Tunas Formation Research Articles

Understanding the burial history and the hydrocarbon potential of the late Paleozoic Claromecó foreland Basin (Southwestern Gondwana, Argentina) by combining organic geochemistry, organic petrology, and thermal modeling

The Claromecó foreland Basin (Carboniferous–Permian; southern Buenos Aires province, Argentina) is key to understanding the paleotectonic evolution of the southwestern Gondwana margin and is relevant to energy resource exploration. This study reconstructs the thermal and burial history of the Claromecó Basin by integrating geochemical data, organic petrology, and thermal modeling techniques. Cores samples of the Tunas Formation (Pillahuincó Group, Early Permian) were studied. A 1D thermal model was constructed, calibrated with vitrinite reflectance measurements (VRo %), and corroborated with fluid inclusion and apatite fission track data from previous studies. Rock-Eval pyrolysis results show TOC% values ranging from 0.13 to 60.35 wt%. The Hydrogen index (HI < 50 mg HC/g TOC) and Oxygen index (OI < 50 mg CO2/g TOC) indicate the dominance of Type III and Type IV kerogens, most likely resulting from the thermal maturation of an original Type III kerogen. Petrologic observations confirm the presence of macerals from the inertinite group, as well as minor amounts of vitrinite and liptinite. The Tmax displays a temperature range mostly from 460 to 610 °C. The VRo % values range from 1.5 to 2.0%. Geochemical data combined with VRo % measurements confirm a late catagenesis to metagenesis stage within the wet to dry gas window for coals and organic-rich strata.In order to constrain the thermal evolution of the basin infill, different scenarios were tested by varying the heat flow and the missing section thickness associated with the uplift and erosion of the basin (Permian–Cenozoic unconformity). The best calibration results were obtained with an erosion thickness of 3000 up to 4200 m and paleo heat flow peaks of either 60 or 80 mW/m2 during the Lower Permian–Lower Cretaceous. The Tunas Formation was deposited and buried during the Permian–Triassic (Gondwanides Orogeny phase), reaching a maximum temperature of 180 °C. The results obtained by combining geochemical analysis, organic petrology, and thermal modeling techniques indicate that the coal beds of the Tunas Formation could have a current potential as gas-prone source rocks. Despite that, the hydrocarbon generation capacity of coal levels is currently low due to the high percentage of residual (Type IV) kerogen. Further research could help clarify if the hydrocarbons potentially expelled by these source rocks have been lost due to migration or could be trapped somewhere in the basin.

The Gondwanide deformation along the southwestern border of the Río de la Plata Craton: Geochemical and geochronological constraints on ductile shear zones from the Ventania System basement, Argentina

This work deals with the study of Gondwanide ductile shear zones developed in Neoproterozoic–middle Cambrian basement rocks of the Ventania System and possibly related to the reactivation of the Sierra de la Ventana Shear Zone. This reactivation was caused by the northeastward migration of the Gondwanide deformation from the North Patagonian Massif. The crustal shortening related to the Gondwanide Orogeny gave rise to thrusting of the Ventania System basement over Paleoproterozoic rocks of the Río de la Plata Craton. This work focuses on the study of muscovite ± quartz phyllonites of the Ventania System basement and quartz ± muscovite veins crosscutting basement and Paleozoic cover rocks of the system. Structural and magnetic surveys were performed in order to characterize the Gondwanide structures of the basement, while petrographic, X-ray diffraction, geochemical, and geochronological studies were carried out in order to investigate the nature, P-T conditions, element mobility, and age of these shear zone-related rocks. In the Sauce Chico Inlier, mylonitized basement rocks crop out along the western edge of the system, near the basement–Paleozoic cover interface, where the main structures are top-to-NNE reverse ductile shear zones with related phyllonites. Ductile shearing under greenschist-grade conditions (125–340 MPa and 300–400 °C) promoted major and trace element mobility. Phyllonites are the result of extreme hydrolysis of feldspars from the acidic igneous rocks of the basement, producing muscovite and releasing aqueous SiO2 later redeposited as quartz veins. These phyllonites developed in connection with reactivated contacts between basement units and also the basement–Paleozoic cover interface. New Rb–Sr quartz-muscovite and Ar/Ar muscovite ages for phyllonites and a quartz vein indicate a protracted tectono-metamorphic history mainly restricted to the Cisuralian (ca. 287 Ma), comprising regional folding and metamorphism, shear zone activity, vein-type mineralization, and syntectonic deposition of the Tunas Formation. Subsequent reactivations during the Lopingian (ca. 256 Ma) and probably in Late Triassic times (ca. 227 Ma) resulted from localized shearing along mylonitic belts.

