A 323 m thick stratigraphic section was measured, described, analysed, and sampled in detail at the Cañadón Lahuincó “A” section (43°30^′59^′′ S, 69°08^′20^′′ W), which is located in the Cerro Cóndor depocentre (Figs. 1 and 2). This section was measured in detail bed by bed, using a Jacob staff, and positioned by a GPS. Each bed was carefully described, with particular attention paid to the primary characteristics, such as lithology, sedimentary structures, grading, bedding contacts, bed geometry, trace fossils, presence of body fossils, and colour. This description was complemented by a detailed rock sampling for palynological studies. The detailed description allows one to perform a facies analysis, focused on recognising the active sedimentary processes during the deposition (facies) and depositional environments (facies associations and sequences). The analysis of the different hierarchically order stacking patterns allows us to review the lake evolution from a sedimentological and stratigraphic point of view.

A total of 51 outcrop palynological samples of very fine-grained and fine-grained sandstones and siltstones from the Cañadón Asfalto Formation were studied (Fig. 2). These samples were prepared according to standard palynological techniques, using non-oxidative acids (i.e. hydrochloric and hydrofluoric acids) to avoid affecting the natural colour of the organic components and the fluorescence of the hydrogen-rich particles (e.g. Tyson, 1995; Oboh-Ikuenobe and de Villiers, 2003). Two slides were mounted, the first made after the treatment with the hydrochloric and hydrofluoric acids to evaluate the total organic content, and the second following sieving the organic residue using a 10 µm mesh, leading to performing the palynofacies count (Batten, 1983; Batten and Morrison, 1983). The total organic carbon (TOC) analysis of selective 25 samples were performed in LABSPA (Soil, Plant and Environment Analytical Services) CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas)/UNS (Universidad Nacional del Sur), Bahía Blanca city, Buenos Aires province, Argentina. The evaluation of the samples according to their carbon richness expressed by TOC in weight per cent (wt %) provides useful information on hydrocarbon source rock potential. The palynological slides are housed in the Paleontological Museum Edigio Feruglio Repository, Trelew city, Chubut province, Argentina. They are identified by the catalogue number, preceded by the abbreviation MPEF-PALIN (Museo Paleontológico Edigio Feruglio, Palinología). The location of the different identified components of the palynological matter (PM) are referred to by England Finder coordinates (EFcos).

A palynofacies is defined as a distinctive palynological matter (PM) assemblage thought to reflect a specific set of environmental conditions or to be associated with a characteristic range of hydrocarbon-generating potential (Tyson, 1995, p. 4). Therefore, the integral palynofacies study involves not only the identification and quantification of the different organic components but also the description of the features of each one (i.e. size, shape, colour, and state of preservation). In this contribution, the evaluation of the PM was made considering two major categories (Table 1): (1) structured organic matter (i.e. palynomorphs, translucent, and opaque phytoclasts) and (2) unstructured organic matter (i.e. amorphous organic matter). The different organic particles were classified using terminology adapted from Batten (1983, 1996), Tyson (1995), and Oboh-Ikuenobe and de Villiers (2003). Cuticles and tracheids are very scarce; therefore, to avoid obscuring the meaningful information and/or real trend of this dataset with noise data, this type of particle has been added up to the woody remains under the “biostructured phytoclasts” category (Table 1). In order to quantify the relative abundance of each palynofacies component, a minimum of 500 particles were counted per sample under transmitted light (TL) microscopy (Olympus BX40). Based on their biological affinities, palynomorphs were classified into six major groups: lycophyte and fern spores, Pteridospermales and Coniferales pollen grains, Chlorophyta algae (Table 2), and fungal spores. A list of the identified palynomorph species is presented in Appendix A. The palynological count considered between 250 and 400 specimens per palynomorph's productive sample. Details of these statistical results were published in Olivera et al. (2015) and are shown in Table S1 in this contribution. The relative frequency (%) diagrams were calculated using TGview 2.0.2 (Grimm, 2004). Palynofacies data were interpreted using the multivariate statistical program PAST (PAleontological STatistics) by Hammer et al. (2001).

