Paganzo Basin Research Articles

The lacustrine ecosystem of the Bajo de Veliz Formation: A wet spot at the Carboniferous-Permian boundary in westernmost Gondwana

During the Carboniferous-Permian transition in the westernmost part of Gondwana, the environment became drier and more seasonal than the preceding humid estuarine and fluvial habitats of the Middle Pennsylvanian of central western Argentina. The Gzhelian-Asselian Pallero Member of the Bajo de Veliz Formation, located to the east of the Paganzo Basin, represents an ecosystem preserved in lake deposits. This unit comprises lacustrine facies, and the fossil flora and fauna contained therein represent a wet refuge developed within a regionally semi-arid context. The flora is assigned to the Gangamopteris Biozone and includes specimens of lycophytes, equisetaleans, cordaitaleans, coniferophytes, and glossopterids that grew around the lake shoreline. Winged insects, arachnids, large eurypterids, and traces of fishes comprise terrestrial and aquatic fauna. This ecosystem is considered a wet spot similar to those that characterize the global paleoclimatic shift in low paleolatitudes.

Coniferophytes from the Bajo de Veliz Formation (Gzhelian-Asselian), San Luis Province, Argentina

In this contribution, we analyze the presence of fossil coniferophytes from the Bajo de Veliz Formation (Paganzo Basin) that were initially referred to Walchia or Paranocladus and have not been described until now. These specimens consist mainly of leafy branches exhibiting foliar heteromorphism characteristic of upper Paleozoic coniferophytes from Gondwana. This group includes Genoites sp. with lax arranged ovules on the last order branches, Krauselcladus sp. represented by a small vegetative branch with biand trifurcated leaves, and Buriadia cf. Buriadia heterophylla Seward et Sahni emend. Serbet, Escapa, Taylor, Taylor et Cúneo, 2010, showed axes with multifid leaves and ovules on terminal branches. Vegetative fragments with small lanceolate leaves are assigned to Ferugliocladus cf. F. riojanum Archangelsky et Cúneo, 1987. In addition, an isolated, probably male reproductive structure similar to those known for th latter species was identified. The presence of Buriadia is recognized for the first time in the Carboniferous-Permian of Argentina, as well as the identification of vegetative buds or shoots, a previously undocumented characteristic for this Paleozoic coniferophyte from Gondwana.

New age, stratigraphic and paleoenvironmental interpretation of the Loma de los Piojos Formation (San Juan Province, Argentina) based on new palynologic and stratigraphic information

A physical stratigraphic study of the Loma de los Piojos Formation (LPF) and related units at the homonymous locality (5 km north of Jachal city, San Juan Province, Argentina) was carried out. The first palynologic result is presented herein from the shale beds of the Loma de los Piojos Formation, just below the deposits bearing floral remains of the Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii (FNC) informal zone of Balseiro et al. (Serpukhovian sensu lato). The Loma de los Piojos does not have an exposed base, and the Guandacol Formation (late Serpukhovian–early Pennsylvanian) overlies it. Also, the field work analysis allowed the association of LPF shales with the last depositional sequence of the recently defined and underlying Don Buenaventura Formation, of late Mississippian–earliest Pennsylvanian age. The four fertile samples obtained from the shale beds of the LPF yielded 45 species of terrestrial plants. Monosaccate pollen grains (Cannanoropollis janakii, Cannanoropollis mehtae, Circumplicatipollis plicatus, Crucisaccites monoletus) represent the Cordaitalean and Coniferalean that varied in their frequency c. 25 % in the lower two samples to 34 % and 7 % in the other two. Spore species of lycophytes dominate in all samples (ca. 50–80 %) along with other spores that appeared in the late Serpukhovian–Bashkirian subzone A of the Raistrickia densa–Convolutispora muriornata Palynozone (DMZ). However, in the lowest sample the key spore Spelaeotriletes ybertii and a well-preserved specimen of the striate bisaccate pollen Illinites unicus allowed the correlation with the late Bashkirian–Moscovian Subzone B of DMZ. Therefore, the informal floral zone Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii is here reallocated to the Nothorhacopteris–Botrychiopsis–Ginkgophyllum (NBG) Zone, which is relevant in the global context of the evolution of the plant groups involved. This dark shale section would correspond to a warmer interval during which the maximum flooding zone was dated c. 320 Ma locally at the Huaco section (also close to Jachal city), also documented across the Paganzo Basin up to the Paraná Basin in Brazil. The three Late Paleozoic mentioned units were deposited within the largest incised valley complex of this region based on the amount of preserved depositional sequences (7), thickness (>1 km), and time span (from Late Mississippian to lower Pennsylvanian). This complex stratigraphic succession formed within a large paleovalley probably acted as a trunk drainage system connecting depositional areas from the continental interior with coastal areas of the Gondwanan continental western margin.

