1. Introduction

 

The Valanginian positive δ¹³C excursion was first recorded by Cotillon and Río (1984) in the Gulf of Mexico (DSDP Site 535). It was then described by many authors in different basins throughout the northern hemisphere (Lini et al., 1992; Weissert et al., 1998; Hennig et al., 1999; Melinte and Mutterlose, 2001; Bartolini, 2003; Erba et al., 2004; Duchamp-Alphonse et al., 2007; Price and Nunn, 2010; Charbonnier et al., 2013; Meissner et al., 2015; Silva-Tamayo et al., 2016), and subsequently in the Neuquén Basin (Aguirre-Urreta et al., 2008; Gómez Peral et al., 2012) (Fig. 1), in the southern hemisphere. This global Valanginian positive δ¹³C anomaly was chronologically situated by Lini et al. (1992) on the basis of calcareous nannofossils. The excursion (~139 My) started in the late early Valanginian Busnardoites campylotoxus Zone and ended in the early late Valanginian Saynoceras verrucosum Zone (Weissert et al., 1998; Hennig et al., 1999; Van de Schootbrugge et al., 2000; Föllmi, 2012). 

 

Fig. 1. To the left, the Neuquén Basin, located in west-central Argentina. Record of the Weissert Event in the Neuquen Basin:  Cerro La Parva area (Aguirre Urreta et al., 2008),  Buta Ranquil area (Gómez Peral et al., 2012 ). To the right, the study area, with the location of the studied sections:  Río Salado,  Puesto Loncoche and  Cuesta del Chihuido.

 

The origin and features of this anomaly has been the subject of controversy. Lini et al. (1992) and Erba et al. (2004) link it to an increase in atmospheric CO₂ as a result of volcanism (especially the basalts of the Paraná-Etendeka emissions), which would have led to global warming. This hypothesis is supported by the mineralogical studies of Duchamp-Alphonse et al. (2011), who analyzed an increased kaolinite content related to a warm, humid climate in sedimentary deposits. On the other hand, Van de Schootbrugge et al. (2000), Pucéat et al. (2003), and Price and Mutterlose (2004) propose a cold climate for the anomaly, based on palaeotemperatures derived from δ¹⁸O data in belemnites and fish tooth enamels. Such hypothesis is supported by glendonites (Kemper, 1987; Tarduno et al., 2002), dropstones, as well as by the nature of calcareous nannofossils (Melinte and Mutterlose, 2001).

Erba et al. (2004) link the positive δ¹³C excursion with the presence of black shales rich in organic carbon in the Southern Alps and in the Pacific Ocean, reinterpreting the Valanginan anomaly as an ocean anoxic event called the Weissert Event. Westermann et al. (2010) propose that the anoxic conditions would be circumscribed only to a few regions. Recently, Föllmi (2012) interprets the Valanginian anomaly as a short period (<1 My) of change in climate conditions from arid to humid in a warm context, classifying the Weissert Event as an episode of environmental change.

The combination of a decrease in shallow-marine carbonate production coupled with the enhanced burial of organic matter on the continent has been suggested to explain the global positive carbon excursion (Westermann et al., 2010). Enhanced detrital and dissolved continental fluxes to the ocean probably boosted marine primary productivity in marginal marine and epicontinental settings (Föllmi, 1995; Duchamp-Alphonse et al ., 2007).

The main objective of this contribution is to provide the first record of δ¹³C data from the Tithonian-Valanginian interval of the Mendoza Shelf (Neuquén Basin), with special emphasis on the positive excursion of the Valanginian. The palaeogeographic position of the section is of particular interest, since it was deposited in a near-shore setting. Palaeotemperature data obtained from δ¹⁸O in calcareous fossils is also shown. Moreover, the integration of stable isotope data and the analyzed sedimentological aspects of the outcrops studied can contribute to the understanding of the climatic context of the Valanginian carbon excursion registered globally.

