1. Introduction

The Sorocayense-Hilario sub-basin is part of the Triassic Cuyana rift basin in western Argentina (Fig. 1). It has been thoroughly explored due to its rich fossiliferous content, mostly plant remains which have allowed to describe plant communities and to infer the paleoclimate (Spalletti, 2001; Bodnar et al., 2019; Ottone et al., 2019; Drovandi, 2020). The rocks of the Sorocayense Group are the main infill of the sub-basin, and despite the many structural, stratigraphical and sedimentological analysis of the area, the status of the Sorocayense Group units remains controversial (Baraldo and Guerstein, 1984; Abarzúa, 2016; Bodnar et al., 2019).

Fig. 1. A. Location map of the Sorocayense-Hilario sub-basin within the Cuyana rift Basin in the west-central region of Argentina showing the location of the master fault bounding the half grabens of the rift. Modified from Benavente et al. (2015). B. Geologic map of the Sorocayense-Hilario sub-basin of the Cuyana rift Basin. From Barredo et al. (2016). C. Location of the active and flexural margin of the Sorocayense-Hilario and Rincón Blanco sub-basins.

The Sorocayense Group is formed by five lithostratigraphic units that from base to top are: Cerro Colorado del Cementerio, Agua de los Pajaritos, Monina, Hilario and El Alcázar formations (Abarzúa, 2016; Bodnar et al., 2019). In the sub-basin, these units include several carbonate rock levels formed in different depositional settings ranging from floodplains of braided and meandering fluvial systems to a lacustrine margin, in the context of the passive margin of the northern half-graben of the Cuyana rift Basin at two localities, Quebrada del Agua de los Pajaritos and Quebrada El Alcázar (Fig. 1B). The development of the carbonate factory therefore spans different tectonic stages of the subbasin, from synrift to postrift in different tectonic settings, the active and the flexural margins of the rift (Abarzúa, 2016).

The main complexity for a clear stratigraphic definition for the lithological units of the sub-basin is the tectonic imprint that affected them through folding and faulting. All lithostratigrafic units host layers of carbonate rocks, which implies a broad time span for the carbonate deposition. Then, the study of their attributes could shed light on the diagenesis undergone by the whole sedimentary sequence and the nature of their principal drivers. Furthermore, characterizing the diagenesis of the carbonates has significant implications in different aspects such as the reconstruction and understating of the paleohydrology drainage patterns of the basin; and more applied aspects as well, such as the potential of the sedimentary rocks as a hydrocarbon source (Brigaud et al., 2009).

The aim of this study is to evaluate if the diagenetic stages recorded in the continental carbonates of the Triassic Sorocayense Group rocks were controlled by the tectonic active phases of the rift. For this purpose we used a detailed microfacies study and cold-cathodoluminescence analysis of carbonate rocks outcropping at the Sorocayense-Hilario sub-basin in the Quebrada del Agua de los Pajaritos and Quebrada El Alcázar localities (Fig. 1B). Such studies are relevant since the tectonic history of basins determines the diagenetic trends of the deposits (Grover and Read, 1983).

2. Geological setting

The Cuyana Basin is the largest Triassic rift basin of western Argentina (Fig. 1A) and is considered to extend over an area of more than 60,000 km². It corresponds to a continental rift (Ramos and Kay, 1991; Ramos, 2000; Legarreta et al., 1993; Barredo, 2004) developed as a consequence of the Gondwana Orogeny (Permian to Late Triassic-Early Jurassic) and the subsequent breakup of this supercontinent during Mesozoic times (Uliana and Biddle, 1988). It is composed of several asymmetric half-grabens (Fig. 1A).

The current geometry of the Cuyana Basin corresponds to a NNW-SSE elongate through with its margins affected by thrusts. Each sub-basin is bounded by mostly steeply deeping normal to oblique-slip faults that strike obliquely to the maximum extension (Barredo, 2012). In detail, these border faults are segmented and have a stepped geometry with sometimes opposite dip directions (e.g., Ramos and Kay, 1991; Legarreta et al., 1993; Barredo, 2004). Sub-basins correspond to asymmetric half-grabens separated by intrabasinal topographical highs that persisted throughout synrift and post-rift sedimentation keeping most of the troughs mostly isolated (Barredo et al., 2012; Abarzúa, 2016). This lack of physical connection between some of the sub-basins leads to different sedimentological conditions in their developments, especially in the Rincón Blanco sub-basin and Sorocayense-Hilario sub-basin, the focus of this study.

