theSierra de Los Colorados.

Uncertainty about the age of the Vinchina Formation is aconsequence of the almost complete absence of lava flows and thescarcity of primary tuff levels for obtaining accurate isotopic ages.In addition, body fossils were not found in the unit up to now andalthough the Vinchina Formation hosts a rich invertebrate andvertebrate ichnofauna (Melchor et al., 2010; Krapovickas and Nasif,2011), there is not enough biostratigraphic definition to refine theage of this unit.

To improve the understanding of the age of the Vinchina For-mation,we report twonewhighprecisionU-Pb zircon ages from theloweranduppermembersof thisunit. Previous isotopic agesare alsoanalyzed in light of the new data presented in this paper and the

significance of these ages for the stratigraphic correlation and tec-tosedimentary models proposed by earlier authors is discussed.

2. Stratigraphic setting

The Vinchina Formation (Turner, 1964) is one of the thickestCenozoic units related to the Andean orogeny in Argentina,reaching a thickness of 5100 m in its type section where the base iscovered by alluvium (Tripaldi et al., 2001; Limarino et al., 2001).Nonetheless, in some other localities the Vinchina Formation restson an erosive surface over eolian sandstones belonging to the lateOligocene-early Miocene Vallecito Formation (Tripaldi andLimarino, 2005; Ciccioli et al., 2010, 2011, 2013b). The VinchinaFormation crops out along the Sierra de Los Colorados (WesternSierras Pampeanas) and the eastern margin of the Precordillera(Fig. 1) in the La Rioja province and records the early distal forelandfill of the northern Bermejo basin (also known as Vinchina and LaTroya basins, respectively). Ramos (1970) divided this unit in twomembers. The Lower one is characterized by sandy shales andarkosic sandstones with highly altered volcanic rock fragments. TheUpper Member is composed of muddy sandstones with in-tercalations of tuffaceous sandstones and conglomerates contain-ing both fresh and altered volcanic clasts (Ramos, 1970). In theQuebrada de La Troya (Sierra de Los Colorados) the contact betweenthe Lower and Upper members is marked by progressive un-conformities draped by several conglomerate levels (Marenssiet al., 2000). The Vinchina Formation is unconformably coveredby the late Miocene-Pliocene Toro Negro Formation (Turner, 1964;Ramos, 1970; Ciccioli et al., 2010, 2011).

3. Sample location and methodology

Two localities were selected in order to obtain ages for theLower and Upper members of the Vinchina Formation. In the south,outcrops at the La Cueva area (Precordillera, Fig. 1) exhibit a wellexposed sequence of the Lower Member containing a thick tuffa-ceous level located 280 m above the base of the unit (Fig. 2). To-wards the north, in the Quebrada de Los Pozuelos (Sierra de LosColorados, Western Sierras Pampeanas, Fig. 1), a second tuff,located ~3470 m above the base of the Vinchina Formation, wassampled (Fig. 3). In both cases 5 kg of rock were collected from thebasal portion of the tuff beds to account for the settling of zirconsduring deposition. Petrographic analysis of these levels demon-strated that they were the product of primary ash falls although theLa Cueva sample shows evidences of partial reworking.

One of our goals was to insert the obtained ages in the previ-ously proposed tectosedimentary models. In this regard detailedstratigraphic sections were measured, and the stratigraphic posi-tions of the dated tuffs were located.

Isotopic analyses were conducted at the Pacific Centre for Iso-topic and Geochemical Research housed in the Department of Earthand Ocean Sciences (EOS) at the University of British Columbia,using a modification of CA-TIMS procedures outlined in Mundilet al. (2004), Mattinson (2005), and Scoates and Friedman (2008).Uncertainties in age are reported at the 95% confidence level.

4. La Cueva area

La Cueva is located to the south of the Sierra de Umango (Fig. 1).This block of crystalline rocks began to be uplifted during theMiddle to Late Miocene (R�e and Vilas, 1990; Reynolds et al., 1990;R�e, 1996; Tedesco, 2007; Ciccioli et al., 2011, 2013a) or early Plio-cene (Ramos et al., 1988, 2002) partitioning the already existingBermejo foreland basin. Therefore, during deposition of the LowerMember of the Vinchina Formation, the Neogene basin was

Fig. 1. A. Location of the study area. B Location of the Vinchina (VD) and La Troya depocenters (LTD) and main morphostructural units and lineaments. C. Geological map of La Troya-Vinchina area, northern Bermejo Basin (modified from Ciccioli et al., 2011).

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213202

Fig. 2. Geologic map of the La Cueva area (La Troya depocenter) showing the location of the dated sample.

Fig. 3. Geologic map of the Los Pozuelos area (Vinchina depocenter) showing the location of the dated sample.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 203

Fig. 4. A. Field photograph of the sampled tuffaceous bed (V1/10) at La Cueva. B and C. Microphotographs of the crystalline tuff. Note zoned plagioclase crystals in a fine-grainedvitric matrix.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213204

continuous along the present day Sierra de Los Colorados to reachthe latitude of La Cueva (Ciccioli et al., 2011).

The geology of the region is shown in Fig. 2 and comprises redbeds included in the Ci�enaga del Río Huaco Formation (Limarinoet al., 2000) for which an age of 108.1 ± 4.4 Ma was reported byTedesco et al. (2007). This sequence is covered by the VallecitoFormation composed of large-scale cross-bedded sandstonesdeposited in a large eolian field. Eolian sandstones of VallecitoFormation are, in turn, covered by fluvial sandstones and mud-stones of the Vinchina Formation by a regional disconformity thatproduces important changes in the thickness of the underlying unit(Tripaldi and Limarino, 2005). At La Cueva, the contact between theVallecito and Vinchina formations is locally marked by an irregularsurface showing evidences of subaerial exposure (paleoweatheringsurfaces, paleosols and bioturbated beds).

At this locality, the Vinchina Formation is made up of sandstonesand mudstones with scarce intercalations of fine-grained con-glomerates and a continuous tuffaceous bed (5 m thick) located280m above the base of the unit. This tuffaceous interval consists ofseveral stacked ash fall deposits that are partially reworked andcomposed of crystalline and, less frequently, vitric tuffs (Fig. 4).Zircons used for the isotopic age determination (sample V1/10)were recovered from the base of a crystalline tuff bed.

Finally, coarse-grained conglomerate, breccias, and lithic sand-stones belonging to the Pliocene El Corral Formation are in faultcontact on top of the Vinchina Formation.

4.1. La Cueva lithostratigraphic units

Four lithostratigraphic units are recognized in the VinchinaFormation at La Cueva (Fig. 5). Their main characteristics and theinterpretation of their depositional environments are givenbelow.

