Cecilia Andrea Benavente

We present a new record of Actinopterygii from continental Triassic rocks from southern South America, Argentina. The material provenance is the Mollar Formation, Santa Clara subbasin, Cuyana rift basin. We studied three specimens preserved in black finely laminated mudstones with different grades of preservation, most of them incomplete but articulated or in casts. These Actinopterygii specimens have been identified as members of the Pseudobeaconiidae family based on their unique combination of diagnostic characters. Pseudobeaconiidae is endemic to southwestern Gondwanan lacustrine settings and in Argentina is restricted to the Ischigualasto-Villa Unión and Cuyana rift basins, being the fishes reported here the first vertebrate remains from the Mollar Formation. Considering Anisian age previously obtained for the overlying Santa Clara Abajo Formation, the fish remains reported here could represent early Anisian or older times, extending the biochron of Pseudobeaconiidae some Myr back. In this context, we revised fish remains from the Middle-to-Upper Triassic Estratos El Bordo unit, in northern Chile, previously assigned to Pseudobeaconiidae, and reassign them as cf. Pseudobeaconiidae because the morphological characters observed hinder the confirmation of most of the diagnostic characters of the family. Further exhaustive preparation of the Mollar Formation material will allow more accurate taxonomic assignments, for example, if these specimens represent a new species of the family Pseudobeaconiidae.

Facies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio‐induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short‐distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

The lake‐basin‐type model classified the stratigraphic record of ancient lake systems according to rates of potential accommodation relative to sediment + water supply. The model convolved all modes and paths of water supply (direct fall, surficial, subsurface) with amounts and types of sediment supply (clastic, biogenic, chemical) into a single basin‐filling volume term (sediment + water); its major strength was its widespread applicability. This was supported by subsequent investigations confirming the utility of this approach, but it also revealed some important limitations due to simplifications in the original model. The model has been expanded here to address all inland waters (lakes, ponds, wetlands, playas) as well as adding two major subdivisions of the sediment + water term: (1) water supply paths and (2) the volume of water supply relative to sediment supply. Water supply flow paths in the subsurface are subdivided into ‘throughflow’, ‘recharge’ and ‘discharge’. Each of these groundwater hydrology states can be defined quite precisely by the ratio of net outflow to inflow, from persistently open to consistently closed. These paths can be deciphered using stable carbonate and oxygen isotope composition of primary lacustrine limestones, detailed sedimentology, stratigraphy, palaeontology and mineralogy. Distinguishing water supply paths provides additional insights into playa systems and the occurrence and character of evaporites and carbonates. The volume ratio of water to sediment supply most directly influences the water depths of lakes, ponds and wetlands, which affect water body hydrodynamics and ecosystem behaviour as well as the details of stratal stacking and depositional sequences. It helps fine‐tune estimates of the distribution of sediment texture, bedding, composition and organic matter content. The aim of this contribution is to address questions about the fundamental types of inland water bodies and to explain the new lake‐basin subtypes and provide examples that illustrate their potential to enable higher‐resolution, robust analysis of inland water systems and their stratigraphic records.

Spinicaudata are freshwater branchiopods. The growth and development of spinicaudata populations in freshwater environments depend on various physical, chemical, and biological factors. The action of volcanism modifies the limnological parameters in the process of fossilization of lacustrine deposits. This study identifies spinicaudatan assemblages in two Triassic lake systems in Argentina, and analyzes the dynamics of shell preservation according to the volcanic activity involved. Two taphonomic grades divided into three different second taphonomic grades were defined. The focus on taphonomic and chemistry studies addresses changes in spinicaudata abundance and richness through time, in different paleoenvironmental conditions, and shell preservation and chemical composition. The discussion highlights the importance of lava flow inflow and volcanic ash fall in the different lake systems. We emphasize the importance of taphonomic studies on the spinicaudata groups to understand their autochthonous and allochthonous populations in Triassic lake systems. In turn, we conclude that the post mortem processes experienced by the shells, the exposure time at the water-sediment interface, and the chemical alteration of the water into the paleolakes produced by the surrounding volcanic activity can explain the development and preservation of spinicaudata in these two Triassic lacustrine systems.

The lithostratigraphic units that conform the Sorocayense Group and fill the Sorocayense-Hilario sub-basin represent an alluvial-fluvial-lacustrine succession with significant volcanic supply during its deposition. They are namely the Cerro Colorado del Cementerio, Agua de los Pajaritos, Monina, Hilario and El Alcázar formations and present several carbonate levels. The genesis, diagenesis, and main controlling factors on continental carbonates present a means to understand basin evolution through the study of their petrography and chemical elemental composition through cathodoluminescence techniques. We have identified six microfacies: a) homogeneous micrite, b) bioclastic micrite, c) dolomicrite, d) laminated micrite, e) oncolitic packstone, and f) sparitic carbonate. Among these microfacies, six cementation and alteration phases have been identified: a) micritization, b) mechanical compaction, c) calcitic cementation, d) sparitic cementation, e) microsparitic cementation, and f) chemical compaction. This analysis allowed establishing a chronology of the diagenetic modifications undergone by the carbonates. Results support diagenesis was controlled mainly by tectonics showing major impact in carbonate facies identified at the flexural margin of the rift. The effect would have been linked to exertion of a paleohydrological effect favoring lateral meteoric water migration through faulting. The presence of dolomite in some of the microfacies is linked to the presence of montmorillonite as the dominant Mg-rich-phyllosilicate in clay assemblages most likely acting as the potential source. In turn, Mg is more likely to be replaced by Mn leading to luminescent carbonate microfacies.

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Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto−Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ¹⁸O_carb −11.6‰ and −15.7‰ (χ average −13.1‰; 1σ = 1.6) and δ¹³C_carb −2.6‰ to −8.0‰ (χ average −5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Model estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

The Santa Clara Abajo Formation in the north of Mendoza Province, Argentina, is part of the Triassic Cuyana rift Basin infill and consists of fluvial, deltaic and lacustrine units. The combination of progradational to aggradational stacking pattern plus the identification of a fluctuating profundal facies association suggests the Santa Clara Abajo palaeolake was a balanced‐fill lake system. Lake‐centre sediments (finely laminated mudrock facies) preserve pseudobeaconiid actinopterygian fish represented primarily by bones and scales, generally undeformed and concordant with bedding planes. We propose that variation of articulation degree of these fish combined with sediment features provides insights into palaeolimnological changes in this single lake system over time, primarily water temperature. The fish fossils are sorted into three taphonomic modes: Mode A, isolated and dispersed scales; Mode B, associated but dispersed scales, loosely to well sorted, and low to moderate density; and Mode C, scale patches and articulated fishes. We conclude these modes represent, respectively, warm, fluctuating warm–cold and cold water conditions. Stratigraphical analysis of the taphonomic modes for the balanced‐fill lake model revealed an up‐section pattern of fluctuating water temperatures (cold and warm), to a stable‐cold pattern, to a stable‐warm pattern and to fluctuating thermal conditions (cold and warm) again. This pattern is linked to lake‐level fluctuations resulting in lacustrine intervals that reflect hydrological closure to opening to closure again.