The Rincón Blanco and Sorocayense-Hilario sub-basins are located at 31°24’-31°33’ south (Fig. 1A,B) conforming a narrow elongated half-graben (60 km N-S and 40 km wide) composed of different depocenters: Barreal, Agua de Los Pajaritos, and Rincón Blanco (Fig. 1A). The sedimentary deposits accumulated in the active margin of the half-graben (Rincón Blanco Group) reach more than 3,000 m in thickness and, today, present a geometry that consists of a north-south tight asymmetric syncline truncated by a series of west and east verging back-thrusts developed since Cenozoic times (Barredo, 2004; Abarzúa, 2016). The sedimentary rocks deposited on the flexural margin (Sorocayense Group) reach ~1,800 m in thickness and are represented by isolated outcrops currently separated from the active margin by a basement high. The Miocene Andean compression led to a positive basin inversion and a gently folding of the rocks formed in the ancient depocenters, in particular the Agua de Los Pajaritos one (Amos et al., 1971; Baraldo and Guerstein, 1984; Bonati et al., 2008; Abarzúa, 2016, among others) (Fig. 1B).

At the beginning of the Permian-Triassic extension the high strength of the crust led to the formation of a series of restricted half-grabens separated by intrabasinal highs and/or transfer faults (e.g., Ramos, 2000; Barredo, 2004; López Gamundí et al., 1994; Japas et al., 2008). The oldest sedimentary rocks correspond to the conglomerate and coarse-grained sandstone of the active margin of the sub-basin that built a narrow fringe of alluvial facies covered and interfingered with sandstone, siltstone and mudstone representing deposition in fluvial and lacustrine/playa-lake settings of the Rincón Blanco Group (Borrello and Cuerda, 1965; Stipanicic, 1979; Barredo and Stipanicic, 2002; Abarzúa, 2016).

In the flexural margin of the sub-basin, instead, the conglomerate, coarse-grained sandstone, siltstone and mudstone of the basin ill are represented by the Sorocayense Group (Stipanicic, 1972, 1979) distributed in two, separated depocenters (Fig. 1A).

2.1. The Sorocayense Group

The sedimentary rock succession in the Agua de Los Pajaritos depocenter on which this study is focused, are assigned to the Sorocayense Group composed of five lithostratigraphic units, with a thickness of 900 m, all of them bearing carbonate rock layers (Fig. 2). These units are from base to top the Agua de Los Pajaritos, Monina, Hilario and     El Alcázar formations (Baraldo and Guerstein, 1984) (Figs. 2, 3A). The Agua de Los Pajaritos Formation is made up by conglomerates and coarse-grained sandstones that are grouped in a facies association (FA-A in Table 1), interpreted as debris flow deposited in an alluvial fan subenvironment (Table 1, Figs. 2, 3B); these lithofacies gradually passes upwards to medium-to coarse-grained sandstones (lithofacies Shg, Sh, Sr, Sm), which together constitute the facies association B (FA-B) that record tractive flows in a fluvial system (Table 1, Figs. 2, 3C). Massive siltstone beds (lithofacies Fm, Cn and Cm) overlays FA-B in a gradual transition recording subaqueous carbonate precipitation. Subjected to pedogenic alteration, most likely in temporary ponds in a floodplain subenvironment (Table 1, Figs. 2, 3D, E). These deposits have been interpreted as the synrift infill within the Sorocayense-Hilario sub-basin (Abarzúa, 2016).

Fig. 2. High resolution sedimentary log of the Sorocayense Group (Agua de los Pajaritos, Monina, Hilario and El Alcázar formations) at the Quebrada del Agua de los Pajaritos and Quebrada El Alcázar creeks.

 

Fig. 3. Field photographs of the facies and facies associations of the Sorocayense Group and the Agua de los Pajaritos Formation. A. General view of the Monina and Hilario formations. B. Lithofacies Gm showing very poorly sorted, clast-supported, rounded and subangular pebble- to cobble-extraformational massive conglomerate interstratified with massive coarse-grained sandstone (Sm). Pole for scale is 1 m. C. Lithofacies Shg showing medium- to coarse-grained horizontally laminated sandstone with clast lags forming tabular to lenticular beds with erosive base. Scale is 25 cm. D. Cm lithofacies showing massive fabric with gleying and circular structures interpreted as rhizolithes. Hammer for scale is 35 cm. E. Carbonate bed representing the Cl lithofacies showing faint horizontal lamination in the base and nodules at the top. Scale is 10 cm.

The Agua de los Pajaritos unit is gradually overlain by the Monina Formation (Baraldo and Guerstein, 1984; Barredo and Stipanicic, 2002; Abarzúa 2016) comprising FA-D with planar-cross stratified sandstone, horizontally laminated sandstone, ripple-cross laminated sandstone, sandstone with low angle lamination and massive sandstone (lithofacies Sp, Sh, Sr, Sl, Sm and Fm) (Table 1, Figs. 2, 4A, B). These deposits are interpreted as tractive flows and suspension settle out from waning flows with incipient paleosol development in a deltaic subenvironment. The upper section of the unit presents FA-E with dark greenish mudstone with very fine lamination (lithofacies Fl) and massive carbonates (lithofacies Cm) interpreted as a combination of suspension settle out processes in the lake (Fl) and subaqueous calcium carbonate precipitation in the lake margin (Cm). The massive features of the carbonate fabric suggest alteration after deposition most likely due to subaerial exposure effect.