In order to form groupings of samples with a similar composition of PM (palynofacies type), cluster analysis (Q-mode) was selected as the most appropriate multivariate analytical method for this dataset. It was performed with the Euclidean distance and the unweighted pair group method (UPGM). The cluster analysis graphics indicate the cophenetic correlation. It is defined as the linear correlation coefficient between the cophenetic distances obtained from the dendrogram and the original distances (or dissimilarities) used to construct this graphic (Anderberg, 1973; Kovach, 1989). The magnitude of this coefficient should be close to 1 for a high-quality solution.

In addition, a palynofacies parameter, i.e. equidimensional : blade-shaped opaque particles (eo : bo) ratio, was considered.

The palynological samples were systematically examined on a standard magnification (×20 objective), under reflected fluorescence light microscope (RFL; Olympus BH2) in order to assess their preservation state and to improve the taxonomic palynological determinations (Tyson, 1995). In those samples with palynomorph content, the colour of the PM was qualitatively assessed applying the thermal alteration index (TAI) method (Staplin, 1969; Pearson, 1990), with the aim of estimating the thermal maturity of the PM. Psilate pollen grains belonging to the Inaperturopollenites Pflug and Thomson fossil-genus were selected for the measurement of the colour of palynomorphs on account of the relatively uniform thickness of its exine. In order to avoid a potential misinterpretation of the colour data caused by unusual thickening resulting from taphonomic folding, only the unfolded areas of the grains were considered. Based on the relative frequencies and fluorescence characteristics (i.e. relative intensity, colour, and heterogeneity) of the PM, the palynofacies assemblages were classified into four main kerogen types: essentially inert material (Type IV), gas-prone material (Type III), oil-prone material (Type II), and highly oil-prone material (Type I) according to Tyson (1995, p. 344).

A total thickness of 323 m of the stratigraphic section was described, sampled and analysed in detail in the Cañadón Lahuincó “A” section (Fig. 2). The lower part of the section is composed of an overall fining – and thinning – upward clastic succession extended up to about 112 m from the base. Internally, this lower interval shows several fining-upward cycles of up to 10 m thick. These cycles conform elementary depositional sequences (Zavala et al., 2024), commonly starting with coarse-grained sandstones and ending with fine-grained (silty-shale) deposits with abundant plant remains and conchostracan valves (Fig. 2). At 45 m from the base, the section is almost entirely composed of black shales. This shale interval is intruded by a more than 50 m thick andesite sill. At roughly 112 m from the base, this basal interval is in turn followed by an overall coarsening-upward succession (Fig. 2), extending up to 198 m from the base (Fig. 2). This succession is composed of shale and fine- to coarse-grained sandstone deposits commonly showing interbedded thin tuff levels. Internally, these deposits compose simple or coarsening-upward parasequences (Van Wagoner et al., 1990) up to 15 m thick, commonly showing abundant plant remains. Mudstone beds commonly bear conchostracan valves (Fig. 2). After a cover interval of about 2 m, the section continues with a fine-grained succession of shales with minor sandstone and limestone beds, each up to 40 cm thick (Fig. 2). At about 221 m from the base, the shales are sharply overlaid by fine- to coarse-grained sandstones, mostly composing coarsening-upward cycles up to 30 m thick, with abundant tractive structures and plant remains. This sandy succession is extended up to the top of the section, showing an overall progradational trend composing a progradational parasequence set (Van Wagoner et al., 1990).

The most conspicuous feature of the PM recognised in Cañadón Lahuincó “A” section is the dominance of terrestrial material, mainly phytoclasts (between 31 % and 100 % of the total PM; Figs. 3–5). However, considering the relative variations of each individual PM's category (Tables 1 and S2), four palynofacies types (A–D), and one outlier sample (MPEF-PALIN117) emerge from the cluster analysis (Fig. 6). The cophenetic correlation coefficient of the dendrogram graphic is 0.735.

