Revista de la Asociación Geológica Argentina

The Jurassic lacustrine record of the subsurface of the Austral-Magellan Basin and its potential as a source rock for hydrocarbons.

Hernán de la Cal — 2024-09-15

The Austral Basin developed as the result of widespread Jurassic rift event that affected an igneous-metamorphic basement of Paleozoic age. During this process, several depocenters were developed and filled with clastic, volcanic and mixed nature sediments. The volcanic deposits originated during this event have been grouped by other authors in the Chon Aike Magmatic Province, which represents a large felsic LIP that can be recognized throughout all of Patagonia. Interdigitated within the volcanic rocks, some clastic continental deposits have
been documented, associated in many cases with swamps and/or lacustrine environments. These deposits have been poorly studied as possible source rocks for the hydrocarbons generated in the basin, so in this paper we present an integral study analyzing the oleogenetic potential of these rocks in a basin wide context. This contribution presents a lithological and geochemical characterization of cutting and drill-cores obtained from exploratory hydrocarbon wells, material that was preliminary assign to a good oleogenetic potential in the original well description files. This study also includes new, previously unpublished, material from other wells across the basin, including wells located in Chile and from the offshore sectors of the Argentinean Sea. Finally, descriptions of the fossil content found in one of the wells is reported, which constitute the first record of this kind for the subsurface of the basin. Results show that the studied rocks have good oleogenetic properties, with a strong terrigenous imprint and participation of high plants-derived material, denoting a lacustrine and/or swamp environment.
As for the reported fossil material, this represents the first mention of this kind of remains for this unit. They are mainly composed of unknown plant remains that are accompanied by typically continental invertebrates composed of crustaceans (Branchiopoda, Diplostraca) and other insect remains (Coleoptera). The identified Crustaceans, commonly known as “estherias” or “conchostracos” were preliminarily assigned to three different families (Palaeolimnadiopseiae, Fushograptidae and ?Eosestheriidae) similar to other South American findings. Finally, the insect remains (Coleoptera) are represented by two sets of elytra that can be assigned to two different families, one (Cupedidae) with a known record in some South American Triassic units and the other (Asiocoleidae) representing the first of this family for the Southern Hemisphere.

Coral-dominated communities from the Jurassic of Argentina: an overview

Javier Echevarría — 2024-09-15

This paper is meant as a synthesis of the knowledge on the Jurassic coral-dominated communities of Argentina, providing novel information for some of them. Scleractinian corals were scarce during the Hettangian-Sinemurian, but were frequent and diverse during the Pliensbachian. Three main communities are recognized for this last age: the earliest Pliensbachian Puesto Araya reef (Mendoza Province), the late Early to early Late Pliensbachian Piedra Pintada coral meadow (Neuquén Province), and the Late Pliensbachian to earliest Toarcian patches from Chubut Basin. After the Early Toarcian biotic crisis, scleractinian corals reappear in the Jurassic basins of Argentina in the latest Aalenian to Early Bajocian. By this last age they appear in great diversity, probably generating meadows. There are also some Callovian coral records spread throughout the Neuquén Basin. During the Oxfordian, coral-dominated communities reached their maximum development in the Jurassic of Argentina. A large pseudobarrier of 13 km of lateral extension was described for Bardas Blancas (Mendoza Province), recognizing different zones within it. Coral meadows were described at Coihue Co and Covunco, while small patches developed at Las Lajas (Neuquén Province). Finally, during the Tithonian, coral meadows developed at Picún Leufú (Neuquén Province), and some corals were recorded in Mallín Cotidiano (Chubut Province, Austral Basin). The general trends observed for corals in Argentina (diversity peaks during the Pliensbachian and Bajocian, severe reduction during the Toarcian, the Oxfordian bioconstruction optimum) match the ‘global’ trends based mostly on the records from the northern hemisphere.

