Geochronologic and paleontologic evidence for a Pacific-Atlantic connection during the late Oligocene-early Miocene in the Patagonian Andes (43-44S)

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Journal of South American Earth Sciences 55 (2014) 1e18

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Geochronologic and paleontologic evidence for a PacificeAtlanticconnection during the late Oligoceneeearly Miocene in thePatagonian Andes (43e44�S)

Alfonso Encinas a, *, Felipe P�erez a, Sven N. Nielsen b, Kenneth L. Finger c, Victor Valencia d,Paul Duhart e

a Departamento de Ciencias de la Tierra, Universidad de Concepci�on, Casilla 160-C, Concepci�on, Chileb Instituto de Ciencias Ambientales y Evolutivas, Universidad Austral de Chile, Casilla 567, Valdivia, Chilec University of California Museum of Paleontology, Valley Life Sciences Building 1101, Berkeley, CA 94720, USAd School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, USAe Servicio Nacional de Geología y Minería, Oficina T�ecnica Puerto Varas, Casilla 613, Puerto Varas, Chile

a r t i c l e i n f o

Article history:Received 14 January 2014Accepted 25 June 2014Available online 4 July 2014

Keywords:North Patagonian AndesLate Oligocene-early MioceneUePb geochronologyBiostratigraphyMarine transgression

* Corresponding author.E-mail address: [email protected] (A. Encinas).

http://dx.doi.org/10.1016/j.jsames.2014.06.0080895-9811/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Cenozoic marine strata occur in the western, eastern, and central parts of the North Patagonian Andesbetween ~43�S and 44�S. Correlation of these deposits is difficult because they occur in small anddiscontinuous outcrops and their ages are uncertain. In order to better understand the age and sedi-mentary environment of these strata, we combined UePb (LA-MC-ICPMS) geochronology on detritalzircons with sedimentologic and paleontologic (foraminifers and molluscs) studies. Sedimentologicanalyses suggest that the Puduhuapi Formation on the western flank of the Andean Cordillera wasdeposited in a deep-marine setting, the Vargas Formation in the central part of the Andes was depositedat outer-neritic or bathyal depths, and the La Cascada Formation on the eastern flank of the range wasdeposited in a shallow-marine environment. Geochronologic and paleontologic results indicate that thethree marine units were deposited during the late Oligocene-early Miocene interval, although it is notclear whether this occurred during one or more marine incursions in the area. The alluvial(?)conglomeratic deposits of the La Junta Formation, exposed in the proximity of the Vargas Formationoutcrops, have a maximum depositional age of ~26 Ma and could have been deposited during the initialstage of subsidence that affected this region prior to the marine transgression over this area. Theoccurrence of both Pacific and Atlantic molluscan taxa in the La Cascada and Vargas formations suggeststhat a marine strait connected both oceans during the accumulation of these units. The new data on theage of the Puduhuapi, Vargas, and La Cascada formations indicate that these units may correlate withlower Miocene marine deposits in the forearc of central and southern Chile (Navidad Formation andequivalent units) and on the eastern flank of the Patagonian Andes (Río Foyel Formation and equivalentunits). A late Oligocene�early Miocene age for these marine deposits is a reliable maximum age for thedeformation and uplift of the North Patagonian Andes.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Andean Cordillera exhibits significant along-strike seg-mentation along its ~7500 km length (Mpodozis and Ramos, 1989;Melnick et al., 2006). One of the most important topographic andgeologic changes in this range takes place at approximately 38�Sbetween the Central and the North Patagonian Andes. South of this

latitude, the broad and high Central Andes narrow and maximumelevation descends from >4000 m to ~2000 m (Herv�e, 1994). TheNorth Patagonian Andes have a crustal thickness of only ~40 km(Lüth andWigger, 2003), mainly consist of the Patagonian batholith(Ramos and Ghiglione, 2008), and compared to the Central Andesshow little deformation (Herv�e, 1994). The North Patagonian Andesalso present a distinct tectonic feature: a dextral strike-slip faultsystem that extends along ~1000 km in a NeS direction and isreferred to as the Liqui~ne-Ofqui Fault Zone (LOFZ) (Herv�e, 1994;Cembrano et al., 2000, 2002; Rosenau et al., 2006). This fault sys-tem accommodates part of the margin parallel component of

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A. Encinas et al. / Journal of South American Earth Sciences 55 (2014) 1e182

oblique subduction and has concentrated most deformation in thisarea since at least the late Miocene (Cembrano et al., 2002;Thomson, 2002; Adriasola et al., 2006; Melnick et al., 2006).

Another singularity of the North Patagonian Andes are itsCenozoic marine deposits exposed in the forearc and on thewestern and eastern flanks of the range (Levi et al., 1966; Ramos,1982; Martinez-Pardo, 1990; Elgueta et al., 2000; Orts et al., 2012;Encinas et al., 2012, 2013; Bechis et al., 2014). Correlation of thesestrata is difficult because they are typically exposed as smalldiscontinuous outcrops (Fig. 1). This is due to the thick vegetationcover that characterizes this area (Cembrano et al., 2002), as well asthe high exhumation rates that affected this chain during the lateCenozoic, causing the erosion of most of the Meso-Cenozoic vol-cano-sedimentary cover and the exposure of the plutonic basement(Adriasola et al., 2006). In addition, fossils from themarine depositsof the Main Andean range are poorly preserved, which hinderstheir recognition at the species level. As a consequence, the tectonicsetting and age of these deposits, and possible connectionswith thePacific or Atlantic oceans during their accumulation, are debatable(see Encinas et al., 2013 and references therein).

In order to study the genetic relationship between Cenozoicmarine strata across the North Patagonian Andes, we focused onthe only area where these deposits crop out in the western, central,and main part of this range (Figs. 1 and 2). Marine deposits exposedon the western flank of the Andes at ~43�S, on a small island nearChait�en (Fig. 1), are informally referred to as the Puduhuapi For-mation (Levi et al., 1966). The age of this unit is uncertain, althoughLevi et al. (1966) correlated it with the Ayacara Formation, anothermarine unit that was considered to be Eocene to Miocene in age,but which Encinas et al. (2013) later confined to the early to middleMiocene. A small outcrop of fossiliferous black shales and minorsandstones exists in the central part of the Andean range at LaJunta, close to the LOFZ (Fig. 1). Urbina (2001) named them theVargas Formation, which he tentatively assigned to the Eocene. Heconsidered the conglomeratic succession exposed in this area to beolder and named it the La Junta Formation. The La Cascada For-mation (Thiele et al., 1978; Castillo, 1983) crops out on the easternflank of the North Patagonian Andes near Futaleufú (Fig. 1) andThiele et al. (1978) assigned to the Eocene.

The ages of the cited marine units remain uncertain becausethey are based on tentative lithologic correlations and poorly pre-served fossils that have not been documented in detail. In thiscontribution, we carried out LA-MC-ICPMSUePb geochronology ondetrital zircons as well as paleontologic and sedimentologic studieson these deposits in order to determine their origin, age, sedi-mentary environment, and possible correlations, as well as thetiming of the uplift of the Patagonian Andes between ~43� and44�S. The intent of this project is to contribute to the understandingof the paleogeographic and tectonic evolution of this region duringthe Cenozoic.

1.1. Geologic setting

The study area is located in the North Patagonian Andes, be-tween ~43� and 44�S (Figs. 1 and 2). Three NeS trending physio-graphic units characterize this region: 1) the Coastal Cordillera, asubdued mountain range located in the western part of Chilo�e Is-land and in the Chonos Islands, where it is dissected by severalfiords; 2) the Longitudinal Depression (a.k.a. Central Valley), a low-lying area between the Coastal Cordillera and the Main AndeanCordillera that it is mostly submerged in this region and graduallydisappears towards the south (Duhart and Adriasola, 2008); and 3)the Main Andean Cordillera that extends from the mainland coastto eastern Argentina and includes active volcanoes and the highestpeaks in the region (Duhart and Adriasola, 2008).

The Coastal Cordillera (Fig. 1) is characterized by a Paleozoic�-Triassic metamorphic basement interpreted as a paleoaccretionarywedge (Duhart and Adriasola, 2008). Locally, Eocene dacitic dikesand sills, as well as granodioritic stocks, intrude the metamorphicbasement on Chilo�e Island (Duhart and Adriasola, 2008). A marine,coal-bearing succession named the Caleta Chonos Formation(Eocene?�Oligocene?) occurs in a small area of northwesternChilo�e (Antinao et al., 2000). Upper Oligocene�lower Miocenevolcanic and subvolcanic rocks of the Complejo Volc�anico de Ancudcrop out in northwestern Chilo�e (Antinao et al., 2000). Miocenemarine strata of the Lacui Formation and equivalent units extendalong the western part of Chilo�e Island and occur on some of theChonos Islands to the south (DeVries et al., 1984; Antinao et al.,2000; Nielsen and Glodny, 2009; Kiel and Nielsen, 2010; Nielsenand Encinas, 2014). Pliocene(?) marine strata of the Caleta GodoyFormation occur in some parts of northern Chilo�e Island (Antinaoet al., 2000). Pleistocene deposits mostly of glacial origin and Ho-locene continental and marine sedimentary beds constitute theyounger successions in the area.