The range of TOC in the entire dataset varies from 0.13 % (PT-D1) to 11.96 % (PT-C) (Fig. 9). The PT-D shows the lowest values of TOC among all studied samples (Fig. 9). Conversely, the PT-C presents the highest percentages of TOC registered in the samples studied here (Fig. 9), with an arithmetic mean of 6.43 %. The PT-B exhibits values that vary from 0.6 % to 4.3 % (Fig. 9), with an arithmetic mean of 2.9 %. The PT-A1 shows the higher values of the two defined sub-palynofacies, with an arithmetic mean of 1.55 %, while in the PT-A2 the average is 0.734 %. The MPEF-PALIN117 sample, widely dominated by opaque phytoclasts, shows a TOC of 2.04 %.

Sedimentological and sequence-stratigraphic analysis of the Cañadón Lahuincó “A” section revealed the existence of two depositional sequences (termed Sequence I and Sequence II), each one characterised by specific lacustrine conditions (Fig. 10).

Sequence I is extended up to 197 m and displays a complete transgressive-regressive cycle (Figs. 10e and 11a). The lower part (TST) of Sequence I (0–112 m; Fig. 10) is characterised by different hierarchical ordered fining-upward cycles (Fig. 11a). Fining-upward cycles are the consequence of the introduction of a substantial volume of water and sediments in a hydrologically closed basin, resulting in an overall punctuated lake transgression. The smaller-scale cycles compose metres-thick normally graded deposits controlled by high-frequency transgressive-regressive climatic cycles (TST-RST in Fig. 10a, b). During these high-frequency transgressive cycles, lake waters dilute and rise during TST periods (Fig. 10a). In contrast, during periods of low water/sediment supply (RST; Fig. 10b), the lake-level falls, favouring the subaerial exposure of marginal areas during a forced regression, with the local development of arid palaeosols (Fig. 11b). The low sediment supply also contributed to creating restricted lake conditions, favouring phytoplanktonic primary productivity (Figs. 10b and 11c). This interpretation is in line with Cabaleri et al. (2005), Volkheimer et al. (2008), and Soreda (2012), who interpreted the lower part of the Cañadón Lahuincó creek's deposits as accumulated in a closed lake. According to the current paradigm of lacustrine sequence stratigraphy (Zavala et al., 2024), these fining-upward cycles evidence an accumulation in a transgressive system tract (TST; Fig. 10) of an underfilled lake, which is extended up to the maximum flooding surface (mfs; Fig. 10).