Equisetaleans from the Bajo de Veliz Formation (Gzhelian-Asselian): a new key in the evolution of Gondwanan reproductive structures

ABSTRACT Equisetaleans are a frequent floral element in the upper Palaeozoic of Gondwana and traditionally have been assigned to a few species of Phyllotheca based mainly on vegetative features. However, studies have suggested that the reproductive organs of Gondwanan equisetaleans may be more complex than previously thought, and new Gondwanan taxa have been reported during recent decades. The evolution of these plants is believed to have been driven by changes in their reproductive organs, including the emergence of simple sporangia or cruciate sporangiophores at the internodes as well as the development of strobilar structures. Here, we describe new records of Cruciatheca Cúneo et Escapa and Conustheca gen. nov. from Gzhelian-earliest Cisuralian deposits in the southeastern boundary of the Paganzo Basin, Argentina. The new species with whorls of strobilus-like structures provides further evidence of a diverse equisetalean flora across the southern continents. These records also suggest that the evolution of these equisetaleans was complex, with multiple lineages and distinct morphologies.

Pennsylvanian palynofloras from the Retamito Formation (Argentina): Their relationship with the marine transgressions in westernmost Gondwana

The westernmost Carboniferous Gondwanan deposits in Argentina recorded the Late Paleozoic Ice Age (LPIA) and the subsequent sea level changes in different fossiliferous stratigraphic units. One of them, the Retamito Formation, located at the southern extreme of the Paganzo Basin in the San Juan Province, contains the first Carboniferous paleoflora described in the country at the end of the 19th century. A recent revision of the outcrops allowed the recognition of invertebrates in marine strata linked to the Moscovian postglacial sea level rise characterized by the Tivertonia jachalensis-Streptorhynchus inaequiornatus (TS Biozone). These deposits overlie Bashkirian coal beds bearing plant remains of the Nothorhacopteris-Botrychiopsis-Ginkgophyllum flora (NBG Biozone), and both sequences provide abundant palynological assemblages. The palynofloras reveal the presence of Subzones B and C of the Raistrickia densa-Convolutispora muriornata (DM) Biozone. Subzone B is represented in the coal beds, and diagnostic palynological species indicate the presence of Subzone C in the transgressive event bearing the invertebrate fauna and microplankton elements. Approximately two tens of meters above the marine strata were recognized seeds linked to the Ferugliocladus Superzone of early Cisuralian age. The continuous succession, bearing palynological assemblages of the two subzones, spans the Bashkirian-Moscovian interval and allows comparisons with coeval palynofloras from Argentina and Brazil.

Sandstone petrofacies, deformational events and the dynamic of the Valle Fértil Lineament during the late Paleozoic (Paganzo Basin, northwestern Argentina)

The behavior during the late Paleozoic of the Valle Fértil Lineament, a first-order structural element placed along the suture between Cuyania and Pampia Terranes in northwest Argentina, is analyzed. The study was undertaken following three different lines of research: 1) the analysis of compositional changes of Carboniferous and Permian sandstones, 2) the interpretation of sizeable soft-sediment deformation, and 3) the analysis of the stratigraphic relationships along and away from the Valle Fértil megashear zone. At the Guandacol Hill, three sandstones petrofacies were recognized in the Carboniferous and Permian Guandacol, Tupe and Patquía formations: quartzolithic (Q-L), quartzofeldspathic (Q-F), and lithic-volcanic (L-V). The vertical distribution of these petrofacies allows distinguishing four stratigraphic intervals. The first, which corresponds to the uppermost part of the Guandacol Formation and the lower Tupe Formation, displays a marked alternation between Q-L and Q-F petrofacies, indicating supply from the metamorphic basement of the local Umango-Maz Block (Q-L) and from granitoids of the Sierras Pampeanas System (Q-F). The second interval, corresponding to the upper Tupe Formation, is widely dominated by Q-F petrofacies owing to the progradation of arkosic wedges from the Sierras Pampeanas System and a limited contribution of the Umango-Maz Block. The third interval, the lower Patquía Formation, shows a sharp change in the composition of the sandstones since they are dominated by acidic and intermediate volcanic fragments (L-V petrofacies), suggesting the establishment of a volcanic field in the area. Finally, the fourth stratigraphic interval, the middle Patquía Formation, is again composed of the Q-F petrofacies indicating a new episode of progradation of arkosic wedges from the Sierras Pampeanas System. Evidence of tectonic activity during the deposition of the Guandacol Formation is also suggested by the presence of different scales of syn-sedimentary deformational structures, from small-scale soft-deformation to large-scale mass transport deposits. This deformational belt only occurs along the Valle Fértil Lineament and is not recorded in other localities of the Paganzo Basin located either to the west or east of the megashear zone. Regional analysis of the late Paleozoic stratigraphy provides additional information about tectonic instability. Firstly, while the contact between the Guandacol and Tupe Formations is concordant throughout the basin, some localities close to the Valle Fértil Lineament show an angular unconformable relationship. Secondly, in the Agua de la Peña creek, located just on the trace of the Valle Fértil Lineament, the Permian Talampaya Formation covers through an angular unconformity the Guandacol Formation, a relation not observed in other areas of the Paganzo Basin. Thirdly, the presence of Carboniferous magmatism, including volcanic rocks in the Guandacol Hill and a granitic intrusion in the Veladero Hill, has been only observed along the Valle Fértil Lineament.

