Basin Evolution of Upper Cretaceous–Lower Cenozoic Strata in the Malargue€ Fold-And-Thrust Belt: Northern Neuquen� Basin, Argentina Elizabeth A

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Basin Research (2014) 1–24, doi: 10.1111/bre.12106 Basin evolution of Upper Cretaceous–Lower Cenozoic strata in the Malargue€ fold-and-thrust belt: northern Neuquen Basin, Argentina Elizabeth A. Balgord and Barbara Carrapa Department of Geosciences, University of Arizona, Tucson, AZ, USA

ABSTRACT The Andean Orogen is the type-example of an active Cordilleran style margin with a long-lived retroarc fold-and-thrust belt and foreland basin. Timing of initial shortening and foreland basin development in Argentina is diachronous along-strike, with ages varying by 20–30 Myr. The Neu- quen Basin (32°Sto40°S) contains a thick sedimentary sequence ranging in age from late Triassic to Cenozoic, which preserves a record of rift, back arc and foreland basin environments. As much of the primary evidence for initial uplift has been overprinted or covered by younger shortening and volcanic activity, basin strata provide the most complete record of early mountain building. Detailed sedimentology and new maximum depositional ages obtained from detrital zircon U–Pb analyses from the Malargue€ fold-and-thrust belt (35°S) record a facies change between the marine evaporites of the Huitrın Formation (ca. 122 Ma) and the fluvial sandstones and conglomerates of the Dia- mante Formation (ca. 95 Ma). A 25–30 Myr unconformity between the Huitrın and Diamante formations represents the transition from post-rift thermal subsidence to forebulge erosion during initial flexural loading related to crustal shortening and uplift along the magmatic arc to the west by at least 97 2 Ma. This change in basin style is not marked by any significant difference in provenance and detrital zircon signature. A distinct change in detrital zircons, sandstone composition and palaeocurrent direction from west-directed to east-directed occurs instead in the middle Diamante Formation and may reflect the Late Cretaceous transition from forebulge derived sediment in the distal foredeep to proximal foredeep material derived from the thrust belt to the west. This change in palaeoflow represents the migration of the forebulge, and therefore, of the foreland basin system between 80 and 90 Ma in the Malargue€ area.

INTRODUCTION behaviour is through investigation of the sedimentary record, which tends to be better preserved than the The Andes Mountains formed as a result of the fold-and-thrust belt. Reconstructing the initiation of subduction of oceanic plates under South America (e.g. crustal shortening, erosion, and foreland basin deposi- Dewey & Bird, 1970; Allmendinger et al., 1997). tion is essential for understanding the timing and rate of Broadly, three different kinematic regimes have been exhumation which can then be used to constrain plate- observed in the Andes: (i) back-arc extension as a result scale geodynamic models of the Andes. of the rate of slab rollback exceeding the margin normal Timing of the onset of contractional deformation along component of ‘absolute’ velocity of the overriding plate; theAndeanmargin(Fig.1)hasbeenatopicofdebatesince (ii) dominant strike-slip with local transtension to trans- Steinmann (1929) first defined three phases of Andean pression during periods of oblique convergence and (iii) shortening in Peru: Peruvian during the Late Cretaceous, contractional deformation caused by the margin normal theEoceneIncaicandtheMiocenetorecentQuechua component of ‘absolute’ velocity of the overriding plate phases. Large discrepancies (tens of millions of years) exist exceeding the rate of slab rollback (e.g. Schellart, 2008). in the literature concerning the timing of initial shortening Such regimes have resulted in different patterns of and foreland basin deposition between the central Andes in deformation and exhumation within the fold-and-thrust Bolivia and the Patagonian Andes in Southern Chile and belt and/or volcanic arc which in turn control subsi- Argentina. In the Bolivian Altiplano, Sempere et al. (1997) dence and sedimentation in the associated retroarc basin. suggest that foreland basin subsidence started in the Late One way to reconstruct kinematic regimes and plate Cretaceous (ca. 85 Ma). Foreland basin deposits preserved in the Eastern Cordillera of central Bolivia suggest that Correspondence: Elizabeth A. Balgord, Department of Geo- flexurally driven subsidence began in the latest sciences, University of Arizona, 1040 E 4th St, Tucson, AZ, Cretaceous–early Cenozoic (Horton & Decelles, 2001; 85721, USA. E-mail: [email protected]. Decelles & Horton, 2003). In Northern and Central

Fig. 1. Map of South America showing the modern extend of the fold-and-thrust belt, location and name of major Andean basins, modern forebulge, as well as timing of foreland basin initiation or hinter- land uplift along-strike (Patagonia segment, Fildani et al., 2003; northern Argentina; Decelles et al., 2011; Carrapa et al., 2012; Bolivia; Sempere et al., 1997; Horton & Decelles, 2001; Decelles & Horton, 2003; Chile, Mpodozis et al., 2005; Arriagada et al., 2006). Box shows location for Fig. 2.

Argentina, well-exposed Cenozoic strata record an active Neuquen Basin, using a multidisciplinary approach that eastward-migrating foreland basin system that initiated in integrates sedimentology, petrology and detrital zircon the Palaeocene (Jordan et al., 1983; Carrapa et al., 2011, U–Pb geochronology to characterize the stratigraphy, 2012; Decelles et al., 2011). To the west, in the Atacama depositional environments, and provenance patterns of Desert of Chile, growth strata in foredeep and wedgetop Cretaceous to Lower Cenozoic strata in the Malargue€ area deposits record Late Cretaceous shortening (Mpodozis (35°S). The northern Neuquen Basin is particularly well et al., 2005; Arriagada et al., 2006). In the same area, inver- suited for this study because it marks a major orographic sion structures documented by Martınez et al. (2012, 2013) transition characterized by a decrease in topography, also record shortening near the Cretaceous–Palaeocene crustal thickness and total shortening (Kley & Monaldi, boundary. In the southern Patagonian Andes, Fildani et al. 1998; Giambiagi et al., 2012). The study area lies between (2003) proposed that the abrupt occurrence of sandstone in the Patagonian Andes and Northwestern Argentinian/ the Punta Barra Formation signified the initiation of con- Bolivian Andes where significantly different ages of fore- tractional deformation at 92 1Ma. land basin strata have been recorded; foreland basin depo- In the Neuquen Basin (32°Sto40°S; Fig. 2) of Argen- sition initiated at 92 1 Ma in the Patagonian Andes tina, early scholars (e.g. Keidel, 1925; Groeber, 1929) used (Fildani et al., 2003; Romans et al., 2010), whereas fore- thechangefrommarinetofluvialdepositionintheLate land basin deposits have been documented at ca. 65 Ma Cretaceous as evidence for initiation of shortening and fore- in the Argentinian/Bolivian Andes (Horton & Decelles, land basin sedimentation. However, subsequent studies 2001; Decelles & Horton, 2003; Carrapa et al., 2011, using various methods in different parts of the basin have 2012; Decelles et al., 2011). Understanding the basin evo- placed the initiation of foreland basin deposition in the Al- lution and timing of foreland basin initiation in the Ma- bian (ca. 115; Howell et al., 2005), Cenomanian largue€ area will fill an important gap in timing constraints (ca. 100 Ma; Cobbold & Rossello, 2003) and Maastrichtian along the Cordilleran margin. (ca. 70 Ma; Barrio, 1990a). Recent geochronological studies (Tunik et al., 2010; Di Giulio et al., 2012) implementing detrital zircon U–Pb technique support a Late Cretaceous GEOLOGICAL HISTORY (100 8 Ma) age of foreland basin initiation in the central Crustal assembly andsouthernNeuquen Basin. No study to date has estab- lished age constraints in the northern-most Neuquen Basin. The basement of Southern Argentina consists of the The goal of this study is to determine the timing of Chilenia, Cuyania, Pampian and Patagonia terranes, foreland basin initiation in the northern part of the which were accreted onto the southwestern margin of

© 2014 The Authors 2 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

Fig. 2. Map showing the location and extent of the Neuquen Basin (grey) the location of Upper Cretaceous exposures (Leanza et al., 2004; Aguirre-Urreta et al., 2011), and a box showing the location of the map in Figure 6. Pink line indicates Cretaceous thrust front from Mescua et al. (2013) and black shows Miocene-modern thrust front from Mosquera & Ramos (2006).

