210 Revista de la Asociación Geológica 75 (2): 210-228 (2018)

Mineralogical and geochemical trends of the -Quintuco system in the Puerta Curaco section, Neuquén Basin

Ignacio A. CAPELLI1, Roberto A. SCASSO1, Diego A. KIETZMANN1, Fernanda CRAVERO2, Daniel MINISINI3 and Juan P. CATALANO1

1Instituto de Geociencias Básicas, Aplicadas y Ambientales de (IGeBA), Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires - Consejo Nacional de Investigaciones Científicas y Técnicas. Email: [email protected], [email protected], [email protected]; [email protected] 2Centro de Tecnología de Recursos Minerales y Cerámica. Email: [email protected] 3Shell Technology Centre Houston. Email: [email protected]

ABSTRACT

The deposits of the Vaca Muerta-Quintuco system (-) have been analyzed in the Puerta Curaco section to study the mineralogical and geochemical variations through the progressive transition from a carbonate ramp (Vaca Muerta Formation) to a mixed shelf (Quintuco Formation). A lithostratigraphic log, gamma-ray measurements (GR), mineralogical (XRD) and geochemical analysis (XRF) allowed to define five intervals: I1-I5. I1 and I2 belong to the Vaca Muerta Formation. They are dominated by carbonate sedimentation and present the highest values of organic proxies of the system. The total GR values are strongly dependent of the U content and the V/V+Ni ratio suggests alternating anoxic and euxinic conditions of the sea bottom. I3 is developed in the transition between both units and exhibits a sudden increase in the clay mineral content and a decrease in the organic proxies, associated to a sea level fall. I4 and I5 belong to the Quintuco Formation. I4 is characterized by a renewed increase in the carbonate sedimentation while I5 records the highest clastic input to the basin, evidencing the settling of the mixed shelf. In the Quintuco Formation, the V/V+Ni ratio suggests anoxia/dysoxia at the sea-bottom. In addition, a decrease in the organic proxies and an excellent correlation between total GR and K and Th is recorded. Decreasing organic matter content in the Quintuco Formation is attributed to a dilution phenome- non caused by the increasing detrital input and not by changes in the oxygenation of the sea-bottom.

Palabras clave: geochemistry, organic matter, shale, sequence stratigraphy, paleoenvironmental analysis

RESUMEN

Tendencias mineralógicas y geoquímicas del sistema Vaca Muerta-Quintuco en la sección de Puerta Curaco, Cuenca Neuquina. Se estudiaron los depósitos del sistema Vaca Muerta-Quintuco (Tithoniano-Valanginiano) en la localidad de Puerta Curaco con el objetivo de analizar las variaciones mineralógicas y geoquímicas de la transición desde una rampa carbonática (Formación Vaca Muerta) a una plataforma mixta (Formación Quintuco). Para ello, se realizaron un perfil litoestratigráfico, mediciones de radioactividad natural (GR), estudios mineralógicos (DRX) y geoquímicos (FRX), definiendo cinco intervalos: I1-I5. I1 e I2 se desarrollan en la For- mación Vaca Muerta, se encuentran dominados por sedimentación carbonática y presentan los mayores valores de proxies orgánicos del sistema. La respuesta del GR se relaciona directamente con el contenido de U y la relación V/V+Ni sugiere una alternancia entre condiciones euxínicas a anóxicas del fondo marino. I3 se desarrolla en la transición entre ambas unidades, presenta un aumento brusco en el contenido de minerales arcillosos y una disminución de proxies orgánicos, asociándose a un descenso del nivel del mar. I4 e I5 se desarrollan en la Formación Quintuco. I4 se encuentra dominada por sedimentación carbonática, mientras que I5 registra los mayores valores de input clástico, evidenciando el establecimiento de la plataforma mixta. En la Formación Quintuco, la relación V/V+Ni sugiere condiciones de anoxia/disoxia, registrándose una notoria disminución en los proxies orgánicos y una fuerte dependen- cia del GR con el K y Th. Por lo tanto, la disminución en el contenido orgánico de la Formación Quintuco es causada por la dilución debida al mayor aporte de detritos inorgánicos y no por cambios en la oxigenación del fondo marino.

Keywords: geoquímica, materia orgánica, shale, estratigrafía secuencial, análisis paleoambiental Mineralogical and geochemical trends 211