The upper part of Sequence I composes an overall thickening and coarsening-upward succession extended up to 197 m (Fig. 10). Internally, this interval shows multiple coarsening-upward cycles of different scales and hierarchies (Fig. 2). The existence of coarsening-upward cycles suggests a coastal progradation in a lake having a limited accommodation space. In clastic lakes, this coastal progradation is only possible with a near stable lake level, which suggests that the maximum lake level was effectively controlled by the spillpoint. According to the sedimentological and stratigraphic evidence (Zavala et al., 2024), this interval is interpreted as accumulated during an overall regressive system tract (RST) in balanced-fill/overfilled lake conditions (Fig. 10c–e) by a stacked succession of littoral delta deposits (Fig. 11d–f). The interval located between 112 and 137 m is interpreted as accumulated in a balanced-fill lake (Fig. 10c), characterised by high-frequency climatic cycles, with alternating transgressions and delta progradations (TST – early RST; Figs. 10c and 11g) followed by a lake restriction (late RST; Fig. 10d). The last lake restrictions favoured the lacustrine primary productivity (Figs. 7g, h, and 10d). The upper part of the RST of Sequence I shows multiple progradational cycles with abundant plant remains, which suggests an accumulation in littoral delta systems in an overfilled lake (Fig. 10e). The RST of Sequence I is extended up to the maximum regressive surface (MRS), which marks the boundary with Sequence II. Sequence II shows an overall finning and then coarsening-upward succession extended up to the top of the section (324 m; Fig. 10). The basal boundary of Sequence II suggests the occurrence of a drastic change in the depositional conditions (paraconformity), evidenced by a sudden increase in basin accommodation, probably driven by an increase in tectonic subsidence. As a consequence, Sequence II starts with a basal shale interval (TST) composed of multiple small-scale cycles of shale interbedded with sandstone and limestone beds. These cycles suggest an accumulation in a partially restricted and closed lake (Fig. 11h, i). Sandstone and shale deposits are mostly associated with high-frequency transgressive periods (Fig. 10f). In contrast, limestone beds probably represent punctuated periods of lake restriction during high-frequency RST periods (Fig. 10f) with alkaline waters in an underfilled lake. This fine-grained interval is in turn sharply overlaid (222 m) by a dominantly sandy succession showing punctuated coarsening-upward cycles (Fig. 10). The basal sharp boundary with the RST of Sequence II is interpreted as a forced regression (FR), resulting from the breaching of the lake spillpoint and the onset into overfilled lake conditions (Zavala et al., 2024; Figs. 10h and 11j). The multiple coarsening-upward successions recognised in this upper section are interpreted as accumulated by progradational littoral deltas developed in an overfilled freshwater lake.

Despite the wide dominance of land-derived material, such as phytoclasts and fibrous AOM, in all the defined palynofacies types (Table S2) that indicate an overall high terrestrial input to the basin, the relative variations in the 12 PM categories (Table 1, Figs. 3–5) suggest the development of small-scale palaeoenvironmental changes into the major recognised sedimentary cycles (Fig. 10). The base of the underfilled succession (TST) of Sequence I is characterised by the intercalation of two PTs, i.e. PT-D and PT-A (Fig. 3). The PT-A shows the largest percentages of phytoclasts (up to 100 % of the total PM; Table S2) among all the defined PT, especially the assemblages belonging to the PT-A2 (Figs. 3 and 4). The PT-A2 PM was mainly recovered from fine-grained sandstone levels (Fig. 2). As previously noted by Fisher et al. (1979), the response of organic particles to bottom shear is usually similar to that of spheres of the same density, and the macrophyte debris have shown to present a similar hydrodynamic equivalence to coarse silt and fine sand. Among the phytoclast group, the black-brown fragments with rounded edges are dominant in this sub-palynofacies (Fig. 7f). High frequencies of dark-coloured rounded edge phytoclasts have been previously interpreted as recycled from older deposits and/or transported far from their source before deposition (e.g. Oboh-Ikuenobe and de Villiers, 2003; Falco et al., 2021; Chalabe et al., 2024). Furthermore, Tyson (1995) pointed out that woody material is selectively preserved and rounded by reworking in the bed load. In addition, the scant recognised AOM shows a poor degree of preservation (weak fluorescence), and the recovered palynomorphs are represented by highly degraded non-fluorescent Botryococcus colonies. The taphonomic attributes of Botryococcus were previously interpreted by Hutton et al. (1980) as the result of penecontemporaneous reworking, i.e. that which occurs over a relatively small time-scale (sensu Cushing, 1964). These features suggest the transport and accumulation of these deposits in a relatively high-energy oxidising environment. This could be related to the intensification of rainfall during a wetter period, leading to extraordinary fluvial discharges (i.e. hyperpycnal flows). The PT-A1, recovered from relatively finer lithologies (Figs. 3–5), shows a higher participation in the assemblages of opaque phytoclasts, mainly blade-shaped ones (Figs. 3–5, Table S2). Based on the physical and morphological characteristics of the equidimensional and blade-shaped opaque phytoclasts, variations in the eo : bo ratio values have been previously interpreted as variations in the flow velocity (Chalabe et al., 2022). When the flow velocity suffers a significant decrease, the relatively lighter, more buoyant material (i.e. blade-shaped opaque particles) is progressively accumulated by fallout. Thus, the PT-A1, which exhibits the lowest values of eo : bo ratio in the lower TST succession (Figs. 3–5, Table S2), could be related with its accumulation in relatively distal settings from a waning flow during the wetter season. Conversely, the PT-D shows a wide dominance of non-fluorescent AOM (Figs. 3 and 8d, e) and a high degree of degradation in all recovered PM. These features could indicate unfavourable environmental conditions for the preservation of the organic matter, with the selective destruction of the less resistant material. It could be related to the exhumation and reworking of these deposits in a context of high-energy fluvial discharges, mainly reflecting the taphonomic history of the palynological assemblages more than a specific depositional setting. However, the greater presence of blade-shaped, opaque phytoclasts in PT-D1 (MPEF-PALIN118) (Fig. 8d) could indicate their accumulation in a more distal position and/or a lower degree of progress in the rotting and coalification processes of the organic matter compared to PT-D2. This may be related to a change in the physical and chemical environmental conditions, at least at a local scale (e.g. more anoxic conditions in the pore spaces).

