Paganzo Basin Research Articles

Palaeomagnetism of the Upper Carboniferous-Lower Permian transition from Paganzo basin, Argentina

A palaeomagnetic study of Upper Palaeozoic rocks was performed on Paganzo basin, central-western Argentina and its western extension, Río Blanco subbasin. The volcanic—volcaniclastic Río del Peñón and marine sedimentary Punta del Agua formations were sampled in the Rincón Blanco syncline. Both units represent the Upper Carboniferous—Lower Permian transition according to palaeontological and geochronological evidence. The Chancaní and Cerro Colorado-Caminiaga formations were sampled in their type localities, in the eastern Paganzo basin. All-reversed, pre-tectonic characteristic magnetizations were isolated in all three localities, spanning a wide lithological variation. Particularly in the Rincón Blanco syncline, the same reversed magnetization is observed in the entire 1500 m-thick sequence in volcanic and sedimentary rocks; at least part of the deformation there is thought to be Permian in age, related to the San Rafael orogenic phase (SROP). The palaeomagnetic poles are: Rincón Blanco (RB) Lat. 77°S, Long. 294°E, N= 19, A95= 4.9, K= 47; Chancaní (CH) Lat. 85°S, Long. 359°E, N= 3, A95= 8.8, K= 196; Cerro Colorado-Caminiaga (CC) Lat. 79°S, Long. 291°E, N= 6, A95= 8.0, K= 71. Neither of the poles is coincident with expected Late Carboniferous—Early Permian directions according to accepted apparent polar-wander paths (APWPs) from South America and from other plates forming Pangaea in Late Palaeozoic times. This discrepancy could be the result of: (i) polar wander affecting the western Gondwana margin, probably related to SROP; (ii) tectonic rotations related to strike-slip faults active in the area since the Palaeozoic and controlling the position of the main depocentres; (iii) incorrect age assignment for the sedimentary sequences or for the remanence acquisition, i.e. local remagnetizations. Tectonic rotations and local remagnetization have both been proven in particular localities and further work needs to be done to establish the Late Palaeozoic APWP for South America, however, SROP seems to have played a major role controlling both processes, rotation and remagnetization.

The Gondwanan Carboniferous-Permian Boundary Revisited: New Data from Australia and Argentina

The identification and correlation of the Carboniferous-Permian (Gzhelian-Asselian) boundary within the sedimentary sequences of Gondwana has always been a topic of debate. Type latest Carboniferous and earliest Permian marine sequences are characterised by warm tropical faunas and come from the Uralian Region of Russia and Kazakhstan. Faunas include conodonts and fusulinid foraminiferids which are prime tools for correlation. Such faunal groups are absent from most Gondwanan sequences where reliance for correlations must be placed primarily on brachiopods, bivalve molluscs and palynology. The Western Australian marine sequences, with their contained ammonoids, provide a pivotal link for the dating and correlating of Early Permian Gondwanan sequences with those of the type regions and their palynostratigraphical record is essential for trans-Australian correlations and correlations elsewhere throughout Gondwanaland. New data from the fully cored DM Tangorin DDHl bore hole, drilled in the Cranky Corner Basin, New South Wales, Australia, reveals a sequence of descending faunal zones. The stratigraphically highest zone with Eurydesrna cordaturn, encompasses the Late Sakmarian (Sterlitamakian). The middle zone with Torniopsis elongata, Sulciplica c r a m and Trigonotreta tangorini straddles the Sterlitamakian-Tastubian boundary, with the palynomorphs Pseudoreticulatispora pseudoreticulata high in the zone and Granulatisporites confluens low in the zone. An impoverished fauna with Trigonotreta nov., low in the Granulatisporites confluens Zone, is probably of latest Asselian or Tastubian age. Significant new data from Argentina has revealed marine faunas from below the occurrence of Granulatisporites confluens. These are considered to be of Asselian age. Outcrops of the Tupe Formation, with a marine fauna, at La Herradura Creek in the western Paganzo Basin, San Juan Province, are best regarded as being of mid to late Asselian age. The Tupe Fauna has been recognised as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone. Previously, this fauna was considered to be of Late Carboniferous or Stephanian age. Three faunal associations are known from the Rio del Peii6n Formation, Rio Blanco Basin, La Rioja Province. The middle assemblage with Tivertonia, Costaturnulus, Kochiproductus and Trigonotreta, appears to correlate well with the Tupe Formation fauna. The lower assemblage, with Streptorhynchus, Etherilosia, Costaturnulus, Trigonotreta and a punctate spiriferid, as well as indeterminate productids, probably of Early Asselian age. The youngest assemblage includes a species ofRhynchopora that is close to Rhynchopora australasica from the latest Asselian-early Tastubian of Western Australia. The marine biostratigraphical data from Argentina has enabled a much greater understanding of the earliest Permian marine faunas to be achieved - a story that is apparently absent from the other cold and cool temperate regions of Gondwana.