INTRODUCTION to analyze the changes underwent in a Structure and stratigraphy of deeper part of the basin and to discuss the the Puerta Curaco section The Vaca Muerta-Quintuco system (Titho- position, sequence-stratigraphic and pa- The Puerta Curaco section is located 30 nian-Valanginian) has received worldwide leoenvironmental meaning of the transition km eastern of the Chos Malal town (Fig. attention during the last decade, since the between Vaca Muerta and Quintuco For- 1b) in the northern part of the Neuquén Vaca Muerta Formation represents the mations. We use mineralogical and geo- province. It is at the southern tip of the most effective source rock of the Neuquén chemical proxies, as well as gamma-ray south-verging La Yesera anticline (Fig. Basin (Uliana and Legarreta 1993) and is measurements in order to characterize 1c), which is part of a series of N-S striking considered as a world-class unconvention- that transition, hoping this would be useful anticlines, probably generated from slips al oil and gas play (Askenazi et al. 2013 in the oil industry, since it may serve as a with east vergence (Kozlowski et al. 1996, and references therein). Although consid- reliable reference for correlation in subsur- Guzman et al. 2011). They are character- erable work has been done regarding the face operations, when information is limit- istic of the thin-skinned Chos Malal fold- main sedimentological, stratigraphical and ed to electrical logs, cuttings or stratigraph- and-thrust-belt, which was formed during paleontological aspects of both units in ic-selected cores. the Andes orogeny on a succession of Me- many locations of the basin, so far there is The aim of the present contribution is to sozoic strata (Sánchez et al. 2015). neither a study of the general mineralogi- define the main geochemical trends of the The late Kimmerdigian-Barremian strati- cal and geochemical trends nor an agree- Vaca Muerta-Quintuco system, to analyze graphic record in Puerta Curaco area in- ment on the stratigraphic position of the the transition between both units as well cludes the Auquilco, Tordillo, Vaca Muerta, boundary between the Vaca Muerta and as to characterize and discuss the main Quintuco, Mulichinco and Agrio Forma- Quintuco Formations in the northern area sedimentological and paleoenvironmental tions (Leanza and Hugo 1977). The Vaca of the Neuquén Basin, mainly because it is aspects in the Puerta Curaco section. Muerta-Quintuco system crops out in the difficult to discriminate both units in distal eastern limb of La Yesera anticline (Fig. positions of the basin (Leanza et al. 2011). 1b) with a total thickness of 716 m. It con- The Quintuco Formation has been subdi- GEOLOGICAL SETTING: cordantly overlies the fluvial, green silt- vided into carbonate-rich (Tithonian-Ber- STRUCTURE AND STRA- stones of the Tordillo Formation (Spalletti riasian) and mixed siliciclastic-carbonate TIGRAPHY and Veiga 2007) and is concordantly cov- (-Valanginian) facies (Olmos et ered by the heterolithic sandstones and al. 2002). On the Chos Malal fold and thrust Overview of the tectonic mudstones of the , belt, Kietzmann et al. (2016a) have estab- evolution of the Neuquén Basin accumulated in a tide- and wave-influ- lished the boundary between Vaca Muerta The Neuquén Basin, western Argentina enced shelf with common storms deposits and Quintuco Formations after a decrease (Fig. 1a), is a Mesozoic-Cenozoic oil- and (Schwarz 1999). of total organic carbon (TOC) and calcite gas- productive basin, with a sedimentary content, with an important increase of clay column up to 7000 m thick. The formation The Vaca Muerta-Quintuco minerals and detrital material, along with of the basin started with a rifting process system the occurrence of dolomitized levels. that lasted from the Late to the Weaver (1931) firstly described the rocks The Puerta Curaco section is a well-known Early . The basin was submitted of the Vaca Muerta Formation naming outcrop of the northern part of the basin to important thermal subsidence (Mitchum them “Tithonian marine strata” (Vaca since the work of Weaver (1931). It shows and Uliana 1985, Legarreta and Gulisano Muerta Formation in plate 1), character- a well exposed section in which the Vaca 1989) and changed to a back-arc basin ized by sandy mudstones with interbed- Muerta Formation passes transitionally to during the Early Jurassic-Early Creta- ded and sandstones up to 200 the Quintuco Formation. The sediments in ceous. A volcanic arc was progressively m thick in the southern part of the basin the transition interval are mostly composed developed to the west and the basin was (southern Neuquén Province) and by of fine-grained, black or gray marlstones, periodically flooded by the paleo-Pacific black calcareous mudstones up to 800 m limestones/dolostones and mudstones and Ocean (e.g., Legarreta and Uliana 1991). thick in the northern part of the basin (in show similar appearance. Subtle sedimen- During the late Tithonian-early Berriasian central/northern Neuquén and Mendoza tological, mineralogical and geochemical a marine transgression generated the de- Provinces). Weaver concluded that the Ti- changes reflect the well-known decrease posits of the Vaca Muerta and Quintuco thonian strata were deposited under shal- in organic matter content in the Quintu- Formations included in the Mendoza Group low to moderate water depth in the south- co Formation (e.g., Uliana and Legarreta (e.g., Mitchum and Uliana 1985, Legarreta ern part of the basin, and under moderate 1993, Stinco and Mosquera 2003, Uliana and Gulisano 1989, Spalletti et al. 2015). water depth in a slowly submerging em- et al. 2014, Fantín et al. 2014, González Finally, during the Late -Ceno- bayment, in the northern part of the basin. Tomassini et al. 2015, Kietzmann et al. zoic, the Neuquén Basin underwent the Weaver (1931) gave the name Quintuco 2016a) and the marked facies change that compressive regimen of the Andes orog- Formation to the Lower Cretaceous ma- take place in shallower, proximal parts of eny. The flexural subsidence generated a rine sediments of the Neuquén Province, the basin (Carozzi et al. 1993, Olmos et fold-and-thrust-belt with a foreland basin to which are constituted by dark grey to black al. 2002). Therefore, this section is ideal the east (Ramos 1999, Howell et al. 2005) mudstones, more calcareous and sandy Mineralogical and geochemical trends 212

Muerta-Quintuco system is characterized by a series of SE-NW or S-N prograding clinoforms (Mitchum and Uliana 1985). The shallow-water deposits in the tops- ets of the the clinoforms accumulated in mixed siliciclastic-carbonate shelfs where- as the deposits in the bottomset corre- spond to outer ramp/basin environments (Mitchum and Uliana 1985, Spalletti et al. 2000, Scasso et al. 2005, Kietzmann et al. 2014b, 2016a). Due to the high or- ganic content, slope and basinal positions (bottomset) are prone to be the best un- conventional targets (Mitchum and Uliana 1985, Desjardins et al. 2016).

Previous stratigraphic studies in the Puerta Curaco area After the work of Weaver (1931), se- quence-stratigraphic and paleontological studies on the Vaca Muerta-Quintuco sys- tem were carried out by Mitchum and Uli- ana (1985) in several localities, including Puerta Curaco area. Previously, Leanza and Hugo (1977) measured a total thick- ness of 325,5 m for the Vaca Muerta For- mation (including the Quintuco Formation) and of 370 m for the overlying Mulichin- co Formation, suggesting an early Titho- nian-early Valanginian age for the system. The Vaca Muerta-Quintuco system in Puerta Curaco area has been interpreted as a shallowing upward system, charac- terized by the outer ramp/basin deposits of the Vaca Muerta Formation, covered by the slope/basin to offshore/transition de- posits of the Quintuco Formation (Mitchum and Uliana 1985, D´Odorico 2009, Legar- reta 2009, Kietzmann et al. 2014c, Kietz- mann et al. 2016). Recently, Kietzmann et al. (2016a) have identified 5 compos- Figure 1. a) Geographic location of the Neuquén Basin, western Argentina. b) Location of the Puerta Curaco ite sequences (CS) and 15 high-ordered section on the eastern limb of La Yesera anticline. c) Geological sketch map of the La Yesera Anticline. sequences (HFS), with a general distri- bution of TOC, calcite, clays and detrital to the south. Because the rocks of the In the northern Neuquén basin the Vaca input. Weger et al. (2017) performed a se- Vaca Muerta and Quintuco Formations are Muerta Formation begins with a charac- quence stratigraphic and organic carbon similar in the northern, depocentral areas, teristic microbial level (Kietzmann et al. distribution analysis on the Vaca Muerta Leanza (1973) incorporated them in the 2014a,b, 2016a,b). The Intravalanginian Formation. In La Yesera section, 11 km to Vaca Muerta Formation. unconformity separates the system from the north of Puerta Curaco (Fig. 1b) de- The Vaca Muerta-Quintuco marine system the overlying shelf deposits of the Muli- tailed facies analysis and paleontological overlies the fluvial Tordillo Formation. This chinco Formation (Valanginian). However, observations on the Vaca Muerta Forma- contact is easily recognized on the entire in distal positions of the basin, this contact tion were done by Spalletti et al. (1999), basin, since it evidences a regional ret- is regarded as concordant and interpret- whereas the sedimentology of Quintuco rogradation pattern in response to a ma- ed as a “relative conformity” in sequence and lower Mulichinco Formations was rine transgression from the paleo-Pacific stratigraphic terms (see Leanza et al. studied by Schwarz et al. (2011). Ocean (Legarreta and Gulisano 1989). 2011 and references therein). The Vaca Mineralogical and geochemical trends 213

