Neogene tectonostratigraphic history of the southern

EAGE

Basin Research (2014) 1–23, doi: 10.1111/bre.12091

Neogene tectonostratigraphic history of the southern n basin (39°–40°300S, Argentina): Neuque implications for foreland basin evolution dric Bonnel,* Bertrand Nivi e re,* Bertrand Fasentieux* and Yves Damien Huyghe,* , † C e €t* Hervou e *UMR CNRS TOTAL 5150 “Laboratoire des Fluides Complexes et leurs Reservoirs”, Universite de Pau et des Pays de l’Adour, PAU Cedex, France †Laboratoire GET, CNRS UMR 5563, Universite Paul Sabatier, OMP, 14 Avenue Edouard Belin, 31400 Toulouse, France

ABSTRACT Although the Neuquen basin in Argentina forms a key transitional domain between the south-central Andes and the Patagonian Andes, its Cenozoic history is poorly documented. We focus on the sedimentologic and tectonic evolution of the southern part of this basin, at 39–40°300 S, based on study of 14 sedimentary sections. We provide evidence that this basin underwent alternating erosion and deposition of reworked volcaniclastic material in continental and fluvial settings during the Neogene. In particular, basement uplift of the Sa~ nico Massif, due to Late Miocene–Pliocene intensification of tectonic activity, led to sediment partitioning in the basin. During this interval, sedimentation was restricted to the internal domain and the Collon Cura basin evolved towards an endorheic intermontane basin. From stratigraphic interpretation, this basin remained isolated 7–11 Myr. Nevertheless, ephemeral gateways seem to have existed, because we observe a thin succession downstream of the Sa~ nico Massif contemporaneous with the Collon Cura basin-fill sequence. Comparisons of stratigraphic, paleoenvironmental and tectonic features of the southern Neuquen basin with other foreland basins of South America allow us to classify it as a broken foreland with the development of an intermontane basin from Late Miocene to Late Pliocene. This implies a thick-skinned structural style for this basin, with reactivation of basement faults responsible for exhumation of the Sa~ nico Massif. Comparison of several broken forelands of South America allows us to propose two categories of intermontane basins according to their structural setting: subsiding or uplifted basins, which has strong implications on their excavation histories.

INTRODUCTION Two foreland basin morphologies can be identified on the eastern retro-arc flank of the Andean mountain chain: flexural foreland (Horton & DeCelles, 1997; Sempere et al., 1997; Giambiagi et al., 2001; DeCelles & Horton, 2003; DeCelles et al., 2011) and broken foreland basins (e.g. Hilley & Strecker, 2005; Hain et al., 2011; Davila et al., 2012; Bilmes et al., 2013; del Papa et al., 2013). At 15–23°S, foredeeps developed as a lithospheric flexural response to surface loads exerted by the fold-thrust belt as it migrates cratonwards, with accumulated sediments from the orogen (e.g. DeCelles et al., 2011). South of 23°S, thick-skinned deformation leads to formation of broken forelands which temporarily trap erosional products in intermontane basins (Hilley & Strecker, 2005; Hain et al., 2011; Davila et al., 2012; del Papa et al., Correspondence: Damien Huyghe, GET UMR CNRS 5563 Universite Paul Sabatier, OMP 14 Avenue Edouard Belin 3140 Toulouse France. E-mail: [email protected]

2013). These two modes of deformation exhibit conflicting basin geometries, associated with paleoenvironmental conditions and sedimentary filling. Broken foreland basins created in retro-arc foreland contexts form in response to basement uplift that generates nonmarine basins often characterized by endorheic drainages (Jordan, 1995). Over the last decade, many studies have focused on characterizing and understanding the dynamics of broken forelands in South America (Davila & Astini, 2003; Hilley & Strecker, 2005; Silvestro et al., 2005; Hain et al., 2011; Davila et al., 2012; del Papa et al., 2013), with new models for the evolution of foreland basins. For example, the Puna and Altiplano forelands of Argentina and Bolivia show basin-fill deposits recording evolution from an undeformed foreland to a crustal domain compartmentalized by basement uplifts, which is then incorporated into the larger scale orogenic structure (e.g. Mortimer et al., 2007; Siks & Horton, 2011). Certain authors have stressed the importance of the reactivation of pre-existing structures along the basin margin in creating east-dipping structures in a generally

© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists

1

D. Huyghe et al. west-dipping domain (Allmendinger et al., 1983; Grier et al., 1991; Ramos et al., 2002). Movement on these opposing faults at the basin margin causes out-ofsequence uplift of an intrabasinal range, fragmenting the foreland. Because the position of the boundary faults is blocked by loads to the west and east, this may lead to increased deformation within the basin, while also enhancing basin fill, uplift and incorporation of sediment into the orogeny, as observed in the Vinchina basin with more than 10 km of Neogene fill (Ramos et al., 2002). This model provides an alternative to the classic view of flexural foreland basins, which reflects a self-similar propagation of deformation into the foreland in a thin-skinned thrust belt governed by wedge mechanics. Whatever the case, foreland basins allow us to couple tectonic activity in the thrust belt with the sedimentary record. In this way, they represent preferential areas for studying the history of orogenic domains through sedimentological investigations (Covey, 1986; DeCelles & Horton, 2003; Hilley & Strecker, 2005). In comparison to NW Argentina and Bolivia, North Patagonian Andes have been poorly studied. The Andes bordering the Neuquen basin (35–41°S) form a transitional area between the central Andes to the north and the Patagonian Andes to the south. The Andes reach lower elevation relative to the northern part of the orogen, with some crustal blocks reaching greater elevations than the main cordillera (Garcıa Morabito et al., 2011). While the Mesozoic Neuquen basin has been widely investigated due to its hydrocarbon potential (Uliana & Legarreta, 1993; Urien & Zambrano, 1994; Cobbold & Rossello, 2003), comparatively few studies have focused on the Cenozoic and particularly the Neogene succession (Ramos & Folguera, 2005; Folguera et al., 2007) and only local studies have addressed tectonic forcing on the nature and distribution of the sedimentary filling (Folguera et al., 2007; Escosteguy & Franchi, 2010). Here, we reconstruct the evolution of the southern Neuquen foreland basin since the Miocene, to characterize the flexural vs. broken foreland nature of the basin. Broken foreland geometries have been described to the south in the Gastre basin (Bilmes et al., 2013) and such geometries could occur in this part of the Andean retroforeland. Since such data are still lacking, we first establish a regional stratigraphic synthesis based on available geochronological results. Then, we reconstruct the evolution of the depositional paleoenvironments and discuss the spatial evolution of depocentres through time to propose paleogeographic maps and define the timing of tectonic events. For this purpose, we studied 14 sedimentary sections in the foothills along the Rio Limay, from the Collon Cura basin in the west to the eastern Neuquen foreland (Fig. 1). Despite remaining in a compressive regime during the Miocene and Pliocene (Garcıa Morabito et al., 2011; Ramos et al., 2014b), no modern depositional foreland basin can be recognized in this part of the basin (Cobbold & Rossello, 2003) and Cenozoic fill is very thin and the basin is now characterized by strong erosion