The geology of the Longitudinal Depression (Fig. 1) in this area ispoorly known because of limited exposure. Subsurface data fromthe 4010 m deep Puerto Montt 1 well drilled by ENAP (The ChileanNational Oil Company) in the homonymous locality immediatelynorth of the study area (Katz, 1965; Elgueta et al., 2000) indicatesthe presence of 1) a lower, 1480 m-thick interval formed by inter-bedded tuff and volcaniclastic breccia of unknown age; 2) 310 m ofMiocene marine rocks; 3) 950 m of Pliocene? continental? or ma-rine? sedimentary rocks; and 4) 1300 m of Pleistocene continentaland marine deposits, mostly of glacial origin, that also crop outalong most of the emerged part of the Longitudinal Depression.Miocene(?) marine strata also occur in some limited areas along thecoast of eastern Chilo�e and on the nearby islands (Quiroz et al.,2004). Upper Cretaceous(?) volcano-sedimentary marine rocksand Eocene plutonic rocks crop out along the western part of theLongitudinal Depression on some of the Chonos Islands in thesouthern part of the study area.

The oldest rocks of the Main Andean Cordillera (Fig. 1) areMiddle to Late Paleozoic�Triassic metamorphics of the ComplejoMetam�orfico Cordillerano, exposed in thewestern part of this range(Duhart, 2008). MesozoiceCenozoic plutonic rocks of the North-Patagonian Batholith (NPB) (Pankhust et al., 1992) are widelyexposed in the Main Andean Cordillera. They are mainly EarlyCretaceous and Miocene in age, although there are also Devonian,Eocene, Lower Oligocene, and Pliocene plutonic rocks (Duhart,2008). Jurassic, Cretaceous, Paleogene, and Neogene volcanic, vol-caniclastic, and sedimentary rocks occur along the eastern andwestern flanks of the Main Andean Cordillera (Giacosa et al., 2005;Duhart, 2008; Ramos and Ghiglione, 2008). Cenozoic marine de-posits include 1) upper Oligocene�lower Miocene marine volcano-sedimentary rocks of the Traigu�en Formation on the western flankof the Andes in the study area and also in the LongitudinalDepression farther south (Herv�e et al., 2001; Riffo et al., 2013); 2)lower Miocene marine sedimentary rocks of the Ayacara and theRío Foyel formations on the western and eastern flanks of the MainAndean Cordillera, respectively; 3) Eocene?�Miocene? marinesedimentary rocks of the Puduhuapi Formation on the westernAndes near Chait�en; 4) Eocene? marine rocks of the Vargas For-mation in the central part of this range; and 5) Eocene?�Miocene?marine rocks of the La Cascada Formation on the eastern flank ofthe Andes near Futaleufú. The youngest successions in the NorthPatagonian Andes consist of Pleistocene and Holocene volcanic,volcaniclastic, and glacial deposits.

As cited above, a significant NeS trending, dextral strike-slip,intra-arc discontinuity referred to as the Liqui~ne-Ofqui Fault Zone(LOFZ) extends for ~1000 km along the North Patagonian Andes.

Fig. 1. Geologic map of the study area. The insets show the location of the studied sections detailed in Fig. 2. After Fuenzalida and Etchart (1975), Castillo (1983), Segemar (1994,1995), De la Cruz et al. (1996), Urbina (2001), Thomson (2002), Silva (2003), Casadío et al. (2004), Giacosa et al. (2005), Duhart (2008), Encinas et al. (2013), and references therein.

Fig. 2. Detailed geologic maps showing the location of sections from Fig. 3.


This fault system decouples the south-central Chilean forearc,which behaves as a northward-gliding sliver (Forsythe and Nelson,1985; Potent and Reuther, 2001; Cembrano et al., 2002). Structuralanalyses, Ar-Ar geochronology, and fission track thermochronologydocument right-lateral transpression along the LOFZ from the lateMiocene to the Pliocene (e.g., Cembrano et al., 2000, 2002;Thomson., 2002; Adriasola et al., 2006; Adriasola and St€ockhert,2008), but the long-term nature and timing of lateral motion ispoorly understood (Thomson, 2002). Alignment of its strike withPlioceneeRecent volcanic centers suggests that the LOFZ is linkedto a zone of magmatic weakening of the crust (Herv�e, 1994).

1.2. Cenozoic marine successions of the study area

This investigation focuses on the marine deposits of the Pudu-huapi, La Cascada, and Vargas formations. With the objective of

exploring a possible relationship between the units, it also includesthe La Junta Formation, which is a continental succession ofconglomerate and sandstone that crops out in the vicinity of theVargas Formation exposures (Fig. 2).

1.2.1. Puduhuapi FormationThis marine unit crops out in the northwestern part of the

Puduhuapi Island, approximately 7 kmwest of Chait�en (Fig. 1). Leviet al. (1966) described the Puduhuapi Formation as a gently foldedsuccession, ~35 m thick, of sandstone, siltstone, and minorconglomerate that is intruded by several andesitic dikes and sills.Levi et al. (1966) referred the Puduhuapi Formation to the Eoce-neeMiocene interval upon correlating it with the Ayacara Forma-tion, a marine unit that crops out north of the study area. Encinaset al. (2013) recently restricted the Ayacara Formation to theearlyemiddle Miocene.

A. Encinas et al. / Journal of South American Earth Sciences 55 (2014) 1e18 5

1.2.2. La Cascada FormationThe La Cascada Formation is a succession of fossiliferous

conglomerate, sandstone, and siltstone that was defined by Thieleet al. (1978). This marine unit crops out discontinuously near thelocality of Futaleufú (Fig. 1). It overlies Cretaceous granitic andLower Jurassic volcanic rocks. The molluscan fossil fauna of the LaCascada Formation was studied by Tavera (in Castillo, 1983) andassigned to the Eocene (Thiele et al., 1978; Castillo, 1983). Tavera (inCastillo, 1983), however, noted the presence of some species typicalof Miocene and Eocene strata that crop out in the forearc of centralChile.

1.2.3. Vargas FormationThe Vargas Formation is a marine unit composed of fossiliferous

black shales and minor sandstones that crop out in a small areaalong the west bank of the Palena River, near the locality of La Juntaand close to the LOFZ (Fig. 1). These strata were first studied bySteffen (1944), who named them “Esquistos Arcillosos (clayeyshales) Formation”. Bobenrieth et al. (1983) subsequently consid-ered them as part of the La Junta Formation, but they did notrecognize their marine origin. Urbina (2001) redefined the marinedeposits as the Vargas Formation. The basal and upper contacts ofthis unit are not exposed. Strata of this unit show significantdeformation (Urbina, 2001), as they are foliated, dip 50e80�, and infault contact with Cretaceous granitoids. Urbina (2001) proposedan Eocene age for the Vargas Formation based on the similarity ofits molluscs with those of the La Cascada Formation that Thieleet al. (1978) and Castillo (1983) had documented. However,Urbina (2001) did not identify any of the Vargas Formation taxa atthe species level.

1.2.4. The La Junta FormationUrbina (2001) described the La Junta Formation as a succession

of continental clastic and volcanic rocks that crops out in the eastbank of the Palena River near the area of the La Junta Formation(Fig. 1). Although we could not observe the stratigraphic relation-ship between the sedimentary and volcanic rocks, we do notdiscard the notion that they are of different age. The La Junta For-mation overlies Cretaceous (AlbianeTuronian) granitoids while itsupper contact is not exposed (Urbina, 2001). The Vargas Formationcrops out nearby but on the opposite side of the Palena River; thestratigraphic relationship between the two sedimentary units hasnot been observed (Urbina, 2001). Bobenrieth et al. (1983) assigneda Paleogene age to the La Junta Formation based on a tentativelithologic correlation with the volcanic rocks of the “Serie Andesí-tica” of Argentina (Rapela et al., 1983). Urbina (2001) noted thatstrata of the La Junta Formation show a higher degree of meta-morphism than those of the Vargas Formation and thereforeconsidered the former to be older. Based on this observation, heassigned a TuronianeEocene age to the La Junta Formation.

2. Results

2.1. Sedimentology

We carried out sedimentologic analyses on the Puduhuapi, LaCascada, Vargas, and La Junta formations. Due to the thick vegeta-tion cover and tectonic deformation that characterize this area,exposures of these units are limited to small outcrops, principally inthe case of the La Junta and Vargas formations (Figs. 3 and 4).

2.1.1. Facies of the Puduhuapi Formation2.1.1.1. Rythmically interbedded sandstones and siltstones (P1).The P1 facies is most abundant and consist on medium- to coarse-grained sandstone interbedded with siltstone (Figs. 3 and 4a).

Sandstone and siltstone beds are typically decimeters in thicknessbut vary from a few centimeters to more than one meter. Contactsbetween beds are usually sharp but occasionally show load-and-flame structures or flute casts. Beds form partial Bouma cycles inwhich sandstone is typically predominant and locally amalgam-ated. Tb and Tbc divisions (paralleleconvolute or, more rarely,ripple cross-lamination) is more common than Ta (massive) andTab (massive-parallel laminate). The sandstone is a lithic arkosecomposed primarily of angular to subrounded grains of volcanicrocks (andesite, basalt, and minor dacite) and plagioclase, andsecondarily of quartz and siltstone. Sandstone beds locally containThalassinoides isp. Fine-grained intervals consist of grey siltstonethat are locally disrupted and transformed into rip-up clasts. Theycontain abundant and mostly planktonic foraminifers.