Didemnoides Bonet y Benveniste-Velasquez (didemnid ascidian spicules) in the Tithonian–lower Berriasian of the Neuquén Basin and its sequence stratigraphic significance

Diego A. Kietzmann — 2024-09-15

In this work, specimens of the genus Didemnoides Bonet and Benveniste-Velásquez are described from different sections of the Tithonian-Berriasian interval of the Neuquén Basin (Vaca Muerta Formation). Didemnoides has been interpreted as spicules of ascidians related to the family Didemnidae Giard, so this record represents the first documentation of tunicates for the Tithonian – Berriasian of the Neuquén Basin. The presence of spicules of didemnid ascidians allow to infer the development of hard substrates, high availability of nutrients, low water turbidity and low fresh water influx. The location of the Didemnoides-bearing levels within the sequence stratigraphic framework of the Vaca Muerta Formation evidences its development during transgressive system tracts, where these conditions prevail.

Analysis of stratigraphic discontinuities of the Lajas Formation (Cuyo Group) in Puesto Seguel, Neuquén Basin

Candela González Estebenet — 2024-09-15

The analysis of the Lajas Formation is carried out in the Puesto Seguel, where two unconformities have previously been recognized: Seguélica and Lohanmahuídica. New sedimentological characteristics are described and allow to refine previous paleoenvironmental interpretations based on four intervals (A-D) that documents new features like prograding platform (A), marginal marine deposits with the Teredolites Icnofacies (B), normal marine deposits with corals, ammonites and bivalves (C) and fluvio-tidal transition and purely fluvial (D). The Seguélica unconformity is located between the interval A and B and exhibits angularity between them. Considering the ammonite known from the interval A corresponds to the Giebeli Zone (lower Bajocian) and the new biostratigraphic indicators (interval C) that includes the first record of ammonites above the unconformity assignable to Choffatia indet. of the Cadomites-Tulitidae Association Zone or the Steinmanni Standard Zone and bivalves from the Retroceramus stehni Association Zone, suggest a middle to Late Bathoninan age. Then, the unconformity evidence a hiatus that make impossible that the Lohanmahuídica unconformity be considered intrabajocian and a Late Bathonian or the Callovian age is proposed.

It allows the correlation with the Formation Lajas outcrops towards the north of the study locality.

El depocentro Alto Atuel: primeras descripciones de las unidades del Jurásico Inferior aflorantes en las nacientes del río Atuel

Federico Exequiel Martos — 2024-09-15

The Alto Atuel depocenter is located on the northwestern margin of the Neuquén Basin. This study focuses on the Jurassic volcano-sedimentary successions cropping out at the headwaters of the Atuel River, which have been scarcely studied over time. This depocenter, located at 34°35’S and 70°10’W in southern Mendoza Province, is of interest due to its position within the basin, which can provide information about the interaction between marine sedimentation and volcanic arc activity during the Early Jurassic. Lithological and biostratigraphic analyses were conducted on a stratigraphic log more than 450 meters thick, allowing the recognition of three sections based on their lithological characteristics. The basal section exhibits high-energy volcaniclastic and pyroclastic deposits. The middle section is dominated by pyroclastic deposits and lava flows interbedded with calcareous sandstones and marls containing marine invertebrate fossils. The upper section reflects a shift towards lower energy marine environments, where interbedded black shales, tuffs, and calcarenites with sparse fossil remains are predominant. The fossil content allowed dating the marine successions as late Pliensbachian–early Toarcian. On the other hand, the lithological analysis of the successions suggests that the Alto Atuel depocenter evolved in isolation, at least during the Early Jurassic, as indicated by the presence of a significant volcanic input, which is not evident in depocenters located immediately to the east, such as Atuel and La Valenciana depocenters.

Geology and tectosedimentary evolution of the Camp Hill Peninsula (Botany Bay), a key Jurassic locality of the Antarctic Peninsula