2

(Fig. 1). There is still a debate concerning the tectonic regime of the basin since 3 Myr, which is considered as extensional (e.g. Zapata & Folguera, 2005; Folguera et al., 2006; Garcıa Morabito et al., 2011; Ramos et al., 2014b) or compressional (Cobbold & Rossello, 2003; Galland et al., 2007; Messager et al., 2010; Niviere et al., 2013). The sedimentological record could provide new insights concerning the recent evolution of the basin. Despite its low hydrocarbon potential, the southern Neuquen basin has been the subject of recent studies (Garcıa Morabito et al., 2011; Garcıa Morabito & Ramos, 2012; Ramos et al., 2014b), but these works focused on the tectonic evolution and were restricted to basins close to the Main Cordillera, because no deformation is observed in the external foreland. Therefore, we utilize a sedimentologic approach to further comprehend the Neogene evolution of the foreland basin system.

GEOLOGICAL SETTING In the Central Andes of Argentina at 35–41°S, the triangular Neuquen basin (Fig. 1) is bounded by the Main Cordillera to the west, the San Rafael Block to the northeast and the North Patagonian Massif to the southeast. The basin developed in several phases in response to widespread stretching followed by opening of the southern Atlantic Ocean and growth of the Andes. It first formed as a rift basin with isolated depocentres during the Late Triassic and Early Jurassic (Manceda & Figueroa, 1995; Vergani et al., 1995; Franzese & Spalletti, 2001). A large retro-arc basin then developed behind the Andean volcanic arc from Early Jurassic to Early Cretaceous before undergoing inversion from latest Early Cretaceous times (100 Myr) onwards (Vergani et al., 1995; Tunik et al., 2010). The obliquity and rate of convergence at the Pacific margin of South America and the dip of the subducting Farallon and then Nazca plate since 26 Myr controlled the style and timing of deformation (Pardo-Casas & Molnar, 1987; Somoza, 1998). The phases of Andean deformation established in Peru by Steinmann (1929) have been recognized in the Neuquen basin (Pardo-Casas & Molnar, 1987; Cobbold & Rossello, 2003), corresponding to the Peruvian (Mid to Late Cretaceous), Incaic (Paleocene to Early Eocene) and Quechua (since the Early Miocene) phases. Inversion of Triassic rifts since the Early Cretaceous (Ramos, 1999; Ramos et al., 2014b) led to shortening of up to 10 km in the study area (Ramos et al., 2014b) and 45–57 to 27–12 km north of the Huincul ridge (Rojas Vera et al., 2014), followed by creation of the Andean relief and uplift of the foreland. The total sedimentary record reaches a thickness of 5000 m (Vergani et al., 1995). The southern Neuquen basin, south of the Huincul Ridge (Figs 1 and 2), can be divided into the Main Cordillera to the west and several NNW-oriented depocentres: from north to south, the Lonquimay, Kilca and Catan Lil depocentres, respectively, and the Collon Cura basin in


basin Neogene evolution of the Neuquen

MAIN

Pampa Curaco

ico Sañ

Rio

20°S 30°S

egro

80°W

Renteria Plateau

La

limit of

Chili Ridge

70°W

in

the bas

Elevation (m)

Cero Policia

Naupa Huen

3500 3000

ssif Ma

Collon Cura Basin

ay Lim

t e en sM am Ramo e go Lin

Rio N

Lim

A’

40°S

CORDILLERA

CaLC Picun Leufu Basin

jia

ay

Neuquen Basin

Southern Andes

NEUQUEN

Rio

Nazca Plate

z rnánde Juan Fe Ridge

Agrio Fold and Thrust Belt

CaC

40°S

n

Huincul Ridge

c oro

ue

Rio Agrio

CLD

A

Ne

uq

LCB

An

Rio Colorado

And Centrlainle Souetshern

Rio

67°W an de

Anticlines

Loncopué Basin

KiD

Junin de los Andes

68°W

Lin Co ea rta m de en ra t s

Chihuidos

LoD

39°S

69°W

50°S

70°W

ino eP c ahu o Blo Copchad Ha

38°S

71°W

Piedra del Aguila

2500

North Patagonian Massif

2000 1500 1000

41°S

500 0 0 km

Elevation (m)

A’

A Collon Cura basin

1500

50

Sañico Massif

Picun Leufu Basin

Renteria Plateau

1000

500

0

40

80

120

160

200

240

280

320

Distance (km) Fig. 1. Topographic map of the Neuquen basin showing the location of morphostructural zones and topographic cross-section along the southern Neuquen Basin and topographic cross-section of the study area. LOD, Lonquimay Depocentre; KiD, Kilca Depocentre; CalC, Catan Lil Cordillera; CaC, Cachil Cordillera; LCB, Las Coloradas Basin; CLD, Catan Lil Depocentre.

the study area (Fig. 2). These basins developed during the Oligocene and Early Miocene in an extensional context and were inverted after the Middle Miocene (Burns et al., 2006; Garcıa Morabito et al., 2011; Ramos et al., 2014b), even if other argue that extension is not observed for this period (Cobbold & Rossello, 2003). These basins shifted to a compressive regime from Late Miocene to Pliocene (Garcıa Morabito et al., 2011; Garcıa Morabito & Ramos, 2012; Ramos et al., 2014b). They became separated from the foreland by structural highs corresponding to Catan Lil and Cachil Cordilleras and the Sa~ nico Massif on the eastern border of the Collon Cura basin (Figs 1 and 2).

