The first description and confirmation of the Vista Alegre impact structure in the Paraná flood basalts of southern Brazil

Abstract– The Vista Alegre structure, centered at 25°57′S and 52°41′W, has been recently proposed as a meteorite impact structure. The 9.5 km‐diameter structure is located in the Paraná state of southern Brazil, within the Paraná Basin, which contains one of the largest and most extensive flood basalt provinces on Earth. The Paraná flood basalts belong to the Serra Geral Formation and are temporally related to the opening of the South Atlantic Ocean, having been dated at about 133–132 Ma. Tholeiitic basalts dominate the western portion of Paraná state, with some minor rhyodacites. Morphologically, Vista Alegre has a prominent circular outline, in the form of an incomplete ring of escarpments, and an inner depression. The presence of a central uplift is not obvious, but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure. These sandstones are possibly related to the Triassic Pirambóia Formation and/or to the Cretaceous Botucatu Formation. These units are normally at stratigraphic depths of about 700–800 m below the present surface in this portion of the Paraná Basin. The structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick, extensively used for agriculture. As a result there are limited outcrops in the interior of the structure, all of polymict breccias, some of them melt‐bearing. We report the extensive occurrence of shatter cones, in the form of fine‐grained rock clasts within the polymict breccias. The shatter cone‐bearing breccias occur at different locations within the structure, separated by several kilometers. The nested shatter cones range in size from about 0.5 to 20 cm for individual cones, and up to half a meter for complete assemblages. The shatter cones formed in fine‐grained Parana flood basalt and might be the first examples of shatter cones in such a rock type. In addition, planar deformation features (PDFs) were found in quartz grains within sedimentary rock clasts of the polymict breccia. These findings confirm the impact origin of the Vista Alegre structure.     

Photo-identification and habitat use of Atlantic humpback dolphins Sousa teuszii around the Río Nuñez Estuary in Guinea,West Africa

Ecological data for the Atlantic humpback dolphin Sousa teuszii are scant. Six on-effort Sousa teuszii sightings were recorded during 817.6?km of boat-based effort in the Río Nuñez region of Guinea during October and November 2013. Two incidental sightings were also reported. Groups comprised 1–25 animals. Photo-identification produced a minimum population estimate of 47 animals. Most sightings (n = 5) were located close (<1?km) to shore along a 5.7?km stretch of coast on the west side of Île de Taïdi, primarily over shallow, sand–mud habitat. Two very distinctive individuals were present in all four Taïdi photo-identification encounters, suggesting high site fidelity and stable associations. Two sightings occurred in the outer Río Nuñez Estuary much farther from the coast (5–12?km) but in relatively shallow water (≤15?m) over sand–mud sediment. Focal follows (n = 5: 0.2–3.8?h duration) produced 9.02?h of behavioural data. Travel (51%), foraging (39%) and feeding (9.2%) dominated, with Taïdi dolphins spending more time foraging and feeding than the outer estuary groups. Three individuals had linear-severed dorsal fins consistent with injuries from fishing line. Some management implications of variation in habitat, site fidelity and movements of Sousa teuszii groups are discussed.     

Gypsum crusts and fly ash particles on carbonatic outcrops

Summary A study of the alterations caused by chemical weathering on carbonatic outcrops in the South of France (near Marseille) has revealed the presence of gypsum crusts. An analogy with carbonatic surfaces (marbles and limestones) located in urban areas (monuments and buildings) has therefore been evidenced. An analytical investigation of the morphology, typology and inner structure of the crusts found is presented in this paper.Furthermore, reference is also made to the identification of fly ash particles embedded in the altered layer: this occurrence enables a limitation of the crust-growth period to the present century. Some hypotheses as to the role played by these particles in the formation of the altered layer have also been advanced. In this regard, the study of particles emitted by coal combustion, one of the major sources of energy, as far as their environment impact is concerned, is of unquestionable importance.