Unit 1: This unit is mainly composed of sandstones andmudstones. Fine-grained conglomerates are very scarce. Twotypes of depositional architecture are identified: 1) interbeddedsandstones and mudstones and 2) channelized sandstones. Thefirst dominates the lower part of the unit where medium- andfine-grained sandstones form 10 to 20 cm-thick tabular beds andshow horizontal lamination, small-scale cross-bedding, ripple-cross lamination, or are massive. Small-scale synsedimentarydeformation (convolute folds) and load-casts are frequent. Muddyveneers to decimetric beds of massive or poorly laminated mud-stones, occasionally showing bioturbation and mud-cracks inter-calate between the sandstone beds. In some cases interbeddedsandstones and mudstones form coarsening-upward sequencesup to 4 m thick (Fig. 5B).

Lenticular bodies of medium-to coarse-grained sandstones andrarely fine-grained conglomerates and gravelly sandstones domi-nate the upper part of the unit. They correspond to channel com-plexes (up to 3 m thick) limited at the base by low-relief, 5th orderbounding surfaces (Miall, 1996). Individual channels (up to 0.5 mthick) are bounded by medium-to high-relief 4c bounding surfaces

Fig. 5. A. Schematic vertical section of the Vinchina Formation at the La Cueva locality. The star indicates the position of the dated tuff level. B. General view of the Unit 1; C. Muddy(a) and sandy facies (b) pile up vertically in the Unit 2; D. Outcrop view of the Unit 4 to the top of the section.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 205

(Tripaldi et al., 2001) indicating recurrent cycles of erosion andfilling of the streams. Periodic channel abandonments are markedby muddy veneers and thin levels of mudstones. Internally, sand-stones show medium-scale (up to 25 cm) cross-bedding or hori-zontal lamination and well developed lateral accretion surfaces (LAsurfaces of Miall, 1996).

The dated level is included in this unit (Fig. 5A). It is a 5 m thickinterval composed of grayish white massive to cross-laminated orhorizontal laminated fine-grained tuffs (Fig. 4).

Unit 1 was deposited by high-sinuosity ephemeral rivers flow-ing along the gentle slope of an alluvial plain. Channels, dominatedby small sandy bars and dunes, hadmiddle to high lateral migration

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213206

rates, as evidenced by the frequent presence of LA surfaces. Inter-bedded sandstones and mudstones would correspond to inter-channel areas. In this context the coarsening-upward cycles suggestthe progradation of small crevasse splays fed during flood stages.

Unit 2: It presents a transitional contact with unit 1 and it isintegrated by two facies which pile up vertically five times (Fig. 5C).The first facies is composed of red mudstones and tabular beds ofyellowish brown fine-to medium-grained sandstones. Mudstonesform thick intervals (up to 6 m) and exhibit parallel lamination orare massive with frequent thin intercalations of current ripple-laminated fine-grained sandstones. Sandstone strata (up to 20 cmthick) show parting lineation and current ripple lamination and,occasionally, heterolithic stratification. Some sandstone beds pre-sent planar bases and convex up tops with horizontal lamination orare massive.

The second facies (up to 3 m thick) corresponds to tabular bedsof yellowish white, medium-grained and occasionally coarse-grained sandstones. They present slightly erosive bases and showhorizontal lamination, rarely planar cross-bedding, or are massive.In some cases, beds appear amalgamated and internal erosivesurfaces are draped by rip-up muddy clasts. Vertebrate footprintsand desiccation cracks are frequent in this interval.

Unit 2 is interpreted as being deposited by ephemeral flowsassociated with a playa lake. The muddy-dominated facies corre-sponds to the lowest energy portion of the system (mudflat tomudflat-sandflat transition) where some ephemeral flows depos-ited crevasse or terminal splays. The sandy facies corresponds to asandflat built up by the progradation of ephemeral flows.

Unit 3: This unit is similar in terms of lithology and sedimentarystructures to the channelized sandstones of unit 1 but lacks tufflevels and fine-grained conglomerates are more frequent. Medium-and coarse-grained, cross-bedded sandstones form most of thisinterval. They occur as lenticular beds, up to 50 cm thick, showinglow-relief erosive basal surfaces. Conglomerates are fine- andmedium-grained with sandy matrix. Although clast-supportedfabrics predominate, matrix-supported strata are not unusual,especially in the case of fine-grained conglomerates. They includeclasts of granites, high-grade metamorphic rocks, quartz, redsandstones and green shales. Parallel-laminated and massivetabular beds of red mudstones, reaching up to 40 cm in thickness,form up to 20% of the unit.

Unit 3 was deposited by ephemeral rivers similar to that of unit1, but the larger proportion of cross-bedded conglomerates sug-gests higher energy of the fluvial systems.

Unit 4: This unit forms the top of the section (Fig. 5) and iscomposed of four lithofacies. Massive, coarse-to medium-grained,occasionally gravelly, sandstones are the most frequent one. Theyform lenticular to lentiform beds (from 10 to 30 cm) which areoccasionally amalgamated (Fig. 5D) presenting sporadically hor-izontal to planar cross-stratification. Massive, matrix-supportedconglomerates (sandy matrix) dominated by volcanic clasts(80%) with minor fractions of metamorphic (10%), quartz (6%) andsandstone (4%) clasts comprise the second lithofacies in abun-dance. Mean grain-size of the clasts is 12 cm and the clast/matrixratio ranges from 10:90 to 30:70. The third lithofacies corre-sponds to clast-to matrix-supported conglomerates with a higherclast/matrix ratio (60:40 to 90:10). They form lenticular beds (upto 40 cm thick) showing crude cross-stratification. Tabular beds ofsiltstones to fine-grained sandstones (up to 50 cm thick) appearbounding all the other lithofacies and form the least abundantlithofacies.

This unit is interpreted as a gravelly-sandy braided plain withgravelly channels and sandy-gravelly interchannel areas and lessfrequently silty-sandy interchannel areas. Longitudinal to trans-verse bars are very common as in-channel deposits.

4.2. U-Pb age of the tuff level from the La Cueva

The dated tuffaceous level located 280 m above the base of theVinchina Formation is continuous along the strike and can belaterally followed for several kilometers in the La Cueva area(Fig. 3). The 5m thick grayish white lensoid interval is massive at itsbase but shows diffuse cross-lamination or horizontal laminationtowards the top. It is composed of several stacked beds of fine-grained tuffs. The sampled bed (Fig. 4) is a crystalline tuff mostlycomposed of euhedral crystals (70%) of plagioclase, K-feldspar,quartz and hornblende within a very fine-grained glassy ground-mass (25%) and minor subangular to subrounded lithic fragmentsand monomineralic clasts (5%).