MATERIAL AND METHODS Mg were excluded of the analysis because except for quartz (the 4.2Ǻ peak of 21 % the low accuracy of the measurements. intensity) and total clays (the 4.5Ǻ peak of Fieldwork, litholog and Measurements lasted 90 seconds and 53 % intensity). Correction factors of 5 and sampling internal standards were measured period- 2 were respectively applied for the mea- A detailed litholog was performed on the ically to check reproducibility. Almost 70 sured surfaces of quartz and total clays entire Vaca Muerta-Quintuco system (716 marlstones and mudstones were studied (e.g., Xu et al. 2017). m). Thickness was measured bed by bed in Vaca Muerta and Quintuco Formations with a Jacob´s staff. Stacking patterns were to determine vertical geochemical varia- used to recognize composite sequences tions. V/(V+Ni) ratio was used to charac- RESULTS (CS), subdivided into high-frequency se- terize the paleoredox conditions of the en- quences (HFS) and very-high-frequency vironment (Hatch and Leventhal 1992). A Litholog and sequence sequences (or parasequences). Sequenc- cut-off value of 25% CaO was used to dis- stratigraphy es were subdivided into a transgressive criminate marlstones and mudstones from The Vaca Muerta Formation is formed by a system tract (TST) and a regressive sys- limestones. Limestones were excluded 407 m thick, marlstone/ rhythmic tem tract (RST) (e.g., Embry and Johan- from the analysis because the high con- succession (Figs. 2a, 6) and both argilized nessen 1992). Almost 70 samples were tent of CaO and CO2 produces a high dilu- (Fig. 2b) and calcitized tuffs are intercalat- collected with a sampling interval about tion of the trace-element abundance with- ed. Sub-spherical carbonate concretions 15 m. The sampling interval was reduced in the sample, masking the main trends in (Fig. 2c), with an average diameter of 10 to 0,5 m within the Vaca Muerta-Quintuco the column (e.g., Tribovillard et al. 2006). cm, are common in some beds, as well as transition interval. Mudstones, marlstones, interbedded beef-veins (e.g., Rodriguez et calcareous marlstones and limestones Mineralogy al. 2009) averaging 1 cm thick (Fig. 2d). were distinguished with field criteria as Microscope analysis: 100 thin sections of 2-7 cm thick microbial laminated bind- more calcareous strata are more resistant marlstones, limestones and mudstones stones (Bml - Kietzmann et al. 2016a,b) are to weathering. Calcareous content in mud- were analyzed under the polarizing micro- commonly observed (Fig. 2e). stones was also estimated with HCl. scope to define microfacies, and to char- The Vaca Muerta Formation comprises acterize and contrast the main textural fea- four entire CS, as well as the TST of the Inorganic Geochemistry tures between Vaca Muerta and Quintuco 5th CS (Fig. 8). Thickness of the CS may Gamma-ray measurements: Spectral Formations. vary from 50 (CS1) to 112 m (CS2). TSTs gamma-ray radioactivity was measured X-Ray diffraction: Both bulk and clay min- are richer in carbonate concretions and every 0,5 or 1 m using a RS 230 Gam- eralogy (< 2 µm) were analyzed from Vaca are usually characterized by the presence maRay Spectrometer. Individual mea- Muerta Formation marlstones and Quintu- of Bml. The Bml are usually located at the surements lasted 120 seconds on the first co Formation calcareous mudstones using lowermost part of the TSTs and have been 483 m of the system. The concentration a Phillips X-Pert PW3710 and a Bruker D2 interpreted as a marker of the sequence of each radioelement (K, U and Th) was Phaser equipments (CETMIC, Argentina). boundaries (Kietzmann et al. 2016a). determined with the equation: ni = SiKCK + Rocks were firstly gently disaggregated TSTs are characterized by a higher marl-

SiU+CU + SiThCTh+niBG, where n represents with an agatha mortar and then, randomly stone/limestone ratio, whereas the oppo- the count rate in the energy window, Si oriented powdered sampled were scanned site is true for the RSTs, which is char- sensitivity of the spectrometer for the from 3-70°, with a step size of 0,04 2θ°. To acterized by a progressive increasing on detection of the element, C the concen- obtain the clay-mineral fraction, carbonate the carbonate content towards the top of tration of the element (% K, ppm U, ppm was removed using a sodium acetate-ace- the sequence (Kietzmann et al. 2016a).

Th) and niBG the background count rate tic acid buffer solution (Jackson 1969). This stacking pattern is also observed on (IAEA 2003). Authigenic U contribution Once the solution was carbonate-free, the 13 HFSs described for the Vaca Muerta was calculated with the equation Uauthigenic organic matter was removed using hydro- Formation. The thickness of the HFS varies

= Utotal - Th/3 following Wignall and Myers gen peroxide (Moore and Reynolds 1997). from 12 to 50 m. (1988). The last 233 m of the outcrop (up- Three oriented clay aggregates, air-dried The contact between Vaca Muerta and per part of the Quintuco Formation) were (AD), ethylene-glycol solvated (EG) and Quintuco Formations has been set in the measured with SGR every 10 m, where- heated (H) to 550°C for 2 hours, were lower dolostone bed (407 m from the base as total gamma-ray (GR) measurements studied. Identification of the clay miner- of the Vaca Muerta Formation), follow- were done every 1 m with a portable Gam- alogy was done after the position of their ing lithostratigraphic criteria (Fig. 6). The ma-Ray radiometer E076-A0 (premarket basal reflections between 2-32° following Quintuco Formation is 309 m thick and prototype by CNEA). Moore and Reynolds (1997). To analyze has been subdivided into two members X-Ray Fluorescence analysis: A Niton vertical mineralogical variations through (Fig. 6): 1) The Puesto Barros Member XL3t Ultra Analyzer was used for deter- the system semi-quantitative determina- (PBMb) which comprises the lower 76 mining major and minor elements (Si, Al, tions of quartz, calcite, plagioclase and m, composed of mudstones, calcareous K, Ca, Fe, S, Ti and P) and trace elements clay-minerals were done accordingly to mudstones, limestones and dolostones. (Zr, Sr, Mo, V, Ni, Zn, Cu) in the lab. Na and the area of the main peak (100% intensity) Five tabular, orange-colored dolostone Mineralogical and geochemical trends 214 beds are prominent in the lower 30 m of with subordinated carbonates, indicating include silt-sized quartz, plagioclase and the member (Fig. 2f), which characterizes a mayor paleoenvironment change (Kietz- feldspar. the transition interval between both units. mann et al. 2016a). - Bioclastic wackestones/packstones

2) The Cerro La Visera Member (CVMb), Vaca Muerta Formation: The main mi- (WPbh) are dark grey to brown and typi- with a total thickness of 233 m. The Cerro crofacies includes laminated mudstones/ cally massive or laminated. They contain La Visera Member is characterized by a marlstones (Fl/Mrh), microbial bindstones abundant disarticulated and fragmented succession of mudstones and calcareous (Bml), radiolaritic wackestones (Wrh), intra- bivalves, including oysters, infaunal arago- mudstones (Fig. 2g), with rare limestones, clastic packstone/grainstone (PGih), bio- nitic bivalves, gastropods and ammonite fine-grained sandstones and bioclastic clastic wackestones/packstones (WPbh), fragments. Other bioclasts include benthic sandstones. The sandstone/mudstone massive to laminated calcite-replaced tuff foraminifera, Saccocoma, and occasion- ratio is markedly low in the base of the (Tch, Tcm), massive to laminated clay-al- ally serpulids. Terrigenous clasts are also member, and trends to increase towards tered tuff (Tah, Tam), and calcite-replaced low within this microfacies. the top of the unit, as well as the bioclas- lapillite (Lm, Lg). - Massive to laminated, calcite-replaced tic content. In the last 80 m of the Cerro - Laminated mudstones/marlstones (Fl, tuffs (Tch, Tcm) forming tabular carbon- La Visera Member, tabular and columnar Mrh) consist of black to dark gray, laminat- ates beds of crystalline appearance. They dolomite concretions are found (Fig. 2h). ed mudstones/marlstones with abundant are usually massive, laminated, or have The Quintuco Formation comprises the organic matter. It is one of the most abun- ripple lamination. They are composed of RST of the CS 5. A correlative conformity dant microfacies in Vaca Muerta Forma- abundant glass shards and pumiceous surface, correlated to the Intravalanginian tion. Particles include silt-sized quartz and fragments immerse in a poikilotopic cal- unconformity (Gulisano et al. 1984, Lean- plagioclase clasts, bivalves and ammonite cite cement (Fig. 3e). Glass shards and za 2009), separates it from the overlying fragments, as well as scarce calcitized pumiceous fragments are partially altered Mulichinco Formation. The Quintuco For- radiolaria (Fig. 3a). The matrix consists to clay minerals. mation is subdivided into three HFS (50 of fine silt-sized quartz and plagioclase - Clay-altered tuff deposits (Tah, Tam) oc- to 118 m thick), with TSTs characterized grains, bioclastic debris, and coccolith cur as tabular argillaceous beds, usual- by carbonate facies and RSTs enriched fragments. Cements include microgranu- ly very weathered. They consist of glass in mudstones (Kietzmann et al. 2016a). lar quartz, and granular calcite. shards and pumiceous fragments com-