Gondwana (Rio de Plata and Amazonia cratons; Fig. 3; and accretionary prism developed along the western and Ramos, 2010 and references therein). The Rio de la Plata southern margin of Gondwana between 300 and 250 Ma craton (Fig. 3) is comprised of igneous and metamorphic (Herve et al., 1988; Willner et al., 2005, 2008). rocks with zircon U–Pb ages ranging from 2200 to The ﬁnal assembly of South America with Patagonia is 2000 Ma (Rapela et al., 2007). The Pampian terrane, not well constrained, but there is some consensus about which contains ca. 1100–1200 Ma basement (Rapela its para-autochthonous character and either its ﬁnal amal- et al., 2007), collided with the Rio de la Plata craton in lat- gamation (Ramos, 1984, 2008; Rapalini, 2005; Pankhurst est Proterozoic–early Cambrian time (Pampean Orogeny, et al., 2006; Rapalini et al., 2013) or intense intraplate Ramos, 1988; Rapela et al., 1998; Ramos, 2010). Cam- deformation (Gregori et al., 2008, 2013) in early Permian brian to Ordovician subduction along the western time. Gondwanan-Patagonian basement (AP and NPM; Gondwana margin led to development of the Famatinian Fig. 3) includes an accretionary prism (AP) located on the magmatic arc (Pankhurst et al., 1998; Sato et al., 2003) western margin of South America, which received sedi- and the subsequent collision and accretion of the Cuyania ment from the east (Willner et al., 2008), arc batholiths terrane (1360–1400 and 1070–1200 Ma; Naipauer et al., and metamorphic basement assemblages associated with 2010) onto the Pampian terrane (Thomas & Astini, 1996, the accretion of the Patagonian terrane along the southern 2003; Ramos, 2004). Following the collision of Cuyania, margin of Gondwana in the Permian (peak metamor- Silurian-Devonian strata were deposited in a foreland phism 375–310 Ma; Pankhurst et al., 2006). Widespread basin (Caminos, 1979; Astini et al., 1995), which was then magmatism associated with the collision of Patagonia and metamorphosed and deformed during a collision with the subsequent orogenic collapse led to the formation of Chilenia terrane in the Late Devonian (Ramos, 1999; abundant Early Triassic rhyolites and granites known as Willner et al., 2008). After this collision, a magmatic arc the Choiyoi igneous complex (Kay et al., 1989;

Formation, Diamante Formation and the Malargue€ Group (Fig. 5). The Upper Valanginian–Lower Barremian Agrio For- mation was defined by Weaver (1931) as a thick succession of marine shales, sandstones and limestones overlying the Mulichinco Formation and underlying the Huitrın Formation. The Agrio Formation was divided into three formal members (Weaver, 1931): the Pilmatue (lower) Member (Leanza et al., 2001) composed of up to 600 m of marine shales, mudstones, thin sandstone beds and carbonates (Aguirre Urreta & Rawson, 1997); the Avile (middle) Member composed of up to 100 m of sandstones with pebble conglomerates and subordinate mudrocks interpreted as aeolian and fluvial deposits (Lazo et al., 2005); and the Agua de la Mula (upper) Member composed of up to 1000 m of open marine shales, mudstones, sandstones and bioclastic carbonates (Spalletti et al. 2001). Regionally, the Huitrın Formation is divided into three members (Fig. 5; Leanza, 2003): the Troncoso Member Fig. 3. Map of various basement provinces of southern South composed of fluvial and aeolian sandstones (Lower America and their expected ages (Rio de la Plata, Naipauer Troncoso) and evaporites (Upper Troncoso); the La et al., 2010; Pampia and Cuyania, Astini et al., 1995; Chilenia, Tosca Member composed mostly of carbonates and the Ramos 1999; Willner et al., 2008; Palaeozoic accretionary prism, Salina Member composed of multicoloured shales alter- Herve et al., 1988; Willner et al., 2005, 2008; Patagonia, Rapa- nating with thin evaporite beds. Detailed analysis of the lini, 2005; Pankhurst et al., 2006; Rapalini et al., 2013; Choiyoi Province, Kay et al., 1989; Rocha-Campos et al., 2011). Base- bivalve fossils within the upper Huitrın Formation (La ment boundary locations modified after Uriz et al. (2011). Tosca Member) by Lazo & Damborenea (2011) indicate a restricted, hypersaline setting with some marine influence. Deposition of the Huitrın Formation occurred dur- Rocha-Campos et al., 2011). The Choiyoi igneous coming final connection of the Neuquen Basin to the Pacific plex is not shown in Fig. 3, because it intrudes and covers Ocean (Lazo et al., 2005). the basement units on all sides of the Neuquen Basin. The contact between the Huitrın and Diamante forma- The volcanic arc (VA) intrudes and covers parts of the tions in the Malargue€ area is a paraconformity, with accretionary prism, Chilenia and the North Patagonian ca. 25 Myr of missing time (Fig. 5). The lower, middle Massif. The main age range includes Jurassic–Cretaceous and upper members of the Diamante Formation (dis- plutonic and volcanic arc-related activity associated with cussed below), exposed in the Malargue€ area, broadly cor- the subduction of the Farallon and other oceanic plates relate with the Rio Limay, Rio Neuquen and Rio beneath South America. Colorado subgroups from the Neuquen Group defined by Leanza & Hugo (2001; Fig. 5). The Diamante Formation € – Stratigraphy of the Neuquen Basin in the Malargue area is age correlative (95 80 Ma) and has many of the same facies as the Neuquen Group stra- The Neuquen Basin contains a ca. 5000–7000-m-thick tigraphy to the south and east, but does not contain all of sedimentary succession of Upper Triassic to Miocene the specific members and formations documented to the strata (Fig. 4; Vergani et al., 1995; Howell et al., 2005). south (Leanza & Hugo, 2001; Leanza et al., 2004). A In the north, the basin forms a narrow belt in the Ma- chronostratigraphic framework was defined by Leanza largue€ fold-and-thrust belt, whereas south of 36°S, the et al. (2004) based on six local tetrapod assemblages, basin expands eastward ca. 250 km, forming a large trian- which are correlated with the Diamante Formation based gular embayment (Fig. 2). The Neuquen Basin is located on maximum depositional ages obtained from detrital zir- in a retroarc position and is characterized by a complex con U–Pb ages (discussed below). tectonic history that includes: (i) a Late Triassic–Middle The Malargue€ Group is comprised of the Allen, Lonc- Jurassic east–west-trending rift basin along the boundary oche, Jaguel,€ Roca and Pircala formations (Fig. 5; Barrio, between the Chilenia, Cuyania, Pampian and Patagonia 1990a). The dominance of continental and deltaic facies, basement blocks (Fig. 3); (ii) a middle Jurassic–Late Cre- and of pyroclastic material in the central Andes sector, taceous marine basin related to post-rift thermal subsi- recorded by Aguirre-Urreta et al. (2011), suggests close dence; and (iii) a Late Cretaceous to modern retroarc- proximity to the volcanic arc in the latest Cretaceous. In foreland basin with flexurally dominated subsidence the central and southern sections of the Neuquen Basin, (Fig. 4; Vergani et al., 1995; Howell et al., 2005). We the Malargue€ Group is dominated by marine deposits, here focus on the stratigraphy of the Upper Cretaceous comprising mainly shales, limestones and evaporites through Lower Cenozoic Agrio Formation, Huitrın (Barrio, 1990b). These rocks correspond to the first Atlan-