Interpretation: The occurrence of rhythmically interbeddedsandstone and siltstone forming partial Bouma cycles indicatedeposition by turbidity currents (Bouma, 1962; Posamentier andWalker, 2006). The presence of abundant planktonic foraminiferscharacterizes deep-water deposition (Van der Zwaan et al., 1999).Convolute lamination and load-and-flame structures indicate rapiddeposition, which traps pore fluid and results in deformation ofprimary structures (Posamentier and Walker, 2006). Rip-up clastsand flute casts evidence turbulent erosion of the substrate imme-diately prior to deposition. The local presence of Thalassinoides istypical of the Skolithos ichnofacies that has beenwidely reported indeep-water settings, and it reflects local environmental conditionssuch as high energy, a sandy substrate, high levels of oxygenation,and an abundance of suspended organic particles (e.g., Buatois andLopez-Angriman, 1992).

2.1.1.2. Conglomerates (P2). This facies is clast-supported andshows a moderate to good sorting. Individual beds are normallydecimeters thick and interbedded with turbidites (facies P1) or finesiltstones. Basal contacts of beds are slightly erosional and locallyshow load-and-flame structures. Clasts are angular to subrounded,centimeters in size, and composed primarily of andesite and basaltand secondarily of dacite. Siltstones and sandstone intraclastscentimeters to decimeters in diameter are locally present andsometimes folded. Conglomerate grading can be normal, inverse, orinverse to normal.

Interpretation: Posamentier and Walker (2006) interpret clast-supported, graded conglomerates associated with classical turbi-dites as deposited by large sandy turbidity currents where thepebble beds represent lags left behind by the main flows. Rip-upclasts evidence turbulent erosion of the substrate immediatelypreceding deposition. The occurrence of load-and-flame structuresindicate rapid deposition.

2.1.1.3. Slumps (P3). This facies is rare and consists of folded in-terbeds of sandstone and siltstone of facies P1 that are underlainand overlain by undistorted, flat beds (Figs. 3 and 4b).

Interpretation: This facies originated by slumping and folding ofunconsolidated turbidites as they were displaced downslope(Posamentier and Walker, 2006).

2.1.2. Facies of the La Cascada Formation2.1.2.1. Basal conglomerate (LC1). The contact of the basalconglomerate of the La Cascada Formation with underlying Creta-ceous granitoids was only observed in Arroyo Grande Creek,19.5 kmwest of Futaleufú (section CAS, Fig. 3). The conglomerate is~2 m thick and comprises decimeter-sized, subrounded to sub-angular granitic clasts. It is overlain by fossiliferous sandstone offacies LC3.

Interpretation: This facies is interpreted as a coarse-grained lagdeposited during the initial stage of the marine transgression.

Fig. 3. Representative sections of the Puduhuapi (PUD), La Cascada (CAS, CHEN, NONO, and PNEG), Vargas (VAR), and La Junta (JUN) formations (see Fig. 2 for location). Grain sizes:(a) clay, (b) silt, (c) very fine sand, (d) fine sand, (e) medium sand, (f) coarse sand, (g) very coarse sand, (h) granule, (i) pebble, (j) cobble, (k) boulder. LT, lithology: (1) clast-supportedconglomerate, (2) matrix-supported conglomerate, (3) breccia, (4) sandstone, (5) siltstone, (6) andesitic dike. Covered intervals are represented by a X. SS, sedimentary structures:


Fig. 4. Characteristic facies of the units studied in this work. (A) Rhythmically interbedded sandstone and siltstone (Puduhuapi Formation, facies P1) at Puduhuapi Island. Noteflame to the right of the pen. (B) Slump (Puduhuapi Formation, facies P3) at Puduhuapi Island. (C) Clast-supported conglomerate and sandstone (La Cascada Formation, facies LC2) atCerro Chenque. Note inversely graded conglomerate with large clasts, some of them imbricated, above the hammer (encircled). (D) Cross-bedded sandstones (La Cascada Formation,facies LC3) at Cerro Chenque. (E) Lam-Scram sandstone (La Cascada Formation, facies LC5) at Arroyo Grande Creek. (F) Clast-supported conglomerates (La Junta Formation, faciesLJ1) at roadcut near La Junta.


2.1.2.2. Clast-supported conglomerates and sandstones (LC2). Asuccession of clast-supported conglomerates interbedded withsandstone occurs at Cerro Chenque (section CHEN, Figs. 3 and 4c).Contact with Lower Jurassic volcanic rocks is inferred, as the lowerpart of the succession is not accessible. This facies is overlain byfossiliferous cross-bedded sandstones of facies LC3. Conglomeratebeds are decimeters to a few meters thick and show a moderate topoor sorting of clasts and an abundant sandy matrix. The clasts areof volcanic and granitic origin, subrounded to angular, and up to~1 m in size. They are generally arranged parallel to bedding, andsome are imbricated. Conglomerates show normal or inverse

(7) parallel lamination, (8) planar cross-bedding, (9) asymmetrical ripples, (10) symmetrica(14) rip- up clasts, (15) convolute lamination, (16) slumps, (17) flames, (18) load structures, ((23) foraminifers, (24) leaves, (25) wood, (26) coal, (27) shark teeth. TF, trace fossils: (28) ThScale is in meters.

grading. Interbedded sandstones are scarce in the basal part of thissuccession but become increasingly predominant toward the top ofit. Sandstones are often very coarse-grained and locally conglom-eratic. They are usually massive but locally exhibit cross-bedding.

Interpretation: The moderate to poor sorting of conglomerates,abundant sandy matrix, clasts parallel to bedding or imbricated,local presence of very large clasts, and inverse grading suggest thattransport by hyperconcentrated flows was the dominant process(Hwang and Chough, 1990; Iverson, 1997). The presence of largeangular clasts suggests short-distance transport. Considering thesefeatures, it is probable that conglomerates were fed to the marine

l ripples, (11) hummocky cross-stratification, (12) LAM-SCRAM, (13) imbricated clasts,19) carbonated concretions. F, fossils: (20) gastropods, (21) bivalves, (22) echinoderms,alassinoides, (29) Skolithos, (30) Planolites, (31) Chondrites, (32) Undetermined burrows.


basin via alluvial fans. Sand beds interbedded with conglomerateswere probably deposited when flow energy decreased after depo-sition of the conglomeratic facies. The local presence of cross-bedding in sandstone indicates migration or dunes and suggestsreworking by waves in the upper shoreface (Clifton, 2006).

2.1.2.3. Cross-bedded sandstones (LC3). Coarse-grained sandstonesoverlying the basal conglomeratic successions of facies LC1 and LC2are usually massive but locally show planar and trough(?) cross-bedding (Figs. 3 and 4d). Oysters, shark teeth, wood fragments,and thin coal laminae are locally present. Sandstone in this and theother sandy facies of the La Cascada Formation are principally ar-koses composed primarily of angular to subrounded grains ofquartz with secondary granite lithics and feldspar.

Interpretation: This facies records deposition from migration of2D and 3D dunes, which is typical of the upper shoreface (Clifton,2006). Fossil oysters and shark teeth indicate a marine origin,whereas wood fragments were derived from terrestrial runoff.

2.1.2.4. Sandstones with hummocky cross stratification (LC4).Medium-grained sandstones at NONO section (Fig. 3) locally showlamination formed into domes and swales that are interpreted ashummocky cross-stratification.

Interpretation. Hummocky cross-stratified sandstone indicatesreworking by storm waves in the lower shoreface (Clifton, 2006).

2.1.2.5. Lam-scram sandstones (LC5). The LC5 facies consists ofdecimeter-thick interbeds of structureless and laminated fine-grained sandstone called “lam-scram” (Howard, 1975) (sectionCAS, Figs. 3 and 4e). Structureless (scrambled) sandstone is highlybioturbated and shows abundant Chondrites. Laminated intervalsshow hummocky cross-stratification, parallel lamination, andpossible wave-ripples. Scarce bioturbation in these intervals con-sists of Skolithos and Thalassinoides burrows internally reworked byChondrites.

Interpretation: The lam-scram facies results from the alternationof storm and fair-weather processes. Stormwaves form laminated-to-hummocky, cross-stratified sand (Harms et al., 1975). Duringnormal low-energy intervals, the opportunistic suspension-feedingfauna recolonizes the upper part of the sandy unit and its intensebioturbation obliterates the primary sedimentary fabric(Pemberton et al., 2001). Lam-scram deposits are typical ofmoderately storm-affected shorefaces of intermediate energy(MacEachern and Pemberton, 1992; Buatois et al., 2007).

2.1.2.6. Interbedded sandstones and siltstones (LC6). This facies oc-curs in association with facies LC4 and LC5. Sandstone beds locallycontain molluscs. Sedimentary structures or trace fossils are notpreserved, probably due to tectonic disruption.