The four parasequences of the HFS-16 - Microbial bindstones (Bml) consist of pletely altered to clay minerals. Silt-sized have similar arrangements of their system thinly laminated, fine-grained limestones, plagioclase grains and disperse pyrite are tracts. rich in organic matter with minor silt-sized also common. A transition interval (42 m) encompassing clasts and clays. The most evident feature - Lapillite (Lm, Lg) deposits are massive the last 7 m of the Vaca Muerta Forma- is the conspicuous submillimeter-to milli- or inverse graded. They are composed of tion and the basal 35 m of Puesto Barros meter-scale horizontal to sub-horizontal coarse sand to granule-sized pumiceous Member is defined here. This interval is lamination (Fig. 3b). Grains consist of thin- fragments, mostly replaced by calcite (Fig. characterized by the presence of black, ly shelled oysters, ammonite and fish re- 3f). They usually have a micritic matrix, laminated mudstones, interbedded dolos- mains, and radiolarians. Silt-sized quartz and contain other particles like crustacean tones and beef veins (Fig. 2f). The mud- and plagioclase clasts also occur in low pellets, bivalves, and radiolarians. stone:dolostone ratio is 30:1. Calcitized proportions. Quintuco Formation: Typical microfacies and argilized tuffs are rare. - Radiolaritic wackestones (Wrh) are well of the Quintuco Formation include lam- laminated, dark grey to black in color, and inated mudstones (Fl), dolostones (D),

Petrographic analysis contain abundant radiolarians, benthic terrigenous wackestones (Wth), bioclastic

Microfacies analysis of the Vaca Muerta foraminifera, Saccocoma, ammonites, floatstones/rudstones (FRbm), and lami- and Quintuco Formations in the Puerta transported infaunal bivalves (Fig. 3c). nated sandstones (Sh). Curaco area reveals a wide range of sedi- Terrigenous clasts occur in low proportions - Laminated mudstones (Fl) in the Quin- mentary and diagenetic processes that are (<10%) and include silt-sized quartz, pla- tuco Formation are gray to light gray. Par- essential for the understanding of this het- gioclase and feldspar. ticles are dominated by silt-to-mud-sized erogeneous unconventional shale system - Intraclastic packstone/grainstone (PGih) quartz, feldspar, and plagioclase, and (e.g., Kietzmann et al. 2014c, Kietzmann et is one of the most abundant microfacies subordinately scarce radiolarians and bio- al. 2016a,b). The Vaca Muerta Formation in Vaca Muerta Formation. It shows typi- clastic detritus. The matrix is composed contain a mixture of components of differ- cally horizontal lamination, and dominant by argillaceous components, showing a ent origins, including epiclastic, carbonate, particles are well sorted, subangular to significant increase in clay minerals with and volcaniclastic particles, where dom- angular, fine to medium sand size mi- respect to the Vaca Muerta Formation. inant microfacies are related to accumu- critic intraclasts (Fig. 3d). Fossil content - Dolostones (D) are horizontaly laminat- lation of sand-size mud-intraclasts, and a include radiolarian, benthic foraminifera, ed and laterally continuous, dolomitized low proportion of terrigenous particles and ammonites, articulated and disarticulat- beds, although they can be also devel- clays. By contrast the Quintuco Formation ed bivalves, and gastropods. Terrigenous oped as horizons with brecciated dolo- consists mostly of epiclastic components clasts occur in low proportions (<10%) and mite septarian concretions. They have a Mineralogical and geochemical trends 215

Figure 2. Main lithologic features of Vaca Muerta and Quintuco Formations in the study area. a) Typical marlstone/li- mestone succession of Vaca Muerta Formation. b) Whitish, argillaceous tuffs intercalated with dark gray marlstones of Vaca Muerta Fm. c) Sub-spheri- cal carbonate concretions developed in the marls- tones of the Vaca Muer- ta Formation d) Fibrous beef-vein within brown calcareous marlstones. Beef veins may present black bituminous layers in the center. e) Laminated microbial bindstone of the Vaca Muerta Formation, usually observed at the base of the TSTs. f) Dolos- tone bed at the base of the Quintuco Formation in the transition interval (Puesto Barros Mb.). Note the la- minated black mudstones surrounding the dolomitic level. g) Succession of fine-grained sandstones with calcareous mudsto- nes in the Cerro La Visera Mb. h) Columnar dolomitic concretions developed within calcareous mudsto- nes in the Cerro La Visera Mb.

characteristic orange color in the Puesto - Terrigenous wackestones (Wth) are typ- ally benthic foraminifera (Lenticulina sp. Barros Member. In thin section this micro- ically laminated and their tops are gener- and Nodosaria sp.). Terrigenous grains facies is characterized by an equigranular ally bioturbated. This microfacies occurs are much abundant than in the other lime- mosaic of anhedral to subhedral crystals, mostly in the Puesto Barros Member. stones, reaching up to 30% of the total containing isolated terrigenous clasts and Particles include small disarticulated os- volume. Clasts include coarse silt-to fine calcispheres (Fig. 4a). tracods, ophiuroid ossicles, and occasion- sand-sized quartz and plagioclase as ma- Mineralogical and geochemical trends 216 jor components, together with muscovite conite are common. The interparticular creases towards the top of the Quintuco and more rarely biotite. (Fig. 4b). space is filled with micrite or granular cal- Formation. The clay minerals/calcite ratio