Fracture frequency and anisotropy of magnetic susceptibility: A case of study in the Claromecó Basin (PANG 0003 well), southwestern Gondwana boundary

The PANG 0003 well records a siliciclastic sedimentary succession corresponding to the Tunas Formation (Permian) in the Claromecó Basin area (Buenos Aires province, Argentina). The aim of this paper is to establish a relationship between fracture frequency in the PANG 0003 exploration well and AMS parameters, as it is a novel research topic that would contribute to the knowledge of the Claromecó basin. Opening-mode I and mode II/III fractures were found in 729 m of unoriented core between 901 m and 170 m recovered. Fracture attributes: inclination, aperture, height, and cement presence are represented by four fracture sets according to the inclination angle; representative kinematic aperture data range from 0 to 5 mm and height constrain by core dimensions. Cement presence is represented by quartz, calcite, and minor pyrite. Fracture description were distributed in lower, middle and upper section of the well according to their fracture frequency (number of fractures per meter). Lower section is represented by a high fracture frequency (13 fractures/meter), which notably diminishes to the middle and upper section of the well (3 fractures/meter). The results indicate that the fracture frequency increases in depth and has a direct relationship with AMS parameters, such as degree of anisotropy (Pj) and magnetic foliation (F). According to previous studies in Sierras Australes outcrops and subsurface for Tunas Formation, tectonic imprint decreases its expression from base to top of the unit and has a positive correlation with fractures studies on cores. This relationship could be used as an alternative methodology to distinguish intervals of high number of fractures in subsurface, in this particular case, for the Claromecó Basin.

Petrography and tectonic provenance of the Permian Tunas Formation: Implications on the paleotectonic setting during the Claromecó Foreland Basin evolution, southwestern Gondwana margin, Argentina

The Claromecó Basin is located at the south-western sector of the Buenos Aires province, Argentina. This basin is considered a foreland basin closely related to the evolution of the southwestern Gondwana margin. This contribution focuses on the provenance analysis of the Tunas Formation (Permian, Pillahuincó Group), which represents the last filling stage for the Claromecó Foreland Basin. Petrographic and tectonic provenance analyses were performed in sandstones recovered from subsurface (PANG 0001 and PANG 0003 exploration wells) and outcrops located close to the basin center (Gonzales Chaves locality). In the subsurface, the analyzed succession is composed of medium- to fine-grained sandstones interbedded with tuffs, mudrocks, carbonaceous mudrocks and coal beds. In outcrops, the succession is dominated by medium-to fine-grained sandstones interbedded with siltstones. Modal composition patterns are distributed into the recycled orogen and transitionally recycled to mixed fields. Petrographic analyses, in addition to provenance and sedimentological studies, confirm that sedimentary material was derived from a mixed source, which largely comes from the Sierras Australes fold and thrust belt, located towards the W–SW, where the sedimentary succession is interbedded with volcanic material. The Tunas Formation shows clear differences in its modal composition, paleocurrent direction and paleoenvironmental conditions with respect to the underlying units of the Pillahuincó Group (Sauce Grande, Piedra Azul and Bonete formations). Source areas changed from cratonic to mixed fold belt/arc-derived material, suggesting variations in the Claromecó Basin configuration during the Late Paleozoic. Changes in the paleotectonic scenario during the deposition of the Tunas Formation have been interpreted as a consequence of a compressive post-collisional deformation event, the product of adjustment, accommodation and translation of terrains towards the equator during the Permian–Triassic to form Pangea.

The Claromecó frontier Basin: Hydrocarbon source rock potential of the Tunas Formation, southwestern Gondwana margin, Argentina

Coal-bearing deposits of the Tunas Formation suggest the existence of potential gas-prone source rocks in the Claromecó Basin, south of Buenos Aires province, Argentina. Two wells, PANG 0001 and PANG 0003, containing Permian rocks assigned to the Tunas Formation were studied through core observation, X-Ray diffraction, petrographic description, geochemical analysis, and organic petrography to investigate the hydrocarbon potential of the Claromecó Basin and define horizons with source rock potential. The analyzed sedimentary successions are composed of medium- to fine-grained sandstones interbedded with tuffs, mudrocks, carbonaceous mudrocks, and coal. Two main horizons with potential source rocks were determined: an upper horizon, at the top of the succession (300–400 m) and a lower one, at the base (740–850 m). These layers are mainly composed of mudrocks, carbonaceous mudrocks, and coal beds up to 4.50 m in cumulative thickness. Total organic carbon (TOC%) content is high, with most samples ranging from 0.5 to 2% in organic-rich mudrocks, and from 26 to 53.9% in carbonaceous mudrocks and coals, characterizing the coal-bearing layers as good-quality source rocks. The organic matter is of terrigenous origin as indicated by the abundance of woody plant debris. Predominant organic matter constituents belong to vitrinite group (type III kerogen), with a low abundance of the inertinite group macerals. Vitrinite reflectance (Ro) values in carbonaceous mudrocks and coals ranged from 1.3 to 1.6% for the upper horizon to 1.9–2.4% at the lower one, reflecting a catagenesis to late catagenesis stage within the wet to dry gas window. The organic matter quantity, type, and maturity in Tunas Formation coal-bearing deposits suggest that this unit has a good potential as gas-prone source rocks.

Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

The Permian Tunas Formation (Claromecó Basin, Argentina): Potential naturally fractured reservoir and/or coal bed methane (CBM) play?

The analysis of structural discontinuities in the Tunas Formation helps characterize this unit as a potential naturally fractured reservoir. The principal discontinuities are 1) fractures parallel to the bedding planes, 2) fractures coplanar to the axial planar cleavage, 3) dilatant fractures filled by quartz, and 4) subvertical shear fractures. Over a thousand measurements on microtectonic structures help infer a stress field whose maximum principal stress (σ1) has a SW-NE orientation. Based on this structural analysis, optimal orientations for exploration and/or production wells in the Claromecó Basin should be a 130° azimuth. Fracture spacing range between 30 and 100 cm in outcrops of the Tunas sandstones along the Ventana foldbelt (VFB). A similar fractured network is postulated for the subsurface of the basin.Organic-rich levels, ranging from coals to carbonaceous shales, occur in the Tunas Formation in the subsurface of the Claromecó Basin and are a potential shale gas resource play. These beds tend to cap successions of interbedded shales and sandstones. The coals range from semi anthracitic to low volatile bituminous. Vitrinite and fusinite prevail as organic matter types, ranging between 1.3 and 2.4 vitrinite reflectance (Ro). The occurrence within condensate-wet gas to dry gas windows reflect subsurface processes such as strong diagenesis and anchimetamorphism. The coal-rich levels in the Tunas Formation can be correlated with other Lower Permian organic-rich units in the Paraná Basin in Uruguay (Tres Islas Formation) and Brazil (Rio Bonito Formation). All are analogous to the lower Upper Permian Vryheid coals of the eastern Karoo Basin in South Africa with shale gas and coal bed methane (CBM) prospectivity.

Anisotropy of magnetic susceptibility analysis in Tunas Formation cores (Permian), Claromecó Basin, Buenos Aires, Argentina: Its relation to depositional and post-depositional conditions

Anisotropy of magnetic susceptibility (AMS) has been used as an important tool to study magnetic fabrics and as a basis of interpretation of depositional conditions and post-depositional processes in sedimentary successions. This contribution presents results of standard AMS and magnetic mineralogy analysis performed in core samples from the PANG 0003 well, located at the Claromecó Basin, Buenos Aires province, Argentina. The sedimentary sequence analyzed has been assigned to the Tunas Formation (Permian), upper Pillahuincó Group, southwestern of the Gondwana boundary. Additionally, obtained results were compared with previous Tunas Formation outcrop data at the Sierras Australes and Claromecó Basin area.Measured AMS parameters vary with depth, reflecting changes in postdepositional conditions such as mechanical compaction and the influence of tectonic stress. Analyses of the mean magnetic susceptibility, thermomagnetic properties and petrographic studies indicate that paramagnetic minerals as Fe-phyllosilicates are the main contributors for the AMS of this sequence. Obtained AMS parameters are mainly characterized by triaxial to oblate AMS ellipsoids and shortening directions (Kmin) with a tendency to cluster in vertical positions, parallel to the bedding plane poles, assigned to sedimentary or weak deformed magnetic fabrics. By contrast, toward the base of the sequence, strongly oblate AMS ellipsoid shapes and Kmin axes that migrate to horizontal positions indicate a high deformation degree product of tectonic stress. The presented results are consistent with geological, AMS and paleomagnetic data from Tunas Formation outcrops and confirm that deformation degree was gradually attenuated upwards in the sequence, to the younger strata and toward the foreland Claromecó basin. The study of AMS parameters and magnetic mineralogy have proven to be important tools to interpret the processes that acted during the burial history of Tunas Formation, increasing the knowledge about the evolution of the Claromecó Basin.