As the PT-A and, possibly, the PT-D, seem to indicate a wet period, the PT-B appears to reflect its accumulation during a drier one. This PT shows the highest values of highly fluorescent, well-preserved Botryococcaceae algae identified in all the studied samples (up to 32.38 % of the total PM) (Figs. 3, 5, and 7g–j). This algal family is a successful coloniser of shallow alkaline oligotrophic to mesotrophic waters under a wide range of climatic conditions throughout the year (Guy-Ohlson, 1992; Tyson 1995; Zippi, 1998). Botryococcus presents a series of adaptive strategies allowing its survival in some adverse conditions in which other types of algae cannot grow, such as the ability to store a large amount of reserve food and a type of wall that is resistant to desiccation (Guy-Ohlson, 1992). These features enable them to withstand relatively dry periods. Medeanic et al. (2010) documented wetter and drier periods in southern Brazil during the 20th century, based on algal palynomorph frequencies. They noted that the lowest sporomorph percentages and the complete Botryococcus predominance in the assemblages indicated drought periods. These authors pointed out that these intervals coincided with the lower water levels in the Mirim Lagoon and related them to variations in regional rainfall. Thus, the highest percentages of Botryococcus recorded in this study, along with a relative decrease in the phytoclast content (Figs. 3 and 5), could be interpreted as a drier period with an elevated evaporation rate and a decrease in the superficial run-off, which would have resulted in a contraction of the water body (Fig. 10b, d). In addition, these blooms (i.e. high primary productivity) would have produced oxygen-depleted conditions in the bottom waters (e.g. Papa et al., 2008; Fantasia et al., 2021), favouring the excellent preservation of the PT-B organic matter.

The PT-C shows transitional features between the PT-A and PT-B depositional conditions (Figs. 3 and 5). The lower frequencies of land-derived PM recorded in the PT-C assemblages compared to the PT-A (Table S2) suggest a reduced supply of water and sediments to the basin during the accumulation of these deposits. These drier climatic conditions are also indicated by a higher Botryococcus content and better preservation of the colonies (Figs. 3 and 5). Conversely, the PT-C assemblages exhibit a higher proportion of terrestrial material and a lower proportion of algae compared with the PT-B (Figs. 3 and 5), suggesting that the maximum aridity reflected in the PT-B PM had not yet been reached. In the upper part of the underfilled succession, an andesite sill body is identified (Fig. 2). The sample MPEF-PALIN117 comes from 1 m of this igneous rock. The recovered PM, which is dominated by dark particles, shows a high degree of oxidation (Fig. 8f). The high maturity and poor preservation of this organic matter may be attributed to the diagenetic effects of the intrusive emplacement.