2. Geological setting

2.1. Neuquén Basin

The Neuquén Basin is located in west-central Argentina, and it constitutes the main oil basin in the country (Fig. 1). Two main depositional areas can be recognized in the Neuquén Basin: the Neuquén Embayment to the south, and the narrow Mendoza Shelf to the north (Doyle et al., 2005). The development of the Mendoza Shelf is favoured by the obliquity between the Andean arc and the axis of the basin, which is oriented NW-SE, and it is characterized by a significant reduction in the width of the basin (Spalletti et al., 2000; Doyle et al., 2005). In this area, an important sedimentary succession (4,000 m) was deposited during the Upper Jurassic-Lower Cretaceous interval, initially called “Mendociano” by Groeber (1947) and currently known as “Mendoza Group”. It comprises the Vaca Muerta, Chachao and Agrio formations of the Mendoza Shelf (Mombrú et al., 1976) (Fig. 2).

 

Fig. 2. Chronostratigraphic scheme of the Neuquén Basin (Upper Jurassic- Lower Crateceous). Based on Groeber (1946) and Gulisano and Gutiérrez Pleimling (1994).

 

The Vaca Muerta Formation was originally defined by Weaver (1931) to designate a set of Tithonian layers consisting of dark grey calcareous shales (Spalletti et al., 2015). This unit represents the stratigraphic succession of the Mendoza Group with the largest extension, the highest degree of lithological uniformity and continuity and of huge regional economic importance due to its potential oil and phosphate content (Leanza et al., 1977). The Vaca Muerta Formation is underlain by the continental deposits of the Tordillo Formation, and the contact between both formations is marked by the Tithonian transgression (Leanza, 1981). In the Mendoza Shelf, overlying the Vaca Muerta Formation, marine carbonates and shales of the Chachao Formation are recorded. The Vaca Muerta Formation was deposited between the end of the early Tithonian (Virgatosphinctes mendozanus Ammonite Zone) (Riccardi, 2015) and the beginning of the early Valanginian (Neocomites wichmanni Ammonite Zone) (Kietzmann et al., 2015).

The Chachao Formation (Groeber, 1946) consists of bioclastic wackestones, packstones, grainstones, floatstones and rudstones. This unit overlies the Vaca Muerta Formation and is overlain by the Agrio Formation, both with concordant contacts (Mombrú et al., 1978; Carozzi et al., 1981). The Chachao Formation was deposited during the early Valanginian and part of the late Valanginian. It contains the Lissonia riveroi Ammonite Zone (beginning of the early Valanginian) (Aguirre-Urreta and Rawson, 1997; Palma and Lanés, 2001) and the Olcostephanus atherstoni Ammonite Zone (end of the early Valanginian and beginning of the late Valanginian) (Legarreta and Kozlowski, 1981; Aguirre-Urreta et al., 2011).

2.2. Geological setting of the studied dataset

Fieldwork was carried out in three localities of the Mendoza Shelf, near the city of Malargüe (Fig. 1): Río Salado, Puesto Loncoche and Cuesta del Chihuido. In these localities, the sedimentary succession studied is composed of mixed ramp deposits (Mitchum and Uliana, 1986; Legarreta and Uliana, 1991; Spalletti et al., 2000; Kietzmann et al., 2008; among others) of the Vaca Muerta and Chachao formations (Tithonian-Valanginian).

2.2.1. Río Salado area

In the Río Salado area (35 ° 11’ S and 69 ° 46 ’ W), a mixed succession of sedimentary rocks with a vertical coarsening-upward trend has been described (Doyle et al., 2005). This unit consists of a 176-m-thick succession composed of an alternation of black shales and carbonates. Carbonates are arranged in 10-70-cm-thick layers composed of bioclastic wackestones, packstones, floatstones and rudstones (Fig. 3). The formational boundaries and environmental characterization of these deposits were proposed by Doyle et al. (2005).

