Tectonic activation,source area stratigraphy and provenance changes in a rift basin: the Early Cretaceous Tucano Basin (NE‐Brazil)

Changes in sandstone and conglomerate maturity in tectonically active basins can be considered either as the product of climatic change or of tectonic restructuring of the feeder drainage system. Besides these regional controls, changes in the configuration of local sources can expressively affect basin fill composition. The Early Cretaceous fluvial successions of the Tucano Basin, a rift basin in northeastern Brazil related to the South Atlantic opening, contain one such case of abrupt change in maturity, marked by the passage from pebbly sandstone and conglomerate rich in quartz and quartzite fragments (Neocomian to Barremian São Sebastião Formation) to more feldspathic pebbly sandstone and conglomerate bearing pebbles of varied composition (Aptian Marizal Formation). Systematic analysis of stratigraphic and spatial variation in palaeocurrents and composition of pebbles and cobbles from both units, integrated with the recognition of fluvial and alluvial fan deposits distribution, revealed an abrupt decrease in maturity during the passage from the São Sebastião Formation to the Marizal Formation. This change is explained by exhumation of basement rocks and erosional removal of originally widespread Silurian to Jurassic sandstone and conglomerate units which were a major source of reworked vein quartz and quartzite pebbles to the São Sebastião Formation. Basin border faults activation during the deposition of the Marizal Formation caused adjacent basement uplift above the local erosional base level at the basin borders, whereas during the São Sebastião Formation deposition, the basin border fault scarps probably exposed mineralogically mature sedimentary units. The proposed model has important implications for interpreting changes in sediment maturity in rift basin successions, as similar results are expected where activation of basin border faults occurs after the erosional removal of older sedimentary or volcanic units that controlled syn‐rift successions composition.     

Low-latitude “dusty events” vs. high-latitude “icy Heinrich events”

It has been proposed that tropical events could have participated in the triggering of the classic, high-latitude, iceberg-discharge Heinrich events (HE). We explore low-latitude Heinrich events equivalents at high resolution, in a piston core recovered from the tropical north-western African margin. They are characterized by an increase of total dust, lacustrine diatoms and fibrous lacustrine clay minerals. Thus, low-latitude events clearly reflect severe aridity events that occurred over Africa at the Saharan latitudes, probably induced by southward shifts of the Inter Tropical Convergence Zone. At a first approximation, it seems that there is more likely synchronicity between the high-latitude Heinrich Events (HEs) and low-latitude events (LLE), rather than asynchronous behaviours.     

Thermal re-emission effects on GPS satellites

Highly precise satellite-derived coordinates depend on accurate orbit predictions, which cannot be achieved with purely empirical models. Global positioning system (GPS) satellites undergo several periodic perturbing forces that have to be modeled and understood. In this scenario, small non-gravitational forces can no longer be neglected when the purpose of the orbital analysis is to obtain accurate results (Vilhena de Moraes 1994). Together with solar radiation pressure, thermal re-emission effects due to solar heating and Earth albedo are the two most important non-gravitational effects. While solar radiation pressure is widely understood, our knowledge about thermal re-emission effects on GPS satellites is in its infancy. Few models have been proposed in recent years and despite the interest of the scientific community, there is a lack of detailed results concerning the magnitude and the behavior of such forces. The aim of this work is to provide a thermal re-emission force model for GPS satellites, simple enough to minimize the problem of modeling a satellite of complex shape with several components on its surface, but accurate enough to provide an estimate of the magnitude and the behavior of these forces, as well as to provide some input to the present knowledge about photon thrust on GPS satellites. Some results of this work point to the fact that thermal re-emission effects are good candidates to partially explain the Y-bias for GPS satellites.     

Sismique marine haute résolution 3D : un nouvel outil de reconnaissance à destination de la communauté scientifique

This note presents the first results of the development of 3D high-resolution marine seismic method designed for scientific application. A particular attention was paid to the realisation of an operational system to be in agreement with the expected goals in term of acquisition and processing. To cite this article: Y. Thomas et al., C. R. Geoscience 336 (2004).     