© 2014 The Authors 4 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

Fig. 4. Generalized stratigraphic column for the central Neuquen Basin showing lithologies, ages and tectonic setting of Neuquen Basin stratigraphy, modified from Mosquera & Ramos (2006) and plate reconstructions from Seton et al. (2012). Note that Neuquen Group on this figure is age equivalent with the Diamante Formation. CHA: Chasca Plate; CAT: Catequi Plate (Seton et al., 2012). tic transgression into the basin (Weaver, 1927; Uliana & (MW; Fig. 6). Fieldwork included a detailed description Dellape, 1981), and define a regional change in the basin of outcrops and measurements (dm scale) of stratigraphic slope associated with eustatic sea level rise (higher eleva- sections beginning at least 200 m below the contact tions to the west and lower to the east; Barrio, 1990a). At between the 136–128 Ma Agrio Formation (Lazo et al., the top of the Malargue€ Group, in the Malargue€ area, a 2005; Archuby et al. 2011) and 128–125 Ma Huitrın For- regional unconformity provides evidence for deformation mation (Veiga et al. 2005) and continuing through the and erosion after ca. 60 Ma; based on cross-cutting rela- 98–76 Ma Diamante Formation (Leanza et al. 2000, tionships between faults and volcanic units, this unconfor- 2004), and the 72–56 Ma Malargue€ Group (Barrio, mity reflects Eocene and Miocene – present deformation 1990b; Aguirre-Urreta et al., 2011) where present (Figs 6 related to modern Andean uplift (Legarreta et al., 1989; and 7). The studied sections span an E–W distance of Ramos & Folguera, 2005; Giambiagi et al., 2008). ca. 50 km and a N–S distance of ca. 45 km (Fig. 6). Twenty-nine standard petrographic thin sections from the Los Angeles, Bardas Blancas and Malargue€ West sec- METHODS tions were stained for Ca- and K-feldspars and point- counted (450 counts per slide) according to the Gazzi– This study reports new sedimentological and zircon U– Dickinson method (G–D) (Gazzi, 1966; Dickinson, 1970; Pb data from the Agrio, Huitrın and Diamante formations Ingersoll et al., 1984) to assess bedrock provenance of the and the Malargue€ Group, which are used to determine Cretaceous–lower Cenozoic deposits of the study area. depositional environments, stratigraphic relations, prove- Petrographic parameters are listed in Appendix S1(A), nance and maximum depositional ages throughout the and recalculated modal data, as well as member averages Cretaceous and early Cenozoic. Three areas were targeted and standard distributions, are listed in Table 1. Ternary for this study, based on previous geological mapping in diagrams with total quartz-feldspar-lithic (QFL) and the region (Nullo et al., 2005), from west to east: the Los quartz-plagioclase-potassium feldspar (QPK) are shown Angeles (LA), Bardas Blancas (BB) and Malargue€ West in Fig. 9 (Dickinson et al., 1983). None of the samples

Fig. 5. Stratigraphic columns compar- ing the stratigraphy from the central Neuquen Basin (Howell et al., 2005) with the formations present in the Malargue€ area (Giambiagi & Ramos, 2002; Giambi- agi et al., 2008). Note the difference in time for the unconformity between the Huitrın and Diamante Formations in the Malargue€ area vs. the Rayoso-Diamante unconformity to the south. The Ma- largue€ area also contains significantly fewer Lower Mesozoic rock units. Spe- cific formation names and ages are shown for the Neuquen and Malargue€ Group stratigraphy in both sections. counted had more than 100 lithic fragments to allow for a taking the youngest age components, generally three or separate quantitative analysis of the lithic fraction, how- more grains (only two for the Huitrın Formation were ever, qualitative descriptions of the lithic fragments are available), that overlap in age within error and calculating provided based on the available data. a mean and standard deviation taking into account the To better constrain the maximum depositional age and original uncertainty in the grain age based on recommen- provenance of the Upper Mesozoic–Lower Cenozoic sed- dations from Dickinson & Gehrels (2009). imentary units in the Malargue€ fold-and-thrust belt, sys- tematic sampling of key formations for U–Pb detrital zircon analysis was undertaken. Samples were processed RESULTS AND INTERPRETATIONS and analysed following the procedures outlined in Geh- Facies analysis, palaeocurrents and rels et al. (2006, 2008). The analytical data and a more depositional environments detailed methodology are reported in Appendix S2(A,B). The age data are displayed in relative age-probability dia- Sedimentological descriptions and interpretations are grams using the routines in Isoplot (Fig. 10; Ludwig, based on three detailed stratigraphic sections (Fig. 7a–c). 2008). Maximum depositional ages were calculated by Lithofacies identified in the measured stratigraphic

© 2014 The Authors 6 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

Fig. 6. Generalized geological map of the study area showing the location of the three measured sections (Los Angeles, Bardas Blancas and Malargue€ West). Fig- ure location shown in Fig. 2. Modiﬁed after Nullo et al. (2005).

sections are based on the lithofacies codes of Miall (1978) siltstone and shale in the BB area (Spalletti et al. 2001; with some modiﬁcations (Table 2) and carbonate facies Lazo et al., 2005). Over the 200 m of measured section, description are based on the carbonate naming scheme of the facies undergo a transition from shale-dominated to Dunham (1962) as modiﬁed by Embry & Klovan (1971) limestone-dominated, becoming lighter in colour, and (Table 2). Palaeocurrent directions were determined for contain more abundant bivalve fossils. Discontinuous the Diamante Formation using limbs of trough cross- 0.5–1-m-thick layers of medium-grained, massive sand- strata (method I of Decelles et al., 1983). stone (Sm) are present in the southernmost (BB; Fig. 6) area; four separate Sm bodies are located about 50 m Agrio Formation below the contact with the overlying Huitrın Formation (Fig. 7b). Description: The upper 200 m of the Agrio Formation was Interpretation: Laminated siltstone and shale (Fcl) are measured in the Los Angeles (LA), Bardas Blancas (BB) interpreted to have been deposited by suspension settling and Malargue€ West (MW) areas to give context and assess in open marine water where waves did not interact with facies changes in the overlying Huitrın and Diamante for- the seabed (Burchette & Wright, 1992). The association mations (Fig. 5). In all three measured sections, the Agrio of black laminated shale (Fcl) with well-preserved fossils Formation consists of thinly laminated to thinly bedded, suggests deposition in a low energy open marine environ- calcareous mudstone (Fcl; see Table 2 for all lithofacies ment. The dark colour indicates high organic content that descriptions) to fossiliferous (containing bivalves and is typical of deposition in a poorly oxygenated or com- ammonites) packstone-grainstone (Lf) arranged in 5–10- pletely anoxic environment (Droste, 1990). The presence m-thick coarsening upwards packages (Fig. 8a). Mud- of ammonites indicates normal marine salinity and oxy- stone throughout all three sections is thinly laminated genation in the upper part of the water column (Batt, (Fcl) and ranges from black and organic rich with sparse 1993). calcareous nodules (Fig. 8a), generally cored by ammon- The upper ca. 1–2 m of each coarsening upward pack- ite fossils, to light grey-white and carbonate rich (Lf) with age contain fossiliferous limestone beds, which range in abundant bivalves that outcrop in resistant tabular, later- texture from wackestone to grainstone. The increase in ally continuous limestone beds (Fig. 8b). Ammonites are concentration of large (>2 mm) fossil fragments, and a common, and marine reptiles and other open marine decrease in micritic matrix up section may indicate sort- fauna have also been documented within the laminated ing by bottom currents (Aigner, 1985; Faulkner, 1988), or