These basement massifs are inherited structures exhumed during the Neogene by tectonic inversion of Triassic normal faults (D’Elia et al., 2012). Finally, in the study area, the Picun Leufu sub-basin east of the Sanico Massif is isolated from the rest of the Neuquen basin by the Huincul Ridge to the north, a structural high formed after the Jurassic (Mosquera & Ramos, 2006). In the following, we consider three main domains in the Picun Leufu sub-basin: (i) the Piedra del Aguila area on east of the Sa~ nico Massif, (ii) the Lago Ramos Mejia area on the SE bank of the Rio Limay at ca. 69–70°W and (iii) the Renteria Plateau area (Fig. 2).


3

D. Huyghe et al.

Legend Caleufu Fm. Collon Cura Fm. Naupa Huen Fm. Chichinales Fm. A

Renteria Fm. Pampa Curaco Fm. Studied section

39°S

68°W

69°W

Picun Leufu basin

Neuquén Renteria Plateau N M

Lago Ramos Mejia

70°W

Collon Cura basin

Piedra del Aguila

71°W

L

J D

40°S

A Sañico

Massif B

C

H E

I

K

G F

Main Main Cordillera North Patagonian Massif 0 km

50

Fig. 2. Geological map of the study area. Only the Neogene sedimentary deposits and the studied sedimentary/stratigraphic sections are shown. The study area is divided into four zones (dashed lines): the Collon Cura basin, the Piedra del Aguila area, the Lago Ramos Mejia area and the Renteria Plateau.

REGIONAL STRATIGRAPHY Although a number of studies have dealt with the Cenozoic stratigraphy of the Neuquen basin (e.g. Marshall et al., 1977; Escosteguy & Franchi, 2010; Folguera & Zarate, 2011), there are few published sedimentary sections and no regional basin-wide correlations. We propose a regional synthesis based on correlation of Neogene formations across the southern Neuquen basin. Many studies of this area since the end of the 19th century were too local and not aimed at regional stratigraphic correlation. We review various Cenozoic units and present a regional stratigraphic synthesis (Fig. 3) and correlation framework for the formations described below. This synthesis is based on units named on geologic maps of the study area. Differences in formation names for a given age mostly arise from the fact that the geologic maps were established by different authors at different periods.

Naupa Huen Formation This formation crops out in the Piedra del Aguila and Lago Ramos Mejia areas, east of the Sa~ nico Massif (Fig. 2). Its first description was from Wichmann (1927), but the first formal subdivision was from Pozzo (1956). Rolleri et al. (1984) considered that the unit now called the Naupa Huen Formation (Fm.) corresponded to a member of the Collon Cura Fm. Near the base, Rolleri et al. (1984) found remains of the mammal genera Protypotherium and Hegetotherium, characteristic of the Early Miocene and correlated the Naupa Huen Fm. with the

4

lower part of the Chichinales Fm. deposited to the east (see below). These deposits constitute filling paleorelief in nonmarine domain (Leanza et al., 1997), which accounts for the strong variations in thickness (from few metres to more than 100 m). Most of the facies correspond to fluvial and alluvial deposits of volcaniclastic sediments and older reworked material. Paleosols developed during quiet periods between deposition of detrital material.

Collon Cura Formation The Collon Cura Fm. is composed of a homogeneously deposited unit extending from the Collon Cura basin in the west to the Lago Ramos Mejia area in the east. It was first described by Roth (1899) and named by Yrigoyen (1969). The attributed stratigraphic position has considerably changed over the last century. Roth (1899) attributed a Middle Miocene age based on its mammal fauna. Pascual & Odreman Rivas (1971) re-evaluated the age of this formation as Middle to Late Miocene, from the mammal fauna, which was corroborated by Marshall et al. (1977), with K/Ar radiometric ages of 15.4 0.3 Myr and 14 0.3 Myr for pyroclastic deposits and confirmed by Gonzalez Diaz & Nullo (1980) with K/Ar age of 11 1 Myr. The Pilcaniyeu Ignimbritic Member locally observed at the base of the formation was dated at 15 Myr (K/Ar age) (Rabassa, 1975) and 13.8 0.9 Myr (K/Ar age) (Mazzoni & Benvenuto, 1990). Cazau et al. (1989) dated the base of the clastic deposits of this formation above the Pilcanyeu ignimbrite, between 11.5 and 10.7 Myr. Thus, Giacosa et al. (2001) attributed a


D. Huyghe et al.

Legend Caleufu Fm. Collon Cura Fm. Naupa Huen Fm. Chichinales Fm. A

Renteria Fm. Pampa Curaco Fm. Studied section

39°S

68°W

69°W

Picun Leufu basin

Neuquén Renteria Plateau N M

Lago Ramos Mejia

70°W

Collon Cura basin

Piedra del Aguila

71°W

L

J D

40°S

A Sañico

Massif B

C

H E

I

K

G F

Main Main Cordillera North Patagonian Massif 0 km

50

Fig. 2. Geological map of the study area. Only the Neogene sedimentary deposits and the studied sedimentary/stratigraphic sections are shown. The study area is divided into four zones (dashed lines): the Collon Cura basin, the Piedra del Aguila area, the Lago Ramos Mejia area and the Renteria Plateau.