Bed thickness, geometry and sedimentary structures suggestmultiple ash fall events and some degree of reworking. Mixedzircon populations composed by anhedral to subhedral (rounded)and euhedral (angular) grains almost certainly comprise detritaland volcanic material (Fig. 6A, B).

The resulting ages are summarized in Fig. 6 and Table 1. Volcaniczircons with ages between 19.8 and 15.6 Ma (Fig. 6C) suggestseveral ash fall events and a maximum depositional age not olderthan 15.6 ± 0.4 Ma. One detrital zircon shed a Late Paleozoic age of334.4 Ma (Fig. 6D) which is close to the published age of 335.99 Mafor the Punta del Agua andesites (Gulbranson et al., 2010) sug-gesting that by Middle Miocene times Carboniferous rocks werealready exposed towards the west-northwest of the study area.

5. Quebrada de Los Pozuelos area

The Quebrada de Los Pozuelos section was measured along awide creek located in the northern Sierra de Los Colorados (north ofBermejo Basin, Figs. 1 and 3) where a thick succession (up to3600 m) of the Vinchina Formation is very well exposed. Here theVinchina Formation overlies a monotonous eolian sequenceincluded in the late Oligocene -early Miocene Vallecito Formation(Ciccioli et al., 2010, 2013b). In this area, the top of the VinchinaFormation is marked by a high-relief unconformity (Ciccioli, 2008;Ciccioli et al., 2010; Limarino et al., 2010) that separates it from theoverlying late Miocene-Pliocene Toro Negro Formation, a thickcontinental succession composed of coarse-grained conglomerates,sandstones and mudstones (Fig. 3).

5.1. Quebrada de Los Pozuelos lithostratigraphic units

At Quebrada de Los Pozuelos, the Vinchina Formation wasdivided into four lithostratigraphic units (Fig. 7, Ciccioli et al.,2013b), each one briefly described below.

Unit 1. This 1000 m-thick unit is made up of sandstones, mud-stones and scarce intra and extrabasinal conglomerates arranged ina fining-upward sequence (Fig. 7). Coarse- and medium-grainedsandstones forming tabular beds with horizontal lamination, low-angle cross-lamination and, less frequently, tabular cross-bedding(sets up to 10 cm thick) predominate in the lower half of the unit.Fine-grained extraformational conglomerates appear at the base ofthe unit, close to the contact with the underlying Vallecito For-mation. A conspicuous feature of this interval is the presence ofwell-sorted fine- and medium-grained sandstones showing welldeveloped cross-stratification. The upper half of the unit presents adecrease in grain-size and is composed of tabular beds of massiveand laminated mudstones and fine-grained sandstones that inter-calate with thin beds of intraformational conglomerates.

The lower half of this unit was interpreted as being deposited byfluvial systems characterized by shallow braided channels andlarge interchannel areas dominated by unconfined sandy flows and

Fig. 6. V1/10 sample A. Microphotograph of the recovered zircon population. Note admixture of euhedral (volcanic) and rounded (detrital) grains. B. Euhedral (volcanic) zirconsselected for dating. C. Concordia diagram and ranked 206Pb/238U age plot for volcanic zircon single grain analyses for the V1/10 sample. D. Concordia diagram and ranked 206Pb/238Uage plot for the Late Paleozoic detrital zircon grain.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 207

small eolian dunes. The upper half shows better developedchannel-bar deposits and thicker sandy-muddy floodplains.

Unit 2. This 1100 m thick interval comprises sandstones, scarceconglomerates, and mudstones arranged in coarsening-upwarddecametric cycles (Fig. 7A, B). The base of unit 2 is shown by alow-relief irregular surface covered by stacked beds of coarse-grained sandstones and intraformational conglomerates which, inturn, pass upward to cross-bedded sandstones and massive orlaminated mudstones, frequently showing mud-cracks at the top ofthe beds. A thick tuffaceos bed occurring at the top of this unit canbe used as a marker level across the basin.

The lower part of unit 2 would correspond to a sandy-gravellybraided fluvial system (Miall, 1996) which is replaced upsectionby anastomosed channels encapsulated within a fine-grainedfloodplain showing well developed crevasse splays.

Unit 3. The base of this unit is a low-relief, but laterallyextensive, erosive surface that separates the Lower and Uppermembers of the Vinchina Formation (Ramos, 1970). It is floored byfine-grained conglomerates or stacked beds of coarse-grainedsandstones and gravelly sandstones. Marenssi et al. (2000)described this surface at the Quebrada de La Troya in detail andsuggested that the Lower Member of the Vinchina Formation wastilted by 9� to the SW prior to the deposition of the UpperMember.

Unit 3 reaches amaximumthickness of 400m in theQuebradadeLos Pozuelos-La Aguada (Fig. 7A). The lower part of the unit is

composed of coarse- and medium-grained sandstones, gravellysandstones and scarce conglomerates, all of them stratified inlenticular beds (Fig. 7C). Clast composition is dominated bygranites,high- and medium-grade metamorphic rocks, quartz, red sand-stones and lesser proportions of volcanic and green sandstones.

The upper part (Fig. 7) is composed of fining-upward cyclo-thems up to 5 m thick. Each of them begins with coarse-grainedsandstones (in some cases bearing muddy intraformationalclasts) resting on an erosive surface, succeeded by medium-tofine-grained sandstones, frequently showing horizontal-beddingand ripple-cross lamination, which in turn pass upward to lami-nated (or massive) mudstones that form the top of the cycle.Lateral accretion surfaces (LA) have been identified in somechannel deposits.

The lower part of the unit 3 was interpreted as being depositedin braided fluvial systems (Miall, 1996) which filled the incisionsurface that separates the Lower and Upper members of the Vin-china Formation. The cyclothems, forming the upper part of theunit, were probably deposited in alluvial plains dominated bymeandering rivers with well developed floodbasin areas.

Unit 4. This 1100 m-thick unit is composed of three megacycleseach of which is a few hundred meters thick (Fig. 7A). The base ofeach megacycle is marked by an erosive surface (incision surface)on which channel-complexes composed of lenticular beds of con-glomerates and coarse-grained sandstones (Fig. 7D) were depos-ited. These rocks represent braided river sedimentation during the

Table 1PbeU age results.