Fig. 4. Field photographs of the lithofacies and facies associations of the Monina and Hilario formations, Sorocayense Group. A. Outcrop view of the stratification of sandstones (yellowish color, Sm, Sp and Sl lithofacies) and very thick siltstones packages (greyish color, Fm lithofacies) of the Monina Formation. B. Detailed of a silicified massive to laminated siltstone of the Monina Formation. C. General view of the lenticular sandstones (brownish color, St, Sr lithofacies) interstratified with the tabular sandstones (yellowish and greenish colors, Sh), Person for scale is 1.7 m. D. Interstratified tabular Fm-Fl lithofacies (eroded, negative relief) with lentiform and pinch out Cn lithofacies (positive relief). Scale is 25 cm. E. Detail of a Cn lithofacies level showing spherical nodules (n) in a disrupted matrix. Hammer point for scale is 10 cm.

Upwards, the Hilario Formation gradually overlies the Monina Formation (Baraldo and Guerstein, 1984; Barredo and Stipanicic, 2002; Abarzúa, 2016) and is characterized by FA-B including clast supported conglomerates, trough cross-stratified sandstone, ripple-cross laminated sandstone and horizontally laminated sandstone (lithofacies Gm, St, Sr and Sh (Table 1, Figs. 2, 4C). FA-C gradually overlies FA-B with massive to faintly laminated siltstone, nodular and domal carbonate and white tuff (lithofacies Fh, Fm, Cn, Cd and Tf) (Table 1, Figs. 2, 4D, E). In this unit, this facies association differs from FA-C in the Agua de los Pajaritos unit in that carbonates present a varied macrofabric from domal (Cd) to nodular (Cn) recording different processes and an ash fall event is recognized. The deposits are interpreted as suspension settle out from waning flows in the floodplain, calcium carbonate precipitation occurred subaqueously with subsequent pedogenic alteration, therefore this subenvironment might represent temporary ponds in the floodplain with ash fall contribution. Sandstone and siltstone to fine conglomerate cover the lacustrine shale and are interpreted as deltaic mouth bars to low energy fluvial plains (Hilario Formation). At its top this sedimentary sequence includes white and green fallout tuffs and fine-grained tuffs, sometimes with siliceous replacement (Barredo, 2012).

The uppermost part of the Sorocayense Group is represented by the El Alcázar Formation (Table 1, Figs. 2, 5A, B) conformed by FA-C with lithofacies finely laminated mudstone (lithofacies Fl) and laminated carbonate (lithofacies Cl) interpreted as tractive flows in a fluvial system; and FA-B with ripple-cross laminated and massive sandstone (lithofacies Sr and Sm); and carbonate precipitation in ponds within the floodplain (Abarzúa, 2016). This unit unconformably overlies the synrift deposits and has been assigned to an early post-rift stage (Barredo, 2004).

Fig. 5. Field photographs of the facies and facies associations of the El Alcázar Formation, Sorocayense Group and microphotographs of the Agua de los Pajaritos, Monina and Hilario carbonate thin sections. A. General view of the tabular sandstones dominated by tuff input of the El Alcázar Formation. B. Detail of Cl lithofacies with disrupted planar lamination. C. Microphotograph of the homogeneous micrite microfacies (Agua de los Pajaritos Formation) showing a clotted micrite matrix with circular concentric laminated structures interpreted as rhizolithes. Scale is 300 µm. D. Detail of a horizontal crack disrupting the matrix, filled by spar. Scale is 250 µm. E. Microphotograph of the homogeneous micrite microfacies (Hilario Formation) showing a clotted micrite matrix with diffuse circular micrite structures and vertical cracks. Scale is 250 µm. F. Detail of a circumgranular crack interpreted as disruption by rhizolithes. Scale is 100 µm. G. Detail of a vein under plane polarized light. Scale is 200 µm. H. Same vein as in G under cold cathodoluminescence showing zonation: the external fringe of the vein is bright luminescent while the interior of the vein is dull. Scale is 200 µm. I. Detail of the circular micrite structures under cross-polarize light with an external rim of blocky calcite. Scale is 50 µm. J. Same structures as in I under cold cathodoluminescence showing the blocky calcite with bright luminescence. Scale is 50 µm.

