SW Gondwana Research Articles

Biogeographic implications of Albian Mohria-like spores (Family Anemiaceae) in SW Gondwana (Patagonia)

The schizaeaceous extant genus Mohria has a distribution restricted to South Africa and Madagascar area. Its cicatricose spores are characterised by their large size and muri with hollow longitudinal channels unknown in other schizaeaceous genera. Fossil spores that sometimes have been referred to Mohria lack this typical character and therefore the determinations were discarded. Early Cretaceous (Albian) sediments from Patagonia, referred to the Piedra Clavada Formation of the Austral Basin, contain spores that have the characters now found in Mohria. These fossils are hereby assigned to the new genus Palaeomohria that includes several morphological types. The paleogeographical proximity of South Africa and Patagonia in Southwestern Gondwana persisted from the Permian to the Early Cretaceous, during a time when floras of both regions had significant similarities. In this context, the finding of Mohria-like spores in Patagonia suggests that the distribution of this fern was more extended in the past and became restricted while the continents shifted apart during the Cretaceous/Tertiary. The two living Mohria species may then represent relict taxa of a much more varied and rich group that flourished during the Early Cretaceous in SW Gondwana.

Provenance of the Lower Paleozoic Balcarce Formation (Tandilia System, Buenos Aires Province, Argentina): Implications for paleogeographic reconstructions of SW Gondwana

Lower Paleozoic moderately sorted quartz–arenites from the Balcarce Formation deposited in eastern Argentina (Tandilia System) comprise mainly detrital material derived from old upper crustal material. The sources were magmatic, sedimentary, and subordinated felsic metamorphic terranes. High concentrations of tourmaline and Ti-rich heavy minerals, including zircon and nearly euhedral chromite, are common. Trace element concentrations (Nb, Cr) on rutile indicate pelitic and metabasaltic sources, respectively. Major element analyses on chromites indicate a basic volcanic protolith of mid-oceanic ridge origin, which was exposed close to the depositional basin. The delivery of chromite may be associated with convergent tectonics causing the consumption and obduction of oceanic crust during pre-Upper Ordovician times. The oblique/orthogonal collision of the Precordillera Terrane with the western border of the Rio de la Plata Craton, west of the Balcarce Basin or source further to the east from a Lower Palaeozoic extensional basin are possibilities. Geochemical and petrographic data exclude the underlying Precambrian and Cambrian sedimentary rocks as dominant sources, and favour the basement of the Río de La Plata Craton, including Cambrian rift-related granites of South Africa and the Sierras Australes (eastern Argentina), as main suppliers of detritus. Trace element geochemistry of recycled pyroclastic material, associated with the quartz–arenites, also suggests volcanic arc sources. The provenance of the pyroclastic material may either be the Puna–Famatina arc, located in north and central Argentina, or a hypothetical active margin further to the south. These ash layers are equivalent in age to volcanic zircons found in the Devonian Bokkeveld Group in western South Africa. The deposition of a glacial diamictite of Hirnantian age (Sierra del Volcán Diamictite) is interpreted as a member of the Balcarce Formation. Based on the stratigraphic re-location of the glacial diamictite and trace fossils, the Balcarce Formation is considered here to be Ordovician to Silurian in age. The Balcarce Formation can be correlated with similar rocks in South Africa, the Peninsula Formation, and the upper Table Mountain Group (Windhoek and Nardouw subgroups), including the Hirnantian glacial deposit of the Pakhuis Formation.

Plant assemblages from SW Gondwana: further evidence for high-latitude vegetation in the Devonian of Argentina

Abstract Plant assemblages are described from the Talacasto and Punta Negra Formations, San Juan Province, the Argentine Precordillera. A detailed facies study involving trace fossil assemblages indicates that the plants were buried in shallow low- to high-energy marine environments, but not in turbidites as previously postulated. Apart from a single specimen resembling Cooksonia from the Lochkovian Talacasto Formation, the abundant compression fossils consist of short lengths of smooth axes, occasional defoliated lycopod stems, and very rare isolated sporangia. It is thus impossible to assign any to existing taxa with confidence nor are there sufficient distinguishing characters to erect new ones. Although the possible age of the formations in the Precordillera remains conjectural, the fossils themselves, even preserved in such fragmentary states, indicate much lower grades of organization than seen in coeval assemblages elsewhere, particularly on the Laurussian and South China palaeocontinents. Palaeocontinental reconstructions of Gondwana indicate that the localities are within 30° of the palaeo South Pole. Thus a global latitudinal gradient in vegetation is postulated for the Early Devonian with decreasing disparity and diversity at higher latitudes. An explanation for the latter is explored in terms of light regime and temperature in highly stressed environments.

Age and geotectonic setting of Late Neoproterozoic juvenile mafic gneisses and associated paragneisses from the Ribeira belt (SE Brazil) based on geochemistry and Sm–Nd data — Implications on Gondwana assembly