UeThePb isotopic data

Sample Compositional parameters Radiogenic isotope ratios Isotopic ages

Wt. U Th Pb 206Pb* mol % Pb* Pbc 206Pb 208Pb 207Pb % err 207Pb % err 206Pb % err Corr. coef. 207Pb ± 207Pb ± 206Pb ±

mg ppm U ppm �10�13 mol 206Pb* Pbc (pg) 204Pb 206Pb 206Pb 235U 238U 206Pb 235U 238U

(a) (b) (c) (d) (c) (e) (e) (e) (e) (f) (g) (g) (h) (g) (h) (g) (h) (i) (h) (i) (h) (i) (h)

V1/10A 0.003 108 1.394 1.2 0.0387 54.92% 0 2.60 41 0.452 0.046662 15.183 0.017359 15.896 0.002698 1.157 0.639 31.97 363.22 17.47 2.75 17.37 0.20B 0.003 70 0.910 1.3 0.0261 40.89% 0 3.10 31 0.366 0.058146 22.681 0.024707 23.626 0.003082 2.052 0.495 535.28 495.83 24.78 5.78 19.84 0.41C 0.004 55 1.270 0.9 0.0221 41.79% 0 2.52 32 0.472 0.053543 31.908 0.019956 32.749 0.002703 1.925 0.461 351.87 720.10 20.06 6.51 17.40 0.33D 0.003 67 1.504 0.5 0.0224 64.87% 1 1.00 53 0.460 0.044033 38.306 0.015336 39.164 0.002526 1.412 0.619 �108.84 941.26 15.45 6.01 16.26 0.23E 0.004 46 1.181 0.7 0.0165 39.70% 0 2.06 31 0.321 0.039036 60.025 0.013031 61.154 0.002421 2.248 0.516 �414.36 1566.23 13.15 7.99 15.59 0.35F 0.003 114 0.933 1.0 0.0337 59.67% 1 1.87 46 0.331 0.051023 14.517 0.017887 15.328 0.002543 1.024 0.805 241.87 333.95 18.00 2.74 16.37 0.17G 0.003 192 0.201 10.1 1.2758 99.03% 29 1.03 1910 0.064 0.053185 0.343 0.390429 0.418 0.053241 0.134 0.675 336.71 7.76 334.68 1.19 334.39 0.44H 0.002 93 1.595 0.7 0.0199 67.93% 1 0.77 58 0.490 0.044185 19.043 0.015603 20.087 0.002561 1.080 0.968 �100.39 467.22 15.72 3.13 16.49 0.18VP1A 0.004 87 1.541 0.2 0.0213 84.96% 2 0.31 123 0.455 0.042266 20.520 0.008412 21.274 0.001444 1.355 0.578 �210.67 513.66 8.51 1.80 9.30 0.13B 0.003 76 1.450 0.4 0.0152 61.50% 1 0.78 48 0.474 0.046773 56.334 0.009156 57.339 0.001420 1.865 0.550 37.65 1344.56 9.25 5.28 9.15 0.17C 0.004 186 0.578 0.4 0.0450 88.35% 2 0.49 159 0.185 0.045542 9.588 0.008983 10.081 0.001431 0.647 0.776 �26.55 231.26 9.08 0.91 9.22 0.06E 0.003 235 0.581 0.5 0.0404 88.98% 3 0.41 168 0.188 0.046032 8.357 0.009120 8.776 0.001437 0.505 0.839 �0.73 200.59 9.22 0.81 9.26 0.05

(a) A, B etc. are labels for fractions composed of single zircon grains or fragments; all fractions annealed and chemically abraded after Mattinson (2005) and Scoates and Friedman (2008).(b) Nominal fraction weights estimated from photomicrographic grain dimensions, adjusted for partial dissolution during chemical abrasion.(c) Nominal U and total Pb concentrations subject to uncertainty in photomicrographic estimation of weight and partial dissolution during chemical abrasion.(d) Model Th/U ratio calculated from radiogenic 208Pb/206Pb ratio and 207Pb/235U age.(e) Pb* and Pbc represent radiogenic and common Pb, respectively; mol % 206Pb* with respect to radiogenic, blank and initial common Pb.(f) Measured ratio corrected for spike and fractionation only. Mass discrimination of 0.23%/amu based on analysis of NBS-982; all Daly analyses.(g) Corrected for fractionation, spike, and common Pb; up to 1 pg of common Pb was assumed to be procedural blank: 206Pb/204Pb ¼ 18.50 ± 1.0%; 207Pb/204Pb ¼ 15.50 ± 1.0%; 208Pb/204Pb ¼ 38.40 ± 1.0% (all uncertainties 1-sigma). Excess over blank was assigned to initial common Pb with Stacey and Kramers (1975) model Pb composition at 16 Ma.(h) Errors are 2-sigma, propagated using the algorithms of Schmitz and Schoene (2007) and Crowley et al. (2007).(i) Calculations are based on the decay constants of Jaffey et al. (1971). 206Pb/238U and 207Pb/206Pb ages corrected for initial disequilibrium in 230Th/238U using Th/U [magma] ¼ 3.(j) Corrected for fractionation, spike, and blank Pb only.

P.L.Ciccioliet

al./Journal

ofSouth

American

EarthSciences

56(2014)

200e213

208

Fig. 7. A. Schematic vertical section of the Vinchina Formation at the Quebrada de Los Pozuelos (modified from Ciccioli et al., 2013b). The star indicates the position of the dated tufflevel. B. Coarsening-upward cycle of the Unit 2; C. Braided fluvial system of the Unit 3 and D. Photo of the Unit 4 near the top of the section.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 209

confinement of the fluvial system. In the middle part of themegacycles the proportion of mudstones and fine-grained sand-stones increases while the thickness of the channel-complexesdecreases (up to 2 m). The uppermost beds of each megacycle arecomposed of coarse-grained sandstones and scarce conglomerates

forming thin channel-complexes, encapsulated within muddy al-luvial plain deposits (wandering fluvial systems). The verticalarrangement of the succession suggests not only a progressive in-crease in the accommodation space but also a gradual loss ofconfinement toward the top of the megacycles.

Fig. 8. A. Field photograph of the sampled tuff bed (VP1) at Quebrada de Los Pozuelos. B. Microphotograph of the vitric tuff showing crystalline material dispersed in a fine-grainedvitric matrix. C. Detailed view showing pumice fragments (arrowed).

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213210

5.2. UePb age of the tuff level from the Quebrada de Los Pozuelos

The dated tuff is located at the top of the third megacycle of unit4, ~3470 m above the base of the Vinchina Formation in the Que-brada de Los Pozuelos (28�36013.400S 68�09048.400W). The tuff bedforms a continuous white level (up to 20 cm thick, Fig. 8A) inter-calated into sandy-muddy floodplain deposits that can be followedfor 5 km from the Quebrada de Los Pozuelos to the northernQuebrada de La Aguada. It is a fine-grained vitric tuff composed ofshards and pumice fragments with up to 25% quartz, plagioclaseand K-feldspar crystals (Fig. 8B, C).

Volcanic zircons collected from this tuff shed a depositional ageof 9.24 ± 0.034 Ma (Table 1 and Fig. 9).