Table 1. Sedimentary petrology tables The contact between the Agrio and Huitrın formations in the BB and LA areas (Fig. 6) is marked by a change Sample Q F L Q P K K P from white, thinly laminated calcareous shale (Fcl; see Upper diamante formation Table 2 for lithofacies codes) below a covered 2–5-m- 20EAB12 38 61 2 38 30 31 51 49 thick interval, to brown, red and yellow rippled siltstone 21EAB12 37 62 1 38 36 26 42 58 (Sr) and mudstone. The basal Huitrın Formation in the 22EAB12 54 25 21 69 18 13 42 58 LA and MW areas contains ca. 10 m package of massive 23EAB12 21 65 13 25 54 21 28 72 green, yellow and red siltstone (Fsm) and mudstone with 25EAB12 43 44 13 49 15 36 71 29 minor bedded gypsum nodules. The overlying ca. 20 m 26EAB12 32 48 20 40 45 15 24 76 this formation transitions from laminated mudstone (Fsl) 27EAB12 20 76 4 21 60 19 24 76 with minor gypsum, to dominantly gypsum with minor 28EAB12 37 45 18 45 50 5 9 91 mudstone, and finally 60 m of massive gypsum (Em) with 29EAB12 41 26 33 61 37 3 7 93 56EAB12 49 18 33 73 24 2 9 91 locally preserved thin laminations in both the LA and 58EAB12 47 22 31 68 27 6 18 82 MW areas, and 5 m of massive gypsum in the BB area 59EAB12 42 28 30 60 35 4 11 89 (Fig. 8c,d). Thinly laminated gypsum beds (El) are wavy 60EAB12 30 31 39 49 32 19 37 63 and irregular, with minor interbedded siltstone, shale and Average 38 42 20 49 36 15 29 71 micrite (Fsl and Fcl). A 2-m-thick section of thinly lami- Std 10 19 13 17 14 11 19 19 nated, sandy limestone (Fsl) near the top of the Huitrın Middle diamante formation Formation (12 m below the contact with the overlying 30EAB12 33 63 4 34 64 2 2 98 Diamante Formation; Fig. 7a) was sampled for detrital 31EAB12 34 61 5 35 65 0 0 100 zircon U–Pb analysis in the LA area. 49EAB12 26 52 22 33 62 4 6 94 In the BB and MW areas (Fig. 7b,c), the massive gyp- 51EAB12 37 41 22 48 44 8 15 85 sum gradually transitions to thinly laminated gypsum 52EAB12 44 49 8 47 33 20 38 62 53EAB12 33 44 23 43 52 5 8 92 (El) that in turn is overlain by a 10-m-thick interval of 54EAB12 45 40 14 53 47 0 0 100 resistant, tabular, laterally continuous, thin medium- Average 36 50 14 42 52 6 10 90 bedded, fossiliferous (bivalves; Lf) and intraclastic (Li), Std 79881271313and pelloidal (Lp) wacke-grainstone, which is locally Lower diamante formation dolomitic, and contains minor interbedded gypsum 15EAB12 34 65 1 34 59 6 10 90 (laminated and nodular). In general, the gypsum layers 16EAB12 35 64 1 36 53 11 18 82 appear to be thickest in the westernmost section (LA, 17EAB12 22 76 2 22 67 10 13 87 80 m) and thin to the south (5 m) and east (40 m), where 34EAB12 29 47 24 38 59 3 5 95 it is potentially replaced by limestone and laminated 35EAB12 23 45 32 34 65 1 2 98 siltstone and mudstone. 47EAB12 34 43 24 44 40 16 28 72 Interpretation: The association of calcareous shale 48EAB12 47 45 8 51 25 24 50 50 Average 32 55 13 37 53 10 18 82 (Fcl) and both laminated (El) and massive (Em) evap- Std 8 13 13 9 15 8 17 17 orate facies suggest deposition in a low energy, restricted environment (James & Kendall, 1992). Thinly laminated siltstone and mudstone require a relatively rapid deposition (high primary productivity) of proximal clastic source area and the combination of large invertebrate faunas (Schlager, 1991). Overall, the horizontal laminations and minor current ripples measured sections comprise repeating shallowing implies generally slow velocities and a unidirectional upwards cycles in a shallowing upwards sequence current (Harms et al., 1975). We interpret the inter- (Fig. 7a–c). Based on facies and fossils within these areas bedded siltstone, mudstone and gypsum as being the Agrio Formation is interpreted to have been deposited deposited in a playa mudflat (e.g. Flint, 1985; Hartley in an open marine environment. et al., 1992). The presence of fine-grained, laminated beds and gypsum is indicative of an arid, or at least Huitrín Formation episodically dry environment (e.g. Hardie et al., 1978). The transition from siltstone and claystone dominated Description: The thickness of the Huitrın Formation is to gypsum dominated suggests an overall decrease in highly variable, ranging from <50 m in the BB area to clastic detritus. The massive (>5 m with no discernible 200 m in the LA area (Figs 6 and 7). The contact between laminations) gypsum (Em) beds indicate a restricted the Agrio Formation and the overlying Huitrın Forma- lagoonal environment, whereas the interbedded lami- tion is covered in all three areas, but appears to be con- nated gypsum (El), limestones (Lm), algal mats formable based on uniform bedding dips and general (Fig. 8d), and nodular gypsum (En), record an open, continuity along-strike. Also, biostratigraphic (Lazo & supratidal environment (Butler et al., 1982). The vari- Damborenea, 2011) and maximum depositional ages from ability in lithologies is a product of fluctuating water this study (discussed later) suggest that there is no signifi- chemistry, biological activity and clastic sedimentary cant time gap between the two formations. input.

© 2014 The Authors 8 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

Fig. 7. (a) Measured stratigraphic section from the Los Angeles area along with the key used in all stratigraphic sections (a–c). Lithol- ogy codes are deﬁned in Table 2. (b) Measured stratigraphic section from the Bardas Blancas area. (c) Measured stratigraphic section from the Malargue€ West area.

The limestones in the BB and MW areas are domi- maintain constant gypsum deposition without other nantly fossiliferous grainstones (Lf) composed of a sin- common evaporite minerals (e.g. halite, dolomite and gle type of bivalve fossil, indicating a high-productivity, carbonate). This was combined with a lack of clastic sedi- low-diversity ecosystem, similar to those seen in other mentation in the basin and slow, steady subsidence parts of the Neuquen Basin and described in detail by (Handford, 1991; Warren, 1991). Lazo & Damborenea (2011). Intraclasts (Li) likely require that the limestone episodically experienced Diamante Formation strong wave and/or storm disturbance (James & Choquette, 1984). Peloidal grainstones (Lp) – comprised The contact between the Huitrın and Diamante forma- of faecal pellets generated by mud ingesting organisms tions is a paraconformity in the Malargue€ area. The con- and micritized by micro-organisms – are indicative of tact is generally covered, but where it is exposed (BB area) relatively low energy, protected lagoonal settings (Reid it is a sharp contact between either tabular limestone or et al. 1992). thinly laminated carbonate rich siltstone and medium- During the deposition of the Huitrın Formation, the grained ripple cross-laminated (Sr) and trough cross- northern Neuquen Basin was a restricted marine environ- stratified (St) sandstone. ment where the rate of evaporation outpaced recharge The Diamante Formation is informally divided into allowing for the precipitation of gypsum. The massive three units based on dominant grain size. The lower, mid- gypsum (Em) facies indicates a semi-continuous connec- dle and upper members are composed of generally fine-, tion to normal salinity sea water, which is required to coarse- and fine-grained sandstone, respectively, which

Fig. 7. Continued. are interpreted to have been deposited under different 10 m to thickly bedded lenses of minor gravel-pebble flow conditions in varying environments, described below conglomerate (Gt), which fines upwards to trough cross- (Fig. 7b,c). stratified (St), horizontally laminated (Sh) sandstone with ripple cross-laminated (Sr) tops (Fig. 7a–c). Beds are lenticular on the outcrop scale generally ranging from 2 to Lower Diamante Formation 5 m wide and 1 to 3-m thick. Description: The lower Diamante Formation is variable The lower Diamante Formation is ca. 100-m thick in between sections, but in general is fine-grained (domi- the BB area and ca. 150 m in the MW area (Fig. 7b,c). nantly siltstone to medium sand size) and contains minor Facies observed in the BB area are similar to those trough cross-stratification (St), horizontally laminated described in the LA area, although the lenticular nature (Sh), ripple cross-laminated (Sr) and massive (Sm and of the beds is less pronounced at the outcrop scale, with Fsm) sandstone. Only the basal 80 m of the Diamante single packages up to 20 m wide. The base of beds tend Formation is preserved in the LA section (Fig. 7a), with to be erosive with medium-grained trough cross-stratified the majority of the section having been removed by ero- (St) and ripple cross-laminated (Sr) sandstone with red sion or covered by Miocene volcanic rocks (Fig. 6). The siltstone and mudstone rip-up clasts that fine upwards lithology is dominated by red, trough cross-stratified (St) over 4–5 m to massive, fine-grained sandstone to and horizontally laminated (Sh), medium- to thick-bed- siltstone (Sm and Fsm). Palaeocurrent measurements ded, fine- to medium-grained sandstone. The Diamante taken from the limbs of troughs indicate west-directed Formation in the LA area coarsens upwards over the first palaeoflow in both the LA and the BB areas (Fig. 7a,b).

© 2014 The Authors 10 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

Fig. 7. Continued.

In the MW area, the base of the Diamante Formation area with medium-grained trough cross-stratified (St) and has a thin (ca. 50 m), laterally discontinuous (ca. 100 m ripple cross-laminated (Sr) sandstone with red siltstone wide) interval of medium red to light green, massive fine- and mudstone rip-up clasts that fine upwards over 3–5m grained sandstone and siltstone (Fsm) with abundant car- to massive and fine-grained to siltstone (Fsm). bonate nodules. The fine interval coarsens upwards into Interpretation: Trough cross-bedded sandstones sandy, lenticular facies similar to that observed in the BB within lenticular sand bodies are interpreted to be