3. Methods

Two high-resolution sedimentary logs were measured in two localities, Quebrada del Agua de los Pajaritos and Quebrada El Alcázar, and lithofacies identified as siliciclastic facies from coarse to fine-grained size and carbonate-dominated facies. Lithofacies code of Miall (1996) has been applied. Every carbonate rock level in the sedimentary logs was sampled including lateral variations within them, obtaining sixteen hand samples. Samples were named with an abbreviation of the formation plus a correlative number. These samples represent all of the limestones found in these rock successions. Thin sections of hand samples were done at the Laboratorio de Corte de Rocas at the Universidad Nacional de San Luis (UNSL) and were studied using a standard petrographic microscope (Olympus BX-51). Carbonate rocks petrographic classification follows criteria from Gierlowski-Kordesch (2010). The Rock Color Chart of the Geological Society of America (GSA) was used for color descriptions. The macrofabric study of the samples was done at the Laboratorio de Paleontología del Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA).

Criteria to differentiate primary carbonate from diagenetic carbonate phases included a thorough multiproxy study with petrography and cathodoluminescence analysis of sixteen samples representing every carbonate layer found in the logs (Murphy et al., 2014; Parrish et al., 2019; Henkes et al., 2018; Benavente et al., 2019, 2021a, b). Those analyses led to the identification of different carbonate phases. Criteria to define diagenetic stages of continental carbonates follow Armenteros (2010).

Cold-cathodoluminescence (TECHNOSYN MKIII) was conducted at the Petrography and Cathodoluminescence Lab, Centro de Investigaciones Geológicas (CIG) in La Plata, Argentina. For this purpose, 10-µm-thick thin sections were cut from each sample; they were double polished and then coated with resin (Marshall, 1988). Analytical conditions were 0.01 and 0.05 Torr vacuum with 14-19 Kv under 370-500 µA.

4. Results

4.1. Carbonate petrography and cathodolumi-nescence

Six microfacies have been identified in the carbonates of the Sorocayense Group (Table 2).

4.1.1.Homogeneous micrite

This microfacies has been identified in the carbonates of the Agua de los Pajaritos, Monina and Hilario formations. The matrix consists of homogeneous clotted micrite with dispersed circular micrite structures with concentric lamination of 250 to 625 µm of diameter (Fig. 5C, D, E) and carbonate intraclasts of 250 µm of diameter. The circular laminated structures are coated by a rim of blocky calcite of 25 µm wide and surrounded by crack systems of circumgranular cracks (Fig. 5F) and vertical cracks that bifurcate downwards and vary in size from 25 to 250 µm. These cracks are associated with reddish, brownish and black organic matter and filled by spar (Fig. 5D). Siliciclasts are angular to subangular quartz grains of 100 to 750 µm. Cements correspond to microspar, spar and microgranular silica (Table 2). This microfacies presents microstylolites and veins filled by spar in the carbonates of the Hilario Formation. Cathodoluminescence study shows that the veins are zoned with an external more luminescent zone and an internal dull zone (Fig. 5G, H). Blocky calcite coating the circular micrite structures presents medium luminescence (1,400 ppm Mn⁺⁺) (Fig. 5I, J) (Table 2).

Interpretation: The clotted micrite matrix suggests primary biogenic induced precipitation (cf. Dupraz et al., 2009; Benavente et al., 2015). The circular laminated micritic structures associated with circumgranular and vertical cracks are interpreted as rhizolites and the organic matter associated with the crack systems as cutans. These structures disrupting the fabric in a paleosol are interpreted as part of rhizobrecciation processes (cf. Freytet and Plaziat, 1982; Alonso-Zarza et al., 1992; Benavente et al., 2015). The presence of angular to subangular siliciclasts dispersed in the matrix indicates proximity to provenance area (Benavente et al., 2015). Non-luminescence of the carbonates of this facies points to no Mn content or either very low Mn content (17-25 ppm) combined with high Fe content (200 ppm or more). This suggests crystal cores with oxygen and therefore might indicate no significant burial (cf. Hiatt and Pufahl, 2014). The blocky calcite forming rims indicates a cementation stage in the phreatic zone (cf. Armenteros, 2010). The presence of different luminescent patterns and microstylolites found exclusively in the Hilario Formation carbonates indicates late mesogenesis probably linked to chemical compaction processes  (cf. Brigaud et al., 2009).

4.1.2. Bioclastic micrite

This microfacies has only been found in the carbonate levels of the Hilario Formation. The matrix consists of homogeneous clotted micrite with dispersed disarticulated sparitic valves of 500 µm, articulated sparitic valves of 300 µm wide and 500 µm long (Fig. 6A), gyrogonites of 250 µm wide and 300 µm long and filamentous structures of 500 µm long with double wall. Gyrogonites consists of spar, and filamentous structures present an inner filament of silica and external wall replaced by spar, they are coated by micrite (Fig. 6B). Articulated valves have been micritized as well and the infill consist of spar (Fig. 6A). There is brownish organic matter dispersed in the matrix. Siliciclasts are angular quartz grains of 125 µm. Cements are micrite, spar, microgranular silica, and chalcedony in patches (Fig. 6C) and dispersed dolomite crystals (Fig. 6D). Chalcedony and dolomite crystals are 250 µm (Fig. 6C, D). There are vein systems with dull luminescence (1,200 and 1,600 ppm of Mn++) (Fig. 6E, F) (Table 2).