6. Stratigraphic implications of the new ages of the VinchinaFormation

Two UePb ages for detrital zircons are now available for theLower Member of the Vinchina Formation. A new maximum depo-sitional age of 15.6 ± 0.4 Ma comes from a tuff located 280 m abovethe base of the Vinchina Formation at La Cueva (Precordillera)whilea previously reported maximum age of 19.1 Ma (D�avila et al., 2008;

Collo et al., 2011) is from a tuffaceous sandstone more than 1100 mabove the base of the Vinchina Formation in the Quebrada de LaTroya (Sierra de Los Colorados, Western Sierras Pampeanas).Because there is no way to physically correlate these two levels atleast two hypotheses should be considered: The first recognizes theexistence of twodifferent volcanic eventswithin the LowerMemberof the Vinchina Formation and the second considers that the samelevel was sampled at two localities. Both suggest large thicknessvariations between the two areas reflecting different locationswithin the basin (i.e margin vs center) and subsidence rates.Regarding the first possibility we noted that in the thick and wellexposed section in the Quebrada de La Troya (and in several othersections to the north) in the Sierra de Los Colorados, there are noother tuffaceous strata within the Lower Member above the con-spicuous bed sampled by D�avila et al. (2008) and Collo et al. (2011).On the other hand, the second hypothesis represents the simplestexplanation taking into account that maximum detrital agesstrongly depend on the chances of an individual representing an agepopulation for being dated. If this assumption is correct the tuff levelmay be considered a marker along the basin with a maximumdepositional age of 15.6 ± 0.4 Ma but with more than 1100 m ofsediments preserved below it in the depocenter.

Fig. 9. VP1 sample. A. Euhedral (volcanic) zircons selected for dating. B. Concordia diagram and ranked 206Pb/238U age plot for zircon single grain analyses.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 211

A new depositional age of 9.24 ± 0.034 Ma comes from a tuffwithin the Upper Member of the Vinchina Formation collected~3470m above the base of the unit at the Quebrada de Los Pozuelos(northern Sierras de Los Colorados, Western Sierras Pampeanas).

These ages and the stratigraphic analysis suggest that the bulk ofthe Vinchina Formation was deposited during the Miocene (fromBurdigalian to Tortonian) but they do not preclude a longer time ofdeposition for the whole unit since along the Quebrada de LosPozuelos the unconformity at the base of the Toro Negro Formationhas eroded ~1000 m of the upper beds of the Vinchina Formation(Limarino et al., 2010).

Magnetostratigraphic studies carried out by Reynolds et al.(1990) indicated that the onset of the sedimentation of the Vin-china Formation occurred around ~18 Ma at Las Juntas (Pre-cordillera) while R�e and Barredo (1993a,b) suggested ~9 Ma in theSierra de Los Colorados (Western Sierras Pampeanas). The first ageis close to the 15.6 ± 0.4 Ma age reported in this paper for La Cueva.Similar early Miocene age (ca. 17 Ma) was obtained by D�avila et al.(2004) by andesite boulders recorded in the lowermost volcano-genic conglomerate of the Del Crest�on Formation in Famatina belt.The thick pyroclastic level of the Lower Member of the VinchinaFormation could be the distal expression of one of the major phasesof the important volcanism recorded in the Andean region (seepaleogeographic stage IV, Alonso et al., 2011; Litvak and Poma,2014). In this sense, it could be linked to the lower andesitic tobasandesitic lower section of the Cerro Las T�ortolas Formation(~90 km to the west in the neighboring Andean Cordillera) datedbetween 16 and 14 Ma (see compilation of K/Ar and 40Ar/39Ar agesby Bissig et al., 2001; Kay and Mpodozis, 2002; Litvak et al., 2007;Alonso et al., 2011; Litvak and Poma, 2014).

Regarding the age of the Upper Member of the Vinchina For-mation attained in the Quebrada de Los Pozuelos (9.24 ± 0.034 Ma)it is important to highlight that it does not represent the age of thetop of the Vinchina Formation but at least permits some certaintyabout the minimum age of the top of the unit. This age allowsestablishing that the incision surface separating Toro Negro andVinchina Formation previously related to the major phase of theuplift of the Nevados del Famatina (Limarino et al., 2010) is youngerthan ~9 Ma but older than the overlying Toro Negro Formationdated as late Miocene (8.6 ± 0.3 Ma and 6.8 ± 0.2 Ma) according toK40/Ar40 ages (Ciccioli et al., 2005). Besides, this age allows datingthe transpressional foreland stage, with an increasing participationof the western orogenic front, established by Ciccioli et al. (2011,2013a) for the Vinchina basin. A similar age of ~9 Ma ismentioned by Reynolds et al. (1990) for the top of the Neogenesequence (Zapallar Formation) at Las Juntas.

From a regional point of view the Vinchina Formation correlateswith the red beds of the Tamberías Formation (Turner, 1967) in theFiambal�a area (~120 km to the northeast of Quebrada de LosPozuelos) in which Carrapa et al. (2008) reported a UePb age of8.2 ± 0.33 Ma. This age comes from the upper portion of theTamberías Formation, some hundreds of meters below the top ofthe unit, and agrees with the youngest age presented in this paperfor the Vinchina Formation. Besides, Tabbutt et al. (1989) publishedsimilar ages (15 ± 2.7 Ma, 9.6 ± 1.5 Ma; 7.54 ± 1.65 Ma and7.4 ± 0.9 Ma) for tuffs collected in a more distal foreland sequencein the Talampaya field (~80 km to the southeast of La Cueva). There,magnetostratigraphic studies indicate Neogene deposition be-tween ~20 and 6.1 Ma (Malizia et al., 1995).

Even though the existence of a volcanic center in Precordillera,Sierras Pampeanas or Famatina should not be ruled out, it isimportant to point out that the age of 9.24 Ma fits well with the latevolcanism of the Cerro Las T�ortolas Formation in the AndeanCordillera (Bissig et al., 2001; Kay and Mpodozis, 2002; Litvak et al.,2007; Alonso et al., 2011; Litvak and Poma, 2014). Still anotherpossibility is that this pyroclastic event (~9 Ma) was derived fromeruptions on the southern Puna, as suggested by Malizia et al.(1995) based on tuffs of similar ages and due to the importantvolcanic activity recorded in the Puna during the late Miocene(Coira et al., 1982; Viramonte et al., 1994; Allmendinger et al., 1997;Montero et al., 2010).