Geochemical, palaeontological, and sedimentological approaches of a syn-orogenic clastic wedge: Implications for the provenance of the Permian (Cisuralian) Tunas Formation, Ventania System (Argentina)

The Tunas Formation crops out in the eastern sector of the Ventania System (southwestern Buenos Aires Province, Argentina) and represents the last unit of the Pillahuincó Group (Pennsylvanian–Cisuralian). This work integrates sedimentological, palaeontological, geochemical, and geochronological studies of the uppermost levels of the unit with the aim to enhance its palaeoenvironmental and stratigraphic framework within the Southwest Gondwana context. The analysed succession, of 250 m thick, is dominated by lithic arkoses and feldspathic litharenites composed of crystalline quartz and minor feldspars and volcanic fragments, interbedded with muddy-siltstones and heterolithic deposits with plant fossils. The succession is organized in three thickening- and coarsening-upward packages that correspond to fluvio-deltaic deposits. The plant fossil assemblage includes fronds of Gangamopteris obovata, Glossopteris angustifolia, Glossopteris wilsonii, and Asterotheca sp. cf. A. andersonii, in association with articulate ribbed stems of Paracalamites australis, which are here assigned to the Gangamopteris Zone. The low plant-diversity of the Tunas Formation could be related to the change from postglacial to semiarid-arid conditions evidenced during the Cisuralian in the Southwest Gondwana region where the study area is located. The obtained geochemical results indicate low recycling and sorting of continentally derived sediments with dominant input from the upper continental crust and acidic to intermediate magmatic precursors related to an active continental margin. Previous studies pointed out that the North Patagonian Massif, located SW of the Ventania System, was the source of such materials. Additionally, new LA-ICP-MS U–Pb zircon ages from two tuff levels of the Tunas Formation were considered together with previous U–Pb zircon ages. These suggest that the source of the ash fall deposits could be as far as the pre-Choiyoi volcanism (Late Mississippian–middle Cisuralian) of the Chilean Precordillera and Frontal Cordillera and as close as the Choiyoi volcanism (middle Cisuralian–Lower Triassic) of the Las Matras and Chadileuvú blocks, all located west of the Ventania System. The integrated analyses of the available U–Pb geochronological data also allowed us to constrain the sedimentation age of the uppermost levels of the Tunas Formation to the Artinskian. However, the age interval of the stratigraphic unit should be wider.

Shrimp zircon geochronology constrains on Permian pyroclastic levels, Claromecó Basin, South West margin of Gondwana, Argentina

Pyroclastic levels are descripted in Sierras Australes outcrops and Claromeco Basin sub-surface records, interbedded with mudrocks and coal beds in the base of the Tunas Formation sequence that correspond to the Permian South West margin of Gondwana. The pyroclastic levels classify as fine tuff. SHRIMP zircon ages obtained are 291.7 ± 2.9 Ma in the outcrop and 295.5 ± 8.0 Ma in the subsurface. These ages are consistent with other zircon SHRIMP ages of other outcrops tuff of the Tunas Formation, with Permian flora, and with tuff ages of correlated Gondwana areas, in the Paraná, Karoo and Paganzo basins. These data, in addition with other geological evidences, support a tectonically active and changing environment during the Permian of Gondwana. The ages allowed calculating a northward latitudinal speed of 2.7 cm/year for Gondwana during the Permian. This latitudinal movement is explained as the consequence of the final coupling of several continental microplates, gradually amalgamated from the southern margins of Gondwana and from the northern of Laurentia to configure the final Pangea during the Triassic. Since the main accretions in the southwestern margin of Gondwana could have started during the Devonian - Carboniferous, this Permian orogeny (San Rafael Orogenic Phase in Argentina) would be representing the post - collisional deformation, with a peak of compression in the Early Permian that was attenuating towards the foreland during the Late Permian - Early Triassic. With these results, it is also possible to constraint the age of the upper Paleozoic glaciation up to 295, previous to the deposition of the Tunas Formation in the Sauce Grande Formation.