REGIONAL STRATIGRAPHY Although a number of studies have dealt with the Cenozoic stratigraphy of the Neuquen basin (e.g. Marshall et al., 1977; Escosteguy & Franchi, 2010; Folguera & Zarate, 2011), there are few published sedimentary sections and no regional basin-wide correlations. We propose a regional synthesis based on correlation of Neogene formations across the southern Neuquen basin. Many studies of this area since the end of the 19th century were too local and not aimed at regional stratigraphic correlation. We review various Cenozoic units and present a regional stratigraphic synthesis (Fig. 3) and correlation framework for the formations described below. This synthesis is based on units named on geologic maps of the study area. Differences in formation names for a given age mostly arise from the fact that the geologic maps were established by different authors at different periods.

Naupa Huen Formation This formation crops out in the Piedra del Aguila and Lago Ramos Mejia areas, east of the Sa~ nico Massif (Fig. 2). Its first description was from Wichmann (1927), but the first formal subdivision was from Pozzo (1956). Rolleri et al. (1984) considered that the unit now called the Naupa Huen Formation (Fm.) corresponded to a member of the Collon Cura Fm. Near the base, Rolleri et al. (1984) found remains of the mammal genera Protypotherium and Hegetotherium, characteristic of the Early Miocene and correlated the Naupa Huen Fm. with the

4

lower part of the Chichinales Fm. deposited to the east (see below). These deposits constitute filling paleorelief in nonmarine domain (Leanza et al., 1997), which accounts for the strong variations in thickness (from few metres to more than 100 m). Most of the facies correspond to fluvial and alluvial deposits of volcaniclastic sediments and older reworked material. Paleosols developed during quiet periods between deposition of detrital material.

Collon Cura Formation The Collon Cura Fm. is composed of a homogeneously deposited unit extending from the Collon Cura basin in the west to the Lago Ramos Mejia area in the east. It was first described by Roth (1899) and named by Yrigoyen (1969). The attributed stratigraphic position has considerably changed over the last century. Roth (1899) attributed a Middle Miocene age based on its mammal fauna. Pascual & Odreman Rivas (1971) re-evaluated the age of this formation as Middle to Late Miocene, from the mammal fauna, which was corroborated by Marshall et al. (1977), with K/Ar radiometric ages of 15.4 0.3 Myr and 14 0.3 Myr for pyroclastic deposits and confirmed by Gonzalez Diaz & Nullo (1980) with K/Ar age of 11 1 Myr. The Pilcaniyeu Ignimbritic Member locally observed at the base of the formation was dated at 15 Myr (K/Ar age) (Rabassa, 1975) and 13.8 0.9 Myr (K/Ar age) (Mazzoni & Benvenuto, 1990). Cazau et al. (1989) dated the base of the clastic deposits of this formation above the Pilcanyeu ignimbrite, between 11.5 and 10.7 Myr. Thus, Giacosa et al. (2001) attributed a


D. Huyghe et al. proposed a new stratigraphic framework that we use here. They differentiated the sediments overlying the tuffaceous material of the Collon Cura Fm., naming them the Caleufu Fm. (formerly known as the Rio Negro and Alicura Fm.). These series were first thought to record several aggradational cycles (Dessanti, 1972; Nullo, 1979; Gonzalez Diaz & Nullo, 1980), but Gonzalez Diaz et al. (1986) instead invoked a single aggradational sequence from sedimentologic arguments. They thus regrouped the Rio Negro and Alicura Fm. into a single unit, the Caleufu Fm., which they divided into the Limay Chico member (Mb.), overlain by the Alicura Mb. Previously, Turner (1973) included the Collon Cura Fm. within the lower member of a new stratigraphic unit called the Chimehuin Fm. Its upper member corresponded to the grey sandstones (Galli, 1954) of the present Limay Chico Mb. of Gonzalez Diaz et al. (1986). Few data allow dating this formation. Nullo (1979) proposed a Middle Pliocene age by comparison with the Rio Negro Fm. in its type locality. Nullo (1979) and Gonzalez Diaz & Nullo (1980) proposed a Pleistocene age for the Alicura Fm. because it should be younger than the Late Pliocene to Early Pleistocene basalts of the Tipilihuque Fm. that overlies this formation in the north of the basin. Gonzalez Diaz et al. (1990) revised the stratigraphic range of the Limay Chico Mb. The basalts observed at the base of the Caleufu Fm. are dated at 20 4 Myr (K/Ar age), whereas two ignimbrites intercalated near the middle of the formation yield ages of 14 1 Myr and an ignimbrite locally marking the contact with the Alicura Mb. is dated at 8 2 Myr. An upper time bracket of 3 0.5 Myr is defined by the Tipilihuque Fm., which covers the entire sedimentary succession. Gonzalez Diaz et al. (1990) acknowledged that the age of these basal basalts is inconsistent with previous youngest K/Ar ages of 14–15 Myr obtained for the Collon Cura Fm. Nevertheless, they argued that the other ages are stratigraphically consistent with the age of the Collon Cura Fm., within the range of experimental errors. However, the Pilcaniyeu Ignimbritic Mb. at the base of the Collon Cura Fm. yields an age between 15 and 14.1 Myr (Rabassa, 1975; Mazzoni & Benvenuto, 1990) and Cazau et al. (1989) dated the base of the clastic deposits of the Collon Cura Fm. between 11.5 and 10.7 Myr. These ages are inconsistent and the stratigraphic framework remains uncertain (Fig. 3). The top of the Limay Chico Mb. ignimbrite at 8 2 Myr (Gonzalez Diaz et al., 1990) could correspond to the Chimehuin Ignimbrite also at 8 Myr (Mazzoni & Rapela, 1991). However, the published data do not allow us to establish if these different studies are dealing with the same ignimbrite because their positions are not clearly indicated on sedimentary sections. The Caleufu Fm. corresponds to siliciclastic and volcanosedimentary material. Dominant lithologies correspond to clay, sand and coarse sands in the lower part of the formation and become coarser with dominant conglomerates

6

in the upper part. These facies are interpreted to have been deposited in fluvial environments, with some lacustrine events (Escosteguy & Franchi, 2010). The Caleufu Fm. could results from the growth of an orogenic front associated with the Quechua phase of the Andean orogeny (Gonzalez Diaz et al., 1986) or could have a fluvio-glacial origin (Gracia, 1958; Galli, 1969; Gonzalez Diaz & Nullo, 1980) or correspond to a pediment (Flint & Fidalgo, 1968).