High-precision U-Pb CA-ID-TIMS calibration of the Permian Lueckisporites-dominated assemblages in westernmost Gondwana: inferences for correlations

Palynology is one of the most useful tools for constructing biostratigraphic charts in the late Paleozoic basins of South America, but the lack of radiometric ages often makes it difficult to establish the precise age of the biozones. This paper focuses on two points: (i) the description of new Argentinian palynological assemblages referred to the Lueckisporites/Weylandites (LW) Biozone and (ii) the application of a new isotopic age that enables more confident large-scale correlations with the neighboring Paraná Basin of Brazil. The palynofloras were recovered from the upper part of the De La Cuesta Formation that crops out in the retroarc basin of the Paganzo Basin, more specifically obtained from mudstones intercalated with limestones, cherts, and tuffs interpreted as being deposited in a shallow water body. The assemblages are characterized by an abundance of bi-taeniate pollen grains of the Lueckisporites complex and multitaeniate pollen grains such as Lunatisporites, Protohaploxypinus, Striatopodocarpidites, Tornopollenites, and Vittatina. Colpate pollen grains are also represented by Marsupipollenites and Pakhapites. A U-Pb CA-ID-TIMS age of 278.84 ± 0.22 Ma, obtained from a tuff level interstratified with the fossiliferous strata, allows the age of the LW Biozone in the Paganzo Basin to be constrained for the first time and the adjustment of the record Lueckisporites virkkiae in Gondwana. Moreover, this date enables precise correlations with other basins, such as the San Rafael Basin (Yacimiento Los Reyunos Formation) in Argentina and the Brazilian Paraná Basin (Iratí Formation).

U-PB ages and environmental evolution of the De La Cuesta Formation (late Paleozoic): Record of the Choiyoi Volcanism in the retroarc area of the Paganzo Basin, Western Gondwana

This paper examines the relationship between the sedimentation in the retroarc Paganzo Basin and the activity of the magmatic arc developed in the neighboring Andean Cordillera during the Late Pennsylvanian-Cisuralian. The temporal relationships between the Choiyoi Volcanism (Andean region) and the retroarc Paganzo Basin were investigated according to three lines of research: 1. Sedimentology and lithostratigraphy of the De La Cuesta Formation, 2. Geochronology and, 3. Biostratigraphy.Paleoenvironmental studies indicate that small alluvial fans (facies association A) and braided river (facies association B) deposits prevailed in the lower third of the De La Cuesta Formation, which includes a thin intercalation of shallow marine sediments only present in the western part of the area (facies association C). A progressive decrease in the energy of the fluvial systems occurred at the middle part of the unit, where dominant finning-upward successions correspond to meandering rivers (facies association D). In contrast, interbedded sandstones and mudstones were subsequently deposited by low-energy ephemeral rivers (facies association E). Finally, the upper third of the De La Cuesta Formation corresponds to eolian sandstones (facies association F) and shallow water bodies deposits, including evaporites, limestones, and tuffs (facies association G) sedimented under semiarid to arid conditions with periods of explosive volcanism.A U–Pb age of 278.84 Ma was obtained from a tuff level at the uppermost part of the De La Cuesta Formation (facies association G). This age, together with one previously obtained of 311.89 ± 0.21 Ma corresponding to the base of the unit, indicate that the De La Cuesta Formation is Moscovian-Kungurian in age and that it was mostly deposited before the main phase of the Choiyoi magmatism, synchronically with the so-called Pre-Choiyoi magmatism. Furthermore, a rich palynological assemblage belonging to the Lueckisporites/Weylandites Biozone (LW), recovered from deposits of the facies association G, is equivalent to the Lueckisporites virkkiae (Lv) Biozone from the Paraná Basin (Brazil), pointing out a Kungurian age for these biostratigraphic units.The new radiometric age and the palynological taxa recovered from the top of the De La Cuesta Formation (facies association G) confirm a Kungurian age for this interval, establishing a key level for correlation between the sedimentation in the retroarc with the onset of the Choiyoi Volcanism in the magmatic arc.