- Bioclastic floatstones/rudstones (FRbm) cite cement (Fig. 4d). is considerably low within the Vaca Muerta forming massive or normally-graded beds. Formation, with a remarkable increase in Particles include oysters, altered aragonit- Mineralogical analysis: X-Ray the transition interval and in the last 150 m ic bivalves, serpulids, echinoderms, ben- Diffraction (XRD) of the Cerro La Visera Member. thic foraminifera, sponge spicules, and Bulk-rock minerals: The Vaca Muer- The dolostone beds developed in the tran- silt-to fine sand-sized terrigenous clasts ta-Quintuco system is characterized by an sition interval (Fig. 2f, 4a) are formed by (quartz, plagioclase and feldspars). Matrix homogenous, non-clay mineralogy. The dolomite (83-95 %), with low contribution is commonly peloidal (Fig. 4c). main constituents are quartz and calcite, of calcite (1-7 %), quartz (2-6 %), pla- - Horizontally laminated to ripple laminated with subordinated plagioclase (Figs. 5, 6). gioclase (2-3 %) and clay minerals (<1 %) sandstones (Sh, Sr) are feldarenites and Gypsum, pyrite, and carbonate-fluorap- (Scasso et al. 2017). volcanic lithoarenites. They are composed atite were recorded in specific intervals Clay-minerals: The clay mineralogy of the of medium to fine sand-sized volcanic and as trace minerals within the Vaca Muerta marlstones of Vaca Muerta Formation is sedimentary lithic fragments, plagioclase, Formation and on the Puesto Barros Mem- dominated by mixed-layer I/S (Fig. 7a), feldspar, micas, and monocrystalline ber (Fig. 6). Total clay content is very low with occasional chlorite as a trace mineral quartz. Clasts are subangular. Isolated, within the Vaca Muerta Formation marl- (Capelli et al. 2017a, b). The mixed-lay- rounded clasts of phosphate and glau- stones (Figs. 5, 6) and progressively in- er I/S is classified as a R3 (Moore and

Figure 3. Main microfa- cies of the Vaca Muerta Formation at Puerta Cu- raco: a) Laminated muds- tones, showing fragments of bivalves (B), radiola- rian (R) and terrigens (T); b) Microbial bindstones showing a characteristic alternation of micritic and microsparitic laminae, that include radiolarian (R), terrigenous (T) and other microfossils; c) Radiolari- tic wackestones showing a radiolarian (R), calcis- pheres (C), and fragments of bivalves (B) and am- monites (A); d) Intraclas- tic packstone/grainstone forming a grain-supported fabric from the accumula- tion of intraclasts (I) and radiolarian (R); e) Calci- te-replaced tuff, showing abundant glass shards (Tr) partially altered to clays; f) Calcite-replaced lapillite which consists of the accumulation of pumi- ceous fragments (P) in a micritic matrix. Mineralogical and geochemical trends 217

Figure 4. Main microfa- cies of the Quintuco For- mation at Puerta Curaco: a) Dolostone from the Puesto Barros Member showing a characteristic subhedral crystalline fa- bric among which some remains of the primary components, such as cal- cispheres (C), are obser- ved; b) Wackestone with terrigenous clasts from the Puesto Barros Mem- ber, showing terrigenous clasts (T) within a mud-su- pported micritic matrix; c) Bioclastic rudstone from Cerro La Visera Member with fragments of bivalves (B), echinoderms forming cortoids (E), terrigenous (Q) and glauconite (G); d) Lithoarenite from Cerro La Visera Member, showing pumiceous fragments (P), plagioclase (Pl) and quartz (Q) and micritic in- traclasts.

Reynolds 1997), since the expandable layers represent less than de 10% of the mixed-layers. Considerable contribution of fine-grained quartz has been recorded in the clay mineral fraction, especially in Vaca Muerta marlstones (Fig. 7a). The transition interval is characterized by the presence of chlorite, mixed-layered I/S and illite. The chlorite pattern presents an important asymmetry on the reflec- tions, with strong reflections on the even peaks and weak reflections on the odd peaks (Fig. 7b), indicating an Fe-rich chlo- rite (Moore and Reynolds 1997). The illite contribution is differentiated from the I/S, recorded on narrower peaks in the 10 and 5 Å positions (Fig. 7b). Finally, the Cerro La Visera Member (Fig. 7c) is dominated by illite, with a subtle contribution of I/S generating an asymmetry the 10Ǻ peak. Chlorite is also observed within the Cerro La Visera Member with poorly developed odd peaks, as previously described for the transition interval (Fig. 7c).