Pampa Curaco Formation This formation, deposited on the eastern border of the Sa~ nico Massif, north of the Rio Limay, consists of conglomerates (max. 20 m thick), with lithoclasts of igneous rocks (granites, gneisses, quartz grains and basalts) and jasper (Cucchi et al., 2005). The clastic supply of this formation is probably derived from the erosion of the Cachil Cordillera (Fig. 1) and represents a single aggradational deposit (Leanza, 1985) between the Late Pliocene and the Early Pleistocene (Leanza et al., 1997).

Renteria Formation The Renteria Fm. (Hugo & Leanza, 2001) is deposited in the Picun Leufun basin, on either side of the Rio Limay, with its main relict on the Renteria Plateau (Fig. 2). It consists of polymictic conglomerates with pebbles including granite, gneiss, quartz, porphyritic clasts, basalt, Mesozoic sediments and silicified wood (Hugo & Leanza, 2001). The Renteria Fm. contains pebbles of the Pliocene El Cuy basalt, deposited a few kilometres south of the Renteria Plateau and is then dated to the end Pliocene– Early Pleistocene (Uliana, 1979; Leanza et al., 1997; Hugo & Leanza, 2001) and correlated with the Pampa Curaco Fm. It was interpreted as a pediment deposit (Uliana, 1979), whereas Hugo & Leanza (2001) considered it as an aggradational deposit, implying sediment accumulation. However, they failed to propose any origin that could explain the existence of such recent conglomerates lying on top of the erosional surfaces.

Synthesis Strong paleontological arguments lead to a good characterization of the regional stratigraphy up until the Middle Miocene but dating of ignimbrites or lavas sometimes yields contradictory results for Middle and Late Miocene. In particular, the stratigraphic position of the Collon Cura Fm. is problematic (Fig. 3), because the dated formations are seldom precisely positioned on the sedimentary sections. According to the stratigraphic ages chosen, the duration of the Collon Cura Fm. ranges from 2 to 5.5 Myr and the Caleufu Fm. from 9 to 7 Myr. These uncertainties have strong implications on the contemporaneous sedimentary and tectonic processes. Nevertheless, this first regional correlation is a major result that allows a better understanding of the stratigraphic


basin Neogene evolution of the Neuquen evolution of the basin. In the following, we use the stratigraphic interpretation of Giacosa et al. (2001), who considered that deposition of the Collon Cura Fm. came to an end during the Tortonian because several K/Ar ages obtained for the top of this formation provided ages between 10 and 12 Myr (Gonzalez Diaz & Nullo, 1980; Cazau et al., 1989) and only one age suggests an age of 14 Myr for the beginning of the Caleufu Fm. (Gonzalez Diaz et al., 1990). Moreover, ages obtained for the Collon Cura Fm. directly come from syn-eruptive deposits that belong to it, whereas ages for the Caleufu Fm., are from basalts deposited under the formation corresponding to a maximal age.

SEDIMENTOLOGY This study is based on 14 sections in four key areas (Fig. 2) representative of the main depositional environments during the Neogene, corresponding to a >1400 m succession. We describe the evolution of facies and sedimentary structures from west to east. Deposits mainly consist of volcaniclastic material reworked by rivers. We use the terminology and classification of volcaniclastic sediments and rocks by White & Houghton (2006), based on Fisher (1961). Interpretation of primary volcaniclastic deposits, i.e. mainly ignimbrites, is based on Branney & Kokelaar (2002).

Collon Cura basin The oldest deposits in the Collon Cura basin are Neogene in age and correspond to the Collon Cura Fm. Syn-eruptive deposits are dominant within this basin, in particular a tens of metre thick ignimbrite at the base of the Collon Cura Fm. (Fig. 4, section C). Within this formation, we also observe volcanosedimentary deposits accumulated in a fluvial context with large (>5 m) channel morphologies. The main sedimentary formation within the Collon Cura basin is the Caleufu Fm. Its base (Limay Chico Mb.) is shown on sections A, B and C (Fig. 4) and corresponds to sandstones, tuffaceous deposits, conglomerates and claystones. Centimetre- to decimetre-sized clasts are made up of reworked Cenozoic and Mesozoic volcanic rocks and Paleozoic plutonic rocks. This member contains abundant pumice fragments with a greyish colour and frequent cross-bedding indicating a fluvial depositional environment (Fig. 5c). A thick white ignimbrite (ca. 130 m) is intercalated within the Caleufu Fm. in section A. This ignimbrite unit is completely homogeneous and is very rich in pumice and quartz. The top of this section also contains an ignimbrite approximately 5 m thick. Above the Collon Cura Fm., section C is made up of grey sandy pumice-rich fluvial deposits similar to those in section B, which can be attributed to the Limay Chico Mb. However, a major difference occurs between sections B and C: the occurrence of red pebbles of igneous rocks within

section C, frequently at the base of fining-upward sequences (Fig. 4). Above the Limay Chico Mb., the Alicura Mb. is composed of conglomerates with sandstones and tuffaceous intercalations in section B. Clasts are similar to the Limay Chico Mb. It is difficult to locate the precise transition between these members because the lithologies are closely similar and the passage is rather transitional in nature (Gonzalez Diaz et al., 1986). Section B ends with a reddish homogenous ignimbrite (Fig. 5f).