Palaeoenvironmental Framework of the Glacial-Postglacial Transition (Late Paleozoic) in the Paganzo-Calingasta Basin (Southern South America) and the Great Karoo-Kalahari Basin (Southern Africa): Ichnological Implications

Sedimentological, palaeontological and geological data from the glacial to postglacial transition in the late Paleozoic successions of the Paganzo-Calingasta Basin (PC) in southern South America and the Great Karoo-Kalahari Basin (GKK) in southern Africa are analysed, revised and reinterpreted. A brackish depositional setting is inferred for main areas previously considered to be nonmarine based upon ichnological interpretations. Three stratigraphic intervals have been defined based on changes in sedimentary facies and trace fossils association: The glacial interval (GI), early postglacial interval (EPI) and late postglacial interval (LPI). The GI and EPI contain a dominance of arthropod trackways, fish trails with and subordinate grazing and feeding traces. The EPI in the PC Basin comprises both nonmarine and brackish-marine ichnocoenoses without significant differences in ichnological composition. Trace fossils are preserved in underflow and turbidite beds of deltaic deposits. Opportunistic grazing traces constitute a post-event ichnocoenosis, while a pre-event ichnocoenosis is preserved at the base of turbidite beds. In the GKK Basin ichnofossils were documented in turbidite fans. The LPI in the GKK Basin contains the first evidence of shallow water deltaic infauna and subordinate grazing traces. Conversely, in the PC Basin the infauna is lacking. The GI and particularly the EPI ichnofauna resemble the nonmarine Mermia ichnofacies but they occur in large and long-lived stable marine basins which received enormous inputs of melt-water that markedly reduced the salinity. They are not ichnologically comparable with brackish marginal marine environments that are affected by frequent, sometimes diurnal, fluctuations in physical parameters such as salinity and temperature. The LPI in the GKK Basin constitutes a typical record of the classical marginal marine ichnofaunas, but in the PC Basin the ichnofacies assignment is problematic. Grazing traces of the Mermia ichnofacies appear in middle estuarine deposits where both palynomorphs and the presence of tidal features suggest brackishness. The Paganzo Basin ichnology, therefore shows some difficulties in differentiating nonmarine from subtle brackish environments without the full integration of sedimentological information. Ichnocoenoses dominated by trackways of arthropods are particularly inappropriate to distinguish between these types of depositional settings.

The Upper Carboniferous postglacial transgression in the Paganzo and Rı́o Blanco basins (northwestern Argentina): facies and stratigraphic significance

During the early Late Carboniferous (Namurian–early Westphalian), an important postglacial transgression took place in the basins of western Argentina. The regional distribution, lithological characteristics, facies arrangement, and age of the transgression are analyzed herein. The postglacial transgressive event was studied in the Rı́o Blanco and Paganzo basins. Seven regionally extensive facies were recognized. Laminated mudstones were deposited during the maximum flooding stage, including some thin marls and black limestone beds interpreted as condensed levels. Laminated mudstones with dropstones facies, due to iceberg melting, point out to deglaciation processes. Bouldery and pebbly diamictites mainly represent gravity flow deposits, which, in some cases, are associated with interbedded sandstones and mudstones sequences interpreted as turbidites. Sandstones with large-scale cross-bedding represent high constructive Gilbert-type deltas. Finally, coarsening and thickening upward sequences result from the progradation of mouth bars deposited in delta-front environments. On the basis of the facies arrangement, three major postglacial facies associations were recognized: open marine, transitional, and continental-dominated. Palynological assemblages suggest a Namurian to early Westphalian age for the postglacial transgression.