A noteworthy feature of the underfilled palynological assemblages is the scant presence of land-derived palynomorphs (i.e. sporomorphs). Only two samples (MPEF-PALIN116 and MPEF-PALIN119, PT-C) show a low percentage of poorly preserved sporomorphs (1.73 % and 9 % of the total organic spectrum, respectively; Fig. 3). Behrensmeyer et al. (2000) pointed out that underfilled basins are not environments that enhance the preservation of land plant fossils. Nevertheless, compression-impression plant specimens with preserved organic tissues are recovered from levels located on the opposite side of the Cañadón Lahuincó creek that can chronocorrelated with this part of the section (Olivera et al., 2015, and bibliography therein). This may suggest that the chemical conditions of the underfilled lake's water (mainly pH conditions) would be highly unfavourable for the preservation of the sporopollenin, the organic compound that forms the sporomorph wall. The highly resistant biopolymer that composes the wall of the Botryococcus alga and the plant cuticles (e.g. Nip et al., 1986,; Zippi, 1998) could explain their presence in these deposits.

Overlying the basal TST of Sequence I, an RST is recognised, composed of balanced-fill/overfilled lake deposits (Fig. 10). The first noteworthy feature of the PT recovered from the finer-grained parasequences (between the MPEF-PALIN120 and MPEF-PALIN138 samples) is that the palynomorphs, and especially the sporomorphs, are present in nearly all the assemblages (Figs. 3 and 5, Table S2). The presence of this PM's group was such that enabled statistical counts (Table S1). Several authors have previously mentioned that the highest percentages of palynomorphs, and particularly the sporomorphs, are observed within the clay- to silt-size fraction as a consequence of its hydrodynamic equivalence with the fine quartz silt. Consequently, sediments comprising over 30 %–40 % sand-sized material generally exhibit lower sporomorph abundances (Tyson, 1995, and bibliography therein). Despite this, in the underfilled succession, siltstone levels were sampled, and in general no sporomorphs were recognised. As was previously discussed, this could be related to the physical-chemical characteristics of the water during the underfilled stage of the lake. Furthermore, the accumulation of the sporomorphs in different basin positions is related to the shape, size, and density of each sporomorph type. In general, all the PTs present high proportions of Hirmierellaceae pollen grains (i.e. Classopollis fossil genus), reaching ∼85 % of the total sporomorph group. The predominance of Classopollis in the palynological assemblages have been previously related to different factors, such as the primary productivity of the parental plants, the habitat in which they are thought to have developed (i.e. proximity to the site of deposition as marginal vegetation), and transportation sorting (Olivera et al., 2015, and bibliography therein). Nevertheless, it is noteworthy to mention that the PT-A1, which shows the highest percentage of land-derived organic components (phytoclasts plus sporomorphs), present a relatively high proportion of araucariacean pollen (up to 23.4 % of the total palynomorph group; Table S1). This pollen type has been previously considered as a water-transport palynomorph, because of the structural characteristic of its exine and its morphological features that enable it to be transported primarily via water (e.g. Caccavari, 2003; Holmes, 1994; Olivera et al., 2015, 2025). It reinforces the idea of the PT-A1 accumulation during periods with an increase in the water influx to the lake. In addition, it is interesting to highlight that the highest frequencies of Podocarpaceae pollen grains recorded here were found in two samples of the PT-A1 (MPEF-PALIN126 and MPEF-PALIN133; Fig. 7e). Saccate grains are generally associated with long-distance transport, particularly by wind, due to the presence of bladders that increase the pollen surface area while adding minimal mass (Schwendemann et al., 2007). Based on that, the relatively increase in the percentages of the saccate group is generally associated with accumulation in a relatively distal position (e.g. Tyson, 1995; Martínez et al., 2016). Thus, these two assemblages, which also show relatively low values of eo : bo ratio (Figs. 3–5), could be interpreted as accumulated in relatively more distal depositional settings.