Presence of charcoal as evidence of paleofires in the Claromecó Basin, Permian of Gondwana, Argentina: Diagenetic and paleoenvironment analysis based on coal petrography studies

The PANG0001 well, situated in the Claromecó Basin, Argentina, involves rocks that belong to the Tunas Formation [29], Permian of Gondwana. It is composed of fine to medium sandstones intercalating with black and green mudrocks and three coal seams up to 3 m thick. In the coals, a petrographic analysis was carried out to analyze the depositional environment and the diagenesis level reached by the Tunas Formation. The coals are composed of mono-maceral bands of Collotelinite and Gelinite, from the Vitrinite Group, or Fusinite, from the Inertinite Group. They were deposited in a wet swampy forest with rises and decreases of the phreatic level. The presence of inertinite or charcoal evidence the occurrence of palaeo-fires during the Permian. The vitrinite reflectance values are between 1.3% and 2.38%. The coals classify as semiantracitic to low volatile bituminous, with 10 to 25% of volatile matter. The vitrinite reflectance values indicate temperatures between 140 °C and 190 °C. The Tunas Formation reaches the oil to methane gas window, into a catagenesis to metagenesis range related to the organic matter diagenesis. Coal petrography is a good method for the environment and diagenesis analyses applied to the Tunas Formation.

Tectonic insight based on anisotropy of magnetic susceptibility and compaction studies in the Sierras Australes thrust and fold belt (southwest Gondwana boundary, Argentina)

The Sierras Australes fold and thrust belt (Buenos Aires Province, Argentina) was in the southwestern Gondwanaland margin during the Paleozoic. The Tunas Formation (Permian) is exposed along the eastern part of it and continues eastward beneath the Claromeco Basin. Anisotropy of magnetic susceptibility (AMS) and compaction studies are described and compared with previous paleomagnetic studies with the aim of determining direction and magnitude of the main stresses acting during the sedimentation of the Tunas Formation. The anisotropy ellipsoids are triaxial with oblate or prolate shapes, reflecting different stages of layer parallel shortening during the evolution of the basin. Kmax axes trend NW-SE, parallel to the fold axes, while Kmin move from a horizontal (base) to a vertical orientation at the top of the succession, showing a change from a tectonic to almost a sedimentary fabric. The magnitude of anisotropy and compaction degree decreases toward the top of the succession. The AMS results are consistent with the outcrop structural observations and the compaction and paleomagnetic data. Regional pattern indicates a compression from the SW along this part of Gondwana, with a migration of the orogenic front and attenuation toward the NE in the foreland basin during the Upper Paleozoic. This deformation, locally assigned to the San Rafael noncollisional orogenic phase, is the result of the latitudinal movements toward the Equator of Gondwana (southern plates) and Laurentia (northern plates) during the Permian. This movement is the result of a rearrangement of the microplates that collided with Gondwana during the Late Devonian, to configure Pangea during the Triassic.

Permian palynostratigraphy of the Claromecó Basin, Argentina

The first palynozonation for Permian strata of the Claromecó Basin (Argentina) is formally proposed, based on palynological assemblages recovered from the UTAL.CMM1 La Estrella.x-1 and UTAL.CMM1 Cruz de Sur.x-1 boreholes, drilled on the Argentinian Continental Platform and correlation with established zones from neighbouring basins. Fifty-five samples were analyzed and 131 species identified from two biozones. The lower Converrucosisporites confluens–Vittatina vittifera (CV) Zone occurs in the Piedra Azul and Bonete formations of both boreholes. This zone can be assigned a Cisuralian–Guadalupian age. The upper Tornopollenites toreutos–Reduviasporonites chalastus (TC) Zone registered in the Tunas Formation can be assigned a Guadalupian–Lopingian age. The CV Zone is dominated by diverse non-taeniate bisaccate, plicate and taeniate pollen. Monosaccate and monosulcate pollen, algae and acritarchs are poorly represented in both biozones. The composition of the TC Zone is broadly similar to the CV Zone, but bisaccate pollen grains are markedly more diverse and abundant in the former.

A provenance study of the Paleozoic Ventania System (Argentina): Transient complex sources from Western and Eastern Gondwana