Roberto Scasso — 2024-12-23

Geology and tectosedimentary evolution of the Camp Hill Peninsula (Botany Bay), a key Jurassic locality of the Antarctic Peninsula. The Camp Hill Peninsula exposes a thick Jurassic clastic sedimentary succession overlying the Late Paleozoic basement of the Trinity Peninsula Group and covered by volcanic and volcaniclastic rocks of the Middle-?Upper Jurassic Antarctic Peninsula Volcanic Group. The Camp Hill Formation was subdivided from base to top in the Dúo, Orca, La Pera and Cascada members. The E-younging monoclinal succession is cut by several E-W and NE-SW oriented faults. Sedimentation took place in well-vegetated alluvial fan, fluvial and lacustrine environments, within tectonically-controlled half grabens formed during a rifting period at the inception of the James Ross Basin. Volcanic and volcaniclastic beds were gradually incorporated in the Camp Hill Formation and finally covered this unit forming the Antarctic peninsula Volcanic Group composed both, by intrusive bodies like sills and dikes, and by effusive products like tuffs, agglomerates, breccias and coladas, of andesitic to latitic composition. E-W and ENE-WSW faults controlled the sedimentation and emplacement of eruptive centers starting in the Middle Jurassic. These faults were reactivated during the Early Cretaceous by N-S directed tectonic compression correlative with strong deformation in the Palmer Land. During the late Early Cretaceous, NE-SW strike-slip and reverse faulting uplitfted the Jurassic rocks that underwent erosion and formed the western margin of the James Ross Basin. Final uplift of the region took place during partial inversion of the basin and E-shift of depocenters during the Late Cretaceous and Cenozoic.

Plant diversity turnovers in the Triassic–Jurassic transition: evidence from the paleobotanical record of Argentina

Josefina Bodnar — 2024-09-15

The Triassic–Jurassic transition was an important interval in Earth’s history, which encompassed two great biotic crises: the End-Triassic mass extinction ant the Early Toarcian Oceanic Anoxic Event. The response of the terrestrial vegetation to these crises has been the subject of debate. In Argentina, a floristic turnover occurring in the Triassic-Jurassic boundary has been suggested, which led to the disappearance of corystoperms and peltasperms, and to a greater influence of ferns, bennettitaleans and conifers. Recent findings have thrown into question the previous evolutionary models. With the purpose of understanding the terrestrial plant turnovers during the Triassic-Jurassic transition in Argentina, we calculated diversity indexes, survivorship proportions, and extinction rates, based on megafossil record. The results indicated that, despite only one lineage disappears in the Late Triassic (Pleuromeiaceae), an important but gradual vegetation change in the Triassic-Jurassic transition. The extinction rates indicate a peak at the end of Rhaetian and the diversity declines in the Hettangian. Late Triassic diagnostic lineages pass through the Jurassic but most of them with few occurrences and going extinct in the Early Jurassic. The Osmundaceae, Dipteridaceae, and Bennettitales diversify in the Sinemurian. In the Toarcian a taxa richness increase is recognized, marked by a conifer diversification. In the Bathonian-Callovian, the conifers and bennetitaleans acquire greater importance. The results show a high correlation between diversity and occurrences. The dataset can be improved in the future by intensifying fossil sampling.

Geographical and temporal distribution of tidalites in the Lajas Formation (Cuyo Group), Early─ Middle Jurassic of the Neuquén Basin

Candela González Estebenet — 2024-09-15

The Lajas Formation is a well-known unit from outcrops and subsurface of the Neuquén Basin (Cuyo Group), traditionally interpreted as a regressive deltaic system prograding up to the shelf edge. In this unit, the dominance of fluvial processes over tidal ones was emphasized in many facies studies. In this paper, a series of tidal indicators are analyzed in several localities located to the north of the Huincul High, like Puesto Seguel (1), Portada Covunco (2) and Sierra de la Vaca Muerta (3). Six types of tidal indicators have been recognized in the succession starting with those considered diagnostic of tidal origin like tidal bundles, and others like rhythmites, heterolithic arrangements, and lateral accretion channels with inclined heterolithic structures, dunes and bars, which are common in tidal dominated and modulated depositional systems, but not exclusive of them. The abundance of tidal indicators in the studied localities is contrasting with the record in those located to the south of the mentioned high, where a microtidal system amplified by structural features has been previously proposed. The reason of the documented differences is explained by the fact that successions situated at both sides of the Huincul High are not coeval in age, being younger those situated to the north of the high, where a delta system is not documented. This paper proposes that between the late Bathonian and early Callovian the sedimentary record to the North of the Huincul High was tide-dominated, and possibly a macrotidal depositional system developed.