Fig. 6. Microphotographs of the Hilario carbonate thin sections. A. Bioclastic micrite microfacies showing clotted micrite with dispersed articulated valves interpreted as ostracods under plain polarized light. Scale is 450 µm. B. Bioclastic micrite microfacies under plain polarized light showing clotted micrite with dispersed poorly preserved gyrogonites and filamentous structures interpreted as Charophyte algae remains and filamentous algae remains respectively. Scale is 300 µm. C. Detail of the cements identified in the microfacies: microgranular silica and zebraic chalcedony under cross-polarized light. Scale is 300 µm. D. Detail of the cements identified in the microfacies: dispersed dolomite crystals of variable size under cross-polarized light. Scale is 300 µm. E. Detail of horizontal and vertical vein systems disrupting the micrite matrix under cross-polarized light. Scale is 300 µm. F. Same vein systems as in E under cold cathodoluminescence showing the horizontal veins are zoned with an external bright luminescent fringe and a dull interior whereas; the vertical veins present dull luminescence. Scale is 300 µm. G. Dolomicrite facies showing aggrading neomorphism. Scale is 300 µm. H. Laminated micrite microfacies with patches of circular to hexagonal structures with double external wall interpreted as vascular plant tissue remains. Scale is 150 µm. I. Detail of articulated overlapping valves interpreted as ostracod remains. Scale is 200 µm.

Interpretation: The clotted micrite matrix suggests primary biogenic induced precipitation (cf. Dupraz et al., 2009; Benavente et al., 2015). Gyrogonites correspond to the reproductive structures of charophyte algae, their presence indicates a soft substrate and a photic zone subenvironment probably with freshwater alkaline conditions. Filaments are interpreted as filamentous algae, possibly cyanobacteria. Both charophytes and cyanobacteria have been found abundantly in Middle Triassic deposits of the north Cuyana Basin (cf. Benavente et al., 2014, 2015). Articulated valves are identified as ostracod remains that have been found in association with charophytes and cyanobacteria before in Middle Triassic deposits of the north Cuyana Basin (cf. Benavente et al., 2014, 2015). Their presence indicates a subaqueous low energy subenvironment (cf. Bustos-Escalona et al., 2019). The presence of angular to subangular siliciclasts dispersed in the matrix indicates proximity to provenance area (cf. Benavente et al., 2015). Therefore, this microfacies most likely represents shallow pond carbonate-rich subenvironments with microorganisms. The presence of dispersed dolomite crystals indicates isolated Mg-rich precipitation sites, since they are irregular a primary origin can be suggested (cf. Armenteros, 2010). In turn, this suggests a high Mg/Ca ratio for the subenvironment during precipitation (cf. García del Cura et al., 2001). Micrite coatings in bioclasts are interpreted as micritization processes that correspond to early diagenetic stages of eogenesis. The presence of veins with spar of dull luminescence indicates late mesogenesis probably linked to chemical compaction taking place after micritization.

4.1.3. Dolomicrite

This microfacies has only been found in the carbonate levels of the Hilario Formation. The matrix consists of homogeneous micrite, microspar and spar arranged in an aggrading increasing crystal size pattern (Fig. 6G). There are dispersed patches of black organic matter and vertical cracks disrupting the fabric. Cements are dolomite and microgranular silica. These cements are non-luminescent (Table 2).

Interpretation: The increasing crystal size pattern is interpreted as a neomorphism process that took place underwater with dissolution occurring contemporaneously to precipitation generating an increase in crystal size in one extreme of the microfacies only (cf. Bathurst, 1975; Armenteros, 2010). Non-luminescent cements indicate very low to low Mn content associated to a moderate or high Fe content, such conditions can be linked to oxidizing conditions that characterizes surficial dolomite precipitation (cf. Armenteros, 2010; Hiatt and Pufahl, 2014).

4.1.4. Laminated micrite

This microfacies has only been found in the carbonate levels of the Hilario Formation. The matrix consists of laminated clotted micrite. Laminae are crenulated and laterally continuous of 25 µm thick. There are patches of circular to hexagonal structures of 100 µm of diameter (Fig. 6H). Dispersed in the matrix there are articulated overlapping valves of 150 µm wide and 250 µm long and disarticulated valves of 200 µm long (Fig. 6I). The valves are sparitic and their infilling is micrite. The circular to hexagonal structures are massive and micritic with a double external wall, the inner layer of the wall is sparitic and they can present micrite coating. Organic matter is concentrated in the weak margins of the structures. There are also carbonate intraclasts of 125 µm and vertical cracks with associated reddish organic matter. Cements are microspar, fibro-radial spar, microgranular silica. The carbonate intraclasts are non-luminescent (Table 2).