The U–Pb and Lu–Hf isotopic analyses of the different sedimentary sequences of the Ventania System, an old Paleozoic orogenic belt exposed in the southern region of the Río de la Plata Craton in the province of Buenos Aires, Argentina, provide new evidence for the understanding of the tectonic evolution of the western sector of the Gondwanides mountain belt. These ranges formed as result of the late Paleozoic collision of the Patagonia terrane against the continental margin of Gondwana. The provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the Tandilia System, a Paleoproterozoic mountain belt formed during the amalgamation of the Río de la Plata Craton at about 1800–2200Ma, and incorporated to Western Gondwana during the Brasiliano Orogeny at 550–530Ma. The local dominant source at the base of the early Paleozoic changed to more distant supplies toward the top of the sequences, when an increasing participation of detritus from first, Cambrian (560–520Ma) zircons from the Pampean Orogen, and later on Ordovician (480–460Ma) zircons from the Famatinian Orogen is recorded. The detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early Paleozoic stratigraphy. The Balcarce Formation, an early Paleozoic sedimentary cover of the Tandilia metamorphic and igneous basement, shows striking differences when compared with the new data from the Ventania System. The two data-sets reveal different sources for the two regions. The late Paleozoic foreland basin deposits mark an abrupt change of 180° in the paleocurrent directions, in the petrographic composition of the sediments, and in the provenance of detrital zircons. These data indicate a southern provenance with the first evidence of Carboniferous and Permian magmatic zircons. The oldest Archean zircons together with the finding of clasts with archeocyathids support the provenance from Patagonia, which was derived from Eastern Gondwana. The U–Pb ages of the ash-fall tuffs in the Tunas Formation confirm the Early Permian age of the Eurydesma Fauna in the Ventania System. The U–Pb data together with the Lu–Hf isotopic data enhance the comprehension of the tectonic evolution of the Ventania System as part of the larger Gondwanides Belt that amalgamated to Western Gondwana during Late Permian times with some independent pieces derived from Eastern Gondwana.

The age of the Tunas formation in the Sauce Grande basin-Ventana foldbelt (Argentina): Implications for the Permian evolution of the southwestern margin of Gondwana

New SHRIMP radiogenic isotope dating on zircons in tuffs (280.8 ± 1.9 Ma) confirms the Early Permian (Artinskian) age of the uppermost section of the Tunas Formation. Tuff-rich levels in the Tunas Formation are exposed in the Ventana foldbelt of central Argentina; they are part of a deltaic to fluvial section corresponding to the late overfilled stage of the Late Paleozoic Sauce Grande foreland basin. Recent SHRIMP dating of zircons from the basal Choiyoi volcanics exposed in western Argentina yielded an age of 281.4 ± 2.5 Ma (Rocha-Campos et al., 2011). The new data for the Tunas tuffs suggest that the volcanism present in the Sauce Grande basin can be considered as the distal equivalent of the earliest episodes of the Choiyoi volcanism of western Argentina. From the palaeoclimatic viewpoint the new Tunas SHRIMP age confirms that by early Artinskian glacial conditions ceased in the Sauce Grande basin and, probably, in adajacent basins in western Gondwana.

Further palaeomagnetic results from the Sierras Australes fold and thrust belt, Argentina

Summary The southwestern Gondwanaland margin during the late Palaeozoic was located between the South America craton and the Patagonia terrane. The Sierras Australes fold and thrust belt of Buenos Aires province (Argentina) has been interpreted as being the result of a crustal collision between these two blocks. A palaeomagnetic study of the less deformed upper part of the Permian Tunas Formation of the Pillahuinco region within the Sierras Australes belt was carried out in order to obtain reliable palaeomagnetic poles to constrain the palaeogeographic and tectonic evolution of this part of Gondwana. More than 300 specimens were measured from the Tunas Formation in the Sierra de Pillahuinco, from 27 sites in four localities of these red sandstones and clay-siltstones. Demagnetization at high temperatures isolated a reversed characteristic remanent magnetization, consistent with magnetization acquired during the Kiaman reverse superchron. Stepwise tectonic tilt correction suggests that the upper part of the Tunas Formation acquired its magnetization before or early in the deformation. The position of the palaeomagnetic pole on the apparent polar wander path of South America suggests that the magnetization was acquired some time between the late Early Permian and the early Late Permian. From the best grouping of the partially tilt-corrected remanence directions, at 90 per cent unfolding for 24 sites, a mean palaeomagnetic pole Tunas II for the Tunas Formation of the Pillahuinco region was calculated. The position is longitude 025.9°E, latitude 74.1°S; A95 = 5.2°, K = 33.6. This pole is consistent with previous poles from South America assigned to a similar age and allows for a more accurate definition of the apparent polar wander path. An earlier study in the lower part of the Tunas Formation (Tunas I) shows syntectonic magnetization acquired during the Early Permian. Therefore the differences in the type and age of magnetization between the lower and upper parts of the Tunas Formation suggest that the deformation was diachronous and propagated more slowly towards the foreland. Palaeogeographically, these two Tunas Formation poles are interpreted as recording a slight counterclockwise movement of South America during the Permian.