Inorganic Geochemistry Figure 5. Bulk rock typical XRD patterns. a) Marlstone of the Vaca Muerta Formation. b) Calcareous mudstone of the PBMb. c) Calcareous mudstone of the CVMb. The non-clay minerals, quartz, calcite and plagioclase present Gamma-Ray spectroscopy variable contribution. The clay mineral content is low in Vaca Muerta Formation and increases in both members of Six intervals with high total GR values are the Quintuco Formation. Abbreviations (IMA): Chl: chlorite, I/S: mixed-layer illite/smectite, Ilt: illite, Ms: muscovite, recorded within the Vaca Muerta Forma- Pl: plagioclase, Qz: quartz, Cal: calcite, Py: pyrite. tion (Fig. 8). The thickness of these inter- Mineralogical and geochemical trends 218 vals varies between 9 and 48 m (Fig. 8). Dowey and Taylor 2017) (Fig. 10). Si (14- posed of Fe, Cu, Zn, S which is associ- They are linked to high values of U (Fig. 32 %) is the most abundant element of this ated to sulfide authigenesis, strongly de- 8), which may reach up to 21 ppm, and group. It shows relatively homogeneous pendent on paleoredox conditions (Algeo are usually located on the TSTs on both values throughout the section (Fig. 11). Al and Maynard 2004, Tribovillard et al. 2006, CS and HFS. K and Th contribution is rel- and K values are variable within the first Calvert and Pedersen 2007). Fe varies be- atively low and stable throughout the Vaca 190 m (T5) of the Vaca Muerta Formation. tween 1 and 5 %, showing relatively high Muerta Formation. Nevertheless, a subtle Afterwards, a general increasing tendency values within the first 200 m of the Vaca increasing trend is observed from the end is observed towards the base of Mulichin- Muerta Formation. Upwards, a general de- of the TST of CS 2 (maximum flooding sur- co Formation, reaching maximum values creasing trend until B4 (Fig. 12). Then a face) to the top of the unit (Fig. 8). When on the upper 150 m of the Cerro La Visera general increasing tendency is recorded, analyzing the authigenic vs detrital contri- Member (Fig. 11). The transition interval with a noticeable increase in the transition bution of U (Wignall and Myers 1988) for is characterized by an important increase interval and a progressive increase for the the Vaca Muerta Formation, it is observed on both elements as well as in Ti, which rest of the column, reaching maximum val- that the U contribution is mostly of authi- shows a similar behavior to K and Al (Fig. ues at the top of the Quintuco Formation genic origin, with a negligible detrital con- 11). (Fig. 12). Cu and S show a similar behav- tribution (Fig. 8). Carbonate group: Ca and Sr are selected ior. Their higher values are recorded in On the other hand, the Quintuco Forma- for the analysis. Ca content is highly vari- Vaca Muerta Formation, especially within tion records 6 anomalies of total GR (Fig. able in the entire system, with a minimum the first 200 m. Then, they present a sub- 8), varying between 10 to 73 m thick. They value of 1 % and a maximum of 25 % (Fig. tle decreasing tendency towards the top of usually correspond to the RST of the HFS 11). Despite local variations, the transition Vaca Muerta Formation (Fig. 12). The tran- and to the RST of the parasequences in interval is characterized by lower contents sition interval is characterized by a deple- the upper 85 m of the formation. Opposite of Ca, as well as the last 150 m of Cerro tion in both elements, Cu and S contents to the Vaca Muerta Formation, the GR La Visera Member (Fig. 11). Sr/Ca ratio are low upwards, despite a local increase anomalies correlate to high values of K shows a steady increase from base to top above the transition interval (Fig. 12). Zn and Th and the U is mainly of detrital origin of the system, with remarkably high Sr val- values are extremely variable (15-1551 (Fig. 8). ues in the upper 150 m of Cerro La Visera ppm), exhibiting the maximum values with- The anomaly of the transition interval Member. The transition interval records a in the lower 220 m of the Vaca Muerta For- between the Vaca Muerta and Quintuco subtle increase in the Sr/Ca ratio (Fig. 11). mation, with an important depletion on the Formations (Fig. 8) present higher con- Organic group: P, Mo, Ni and V, repre- transition interval and a subtle increase on tribution of K and Th, whereas U values sent the elements with high affinity to the the upper 200 m of the Cerro La Visera are remarkably low and mainly of detrital organic matter (e.g., Algeo and Maynard Member (Fig. 12). Fe/S ratio remains in- origin. The total GR correlates well to the 2004, Tribovillard et al. 2006). Mo, Ni and variable throughout the Vaca Muerta For- U spectrum in the Vaca Muerta Formation, V contents are characterized by an im- mation, with a noticeable increase on the whereas no correlation is observed in the portant variability within the first 200 m of transition interval, and a general increas- Quintuco Formation (Fig. 9a). The oppo- the Vaca Muerta Formation, where they ing trend from the base to the top of the site is observed for K (Fig. 9b). reach the highest values: 363, 334 and Cerro La Visera Member (Fig. 12). 1047 ppm respectively (Fig. 12). Then, a Major and trace elements general decreasing tendency is observed The elements were grouped according to in these elements towards the top of the DISCUSSION AND INTE- their affinity (Calvert and Pedersen 2007, Vaca Muerta Formation. They are notably GRATED ANALYSIS Montero-Serrano et al. 2010, 2015). Main depleted in the transition interval, a ten- elements of the siliciclastic, carbonate, or- dency that continues upwards in the Quin- Mineralogy and diagenesis ganic and sulfide groups were plotted in tuco Formation (Fig. 12). Mo is below the of the Vaca Muerta-Quintuco figures 11 and 12. detection limit in specific samples within system Siliciclastic group: Si, Al, K, Ti and Zr are the transition interval and on the upper 150 In the study area, the Vaca Muerta-Quin- typically representative of this group. They m of Cerro La Visera Member (Fig. 12). P tuco system underlies the Mulichinco and are usually associated with clay, silt and content is markedly low and is beneath Agrio Formations (upper Mendoza Group), sand particles, either as the main chemi- the lower detection limit in 9 samples (Fig. and the Bajada del Agrio and Neuquén cal constituents of the detrital grains (e.g., 12). Although maximum values are locat- Groups (Leanza and Hugo 1977, Rojas quartz, K-feldspar) or adsorbed on their ed within the Vaca Muerta Formation (0,32 Vera et al. 2015). Consequently, it un- surface (see Calvert and Pedersen 2007, %), no clear trend for the entire system is derwent considerable overburden which Montero-Serrano et al. 2010, 2015). Si is recognized. Nevertheless, an important is reflected in the overmatured organic associated to quartz. As quartz may or increase on the P content (> 0,17 %) is matter, which bear a Tmax about 520°C may not present a detrital origin, Si vs Zr recorded within the transition interval (Fig. under Rock-Eval pyrolysis (Kietzmann correlation helps to unravel its origin (e.g., 12). et al. 2016a). Accordingly, a high-diage- Calvert et al. 1996, Ross and Bustin 2009, Sulfide group: The sulfide group is com- netic mineralogical association mainly Mineralogical and geochemical trends 219

Figure 6. Schematic litholog of the Vaca Muerta-Quintuco system in the Puerta Curaco section. Mineral abundance in the bulk rock was calculated for quartz, calcite, clay minerals, plagioclase and gypsum. Clay mineral/calcite ratio is not represented in samples lacking calcite (22.3.16_14; 22.3.16_26; QC_995). T1, T3, T5, B4, V2, V4: regio- nal stratigraphic surfaces from Desjardins et al. (2016). Mineralogical and geochemical trends 220

Diagenesis may also modify Sr/Ca ratio (e.g., Stoll and Schrag 2001). Primary biogenic calcites present a much higher Sr content than the secondary equilibri- um abiogenic calcite. Therefore, diage- netic recrystallization (occurred during early diagenesis) results in loss of Sr to porewaters (Stoll and Schrag 2001). This may lead to the diagenetic precipitation of calcite with low Sr/Ca ratio. Increasing diagenesis may have produced a down- ward decreasing trend of the Sr/Ca ratio in the column (Fig. 11). Isolated maxima on the Sr/Ca ratio would represent prima- ry variations in the bioclastic content.

Detrital input and organic productivity: implications on lithostratigraphy The petrographic, geochemical, miner- alogical and lithological variations record- ed throughout the Vaca Muerta-Quintuco system suggest that sedimentation was controlled by a combination of processes: carbonate production, detrital contribution, mineral authigenesis and organic produc- tion within the water mass. The analysis allowed to discriminate 5 intervals (I1-I5) for the Vaca Muerta-Quintuco system in the Puerta Curaco section (Fig. 13). -I1: The lower 170 m (CS 1 and CS 2) are characterized by relatively stable, low val- ues of K, Th, Zr and Ti, suggesting a low contribution of detrital sedimentation to the outer carbonate ramp. The low correlation of Si-Zr and the low amounts of detrital quartz observed for the Vaca Muerta For- mation marlstones, suggest that quartz Figure 7. XRD patterns for the clay fraction below 2 μm. a) Marlstone of the Vaca Muerta Fm. dominated by detrital contribution is negligible (e.g., high-ordered (R3) I/S. b) Calcareous mudstones of the PBMb (transition interval), dominated by chlorite and I/ S+I. c) Mudstone of the CVMb, characterized by the presence of illitic material with some expandable phase and Blood et al. 2013, Dowey and Taylor chlorite. AD: air-dried, EG: ethylene-glycol solvated, H: heated. 2017). Quartz content can be associated to a biogenic source, probably linked to the composed of quartz, plagioclase, calcite, The high-ordered I/S is a consequence abundance of radiolaria, combined with high-ordered I/S and chlorite was found in of the progressive illitization process suf- other factors such as the progressive illiti- the system (Capelli et al. 2017a). fered by smectite layers (e.g., Hower et al. zation of smectite layers that yields silica Plagioclase was found throughout the en- 1976). The progressive transformation of for the formation of microcrystalline quartz tire column (Fig. 6). The XRD pattern pre- smectite to illite via mixed-layer releases (Fig. 7a) (e.g., Hower et al. 1976). The im-

liminary suggests a Na-rich plagioclase, SiO2 and Fe, favoring the precipitation of portant variations in calcite content are ex- probably albite. Albite is a consequence of overgrowths on quartz and Fe-rich chlorite plained by the existence of high-frequency the complete transformation of Na-rich ze- (e.g., Hower et al. 1976, Boles and Franks sequences, in which HSTs are enriched in olites or the alteration of K-feldspar (e.g., 1979, Lee et al. 1985). Recently, Wilson et calcite when compared to the TSTs (Kietz- Coombs 1954, Land and Milliken, 1981). al. (2016) proposed that high-ordered I/S mann et al. 2016a). Since no significant K-feldspar is found from shale reservoirs may precipitate under U, V, Ni are insoluble under reducing con- throughout the system, K content is mainly specific conditions from pore waters. This ditions. So, these elements are usually assigned to the illitic minerals (e.g., Adams origin in the Vaca Muerta-Quintuco system richer in oxygen-depleted environments and Weaver 1958, Cowan and Myers 1988). should not be fully discarded. (e.g., Tribovillard et al. 2006). Maximum Mineralogical and geochemical trends 221