In the southern Huaco and Mogna sections (San Juan province),several magnetostratigraphic studies complemented by isotopicdatings were performed (Johnson et al., 1986; Jordan et al., 1990,1997; Milana et al., 2003). They provided similar ages(~19e18Ma) for the beginning of the sedimentation in the Bermejoforeland basin in Precordillera. Comparison of our new isotopicages with the previously existing chronological data indicates thatsedimentation inWestern Sierras Pampeanas (Vinchina depocenterand Talampaya) was very nearly coetaneous to the Precordillera (LaTroya depocenter and the southern Huaco-Mogna sections) alongthe Bermejo foreland basin.

Finally, the new isotopic age for the Lower Member of the Vin-china Formation has also an important paleoclimatic implicationsince it was obtained from the basal fluvial strata at La Cueva. Thesefluvial deposits cover the thick eolian succession included in theVallecito Formation. This last unit, together with others of similarage in the region (i.e. Mari~no Formation, Tripaldi and Limarino,2005), represent the severe arid climatic conditions that pre-vailed along the Precordillera and Sierras Pampeanas during theearly Miocene. Until now the age of this arid episode had beenestimated only based on the date of 32.7 ± 2.6 Ma and 21.6 ± 0.8 Ma

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213212

obtained by Jordan et al. (1993) from fluvial strata underlying theVallecito Formation in San Juan Precordillera. The new age of15.6 ± 0.4 Ma, from beds above the eolian deposits, indicates thatduring the Langhian severe arid conditions had disappeared andwere replaced by a semiarid climate (probably similar to the pre-sent day). Therefore the arid episode in Sierras Pampeanas andPrecordillera can be now constrained to the Oligocene to Aquita-nian e early Burdigalian (early Miocene).

7. Conclusions

1) Two high-precision isotopic UePb ages are presented for theVinchina Formation, one of the thickest foreland sequences innorthwestern Argentina. The first one of 15.6 ± 0.4 Ma corre-sponds to the lower part of the Lower member at La Cueva(Precordillera) while the second one of 9.24 ± 0.034 Ma is fromthe Upper member in the Quebrada de Los Pozuelos (Sierras deLos Colorados, Western Sierras Pampeanas).

2) The maximum depositional age of 15.6 ± 0.4 Ma indicates thebeginning of the sedimentation in the foreland basin at 29�S andagrees well with previously obtained ages reported by severalauthors for the Bermejo Basin. This pyroclastic event could begenetically related to the Cordillera volcanism (Cerro LasT�ortolas Formation).

3) The 15.6 ± 0.4 Ma age also constrains the end of the severe aridconditions recorded in the underlying eolian Vallecito Forma-tion in the Sierras Pampeanas and Precordillera region.

4) The depositional age of 9.24 ± 0.034 Ma allows establishing thatthe incision surface separating the Toro Negro and Vinchinaformations, previously related to the major phase of the uplift ofthe Nevados del Famatina, occurred between ~8 and 9 Ma. Thispyroclastic event was recorded in several locations within theforeland basin and it could be genetically related to a volcanicevent at either the Andean Cordillera or the southern Puna.

5) Finally, the new isotopic ages suggest that nearly coetaneoussedimentation occurred along the Bermejo foreland basin fromthe Precordillera in the south to the Western Sierras Pampeanasin the north.

Acknowledgments

This study was funded by UBACyT GC01/W321, CONICET PIP252, ANPCyT PICT 0727/2012 research grants. The authors thankthe constructive comments of the two reviewers (Dra. C.I. Galli andone anonymous reviewer) as well as to Regional Editor V.A. Ramosthat helped to improve the manuscript.

References

Allmendinger, R.W., Jordan, T.E., Kay, S.M., Isacks, B.L., 1997. The evolution of thealtiplano-puna plateau of the Central Andes. Annu. Rev. Earth Planet. Sci. 25,139e174.

Alonso, M.S., Limarino, C.O., Litvak, V.D., Poma, S.M., Suriano, J., Remesal, M.B., 2011.Paleogeographic, magmatic and paleoenvironmental scenarios at 30�S duringthe Andean orogeny: cross sections from the volcanic-arc to the orogenic front(San Juan, Argentina). In: Salfity, J.A., Marquillas, R.A. (Eds.), Cenozoic Geology ofthe Central Andes of Argentina. SCS publisher, Salta, pp. 23e46.

Bissig, T., Clark, A.H., Lee, J.K.W., Heather, K.B., 2001. The Cenozoic history ofvolcanism and hydrothermal alteration in the Central Andean flat-slab region:new 40Ar-39Ar constrains from the El Indio-Pascua Au (-Ag, Cu) belt,29�200e30�300S. Int. Geol. Rev. 43 (4), 312e340.

Carrapa, B., Hauer, J., Schoenbohm, L., Strecker, M.R., Schmitt, A.K., Villanueva, A.,Sosa Gomez, J., 2008. Dynamics of deformation and sedimentation in thenorthern Sierras Pampeanas: an integrated study of the Neogene Fiambal�abasin, NW Argentina. Geol. Soc. Am. Bull. 120, 1518e1543.

Ciccioli, P.L., 2008. Evoluci�on paleoambiental, estratigrafía y petrología sed-imentaria de la Formaci�on Toro Negro, Sierras Pampeanas Noroccidentales,provincia de La Rioja. Ph.D. Thesis. Facultad de Ciencias Exactas y Naturales,Universidad de Buenos Aires, Argentina.

Ciccioli, P.L., Limarino, C., Marenssi, S.A., 2005. Nuevas edades radim�etricas para laFormaci�on Toro Negro en la Sierra de Los Colorados, Sierras Pampeanas Nor-occidentales, provincia de La Rioja. Rev. la Asoc. Geol�ogica Argent. 60 (1),251e254.

Ciccioli, P.L., Limarino, C.O., Marenssi, S.A., Tedesco, A.M., Tripaldi, A., 2010. Estra-tigrafía de la Cuenca de Vinchina (Terciario), Sierras Pampeanas, provincia de LaRioja. Rev. la Asoc. Geol�ogica Argent. 66 (1), 146e155.

Ciccioli, P.L., Limarino, C.O., Marenssi, S.A., Tedesco, A.M., Tripaldi, A., 2011. Tecto-sedimentary evolution of the La Troya-Vinchina depocenters (Northern Ber-mejo Basin Tertiary), La Rioja Province, Argentina. In: Salfity, J.A.,Marquillas, R.A. (Eds.), Cenozoic Geology of the Central Andes of Argentina. SCSpublisher, Salta, pp. 91e110.

Ciccioli, P.L., Marenssi, S.A., Rossello, E., Limarino, C.O., 2013a. Sedimentary patternsin the Vinchina Basin: interplay between compressional and transcurrenttectonism during the Andean Orogeny. Boll. Geofis. teor. appl. 54 (suppl. 2),217e220.

