Paleocurrents and the problem of continental drift

The hypothesis of continental drift is examined on the light of paleocurrent analysis. A peripherical radial transportation pattern was obtained in South America and Africa from marine sandstones deposited during Ordovician to Devonian times. This pattern is suggestive of a former adjacent position between both continents, being the source area located somewhere in eastern Brazil and western Africa (Figs. 8 and 9). The paleocurrents in both continents flowed away from this common area. Evidences of Ordovician and/or Silurian glaciation indicated a radial movement for the ice-sheets, away from the same source area. The Caradocian ice movement in Sahara was to the north, while the ice-flow in the Gape Province was to the south, and in Northeastern Brazil it was possibly to the west.The Carboniferous paleocurrent and ice-movement trends are suggestive of a territorial contiguity between Africa and South America. However, several data must be revised for stratigraphic control.The Permian formations in the Paraná and Karroo basins had independent development without any direct connection as suggested by cross-bedding analysis. The occurrence ofMesosaurus in both basins is a strong argument of their close position at the time of sedimentation.Mesozoic wind laid deposits occur in both continents from Upper Triassic to Lower Cretaceous time. The Triassic eolian sandstones were deposited by westerly paleowinds in middle latitude region at the time of continental contiguity. The Upper Jurassic-Lower Cretaceous eolian sandstones were deposited after the separation between the continents had started. The paleowind pattern includes westerlies, trade-,and return tradewinds. This pattern is suggestive for the presence of the Southern Atlantic Ocean in Upper Jurassic-Lower Cretaceous time.

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Zusammenfassung Die Hypothese der Kontinentaldrift der Südkontinente Afrika und Südamerika wird mit Hilfe der Analyse von PalÄoströmungen untersucht.Marine Sandsteinablagerungen vom Ordoviz bis zum Devon zeigen in Südamerika und auch in Afrika peripher-radiales Transportnetz.Dies weist auf eine früher zusammenhÄngende Position zwischen beiden Kontinenten hin, deren Ursprung in Ost-Brasilien und West-Afrika gelegen haben mu\, da die Strömungsrichtungen in beiden Kontinenten von diesem Gebiet ausgehen (Fig. 8 und 9).Die Vereisung im Ordoviz und/oder Silur zeigt eine sternförmige Bewegungsrichtung des Eises vom gleichen Ursprungsgebiet aus. Die Strömungsrichtung zur Caradoc-Zeit war in der Sahara nach N gerichtet, wÄhrend sich das Eis in der Kap-Provinz nach S und in NE-Brasilien möglicherweise nach W bewegte.Die Strömungsrichtungen und Eisbewegungen im Karbon deuten auf einen territorialen Zusammenhang zwischen Afrika und Südamerika hin, jedoch bedürfen einige Daten einer stratigraphischen überprüfung.Das Perm im Paraná- und Karroo-Becken zeigt eine unabhÄngige Entwicklung ohne direkten Zusammenhang, wie er aus SchrÄgschichtungsmessungen angedeutet wurde. Das Vorkommen vonMesosaurus in beiden Becken ist ein gewichtiges Argument für ihre benachbarte Lage wÄhrend der Sedimentation.Im Mesozoikum finden wir von Obertrias bis Unterkreide in beiden Kontinenten Äolische Ablagerungen. Die der Trias wurden durch westliche Winde in mittleren geographischen Breiten sedimentiert, zu einer Zeit, als die beiden Kontinente noch zusammenhingen. Die Sandsteine des Oberen Juras und der Unteren Kreide wurden abgelagert, nachdem die Trennung der Kontinente begonnen hatte.Das PalÄowindsystem zeigt westliche und Passatwinde. Dieses System spricht bereits für das Vorhandensein des Südatlantiks wÄhrend dem Oberen Jura und der Unteren Kreide.

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Sumário No presente trabalho o problema da deriva continental é examinada do ponto de vista da análise das paleocorrentes. MediÇÕes de estratos cruzados em arenitos marinhos do Ordoviciano ao Devoniano, realizadas na áfrica e na América do Sul indicaram urn padrÃo de transporte periférico-radial. Êste padrÃo é sugestivo de uma justaposiÇÃo pretérita entre ambos continentes. A área fonte estaria localizada na regiÃo oriental da América do Sul e Ocidental da áfrica (figs. 8 e 9). Em ambos continentes as paleocorrentes fluiram a partir desta área comum. EvidÊncias de glaciaÇÃo Ordoviciana e/ou Siluriana indicam um movimento radial das massas de gÊlo a partir da mesma área fonte. No Sahara, o movimento do gÊlo no Caradociano fazia-se para o norte, enquanto que, na Provincia do Cabo era para o Sul, e no Nordeste do Brasil Êle provàvelmente dirigia-se para o oeste.No Carbonifero, as paleocorrentes e tendÊncia do movimento das geleiras sÃo sugestivas de uma contiguidade territorial entre a América do Sul e áfrica. Contudo, várlos dados devem ser revisados para controle estratigráfico.As formaÇÕes Permianas nas bacias do Paraná e do Karroo apresentam desenvolvimento independente. O estudo das paleocorrentes nÃo sugere qualquer ligaÇÃo direta. Entretanto, a ocorrÊncia deMesosaurus em ambas bacias, constitui forte argumenta a favor de maior proximidade entre elas na época de sedimentaÇÃo.Os depósitos eólicos mesozóicos ocorrem em ambos continentes desde o Triássico Superior até o Cretáceo Inferior. Os arenitos eólicos Triássicos foram depositados em regiÃo de latitude média pelos paleoventos de oeste. Nésta época, ambos os continentes encontravam-se unidos. Os arenitos eólicos do Jurássico Superior-Cretáceo Inferior foram depositados após o início da separaÇÃo. O sistema de paleoventos inclui ventos de oeste, aliseos e aliseos de retÔrno. Êste padrÃo de circulaÇÃo aérea é sugestivo da presenÇa do Atlântico sul durante o Jurássico Superior — Cretáceo Inferior.