Mafic gneisses and associated paragneisses from the Cabo Frio Tectonic Domain in the southeastern part of the Ribeira Belt, along the coast of Rio de Janeiro State in southeast Brazil, were subjected to a geochemical and Sm–Nd isotope study. Four lithotypes are distinguished: aluminous paragneisses (mainly sillimanite–kyanite–garnet–biotite gneiss), calcsilicate lenses, quartzo–feldspathic metasedimentary gneisses and mafic–ultramafic lenses. The whole-rock major and trace, including rare earth element distributions in the mafic–ultramafic intercalations indicate that derivation from subalkaline basalt/gabbro of tholeiitic affinity with E-MORB signature from a non-subduction environment. These mafic rocks have positive ε Nd(t) and T DM of 1.1 Ga. The metasedimentary rocks have negative ε Nd(t) and T DM of 1.7 Ga. A Sm–Nd whole rock isochron of mafic rocks yielded an age of 604 ± 38 Ma for the crystallization. This matches with the age of some detrital zircon grains from the paragneisses. The depositional basin, named Buzios–Palmital, was active at least until 620 Ma (age of the youngest detrital zircon) and was subsequently deformed and metamorphosed at ca. 525 Ma (age of metamorphic zircons) during the Buzios Orogeny. It is interpreted as a back arc basin with relation to the 630 Ma magmatic arc of the Oriental Terrane in the Ribeira Belt to the NW. However, after 600 Ma, the Buzios–Palmital basin changed to an active margin setting because the arc collided with the continental margin and the subduction shifted to the back arc environment. By 610 Ma, most of the Brasiliano belts registered collisional events related to multiple convergent blocks. The stress fields and paleocontinent shapes would have allowed the occurrence of extensional areas with not only sedimentary deposition but also ocean floor spreading. Its remnants are preserved in this Brazilian coastal region as an ancient suture, reworked intensively during the Mesozoic rifting events. The reconnaissance of Late Neoproterozoic basins in the Brasiliano–Pan-African belts is of major importance to partially unravel the final amalgamation events of SW Gondwana. Considering that the Buzios–Palmital basin rock units are mostly covered by the marginal Atlantic basins, it is possible that other evidence could be preserved in the coastal regions of SW-Africa and SE-South America.

Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies – Response to climate evolution and sedimentary environment

Records of incisive climate changes during the late Palaeozoic Era are best documented in sedimentary basins of South Africa. Glacial (Upper Carboniferous–Early Permian Dwyka Group and equivalents) and postglacial deposits (Early to Middle Permian Ecca and Beaufort Groups) are known from the Main Karoo Basin in South Africa, from the Kalahari Basin (Namibia and Botswana) and other late Palaeozoic Basins of south Gondwana. Glacial deposits comprise diamictites and tillites followed by postglacial shales with variable organic matter contents and intercalated carbonate horizons. This study covers the final glaciation phase (Upper Pennsylvanian–Asselian/Sakmarian) and the transition into the postglacial stage of the Middle to Late Permian. For sedimentary successions from the Karoo and eastern Kalahari Basin element geochemical proxies (Chemical Index of Alteration, Zr/Ti, Rb/K, V/Cr and organic carbon content) supported by mineralogical data record pronounced variation in sedimentary conditions. These took place in the course of glacier retreat at the Dwyka/Ecca boundary where climate and weathering processes are known to have changed. The proxy signals indicate an increase in sea level and salinity, and a shift in redox conditions from oxygenated to temporary anoxic/euxinic conditions. Due to the synchronicity of these events, distant sampling localities in the Karoo Basin can be correlated. Despite facies differences between the Eastern Kalahari and Karoo Basin, the geochemical proxies point to a comparable climatic evolution during the postglacial phase. For the Early Permian, proxy signals indicate warm–humid conditions in the Karoo and Kalahari Basin. During the Middle Permian increased aridity hampered chemical weathering processes as indicated by decreasing CIA values. Results show that: (i) proxies reflect provenance, synsedimentary redox regime and late diagenetic alteration, (ii) by combination of different independent geochemical proxies primary conditions of sedimentation and climate can be deduced, (iii) the geochemical proxies applied are independent of local environments and (iv) the data reveal supra-regional or even hemispherical changes in climate and sedimentary conditions. Thus geochemical proxies can be used as reliable markers for basin wide correlations. Knowledge about synchronicity on basin scale contributes to an improved understanding of SW Gondwana's interaction with and influence on global climatic evolution.

First Late Ordovician Paleomagnetic Pole for the Cuyania (Precordillera) Terrane of Western Argentina: a Microcontinent or a Laurentian Plateau

Time and tectonic processes involved in docking of the Argentine Precordillera (Cuyania terrane) against SW Gondwana has been a matter of much debate. A paleomagnetic study on the Early Caradoc Pavón Formation, exposed in the San Rafael block, province of Mendoza, Argentina, is presented. After detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined. A widespread post-tectonic component (A) is present in most sites of the Pavón Formation, with dual polarities, and is coincident with the characteristic remanence isolated from a Permo-Triassic rhyolitic dome intruding the sediments. Its pole position (83.7°S, 271.0°E, dp = 6.8°, dm = 9.0° N = 11 sites) falls on the Late Permian-Early Triassic South American reference poles suggesting that this component was acquired during the Choiyoi magmatic phase. A second component (B) also shows dual polarities and a positive fold test suggesting a primary origin. Unblocking temperatures and rock magnetic experiments indicate that B is carried either by hematite or magnetite at different sites. Anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction. A paleomagnetic pole computed from this remanence (PV) falls on 3.6°N, 346.4°E (dp = 2.9°, dm = 4.6° n = 22 samples). It indicates a paleolatitude around 26°S for deposition of Pavón sediments and constrains the paleogeographic evolution of Cuyania during the Ordovician, which was still at subtropical latitudes by the Early Caradoc. PV is consistent with the Laurentian Late Ordovician reference pole if Cuyania remains attached to SE Laurentia for the Early Caradoc, while it shows a significant cw rotation with no paleolatitude anomaly respect to the Gondwana reference pole when kept in its present position in SW South America. These comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of Cuyania in the Late Ordovician.

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential. Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift. During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature. There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality. In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Recôncavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge. Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.