The Late Carboniferous Glacial to Postglacial Transition: Facies and Sequence Stratigraphy, Western Paganzo Basin, Argentina

Namurian-early Westphalian glacial-postglacial deposits have been documented in the Paganzo Basin, Argentina, but integrated sequence stratigraphic and sedimentological analyses of the glacial-postglacial transition are rare. In the Cuesta de Huaco section the glacial-postglacial transition encompasses the lower sequence of the existing schemes. Glacioeustatic changes in sea-level controlled the architecture and vertical arrangement of facies. The basal sequence boundary is coeval with the onset of glaciation when sea level fell, contemporaneous with glacial erosion and bypass of sediment. The upper sequence boundary is a high-frequency sequence boundary. The basal deposits are interpreted as an early transgressive systems tract (ETST) initiated during glacial retreat. The interval fines upward, contains current reworked diamictites, conglomerates and sandstones deposited in a subglacial to proglacial environment (Facies I-II) and ice-rafted glaciomarine rhythmites (Facies III). Postglacial brackish-marine black shales (Facies IV) represent the late transgressive systems tract (LTST). Micritic-limestone beds were deposited during a period of sediment starvation coeval with the maximum marine inundation in the basin (MFZ). It marks a turnaround in the stacking pattern that became coarsening and thickening upward during the overlying highstand systems tract (HST). It consists of fine-grained turbidites (Facies V) and classical turbidites with wave reworked beds at the top (Facies VI) deposited in a shallow deltaic systems. Intervening postglacial brackish deposits contain an ichnofauna dominated by trackways of arthropods and minor grazing traces, preserved in bedding planes, that resemble nonmarine ichnocoenoses and differ from the archetypal brackish ichnofaunas, even when palynological data support the saline-depleted environment.

Palynology of glacial sediments from the Guandacol Formation (Middle Carboniferous) in the Cerro Bola area, Paganzo Basin, Argentina

The palynoflora of surface samples of glacial deposits from the Guandacol Formation in the Cerro Bola area of the Argentinian Paganzo Basin is documented. The existence of glacial related diamictites in this area has been previously ignored. The fossiliferous levels are located in section B of the formation, which corresponds to the earlier stages of deglaciation. The assemblage contains abundant monosaccate pollen grains whose global incoming is not older than early Namurian (Middle Carboniferous). Many species of this palynoflora are present in equivalent stratigraphic sequences of Argentina and can be referred to Sub-biozone A of the Raistrickia densa-Convolutispora muriornata Biozone (Namurian-Stephanian). New species described here are Cyclogranisporites rinconadensis and Tricidarisporites gutii.

Permian brachiopods from the Tupe Formation, San Juan Province, Precordillera, Argentina

The brachiopod fauna from the Tupe Formation at La Herradura Creek, located on the west flank of Perico Hill, San Juan Province, Argentina, palaeogeographically belongs to the western sector of the Paganzo basin (‘Guandacol embayment’). The stratigraphical section of the Tupe Formation at La Herradura Creek is the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone, was previously regarded as being of Late Carboniferous age but here is assigned to the earliest Permian (Asselian). We describe and review the biozone assemblage, which consists of Streptorhynchus inaequiornatus, Tivertonia jachalensis, Kochiproductus sp., Costatumulus sp., Coronalosia argentinensis, Tupelosia paganzoensis, Trigonotreta pericoensis, Septosyringothyris sp. aff. Septosyringothyris jaguelensis and Crurithyris? sp. This brachiopod assemblage is related to Indian and Australian Early Permian faunas and its presence in the La Herradura Creek section provides new evidence in support of an Asselian (Early Permian) age for the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone. This assemblage is also important for intra- and inter-basinal correlation because several of its characteristic species have been identified from other sections of the Paganzo basin and the Río Blanco basin. The proposed age for this biozone is consistent with the age of palynological data from slightly above the marine faunas from the stratotype locality.