Palaeomagnetic constraints on the age of deformation of the Sierras Australes thrust and fold belt, Argentina

SUMMARY A palaeomagnetic study has been carried out on late Palaeozoic rocks exposed in the Sierras Australes thrust and fold belt of Buenos Aires province (Argentina), in the early Permian red sandstones and clay siltstones of the Tunas Formation. The sections sampled are exposed in the eastern parts of the belt, in Sierra de las Tunas (north) and Sierra de Pillahuinco ´ (south). More than 300 specimens were collected from 25 sites, in three localities with diVerent structural attitudes. Demagnetization at high temperatures isolated a characteristic remanence at 20 sites. All the localities have a reverse characteristic remanence, suggesting that the magnetization was acquired during the Kiaman interval. Stepwise tectonic tilt correction suggests that the Tunas Formation in these localities acquired its magnetization during folding in early Permian times. Palaeomagnetic poles were computed for each locality based on partial tiltcorrected remanence directions. Taking into account the fact that these localities are close to one another and that the rocks are all of reverse polarity, a group syntectonic palaeomagnetic pole called Tunas was calculated: longitude: 13.9°E, latitude: 63.0°S; A 95 =5.4°, K=39.7, N=19. This pole is consistent with previously calculated poles from South America assigned to the early Permian. In age it corresponds to the early Permian San Rafaelic tectonic phase of the Sierras Australes. Independent geological evidence indicates that the Tunas Formation underwent syndepositional deformation. We conclude that the Tunas Formation was deposited, deformed and remagnetized, all during the early Permian.

Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) revisited

As integral parts of du Toit’s (1927) “Samfrau Geosyncline”, the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) share similar paleoclimatic, paleogeographic, and paleotectonic aspects related to the Late Paleozoic tectono-magmatic activity along the Panthalassan continental margin of Gondwanaland. Late Carboniferou-earliest Permian glacial deposits were deposited in the Sauce Grande (Sauce Grande Formation) and Karoo (Dwyka Formation) basins and Falkland–Malvinas Islands (Lafonia Formation) during an initial (sag) phase of extension. The pre-breakup position of the Falkland (Malvinas) Islands on the easternmost part of the Karoo basin (immediately east of the coast of South Africa) is supported by recent paleomagnetic data, lithofacies associations, paleoice flow directions and age similarities between the Dwyka and the Lafonia glacial sequences. The desintegration of the Gondwanan Ice Sheet (GIS) triggered widespread transgressions, reflected in the stratigraphic record by the presence of inter-basinally correlatable, open marine, fine-grained deposits (Piedra Azul Formation in the Sauce Grande basin, Prince Albert Formation in the Karoo basin and Port Sussex Formation in the Falkland Islands) capping glacial marine sediments. These early postglacial transgressive deposits, characterised by fossils of the Eurydesma fauna and Glossopteris flora, represent the maximum flooding of the basins. Cratonward foreland subsidence was triggered by the San Rafael orogeny (ca. 270 Ma) in Argentina and propogated along the Gondwanan margin. This subsidence phase generated sufficient space to accommodate thick synorogenic sequences derived from the orogenic flanks of the Sauce Grande and Karoo basins. Compositionally, the initial extensional phase of these basins was characterized by quartz-rich, craton-derived detritus and was followed by a compressional (foreland) phase characterized by a paleocurrent reversal and dominance of arc/foldbelt-derived material. In the Sauce Grande basin, tuffs are interbedded in the upper half of the synorogenic, foldbelt-derived Tunas Formation (Early–early Late? Permian). Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late Early Permian (260 Ma) age. Silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) peaked between the late Early Permian and Late Permian. A link between these volcanics and the consanguineous airborne tuffs present in the Sauce Grande and Karoo basins is suggested on the basis of their similar compositions and ages.

Late Paleozoic transpression in Buenos Aires and northeast Patagonia ranges, Argentina