Table 2. Lithology codes

Facies code Lithofacies Sedimentary structures Interpretation

Lf Limestone, fossiliferous Generally massive, sometimes Shallow marine to shelf deposits graded >2 m fossils Li Limestone, intraclastic Generally massive, sometimes graded High energy, shallow marine Lp Limestone, peloidal Thinly bedded Low energy, protected lagoonal setting Lo Limestone, oolitic Generally massive, sometimes graded High energy, waver reworked, restricted, shallow marine or lacustrine Em Evaporite, massive Massive Continually recharged lagoon En Evaporite, nodular Nodular Supratidal banded layers El Evaporite, laminated Thinly laminated to thinly bedded Supratidal deposition, with alternating layers of evaporite beds, limestones, algal mats Fcl Laminated black or brown Thinly laminated to thinly bedded Suspension settling in ponds and lakes claystone Fsl Fine-grained sandstone, Thinly laminated to thinly bedded Suspension settling in ponds and lakes siltstone, mudstone Fsm Fine-grained sandstone, Massive, bioturbated, with common Palaeosols, usually calcic siltstone, mudstone carbonate nodules Sm Sandstone, fine- to coarse-grained Massive, bioturbated, with common Palaeosols, usually calcic carbonate nodules Sr Fine- to medium-grained With small, asymmetric, 2D and Migration of small ripples under sandstone 3D current ripples weak (ca. 20–40 cm s1), unidirectional flows in shallow channels Sh Fine- to medium-grained Plane-parallel lamination Upper plane bed conditions under sandstone unidirectional flows, either strong (>100 cm s1) or very shallow Sp Medium- to very Planar cross-stratified Migration of large 2D ripples under coarse-grained sandstone moderately powerful (ca. 40–60 cm s1), unidirectional channelized flows; migration of sandy transverse bars St Medium- to very Trough cross-stratified Migration of large 3D ripples (dunes) coarse-grained sandstone under moderately powerful(40–100 cm s1), unidirectional flows in large channels Gt Pebble conglomerate, well sorted, Clast-supported, trough cross-stratified Deposition by large gravelly ripples under traction flows in relatively deep, stable fluvial channels Gm Pebble conglomerate, M, bioturbated, matrix-supported, poorly Bioturbated gravel-fine pebble matrix supported sorted, disorganized, unstratified conglomerate

deposited by the migration of dune bed forms within ing or braided river systems (Miall, 1978; Smith, 1987). fluvial channels (Miall, 1996). Massive fine-grained The prominence of the palaeosol facies in combination sandstone and siltstone lithofacies (Sm and Fsm) com- with minor trough cross-bedded sandstones, and climb- monly contain evidence for weakly to moderately devel- ing ripples suggests a meandering fluvial environment oped soils including root casts, mottling, absence of (Miall, 1978). stratification, and carbonate nodules, which are interpreted as floodplain deposits (Retallack, 1988). The red Middle Diamante Formation colour and abundant carbonate nodules indicate an oxidized subsurface and an arid to semi-arid environment Description: The contact between the lower and middle (Hardie et al., 1978), whereas green units likely formed Diamante Formation is marked by the first thick (>2m) under reduced conditions (Bowen & Kraus, 1981). The conglomerate bed of the formation and exhibits similar close vertical association between channel and floodplain facies in both the BB and MW areas (not present in LA deposits suggests deposition onto a fluvial plain (Miall, area), which have a total thickness of 240 and 440 m 1996), which could be either representative of meander- respectively (Fig. 7b,c). The middle Diamante Forma-

© 2014 The Authors 12 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

(a) (b)

Fig. 8. Photo panel of important facies (e) (f) and sedimentary structures in the Agrio (a, b), Huitrın (c, d), Diamante (e, f) formations and Malargue€ Group (g, h). (a) Is thinly laminated organic-rich shale with nodular thickly bedded carbonate beds. (b) Bivalve packstone at the base of the Malargue€ West section. (c) Massive gypsum from the Malargue€ West section. (g) (h) (d) Laminated gypsum. (e) Lateral accretion sets from the Malargue€ West section. (f) Example of bioturbation common in the Malargue€ West section. (g) Contact between the red beds of the upper Dia- mante Formation and the yellow Ma- largue€ Group. (h) Cross-bedded, glauconitic sandstones from the Ma- largue€ Group. tion is thicker bedded, coarser grained (dominantly sand 5–10 m (Fig. 8e), and commonly cut one another stack- to pebble grain size), and contains more prominent sedi- ing both vertically and diagonally. Beds fine upwards mentary structures (St, Sr, Sh, Sp, Gt) than the lower ranging from pebble and gravel at the base to fine sand and upper Diamante Formation (Fig. 7c). The middle to siltstone size and are 2–5-m thick. Medium- to thick- Diamante Formation comprises an overall fining upward bedded massive sandstones, with minor parallel lami- sequence that is characterized by 2–3-m-thick packages nated sandstone (Sp) at the base of the beds, are also of both clast- (Gt; Fig. 7b,c) and matrix-supported present. Palaeocurrent measurements indicate east- (Gm) conglomerate, interbedded trough cross-stratified directed flow in both the BB and MW areas throughout gravelly (St and Gt) sandstone (Fig. 8e) along with the middle Diamante Formation (Fig. 7b,c). minor, medium-grained, massive sandstone (Sm) with Interpretation: The interbedded massive and planar abundant bioturbation and carbonate nodules, which laminated sandstones (Sm and Sp) are interpreted as increases in abundance up section (Fig. 7b,c). The being deposited by high-density flows during waning matrix-supported conglomerate (Gm) beds tend to be flow conditions (Rasmussen, 2000). Matrix-supported massive (2–3-m thick) with nonerosive bases, whereas conglomerate (Gm) beds are interpreted as high viscos- gravel- to pebble-rich trough cross-stratified (Gt and St) ity debris flows due to their nonerosive bases and mas- sandstone beds exhibit erosive bases with abundant mud sive bedform (Buck, 1983; Schultz, 1984). We interpret rip-up clasts and fine upwards to medium- to fine- the lenticular conglomerates (Gt) as resulting from grained sandstone with ripple cross-lamination (Sr) and deposition in shallow, gravely, bed load channels, with planar laminated sandstone (Sp). Trough cross-stratified (e.g. Nemec & Steel, 1984). Trough cross-stratified beds are lenticular in nature, over a horizontal scale of sandstones and pebble conglomerates (St and Gt) are

© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 13 E. A. Balgord and B. Carrapa interpreted as being derived from the migration of dune € bed forms within sand–gravel bed channels in a fluvial Malargue Group depositional environment (Miall, 1996). Scour-based, Lower Malarg€ue Group trough cross-stratified sandstone (St) grading into climbing ripples (Sr) represents a lower flow regime Description: A distinct shift in facies marks the contact € within a fluvial environment (e.g. Allen, 1963). The between the Diamante Formation and the Malargue minor mudstones are likely to be floodplain deposits. Group with a change from the massive red beds (Fsm) Due to the combination of debris flow deposits, verti- of the Diamante Formation to the laminated green silt- € cally stacked channel deposits, climbing ripples, and an stone and shale (Fcl) of the Malargue Group (Fig. 9g). € overall lack of fine-grained material and soil formation, Although most of the lower Malargue Group (Lonc- € we interpret this facies association to indicate a mixed oche, Jaguel and Roca Formations; Fig. 5) is recessive – braided fluvial and distal alluvial fan environment and exposures are limited, there are 5 10 laterally con- (Miall, 1978). tinuous, thin-bedded limestone beds (Fcl) within the green and yellow, recessive siltstone and shale (Fcl and Fsl) sequence. Contacts and facies variation are difficult Upper Diamante Formation to follow along-strike, but generally the section seems to Description: The total thickness of the upper Diamante be arranged in coarsening upwards packages that range Formation, where it is completely preserved in the MW from 5 to 10-m thick and define an overall fining area, is 480 m, whereas in the BB area, where the top of upwards succession with a general increase in shale and the upper Diamante Formation is not present, the total decrease in sandstone (Fig. 7c). Two resistant medium- measured thickness is 412 m. The upper Diamante For- to thick-bedded marker intervals, composed of trough mation is dominantly massively bedded (Sm and Fsm) cross-stratified, glauconitic sandstone can be followed due to a large amount of thick, bioturbated, heavily for hundreds of m along-strike (Fig. 9h). After a cov- weathered (covered in the sedimentary logs) intervals of ered interval of ca. 200 m, there is a resistant ridge that sandstone and siltstone (Fig. 7b, c and f); although contains thin-bedded oolitic grainstone (Lo) and med- minor cross-stratification and lenticular- to wedge- ium-bedded fossiliferous (bivalves and gastropods) pack- shaped bedding are still sporadically preserved, espe- grainstone (Lf). cially in the uppermost deposits in the BB area Interpretation: The combination of laminated mud- (Fig. 7b). Abundant in situ and reworked carbonate nod- stone, siltstone and limestone at the base of the € ules, at the base of conglomerate beds, are found in both Malargue Group in the MW are interpreted to have the MW and BB areas (Fig. 7b,c). Rip-up clasts are also been deposited mostly by suspension settling in either common at the base of individual beds. Amalgamated a marginal marine or lacustrine setting. Oolitic grain- beds range from 1 to 15-m thick, and are arranged into stone (Lo) requires warm surface temperatures, thinning- and fining-upward patterns (Fig. 7b,c). Len- constant wave action, and water saturated to super- ticular sand bodies are infrequent, but where present saturated in calcium carbonate (Tucker & Wright, they are 1–3-m thick, up to 20 m wide, and tend to 1990) in shallow marine or lacustrine settings. Within have erosive bases comprising St with very minor Gt, the ridge-forming limestone beds, marine fossils have which then transition into Sr and Sm/Fsm. Overall, the been documented by Aguirre-Urreta et al. (2011) and upper Diamante Formation in the MW and BB area Parras & Griffin (2013), which along with distinctive € fines upwards with an increase in the thickness of silt- glauconitic sandstones in the lower Malargue Group stone and mudstone intervals and a decrease in sand- indicate a marine environment in the latest – stone with little to no conglomerate in the middle Cretaceous early Cenozoic. ca. 300 m and a slight increase again in the uppermost 100 m. All palaeocurrent measurements taken on the Upper Malarg€ue Group limbs of trough cross-stratified sandstones in the upper Diamante Formation indicate east-directed flow in both Description: In the Malargue€ area, the upper Malargue€ the BB and MW areas (Fig. 7b,c). Group comprises the continental Pircala Formation Interpretation: The massively bedded sandstone and that sits above a thick (ca. 300 m) covered interval, siltstone lithofacies (Sm and Fsm) commonly contain likely comprised of the recessive Roca Formation evidence of weakly to strongly developed palaeosols (Fig. 7c; fig. 5, Barrio, 1990a). The upper Malargue€ (Retallack, 1988) including: root casts, mottling and car- Group is poorly exposed but a generalized section was bonate nodules. The combination of closely stacked chan- measured in the MW area (Fig. 7c). There are discon- nel and floodplain deposits combined with the dominance tinuous exposures of red, interbedded siltstone (Fsr) and strongly developed nature of palaeosol facies, minor and fine medium-grained, trough cross-stratified (St), trough cross-bedded sandstones and climbing ripples thin medium-bedded, bioturbated sandstone (Sm and indicates a meandering or anastamosing fluvial environ- Fsm) with minor carbonate nodules. Beds are lenticular ment for the upper Diamante Formation (Miall, 1978; over a horizontal distance of 10–20 m. The uppermost Smith, 1987). exposures are massive silt and mudstone (Fsm and Sm)