Interpretation: The clotted micrite matrix suggests primary biogenic induced precipitation (cf. Dupraz et al., 2009; Benavente et al., 2015). Articulated and disarticulated valves are interpreted as ostracod remains. Their presence indicates a subaqueous low energy subenvironment (cf. Bustos-Escalona et al., 2019). The circular to hexagonal structures are doubtful. When observed in a more dispersed preservation they have circular shape and resemble coccoid algal or bacterial colonies (cf. Benavente et al., 2012, 2015). When they are preserved more organized and in connection to one another their shape is hexagonal and they resemble reticulate tracheid from plant tissue remains (cf. Bodnar and Falco, 2018). If they are indeed plant tissues, they indicate that their organic matter (OM) was partially replaced by calcium carbonate permineralization under rapid burial and low pressure conditions (cf. Scott and Rex, 1985) and OM was only retained in the weak margins of the structures. Micritization indicates an eogenesis diagenetic stage due to early precipitation of crusts around biogenic allochems (cf. Armenteros, 2010).

4.1.5. Oncolitic packstone

This microfacies has only been found in the carbonate levels of the Hilario Formation. The matrix  is formed by microspar, spar, microgranular silica, chalcedony and dolomite. Carbonate grains are micrite oncolites spherical, oval, irregular in shape or fragmented and of 1,500 µm of diameter (Fig. 7A). The cortex consists of several concentric laminae and the nuclei are formed by bioclasts or replaced by spar or chalcedony. There are also spherical to irregular massive micrite structures of 500 µm of diameter. They are surrounded by an external layer of blocky calcite (Fig. 7B). There are articulated valves of 750 µm with syntaxial cement. Disrupting the fabric there are cracks of 1,250 µm. Under cold-cathodoluminescence, oncolites cortex shows alternating laminae of dark (Mn+2:~1,400-1,800 ppm) and light (Mn+2:~1,800-2,100 ppm) orange color (Fig. 7C, D). The nuclei are very low to non-luminescent. Blocky calcite is bright luminescent (Fig. 7E, F) (Table 2).

Fig. 7. Microphotographs of the Hilario and El Alcázar formations carbonate thin sections. A. Oncolitic packstone microfacies with complete or fragmented oncolites of variable morphology. Scale is 750 µm. B. Detail of massive micrite structures with an external blocky calcite rim. Scale is 750 µm. C. Detail of spherical to irregular structures under plane polarized light with empty centers, micrite and microspar rims and blocky calcite infilling the inter-structural spaces. Scale is 500 µm. D. Same microphotograph as in C under cold cathodoluminescence showing bright luminescent blocky calcite and dull luminescent spar and microspar. Scale is 500 µm. E. Detail of concentric laminae in the cortex of oncolites and nuclei replaced by spar under plane polarized light. Scale is 700 µm. F. Same oncolites as in E under cold cathodoluminescence showing non-luminescent nuclei and very bright luminescent cortex. Scale is 700 µm. G. Sparitic carbonate microfacies of the El Alcázar Formation with dominant cement replacement. Scale is 250 µm.

Interpretation: The different carbonate grains observed resemble a diagenetic trend in which the original component is interpreted to be the oncolites. Oncolites are interpreted as biogenically formed, probably linked to the microbial buildups of filamentous cyanobacteria that have been described in the bioclastic micrite microfacies. These microorganisms form biofilms over the substrate under low energy conditions but with higher energy conditions the biofilms are detached from the surface and move freely in the water body obtaining spheroidal or irregular structures. Similar oncolites and microorganism have been observed in other carbonate microfacies of the Middle Triassic north Cuyana Basin (cf. Benavente et al., 2014, 2015). Fragmented and extremely ovoal oncolites are interpreted as the result of diagenetic compaction. The irregular and massive micrite structures are considered diagenetically altered oncolites that possibly underwent micritization. Different luminescence patterns indicate a range of diagenetic processes from late eogenesis to late mesogenesis.

4.1.6. Sparitic carbonate

This microfacies has only been found in the carbonates of the El Alcázar Formation. It is composed of spar mosaic with crystals of 250 µm. There are also microgranular silica patches and faint biogenic remains dispersed in the mosaic (Fig. 7G) (Table 2). The microfacies is non-luminescent.

Interpretation: the presence of significantly large spar crystals and faint biogenic remains suggests that spar cementation occurred obliterating the original carbonate microfabric. Sparitic cementation is considered a mesogenesis process (cf. Armenteros, 2010). Non-luminescence points to very low to low Mn combined with high Fe content (cf. Hiatt and Pufahl, 2014).

4.2. Relative chronology of diagenetic processes

This section summarizes the identified diagenetic phases in the Sorocayense Group microfacies. Based on petrography and cathodoluminescence we have stablished a relative chronological order of alteration of the carbonate primary fabric following the diagenetic environments proposed by Armenteros (2010).