Evidence of glacial abrasion in the Calingasta–Uspallata and western Paganzo basins, mid-Carboniferous of western Argentina

Evidence of glacial abrasion is present in the basal part of the Carboniferous section of the Calingasta–Uspallata and Paganzo (western Argentina), where subglacial and proximal glacial-marine diamictites with pebbly (dropstone) shales constitute the dominant facies association. Intraformational striated pavements are exposed in the Hoyada Verde and Leoncito Formations of the Calingasta–Uspallata basin. Marine invertebrate fossils ( Levispustula zone) in associated shales indicate a mid-Carboniferous age for both units. A single layer striated boulder pavement is superbly exposed in the Hoyada Verde Formation. Local a-axis clast imbrication in the underlying bouldery diamictite and strong parallel striae on the flattened upper surfaces of boulders indicate a subglacial origin for the pavement. Paleo-ice flow directions have an NNE–SSW strike. The Leoncito intraformational pavement is shaped on bioturbated, fine-grained sandstones with plant remains. Glacial striations are straight and parallel with subcircular cross-sections. Dome-like features are present in the surface, which is covered in turn by a sandy bouldery diamictite. Four striated surfaces are present in the western Paganzo basin. Three of them (pavements at the San Juan river valley, Loma de Los Piojos and Agua Hedionda) are developed on basement rocks. They contain parallel to subparallel striations, nailhead striations, crescentic gouges, impact marks and furrows. The pavements are overlain by thin (up to 1.5 m), laterally discontinuous diamictite beds with striated clasts, some of them bullet-shaped. Paleo-ice flow directions are N–NE-directed. An intraformational pavement is also present at the San Juan river valley section; it is carved on the top of the basal diamictite and exhibits furrows, probably caused by subglacial soft-sediment grooving. The dispersion of paleo-ice flow directions in the Calingasta–Uspallata and Paganzo basins indicates: (i) regional slopes toward the north into a brackish water embayment and northwest into the open marine conditions; (ii) glacial erosion by ice grounded below sea level and confined to the margins of the basins.

Sedimentary facies and environmental ichnology of a ?Permian playa-lake complex in western Argentina

Abstract A moderately diverse arthropod ichnofauna occurs in ?Permian ephemeral lacustrine deposits of the Paganzo Basin that crop out at Bordo Atravesado, Cuesta de Miranda, western Argentina. Sedimentary successions are interpreted as having accumulated in a playa-lake complex. Deposits include three sedimentary facies: (A) laminated siltstone and mudstone; (B) current-rippled cross-laminated very fine grained sandstone; and (C) climbing and wave-rippled cross-laminated fine-grained sandstone deposited by sheet floods under wave influence in the playa-lake complex. Analysis of facies sequences suggests that repeated vertical facies associations result from transgressive-regressive episodes of variable time spans. The Bordo Atravesado ichnofauna includes Cruziana problematica, Diplocraterion isp., cf. Diplopodichnus biformis, Kouphichnium ? isp., Merostomichnites aicunai, Mirandaichnium famatinense, Monomorphichnus lineatus, Palaeophycus tubularis, Umfolozia sinuosa and Umfolozia cf. U. longula . The assemblage is largely dominated by arthropod trackways and represents an example of the Scoyenia ichnofacies. Trace fossils are mostly preserved as hypichnial ridges on the soles of facies C beds, being comparatively rare in facies A and B. Ichnofossil preservation was linked to rapid influx of sand via sheet floods entering into the lake. Four taphonomic variants (types 1–4) are recognized, each determined by substrate consistency and time averaging. Type 1 is recorded by the presence of low density assemblages consisting of poorly defined trackways, which suggests that arthropods crawled in soft, probably slightly subaqueous substrates. Type 2 is represented by low to moderate density suites that include sharply defined trackways commonly associated with mud cracks, suggesting that the tracemakers inhabited a firm, desiccated lacustrine substrate. Type 3 displays features of types 1 and 2 and represents palimpsestic bedding surfaces, resulting from the overprint of terrestrial ichnocoenoses over previously formed softground suites. Type 4 differs from type 2 only in that assemblages display a high density of traces, recorded by numerous superimposed trackways, which suggests a major time gap of subaerial exposure before sheet flood entrance. Therefore, type 4 surfaces are mostly interpreted as track imprinted omission surfaces.