Ciccioli, P.L., G�omez O'Connell, M., Limarino, C.O., Marenssi, S.A., 2013b. La sucesi�onterciaria de la quebrada de Los Pozuelos (Cuenca de Vinchina): su importanciaestratigr�afica y paleogeogr�afica para el antepaís andino. Rev. la Asoc. Geol�ogicaArgent. 70 (4), 451e464.

Coira, B., Davidson, J., Mpodozis, C., Ramos, V., 1982. Tectonic and magmatic evo-lution of the Andes of Northern Argentina and Chile. Earth Sci. Rev. 18,302e332.

Collo, G., D�avila, F.M., N�obile, J., Astini, R.A., Gehrels, G., 2011. Clay mineralogy andthermal history of the Neogene Vinchina Basin, central Andes of Argentina:analysis of factors controlling the heating conditions. Tectonics 30 (4), TC4012.http://dx.doi.org/10.1029/2010TC002841.

Crowley, J.L., Schoene, B., Bowring, S.A., 2007. U-Pb dating of zircon in the BishopTuff at the millennial scale. Geology 35 (12), 1123e1126.

D�avila, F.M., Astini, R.A., Jordan, T.E., Kay, S.M., 2004. Early Miocene andesite con-glomerates in the Sierra de Famatina, broken foreland region of westernArgentina, and documentation of magmatic broadening in the south-centralAndes. J. South Am. Earth Sci. 17, 89e101.

D�avila, F.M., Collo, G., N�obile, J., Astini, R.A., Gehrels, G., 2008. U-PB detrital age on atuffaceous sandstone sheet of the Vinchina Formation, La Rioja: depositionaland exhumation implications. In: 17th Congreso Geol�ogico Argentino, S.S. deJujuy, pp. 95e96.

Gulbranson, E.L., Monta~nez, I.P., Schmitz, M.D., Limarino, C.O., Isbell, J.L.,Marenssi, S.A., Crowley, J.L., 2010. High-precision UePb calibration of Carbon-iferous glaciation and climate history, Paganzo Group, NW Argentina. GSA Bull.122, 1480e1498.

Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971. Precisionmeasurement of half-lives and specific activities of 235U and 238U. Phys. Rev.C4 1889e1906.

Johnson, N.M., Jordan, T.E., Johnsson, P.A., Naeser, C.W., 1986. Magnetic polaritystratigraphy, age and tectonic setting of fluvial sediments in an eastern Andeanforeland basin, San Juan Province, Argentina. In: Allen, P., Homewood, P. (Eds.),Foreland Basins, vol. 8. IAS Special Publication, pp. 63e75.

Jordan, T.E., Rutty, P.M., McRae, L.E., Beer, J.A., Tabbutt, K., Damanti, J.F., 1990.Magnetic polarity stratigraphy of the Miocene Rio Azul section, Precordillerathrust belt, San Juan province, Argentina. J. Geol. 98, 519e539.

Jordan, T.E., Allmendinger, R.W., Damanti, J.F., Drake, R.E., 1993. Chronology ofmotion in a complete thrust belt: the Precordillera, 30�-31�S, Andes Mountains.J. Geol. 101, 137e158.

Jordan, T.E., Reynolds, J.H., Erikson, J.P., 1997. Variability in the age of initial short-ening and uplift in the central Andes, 16�-33�30' S. In: Ruddiman, W.F. (Ed.),Tectonic Uplift and Climate Change. Plenum Press, New York, pp. 41e61.

Kay, S.M., Mpodozis, C., 2002. Magmatism as a probe to the Neogene shallowing ofthe Nazca plate beneath the modern Chilean flat slab. J. South Am. Earth Sci. 15,39e57.

Krapovickas, V., Nasif, N.L., 2011. Large caviomorph rodent footprints of the lateOligocene Vinchina Formation, Argentina. Palaeontol. Electron. 14 (2), 12Ae13A.palaeo-electronica.org/2011_2/256/index.html.

Limarino, C.O., Net, L., Guti�errez, P., Barreda, V., Caselli, A., Ballent, S., 2000. Defi-nici�on litoestratigr�afica de la Formaci�on Ci�enaga del Río Huaco (Cret�acico Su-perior), Precordillera Central, San Juan, Argentina. Rev. la Asoc. Geol�ogicaArgent. 55 (1e2), 83e99.

Limarino, C.O., Tripaldi, A., Marenssi, S.A., Net, L.I., Re, G., Caselli, A.T., 2001.Tectonic control on the evolution of the fluvial systems of the VinchinaFormation (Miocene), Northwestern Argentina. J. South Am. Earth Sci. 14 (7),751e762.

Limarino, C.O., Ciccioli, P.L., Marenssi, S.A., 2010. An�alisis del contacto entre lasFormaciones Vinchina y Toro Negro, sus implicancias tect�onicas (Sierra de LosColorados, provincia de La Rioja). Lat. Am. J. Sedimentol. Basin Anal. 17 (2),113e132.

Litvak, V.D., Poma, S., 2014. Petrogenesis of Miocene volcanic arc rocks over theChilean-Pampean flat-slab segment of the Central Andes constrained by min-eral chemistry. Geol. Acta 12 (2), 151e170.

Litvak, V.D., Poma, S., Kay, S.M., 2007. Paleogene and Neogene magmatism in theValle del Cura region: a new perspective on the evolution of the Pampean flatslab, San Juan province, Argentina. J. South Am. Earth Sci. 24, 117e137.

Montero L�opez, M.C., Hongn, F., Brod, J.A., Seggiaro, R., Marrett, R., Sudo, M., 2010.Magmatismo �acido del Mioceno Superior- Cuaternario en el �area de CerroBlanco-la Hoyada, Puna Austral. Rev. la Asoc. Geol�ogica Argent. 67 (3),329e348.

P.L. Ciccioli et al. / Journal of South American Earth Sciences 56 (2014) 200e213 213

Malizia, D.C., Reynolds, J.H., Tabbutt, K.D., 1995. Chronology of Neogene sedimen-tation, stratigraphy, and tectonism in the Campo de Talampaya region, La RiojaProvince, Argentina. Sediment. Geol. 96, 231e255.

Marenssi, S.A., Net, L., Caselli, A., Tripaldi, A., Limarino, C.O., 2000. Hallazgo dediscordancias intraformacionales en la Formaci�on Vinchina (Ne�ogeno), que-brada de La Troya, La Rioja, Argentina. Rev. la Asoc. Geol�ogica Argent. 55 (4),414e418.

Mattinson, J.M., 2005. Zircon U-Pb chemical abrasion (CA-TIMS) method: combinedannealing and multi-step partial dissolution analysis for improved precisionand accuracy of zircon ages. Chem. Geol. 220, 47e66.