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Under the auspices of the Conselho Nacional de Pesquisas (Brasil), Geological Survey of South Africa, and Institute Algerien du Pétrole.     

Gully-head morphology and implications for gully development on abandoned fields in a semi-arid environment,Sierra de Gata,southeast Spain

In this paper we examine whether gully-head morphology can be used as an indicator for gully development and, hence, for sediment production. A survey was conducted at five hillslopes in the Sierra de Gata where different types of channel heads occur close to each other. The survey included measurements of morphologic and pedologic properties, ground surface, channel and catchment characteristics of every gully head present (n = 59). On the basis of the observed morphologies, the heads were subdivided into four types: gradual, transitional (a short inclined section), abrupt and rilled-abrupt. The analyses showed that it is possible to explain the differences of gully heads and the role of some environmental factors on the basis of their morphologies, at least for the gradual and the abrupt types. The results suggested that steep headcuts (abrupt) were formed from secondary headcuts in the channel, which migrated upstream. The abrupt headcuts were always formed in more than one soil layer of which one was a resistant (stony) layer. However, shear strength measurements (at saturation) showed that the top layer was not always the most resistant one. Width–depth relationships indicated that gradual type headcuts were controlled by fluvial processes and abrupt headcuts by a combination of fluvial and mass-wasting processes. Gradual types occurred more downslope than the abrupt types suggesting that the incisions started by fluvial processes and migrated upwards when knickpoints developed in the channel. The rilled-abrupt types are still actively retreating. Thus, the abrupt types correspond to slower retreat rates. Abrupt gully heads may deteriorate into transitional types when plunge-pool erosion becomes less effective. The conceptual model is supported by data from ephemeral gullies in two other study areas (Sierra de la Torrecilla, Spain, and Alentejo, Portugal). Copyright © 1999 John Wiley & Sons, Ltd.     

Lithospheric mantle signatures as revealed by zircon Hf isotopes of Late Triassic post‐collisional plutons from the central Korean peninsula,and their tectonic implications

The Mesozoic tectonic architecture of the Korean peninsula is largely governed by the continental collision between the North and South China blocks. Zircon Hf isotopic compositions presented in this study and whole‐rock geochemical and Sr‐Nd isotope data in the literature collectively suggest that the lithophile‐elements‐enriched signature of the Late Triassic post‐collisional plutons from the Gyeonggi massif in central Korea is a primary feature inherited from the metasomatized mantle lithosphere. Highly negative zircon ε_Hf (t) values (?23 to ?19) of plutons from the middle and eastern parts of the massif indicate an ancient metasomatism of their mantle source. Distinctly higher zircon ε_Hf (t) values (?15 to ?12) from the southwestern plutons are ascribed to a contribution from an accreted component of the South China‐like block. The involvement of asthenospheric mantle is not recognized in zircons from either group. The implications of these isotopic features are discussed in the context of Mesozoic collisional tectonics.     

A New Family of Moths from the Middle Jurassic (Insecta: Lepidoptera)

<正>Three lepidopteran species,from the Middle Jurassic Daohugou beds(inner Mongolia,China),are described in a new family,Mesokristenseniidae,and new genus,Mesokristensenia,which could represent the sister group of the Micropterigidae.Mesokristensenia differs from all extant Lepidoptera,but one genus(Agathiphaga, Agathiphagidae),in retaining four median veins in the forewing,a plesiomorphy also present in many Trichoptera. Evidence for placing Mesokristensenia in the Lepidoptera includes four traits,notably a previously unrecorded autapomorphy of this insect order:beyond stem M1+2,vein M1 is bent and connected to cross-vein r-m(in both wing pairs).Among 24 characters taken into account to assess the systematic position of Mesokristensenia,12 are considered informative for a cladistic analysis involving this fossil taxon and the four suborders recognized in present-day Lepidoptera(Zeugloptera,Aglossata,Heterobathmiina,and Glossata).     