Interpretation. The presence of interbedded sandstone and silt-stone in association with sandstones showing lam-scram or HCS(facies LC4 and LC5) suggest deposition in the transition zone of ashallow-marine setting.

2.1.2.7. Siltstone and minor sandstone (LC7). A succession domi-nated by dark-grey siltstone and minor sandstone intercalationsoccurs in Arroyo Grande Creek (section PNEG, Fig. 3), 19.5 kmwestof Futaleufú. No sedimentary structures or trace fossils are evident,but molluscs occur locally.

Interpretation. The presence of coastal facies in the La CascadaFormation suggest that siltstone and minor sandstone are outer-shelf facies deposited during a relative rise of sea-level (Clifton,2006), but the absence of sedimentary structures and trace fossilsprecludes a more refined interpretation.

2.1.3. Facies of the Vargas Formation2.1.3.1. Silstones and minor sandstones (V1). The Vargas Formationis exposed in a small area on the west bank of the Palena River.Outcrops of this unit are small and discontinuous and consist of afewmeters thick successions of black siltstone and minor medium-grained sandstone (section VAR, Fig. 3). Fossils of gastropods, bi-valves, echinoderms, and planktic foraminifers are present, prin-cipally in siltstones. Sedimentary structures are not evident,probably as a consequence of the strong tectonic deformation thatcharacterizes these strata. The sandstone is a sedarenite composedof angular to subrounded grains of quartz, sedimentary and vol-canic lithics, and minor feldspar.

Interpretation: The small size of the outcrops and the lack ofsedimentary structures prevent a refined interpretation. Fossilif-erous siltstones occur in low-energy marine settings that can belocated either above or below fairweather wave base. The presenceof planktic foraminifers, however, suggests offshore deposition. Theblack color of these facies indicates a substrate rich in organicmatter and low in oxygen.

2.1.4. Facies of the La Junta FormationA succession of conglomerates and sandstones ~7 m thick crops

out in a small roadcut near La Junta. We observed the followingfacies in this short succession:

2.1.4.1. Clast-supported conglomerates (LJ1). Conglomerates in thisfacies present well-rounded to, more rarely, subangular clasts ofandesite, granitoid, or quartz that vary in size from 1 to 20 cm(section JUN, Figs. 3 and 4f). Selection is good to moderate. Clastsare locally imbricate. Conglomerate beds typically have erosionalbases and gradational contacts with the overlying sandstones of theLP3 facies. They locally show thin layers of coal that are a fewcentimeters in thickness and irregularly shaped.

Interpretation. The presence of conglomerates with a good tomoderate selection and well-rounded, locally imbricated clastsindicate transport by fluidal flows (Hwang and Chough, 1990). Theoccurrence of coal laminae suggests a subaerial or transitionalsetting for the LJ1 facies.

2.1.4.2. Matrix-supported conglomerates (LJ2). Some conglomeratebeds are matrix-supported, display moderate selection, and have asandy matrix. Clasts are well-rounded to subangular and have asimilar composition to those of clast-supported conglomerates.

Interpretation. Matrix-supported conglomerates with a sandymatrix suggest that they are non-cohesive debris-flow deposits(e.g., Nemec and Steel, 1984; Scott et al., 1995).

2.1.4.3. Sandstones (LJ3). Very coarse-grained sandstones, locallyconglomeratic, are interbedded or gradational with conglomerates.No sedimentary structures were observed. The sandstone is a vol-carenite composed of angular to subrounded volcanic and sedi-mentary lithics with minor quartz and feldspar grains.

Interpretation: The lack of sedimentary structures prevents arefined interpretation of this facies. Sands were probably depositedwhen flow energy decreased after deposition of the conglomeraticfacies.

2.1.5. UePb geochronologyHeavy mineral concentrates of the <350 mm fraction were

separated using traditional techniques at ZirChron LLC. Zirconsfrom the non-magnetic fraction were mounted in epoxy andslightly ground and polished to expose the surface and keep asmuch material as possible for laser ablation analyses.

LA-ICP-MS UePb analyses were conducted at Washington StateUniversity prior to CL imaging using a New Wave Nd:YAG UV 213-


nm laser coupled to a ThermoFinnigan Element 2 single collector,double-focusing, magnetic sector ICP-MS. Operating proceduresand parameters are a modification of Chang et al. (2006). Laser spotsize and repetition rate were 30 nm and 10 Hz, respectively. He andAr carrier gases delivered the sample aerosol to the plasma. Eachanalysis consisted of a short blank analysis followed by 250 sweepsthrough masses 204, 206, 207, 208, 232, 235, and 238, takingapproximately 30 s. Time-independent fractionation was correctedby normalizing U/Pb and Pb/Pb ratios of the unknowns to the zirconstandards (Chang et al., 2006). We used three zircon standards:Peixe, with an age of 564 Ma (Dickinson and Gehrels, 2003); Ple-sovice, with an age of 338Ma (Sl�ama et al., 2008); and FC-1, with anage of 1099 Ma (Paces and Miller, 1993). Uraniumelead ages werecalculated using Isoplot (Ludwig, 2003).

Seven samples were selected for UePb zircon geochronology.Table DR1 presents the analytical results. Maximum depositionalages are defined bymore than three grains of overlapping analyses,which are interpreted to represent a valid population and not leadloss or mixing trajectories.

The two samples from the Puduhuapi Formation are probably ofvolcanic origin, as they are characterized by abundant volcaniclithics and plagioclase, and cathodoluminiscence (CL) images showthe zircons are characterized by a single oscillatory zoning patternwith some fragmentary morphology. The 104 analyzed zirconsfrom sample PUD-7 have ages that overlap within error, suggestinga single source area. An age of 23.3 þ 0.4e0.5 Ma (2 sigma, n ¼ 97,Figs. 5 and 6) was calculated with the Tuffzirc algorithm (seeLudwig, 2003) that eliminates both inheritance and lead loss. A15.8 ± 1.0 Ma age was obtained from a single grain, however thisage is statistically meaningless as it could be either lead loss or ameaningful zircon crystallization age. The other sample (PUD-1)also presents a predominantly single age population and is alsocharacterized by oscillatory zoned zircons with some fragmentary,euhedral morphologies and a 3:1 length/width ratio. One-hundredone zircons were analyzed from this samplewith a calculated age of22.7 þ 0.3e0.4 (2 sigma, n ¼ 98) (Figs. 5 and 6) representing amaximum depositional age that overlaps with the age obtained insample PUD-7. A single zircon grain yielded a 19.4 ± 1.6 Ma age thatis statistically meaningless.

Fig. 5. TuffZirc ages calculated from single grain ages using methods and algorithms of Isopincluded.

Two samples from the La Cascada Formation were selected foranalysis. One-hundred five grains from the PNEG sample exhibitedvariable CL patterns, including single oscillatory zoned and mix-zoned oscillatory with homogeneous rims, which suggestmagmatic and mixed magmatic-metamorphic? sources. However,all the analyzed grains have Th/U > 0.1 characteristic of an igneousorigin (Rubatto, 2002). The youngest detrital grain yielded a ca.48.5 Ma age whereas the oldest zircon is of ca. 1810.8 Ma (Fig. 6).The two major age populations are of 51 Ma (15 grains) and 84 Ma(42 grain) and also confirm a post-Eocene depositional age. Theother sample from the La Cascada Formation (NONO-2) yielded 96zircon grains with Th/U ratios >0.1, typical of igneous-related zir-cons. The prominent Early Cretaceous populations of 110e117 Ma(Fig. 6) contrast with the Late Cretaceous and Eocene zircons of theLa Cascada sample. Six zircon grains overlap within error anddefine a UePb age of 17.7 þ 0.4e0.2 Ma (Fig. 5) that is consideredthe maximum age of deposition.

Two samples were selected from the Vargas Formation. Detritalzircons from sample VAR-2 (n¼ 82) are heterogeneous and includezircon fragments, euhedral zircons with 2:1 to 3:1 length/widthratios and predominantly oscillatory-zoned zircons indicative ofigneous origin and some composite oscillatory and homogenousrim zircons that also suggest the presence of complex sources. Th/Uratios for the analyzed zircons are >0.1, which suggests a typicaligneous origin (Rubatto, 2002). Zircons are predominantly of LateCretaceous age and can be divided into populations of 82Ma and 93Ma (Fig. 6); however, a small group ~39 Ma (n ¼ 4) indicates amaximum depositional age in the Eocene. The other sample VAR-5also has a similar variable CL pattern. The analyzed zircons arecharacterized by Th/U ratios >0.1, which are characteristic ofigneous-derived zircons. Two main Cretaceous-age populations of84 Ma and 107 Ma characterize this sample (Fig. 6).