Estratigrafía, paleoambientes y correlación del Paleozoico Superior en la localidad Río Francia, Cuenca Paganzo, Argentina

The Tupe and Patquía formations are known for their marine, deltaic, river, and eolian deposits that record different Late Paleozoic events in the Paganzo Basin. However, in the area of the Río Francia (Province of San Juan, Argentina), these units are relevant because they are located in a strategic place of connection with the southernmost sedimentary sequences in the Paganzo Basin on the Central Precordillera. The Tupe Formation (which by palynological data would represent the late Serpukhovian-Gzhelian interval) has a minimum thickness of 140 m of bioturbed mudstones with a minor percentage of coarse to fine sandstones intercalated. In this work they are grouped into 3 facies associations: I lagoon; II barrier island and III shoreface. The Patquía Formation (from palynological data, in this locality, of Cisuralian in age), it is a typical succession of red beds with deposits of fine -grained sandstones with cross-bedding, and alternation of sandstones with mudstones. 3 facies associations were differentiated: IV anastomosed fluvial; V eolian and VI playa lake. A correlation analysis between these formations and their lithostratigraphic equivalents provided possible scenarios for the western sector of the Paganzo Basin, showing a lineament of units with coastal environment features for the Tupe Formation and equivalents at these latitudes. On the other hand, the Patquía Formation and their equivalents display a progressive continentalization from north to south.

The late Paleozoic Ice Age along the southwestern margin of Gondwana: Facies models, age constraints, correlation and sequence stratigraphic framework

The southwestern margin of South America offers a complete record of the Late Paleozoic Ice Age (LPIA) that affected the Gondwana supercontinent. The tripartite division of LPIA glacial episodes has been refined with the help of new radiometric dates and biostratigraphic (flora and fauna) zonations in recent years to five shorter-lived discrete events: 1. Latest Devonian-earliest Tournaisian, 2. Tournaisian, 3. Visean, 4. Serpukhovian-Early Bashkirian, 5. late Pennsylvanian-earliest Permian. These glacial events are capped by postglacial transgressive deposits with marine fauna. The unbalanced preservation potential of the glacial deposits, skewed toward the glaciomarine sediments, provides an uneven stratigraphic record with few cases of continental glacial sedimentation, confined to the Serpukhovian-Early Bashkirian event, and numerous examples of glacial sedimentation in marine environments. Glacial sedimentation in marine settings has been grouped in two main facies associations: a valley-glacier-retreat (fjord) facies association and a submarine-retreat (glaciomarine apron) facies association in open-marine areas.Transitional facies, correspondent to those formed by the flooding of valleys during postglacial transgressions, are widely distributed along the Protoprecordillera in western Argentina, where paleofjord successions are well exposed particularly in western Paganzo Basin, and mapped in subsurface in the Tarija basin. A general paleofjord model includes (from base to top) the following stages: (i) Incision of paleovalley and deposition of subglacial diamictites in ice contact deltas, (ii) Early Transgressive stage characterized by resedimentation of subglacial material by subaqueous sediment gravity flows and slumps in proglacial settings, (iii) Maximum flooding (late transgressive stage) dominated by black shales or laminated mudstones related to a marine incursion that flooded valleys; normal marine or brackish conditions may dominate this stage and (iv) Highstand: progradation of a fluvial-deltaic system including in some cases Gilbert-type deltas. In glaciomarine apron environments, the basal facies includes massive clast-supported conglomerates, with few striated and polished clasts, followed by fining-upward successions including thinly bedded diamictites with ice-rafted debris (IRD) and locally contorted sandstone masses in diamictite beds, indicative of mass-emplacement mechanisms. The presence of inter- and intratill pavements suggests glacial advance/retreat fluctuations along the basin margins.Deglaciation sequences, reflecting deposition mainly during the retreat of ice sheets, ice caps and alpine glaciers and successive deglaciation, can be used as operational tools for the analysis of glacial successions in southwestern Gondwana. They are characterized as rather simple upward-fining successions in open marine settings as exemplified in most of the Calingasta-Uspallata Basin,Claromecó Basin (and adjacent Ventana foldbelt, VFB) and central portions of the Paraná and Karoo basins. In more proximal areas (i.e., paleofjords) this vertical trend is commonly punctuated with deltaic wedges fed by nearby provenance areas.The late Paleozoic glacial-related successions of southwestern Gondwana exhibit a common tripartite motif, equivalent to second-order sequences with estimated durations of 10–80 Myr. The lower section corresponds to glacial and glacially-influenced diamictites; the middle interval is initiated with postglacial transgressions. The lower and middle intervals correspond to the deglaciation sequence as described and identified in several basins of Gondwana. Finally, the upper term includes coastal progradation, followed in some places by continentalization, accompanied in many sectors by increasing aridization. Examples of second-order sequences can be identified in the thick late Paleozoic successions of the Paraná and Karoo basins and in the VFB. Thinner second order sequences can be identified in the Calingasta-Uspallata, Rio Blanco, Paganzo and San Rafael basins. In the Paganzo and San Rafael basins the middle interval is also punctuated by short lived marine ingressions. The basal sequence boundary is commonly an abrasion surface (glacial erosion surface, GES) developed on bedrock. Deglaciation sequences are assigned to third order sequences made up, when present, of a thin lowstand system tract (LST) of subglacial deposits followed upward by thick glaciomarine and glacially influenced sediments. These facies are part of a thick transgressive systems tract (TST) that culminates with marine shales that reflect interglacial or postglacial conditions during ice retreat. Thus, the deglaciation sequences are proposed to be third order sequences made up of LST-TST or exclusively TST.