Palinologia dos carvões paleozóicos do Estado de São Paulo: II - o carvão de Monte Mor

Este trabalho se constitui na segunda contribuição a partir do desenvolvimento dos estudos palinológicos dos carvões paleozóicos do Estado de São Paulo, que inclui os carvões de Buri, Monte Mor e Cerquilho. Os resultados das análises palinológicas do carvão de Monte Mor são apresentados, com a descrição de vinte e seis táxons, dos quais dois são inéditos para a Bacia do Paraná (Punctatisporites resolutus Playford, 1971 e Apiculiretusispora cf. A. coalescens Menéndez & Azcuy, 1973). A maioria dos táxons descritos ocorrem, também, em Buri e em Araçoiaba da Serra, permitindo assim a correlação palinológica com as associações ali apresentadas e, conseqüentemente, sua correlação estratigráfica. Uma idade westphaliana é sugerida para o carvão de Monte Mor, assim como já fôra proposto para Buri e Araçoiaba da Serra; referiveis ao intervalo bioestratigráfico informal "Pré-G". Fora do Brasil, a única possibilidade de correlação é com as biozonas westphalianas da Bacia de Paganzo, Argentina. As informações paleoambientais são apresentadas com base na freqüência dos morfogrupos palinológicos considerados e os contextos geológico e estratigráfico são discutidos

Post glacial lacustrine event sedimentation in an ancient mountain setting: Carboniferous Lake Malanz�n (Western Argentina)

Lacustrine deposits of the Malanzan Formation record sedimentation in a small and narrow mountain paleovalley. Lake Malanzan was one of several water bodies formed in the Paganzo Basin during the Late Carboniferous deglaciation. Five sedimentary facies have been recognized. Facies A (Dropstones-bearing laminated mudstones) records deposition from suspension fall-out and probably underflow currents coupled with ice-rafting processes in a basin lake setting. Facies B (Ripple cross-laminated sandstones and siltstones) was deposited from low density turbidity currents in a lobe fringe environment. Facies C (Massive or graded sandstones) is thought to represent sedimentation from high and low density turbidity currents in sand lobes. Facies D (Folded sandstones and siltstones) was formed from slumping in proximal lobe environments. Facies E (Wave-rippled sandstones) records wave reworking of sands supplied by turbidity currents above wave base level.

The ichnotaxonomic status ofPlangtichnusandTreptichnus

A new ichnospecies, Treptichnus pollardi, is described from Carboniferous lacustrine rocks of the Paganzo Basin, northwestern Argentina. Accordingly, the ichnotaxonomic status of the ichnogenera Plangtichnus and Treptichnus is reviewed. Because these ichnotaxa represent different parts of the same 3D zigzag structure, they are considered to be synonymous. Considering historical usage, Plangtichnus is regarded as a nomen oblitum and Treptichnus as a nomen conservandum. Four Treptichnus ichnospecies have been recognized in the stratigraphic record: T. bifurcus, T. triplex, T. lublinensis, and T. pollardi isp. nov. Treptichnus pollardi differs from the ichnospecies described hithertoo by the absence of segment projections and the presence of shafts vertical to the zigzag trace, expressed at the bedding plane by pits either at the angle of juncture between burrow segments or within burrow segments. It is also distinguished from T. lublinensis and T. triplex by its thinner burrow segments.