---

Gipskrusten und Flugascheteilchen auf Carbonat-Schichtköpfen

Zusammenfassung Die Untersuchung der durch chemische Verwitterung verursachten Änderungen an Carbonat-Schichtköpfen im Süden von Frankreich (bei Marseille) hat das Vorhandensein von Gipskrusten gezeigt. Eine Analogie zu carbonaten Oberflächen (Marmor und Kalkstein) in Stadtgebieten (Monumente und Gebäude) ist damit erwiesen. Eine analytische Erforschung der Morphologie, Typologie und inneren Struktur der gefundenen Krusten wird in dieser Arbeit vorgelegt.Ferner wird auch Bezug genommen auf die Identifizierung der in die geänderte Schicht eingelagerten Flugascheteilchen. Dieses Vorkommen ermöglichte eine Begrenzung der Periode des Krustenwachstums auf das gegenwärtige Jahrhundert. Einige Hypothesen über die Rolle, die von diesen Teilchen bei der Bildung der geänderten Schicht gespielt wird, werden auch vorgebracht. In dieser Hinsicht ist die Untersuchung der bei der Kohleverbrennung — das ist eine der größten Energiequellen — ermittelten Teilchen, insofern ihr Einfluß auf die Umgebung betrachtet wird, unzweifelhaft von Bedeutung.

---

---

With 4 Figures     

Method of calculating tsunami travel times in the Andaman Sea region

A new model to calculate tsunami travel times in the Andaman Sea region has been developed. The model specifically provides more accurate travel time estimates for tsunamis propagating to Patong Beach on the west coast of Phuket, Thailand. More generally, the model provides better understanding of the influence of the accuracy and resolution of bathymetry data on the accuracy of travel time calculations. The dynamic model is based on solitary wave theory, and a lookup function is used to perform bilinear interpolation of bathymetry along the ray trajectory. The model was calibrated and verified using data from an echosounder record, tsunami photographs, satellite altimetry records, and eyewitness accounts of the tsunami on 26 December 2004. Time differences for 12 representative targets in the Andaman Sea and the Indian Ocean regions were calculated. The model demonstrated satisfactory time differences (<2 min/h), despite the use of low resolution bathymetry (ETOPO2v2). To improve accuracy, the dynamics of wave elevation and a velocity correction term must be considered, particularly for calculations in the nearshore region.     

The complex impact structure Serra da Cangalha,Tocantins State,Brazil

Abstract– Serra da Cangalha is a complex impact structure with a crater diameter of 13,700 m and a central uplift diameter of 5800 m. New findings of shatter cones, planar fractures, feather features, and possible planar deformation features are presented. Several ring‐like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata. The target at Serra da Cangalha is composed of Devonian to Permian sedimentary rocks, mainly sandstones that are interlayered with siltstone and claystones. NNE–SSW and WNW–ESE‐striking joint sets were present prior to the impact and also overprinted the structure after its formation. As preferred zones of weakness, these joint sets partly controlled the shape of the outer perimeter of the structure and, in particular, affected the deformation within the central uplift. Joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed. These planes were used as major displacement zones. The asymmetry of the central uplift, with preferred overturning of strata in the northern to western sector, may suggest a moderately oblique impact from a southerly direction. Buckle folding of tilted strata, as well as strata overturning, indicates that the central uplift became gravitationally unstable at the end of crater formation.     

The Colônia structure,São Paulo,Brazil

Abstract– The near‐circular Colônia structure, located in the southern suburbs of the mega‐city of São Paulo, Brazil, has attracted the attention of geoscientists for several decades due to its anomalous character and the complete absence of any plausible endogenous geologic explanation for its formation. Origin by impact cratering has been suggested repeatedly since the 1960s, but no direct evidence for this has been presented to date. New seismic data have been recently acquired at Colônia, providing new insights into the characteristics and possible layering of infill of the structure, as well as into the depth to the underlying basement. We review the current knowledge about the Colônia structure, present the new seismic data, and discuss the existing—as yet still indirect—evidence for a possible origin by an impact. The new data suggest the existence of a sedimentary fill of approximately 275 m thickness and also the presence of two intermediate zones between sediment and basement: an upper zone that is approximately 65 m thick and can be interpreted as a possible crater‐fill breccia, whereas the other zone possibly represents fractured/brecciated basement, with a thickness of approximately 50 m. Although this depth to basement seems to be inconsistent with the expected geometry of a simple, bowl‐shape impact structure of such diameter, there are a number of still unconstrained parameters that could explain this, such as projectile nature, size and velocity, impact angle, and particularly the current erosion depth.