 

Fig. 3. Sedimentary sections of the studied localities; notice their formational boundaries and ammonites biozones. V. men: Virgatosphinctes mendozanus; P. zit: Pseudolissoceras zitteli; A. pro: Aulacosphictes proximus; W. int: Windhauseniceras internispinosum; C. alt: Corongoceras alternans; S. koe: Substeuroceras koeneni; A. nod: Argentiniceras noduliferum; S.d.: Spiticeras damesi N. w: Neocomites wichmani; L. r: Lissonia riveroi; O. atherstoni: Olcostephanus atherstoni; Silt/mud: silt/mudstone; vf/wacke: very fine sand/wackestone; m/pack: medium sand/packstone; vc/grain: very coarse sand/grainstone; gran: granule; pebb/rud/bound: pebble/rudstone/boundstone.

 

The Vaca Muerta section at Río Salado area is characterized by the presence of black shales interbedded with marls and scarce wackestones and packstones near of the base. Based on these lithological attributes and the recognized trace fossil associations, Doyle et al. (2005) interpreted this section as the result of sedimentation under an anoxic to suboxic seafloor in a basinal environment. This siliciclastic and carbonate particles may have been deposited from suspension under an anoxic to suboxic seafloor, with evidences of storm reworking at the top. The middle interval consists of black shales, marls, heterolithic beds and packstones, with primary sedimentary structures (such as lamination and hummocky cross-stratification), and deformational structures (water escape features, ball-and-pillow), all this features strongly suggest deposits of a distal outer ramp system. The upper part of Vaca Muerta Formation is characterized by black shales and marls, marking a restoration of the basinal conditions. Meanwhile, the Chachao Formation consists of floatstones, rudstones, shales and marls. The skeletal carbonates are interpreted as oysters shell beds that developed in a proximal outer ramp (Doyle et al., 2005).

2.2.2. Puesto Loncoche area

In the Puesto Loncoche area (35° 36’ S and 69° 37’ W), the studied deposits were previously described in detail by Kietzman et al. (2008). The same vertical coarsening-upward trend observed in the Río Salado section is recognized here, accompanied by a thickening-upward arrangement that culminates in a 25-m-thick oyster bank, corresponding to Chachao Formation (Fig. 3).

A detailed palaeoenvironmental analysis of the Vaca Muerta Formation in the Puesto Loncoche area proposed by Kietzmann et al. (2008) divides the unit, from base to top, into four sub-environments: basin, distal outer ramp, proximal outer ramp and distal middle ramp. The basin sub-environment is composed of black shales, massive marls and siltstones, together with laminated mudstones and wackestones deposited by decantation in anoxic-suboxic environments. The distal outer ramp consists of black shales, marls, nodular mudstones and bioclastic wackestones (massive and laminated). The fine-grained siliciclastic and mixed siliciclastic carbonate deposits were essentially accumulated from suspension fall out. The outer ramp bioclastic wackestones represent distal storm-induced layers. In the proximal outer ramp, in addition to the association described previously, the above mentioned authors identified floatstones and packstones interpreted as proximal tempestites. Finally, the distal middle ramp is composed by an association of suspension fall out deposits (shales, marls, nodular mudstones) and storm induced deposits (floatstones, massive bioclastic packstones and wackestones). Subsequently, the palaeoenvironmental interpretations were later reinforced with taphonomic, biofacies and sequence stratigraphic studies (Kietzmann and Palma, 2009; Kietzmann et al., 2014; Kietzmann et al., 2015). 

The Chachao Formation is represented, in this area (Fig. 3), by an inner ramp environment, constituted by a coarse oyster bioclastic buildup of rudstones and floatstones, which shows vertical accumulation and a lateral stacking pattern after described by Palma and Lanés (2001). Within these bodies, slight signs of reworking were recognized from the original oyster banks, and can be characterized as an autoparabiostrome according to Kershaw (1994). Palma and Lanés (2001) performed a taphofacies analysis of carbonate bodies of the Chachao Formation, describing patterns of accumulation and dividing this unit into two sections: the lower section, with a low degree of reworking of the oyster beds, and the upper section, with a progressive increase in the intensity of disarticulation, bioerosion and fragmentation of the remains, accompanied by an increase in the matrix content.