Figure 8. Spectral Gamma Ray log together with the lithostratigraphic and sequence stratigraphic subdivision for the Vaca Muerta-Quintuco system. Mineralogical and geochemical trends 222 values of U, V and Ni are observed with- in the I1 interval (Figs. 12, 13), indicating high organic productivity in accordance to the TOC vertical trend (Kietzmann et al. 2016a, Eberli et al. 2017). This tendency presents a good correlation with Mo con- tent, which is as a good proxy for redox conditions (see Tribovillard et al. 2006). The excellent correlation of Total GR and U (Fig. 9a) indicates that, for Vaca Muerta Formation (I1 and I2) the GR signal essen- tially responds to the organic content. The (V/V+Ni) curve indicates that I1 was sedimented in an anoxic to intermittently euxinic environment (Fig. 12) (Hatch and Leventhal 1992), consistently with the lack Figure 9. a) Cross-plot of To- of significant bioturbation observed in the tal GR vs U (ppm). They show good, positive correlation for Vaca Muerta Formation. Euxinic conditions the Vaca Muerta Formation were reached on the Tithonian transgres- (R2: 0,75) and no correlation 2 sion, leading to the formation of organ- for the Quintuco Formation (R : 0,01). b) Cross-plot of Total GR ic-rich levels found in the base of the Vaca vs K (%) showing no correla- Muerta Formation. An euxinic environment tion for the Vaca Muerta For- was proposed for that unit in the Picún mation (R2: 0,28) and good, positive correlation for the Leufú depocenter (southern Neuquén Ba- Quintuco Formation (R2: 0,81). sin) (Legarreta and Villar 2015). Recent studies, also proposed an euxinic envi- ronment for the very basal deposits of the Vaca Muerta Formation in this depocenter, whereas the rest of the column was depos- ited under oxic/anoxic or fully oxygenated marine waters (Krim et al. 2017). In I1, euxinic conditions were reached on TST of CS-1 and on TST of HFS-7, suggesting limited water circulation through the basin. Figure 10. Zr vs Si cross-plot, showing different correlation Within this interval, Zn and Cu present the in Vaca Muerta and Quintuco higher values, associated to high organic Formations. matter content. Those metals, under anox- ic conditions and during the sulfate reduc- tion stage, are released from organic mat- ter to pore waters, being usually present as solid solutions of pyrite crystals (Morse and ty, reflected in the high content of the ele- sition interval and it is characterized by a Luther 1999, Tribovillard et al. 2006). ments of the organic group. sudden increment of Zr, Ti, Al and K, and -I2: Within this interval, a general increase The Fe/S ratio remains almost invariable, a decrease of organic proxies and car- of clastic material is observed, which is indicating that both elements are strongly bonate content, as shown in figure 11. indicated by the progressive increase of correlated to the authigenic pyrite content. These elements represent an important K (Figs. 8, 11). On the other hand, a de- The V/V+Ni ratio indicates predominantly contribution of detrital material (Calvert creasing trend of the organic-rich proxies is anoxic to dysoxic conditions (Fig. 11), al- and Pedersen 2007) mainly associated to observed. An exception to this trend is re- though euxinic conditions were reached an increase of clay minerals (Fig. 6). The corded on the TST of the CS 4 (B4), where on the TSTs of both CS-4 and CS-5. P, correlation of Si-Zr (Fig. 10) suggests a a 50 m thick interval with high U content usually associated to high-productivity detrital origin for quartz. Also, the signifi- is recorded. This level corresponds to an stages, may be not directly related to the cant increase on the Fe/S ratio reflects the important regional transgressive event ex- supply of organic matter (e.g. Tribovillard increase of illite and chlorite contents, both tensively represented on the basin (e.g., et al. 2006). Indeed, within the Vaca Muer- with important amount of Fe as evidenced Desjardins et al. 2016), also called “upper ta Formation, there is not a vertical cor- by their asymmetric XRD traces. Never- enriched zone” (e.g., Marchal et al. 2016). relation between P and Mo (Fig. 12). theless, Fe may also be present in amor- This event led to high organic productivi- -I3: this interval is coincident with the tran- phous Fe-oxides not determined by XRD. Mineralogical and geochemical trends 223

Figure 11. Major and trace elements of marlstones and mudstones associated to the siliciclastic and carbonate groups. Note the transition interval enriched in Al, K and Ti and Sr/Ca, and relative impoverished in Ca.

The high contribution of clastic material of the carbonate ramp environment. I4 is abundance of organic matter. A progres- may be the consequence of an important then similar to I2, but with less contribu- sive input of detrital material is evidenced base level fall, as observed on the seis- tion or organic matter, as suggested by the in I2, (Figs. 8, 13), generating a relative re- mic lines for this interval (see Sattler et al. content of the organic-related elements. duction of organic matter content, as sug- 2016). The subtle increment of the Sr/Ca -I5: this interval comprises the rest of gested by the decrease of organic-proxies. ratio reflects an important contribution of Cerro La Visera Member (Fig. 13) and An anoxic environment prevailed during biogenic carbonate in the carbonate par- represents the final establishment of the Vaca Muerta Formation sedimentation, al- ticles of the calcareous mudstones (e.g. mixed siliclastic-carbonate shelf of the though euxinic conditions were occasion- Stoll and Schrag 2001). The increase on Quintuco Formation (Kietzmann et al. ally reached, especially in I1 (Fig. 12). Or- the Sr/Ca ratio suggests that the carbon- 2016a). It is characterized by an import- ganic matter accumulation is concentrated ate of the matrix presents a richer bioclas- ant increase of K, Th, Al, Ti and Zr, con- within TSTs of both, CS and HFS for the tic contribution. The I3 is characterized by current with the appearance of siltstones Vaca Muerta Formation outer ramp depos- relative high and stable values of P, indi- and occasional, fine-grained, rippled sand- its, which were submitted to relative low cating an enrichment in P during the tran- stones, as well as the abundant bivalve sedimentation rates (25 cm/1000 years, sition from Vaca Muerta to Quintuco For- fragments and sponge spicules. V/(V+Ni) Kietzmann et al. 2016). Furthermore, high mations (Fig. 12), also evidenced by the ratio indicates that, although the water sea-level and lower sedimentation rates presence of fluorapatite. The enrichment conditions were still anoxic, fluctuations to generated oxygen-depleted conditions of P in this transition interval may also be more dysoxic or even oxic environments that allowed the accumulation of organic linked to a sea-level fall, where reworking were recorded (Fig. 12). The high Sr/Ca matter with insignificant terrigenous dilu- and concentration of phosphatic particles ratios in this interval reflect the important tion. may occur (e.g., Föllmi 1996 and refer- contribution of bioclastic carbonate, a con- The carbonate ramp environment (Vaca ences therein). For example, in the Cerro sequence of the shallowing upwards envi- Muerta Formation) was suddenly submit- Salado area (southern of Puerta Curaco), ronment. The Sr content of seawater was ted to a significant contribution of detrital important accumulation of phosphatic par- probably increased by the release of pore material, especially illite, chlorite and also