Revision of Botrychiopsis plantiana: A Key Species of the Gzhelian–Cisuralian in Westernmost Gondwana

Botrychiopsis is an important biostratigraphic index taxon for Carboniferous–early Permian plant fossil successions of Gondwana. Its species have wide geographic distributions, restricted chronostratigraphic ranges, and are associated with diverse groups of plant associations. They have been described from sedimentary sequences of Australia, India, South Africa, Brazil, and Argentina. A whole-frond reconstruction of Botrychiopsis plantiana is presented based on a study of very well preserved specimens from the Gangamopteris flora (Ghzelian–Asselian) of the Bajo de Véliz Formation (Paganzo Basin, Argentina). The material consists of impressions of complete fronds and basal fragments, which are rarely represented in the Gondwanan upper Paleozoic fossil record.

Late Paleozoic coal beds and coaly mudstones in northwestern basins of Argentina: Paleoenvironmental context and paleoclimatic significance

Coals and coaly mudstones are found in the late Paleozoic Paganzo Basin, forming a key stratigraphic level deposited during the Bashkirian – Moscovian interval of relative tectonic quiescence. Coal beds vary from bituminous high volatility type C to sub-bituminous type A and, the coaly mudstones are kaolinitic-rich and presently used in the ceramic industry. Taking in account the stratigraphic position, facies associations, and the characteristics of the organic-rich levels, three domains were recognized within the basin: 1) The western domain (Precordillera) where paralic coals formed in transitional (mainly estuarine) environments, 2) The central domain (western Sierras Pampeanas) with limnic coals formed in floodplain areas, and 3) The eastern domain (eastern Sierras Pampeanas and Nevados del Famatina), in which coals are almost absent and are replaced by kaolinitic-rich coaly mudstones. Sea-level changes and climatic conditions related to the end of the Gondwanan glaciation played critical roles in the formation of coals and coaly mudstones. Regarding sea-level changes, three main transgressive events were identified in the Paganzo Basin. The oldest one corresponds to the “Namurian” postglacial transgression that produced no coal beds. The second event is the Pennsylvanian transgression 1, which favored the formation of coals and coaly mudstones in both coastal (i.e estuarine) and inland (i.e fluvial) environments. The third transgression, Pennsylvanian transgression 2, produced only very thin and scarce coal seams. During the transition from glacial (late Serpukhovian) to early postglacial climates (early Bashkirian), the vegetation was scarce and not suitable for the development of extensive mires. The post-glacial (late Bashkirian-early Moscovian) climatic amelioration, characterized by humid and temperate conditions, facilitated the proliferation of the NBG flora composed of hygrophilous elements as equisetalean, arborescent to sub-arborescent lycophytes and pteridosperms fronds, together with some gymnosperm that produced large amounts of organic detritus. On the contrary, the transition from subhumid to semiarid climate (late Moscovian-Kasimovian) led to the declination of the arborescent floras and the disappearance of coal beds. Although climate can be assumed to have simultaneously affected the whole basin, fluvial systems needed time to adjust to the sea-level changes and, therefore, at least two possibilities should be considered regarding the chronostratigraphic significance of the coal horizon. Firstly, that sea-level rise effectively controlled the equilibrium profile of channels well inland and coals and coaly mudstones across the Paganzo Basin form a time-transgressive horizon with a lag time not yet quantified based on the available paleontological and radiometric data. Secondly, that the growing of the riparian vegetation led by the ameliorating climatic conditions resulted in the almost synchronic formation of coals in the three domains.