2.2.3. Cuesta del Chihuido area

In the 63-m-thick Cuesta del Chihuido study section (35°44’ S and 69°34’ W), only the top of the Vaca Muerta Formation and the entire Chachao Formation crop out (Kietzmann et al., 2015). This section shows vertical arrangement, similar to the one described in Puesto Loncoche (Fig. 3). The Vaca Muerta Formation was studied in this region by Kietzmann et al. (2015), the cusp deposits were described as an oyster intercalation forming autoparabiostromes composed of bioclastic floatstones and rudstones, bioturbated wackestones, packstones, laminated wackestones and marls. This section is interpreted as a bioclastic middle ramp to proximal outer ramp, and an oyster biostrome dominated middle ramp.

The Chachao Formation in the Cuesta del Chihuido area has been extensively studied, from both the sedimentological and palaeontological points of view (Leanza et al., 1977; Mombrú et al., 1978; Legarreta and Koszlowsky, 1981; Uliana et al., 1979; Legarreta et al., 1981; Palma and Angeleri, 1992; Palma and Lanés, 2001), as well as by means of diagenetic analysis (Carozzi et al., 1981; Palma et al., 1999; Palma et al., 2008). It was described as an accumulation of floatstones and rudstones rich in benthic fauna (serpulids, oysters, brachiopods, and occasional ammonites) (Palma and Lanés, 2001), forming thick bioclastic accumulations associated with an inner carbonate ramp, with similar characteristics to those described for Puesto Loncoche.

3. Materials and methods

3.1. Sampling

Three sedimentary sections from three selected areas were studied in detail in order to recognize the sedimentary facies, fossil content and their palaeoenvironmental features. Fifty-seven (57) samples of fossil oysters (11 from Río Salado, 29 from Puesto Loncoche and 17 from Cuesta del Chihuido) were selected from sedimentary logs, taking as the main criteria the sampling equidistance (subject to fossil presence) and the degree of fossil preservation. Samples of Aetostreon sp. were selected to obtain accurate C and O isotope data and also to avoid the variations due to the “vital effect” (Fig. 4). Moreover in order to constrain these data with previous results obtained from samples of the same genus in other localities of the Neuquén Basin (Aguirre-Urreta et al., 2008).

Samples were carefully selected based on their mesoscopic features. The best criteria for choosing the fossil material were the taphonomic characteristics (avoiding material which shows a high degree of fragmentation and corrasion), and the size (microscopic observations showed that the larger individuals had better preservation of the internal layers); therefore, specimens larger than 5 cm were selected.

 

Fig. 4. Selected samples of Aetostreon sp. with a high degree of preservation. Notice the low grade of corrasion on external walls, this taphonomic feature suggest a better conservation of the inner layers.

 

3.2. Petrography

Chips from the innermost oyster shell were covered with Au and analysed by SEM-EDS with a FEI Quanta 200 SEM (Laboratorio de Investigaciones de Metalurgia Física, Universidad Nacional de La Plata (UNLP), Argentina) with the objective of recognizing the original microtextures. In addition, sixteen thin sections in dorsoventral position were analysed under cathodoluminescence CITL Technosyn MKIII microscope of the Centro de Investigaciones Geológicas (CONICET-UNLP). Manganese emits a characteristic dull to bright luminescence during cathodic excitation of the calcitic shell layers and induces a typical orange colour, indicative of diagenetic alteration, while primary calcite sectors appear neither luminescent nor blue (Ullmann and Korte, 2015; Marshall, 1992).

3.3. Geochemistry

Twenty-eight elemental analyses were performed in ~50 mg of powdered oyster samples (12 from Puesto Loncoche, 6 from Río Salado and 10 from Cuesta del Chihuido). The dissolution of the carbonate phase (5% HNO₃ solution for 2-3 h) was analyzed by ICP-MS, using a Perkin-Elmer ICP-MS fitted with a Meinhardt concentric nebulizer of the Centro de Investigaciones Geológicas geochemistry laboratories (CONICET- Universidad Nacional de La Plata) to determine Mn and Sr concentrations in ppm. Precision and reproducibility for all elements analyzed are better than 10%, based on replicate measurements of laboratory calcite and dolomite standards.