A single sample was selected from the La Junta Formation (JUN-1). It yielded zircons that are mainly oscillatory zoned andmagmatic related, and with a 3:1 length-width ratio. Analyzedgrains have Th/U ratios >0.1 that are characteristic of an igneousorigin. Three main age peaks characterize this sample: (1) aprominent Miocene peak at 26 Ma and (2) two Cretaceous pop-ulations of 97 Ma and 108 Ma (Fig. 6) that are similar to age

lot 3 (Ludwig, 2003). All uncertainties are 2 sigma. Systematic and analytical errors are

Fig. 6. Probability density function of detrital zircon grain ages from Padahuapi Formation (A and B), La Cascada Formation (C and D), Vargas Formation (E and F) and La JuntaFormation (G).


populations seen in other samples. An age of 26.0 þ 0.9e0.4 Ma(Fig. 5) calculated from 21 grains is the maximum depositional age.

2.1.6. ForaminifersWe processed five siltstone samples for foraminifers: two from

the Puduhuapi Formation, two from the Vargas Formation, and onefrom the La Cascada Formation. To separate the foraminifers, weinitially soaked the rocks in H2O2, but they were too firmlycemented and did not disaggregate. Subsequently, we tried severalother methods, including the use of an ultrasonic water bath andimmersions in HCl and HF. Of these, only 50% HF succeeded infreeing any foraminifers from the indurated rocks. That methodfailed to produce a single specimen from the La Cascada Formation,but yielded 8 planktic foraminifers from the two samples of the

Vargas Formation and >100 planktic foraminifers and 5 benthicspecimens from the two samples of the Puduhuapi Formation. Allof the isolated specimens, however, show some degree of diage-netic alteration that masks those diagnostic features needed foraccurate taxonomic identification.

None of the 8 planktic foraminifers from the Vargas Formationare recognizable at the genus or species level. Although most of the>100 planktic specimens of the Puduhuapi Formation were simi-larly unrecognizable, we were able to identify Globigerina ven-ezuelana Hedberg and Paragloborotalia mayeri (Cushman andEllisor) among them. Both species have relatively long ranges:G. venezuelana spans the middle Eoceneeearly Pliocene, withinwhich Pg. mayeri ranges middle Oligocene to late middle Miocene;therefore, the latter range is also their concurrent range that

Fig. 7. (A). SNGM 1348, ‘Trachycardium cf. puelchum’, height 84 mm. (B). SNGM 1360,‘Chione sp.’, length 72 mm. (C). SNGM 1352, ‘Trochus aff. laevis’, diameter 36 mm. (D, E).SNGM 1339, ‘Ficus distans’, height 12.4 mm (F). SNGM 1333, ‘Turritella ambulacrum’,height 12.4 mm (G). SNGM 1355, ‘Proscaphella sp.’, height 76 mm. (H). SNGM 1362,‘Turritella ambulacrum’, height 28.6 mm (AeG). La Cascada Formation. (H). VargasFormation.


restricts the Puduhuapi Formation. Benthic specimens obtainedfrom this unit are Lagena sp., Triloculina sp., andMelonis barleeanus(Williamson). The presence of the latter species suggests deep-water deposition, as modern M. barleeanus typically occur at up-per middle-bathyal or greater depths. The high planktic:benthicratios (>9:1) of foraminiferal assemblages in the Puduhuapi For-mation are also characteristic of deep-water deposits (e.g., Van derZwaan et al., 1999).

2.1.7. MolluscsOf the investigated units, only the La Cascada and Vargas for-

mations yielded relatively well-preserved macrofossils. The pres-ence of molluscs in the La Cascada Formation was previouslyreported by Tavera (in Castillo, 1983) but apparently never pub-lished. The fauna determined by Tavera includes Cucullaea sp.,Trochita costellata (?), Fissurella sp. alternula [sic], and Pitaria sp.from a section at La Bajada, and a rich fauna from loose blocks at LaCascada and Arroyo Pinilla (Table DR2). These present a curiousmixture of early Miocene Navidad and younger taxa. Excluding thestratigraphic confusion created by Tavera (1942), who mixed upMiocene and Eocene faunas from Arauco that Tavera (in Castillo,1983) later used for biostratigraphic correlation, typical Miocenetaxa include Venus cf. V. navidadis, Cucullaea alta, Ficus carolina,Turritella ambulacrum, and Distorsio sp. Mytilus and Fissurella arerecorded with certainty only from shallower deposits that areyounger than the Navidad Formation (Tavera and Veyl, 1958;McLean, 1984; Nielsen, unpublished data).

Two Sernageomin (Servicio Nacional de Geología y Minería deChile) internal paleontological reports (Rubilar et al., 2001, 2002)record the presence of the bivalves Neilo (?) sp. (SNGM 1053),Trachycardium philippii (SNGM 1342), Trachycardium cf. puelchum(SNGM 1348), Callista (Costocallista) sp. (SNGM 1332), Chione aff.C. argentina (SNGM 1343), Chione aff. C. patagonica (SNGM 1344),and Panopea sp. (SNGM 1350), the gastropods Trochus aff. T. laevis(SNGM 1352), ‘Gibbula' sp. (SNGM 1336), Turritella ambulacrum(SNGM 1055, 1328, 1333, 1337, 1346, 1353, 1362), Polinices sp.(SNGM 1329), Ficus distans (SNGM 1339), Trophon sp. (SNGM 1347,1354), and Proscaphella sp. (SNGM 1340, 1355), the coral Flabellumsp. (SNGM 1361), the bryozoan Heteropora pelliculata (SNGM 1335),a lamniform shark tooth (SNGM 1057), and plant debris (SNGM1359). While most of these come from the La Cascada Formation,Chione (?) sp. (SNGM 1363), Chione sp. (SNGM 1360), three speci-mens of Turritella ambulacrum (SNGM 1362), and Flabellum sp.(SNGM 1361) are from the Vargas Formation. Based on this asso-ciation, Rubilar et al. (2001, 2002) concluded a late Oligocene toMiocene age, which would apply to both units.

We could not locate the specimens identified by Tavera (inCastillo, 1983), so they are excluded from further consideration.Those of Rubilar et al. (2001, 2002) arewell-curated at Sernageominin Santiago, Chile and presented here as the first publishedmacrofossil evidence for the La Cascada and Vargas formations. Theavailable material consists of internal and external molds that oftenare insufficient for precise determinations (see Fig. 7). Some of thespecimens warrant some comments if they are to be used forbiostratigraphic or biogeographic purposes.

If correctly identified, both species of Trachycardium are ofAtlantic affinity. Cardium puelchum Sowerby, 1846 was placedtentatively in Hedecardium by Griffin and Nielsen (2008); however,there is at least one similar species present on the Pacific side, i.e.Trachycardium? multiradiatum (Sowerby, 1846) (see Griffin andNielsen, 2008). Better-preserved material is needed to clarify ifthe La Cascada species (Fig. 7A) is of Atlantic or Pacific origin.

Nielsen et al. (2004) and Griffin and Nielsen (2008) reviewedTrochus laevis Sowerby, 1846 and corrected its preoccupied name toAstele chilensis (d'Orbigny, 1852). Specimen SNGM 1352 (Fig. 7C)

appears to belong to this species, which is known from the NavidadFormation and the coeval Ranquil Formation on the Araucopeninsula. Frassinetti and Covacevich (1999) reported this speciesas T. laevis from the Atlantic incursion of the Guadal Formation atChile Chico.

Turritella ambulacrum Sowerby, 1846 is an Argentinean speciesthat was originally described from Santa Cruz and possibly SanJuli�an (see Griffin and Nielsen, 2008). It is not present in Chile, but asimilar species, Turritella pseudosuturalis d'Orbigny, 1852, occurs inthe Navidad Formation. Currently, these cannot be separated withconfidence (Griffin and Nielsen, 2008); thus, it cannot be deter-mined if the species present in the La Cascada Formation (Fig. 7F)and the Vargas Formation (Fig. 7H) is of Atlantic or Pacific origin.

Ficus distans (Sowerby, 1846) is an exclusively Pacific speciesknown from Chile and Peru (Covacevich and Frassinetti, 1980;DeVries, 1997; Griffin and Nielsen, 2008). It differs from Ficus car-olina (¼Pyrula carolina d'Orbigny, 1847), which was reported fromAtlantic sites in Argentina and Chile (Covacevich and Frassinetti,1980; Frassinetti and Covacevich, 1999). However, P. carolina wasdescribed from the Cretaceous of India (see d'Orbigny, 1850) anddoes not belong in Ficus. The South American specimens identifiedas F. carolina clearly belong in Ficus but need to be evaluated becausethe genus is unknown from the Cenozoic of Argentina. Specimensrecorded as F. carolina resemble F. allemanae DeVries, 1997 ratherthan F. distans in their sculpture and more slender shape, but the LaCascada specimens (Fig. 7DeE) are more similar to F. distans.

Specimen SNGM 1355, identified as Proscaphella sp. (Fig. 7G),belongs either in Palaeomelon or Pachycymbiola but the poorpreservation hinders specific determination. Several species areknown from Argentina (del Río and Martínez, 2006) and Chile(Nielsen and Frassinetti, 2007).