© 2014 The Authors 14 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

(a) (b) trough cross-bedded sandstones and climbing ripples, we interpret this association as typical of a meandering or anastomosing ﬂuvial environment (Miall, 1978), similar to the ﬂuvial setting interpreted for the upper Dia- mante Formation.

Sandstone Petrology and Provenance Description: Sandstones from the Diamante Formation are dominantly feldspathic and lithofeldspathic with minor amounts of lithic arenite and contain abundant monocrystalline quartz (Qm) grains, with lesser amounts of polycrystalline quartz (Qp) and quartzose sedimentary lithic fragments (Qss). Plagioclase is much more abundant than K-feldspar. K-feldspar types include ortho- clase (dominant), microcline and perthite. Sedimentary lithic fragments comprise dominantly chert, quartzose sandstone (Qss) and siltstone with minor limestone and shale. Volcanic grains include lathwork (Lvl), microlitic (Lvm), vitric (Lvv), felsic (Lvf) and rare mafic (Lvma) varieties. Minor amounts of micas, zircon, magnetite and Fe-oxide altered fragments are present. Modal sandstone compositions are regionally consistent, with monomineralic fractions dominated by either plagioclase or quartz, and lithic compositions dominated by volcanic grains (Fig. 9; Table 2). Sandstones in the upper Diamante Formation are also generally feldspathic, but are more lithic-rich (especially in the MW area) and have a higher potassium feldspar to plagioclase ratio in the BB area than the lower and middle Diamante Formation (Fig. 9). The lithic component of the upper Diamante Formation is still dominated by volcanic lithics, but there is also a larger component (20% Fig. 9. Ternary diagrams plotting sedimentary petrography for of total lithics) of sedimentary lithics, which include the Diamante Formation. (a) Diagram showing modal abun- chert, siltstone, as well as a larger component of quartz in dances of total quartz (Q), feldspar (F) and lithic fragments (L), provenance field are from Dickinson et al. (1983). (b) Diagram large sedimentary lithics as opposed to lower in the showing modal abundance of quartz (Q), plagioclase (P) and Diamante Formation where the majority of the quartz potassium feldspar (K). in lithics is plutonic. Interpretation: In general, sandstones from the lower and middle Diamante Formation are plagioclase-rich, and form red and white amorphous mounds in the cen- especially in the LA area (Fig. 9). The minor lithic com- tre of the syncline in the MW area and are up to 10-m ponent (<50%) is almost entirely volcanic, although there thick. are some large (>60 lm) plutonic grains, and few to no Interpretation: The thickness, mottling, root casts, metamorphic fragments. Quartz grains are dominantly absence of stratification, bleached horizons and carbon- monocrystalline with parallel extinction indicating ate are evidence for moderate to strongly developed dominantly igneous sources (Tortosa et al., 1991). The palaeosols (Retallack, 1988). The alternation between presence of fine-grained plutonic fragments and dark red and bleached horizons and abundant carbonate plagioclase crystals that exhibit very little chemical nodules require an oxidized subsurface and an, at least decomposition require a local source and/or a semi-arid episodically, arid environment (Hardie et al., 1978). to arid environment. Samples from the LA area (only The minor trough cross-bedded (St) lenticular sand- lower Diamante Formation) plot in the continental block stones are interpreted as being formed by migrating uplift field of Dickinson et al. (1983), whereas the lower dunes within a fluvial channel, which are at times and middle Diamante Formation from the BB and MW capped by ripple cross-laminated (Sr) and planar lami- areas plot into both the Continental Block and the nated sandstone (Sp) caused by differing flow condi- Dissected Arc fields (Fig. 9). Sandstones from the upper tions/water depths on the upper parts of the migrating Diamante Formation plot in the Continental Block, dune-forms (Miall, 1996). Due to the dominance and Dissected Arc and Recycled Orogen provenance fields strongly developed nature of palaeosol facies, minor (Fig. 9).

Detrital U–Pb geochronology and source, and could be from air-fall or minor east-directed Provenance tributaries coming off the volcanic arc. Pre-Jurassic detrital zircon grains could conceivably have originated from Agrio Formation the north, but the abundance of Gondwanan-Patagonian Description: Detrital zircons from the Agrio Formation and Andean-aged grains suggests that the Palaeozoic were collected from m-scale sand bodies interbedded with accretionary prism to the west is a likely source (AP, the organic-rich shale in the BB area (Figs 6 and 7b). Fig. 3; Willner et al., 2008). Although it has been sug- Maximum depositional age, based on the mean age of the gested that the Neuquen Basin experienced a post-rift youngest population present within the sample is thermal sag stage during the Late Jurassic–Early Creta- 134 3.8 Ma (n = 6). Peaks in the age distribution ceous (Howell et al., 2005 and references therein), multi- include: (i) a 150–200 Ma component, (ii) a dominant ple authors (e.g. Vergani et al., 1995; Mosquera & Ramos, 250–300 Ma component and (iii) a minor Cambrian to 2006; Garcıa Morabito et al., 2011; Naipauer et al., 2012) Neoproterozoic component (ca. 450–650). In addition, a have documented Upper Jurassic deformation on a struc- few Mesoproterozoic (ca. 1100 Ma) and Palaeoproterozo- tural high within the southern Neuquen Basin designated ic grains (ca. 1790–1900 Ma) are present (Fig. 10). the Huincul Rise (Fig. 2). This arch may have continued Interpretation: The majority of the grains from the Ag- to act as a structural high and as a sediment source to the rio Formation fall within the young end of the Gondwa- northern Neuquen Basin throughout the Early Creta- nan-Patagonian ﬁeld (Fig. 10), which could be coming ceous (Fig. 11). A combination of thickening and coars- from the south or west (Franzese & Spalletti, 2001; Ra- ening of sandstone facies within the Agrio Formation to mos, 2008; Naipauer et al., 2012). Jurassic- and Creta- the south (Lazo et al., 2005; Archuby et al. 2011) imply a ceous-age detrital zircon populations require a western source of clastic material somewhere in the northern Pata-

Fig. 10. Detrital zircon U–Pb data plot- ted as relative age-probability curves (red thick line) and age histograms (blue rect- angles) for the Agrio, Huitrın and Dia- mante formations and the Malargue€ Group. Important source areas are high- lighted in different colours. The number of grains in each sample is denoted with n = x below sample name. Maximum depositional age is denoted immediately adjacent to the youngest peak on samples with syndepositional zircons.