Fifty-seven microsamples of 0.1 gr of oysters (one sample per oyster) were taken with a microdrill. The microsamples were analyzed for C and O isotopes (29 from Puesto Loncoche, 11 from Río Salado and 17 from Cuesta del Chihuido). These analyses were performed at the Núcleo de Estudos Geoquímicos-Laboratório de Isótopos Estáveis (NEG-LABISE) of the Departamento de Geología, Universidade Federal de Pernambuco, Brazil. Extraction of CO₂ gas from selected unaltered samples was performed in a high-vacuum line after reaction with 100% orthophosphoric acid at 25 °C for one day. Released CO₂ was analyzed after cryogenic cleaning in double inlet, triple-collector SIRA II or Delta V Advantage mass spectrometers and results are reported in δ notation in per mil (‰) relative to the VPDB standard. The uncertainties of the isotope measurements were better than 0.1‰ for carbon and 0.2‰ for oxygen, based on multiple analyses of an internal laboratory standard (BSC). The δ¹³C and δ¹⁸O values obtained were calibrated against the internationally accepted International Atomic Energy Association carbonate standard NBS-19.

3.4. Palaeotemperatures

Palaeotemperature values obtained from the calcite samples were estimated using the modified version of the Epstein palaeotemperature equation (Epstein et al., 1953), given by Anderson and Arthur (1983), and then shown by Pirrie et al. (2004) as:

T(°C)=16.0-4.14(δc-δw)+0.13(δc-δw)²

In this equation, δc=δ¹⁸O (VPDB) of the analyzed carbonate at 25 °C, and δw=δ¹⁸O (SMOW) of the water in which the carbonate precipitated, relative to the Standard Mean Ocean Water international standard. While δc is measured, δw has to be estimated as -1.2‰, the global average for periods of limited or no glaciation (Shackleton and Kennett, 1975). The δ¹⁸O values higher than -5‰ VPDB were considered appropriate to estimate palaeotemperatures as seen in similar works (e.g., Zakharov et al., 2011). Those higher values indicate a low degree of diagenetic alteration (Brand and Veizer, 1981) and are therefore more accurate to show original features.

4. Results

From the photomicrographs obtained by SEM, oysters show foliated growth in layers with smooth-textured surfaces (Fig. 5). Cathodoluminescence photomicrographs make it possible to distinguished luminescent and non-luminescent zones (Fig. 6). Non-luminescent areas coincide with the growth of a foliated type of oysters, and were particularly selected for isotope microsampling.

Manganese, Sr, δ¹³C, and δ¹⁸O values are shown in Table 1. The Mn/Sr ratio is below 1.5 (Table 1), except for three samples (PL88, CDC35 and CDC38, excluded from further interpretation), which are as high as Mn/Sr=6 and are considered as probably being diagenetically altered. Strontium concentrations vary between 300 and 700 ppm, very close to the data recorded by Korte et al. (2009) and Korte and Hesselbo (2011) for Jurassic oysters, with anomalously low values of up to 135 ppm.

Manganese concentrations reach 100 ppm on average, with some values of up to 282 ppm, which are also similar to those obtained by Korte et al. (2009) and Korte and Hesselbo (2011) for Jurassic oysters.

The δ¹³C values vary between -4.93 and 2.66‰ VPDB, while δ¹⁸O values are between -1.92 and -4.76‰ VPDB, with anomalous values of up to -11.56‰ VPDB (samples CDC 23 and 35-41, excluded from further interpretation (Table 1).

 

Fig. 5. Scanning electron microscopy images of oysters from the Puesto Loncoche sedimentary succession. It is possible to observe the “smooth textured” surfaces.

 

 

Fig. 6. Cathodoluminescence microscope images of oyster samples. Luminescent (right) and non-luminescent (left) areas are recognized; the non-luminescent areas match the foliated growth of oysters.