Based on this fauna and in agreement with the conclusions ofRubilar et al. (2001, 2002), we confirm a Miocene age for the LaCascada and Vargas formations, which could be contemporaneouswith the Navidad Formation or with the younger Tres Montes de-posits in the Golfo the Penas, southern Chile (see Frassinetti, 2006;


Forsythe et al., 1985). The currently available fauna is not suffi-ciently well-preserved to conclude an Atlantic or Pacific affinity, butit seems that Atlantic taxa dominate. Some Pacific taxa are alsopresent, suggesting an AtlanticePacific connection, as previouslypostulated by Nielsen (2005). A paleo-Magellanic faunal provincereaching around the southern tip of South America is also possible,but not a single Atlantic taxon is reported from the Chonos Islands(see Kiel and Nielsen, 2010). Better-preserved faunas need to becollected to satisfactorily answer these important questions.

3. Discussion

3.1. Sedimentary environment

The sedimentologic analyses of the sedimentary successionsdescribed in this study are hampered by the small size of the out-crops, the lack of continuous sections, and the poor preservation ofsedimentary structures that some of these units present. Therefore,some sedimentologic interpretations, in particular those for theVargas and La Junta formations, are tentative.

The Puduhuapi Formation is interpreted as deposited in a deep-marine environment based on the presence of interbedded sand-stone and siltstone with partial Bouma cycles (i.e., turbidites) thatinclude minor intercalations of breccia and slumps (Figs. 3 and 4,Table 1). The occurrence of benthic foraminifers indicative ofbathyal depths and a predominance of planktic foraminifers (>90%)supports this interpretation (see Van der Zwaan et al., 1999). Thepresence of angular clasts in the conglomerates of this unit in-dicates that the source area was close to the depositional basin. TheLa Cascada Formation shows facies typical of a shallow-marineenvironment (Figs. 3 and 4, Table 1). Alluvial fans were the likelyprovenance of the coarse sediment forming the thick basalconglomeratic succession at Cerro Chenque (section CHEN). Theoverlying cross-bedded sandstones are typical of the uppershoreface, whereas those with hummocky cross-stratification orlam-scram characterize deposition in the lower shoreface. Localintervals of interbedded sandstone and siltstone, as well as silt-stone and minor sandstone, characterize deposition in the transi-tion zone and outer shelf, respectively, as a consequence of relativerises of the sea level. The poor preservation of the Vargas Formationstrata hinders a refined interpretation, but the predominance ofblack siltstone (Fig. 3, Table 1) and the relative abundance ofplanktic foraminifers suggests offshore (outer shelf or deeper)deposition. The La Junta Formation is interpreted as an alluvial fandeposit based on the abundance of clast- and matrix-supportedconglomerates that contain subangular clasts and on the occur-rence of coal laminae (Figs. 3 and 4, Table 1). A marine origin,however, cannot be discarded.

3.2. Age and possible correlations

Ages for the Puduhuapi, La Cascada, Vargas, and La Junta for-mations were determined by UePb geochronology (Table DR1,Figs. 5 and 6). Recognizable species of planktic foraminifers were

Table 1Sedimentary environment, age, and relative location of possible provenance areas of the zages are maximal in Ma. Under Geographic Provenance, the LM to > P sequence refers to parea.

Formation Sed. Envir. Age

UePb Forams Molluscs

Puduhuapi Deep marine 22.7 M Oligo. e M Mio. e

La Cascada Shallow marine 17.7 e Early MioceneVargas Outer shelf? 39.4 e Early MioceneLa Junta Alluvial fans? 26.0 e e

found in the Puduhuapi Formation, but they were relatively longranging. A few molluscs of biostratigraphic utility were found instrata of the La Cascada and Vargas formations.

In determining ages of the studied units, the following aspectsmust be taken into consideration: 1) UePb geochronology ondetrital zircons yields maximum ages; 2) identifications of fora-minifers and molluscs that are not especially well preserved mustbe taken with some caution; and 3) the absence of a completesection for any of the units renders it impossible to determine theirrespective age intervals (from base to top).

Two samples from the Puduhuapi Formation were analyzed byUePb geochronology (Figs. 5 and 6). The youngest populations ofdetrital zircons from both samples indicate maximum depositionalages of 22.7 þ 0.3e0.4 and 23.3 þ 0.4e0.5 Ma (latest Oligocene-eearliest Miocene). Strata of this unit contain abundant volcanicmaterial and therefore sedimentation probably was contempora-neous with nearby volcanism, as previously suggested by Levi et al.(1966). The UePb ages are not contradicted by the occurrences ofG. venezuelana and Pg. mayeri.

Recent studies on the age of other Cenozoic marine successionsthat crop out in the forearc andwestern flank of the Andes of south-central Chile (~33�e47�S) indicate their possible temporal corre-lation with the Puduhuapi Formation (Fig. 8). Levi et al. (1966)correlated the Puduhuapi and the Ayacara formations based ontheir similar facies. UePb dating of detrital zircons from the AyacaraFormation indicates ages between ~17.6 and 21.8 Ma, whereasplanktic foraminifers indicate a middle Miocene age for this unit(Encinas et al., 2013). Yet, the later age must be taken with somecaution because the microfossils are not well preserved. UePb dataon detrital zircons from the Traigu�en Formation, a volcanosedi-mentary unit that crops out farther south, between approximately44� and 46�S, indicate probable depositional ages of ~23e26Ma forthis unit (Herv�e et al., 2001; Riffo et al., 2013). Strata of the LagoRanco Formation, a marine unit that crops out on the western flankof the Andes at ~40�S, yielded a UePb (on detrital zircons)maximum age of ~20 Ma (Bernab�e et al., 2009). The age of theNavidad Formation and equivalent units in the forearc of centraland southern Chile (33�e47�S; see Encinas et al., 2008 and refer-ences therein) has been largely debated but recent data indicate alikely early Miocene age for these strata (see Nielsen and Glodny,2009; Guti�errez et al., 2013; Finger, 2013; Finger et al., 2013) fordetails). In summary, the Puduhuapi Formation is probablycorrelative with the cited late Oligoceneemiddle? Miocene unitsthat crop out in the forearc and western flank of the AndeanCordillera between ~33� and 47�S. Yet, small differences exist be-tween the ages of some of these units, which opens-up the possi-bility of multiple depositional sequences.

The youngest zircon peak for one of the samples from the LaCascada Formation (NONO-2) indicates a maximum depositionalage of 17.7 þ 0.4e0.2 Ma for this unit (Figs. 5 and 6), which discardsthe proposed Eocene age (Thiele et al., 1978; Castillo, 1983). Thepresence of the gastropod species Ficus distans, otherwise onlyknown from the Navidad Formation of central Chile and the Chil-catay Formation of southern Peru (Covacevich and Frassinetti,

ircon groups from the Puduhuapi, La Cascada, Vargas, and La Junta formations. UePbrogressively older geologic intervals from lower Miocene to pre-Permian); s/a, same

Geographic Provenance

LM UO LO E UK LK LJ P >P

s/a e e e e e e e e

W e e W, S, SW W s/a s/a W We e e s/a?, W N, E e N e We NW W e N, E NW e e e

Fig. 8. Distribution of upper Oligocene-early Miocene marine deposits in central and southern Chile and Argentina. After Fuenzalida and Etchart (1975), Ramos (1982), Castillo(1983), Segemar (1994, 1995), De la Cruz et al. (1996), Frassinetti and Covacevich (1999), Urbina (2001), Thomson (2002), Sernageomin (2003), Silva (2003), Casadío et al.(2004), Giacosa et al. (2005), Encinas et al. (2006), Duhart (2008), Encinas et al. (2008), Asensio et al., 2010, Encinas et al. (2012), Encinas et al. (2013), and Bechis et al. (2014).


1980; DeVries, 1997), and ‘Turritella ambulacrum’ (Griffin andNielsen, 2008), suggests that the La Cascada Formation is also up-per Oligocene-lower Miocene; however, as discussed above, amiddle Miocene age cannot be ruled out. A better-preserved fauna

from the La Cascada Formation and better knowledge of the middleMiocene faunas from the Golfo the Penas (see Frassinetti, 2006) arerequired to clarify this. Recent studies indicate a loweremiddle?Miocene age for the Cenozoic marine deposits that crop out on the


eastern flank of the North Patagonian Andes of Argentina (seebelow). Thus, the tentative correlation of the La Cascada and the RíoFoyel formations proposed by Ramos (1982) appears to be correct(Fig. 8). It is likely that the La Cascada Formation is also contem-poraneous with the uppermost Oligoceneelower Miocene succes-sions that crop out in the forearc and on the western flank of theAndes of south-central Chile between ~33� and 47�S (see above), aswell as the upper Oligoceneelower Miocene marine deposits ofAtlantic origin of the “Patagoniense” in Argentina (see Cuiti~no et al.,2012 and references therein).

The two samples of the Vargas Formation analyzed by UePbgeochronology yieldedmaximumdepositional ages of ~85Ma (LateCretaceous) and 39.4 Ma þ 1.3e1.9 Ma (Eocene) respectively(Fig. 6). According to these data, the Vargas Formation is Eocene(Bartonian) or younger. The presence of T. ambulacrum, however,indicates a late Oligocene-early Miocene age that correlates withthe La Cascada Formation. No similar turritellids are known fromEocene deposits in Chile (Nielsen, unpublished data). Therefore, thedetrital zircon UePb ages are significantly older than the deposi-tional age of this unit. An upper Oligoceneelower Miocene age forthe Vargas Formation and the presence of coeval deposits of thePuduhuapi and La Cascada formations in the same region (Fig. 8)support the notion that these three units were deposited by thesame transgressive event.