Conversely, the PT-C exhibits the highest proportion of Hirmeriellaceae pollen grains recognised in all the PTs, along with a greater presence of tetrads and polyads (i.e. pollen grain agglomerates) of Classopollis (Fig. 7m, o). The record of tetrads and even polyads of any type of sporomorph has previously been considered as an indication of their accumulation near the source of vegetation (Maher, 2025, and bibliography therein). Moreover, the blade-shaped opaque phytoclasts present in these assemblages are larger than the equidimensional ones, which are regarded as more indicative of proximal positions (Tyson, 1995; Martínez et al., 2008b). All these features suggest the accumulation of this PM in a relatively proximal sub-environment of the lake system closer to the parental plants. The upward shallowing, progradational trend of the deposits (Fig. 2) yielding these assemblages, and the characteristics of these PM suggest its accumulation in a delta plain environment (Fig. 11f, g). The water characteristics of this particular depositional setting have been suggested as encouraging the development of fungal and bacterial aerobic biocenosis (Hart, 1994), and in general the palynomorphs identified in the PT-C exhibit a high degree of corrosion and degradation (Fig. 7n–p). The local accumulation of coal levels related to the swampy areas are previously pointed out as a common feature of the delta plain environment (Zavala et al., 2024), and the only coal level recognised in this study belongs to this PT (MPEF-PALIN122 sample; Figs. 4 and 11d). In addition, this assemblage presents the highest frequencies of Annella capitata Srivastava fungal spores identified in the samples studied herein (Fig. 7o, p).

In this part of the section, the PT-B shows a participation in the palynomorph assemblages of sporomorphs. Unlike the PT-C and PT-A1, these sporomorphs show an overall good degree of preservation. It could be a response to the development, at least temporarily, of the dysoxic to anoxic bottom-water conditions due to the algal bloom during drier periods (Fig. 10d). The presence of PT-B, and also the PT-C, in this first overall thickening and coarsening-upward succession (until sample 135) can be interpreted as showing fluctuating climatic conditions, i.e. wetter and drier periods. This palaeoenvironmental framework is in agreement with a balanced-fill lake stage (Fig. 10c, d), in which the PT-B would indicate the underfilled period (late RST) and the PT-A the overfilled one (TST – early RST; Fig. 10c, d). From this level upwards, the PT-B is not recognised, and the samples belonging to the PT-C only include a very low percentage of badly preserved algal colonies (Table S2). Thus, these palynomorph assemblages in the upper part of Sequence I could indicate the transition to an overfilled lake stage. From the MPEF-PALIN136 sample upwards to the section, the palynomorphs are only recorded in very low proportions in a few assemblages (Figs. 3 and 4). This may be related to the relatively coarser granulometry of these levels. The palynological assemblages recovered from Sequence II, after the MRS, belong to the PT-A (Figs. 4 and 10). The underfilled succession (TST) is characterised by the PT-A2 sub-palynofacies (Fig. 5), which would reflect the accumulation of these levels from relatively high-energy flows during intervals of rainfall intensification (i.e. wetter periods). In contrast, the limestone levels could indicate low clastic input to the lake during drier periods (punctuated by high-frequency RST; Figs. 10f and 11h, i). The PM assemblages recovered from the overfilled deposits belong to the PT-A1 (Fig. 5). This sub-palynofacies, which exhibits a relatively high proportion of opaque blade-shaped phytoclasts and weakly fluorescent AOM, could indicate more distal depositional settings and/or waning phases of the incoming fluvial flows. The unique presence of this sub-palynofacies in this part of the section, alongside the overall progradational trend of this succession, suggests a transition towards a more humid climatic condition during the infill of the lake.