Palinologia dos carvões paleozóicos do Estado de São Paulo, Brasil: I - O carvão de Buri

São inúmeros os problemas relacionados à estratigrafia do Subgrupo ltararé, unidade representativa da sedimentação glacial paleozóica da Bacia do Paraná. Tais dificuldades são decorrentes do fato de a seqüência em questão constituir um conjunto de grande diversidade litológica e variação faciológica, no qual os fósseis são relativamente raros. Dentre os diversos termos litológicos presentes no Subgrupo ltararé, destacam-se os carvões pela potencialidade bioestratigráfica. Conhecidos há longo tempo, representam pelo menos três ocorrências: Buri, Monte Mor e Cerquilho. O trabalho ora apresentado refere-se especificamente aos resultados palinológicos já obtidos a partir do estudo de amostras do Carvão de Buri, que revelam ser ele o mais antigo entre os três níveis estudados. Trinta e quatro taxa foram identificados nas associações, cujo predomínio absoluto é de esporos triletes zonados. Diversas espécies presentes permitem sua correlação com sedimentos da Bacia de Paganzo (Formação Malanzán - Membro Estratos Carbonosos), de idade carbonífera, situada provavelmente no Westfaliano, em condições de deposição continentais.

Late Paleozoic paleoclimatology of central west Argentina

Based on sedimentologic and paleontological characteristics present in the sedimentary record of the Calingasta-Uspallata and Paganzo basins, a model of paleoclimatic evolution for the Late Paleozoic of central west Argentina is proposed. Additional evidence is also provided by data from other basins of southern (San Rafael and Tepuel basins) and northern (Tarija basin) west Argentina. Five paleoclimatic stages have been defined: Paleoclimatic stage I (Tournalism-Visean, Early Carboniferous) is characterized by temperature and humid preglacial conditions as evidencedin the El Ratón and Malimán formations (Calingasta-Uspallata basin). Plant remains of Archeosigillaria and Lepidodendropsis zone are present in both units. The paleoclimatic stage II (Namurian-Early Westphalian, Middle Carboniferous) comprises widespread glacial deposits including glaciomarine sequence in the westernmost basins (Calingasta-Uspallata, western Paganzo, San Rafael and Tepuel) and terrestrial tillites and glaciolacustrine fine clastics in some areas of the Paganzo basin. Fossils of the Levipustula fauna are distinctive in the glaciomarine deposits of this stage and in the overlying postglacial fine clastics of the basal portion of stage III (substage IIIa) The paleoclimatic stage III, of Middle Carboniferous (Late Westphalian) age, corresponds to postglacial transgressions that flooded the previously glaciated shelves in the Calingasta-Uspallata and Tepuel basins, terminal glaciolacustrine deposits (substage IIIa), indicate of the demise of glacial conditions, and coal measures of fluvial affinity in the Paganzo basin (substage IIIb). Probably this postglacial stage was characterized by high humidity and moderate temperatures. This stage is paleobiologi defined by a floral assemblage known as NBG ( Nothorhacopteris, Botrychiopsis and Ginkgophyllum). The paleoclimatic stage IV (Late Carboniferous (Stephanian)-Early Permian) corresponds to a time-interval characterized by seasonally dry climates and a progressive amelioration in humidity. Clear evidence of these climatic conditions is present in several localities in the Paganzo basin. The first Astherothecae and Glossopteris leaves are found. Finally, paleoclimatic stage V (Middle-Late? Permian) is dominated by red-beds, thick eolian sequences and saline lake facies deposited under arid to semi-arid conditions. Coeval volcanism derived from the west is expressed by interbedded tuffs and agglomerates (Calingasta-Uspallata, Paganzo and San Rafael basins).