Paleozoic sediments are present in three regions in eastern central Argentina: (1) the Sierras Australes of Buenos Aires, (2) Sierras Septentrionales of Buenos Aires and (3) Northeast Patagonia. All of these deposits share a common deformational imprint imparted by late Paleozoic Gondwanan deformation. Exposures of these rocks are scattered, variably deformed, and isolated by younger sediments deposited in basins related to the Mesozoic through Tertiary opening of the South Atlantic such as the offshore Colorado Basin. The Sierras Australes of Buenos Aires outcrops are the best preserved. They are mostly located along the Sierras Australes foldbelt, with minor outliers distributed in the adjacent Claromec-basin. The Tunas Formation (early-early late? Permian) is the uppermost unit of the Pillahuincó Group (late Carboniferous-Permian) and is crucial to the understanding of the tectono-sedimentary evolution of the region during the late Paleozoic. The underlying units of the Pillahuincó Group (Sauce Grande, Piedra Azul and Bonete Formations) exhibit a depositional and compositional history characterized by glaciomarine sedimentation and postglacial transgression. They are also characterized by rather uniform quartz-rich compositions indicative of a cratonic provenance from the La Plata craton to the NE. In contrast, the sandstone-rich Tunas Formation has low quartz contents, and abundant volcanic and metasedimentary fragments; paleocurrents are consistently from the SW. Glassrich tuffs are interbedded with sandstone in the upper half of the Tunas Formation. The age of the deformation in the Sierras Australes is Permian and early-middle Triassic. This is based on metamorphic events indicated by formation of illite at 282 ± 3 Ma, 273 ± 8 Ma, 265 ± 3 Ma, and 260 ± 3 Ma ( K Ar illite) in the Silurian Curamalal Group. Evidence of syntectonic magmatism is provided by a radiometric date of 245 ± 12 Ma ( K Ar hornblende) for the López Lecube Granite, immediately west of the Sierras Australes. In the Sierras Septentrionales of Buenos Aires, Precambrian through early Paleozoic deposits of La Tinta, Sierras Bayas, Las Aguilas and Balcarce Formations rest on Precambrian crystalline basement of the La Plata craton. These exposed rocks are affected by subordinate, right lateral wrench faulting; some thrusting indicates tectonic transport toward the NE. In northeast Patagonia (Sierra Grande region) synkinematic deformation of early Permian (261 ± 5 Ma, Rb Sr whole rock) age has been identified in Silurian metasediments of the Sierra Grande Formation. Bands of deformation in Sierra Grande quartzites indicate right lateral wrenching in a N-S direction. Contraction in a NE-SW direction is evidenced by folding. Three stages of tectonic evolution can be discerned for the above regions: (1) Early Paleozoic platform sedimentation, punctuated by episodes of accelerated subsidence during the Silurian and early Devonian, as shown by transgressive episodes, (2) late Paleozoic sedimentation and deformation, and (3) Meso-Cenozoic extensional inversion due to the South Atlantic opening. The late Paleozoic sedimentation and deformation (stage 2) includes late Carboniferous-earliest Permian glacial deposits of the Sierras Australes and Colorado offshore basin, deposited during an initial phase of extension, and cratonward foreland subsidence triggered sedimentation of the synorogenic deposits of the Permian Tunas Formation. Tuffs are intercalated in the upper half of this unit. These tuffs are associated with the silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) that peaked between the late early Permian and late Permian. Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late early Permian (260 Ma) age. The deformation described in this paper can be considered as part of a large scale intracontinental deformation in SW Gondwanaland inboard of an Andean-type compressive margin. This deformation is characterized by transpression (right lateral wrenching) combined with overthrusting to the NE and N-S horizontal contraction.

The Tunas Formation (Permian) in the Sierras Australes foldbelt, east central Argentina: evidence for syntectonic sedimentation in a foreland basin

The Tunas Formation, extensively exposed in the Sierras Australes foldbelt of eastern central Argentina, completes the sedimentation of the Gondwanan (Late Carboniferous-Permian) sequence, locally known as the Pillahuincó Group. The underlying units of the Group show an integrated depositional history which can be explained in terms of glaciomarine sedimentation (Sauce Grande Formation) and postglacial transgression (Piedra Azul and Bonete Formations). This succession also has a rather uniform quartz-rich, sand-sized composition indicative of a cratonic provenance from the Tandilia Massif to the northeast. Early to Late Permian deformation folded and thrusted the southwestern basin margin (Sierras Australes) and triggered the deposition of a 1,500 m — thick, synorogenic prograding wedge, the Tunas Formation, in the adjacent foreland basin (Sauce Grande or Claromecó Basin). Sandstone detrital modes for the Tunas deposits show moderate to low contents of quartz and abundant lithics, mostly of volcanic and metasedimentary origin. Paleocurrents are consistently from the SW. Tuffs interbedded with sandstones in the upper half of Tunas Formation (Early — early Late? Permian) are interpreted as being derived from volcanic glass-rich tuffs settled in a body of water. Extensive rhyolitic ignimbrites and consanguineous airborne tuffaceous material erupted in the northern Patagonian region during that period. The age constraints and similarities in composition between these volcanics and the tuffaceous horizons present in the Sauce Grande, Parana and Karoo Basins suggest a genetic linkage between these two episodes. The intimate relationship between volcanic activity inboard of the paleo-Pacific margin, deformation in the adjacent orogenic belt and subsidence and sedimentation in the contiguous foreland basin constitutes a common motif in the Sauce Grande and Karoo Basins of southwestern Gondwana.