Palaeoenvironmental significance of beds bioturbated by Haentzschelinia ottoi (Geinitz) in delta front facies, Lajas Formation (Middle Jurassic)

Graciela S. Bressan — 2024-09-15

The distribution of Haentzschelinia ottoi (Geinitz) is studied from the Lajas Formation (Middle Jurassic) of the Neuquén Basin in Arroyo Carreri. For this work were considered more than 200 specimens and their relation with the facies and other trace fossils, as well as the ichnofabric index (ii) and the bedding plane bioturbation index (bpbi) were determined. H. ottoi appears in fluvio-dominated delta front (FA2) and fluvio-dominated delta front to delta plain with tidal influence (FA3). In general, trace fossils correspond to deposit-feeders (H. ottoi, Planolites, ?Taenidium, ?Parahaentzschelinia), suggesting the development of an impoverished Cruziana ichnofacies. In particular, H. ottoi constitutes monoichnospecific associations with ii/bpbi = 2-3 (in FA2 and FA3) or more diverse associations with other trace fossils and ii/bpbi = 3-4 (only in FA3). The development of one association or another depends on the interplay between sediment discharge and salinity conditions. More diverse associations with ii/bpbi = 3-4 would be favoured by a rise in salinity under the influence of tides and enough time for substrate colonisation by other deposit-feeders. Lower salinity conditions or high frequency of sediment discharge would have given rise to monoichnospecific associations of H. ottoi with ii/bpbi = 2-3.

Integration of geochronological, lithofacial and paleontological data to refine the context of the Marifil Complex (Jurassic), Río Negro, Argentina

Cecilia Pavón Pivetta — 2024-09-15

In northeastern Patagonia, outcrops of the Marifil Complex located three km north of Mina Delta XXI expose a succession of volcaniclastic and sedimentary rocks 160 m thick. This succession, along with other correlatable deposits, were previously mapped as undifferentiated ignimbrites, tuffs, and sandstones in recent regional geological surveys. The Marifil Complex in the study area comprises ten distinct lithofacies, including paraconglomerates forming the basal layer of the volcano-sedimentary succession; coarse-grained arkosic sandstones; volcanic sandstones and siltstones containing fossil flora; mixed volcanic-clastic rocks; massive brecciated and bituminous limestones; massive lapilli-rich and crystal-rich eutaxitic tuffs; and porphyritic rhyolite intrusions that crosscut the other facies. The volcano-sedimentary succession studied is interpreted as having been deposited in a small lacustrine basin adjacent to an explosive volcanic center. The macrofloral assemblage includes equisetalean stems assigned to Equisetites sp., conifer vegetative and reproductive structures (Pagiophyllum spp., a probable bract/seed-scale complex and a probable pollen cone), and an incomplete leaf fragment with reticulate venation of uncertain affinity. Additionally, a gymnosperm pollen grain assigned to Inaperturopollenites indicus Srivastava was identified from the same levels. The age of this geological unit, determined by U-Pb zircon geochronology, is 189.5 ± 2.2 Ma, constraining the deposition and associated paleoflora to the Sinemurian-Pliensbachian boundary. These volcanic and volcano-sedimentary deposits are associated with the development of an extensional basin, possibly linked to the breakup of Gondwana.

Prologue to the special issue on the Jurassic of Argentina

Susana Damborenea — 2024-09-15

This special issue is dedicated to Dr. Alberto Carlos Riccardi, who is an active promoter of collaboration between researchers in different aspects of the Jurassic of South America. As such, he was the organizer of the “4th International Congress on Jurassic Stratigraphy and Geology” (Neuquén-Mendoza, 1994) and mentor, as President of the Argentine Committee on Stratigraphy of the AGA, of the Argentine Symposia on the Jurassic, which have been held every 3 or 4 years since 2003 in different locations in Argentina. This special issue contains a series of works that emerged precisely from the last Argentine Symposium on the Jurassic (2023). There are 10 contributions made by more than 40 specialists, covering various basins of the territory of the Argentine Republic, including the Antarctic Peninsula, under very varied approaches, including academic and applied aspects. Sedimentological and lithofacial, stratigraphic and geochronological, tectonic evolution, ichnological and paleoenvironmental analyses are addressed in sedimentary (marine and continental), volcanic, or mixed contexts. Paleontological studies with a southern focus are presented, both with synthesis works for various fossil groups and with descriptions of the first records. All this demonstrates the current interest in these meetings and the permanence and relevance of their initial objectives.