Piedra del Aguila area We studied four sections in this area (D–G; Fig. 6). Neogene sedimentation, which started contemporaneously with the deposition of the volcanic Auca Pan Fm. in the Collon Cura basin, begins with the Naupa Huen Fm. in the Early Miocene, in sections D and F. Section D overlies Early Cretaceous nonmarine deposits. We observe a reduced angular unconformity between these two formations, the Cretaceous being tilted of ca. 5° to the NE. The base of the sedimentary succession is composed of orange-reddish coarse sandstones with pebble beds (ca. 5 m), overlain by a thick interval of homogenous silts with channelized sandstone units (ca. 50 m). This passes up into a thick unit of coarse sandstones with erosive bases and channel morphologies. The channels are filled by fining-upward sequences, with cm-scale layers of rounded pebbles at their base. Above these deposits, white homogeneous sediments correspond to the Collon Cura Fm., with some thin intercalations of sandstone. The grain size of the clasts becomes coarser at the top of the section, beds showing a channelized structure eroding the underlying sediments. Section E is rather short, exposing only the Collon Cura Fm., which again corresponds to homogenous white tuffaceous deposits (Fig. 6). Sections F and G are close, and section G is laterally offset from section F (Figs 2 and 7). There is no lateral continuity between the Naupa Huen and Collon Cura Fm. here. On the contrary, deposits in section G downcut into the succession of section F, corresponding to the Naupa Huen Fm. (Fig. 7). Base of section F is composed of coarse conglomerates containing cm-scale rounded pebbles of dominantly igneous origin and some reworked clasts of Cretaceous sediment (Fig. 6). There are also thin intercalations of sandstone, overlain by a thick interval of orange-coloured homogenous clays, passing upwards into lighter coloured lithofacies. The depositional environments are dominantly fluvial, with alternations of alluvial plain deposits and paleosols (Fig. 5d). Section G is mostly composed of grey sands exhibiting channelized morphologies and cross-stratification, with a large proportion of pumice that picks out the internal stratification in each bed (Fig. 5a) that highlight the fluvial mode of deposition of this formation. Sands also contain red sub-angular pebbles of igneous rocks, with grain size ranging from few mm to several


7

D. Huyghe et al. proposed a new stratigraphic framework that we use here. They differentiated the sediments overlying the tuffaceous material of the Collon Cura Fm., naming them the Caleufu Fm. (formerly known as the Rio Negro and Alicura Fm.). These series were first thought to record several aggradational cycles (Dessanti, 1972; Nullo, 1979; Gonzalez Diaz & Nullo, 1980), but Gonzalez Diaz et al. (1986) instead invoked a single aggradational sequence from sedimentologic arguments. They thus regrouped the Rio Negro and Alicura Fm. into a single unit, the Caleufu Fm., which they divided into the Limay Chico member (Mb.), overlain by the Alicura Mb. Previously, Turner (1973) included the Collon Cura Fm. within the lower member of a new stratigraphic unit called the Chimehuin Fm. Its upper member corresponded to the grey sandstones (Galli, 1954) of the present Limay Chico Mb. of Gonzalez Diaz et al. (1986). Few data allow dating this formation. Nullo (1979) proposed a Middle Pliocene age by comparison with the Rio Negro Fm. in its type locality. Nullo (1979) and Gonzalez Diaz & Nullo (1980) proposed a Pleistocene age for the Alicura Fm. because it should be younger than the Late Pliocene to Early Pleistocene basalts of the Tipilihuque Fm. that overlies this formation in the north of the basin. Gonzalez Diaz et al. (1990) revised the stratigraphic range of the Limay Chico Mb. The basalts observed at the base of the Caleufu Fm. are dated at 20 4 Myr (K/Ar age), whereas two ignimbrites intercalated near the middle of the formation yield ages of 14 1 Myr and an ignimbrite locally marking the contact with the Alicura Mb. is dated at 8 2 Myr. An upper time bracket of 3 0.5 Myr is defined by the Tipilihuque Fm., which covers the entire sedimentary succession. Gonzalez Diaz et al. (1990) acknowledged that the age of these basal basalts is inconsistent with previous youngest K/Ar ages of 14–15 Myr obtained for the Collon Cura Fm. Nevertheless, they argued that the other ages are stratigraphically consistent with the age of the Collon Cura Fm., within the range of experimental errors. However, the Pilcaniyeu Ignimbritic Mb. at the base of the Collon Cura Fm. yields an age between 15 and 14.1 Myr (Rabassa, 1975; Mazzoni & Benvenuto, 1990) and Cazau et al. (1989) dated the base of the clastic deposits of the Collon Cura Fm. between 11.5 and 10.7 Myr. These ages are inconsistent and the stratigraphic framework remains uncertain (Fig. 3). The top of the Limay Chico Mb. ignimbrite at 8 2 Myr (Gonzalez Diaz et al., 1990) could correspond to the Chimehuin Ignimbrite also at 8 Myr (Mazzoni & Rapela, 1991). However, the published data do not allow us to establish if these different studies are dealing with the same ignimbrite because their positions are not clearly indicated on sedimentary sections. The Caleufu Fm. corresponds to siliciclastic and volcanosedimentary material. Dominant lithologies correspond to clay, sand and coarse sands in the lower part of the formation and become coarser with dominant conglomerates

6

in the upper part. These facies are interpreted to have been deposited in fluvial environments, with some lacustrine events (Escosteguy & Franchi, 2010). The Caleufu Fm. could results from the growth of an orogenic front associated with the Quechua phase of the Andean orogeny (Gonzalez Diaz et al., 1986) or could have a fluvio-glacial origin (Gracia, 1958; Galli, 1969; Gonzalez Diaz & Nullo, 1980) or correspond to a pediment (Flint & Fidalgo, 1968).