 

4.1. Isotopic preservation

The degree of isotopic preservation was evaluated considering the following parameters: (i) preservation of the original valve microtextures (Fig. 5); (ii) presence of non-luminescent areas (Fig. 6); (iii) δ¹⁸O≥ -5‰ VPDB; (iv) lack of correlation between δ¹³C versus Mn/Sr (≤1.5), δ¹³C versus Sr (ppm), δ¹⁸O versus Mn (ppm) and δ¹³C versus δ¹⁸O.

Selected layers with smooth-textured surfaces in oysters with foliated growth (observed by SEM) are considered as diagenetically unaltered (Korte and Hesselbo, 2011; Ullmann and Korte, 2015). Similarly, non-luminescent areas, which coincide with those of foliated growth, indicate a better degree of preservation, as mentioned above.

The data obtained from Mn and Sr showed there was no relative Sr impoverishment or Mn enrichment produced by diagenetic alterations in most of the samples, with three exceptions (samples PL 88, CDC 35 and CDC 38, Table 1) which show higher values of Mn, and depleted Sr, which can be associated with probable diagenetic alteration. Furthermore, the degree of correlation between the different variables was calculated considering the values of stable isotopes and Mn and Sr concentrations (Fig. 7 and Table 2).

The lack of covariation between δ¹³C and Mn/Sr (Fig. 7a and Table 2) and δ¹³C versus Sr (Fig. 7b and Table 2) for the three localities studied suggest a high degree of isotopic preservation for the stable carbon isotopes (Marshall, 1992). Furthermore, the diagram of δ¹⁸O versus Mn shows no correlation in the Puesto Loncoche and Cuesta del Chihuido localities, while in the Río Salado area it shows a slight positive correlation (Fig. 7c and Table 2).

 

Fig. 7. Isotopic and element cross plots: a. Mn/Sr versus δ13C; b. Sr versus δ13C; c. δ18O versus Mn; and d. δ13C versus δ18O.  Río Salado samples,  Puesto Loncoche samples and  Cuesta del Chihuido samples.

 

4.2. Chemostratigraphy

The δ¹³C values were represented in curves parallel to the sedimentary logs in order to compare the segments of the same age from the three localities under study (Fig. 8). The δ¹³C curve for the Río Salado sector shows values obtained from the uppermost strata of the Chachao Formation, taking into consideration the presence and preservation of the fossil samples. In Puesto Loncoche, the δ¹³C curve represents the complete section; however, data from the basal section of the Vaca Muerta Formation is not represented due to the absence of fossils of the genus Aetostreon sp. In Cuesta del Chihuido, the δ¹³C curve represents the complete section of the Chachao Formation and the upper part of the Vaca Muerta Formation.

 

Fig. 8. Sedimentary sections with the δ13C curves of the three areas under study (Río Salado in green, Puesto Loncoche in red and Cuesta del Chihuido in blue). Towards the top of the sequence, a carbon positive excursion can be recognized.

 

The δ¹³C values from the early Tithonian (V. mendozanus Ammonite Zone) to the early Valanginian (N. wichmanni Ammonite Zone) range from slightly negative to ~0‰, with a single value that reaches 1‰. In the early Valanginian-early late Valanginian succession (O. atherstoni Ammonite Zone), a marked positive excursion in δ¹³C values is recognized throughout the three studied sections, ranging between 0.94 and 2.53‰ (Río Salado), from 1.16 to 2.66‰ (Puesto Loncoche) and from 0.91 to 2.13‰ VPDB (Cuesta del Chihuido), with a decrease towards the upper sedimentary section (top of the Chachao Formation).

Estimated seawater palaeotemperatures, obtained from the δ¹⁸O values (Table 1, Fig. 9), are on average 25 °C (26 °C in Río Salado, 25 °C in Puesto Loncoche and 25 °C in Cuesta del Chihuido); this value coincides with those specified for the Neuquén Basin by Lazo et al. (2008), based on isotopic studies in fossil oysters of the same genus, and Lazo et al. (2005), on the basis of fossiliferous associations of the Pilmatué Member of the Agrio Formation located to the south of the basin, in Province of Neuquén.