Constraining late paleozoic ice extent in the Paganzo basin of western Argentina: Provenance of the lower Paganzo group strata

The western margin of Gondwana records evidence of mid-Carboniferous glaciation (Visean- Bashkirian) in the strata of the Paganzo Basin and adjacent areas. Previous studies focused on constraining the orientations of ice flow and generalizing the extent and occurrence of glacial ice in the basin. However, there is uncertainty occurs concerning the location and extent of glaciation and the locations of glacial centers during deposition of the Guandacol Formation located in the western portion and the time correlative Malanzán Formation in the eastern parts of the basin respectively. Understanding these strata and the conditions that led to the deposition of these strata has important implications for understanding glaciation extent along the western margin of Gondwana during the late Paleozoic ice age. To refine the glaciation history, we present new paleocurrent, facies, and a comparative analysis of previously published and one new detrital zircon geochronology data set. Together, these data provide new insight into sediment dispersal patterns and glacial centers within the basin. Our data indicate that both units were deposited under cold climatic conditions that occurred across the entire basin, but that glaciers were restricted to the western portion of the basin. The facies analysis for both formations indicates very different depositional environments for the two units: glacial and glaciomarine environments in the west and non-glacial alluvial, alluvial fan and lacustrine and/or marine environments in the east. Detrital zircon geochronology indicates separate localized provenance signatures for the two formations. Furthermore, the detrital zircon populations allude to disconnected depositional centers. At Huaco, glacial flow in the Guandacol is oriented toward the northwest (i.e. 313°) and appears to be draining from an uplift that contains similar zircon age populations as the Sierra de Valle Fértil and Sierra de Pie de Palo ranges. In the Olta-Malanzán paleovalley system, drainage was off valley walls and down the valley axis (i.e. south and southwest) within the Sierras de Chepes region with limited sediment sourced from the east just beyond the paleovalley system. Paleoflow measurements reported from other known glacial localities along the western portion of the basin reflect a radial flow pattern within and away from deeply incised valleys that clearly point to upland glacial centers within the Cuyania and Precordilleran terranes.

Internal anatomy of an erg sequence from the aeolian-fluvial system of the De La Cuesta Formation (Paganzo Basin, northwestern Argentina)

Permian red beds of the De La Cuesta Formation in the Sierra de Narvaez (Paganzo Basin, northwestern Argentina) are essentially composed of sandstones associated with mudstones and subordinate conglomerates. Facies distributions and stacking patterns indicate that these sediments resulted from the interaction between aeolian and ephemeral fluvial systems, and are represented by aeolian dune, dry aeolian interdune and aeolian sand sheet, mudflat, wet aeolian interdune, and fluvial deposits. The De La Cuesta Formation is characterised by aeolian (erg) sequences alternating with non-aeolian (terminal alluvial fan – mudflat) sequences. Each erg sequence is bounded at its base by a regionally extensive sand-drift surface and at the top by an extinction surface. A number of architectural elements, including aeolian dunes limited by interdunes, grouped crescentic aeolian dunes, longitudinal dunes, and draa with superimposed crescentic dunes are recognised in the erg sequences. The sand sea developed during phases of increasing aridity, whereas non-aeolian deposition might have occurred during more humid phases. Thus, the styles of aeolian-fluvial interaction are considered to result from cyclical climatic changes. Within the drier hemicycles, the rhythmic alternation between draa deposits and aeolian dune and interdune deposits indicates higher frequency cycles that could be attributed to subtle climatic oscillations and/ or changes in sand supply and availability. The development of the Permian sand sea in the inland Paganzo Basin seems to be related to the growth of a volcanic chain to the west. This topographic barrier separated the Paganzo Basin from the Chilean Basin, located along the western margin of Gondwana and characterised by shallow marine carbonate sedimentation. The correlation between the Permian erg and the shallow marine carbonates suggests a regional warming period during the Middle Permian in western Gondwana. 1

Timing of the Late Palaeozoic glaciation in western Gondwana: New ages and correlations from Paganzo and Paraná basins