One of the primary stages in the study of a rock's hydrocarbon potential is the determination of its organic content. The TOC analysis is generally used as a proxy for determining this parameter. Despite the fact that TOC percentages are not strictly equivalent to the organic matter amount, it results in a very good approximation (Tyson, 1995, p. 81). Further discussion can be found in the literature explaining, or trying to explain, the different environmental factors that could condition TOC values (i.e. organic matter accumulation) with the main aim of predicting possible source hydrocarbon rocks (e.g. Tyson, 1989; Mustafa and Tyson, 2002; Li et al., 2021). The sediment granulometry and the predominant nature of the organic matter emerge as two important factors that condition the organic matter accumulation (Tyson, 1995, and bibliography therein). It is widely accepted that the finest lithologies contain higher TOC values than the coarser ones, because of the organic components show similar settling velocities (i.e. hydraulic equivalence) to the fine-grained mineralic particles (in Tyson, 1995). When the percentages of TOC in the studied samples are observed in relation to the defined PTs, with the exception of one sample (i.e. MPEF-PALIN134, PT-A1), an overall trend can be distinguished (Fig. 9). The PT-D, dominated by non-fluorescent AOM (Figs. 3 and 9), exhibits the lowest TOC values, which are probably related to the accumulation of these palynological assemblages in a dynamic system, with sediment reworking under oxidising conditions that do not promote the preservation of the organic components. Conversely, the PT-C, followed by the PT-B, shows the highest TOC values registered in the samples studied here (Fig. 9). There is a general consensus that elevated percentages of TOC are related to low depositional rates that promote a minimal dilution of the organic matter with the clastic material (e.g. Tyson, 1995; Zavattieri and Prámparo, 2006; Zavala et al., 2024). Both palynofacies types have been interpreted as accumulated in periods of a relatively decreased water supply, and hence sediment influx to the basin. In the context of highly variable climatic conditions, the PT-B would represent the driest periods, and the PT-C could be pointed out as the transition between this condition and others much wetter, in which extraordinary precipitations could promote extraordinary fluvial discharges (PT-A). It is interesting to highlight that, in contrast with other lacustrine basins, such as the Cacheuta Basin (northwestern Mendoza, Argentina) in which the organic-rich shales (average TOC of 4 %, locally reaching 20 %) have a PM dominated by algal-derived organic matter (Zavattieri and Prámparo, 2006), the highest TOC percentages (MPEF-PALIN129 and MPEF-PALIN134 samples) are recorded in samples containing a kerogen of predominantly terrestrial origin (Tables 1 and S2), i.e. sporomorphs, phytoclasts, and fibrous AOM (Table 1, Fig. 9). This could be related to the relatively low initial H : C ratio (usually less than 1.0) and a high initial O : C ratio (up to 0.3; Tyson, 1995) in this type of organic matter. The preservation of this PM could be related to meromictic intervals in the lake. Tyson (1989) pointed out that during meromictic periods, highly organic-rich kerogenous shales are accumulated. The lack of mixing during these intervals could have favoured the intrinsic preservation potential of the organic matter in the basin. This could explain the two highest percentages registered in samples with a minimal autochthonous organic content (MPEF-PALIN129 and MPEF-PALIN134 samples). On the one hand, the complete predominance of terrestrial material in the PT-A, PT-C, and PT-D (Table S2, Fig. 9) mainly reflects a kerogen of Type III composition. On the other hand, the presence of strongly fluorescent organic matter including structured (i.e. Botryococcus colonies) and algal-derived amorphous material (i.e. spongy AOM, Table 1 organic categories) reaching up to 39.43 % of the total organic spectrum identified in the PT-B, suggest a mixed Type I/III kerogen composition.

The TAI values observed in the samples containing the palynomorphs selected for this analysis range from 2+ (PT-A1; Fig. 7d) to 3- (PT-C; Fig. 8a), indicating an early to middle-mature phase (oil/gas-prone; Pearson, 1990; Tyson, 1995). The fluorescence of the AOM and the sporomorphs is, in general, weak to moderate, with the exception of those recorded in the PT-B. Nevertheless, as the main proportion of the recovered PM is in accordance with the definition of kerogen Type III (sensu Tyson, 1995) – in terms of the nature of the organic matter, degree of preservation, and thermal maturity – these deposits appear to be a potential unconventional shale gas reservoir.