Development of the Late Paleozoic glaciations of the South American Gondwana in western Argentina

The Late Paleozoic Ice Age in the South American segment of Gondwana consisted of several glaciations, which can be grouped into two main glacial periods. The oldest of them occurred in late Early to early Late Carboniferous time and the younger in Early Permian time. Paleontological evidence in the western Argentine Precordillera supports a late Visean age for the beginning and a probable early Westphalian age for the end of the Carboniferous glacial period. In this span of time, subsidence and deposition of glacial debris took place in the western mobile belt and in Patagonia, but no sediments of that age were deposited in the eastern portion of the South American Gondwana, where subsidence started later. A Carboniferous glacial sequence is remarkably well preserved in the San Eduardo Group on the western flank of the Precordillera, where at least three distinct glaciations are recognized. This sequence can be taken as a reference section for the Carboniferous glacial deposits. Shorter than the former, and probably restricted to the Asselian Epoch, the Permian glacial period is better known. The evidence for it is well developed in eastern South American Gondwana, although a relic is also present in the Precordillera. The recognition of two glacial periods within the Late Paleozoic Ice Age is supported by the existence of a long-lasting interglacial, which seemingly began in the late Westphalian and lasted until the end of the Carboniferous Period. This major interglacial period is inferred from the total absence of glacial features in the Upper Carboniferous sequences. In spite of the existence of a mountainous belt parallel to the “Pacific” during the Carboniferous Period, the presence of marine beds and glacial drift well to the east of these ranges, suggests that it was not altitude that was the main factor in determining the formation of glaciers, but latitude plus a global change in temperature. It is argued that the ice sheets extended greatly during the Carboniferous glaciations and reached neighboring areas of the Paganzo basin in central Argentina.

Sandstone composition changes and paleocurrent reversal in the Upper Paleozoic and Triassic deposits of the Huaco area, western Paganzo Basin, west-central Argentina

Sandstone detrital modes and paleocurrents in the Upper Paleozoic and Triassic deposits exposed near Huaco on the western sector of the Paganzo basin, western Argentina, reveal two distinctive petrofacies. A quartzofeldspathic petrofacies represented in the lacustrine and fluvial deposits of Carboniferous and early Permian age (Guandacol, Tupe and lower Patquía Formations) is characterized by a high content of monocrystalline quartz, low lithic content and high K-feldspar to total feldspar ratios. The fluvial facies of the lower Patquía grade vertically into eolian sandstones (upper Patquía Formation) which show a significant content of volcanic lithic fragments along with high proportions of monocrystalline quartz. This eolian succession is overlain by a volcaniclastic (lithofeldspathic) petrofacies represented by alluvial-fan and fluvial deposits of the late Permian to Triassic age (Cerro Morado and Cauquenes Formations). These sandstones are characteristic by high proportions of volcanic lithic fragments, plagioclase grains, low content of monocrystalline quartz and low K-feldspar to total feldspar ratios. This change in sandstone composition is accompanied by a reversal in paleocurrent patterns. The quartzofeldspathic petrofacies present in the Carboniferous and Lower Permian deposits show a general paleocurrent pattern from east to west with inferred cratonic provenances of plutonic and metamorphic composition. The volcaniclastic deposits indicate paleocurrents from west to east suggesting source areas associated with the magmatic arc developed on the Paleo-Pacific Gondwana margin during Permian and Triassic times.

Thermochronometric data on the development of the basement peneplain in the Sierras Pampeanas, Argentina