During the Late Palaeozoic, the Gondwana supercontinent formed an extensive Southern Hemisphere landmass that was affected by multiple glacial episodes, known collectively as the Late Palaeozoic Ice Age (LPIA). This resulted in the deposition of glacial, periglacial and deglacial sediments over much of the supercontinent. The Mississippian to early Pennsylvanian phase of glaciation is widely represented along the western margin of Gondwana. This constitutes one of the largest glaciations of the Phanerozoic in terms of its recorded extent and the widespread erosional hiatus it produced in the stratigraphic record. It was this mid Carboniferous glaciation, recorded in the Paganzo Basin of NW Argentina, that carved most of the paleovalleys and paleofjords present there. We report new U-Pb zircon ages from a single glacial succession (Guandacol Formation) of 326 ± 3 Ma and 320 ± 5 Ma, that in comparison with neighbouring dated sequences allow for the first time a reliable estimate of the timing and duration of glacial cycles. Palynological studies of these glacial-deglacial events yielded palynoassemblages of the MQ (Late Visean) and DMa (Sepukhovian-Bashkirian) Palynozones. The Carboniferous glacially-related strata and glacial cycles of the Paganzo Basin are compared here with equivalent units of the Paraná Basin of SE Brazil, suggesting a similar climate record for most of western Gondwana. We propose a new correlation between these two basins. The new U-Pb zircon ages reported here indicate that a regional glacial peak occurred almost coincident with the Mississippian-Pennsylvanian boundary, suggesting that the ensuing postglacial transgression is the best regional marker to differentiate the Mississippian and Pennsylvanian as it forms an identifiable interbasinal horizon. The biostratigraphic and chronostratigraphic comparison with other LPIA successions from South America reinforces that the First Appearance Datum of monosaccate pollen grains occurs in the late Serpukhovian. Their correlation confirms that similar climate conditions prevailed across most of western Gondwana during this phase of the LPIA.

Stratigraphy and depositional architecture of lobe complexes across a range of confinements: Examples from the Late Paleozoic Paganzo Basin, Argentina

Abstract The well-exposed, Late Paleozoic glacially-influenced Paganzo Basin offers a unique opportunity to compare and contrast the stratigraphy and depositional architecture of turbidite sheet/lobe complexes with distinct degrees of basin-scale confinements. These include sandstones deposited in narrow paleofjord (Quebrada Las Lajas), moderately narrow paleovalley (Quebrada Grande) and broad depression (Cerro Bola) that are part of the same synchronous transgressive-regressive succession recorded in the basin. This work emphasizes the importance of scales of observation and the importance in make the distinction between the degree of confinement experienced by individual flows and the degree to which the system as a whole is limited by the basin size and geometry. In Quebrada Las Lajas, sheet-like sandstones form lobe complexes largely reflecting the basin physiography. The succession was subdivided into two turbidite stages (Ts1 and 2), separated by a finer-grained interval (F1) that together represents lobe complexes and interlobe systems. Individual sandstone beds are laterally and longitudinally continuous and onlap the paleofjord walls, suggesting vertically-stacked beds. In Quebrada Grande paleovalley, the succession was subdivided into three stages (Ts1 to Ts3), representing lobe complexes, also separated by finer-grained packages (F1 and 2) ascribed to interlobe phases. Individual sandstone beds are truncated, thinning and pinching out, making them hard to correlate between logged sections. This depositional pattern may suggest that lateral stacking dominates during the construction of lobe complexes. At Cerro Bola, the succession was subdivided into five stratigraphic units (Ts1 to Ts5). These units also onlap the basin margin. Individual sandstone beds of Ts1 and Ts2 are similar to those recorded in Quebrada Grande area, also suggesting compensational stacking dominated systems. However, individual sandstone beds within Ts4 are thicker relative to the other units and more continuous longitudinally, reaching the basin margin thus pointing to larger volume flows ascribed to a vertically-stacked pattern. On the other hand, Ts3 and Ts5 correspond to the thinner sandstone beds in the succession and, as these intervals are as thick as the others, they may be attributed to lobe off-axis to fringe settings. In all three areas, the lobe complex geometry is determined by the basin-scale confinement and, at this scale, all of them are represented by more or less tabular, large sandstone bodies showing more similarities than differences. Each of these three successions record an upward increase in the sand-to-mud ratio, which may denote flow ponding or progradation of up-dip deltas that fed these systems.

A paleoclimatic reconstruction of the Carboniferous-Permian paleovalley fill in the eastern Paganzo Basin: Insights into glacial extent and deglaciation of southwestern Gondwana