Geoeconom ic Processesin the Northwestern Adriatic CoastalPlain

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Discordant isotopic ages and excess argon in biotites

Major isotopic age discordances are found at the eastern margin of a terrane in SW Montana which underwent metamorphism 1.6 AE ago (1AE= 10⁹years). In Portal Creek, a one-mile portion of a traverse across this margin yields: (1) discordant whole rock Rb-Sr ages which approximate 2.8 AE; (2) seven biotite Rb-Sr ages concordant at 1.68 AE; (3) fifteen biotite K-Ar ages which are highly discordant and range from 1.63 to 3.25 AE; (4) ten generally concordant hornblende K-Ar ages at 1.9 AE. It is concluded that there was a major resetting event for the hornblendes at 1.9 AE ago. This was followed by a milder (?) event at 1.6 AE when the biotites became open systems while the hornblende did not. During this time, the biotites lost all their radiogenic⁸⁷Sr, but actually gained radiogenic⁴⁰Ar in amounts up to 16.0 × 10⁻⁴ scc Ar/g biotite. There is clear evidence of the incorporation of⁸⁷Sr into minor amphibolitic layers which occur occasionally in these generally quartzo-feldspathic rocks.     

On the system Ba [Al2Si2O8]-Ca[Al2Si2O8]

First results obtained in the study of the Ca-Ba diadochic substitution in the several polymorphic modifications of BaAl₂Si₂O₈ are reported:

1. in the hexagonal modifications synthesized at 1200° C by a solid state reaction, Ca replaces Ba up to 37% (atomic fraction);
2. in the hexagonal modifications obtained by crystallization of a melt, the replacement is limited to 25%;
3. in the monoclinic modifications obtained by heating the above mentioned modifications to 1450° C, the replacement is limited to 25% again.

A remarkable feature of the low and high hexagonal modifications is that their unit-cell parameters show no variation with composition. These hexagonal phases, furthermore, seem identical irrespective of the method of synthesis. The unit-cell parameters of the monoclinic phases vary very little with composition.     

Die Struktur des K2MgF4

Zusammenfassung Die Struktur des K₂MgF₄ wurde untersucht. Es ergab sich: Raumgruppe I 4/m m m ₂,a ₀ = 3,977 Å,c ₀ = 13,16 Å, Inhalt der Elementarzelle zwei Formeleinheiten. Die Struktur besteht aus Schichten zweidimensional-unendlich miteinander verknüpfter MgF₆-Koordinationsoktaeder, was rich auch in der Art der Spaltbarkeit und Optik äußert.

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Herrn Professor Dr.Carl W. Correns zum 60. Geburtstag gewidmet.     

Die Kristallstruktur des KZnCl3 · 2H2O

Zusammenfassung KZnCl₃ · 2H₂O kristallisiert in der Raumgruppe P 2₁/a-C _2h ⁵ mit a =12,03 Å ; b =10,09 Å c =6,26 Å, =107,5° und Z =4. Die Kristallstruktur wurde aus Patterson-Schnitten bestimmt. Die Zink werden von drei Chlor und einem Wassermolekül tetraedrisch umgeben; die Kalium von sieben Chlor und einem Wassermolekül.Die Parameter wurden nach der Methode der kleinsten Quadrate verfeinert.

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KZnCl₃ · 2H₂O crystallizes with unit cell dimensions a=12,03 Å; b=10,09 Å; c=6,26 Å, =107,5°. The space group is P2₁/a-C _2h ⁵ with Z=4.The structure has been determined by Patterson methods. Zinc is tetrahedrally surrounded by three chlorine and one H₂O, potassium is surrounded by seven chlorine and one H₂O. The structure has been refined three-dimensionally by the method of least-squares.

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Herrn Prof. Dr.C. W. Correns zum 70. Geburtstag gewidmet.