4.2.1. Eogenetic

Very early diagenesis

Micritization. The very first process observed as result of diagenesis is the micritization of calcitic elements (spar) like the ostracod valves present in the bioclastic micrite microfacies of the Hilario Formation (Table 2, Figs. 1B and 8). This is a chemical process most likely due to pH changes in fluids in contact with the precipitating carbonates that cause frequent dissolution and re-precipitation (Blatt et al., 1980; Reid and Macintyre, 1998). Therefore, this process is part of the very early diagenesis and can occur simultaneously with primary carbonate precipitation.

Fig. 8. Diagram showing the diagenetic stages identified and their distribution throughout the Sorocayense Group units.

Early diagenesis

Mechanical compaction. The following indicator of diagenesis observed is mechanical and it is identified as light compaction. This process is evidenced in the tangential contact between grains of the oncolitic packstone microfacies of the Hilario Formation (Table 2, Fig. 8), in one case the oncolites present sutured margins. This process is interpreted as early diagenesis (Bathurst, 1975).

4.2.2. Shallow mesogenetic

Cementation

Subsequent processes identified correspond to cementation. Three different types of cements have been identified.

Calcitic cement. It is composed by blocky calcite. It can present equidimensional or scalenohedral crystals of approximately 30 µm that occlude the porosity. This cement is luminescent and orange in color (Mn+2:1,400-1,800 ppm). It has been recognized in the homogeneous micrite microfacies of the Agua de los Pajaritos and Hilario formations, in the bioclastic micrite and laminated micrite microfacies of the Hilario Formation, and in the sparitic carbonate microfacies of the El Alcázar Formation (Fig. 1B). The cement has not been found in the Monina Formation carbonates (Table 2, Fig. 8). It postdate micritization and mechanical compaction and corresponds to diagenesis in the meteoric phreatic environment during late diagenesis (James and Choquette, 1990).

Sparitic cement. It is formed by a mosaic of drusyform crystals of 0.04 a 0.7 µm that occlude the porosity. It is found inside the intercrystalline porosity. Some crystals are zoned, one is non-luminescent and the other presents dull luminescence. This phase indicates late cementation in a reducing environment with high Fe+2 concentrations that cause low to non-luminescence. It has been identified in the oncolitic packstone microfacies of the Hilario Formation (Table 2, Fig. 8). This cement follows the calcitic cement and it is genetically linked to a first fracture formation event. The mechanical fracturing event would have allowed fluids with different chemical composition to interact with the precipitated carbonates generating cement precipitation in available pores.

Microsparitic cement. It presents moderate luminescence of light orange color and its genesis is linked to a second event of fractures. This cement is present in all the carbonate microfacies except in the El Alcázar Formation (Table 2, Fig. 8). This implies that this stage was common to the entire analyzed deposits suggesting a regional control such as tectonics.

4.2.3. Deep mesogenetic

Chemical compaction. It is characterized by the identification of microstylolites interpreted as the result of chemical compaction processes (Bathurst, 1975; Choquette and James, 1987). This has been observed in the homogeneous micrite microfacies of the Agua de los Pajaritos and Hilario formations at the Quebrada del Agua de los Pajaritos locality (Fig.1B). Also, the presence of crossed veins observed in the homogeneous micrite microfacies and the partial dissolution of bioclasts observed in the bioclastic micrite microfacies of the Hilario Formation along with the growth of dolomite crystals indicate late mesogenesis (Armenteros, 2010). This suggests that diagenetic effects were more pervasive at the Quebrada del Agua de los Pajaritos locality representing the flexural margin of the basin. This could be due to a paleohydrogeological effect by favoring the lateral inflow of meteoric waters.

5. Discussion

The tectonic configuration and stages of the rift are recorded by each unit deposited in the Sorocayense-Hilario sub-basin, and they have a direct link to the carbonate factory development in the subenvironments through time. In the initial stage of the synrift, when the sediments of the Agua de Los Pajaritos Formation were deposited, sediment supply exceeded the creation of accommodation. This was not the best scenario for the development of the carbonate factory (Benavente et al., 2021a). The most favorable conditions for the carbonate factory to develop occurred during the deposition of the Hilario Formation in which the accommodation and sediment supply were roughly at balance (Bohacs et al., 2000). During the deposition of the Monina Formation sedimentary rocks, accommodation surpassed water+sediment supply and therefore the carbonate factory could not be maintained, which is expressed by very few carbonate levels hosted in this unit (Fig. 2). During the El Alcázar Formation deposition the increased volcanic supply most likely was unfavorable for the carbonate factory to conspicuously develop. 