© 2014 The Authors 16 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

arc grains as well as a mix of older Palaeozoic and Protero- zoic grains from the Palaeozoic accretionary prism (Will- ner et al., 2008).

Huitrın Formation Description: The maximum depositional age of the Huitrın Formation, based on the youngest detrital zircon component, is 124 1.3 Ma (n = 2, Fig. 10). The age distribution for the Huitrın Formation contains minor peaks at ca. 150 and 200 Ma. The majority of grain ages fall between 250 and 500 Ma with prominent components at 260, 315, 375, 420 and 480 Ma. Minor Proterozoic peaks (ca. 1000 Ma and a few older grains) are also present (Fig. 10). Interpretation: The detrital zircon spectra of the Hu- itrın Formation differs from that of the Agrio Formation with a stronger Choiyoi signature (250–280 Ma), along with older (ca. 350–400 Ma) Gondwanan-Patagonian ages and fewer early Palaeozoic and Proterozoic grains (Fig. 10). The Huitrın Formation was likely receiving material dominantly from western sources, and noticeably less from southern sources. The change from a dominantly southwestern source in the Agrio Formation to western sources in the Huitrın Formation requires a palaeodrainage reorganization which may be related to uplift of the volcanic arc, aridiﬁcation and restriction of the Neuquen Basin from open marine Paciﬁc Ocean water in Aptian time (Figs 10 and 11).

Lower Diamante Formation Description: The Diamante Formation shows variability in detrital zircon age distributions between different locali- ties. The basal Diamante Formation in the LA area (Fig. 10) has a nearly identical age distribution to the underlying Huitrın Formation except for a signiﬁcantly younger, 97 2 Ma, maximum depositional age (n = 5, Fig. 10). The detrital zircon age spectra dominantly fall between 250 and 500 Ma, with a few Proterozoic grains. Samples from the lower Diamante Formation in the MW area contain a signiﬁcant number of grains spanning ca. 230–300 Ma with almost no older grains and a very minor population of Mesozoic grains. Samples of the lower Diamante Formation from the MW area have a different signature from the samples in the LA area. Most grains from the MW sample are around ca. 230–300 Ma with a very minor population of Fig. 11. Palaeogeographic reconstructions showing generalized Mesozoic grains that record smaller input from the Juras- depositional and tectonic environments during deposition of sic to Cretaceous Andean arc. Overall, ages typical of the each unit (130 Ma, Agrio Formation; 125 Ma, Huitrın Forma- Choiyoi and late Gondwanan-Patagonian source areas tion; 100 Ma, unconformity between the Huitrın and Diamante dominate this sample. formations; 95 Ma, Diamante Formation; 65 Ma, Malargue€ Interpretation: The detrital zircon spectra recorded Group) described in this paper. from the lower Diamante Formation sample collected in the LA area require that the source material was either gonian Massif. The southern signature (Gondwanan-Pat- directly recycled from the Huitrın Formation, or that the agonian) was combined with a minor western source, Diamante Formation was receiving sediments from the which contained a combination of Jurassic–Cretaceous same western source area. High plagioclase to total feld-

© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 17 E. A. Balgord and B. Carrapa spar ratios (generally greater than .8; Fig. 9; Table 1) the lower Malargue€ Group, and a younger one, at imply a volcanic source terrane (Dickinson, 1970), which 61.7 2.1 Ma which is likely synchronous with deposi- is difficult to reconcile with an eastern source, unless Cho- tion of the upper Malargue€ Group. iyoi igneous province rocks were exposed in the vicinity Interpretation: The age range between 60 and 72 Ma of the future San Rafael uplift, or if arid conditions implies continuous activity and input from the volcanic allowed for the preservation and recycling of plagioclase arc in the latest Cretaceous and early Cenozoic. The between the Huitrın and Diamante formations. The near- older, ca. 300 Ma component, is consistent with what is depositional zircon ages from the lower Diamante Forma- observed in the lower Diamante Formation from the MW tion indicate an active volcanic source. Palaeozoic–lower area suggesting that a similar drainage pattern continued Triassic basement blocks uplifted immediately to the west in the MW area during deposition of the Diamante For- of the MW section (Fig. 6) may have been potential mation and Malargue€ Group. sources for the pronounced Choiyoi and late Gondwanan- Patagonian signature; however, there is no evidence that they were exposed in the Late Cretaceous. DISCUSSION Basin evolution in the Malargue€ area Upper Diamante Formation Our new sedimentological and provenance data show two Description: The detrital zircon signature of the upper- major changes in depositional environments: (i) the mar- most Diamante Formation sample in the BB area is dis- ine-marginal marine/evaporite transition between the tinct (Fig. 10). It does not have any grains near Agrio and Huitrın formations; and (ii) the unconformity depositional age, or any grains younger than 200 Ma. and change to fluvial-alluvial environment in the Dia- There are major age components ranging from 200 to mante Formation. The unconformity between the Hu- 280 Ma and another at ca. 400 Ma. There are also signifi- itrın Formation and the Diamante Formation records a cant Proterozoic components that are not present in the significant period (25 Myr) of erosion and/or nondeposi- two lower Diamante Formation samples. tion in the Neuquen Basin. In a foreland basin system, the Interpretation: The strong Choiyoi and Gondwanan- only depozone where uplift is expected is the forebulge Patagonian signatures may be locally sourced immediately (Decelles & Giles, 1996). Palaeocurrents measured in the to the west of the BB area (Figs 4 and 6). The abundance Diamante Formation show a shift in palaeoflow from of rhyolitic fragments in the conglomerates likely indi- west-directed in the lower Diamante Formation to east- cates local Choiyoi basement as a source. The re-emer- directed in the middle and upper Diamante Formation. gence of Proterozoic grains either implies input from a When considered in a foreland basin setting, the change northeastern source or recycling from older sedimentary in flow direction is likely a product of forebulge migration units. at ca. 90 Ma. During deposition of the lower Diamante Formation, the forebulge was a topographic high, supply- ing sediment to the distal foredeep. As the foreland basin Lower Malarg€ue Group system moved eastward, the majority of the sediment Description: The detrital zircon signature of the lower, came from the fold-and-thrust belt to the west and was marine, Malargue€ Group is a unimodal 69.1 1.9 Ma deposited in the proximal foredeep (Fig. 11). A switch peak; the signature also contains a few older grains, none from eastern to western sources is supported by a change of which form a distinct population. in sandstone composition containing basement and volca- Interpretation: This unimodal, ca. 70 Ma, population is nic lithic detritus in the lower Diamante Formation to syndepositional and derived from the volcanic arc. This more sedimentary rock sources in the middle and upper sample does not provide any information about drainage Diamante Formation. Provenance data from this study do patterns in the Maastrichtian, but it indicates that there not show a major change in dominant sediment sources was significant volcanic activity during Malargue€ Group between the deposition of the Aptian Huitrın Formation deposition that did not occur during deposition of the and the Cenomanian Diamante Formation. The similarity upper Diamante Formation as a result of the continued in detrital zircon signatures can be explained by the uplift eastward migration of the volcanic arc or higher volcanic and erosion of Huitrın Formation stratigraphy which was activity during that time (Fig. 11). then recycled into the lower Diamante Formation. A large shift in provenance is observed instead between the lower and upper Diamante Formations, which is here inter- Upper Malarg€ue Group preted as the product of a change from eastern to western Description: The detrital zircon signature of the upper sources. The combination of palaeocurrent, sedimentary Malargue€ Group is dominated by two main peaks, one at petrography and detrital zircon analysis indicate an east- ca. 300 Ma and another ca. 65 Ma (Fig. 10). Upon clo- ern and southern – dominantly volcanic and igneous base- ser inspection, (Fig. 10) the younger ca. 65 Ma age com- ment – source for the lower Diamante Formation and a ponent likely represents two subgroups of grains, one at western – composed of a combination of volcanic, base- 69.5 2.3 Ma, similar to the syndepositional peak from ment and sedimentary rocks – source for the middle and