Effect of alkaline solutions on bentonite properties

The influence of alkaline aqueous solutions on the properties of bentonite was investigated to evaluate the performance of bentonitic engineered barriers when contacted with alkaline groundwater. Batch and hydraulic conductivity tests were conducted on Na-bentonite using six different alkaline aqueous solutions. For the batch tests, almost no change in the montmorillonite fraction of the bentonite was observed after reacting with alkaline solutions (pH = 8.4–13.1), regardless of the solution type. On the other hand, aluminosilicate minerals (e.g., albite) were dissolved and secondary minerals (e.g., anorthite) were formed in alkaline NaOH solutions (pH > 13). The cation (Ca or Na) concentration primarily affected the swelling properties of bentonite rather than the pH of the solution, which was comparable to the results of the hydraulic conductivity tests. For the Ca solutions, the hydraulic conductivity of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ solution (6.5 × 10^?9 cm/s) was approximately an order of magnitude lower than that of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (5.0 × 10^?8 cm/s), whereas the hydraulic conductivity to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (pH = 11.3) (5.0 × 10^?8 cm/s) was slightly higher than that to the 1 mol/L CaCl₂ solution (pH_i = 8.4) (4.4 × 10^?8 cm/s). For the NaOH solutions with pH > 13, the hydraulic conductivity of the bentonite specimen decreased with increasing Na concentration, suggesting that the effect of Na concentration was more dominant than that of permeant pH.     

Overwash vulnerability assessment based on long-term washover evolution

An integrated methodology for evaluation of overwash vulnerability is developed with respect to the historical washover evolution of a barrier island system. Three different aspects of overwash are addressed in the vulnerability indices developed: overwashed shoreline ratio, maximum overwash intrusion recurrence, and complete barrier overwash. The indices were applied to the barriers in the Ria Formosa system in Southern Portugal using an aerial photography catalogue covering the period 1947–2001. Historical trends of washover evolution were observed to be different between the barriers analysed, but generally, there was a decrease in washover number and dimensions throughout the analysed period. The final overwash diagnostic obtained allowed an integrated overwash vulnerability rating to be defined for each barrier, with vulnerabilities ranging from low to extreme. The methodology has produced results that may assist coastal managers with information concerning barrier island system overwash hazard, define the temporal and geographical distributions of overwash, and provide indications as to where overwash is most likely to occur in the future.     

Sangay volcano, Ecuador: structural development, present activity and petrology

Sangay (5230 m), the southernmost active volcano of the Andean Northern Volcanic Zone (NVZ), sits 130 km above a >32-Ma-old slab, close to a major tear that separates two distinct subducting oceanic crusts. Southwards, Quaternary volcanism is absent along a 1600-km-long segment of the Andes. Three successive edifices of decreasing volume have formed the Sangay volcanic complex during the last 500 ka. Two former cones (Sangay I and II) have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the Amazon plain. Sangay III, being constructed within the last avalanche amphitheater, has been active at least since 14 ka BP. Only the largest eruptions with unusually high Plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano. However, given the volcano's relief and unbuttressed eastern side, a future collapse must be considered, that would seriously affect an area of present-day colonization in the Amazon plain, 30 km east of the summit. Andesites greatly predominate at Sangay, there being few dacites and basalts. In order to explain the unusual characteristics of the Sangay suite—highest content of incompatible elements (except Y and HREE) of any NVZ suite, low Y and HREE values in the andesites and dacites, and high Nb/La of the only basalt found—a preliminary five-step model is proposed: (1) an enriched mantle (in comparison with an MORB source), or maybe a variably enriched mantle, at the site of the Sangay, prior to Quaternary volcanism; (2) metasomatism of this mantle by important volumes of slab-derived fluids enriched in soluble incompatible elements, due to the subduction of major oceanic fracture zones; (3) partial melting of this metasomatized mantle and generation of primitive basaltic melts with Nb/La values typical of the NVZ, which are parental to the entire Sangay suite but apparently never reach the surface and subordinate production of high Nb/La basaltic melts, maybe by lower degrees of melting at the periphery of the main site of magma formation, that only infrequently reach the surface; (4) AFC processes at the base of a 50-km-thick crust, where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated, producing andesites with low Y and HREE contents, due to garnet stability at this depth; (5) low-pressure fractionation and mixing processes higher in the crust. Both an enriched mantle under Sangay prior to volcanism and an important slab-derived input of fluids enriched in soluble incompatible elements, two parameters certainly related to the unique setting of the volcano at the southern termination of the NVZ, apparently account for the exceptionally high contents of incompatible elements of the Sangay suite. In addition, the low Cr/Ni values of the entire suite—another unique characteristic of the NVZ—also requires unusual fractionation processes involving Cr-spinel and/or clinopyroxene, either in the upper mantle or at the base of the crust.