Pampa Curaco Formation This formation, deposited on the eastern border of the Sa~ nico Massif, north of the Rio Limay, consists of conglomerates (max. 20 m thick), with lithoclasts of igneous rocks (granites, gneisses, quartz grains and basalts) and jasper (Cucchi et al., 2005). The clastic supply of this formation is probably derived from the erosion of the Cachil Cordillera (Fig. 1) and represents a single aggradational deposit (Leanza, 1985) between the Late Pliocene and the Early Pleistocene (Leanza et al., 1997).

Renteria Formation The Renteria Fm. (Hugo & Leanza, 2001) is deposited in the Picun Leufun basin, on either side of the Rio Limay, with its main relict on the Renteria Plateau (Fig. 2). It consists of polymictic conglomerates with pebbles including granite, gneiss, quartz, porphyritic clasts, basalt, Mesozoic sediments and silicified wood (Hugo & Leanza, 2001). The Renteria Fm. contains pebbles of the Pliocene El Cuy basalt, deposited a few kilometres south of the Renteria Plateau and is then dated to the end Pliocene– Early Pleistocene (Uliana, 1979; Leanza et al., 1997; Hugo & Leanza, 2001) and correlated with the Pampa Curaco Fm. It was interpreted as a pediment deposit (Uliana, 1979), whereas Hugo & Leanza (2001) considered it as an aggradational deposit, implying sediment accumulation. However, they failed to propose any origin that could explain the existence of such recent conglomerates lying on top of the erosional surfaces.

Synthesis Strong paleontological arguments lead to a good characterization of the regional stratigraphy up until the Middle Miocene but dating of ignimbrites or lavas sometimes yields contradictory results for Middle and Late Miocene. In particular, the stratigraphic position of the Collon Cura Fm. is problematic (Fig. 3), because the dated formations are seldom precisely positioned on the sedimentary sections. According to the stratigraphic ages chosen, the duration of the Collon Cura Fm. ranges from 2 to 5.5 Myr and the Caleufu Fm. from 9 to 7 Myr. These uncertainties have strong implications on the contemporaneous sedimentary and tectonic processes. Nevertheless, this first regional correlation is a major result that allows a better understanding of the stratigraphic


basin Neogene evolution of the Neuquen (A)

(B)

(C)

(D)

(E)

(F)

Fig. 5. Illustrations of sedimentary deposits encountered in the field. (A) detail of the Caleufu Fm. (Limay Chico Mb.) observed in section G (Piedra del Aguila area), (B) meandering channel in the Caleufu Fm. (Limay Chico Mb.) observed in section G (Piedra del Aguila area), (C) Caleufu Fm. (Limay Chico Mb.) observed in section B in the Collon Cura basin, (D) Naupau Huen Fm. observed in section H. (E) Caleufu Fm. with seismically induced deformational structures in the Collon Cura basin and (F) ignimbrite located at the top of section B.

dm (Fig. 5a and b). Some layers are more thinly bedded and contain root traces. These deposits, which lie stratigraphically above the Naupa Huen and Collon Cura Fm., have a very similar facies to the Limay Chico Mb. of the Collon Cura basin, except for the presence of igneous pebbles (Fig. 5a). From our field observations, we can also conclude that deposition of the Limay Chico Fm. was not restricted to the Collon Cura basin, but also extended into the Picun

Leufu basin, even if the deposited volumes are not comparable. The Collon Cura Fm. is thus locally absent in this exposure, but we note that the Limay Chico Mb. cuts down into the Naupa Huen Fm (Fig. 7) and it is likely that the Collon Cura Fm. covers the Naupa Huen Fm. on the scale of the region, and the top of the latter formation was eroded before deposition of the Limay Chico Mb. in section G.


9

D. Huyghe et al. LEGEND Sandstone Cross-stratified sandstone

Conglomerate

c s fs cscg

Igneous rock pebble

Clay Siltstone Fine sandstone Coarse sandstone Conglomerate

Section D

Volcanic

Compaction figure Sismite

100

Section G

Grain size key:

Pumice

Ignimbrite

2D current ripples

S 40°06.11' W 69°55.45' 720 m 100

Channelized morphology 95

95

90

90

85

85

60

55

55

50

Huen

60

50

75 70 65 60 55 50

45

45

40

40

40

35

35

30

30

20

120

15

115

10

110

5

105

20 15 10 5

100

0 c fs cg s cs

25

25

20

20

15

15

10

10

5

0 c fs cg s cs

Huen

125

Collon Cura Fm.

25

S 40°00.248' W 70°02.834' 660 m

Naupa

Section E

130

Collon Cura Formation

30

S 39°57.17' W 69°37.52' 820 m

Naupa

35

45

0 c fs cg s cs

Caleufu

65

Caleufu Fm

65

S 40°07.08' W 69°56.39' 580 m

Formation

70

Formation

75

80

Formation

Section F

80

5

c fs cg s cs

0

c fs cg s cs

Fig. 6. Studied sedimentary sections in the Piedra del Aguila area.

Lago Ramos Mejia area The onset of Neogene nonmarine sedimentation in this area corresponds to the Naupa Huen Fm., only observed in section J (Fig. 8) where it overlies Cretaceous bedrock. The main lithofacies are orange-red in colour and correspond to clay, silt, sandstones and conglomerates. These conglomerates have a poor content in matrix and are not strongly consolidated. They have erosive bases and channelized morphologies. Pebbles range in size from several cm up to a few dm and have a rounded shape, belonging to various different lithologies (i.e. quartz, igneous basement rocks, red Cretaceous sandstones and volcanic

10

rocks). Sandstones are also composed of heterogeneous lithoclasts with calcareous white cement. A 25-m thick interval of reworked volcanosedimentary material occurs in the middle of the section (Fig. 8). The Collon Cura Fm. lies above the Naupa Huen Fm. (sections I and J; Fig. 8). Sediments of these two sections mostly consist of tuffaceous material with pumice and volcaniclastic particles, alternating with marls, sands and conglomerates. The thickness of beds is of the order of metres, with channelized morphologies. Contrary to the situation upstream, primary volcanosedimentary deposits are not observed and the sedimentary deposits consist exclusively of reworked volcanosedimentary material.