© 2014 The Authors 18 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin upper Diamante Formation. Growth structures docu- Diamante formations) is preserved in the Cordillera Prin- mented by Mescua et al. (2013) in Upper Cretaceous red cipal all the way to the southern San Juan Province (31°S; beds in Chile imply movement on a thrust fault immedi- Fig. 2). Although the stratigraphy is the same, it is ately to the west of the stratigraphic sections measured in unclear if the depositional system remained similar to the the Malargue€ area for this study (Fig. 2) and support our north. Growth structures identified in Cretaceous stratig- model of foreland basin initiation and migration. raphy remain the only documented evidence of Creta- According to Giambiagi et al. (2012), there has been a ceous shortening in the Cordillera Principal at 33°S (Orts minimum of 20 km of shortening in the Malargue€ area & Ramos, 2006). Mpodozis et al. (2005) and Arriagada and the location of the thrust front migrated ca. 180 km et al. (2006) present evidence for mid to Late Cretaceous since the Cretaceous when it was located in Chile (Fig. 2; shortening in the Cordillera de Domeyko in the Atacama Mescua et al., 2013) leading to an estimated 200 km of Basin (Fig. 1); although these units lack good age control, flexural wave migration of the foreland basin depozones they potentially represent the northern equivalent of the since the Cretaceous. The geomorphic expression of the Diamante Formation. modern forbulge as documented by Niviere et al. (2013), At 26°S, stratigraphy in the Eastern Cordillera and Sal- has a length of 150 km and a height of 250 m and sits in ta region, ca. 100 km east of the Cordillera de Domeyko, the La Pampa High (Fig. 1; Chase et al., 2009). Based on record foreland basin initiation and migration in the the aforementioned shortening values and the modern Paleocene (Fig. 1; Decelles et al., 2011), and active rifting location of the Andean forebulge at 35°S (Chase et al., was occurring in the back-arc region during the Creta- 2009; Niviere et al., 2013), it is plausible that the Ma- ceous (Grier et al., 1991). The ca. 30 Myr age discrepan- largue€ area was in the forebulge position during the Late cies between northern Argentina and central-southern Cretaceous, and as the flexural wave moved eastward the Argentina could either be a result of (i) different boundary depositional system migrated into the foredeep and conditions along the Pacific margin during the Creta- wedgetop depozones. Eastward migration of the thrust ceous, potentially due to multiple oceanic plates with vari- front seems to have occurred mainly in the Miocene in able convergence vectors; (ii) a lack of precise age control association with potential flattening of the Nazca Plate, on Cretaceous terrestrial units along-strike; or (iii) which led to the uplift of the San Rafael Block (Pascual expected changes in timing of foreland basin initiation as et al., 2002), whereas the amount and magnitude of Cre- the thrust front migrates east. taceous and Palaeogene shortening is less well documented and understood. Thus, if our foreland basin Global context model is correct it raises issues associated with the amount and timing of shortening and thrust front migration, Recent plate reconstructions since 200 Ma by Seton et al. which will need to be considered in future studies. (2012) appear to be in agreement with Early Cretaceous shortening in the central and southern Argentinian An- Along-strike variation in timing of foreland des. Rifting in the South Atlantic progressed from south basin initiation to north in the Late Cretaceous (Daly et al., 1989) and was associated with substantial intracontinental deforma- Initial uplift of the South American Cordillera ranges tion within Africa and South America (Unternehr et al., from Late Cretaceous to Palaeogene along the length of 1988; Nurnberg€ & Muller,€ 1991; Eagles, 2007; Torsvik the Andes (Fig. 1). Our study suggests that the unconfor- et al., 2009; Moulin et al., 2010). The South Atlantic mity between the Huitrın and Diamante formations Ocean was opening at the latitude of Malargue€ around marks the passage of the forebulge and that the Diamante 130 Ma with absolute motion of South America towards Formation was deposited in the foredeep implying that the northwest (Fig. 4). Complex spreading patterns in shortening in the Malargue€ area began by at least the west Pacific Ocean led Seton et al. (2012) to add two 97 2 Ma (maximum age). This age is in relative agree- plate segments along the western margin of South Amer- ment with the ca. 92 1 Ma (Fildani et al., 2003; ica (Chasca and Catequil plates; Fig. 4), breaking up the Romans et al., 2010) timing of foreland basin initiation in Farallon Plate into multiple segments in the Late Creta- the Patagonian Andes to the south. In the central ceous. The Chasca Plate appears to subduct nearly Neuquen Basin, timing of foreland basin initiation is orthogonally beneath the western margin of South either interpreted to coincide with the base of the Neu- America at ca. 120 Ma (Fig. 4; Seton et al., 2012; Malo- quen Group (100 8; Tunik et al., 2010; Di Giulio ney et al., 2013). Apparent motion between the Chasca et al., 2012) or with the unconformity between the Rayoso and South American plates looks to be convergent Formation and the Neuquen Group (Cobbold & Rossello, between 120 and ca. 80 Ma, which would be consistent 2003), anywhere from 110 to 90 Ma. These ages also with shortening beginning in the Malargue€ area in the correlate with timing of foreland basin initiation in the Late Cretaceous along with a migration of depozones. By Malargue€ area. the Cretaceous–Palaeogene boundary there is no longer Timing relationships north of the Malargue€ area are differential movement between the Chasca and Catequil less well known. The Neuquen Basin narrows signifi- plates, so the Farallon Plate has a single vector along the cantly to the north, but similar stratigraphy (Huitrın and entire margin of South America (Fig. 4). The Farallon

Plate has a similar absolute motion to South America in contemporaneous with shortening to the north requiring a the latest Cretaceous to early Palaeogene which should plate-scale mechanism. If shortening to the north does not have led to either a neutral boundary or extension (Pardo- begin until the Palaeogene, the transition from extension to Casas & Molnar, 1987; Seton et al., 2012), and may compression requires a regional-scale mechanism and a explain why the Malargue€ area stays in the foredeep posi- sharp boundary between older (ca. 95 Ma) ages in the tion during the entire deposition of the Diamante Forma- south and younger (ca. 65 Ma) ages to the north. tion and Malargue€ Group. However, the central Andean rotation pattern of Arriagada et al. (2008) still predicts dominantly orthogonal convergence in the Cenozoic, so ACKNOWLEDGEMENTS more work will need to be done to fully constrain plate boundary condition during Andean mountain building. The authors acknowledge contributions from the Exxon- The next two major pulses of shortening coincide with Mobil COSA project, National Geographic Young high convergence velocities in the Eocene and Miocene Explorer Grant, and the Geological Society of America (Pardo-Casas & Molnar, 1987) and likely caused major Student Research Grant. The Arizona Laserchron center uplift and an eastward migration of the thrust front, is in part supported by NSF-EAR 1032156 and NSF- which incorporated the Malargue€ area into the thrust belt, EAR 0929777. Thoughtful discussions with P. DeCelles, uplifting and potentially eroding much of the foreland A. Cohen, P. Kapp and G. Gehrels greatly helped this basin stratigraphy. manuscript. Laura Giambiagi, Patricia Alvarado and Marco Sobol are thanked for their help with logistics and fieldwork. Helpful reviews from C. Arriagada, R. Spi- CONCLUSIONS kings, A. Fildani, and Editor S. Castelltort greatly bene- fited this manuscript. The Mesozoic to early Cenozoic history of the Malargue€ area records the transition from thermally to flexurally controlled subsidence by at least 97 2 Ma. This SUPPORTING INFORMATION change coincides with a shift in facies from marginal marine to continental sedimentation, and an increase in Additional Supporting Information may be found in the clastic input into the northern Neuquen Basin. Palaeo- online version of this article: currents indicate a change from east- to west-derived sediment between the lower and middle Diamante For- Appendix S1. Tables with point counting information. mation which is interpreted as a change in sediment (A) Abbreviations used and equations for recalculated val- source area from the forebulge to the fold-and-thrust ues shown in Table 1. (B) Raw point count data. belt, requiring forebulge migration between ca. 95 and Appendix S2. Detrital zircon extended methods (A) 80 Ma (Fig. 11). Following the Late Cretaceous migra- and datatables (B). tion of the foreland basin system, it remains relatively stable during deposition of the Malargue€ Group in the latest Cretaceous and early Cenozoic, however the volcanic arc appears to increase in activity between 70 and REFERENCES 60 Ma (Figs 10 and 11). Following the deposition of the € AGUIRRE URRETA, M.B. & RAWSON, P.F. (1997) The ammonite Malargue Group, continued eastward migration of the sequence in the Agrio Formation (Lower Cretaceous), Neu- fold-and-thrust belt and volcanic arc in the Eocene and/ quEn Basin, Argentina. Geol. Mag., 134(4), 449–458. or Miocene cause uplift and erosion of much of the AGUIRRE-URRETA, M.B., TUNIK, M., NAIPAUER, M., PAZOS, P., € Upper Cretaceous foreland basin stratigraphy. OTTONE, E., FANNING,M.&RAMOS, V.A. 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© 2014 The Authors 20 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cretaceous strata of the northern Neuquen Basin

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