 

Fig. 9. Estimated palaeotemperatures (T °C) and results of δ18O (‰VPDB) of samples from the three sedimentary sections.  Río Salado samples,  Puesto Loncoche samples and  Cuesta del Chihuido samples.

 

5. Discussion

The C and O-isotope chemostratigraphic curves compare variations from three different sections of the Chachao Formation (Fig. 7), Mendoza shelf area, which are well correlated with previous data presented by Aguirre-Urreta et al. (2008) and Gómez Peral et al., (2012) from other localities of the central area of the Neuquén Basin. Aguirre-Urreta et al. (2008) mentioned such a positive carbon anomaly (+2-3‰ VPDB) from the analysis of oysters from the Cerro La Parva locality (37º15’30’’- 70º26’30’’), characterized by the ammonite subzones of O. (O.) atherstoni-Karakaschiceras attenuatum (O. atherstoni Ammonite Zone). Subsequently, Gómez Peral et al. (2012) obtained a δ¹³C curve with values that on average are of +1.6‰ with a positive excursion of +2.9‰. This analysis of oysters and micrite corresponds to deposits from Buta Ranquil (37º06’00’’- 69º49’00’’) characterized by the subzone of K. attenuatum (Schwarz et al., 2011). Therefore, the positive carbon anomaly recorded in O. arthestoni Zone deposits is clearly established in different areas of the Neuquén Basin, from the Mendoza Platform (this study) to the central area of the basin (Aguirre-Urreta et al., 2008; Gómez Peral et al., 2012).

According to Aguirre-Urreta and Rawson (1997) and Aguirre-Urreta et al. (2005), the ammonite faunas of O. (O.) atherstoni-K. attenuatum correlates with the B. campylotoxus to S. verrucosum Ammonite Zonedescribed for the Mediterranean Province, where the positive carbon anomaly was defined by Weissert et al. (1998) and Hennig et al. (1999), among others (Fig. 10).

 

Fig. 10. Chemostratigraphic correlation between δ13C curves and biozones of the Mendoza Shelf in Río Salado (green), Puesto Loncoche (red) and Cuesta del Chihuido (blue) here obtained, with curves reported for the Tethys (Hennig et al., 1999) and other localities of the Neuquén Basin in Cerro La Parva (Aguirre-Urreta et al., 2008) and Buta Ranquil (Gómez Peral et al., 2012).

 

The origin of the carbon positive anomaly has been the cause of many controversies (Lini et al., 1992; Weissert et al., 1998; Hennig et al., 1999; Melinte and Mutterlose, 2001; Bartolini, 2003; Erba et al., 2004; Duchamp- Alphonse et al., 2007; Aguirre-Urreta et al., 2008; Price and Nunn, 2010; Gómez Peral et al., 2012; Charbonnier et al., 2013; Meissner et al., 2015; Silva-Tamayo et al., 2016). This contribution provides new C and O isotope data, as well as a record of the palaeotemperature data, which makes it possible to understand better the palaeoenvironmental context, considering the sedimentological and palaeontological features of the studied deposits.

The isotopic results indicate an average seawater palaeotemperature of 25º C, indicating a warm context for the Tithonian- Valanginian interval in the Neuquén Basin, which is consistent with data provided by Lazo et al. (2005, 2008) for other locality in the same basin. Duchamp-Alphonse et al. (2011) also proposed warm temperatures based in studies of kaolinite content of Valanginian sequences. Lini et al. (1992) and Erba et al. (2004) arrived at the same conclusion but linked to increase in pCO₂ from volcanism, especially the Paraná-Ethendeka continental flood-basalts. In the analyzed sections there is no evidence of vertical changes in temperature, so the enrichment in the δ¹³C ratio to the top of the succession cannot be related to sudden changes in temperature.

Furthermore, black shales rich in organic matter or other evidence of anaerobic conditions are absent in the studied sections, which make to avoid a connection of the δ¹³C anomaly with an ocean anoxic event (OAE).