The La Junta Formation was tentatively assigned to the Paleo-gene by Bobenrieth et al. (1983) and to the TuronianeEocene byUrbina (2001). However, in the present study, the sample analyzedby UePb geochronology indicates a maximum depositional age of26.0 þ 0.9e0.4 Ma (late Oligocene) for the conglomeratic succes-sion included in this unit (Figs. 5 and 6). Correlation of the La JuntaFormation is hindered by the absence of late Oligocene or youngercontinental successions in the area, although they have beendefined near Bariloche in eastern Argentina (e.g., Bechis andCristallini, 2005). Considering that the marine deposits of thePuduhuapi, La Cascada, and Vargas formations crop out in the sameregion (Fig. 8) and appear to be coeval with the Oligocene La JuntaFormation, and a thick conglomeratic succession is at the base ofthe La Cascada Formation, it is conceivable that the conglomeratesof the La Junta Formation were deposited during the initial stage ofbasin subsidence and prior to the marine transgression that led todeposition of the Vargas Formation in the area.

Summarizing, the Puduhuapi, La Cascada, and Vargas forma-tions were deposited in the upper Oligoceneelower Miocene in-terval. They are likely correlative, although our data are insufficientto determine whether they were deposited by a single or severalmarine incursions in the area. These units possibly correlate withlower Miocene marine deposits in the forearc and western AndeanCordillera of south-central Chile (Navidad Formation and equiva-lent units) and the eastern flank of the Patagonian Andes (Río FoyelFormation and equivalent units and those of the “Patagoniense”).

3.3. Provenance analysis

Analysis of provenance based on the age of detrital zirconpopulations for the different samples analyzed in this study pro-vides information for paleogeographic reconstruction of the studyarea during deposition of the studied sedimentary units(Table DR1; Figs. 1, 2, 5 and 6). Provenance inferences, however, arehampered by the high exhumation rates that affected the NorthPatagonian Andes from the late Miocene onwards and gave way tothe erosion of the majority of the Meso-Cenozoic volcano-sedi-mentary rocks that covered their plutonic roots. In addition,although the plutonic rocks that form part of the Patagonianbatholith are the most extensively exposed rocks in the area, theirradiometric dates are scarce and those obtained by the K/Ar

method must be taken with caution because their isotopic signalscould have been modified by younger intrusions.

The two samples of the Puduhuapi Formation analyzed in thisstudy contain zircon populations with ages of 22.7 and 23.3Ma thatare indistinguishable within errors and close to the Oligocene/Miocene boundary (Table DR1, Fig. 5, Table 1). These data, thepresence of abundant volcanic material, and the occurrence ofinterbedded conglomerates with angular to subrounded clasts inthis unit suggest that the sediment provenance was a volcanic arcclose to the marine basin. Although no volcanic outcrops have beenrecognized in the vicinity of the Puduhuapi Formation exposures,lower Miocene plutonic rocks crop out near Puduhuapi Island(Fig. 1) (Duhart, 2008). Interestingly, the cited UePb data coincidewith those of Encinas et al. (2013) from detrital zircons of theAyacara Formation. The lower, shallow-marine member of this unitcontains zircon populations of the Oligoceneemiddle Miocene,Lower Cretaceous, Lower Jurassic, Carboniferous, Cambrian, andProterozoic. In contrast, samples from the upper, deep-marinemember of the Ayacara Formation yielded detrital zircon pop-ulations that are exclusively late Oligoceneeearly Miocene. Encinaset al. (2013) suggested that this marked change in provenance mayhave been consequence of 1) the Miocene transgression in whichdeposition of the deep-marine upper member of the Ayacara For-mation covered the former source area to the east except for anemergent volcanic arc, or 2) an important episode of magmatismcreated large volcanic edifices that blocked river channels drainingMesozoicePaleozoic terrains. Similar scenarios could explain theOligo-Miocene zircon populations of the Puduhuapi Formation.

The two samples from the La Cascada Formation yielded zirconpopulations of the lower Miocene (~17 Ma), Eocene, Upper Creta-ceous, Lower Cretaceous, Lower Jurassic, Permian, and olderPaleozoic, Grenville, and Archean components (Table DR1, Fig. 5,Table 1). At present, rocks with similar ages are located principallyto the west, south, southwest and nearby the La Cascada Formationexposures (Sernageomin, 2003; Duhart, 2008) (Table 1, Fig. 1).Grenville and Archean rocks do not occur in the area, which in-dicates that zircons of these ages are probably recycled.

The two samples from the Vargas Formation contain zirconpopulations of the upper Eocene (39 Ma), Upper Cretaceous, andminor zircon population of the Lower Cretaceous, Lower Jurassic,Paleozoic and Grenville (Table DR1, Fig. 5). At present, rocks withsimilar ages in the region occur to the west, north, east, and nearbythe Vargas Formation exposures (Bobenrieth et al., 1983; Thomson,2002; Duhart, 2008) (Table 1, Fig. 1). Grenville zircons are probablyrecycled.

The sample from the La Junta Formation contains zircon pop-ulations of the upper Oligocene (~26 Ma), lower Oligocene, andUpper to Lower Cretaceous (Table DR1, Fig. 5). At present, rockswith similar ages occur to the west, northwest, north, and east ofthe La Junta Formation outcrops (Thomson, 2002; Duhart, 2008)(Table 1, Fig. 1).

3.4. Local marine incursions and deposition

The location of the Puduhuapi Formation outcrops on thewestern flank of the Andean range (Fig. 1) dictates a Pacific originfor this unit. On the other hand, the age and Atlantic versus Pacificorigin of the Cenozoic marine outcrops on the eastern flank of thePatagonian Andes (i.e., the Río Foyel, La Cascada, and their regionalequivalents) between ~41�300 and 43�300S (Fig. 1) remainsdebatable.

The most accepted age for the Río Foyel Formation and correl-ative units in Argentina has been lower middle OligoceneelowerMiocene(?), as proposed by Bertels (1980) based on foraminifers.However, these strata have also been assigned to the Eocene based


on molluscs (Chiesa and Camacho, 2001), to the lower Oligocenebased on 87Sr/86Sr (Griffin et al., 2004) and K/Ar dating of cross-cutting dikes (Giacosa et al., 2001), and to the upper Oligoce-neelower Miocene based on palynomorphs (Barreda et al., 2003).Asensio et al. (2010) and Malumi�an and N�a~nez (2011) point out thepresence of Transversigerina cf. T. transversa in the Río Foyel For-mation, a benthic foraminifer species typical of the Navidad andequivalent formations in the Chilean forearc that indicates a lateOligoceneeearly Miocene age (Finger, 2013). The La Cascada For-mation, on the other hand, contains a molluscan fauna that hasbeen correlated with the Eocene (Thiele et al., 1978) which, how-ever, is based on stratigraphic confusion created by Tavera (1942)on Arauco and can be discarded. Recent radiometric dating(UePb, LA-MC-ICPMS) of detrital zircons, including those pre-sented in this study, resolves this disparity by indicating maximumdepositional ages between ~22 and ~17 Ma for the Río Foyel, LaCascada, and equivalent units (Encinas et al., 2012; Bechis et al.,2012, 2014; Orts et al., 2012). These data are supported by calcar-eous nannofossils from the Río Foyel Formation that indicate anearly to middle Miocene age (Bechis et al., 2012, 2014). The volcanicsuccession overlying lower Miocene (Orts et al., 2012) marine de-posits of the Cerro Plataforma in Argentina (42�300S), which yieldeda 15 Ma (K/Ar) age (Lizuaín, 1983), restricts marine deposition onthe eastern flank of the Patagonian Andes in this area to the earlyMiocene.

The marine transgression that led to the deposition of theseunits is also debated as there are data favoring both Atlantic andPacific origins. Miocene marine deposits of Pacific origin (the Aya-cara Formation) crop out on the western flank of the Andes at only~70 km west of the Río Foyel Formation and equivalent units (Leviet al., 1966; Rojas et al., 1994; Encinas et al., 2013) (Fig. 1). UpperOligoceneelower Miocene marine deposits of Atlantic origin,popularly known as the “Patagoniense”, reached the eastern flankof the Patagonian Andes ~250 km south of the study area (e.g.,Ramos, 1982; Frassinetti and Covacevich, 1999). Ramos (1982)proposed a Pacific origin for the Río Foyel Formation and equiva-lent units based on the occurrence of Oligocene to middle Miocenecontinental deposits north, east, and south of the marine deposits(i.e., according to this hypothesis, a marine ingression could haveonly advanced from the Pacific). Similar paleogeographic schemesproposed by Asensio et al. (2010) and Bechis et al. (2014) are sup-ported by the presence of upper Oligoceneelower Miocene marinedeposits of the Puduhuapi, Vargas, and La Cascada formations at43e44�S on thewestern, central, and eastern part of the PatagonianAndes, respectively, that marks a likely path followed by the Pacificingression, although it is not necessarily the only one possible. Incontrast, Bertels (1980), who studied foraminifers of the Río FoyelFormation, and Griffin et al. (2002), who worked on molluscs fromthe Cerro Plataforma Formation, correlated their faunas with thoseof the “Patagoniense” of Argentina, which prompted them to pro-pose an Atlantic origin for these deposits. Feldmann et al. (2011),however, proposed a Pacific origin for the Río Foyel Formationbecause it contains crustaceans similar to those of the NavidadFormation and equivalent units in the Chilean forearc. Nielsen(2005) proposed an AtlanticePacific connection based on theoccurrence of a gastropod of Atlantic origin, Perissodonta ameghinoi(Ihering, 1897), in Miocene marine deposits located near the Pacificcoast in the proximity of Valdivia (~40�S). That connection is sup-ported by the apparent presence of both Atlantic and Pacific taxa inthe La Cascada Formation (see above). Bechis et al. (2014) consid-ered that a PacificeAtlantic connection, if ever existed, could haveonly taken place at ~45�S. At that latitude, the upper Oligocene-eupper Miocene marine deposits of Pacific origin that crop out inthe Chilean forearc at the Golfo the Penas and Traigu�en areas (Stottand Webb, 1989; Herv�e et al., 2001; Riffo et al., 2013) are closest to