ticles (P2O5 <8 %) has been recorded in waters enriched in Sr due to the recrystalli- detrital quartz. The contribution of detrital regressive system tracts of the Quintuco zation of shelf aragonite to calcite on inner material was a consequence of a relative Formation (Medina et al. 2016). exposed platforms (e.g. Schlanger 1988, sea-level fall and it is characteristic of the -I4: this interval spans the upper part of Stoll and Schrag 2001). the transition interval (I3), representing the the Puesto Barros Member and the bas- first evidence in Puerta Curaco area of al meters of the Cerro La Visera Member Paleoenvironmental changes the establishment of the mixed siliciclas- (Fig. 13). It is characterized by a relative inferred through geochemical tic-carbonate shelf, typical of the Quintuco decrease of detrital proxies, (Fig. 11) and and mineralogical variations Formation. V/V(+Ni) ratio does not sug- a subtle increase of organic proxies when gest significant paleoredox changes for compared to I3. Within this interval, a sub- The Vaca Muerta Formation is character- the transition interval and for the rest of the tle increment of U, Mo, S and Zn is record- ized by relative low values of detrital prox- Quintuco Formation, where anoxic condi- ed, mainly developed on the TST of the ies (Figs. 11,13). On the other hand, the tions still prevailed. Consequently, the low- HFS-15 (Figs. 8, 12). A parallel increase of high content of sulfide-associated metals er content of organic matter was caused Ca and carbonate suggest the restoration (Zn, Cu) and the organic-proxies indicates by the increased clastic input and not by Mineralogical and geochemical trends 224

Figure 12. Major and trace elements of organic and sulfide groups. The transition interval is characterized by an increase on P, Fe, Cu and Fe/S, with a notable decrease of V, N, Mo and S. significant changes in the oxygenation Th content, and U contribution is almost that the anoxic environment still prevailed of the environment. This is in agreement negligible. during the accumulation of the Quintuco with the higher sedimentation rate for the Mineralogical, geochemical and sedimen- Formation. Quintuco Formation, calculated in 100-120 tological features allow us to discriminate cm/1000 years (Kietzmann et al. 2016a). 5 intervals in the Vaca Muerta - Quintuco system. The I1 and I2 are developed with- ACKNOWLEDGMENTS in the Vaca Muerta Formation, character- CONCLUSIONS ized by carbonate sedimentation, essen- Financial support was provided by the tially deposited under anoxic conditions, National Council of Scientific and Tech- The Vaca Muerta Formation in the Puerta with intermittent euxinic periods. I1 is en- nical Research (CONICET) of Argentina, Curaco section presents a thickness of riched in organic-matter, with insignificant and the Vaca Muerta Consortium (LOU 407 m. It is characterized by a succes- contribution of detrital material and quartz 4900023748) integrated by the University sion of black, laminated marlstones and of non-detrital origin. The anomalous val- of Buenos Aires, Shell, Total and Chevron. limestones, with commonly interbedded ues of U are associated to organic-rich in- We would like to extend our appreciation tuffs and beef veins. Overlaying the Vaca tervals, developed within TSTs, that may to the members of the consortium for al- Muerta Formation, the Quintuco Forma- have resulted from euxinic conditions. I2 lowing us to publish this study, to Cecilia tion presents a thickness of 309 m and is also enriched in organic material, al- Genazzini, Martín Morosi and Jorge Maggi consists on calcareous mudstones, silt- though a progressive increment on detri- (CETMIC) for providing support regarding stones, limestones, dolostones and fine- tal contribution is evidenced. XRD measurements and to Oscar Comito grained sandstones. I3 is developed within the transition in- (CNEA) for his advice and support regard- The Quintuco Formation is subdivided terval between Vaca Muerta and Quin- ing natural radioactivity measurements. into two members: the lower Puesto Bar- tuco Formations and is characterized by We would like to thank Barros family for ros Member (76 m), composed by black a noticeable increment on K and Th, an their kindness and for allowing us to work calcareous mudstones, dolomite concre- increase in the clay minerals/calcite ratio, on their lands. tions and limestones, and the upper Cer- as well as an increase on other detrital ro La Visera Member (233 m), character- proxies like Al, K and Ti, combined with REFERENCES ized by calcareous mudstones, dolomite detrital quartz and a general decrease on Adams, J.A.S., and Weaver, C.E. 1958. Tho- concretions and interbedded siltstones the organic-related elements. The high rium-to-Uranium ratios as indicators of sedi- and fine-grained sandstones. abundance of detrital material is inter- mentary processes: example of concept of The boundary between the Vaca Muer- preted as a consequence of a sea level geochemical facies. American Association of ta and Quintuco Formations is set at the fall. I4 is coincident with the Puesto Bar- Petroleum Geologists Bulletin 42: 387-430. lower dolostone bed (407 m from the ros Member in Quintuco Formation and Algeo, T.J., and Maynard, J.B. 2004. Trace-ele- base of the Vaca Muerta Formation). The presents a lithology similar to Vaca Muer- ment behavior and redox facies in core sha- dolostone bed is easily recognizable in ta Formation (I2), suggesting that the car- les of Upper Pennsylvanian Kansas-type cy- the outcrops and can be extrapolated to bonate ramp sedimentation was partially clothems. Chemical Geology 206: 289-318. the subsurface in neighboring blocks. restored. Finally, I5, is developed in the Askenazi, A., Biscayart, P., Cáneva, M., Mon- Natural radioactivity of the Vaca Muerta Cerro La Visera Member, below the In- tenegro, S., and Moreno, M. 2013. Analogía Formation is strongly dependent on the travalanginian unconformity, being char- entre la Formación Vaca Muerta y shale gas/ U content, while K and Th contributions acterized by an important contribution of oil play in the EEUU. Society of Petroleum are less important. The opposite is true detrital quartz and clay minerals. In spite Engenieers, young professional Comitee, within the Quintuco Formation, where to- of the progressive increase on the detri- 1-20. tal GR is strongly influenced by the K and tal material, geochemical proxies suggest Blood, R., Lash, G., and Bridges, I. 2013. Bio- Mineralogical and geochemical trends 225

Figure 13. Schematic litholog of the Vaca Muerta-Quintuco system in Puerta Curaco area, exhibiting the main geochemical/sedimentological intervals (I1-5). Mineralogical and geochemical trends 226

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Facies sedimentarias y ciones Vaca Muerta y Los Molles, Cuenca modelo deposicional de la Formacion Mu- Neuquina, Argentina. Boletín de Informacio- lichinco (Valanginiano), Cuenca Neuquina nes Petroleras: 18-30. Septentrional. Latin American Journal of Se- Stoll, H.M., and Schrag, D.P. 2001. Sr/Ca varia- dimentology and Basin Analysis 6: 37-59. tions in Cretaceous carbonates: relation to Recibido: 5 de marzo, 2018 Schwarz, E., Spalletti, L.A., and Veiga, G.D. productivity and sea level changes. Palaeo- Aceptado: 21 de mayo, 2018