Melchor, R.N., Genise, J.F., Farina, J.L., S�anchez, M.V., Sarzetti, L., Visconti, G., 2010.Large striated burrows from fluvial deposits of the Neogene Vinchina Forma-tion, La Rioja, Argentina: a crab origin suggested by neoichnology and sedi-mentology. Palaeogeogr. Palaeoclimatol. Palaeoecol. 291, 400e418.

Miall, A.D., 1996. The Geology of Fluvial Deposits. Springer.Milana, J.P., Bercowski, F., Jordan, T., 2003. Paleoambientes y magnetoestratigrafía

del Ne�ogeno de la Sierra de Mogna, y su relaci�on con la Cuenca de AntepaísAndina. Rev. la Asoc. Geol�ogica Argent. 58 (3), 447e473.

Mundil, R., Ludwig, K.R., Metcalfe, I., Renne, P.R., 2004. Age and timing of the end-Permian mass extinction: U/Pb geochronology on closed system zircons. Sci-ence 305, 1760e1763.

Ramos, V.A., 1970. Estratigrafía y estructura del Terciario en la Sierra de Los Colo-rados (Provincia de La Rioja), Republica Argentina. Rev. la Asoc. Geol�ogicaArgent. 25, 359e382.

Ramos, V., Reynolds, J., Jordan, T., Tabbutt, K.D., 1988. Time constraints for the upliftof the Sierras de Toro Negro, Umango and Espinal, Western Sierras Pampeanas,Argentina. Geol. Soc. Am. Abstr. Prog. 20 (7), A61. Denver.

Ramos, V.A., Cristallini, E.C., P�erez, D.J., 2002. The Pampean flat-slab of the CentralAndes. J. South Am. Earth Sci. 15 (1), 59e78.

R�e, G.H., Barredo, S.P., 1993a. Esquema de correlaci�on magnetoestratigr�afica deformaciones terciarias aflorantes en las provincias de San Juan, La Rioja y Cat-amarca. Rev. la Asoc. Geol�ogica Argent. 48 (3e4), 241e246.

R�e, G.H., Barredo, S.P., 1993b. Esquema de correlaci�on de las Formaciones terciariasaflorantes en el entorno de Sierras Pampeanas y la Precordillera. In: 12thCongreso Geol�ogico Argentino y 2th Congreso de Exploraci�on de Hidrocarburos,Actas, vol. 2, pp. 172e179. Mendoza.

R�e, G.H., Vilas, J.F., 1990. An�alisis de los cambios paleogeogr�aficos ocurridos duranteel Cenozoico tardío, en la regi�on de Vinchina (provincia de La Rioja); a partir deestudios magnetoestratigr�aficos. In: 11th Congreso Geol�ogico Argentino, Actas,vol. 2, pp. 267e270. San Juan.

R�e, G.H., 1996. Relaci�on entre tect�onica andina y posici�on del depocentro de lacuenca del Bermejo (27� a 33� S). In: 13th Congreso Geol�ogico Argentino y 3thCongreso de Exploraci�on de Hidrocarburos, Actas, vol. 2, p. 209. Buenos Aires.

Reynolds, J.H., Jordan, T.E., Johnson, N.M., Damanti, J.F., Tabbutt, K.D., 1990. Neogenedeformation of the flat - subduction segment of the Argentine - Chilean Andes:magnetostratigraphic constrains from Las Juntas, La Rioja province, Argentina.Geol. Soc. Am. Bull. 12, 1607e1622.

Schmitz, M.D., Schoene, B., 2007. Derivation of isotope ratios, errors, and errorcorrelations for U-Pb geochronology using 205Pb-235U-(233U)-spiked isotopedilution thermal ionization mass spectrometric data. Geochem. Geophys.Geosyst. 8, Q08006. http://dx.doi.org/10.1029/2006GC001492.

Scoates, J.S., Friedman, R.M., 2008. Precise age of the platiniferous Merensky Reef,Bushveld Complex, South Africa, by the U-Pb zircon chemical abrasion ID-TIMStechnique. Econ. Geol. 103, 465e471. þ digital supplement (analyticaltechniques).

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotopic evolutionby a two-stage model. Earth and Planetary Science Letters 26, 207e221.

Tabbutt, K.D., Naeser, Ch.W., Jordan, T.E., Cerveny, P.F., 1987. Edades nuevas porm�etodo de trazas de fisi�on de tobas mio-pliocenas en las Sierras Pampeanas y laPrecordillera de Argentina. In: 10th Congreso Geol�ogico Argentino, Actas, vol. 4,pp. 22e24. Tucum�an.

Tabbutt, K.D., Naeser, Ch.W., Jordan, T.E., Cerveny, P.F., 1989. New fission-track agesof Mio-Pliocene tuffs in the Sierras Pampeanas and Precordillera of Argentina.Rev. la Asoc. Geol�ogica Argent. 44 (1e4), 408e419.

Tedesco, A.M., 2007. Estratigrafía, evoluci�on paleoambiental y tectosedimentaria dela Cuenca de La Troya (Pale�ogeno y Ne�ogeno, Precordillera de La Rioja). Ph.D.Thesis. Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,Argentina.

Tedesco, A.M., Limarino, C.O., Ciccioli, P.L., 2007. Primera edad radim�etrica de losdep�ositos Cret�acicos de la Precordillera Central. Rev. la Asoc. Geol�ogica Argent.62 (3), 471e474.

Tripaldi, A., Limarino, C., 2005. Vallecito Formation (Miocene): the evolution of aneolian system in an Andean foreland basin (northwestern Argentina). J. SouthAm. Earth Sci. 19, 343e357.

Tripaldi, A., Net, L., Limarino, C., Marenssi, S., R�e, G., Caselli, A., 2001. Paleoambientessedimentarios y procedencia de la Formaci�on Vinchina, Mioceno, noroeste de laprovincia de La Rioja. Rev. la Asoc. Geol�ogica Argent. 56, 443e465.

Turner, J.C.M., 1964. Descripci�on geol�ogica de la Hoja 15c, Vinchina, provincia de LaRioja: Buenos Aires. Direcci�on Nac. Geol. Minería Boletín 100, 81.

Turner, J.C.M., 1967. Descripci�on Geol�ogica de la Hoja 13b, Chaschuil, Provincia deCatamarca y La Rioja. Direcci�on Nac. Geol. Minería Boletín 106, 79.

Viramonte, J.G., Reynolds, J.H., Del Papa, C., Disalvo, A., 1994. The Corte Blancogarnetiferous tuff: a distinctive late Miocene marker bed in northwesternArgentina applied to magnetic polarity stratigraphy in the Río Y�acones, SaltaProvince. Earth Planet. Sci. Lett. 121, 519e531.