the coeval deposits of Atlantic origin of the Centinela and Guadalformations (Ramos, 1982; Frassinetti and Covacevich, 1999)exposed in the eastern part of the Andean Cordillera. Problemati-cally, this proposition is not supported by paleontologic studies asno mixed faunas of Atlantic and Pacific origin has been reported inany of thesemarine deposits of either side of the Andes. The erosionof most of the Meso-Cenozoic volcano-sedimentary cover of thePatagonian Andes during the late Cenozoic and the relatively poorpreservation of the fossils in many of the upper OligoceneelowerMiocene marine deposits of this area precludes any reliablepaleogeographic reconstruction that resolves the issue concerningPacificeAtlantic connections in this region. Paleontologic data,however, including those presented in this study, suggest such aconnection did exist, although its path remains uncertain.

3.5. Tectonic implications

Our data indicate that the Puduhuapi, La Cascada, and Vargasformations were probably deposited during the late Oligocene-eearlyMiocene interval as a consequence of a marine transgressionthat connected the Pacific and Atlantic oceans. The conglomeratesof the La Junta Formation (maximum UePb depositonal age of ~26Ma) could have been deposited during the initial stage of subsi-dence that affected this region during the late Oligocene?eearlyMiocene, prior to the marine transgression over this area. Prove-nance analysis (see above), on the other hand, suggests that some ofthe zircon populations from the studied units were derived fromrocks located between the La Cascada and Puduhuapi outcrops(Fig. 1, Table 1). This implies that the Pacific transgression thatreached the La Cascada depositional area probably did not cover thewhole region but instead advanced eastward through channelsdissecting higher terrain, although it is not clear if these passageswere in a compressional or extensional setting (see below anddiscussion in Encinas et al., 2013). The Puduhuapi depocenter wasprobably fed by sediment derived from nearby coeval volcanicedifices located principally to the east and which blocked riverchannels draining fromMesozoicePaleozoic terrains. As previouslynoted, these paleogeographic scenarios must be takenwith cautionbecause the high exhumation rates that affected the study areaduring the Neogene have modified the geological configuration ofthis region since the middleelate Miocene (Thomson, 2002).

The upper Oligoceneelower Miocene age of the Puduhuapi, LaCascada, and Vargas formations indicates that these units are likelycoeval with lower Miocene marine strata that crop out in theforearc of south-central Chile (see Encinas et al., 2008; Nielsen andGlodny, 2009; Encinas et al., 2012; Guti�errez et al., 2013; Finger,2013; Finger et al., 2013), the western flank of the North Patago-nian Andes (Bernab�e et al., 2009; Encinas et al., 2013), and theeastern flank of this range (Río Foyel and equivalent units exposedon the eastern flank of the North Patagonian Andes) in Argentina(Encinas et al., 2012; Bechis et al., 2012, 2014; Orts et al., 2012).Given the significant areal extension of the Miocene transgression,this must have been caused by a major event of regional subsidencethat cannot be associated with the Liqui~ne Ofqui Fault Zone or anyother local tectonic discontinuity. Encinas et al. (2013) thoroughlydiscuss the possible origin of this event and conclude that the twomost probable causes of subsidence are 1) a major event of basalsubduction erosion that caused the thinning of the overriding plate(Encinas et al., 2008), and 2) a regionally widespread episode ofextension that affected the Chilean margin between ~33� and 43�Sduring the late Oligoceneeearly Miocene that was previouslyproposed byMu~noz et al. (2000), and Jordan et al. (2001) andwhichwas probably caused by negative rollback of the subducting slab(Mu~noz et al., 2000). The second cause could be considered morelogical because subsidence driven by subduction erosion is


normally described for offshore and coastal regions, and Miocenemarine deposits also occur in the eastern flank of the Andes (seediscussion in Encinas et al., 2013). However, data provided by Ortset al. (2012) seem to refute this hypothesis as they describe thepresence of progressive unconformities in the lower Miocene (~18Ma) marine strata of the Cerro Plataforma succession, which theyinterpret as evidence of ongoing contraction at the time of sedi-mentation. If that scenario is correct, a Pacific ingression in this areahad to occur through narrow seaways that transected an incipientCordillera (Encinas et al., 2013). Provenance analysis presented inthis study does not contradict this hypothesis because zirconpopulations from the La Cascada, Vargas, and La Junta formationswere probably derived from rocks located east and west of thedepositional areas of these units as mentioned above. Yet, Bechiset al. (2011) cited the presence of growth strata that they inter-preted as synextensional in the Troncoso Formation, a continentalsedimentary unit that overlies the Río Foyel Formation and wasdated as 16.6 ± 0.5 Ma (UePb on zircons; Bechis et al., 2012). In-terpretations of the tectonic setting for the deposition of theMiocene marine strata on the Patagonian Andes are complicated bythe scarcity of structural data, the small size and isolation of most ofthese outcrops, and the deformation of the strata. Additionalstudies are therefore needed to resolve this issue that has impor-tant geodynamic implications.

The upper Oligoceneelower Miocene ages of deposition for themarine strata of the Puduhuapi, La Cascada, and Vargas formationsindicate a maximum age for the deformation and uplift of thePatagonian Andes between 43� and 44�S. Agreeing with these dataare the loweremiddle(?) Miocene age obtained for the marinedeposits of the Ayacara Formation that crop out in the westernflank of the Andes farther north (~42�S) and the 22 to 18 Ma lowerMiocene age (Encinas et al., 2012; Bechis et al., 2012, 2014; Ortset al., 2012) of the Río Foyel marine strata and equivalent unitson the eastern flank of this range at the same latitudes. The angularunconformity between the continental ~Nirihuau and Coll�on-Cur�aformations on the eastern flank of the North Patagonian Andes andthe presence of synorogenic deposits in the upper member of the~Nirihuau Formation evidence the onset of contractional tectonics inthis range during the middle to late Miocene (Bechis and Cristallini,2005). Accordingly, zircon and apatite fission-track thermochro-nology indicate the significant denudation of the Patagonian Andes(41��46�S) during the late MioceneePliocene that commenced16e10 My (Thomson, 2002; Adriasola et al., 2006). In addition,studies based on carbon and oxygen isotopes indicate significantuplift of the South Patagonian Andes at ~16.5 Ma (Blisniuk et al.,2005). All these data suggest that contraction and uplift of thePatagonian Andes commenced at ~16 Ma. As discussed previously,it is not clear yet if this contractional tectonic regime developedprior to deposition of the lower Miocene marine deposits exposedon the western and eastern flanks of the North Patagonian Andes.

4. Conclusions

The results of this study indicate that the marine deposits of thePuduhuapi, La Cascada, and Vargas formations exposed in thewestern, eastern, and central parts of the North Patagonian Andesbetween ~43� and 44�S were deposited by a marine transgressionthat possibly connected Atlantic and Pacific waters during the lateOligoceneeearly Miocene. It is not clear yet if the sea advanceoccurred in a compressional or extensional setting as publisheddata in this regard are contradictory. Also uncertain is the possibleexistence of one or more marine incursions during deposition ofthese units. The alluvial(?) deposits of the La Junta Formation thatcrop out close to the Vargas Formation exposures could have beendeposited during the initial stage of subsidence that affected this

region in the late Oligoceneeearly Miocene, prior to the marinetransgression over this area. In accordance with recent studies, thelate Oligoceneeearly Miocene age for the marine deposits in theNorth Patagonian Andes is a reliable maximum age for the defor-mation and uplift of this mountain range.

Acknowledgments

This research was funded by Fondecyt Projects 11080115 and1110914. We thank the support of the Servicio Nacional de Geologíay Minería of Chile (Sernageomin). Alfonso Rubilar (Sernageomin) isespecially thanked for permission to use internal reports and accessto collections under his care.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jsames.2014.06.008.

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http://dx.doi.org/10.1029/2011TC003084

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Geochronologic and paleontologic evidence for a Pacific-Atlantic connection during the late Oligocene-early Miocene in the Patagonian Andes (43-44S)

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