During the mid-Carboniferous, ice centers located in present-day western Argentina disappeared until the late Cenozoic with glaciation of the Andes. The disappearance of mid-Carboniferous glaciers and the subsequent climate shift, recorded in the Paganzo Basin, has been attributed to global events and drivers, such as increased atmospheric CO2 concentrations and the shifting position of Gondwana across the South Pole. However, glaciers continued at the same paleolatitude in eastern South America and did not disappear from Gondwana until the Late Permian. This study investigates links to local drivers that acted in combination with other global drivers to explain the early deglaciation along the western margin of Gondwana. To do this, several outcrops within the eastern portion of the Paganzo Basin in western Argentina were sampled for the Chemical Index of Alteration (CIA) geochemical analyses. Here, we test the applicability of the CIA as a paleoclimate proxy on strata in the Olta-Malanzán paleovalley that historically was thought to have been glaciated. A recent study by the authors has shown that the paleovalley was not glaciated, but owes its origin to extension and excavation by fluvial processes. However, the late Paleozoic stratigraphy of this paleovalley system is similar to the rest of the Paganzo Basin. The results from the paleovalley samples show that this area was intermittently humid and arid through time, but with an overall arid profile. This signature is predominantly due to the nature of the paleovalley, which was subjected to rapid burial from frequent rock falls, progradation alluvial fans/fan deltas, and lacustrine sediment gravity flows (Malanzán Fm.), which prevented any significant chemical weathering. While the overall Pennsylvanian climatic signature appears to be relatively arid (Malanzán, Loma Larga, and Solca Fms.), it seems that the climate during the deposition of the late Pennsylvanian and Permian La Colina Formation was more humid than previously thought.

New U[sbnd]Pb zircon age for the Pennsylvanian in Argentina: Implications in palynostratigraphy and regional stratigraphy

A continuous Pennsylvanian succession belonging to the Agua Colorada and De La Cuesta formations is exposed in the Las Angosturas area (Catamarca Province) of the Paganzo Basin. Glacial, postglacial, fluvial and shallow marine deposits are identified and referred to seven stratigraphic stages. The Agua Colorada Formation provides a persistent record of palynological assemblages in the first five stages, which are assigned to the Subzones A and B of the Raistrickia densa/Convolutispora muriornata Biozone. These palynofloras span the late Serpukhovian-Bashkirian interval. The first isotopic age for the basal section of the De La Cuesta Formation was obtained from shallow marine deposits. Zircons from a tuff intercalated with mudstones and sandstones in the marine interval were analyzed by the CA-TIMS technique providing an age of 311.89 ± 0.21 Ma, based on the weighted average of 206Pb/238U dates. This new age is compared with previous isotopic ages obtained from different successions in the Paganzo Basin, resulting in the youngest known radiometric date, for the Moscovian transgression. Indeed, a late Moscovian age is already known by isotopic dating of the reddish fluvial deposits of the lower section of the Patquía Formation, which overlies the marine transgression in the Huaco area (San Juan Province). The record of this marine flooding surface in reddish strata at Las Angosturas area provides a cautionary note on considering the appearance of red beds as indicators of the onset of continental conditions in the Paganzo Basin.

Quantitative comparisons of depositional architectures of unconfined and confined turbidite sheet systems

Degree of confinement refers to the degree to which sediment gravity flows are influenced by the surrounding basin relief and interact with the basin margins, and is a key controlling factor in determining the depositional architectures of turbidite sheet systems. Two well exposed late Palaeozoic sandy turbidite sheet systems in the Paganzo Basin at Cerro Bola, north-west Argentina, termed Stages II and IV (unconfined and highly confined respectively), provide three-dimensional constraints on facies associations, stacking patterns, depositional geometries and evolution of these turbidite sheet systems. Cluster analysis has been applied to define facies associations in these two systems at an appropriate hierarchical level depending on the overall nature of the system. The overlap index (OI), OI = A0/A1, where A0 is the overlap area and A1 is the area of underlying depositional element, is adopted to quantitatively characterise the stacking patterns. The two turbidite sheet systems show contrasts in that: (1) Stage II is built up by four hierarchical elements, each stacked in a shingled or compensational way with the majority of overlap index values of 0.7 and 0.8, to form higher hierarchical elements, whereas in Stage IV, individual beds stack vertically with OI = 1, together forming the stage and (2) Four facies associations have been identified in Stage II by cluster analysis and they occur in a spatial sequence from axis to fringe; one facies association has been identified in Stage IV and it shows little variation spatially across the basin. In unconfined systems, depositional elements are self-organized, and autocyclic controls are dominant, whereas in highly confined systems, confinement has suppressed the autocyclic signal and systems are allocyclically controlled.

Hepaticites(Marchantiophyta) in the Lower Permian Arroyo Totoral Formation (La Rioja Province, Argentina)

In this contribution we present the first record of a liverwort from the Arroyo Totoral Formation in the Paganzo Basin. The sample is preserved as a compression, founded in a greenish-gray shale with parallel lamination, associated with gymnosperms. The delicate thallus differentiated into wings and midrib with elongate cells but lacking evidence of air-pores like those developed in complex thalloid liverworts, suggests that the fossil is a simple thalloid liverwort. The lack of reproductive traits restricted the assignation to Hepaticites sp. This record supports the interpretation of local humid conditions and improves the Gondwanan Paleozoic record of Bryophytes.