D. Huyghe et al. Structures

Section A

S 39°59.966' W 70°49.539' 980m

LEGEND

Grain size key:

Pumice

Ignimbrite

Volcanic

Sandstone Cross-stratified sandstone

Conglomerate

c s fs cscg

Igneous rock pebble


Formation Thickness Lithology (m) 160

Compaction figure Sismite

155

315

150

310

150

145

305

145

140

300

140

290

135

295

135

285

130

290

130

280

125

285

125

275

120

280

120

270

115

275

115

265

110

270

110

260

Section B

250

Section C S 40°10.27' W 70°39.39' 1070 m 100 95

90

250

90

240

90

85

245

85

235

85

80

240

80

230

80

75

235

75

225

75

70 65

230 225

70 65

220 215

Caleufu

245

Caleufu

95

Caleufu

255

Caleufu

95

Formation

100

255

S 40°07.121' W 70°55.212' 1030m

Formation

260

105

Channelized morphology

Formation

100

265

Formation

105

Formation

2D current ripples

70

?

65

60

220

60

210

60

55

215

55

205

55

50

210

50

200

50

45

205

45

195

45

40

200

40

190

40

35

195

35

185

35

30

190

30

180

30

25

185

25

175

25

20

180

20

170

20

15

175

15

165

15

10

170

10

160

10

5

165

5

155

5

0

160 c fs cg s cs

c fs cg s cs

150 c fs cg s cs

0 c fs cg s cs


0

Caleufu

?

c fs cg s cs

Fig. 4. Studied sedimentary sections in the Collon Cura basin.

8


D. Huyghe et al.

Conglomerate

Grain size key: c s fs cscg Clay Siltstone Fine sandstone Coarse sandstone Conglomerate

LEGEND Sandstone Cross-stratified sandstone Pumice Igneous rock pebble Compaction figure Sismite

Section J

50

2D current ripples Channelized morphology

45

S 39°47.623' W 69°17.027' 580 m

Section H

15 10 5

15 10 5

c fs cg s cs

70 65

15 10

60 55

5

c fs cg s cs

0

Collon Cura Fm.

75

Formation

25 20

0

0

80

Huen

20

30

Naupa

20

85

Huen

25


25

35

90

Naupa

30

Caleufu Fm

S 39°57.48' W 69°36.20' 760 m


30

Caleufu Fm

35

Section I

Formation

40

S39°57.17' W69°37.52' 755 m

50 c fs cg s cs

c fs cg s cs

Fig. 8. Studied sedimentary sections in the Lago Ramos Mejia area.

Miocene and end Pliocene, except for deposits corresponding to the distal part of the Caleufu Fm. This reconstruction highlights the relative subsidence of the Collon Cura basin compared to the Picun Leufu basin during uplift of the Sa~ nico Massif (Fig. 10). During the Early Miocene, maximum deposition in the Picun Leufu Basin occurred on the eastern side of the Sa~ nico Massif (Fig. 10). The Collon Cura basin was characterized by a phase of extension and filled by volcanic products of the Auca Pan Fm. (Garcıa Morabito et al., 2011; Garcıa Morabito & Ramos, 2012; Ramos et al., 2014b). Downslope, in the Lago Ramos Mejia area and near the Renteria Plateau, sedimentary deposits are thin. Maximum Middle Miocene thicknesses are observed in the Lago Ramos Mejia area and sedimentation began in the Collon Cura basin. During Late Miocene and Early Pliocene, sedimentation rates reached a maximum in the Collon Cura basin, with deposition of several hundred

12

metres of volcaniclastic material in the Caleufu Fm. Elsewhere, a major phase of nondeposition and possible erosion happened, even if sedimentological evidence suggests the contemporaneous deposition of conglomerates in the Picun Leufu basin, implying that the Caleufu Fm. was deposited downstream of the Collon Cura basin.

PALEOGEOGRAPHIC EVOLUTION By placing the sedimentological data in the established chronostratigraphic framework of the southern Neuquen basin, we construct paleogeographic maps for four time intervals corresponding to Early Miocene, Middle Miocene, Late Miocene to Early Pliocene and Late Pliocene to Early Pleistocene (Fig. 11). Maps are constructed given the current distribution of the deposits and are not palinspasticsally restored though time because shortening


basin Neogene evolution of the Neuquen LEGEND

Section L

Volcanic Sandstone Cross-stratified sandstone Conglomerate

S 39°43.984' W 68°29.474' 850 m

95

Section N Pumice

90

Igneous rock pebble 85

Root traces 2D current ripples Channelized morphology Grain size key:

80 75

S 39°08.40' W 67°40.43' 460 m

Caleufu Fm.

Carbonate nodule

80 75

70

70

65

65

30 25

Caleufu Fm.

35

10 5 0

45

45

40

40

35

35

c fs cg s cs

40 35

30

30

25

25

20

20

20

15

15

10

10

5

5

0

15 10 5

0 c fs cg s cs

Formation

45

25

Chichinales Fm

15

50

30

20

55

50

50

Formation

S 39°59.73' W 68°53.08' 850 m

S 39°20.299' W 68°26.781' 745 m

Chichinales

Section K

60

Chichinales

55

Section M

Formation

60

Chichinales


c s fs cscg

c fs cg s cs

0

c fs cg s cs

Fig. 9. Studied sedimentary sections in the Renteria Plateau area.

is supposed to be low in this part of the basin (