In the Sierras Pampeanas geomorphic province of central Argentina there are regions where exposed crystalline basement has had no sedimentary cover older than Neogene strata, and other areas where upper Paleozoic and Mesozoic strata overlie the basement. Both types of areas are characterized by a smooth upper basement surface, which has been interpreted as a peneplain that has been deformed and exhumed during the Neogene. The history of generation of the basement peneplain can be determined from thermochronometric and stratigraphic data. This reconnaissance study produced new data on the cooling history of rocks from seven sites in the Sierras Pampeanas. Fission-track cooling ages of apatite crystals, the 40Ar/ 39Ar age of potassium feldspar, and the KAr age of a muscovite have been determined from the basement rocks at sites which do not have the cover strata. The dating shows that what is now the smooth upper surface of basement at those sites was near the depth of the 100° iotherm during the Triassic. Samples from sites located at deeper structural levels yield younger Mesozoic ages. The fission-track ages, fission-track lengths, and 40Ar/ 39Ar ages are best interpreted to demonstrate slow cooling of the crust since the end of the Paleozoic, which implies continued slow denudation. At the sites studied, the smooth basement surface formed as the cooling and unroofing progressed and is not a relict of the late Paleozoic or early Mesozoic. The chronological data for sites not overlain by strata can be reconciled with the stratigraphic data if the surface is diachronous. In that case, the sampled sites were covered during the Triassic by thick columns (a maximum of 2.8 to 4.2 km cover estimated for 20–30°C/km geothermal gradient) of basement (possibly capped by sedimentary) rocks. This implies that there has been a large amount of vertical motion between the Paganzo basins and the contemporaneous highlands since the late Paleozoic, and that the region was mountainous at the time that the upper Paleozoic and Triassic strata accumulated.

Comparacion de palinofloras carboniferas de las cuencas paganzo (Argentina) y Paraná (Brasil)

Recent studies of Ancistrospora and Potonieisporites palynozones in the Paganzo Basin (Argentina) have shown that they belong to a single biostratigraphic unit for which the name Raistrickia-Plicatipollenites subzone has been proposed. This unit has been referred to the Potonieisporites-Lundbladispora zone from the Chacoparaná Basin as a lower subzone; the upper subzone would then be an Interval sub-zone. On the other hand, recent studies in Brazil have shown that a new pollen association can be defined beneath the basal G-H1 zone; it has been compared with the Paganzo association on the basis of common taxa. This similarity is now strengthened by the new findings in Argentina, and the pre-G-H1 interval probably can be correlate d with the Raistrickia-Plicatipollenites subzone. It thus seems that floras from both áreas had common, basic components and that variations between them were controlled by paleogeography, paleoclimate and paleoenvironment.

Fedekurtzia, a New Carboniferous Frond from Gondwanaland and Its Fructification

Fedekurtzia gen. nov. is proposed for specimens referred to Archaeopteris argentina (Kurtz, 1921) from middle Carboniferous strata of the Paganzo Basin in Argentina. Similar materials from the same area were assigned to at least six different genera by later authors. A revision of specimens from various collections shows that they all belong to a single type of polymorph frond. A progymnospermous affinity is suggested by the small fructifications resembling spikes which are attached to main rachides, as well as by the helicoidal insertion and bilateral disposition of the pinnae on the main axis. Similar fronds and fructifications have been found in Australia. These are referred in part to the new genus which seems to have had a wide distribution over Gondwanaland. Paleoclimatic and palaeogeographic evidence is considered for the Carboniferous in the Gondwana Realm. Rather poor and primitive palaeofloristic associations are usually found.

FEDEKURTZIA, A NEW CARBONIFEROUS FROND FROM GONDWANALAND AND ITS FRUCTIFICATION

Fedekurtzia gen. nov. is proposed for specimens referred to Archaeopteris argentina (Kurtz, 1921) from middle Carboniferous strata of the Paganzo Basin in Argentina. Similar materials from the same area were assigned to at least six different genera by later authors. A revision of specimens from various collections shows that they all belong to a single type of polymorph frond. A progymnospermous affinity is suggested by the small fructifications resembling spikes which are attached to main rachides, as well as by the helicoidal insertion and bilateral disposition of the pinnae on the main axis. Similar fronds and fructifications have been found in Australia. These are referred in part to the new genus which seems to have had a wide distribution over Gondwanaland. Paleoclimatic and palaeogeographic evidence is considered for the Carboniferous in the Gondwana Realm. Rather poor and primitive palaeofloristic associations are usually found.