Föllmi (2012) postulated that the δ¹³C anomaly was in response to a change in the Cretaceous climate from warm and dry to warm and wet, which had occurred in the mid-Valanginian (Busnardoites campylotoxus Zone- Saynoceras verrucosum Zone, in the Tethys area). Among other factors, this variation in humidity regime would have produced a significant increase in the detrital input from the continent to the sea, with the consequent drowning of the carbonate platform (Westermann et al., 2010). The fact that the δ¹³C anomaly is located just on the top of the buildup of Chachao Formation, reinforce the approach that this ultimate demise was in response to the climatic change suggested by Föllmi (2012). In this sense, Gómez Peral et al. (2012) showed a C isotope curve for Mulichinco Formation, located in a deeper portion of the basin (north of Neuquén). They place the Valanginian positive δ¹³C anomaly in the top of the unit, which coincides with the basal siliciclastic parasequence immediately above the carbonate bodies that characterize the middle section of Mulichinco Formation. Furthermore, it is noticeable that the positive δ¹³C excursion in the Neuquén Basin occurs as a regional event, which can be correlated with an increase in the detrital supply to the marine system, and as consequence this cause the cease in the development of the carbonate deposits. Therefore, it should not be ruled out, this event reflects a change towards higher humidity conditions as suggested by Föllmi (2012).

On the other hand, in the studied sections of the basin represented in the Mendoza Shelf area, the δ¹³C anomaly shows no correlation with a facies change. In this case, the anomaly is recorded at the top of the biogenic carbonate bodies of the Chachao Formation and it is considered as signal of its imminent collapse.

6. Conclusions

Based on the foregoing, it can be concluded that:

The samples of Aetostreon sp. selected for C-O isotope analysis show a high degree of preservation validated by geochemical, cathodoluminescence and scanning electron microscopy studies.

The water palaeotemperature estimations for the Mendoza Shelf in the Neuquén Basin area during the Tithonian-Valanginian indicate a warm period, with seawater temperatures around ~25 °C on average, and sudden temperature changes are not recorded.

The occurrence of the well-established mid-Valanginian positive δ¹³C excursion is documented, and it is recognized in the upper section of the Chachao Formation and characterized by the O. atherstoni Ammonite Zone in the Río Salado, Puesto Loncoche and Cuesta del Chihuido areas.

The positive δ¹³C anomaly can be temporally correlated with the previously defined chemostratigraphic studies in other localities of the Neuquén Basin and can be also related to the event defined for the Tethys area.

The absence of black shale deposits or other indications of anaerobic conditions in Chachao Formation make it impossible to link the anomaly with an ocean anoxic event in the Mendoza Shelf.

The Chachao Formation, especially the upper section constituted by thick carbonates with a wide faunal diversity, can be correlated with the contemporary deposits of Buta Ranquil, where a marked decrease in carbonate content with the consequent drowning of the carbonate platform can be observed.

Finally, it can be concluded that the Upper Chachao Formation may constitute a scenario linked to an episode of environmental change, in which the positive δ¹³C excursion must be related to a change in a warm context from arid to humid conditions. 

Acknowledgments
The authors would like to thank Dr. E. Schwarz for his comments and suggestions, which improved the manuscript significantly. Special thanks to the NEG-LABISE, Departamento de Geología, Universidade Federal de Pernambuco, Recife, Brazil. We thank D. Mártire for the preparation of the thin sections, and Lic. C. Cavarozzi and Dr. M. Pedemonte for the ICP-MS analysis. We are also very grateful to Lic. M. Zalazar for assisting in the field and for their support in cathodoluminiscence. This research is part of the Ph.D. thesis of the first author. Fieldwork and laboratory analyses and materials were financially supported by LAS (PIP 112-201101-00322), DGP (PIP 112-201501-00866) and LEGP (PIP-0134 and PICT Pres. BID 2012) grants. Helpful reviews from Dr. G.D Price, and Editor Dr. W. Vivallo greatly benefited this manuscript..

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