All the carbonates analyzed in the lithostratigraphic units of the Sorocayense-Hilario sub-basin show features of very early and early diagenesis and a few record processes that indicate mesodiagenesis. The carbonate microfacies that present almost every diagenetic stage mentioned above are the ones of the Hilario Formation representing the complete array of very early, early and late diagenesis (Fig. 8). This could be due to the fact that this unit is the one with most carbonate rocks. Since there are many different microfacies identified in these carbonate levels it is likely that a variety of diagenetic stages was recorded in such microfabrics since they would present different susceptibility to different diagenetic processes. Probably this is why, even though the Hilario Formation is not the base of the Sorocayense Group presents more diagenetic effects than the deposits of the Agua de los Pajaritos and Monina formations. When comparing the carbonates of the Hilario Formation in both studied localities, differences are notorious and the samples from the Quebrada del Agua de los Pajaritos present the most complex diagenetic evolution recording three diagenetic stages (very early, early and late diagenesis). This is interesting since that locality is the one linked to the most faulted section of the subbasin having been affected by several thrusts and folds (Fig. 1). This may suggest that tectonics was a main factor controlling diagenesis. Nevertheless, since the carbonates from other units of the Agua de los Pajaritos locality display a simpler diagenetic history, it is most likely that the susceptibility of the microfacies to diagenesis also played a significant role in the alteration processes. We interpret that tectonics, through the effect of faulting allowed circulation of fluids through the sedimentary succession and generated chemical compaction while being shallowly buried (Abarzúa, 2016). One significant feature to highlight, is that only one microfacies showed signs of mechanical compaction (Fig. 8), supporting previously proposed minor burial effect for the Sorocayense Group deposits (Abarzúa, 2016) and for the northern Cuyana rift Basin (Benavente et al., 2019). As it has been proposed by Abarzúa (2016) after infilling of the Triassic rift, sediments were shallowly buried by a thin sedimentary fill of the Cenozoic foreland basin in the northern part of the rift and then exposed only during Andean uplift episodes.

Another factor to suggest low burial effect is the presence of primary montmorillonite dominating clay assemblages of the fine-grained deposits of the Sorocayense Group (Tettamanti, 2020; Brigaud et al., 2009). This clay composition also has relevant implications for the diagenetic processes, since it generally supplies Mg which could be involved in dolomitization processes. Dolomite-rich deposits and interpretation of dolomitization are controversial for the record and even more for the continental carbonates record since modern analogues are scarce (Armenteros, 2010). However, the occurrence of dolomite in continental carbonate successions is widespread and moreover, also linked to primary biogenic processes mostly in lacustrine subenvironments (Sanz-Montero et al., 2006, 2008; Armenteros, 2010). Primary and very early diagenetic dolomite has been registered at modern permanent lakes (Anadón et al., 2000; Bustillo et al., 2002) and it can occur in different lake types such as ephemeral or meromictic. Such is the case of the Santa Clara Arriba lacustrine system of the adjacent Triassic Santa Clara sub-basin in Argentina (Benavente et al., 2021b) in which dolomite mosaics of 5 μm crystal size have been identified associated with organic matter and calcite spheroids and pyrite concretions. In that case, the geologic context and other proxies led to the interpretation that dolomite most likely precipitated as a primary phase in a Mg-rich meromictic lake (Benavente et al., 2021b).

In the case of the Hilario Formation, dolomite is found as dispersed crystals in microfacies that suggest a Ca-rich palustrine subenvironment. Other features such as subaerial exposure and weathering indicators, would suggest that most likely those dolomite crystals precipitated as a very early diagenetic stage. However, with current evidence it is not possible to discard a primary origin. Regarding the dolomicrite microfacies of this stratigraphic unit, the disposition in an aggrading neomorphism pattern suggests that dolomite is an early diagenesis replacement (Hay et al., 1968).

6. Conclusions

The continental carbonate rocks of the Sorocayense Group in the Sorocayense-Hilario sub-basin have been deposited and precipitated subaqueously in Ca-rich ponds of fluvial floodplains and in shallow lacustrine settings; they have undergone diagenetic processes that represent very early diagenesis, eogenesis and mesogenesis, whereas no evidence for telogenesis has been found. The more complex diagenetic evolution of the carbonates is found in the microfacies of the Agua de los Pajaritos locality, which is the most significantly thrusted and folded area of the sub-basin suggesting a tectonic control on some diagenetic processes.

Signs of mechanical compaction are scarce and, in agreement with previous tectonic studies in the sub-basin, indicate that the Triassic sedimentary column did not experience deep burial. The diagenetic study of these carbonates confirms those previous large scales analysis in a higher resolution and fine scale.

The presence of dolomite could be interpreted as primary in one microfacies and as the result of very early diagenesis in other. In any case, the supply of Mg for dolomite precipitation could be linked to montmorillonite clay dominated assemblages.

Akcnowledgements
Authors wish to thank Lic. González Pelegrí and Dr. Cuitiño for improving the manuscript through their revision; and Geól. Federico Ferri and Lic. Tomás Heredia for their assistance during field work. The study was funded by PICT2014-0489 (CAB).

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