E. Buchner

Upheaval Dome, Utah, USA: Impact origin confirmed

Upheaval Dome is a unique circular structure on the Colorado Plateau in SE Utah, the origin of which has been controversially discussed for decades. It has been interpreted as a crypto volcanic feature, a salt diapir, a pinched-off salt diapir, and an eroded impact crater. While recent structural mapping, modeling, and analyses of deformation mechanisms strongly support an impact origin, ultimate proof, namely the documentation of unambiguous shock features, has yet to be successfully provided. In this study, we document, for the first time, shocked quartz grains from this crater in sandstones of the Jurassic Kayenta Formation. The investigated grains contain multiple sets of decorated planar deformation features. Transmission electron microscopy (TEM) reveals that the amorphous lamellae are annealed and exhibit dense tangles of dislocations as well as trails of fluid inclusions. The shocked quartz grains were found in the periphery of the central uplift in the northeastern sector of the crater, which most likely represents the cross range crater sector.

Age distribution of cinder cones within the Bandas del Sur Formation, southern Tenerife, Canary Islands

The Quaternary Bandas del Sur Formation in the south of Tenerife comprises a complex sequence of pyroclastic rocks and lavas. In contrast to the NW- and NE-Rift zone on Tenerife, the S-Rift zone comprises a number of characteristics with respect to the morphological features, eruption cyclicity and the geochemistry of the volcanic deposits. Various flank eruptions of the Las Canadas volcano associated with basaltic lavas and the formation of cinder cones within the Bandas del Sur are important volcanic units for understanding the explosive volcanic cycles during the Pleistocene on Tenerife. A number of palaeomagnetic studies, as well as major and trace element geochemistry and two radio-isotope dates (K–Ar), have been carried out on prominent cinder cones, in order to discover their stratigraphic position. Combining our results with previous K–Ar data, the cones and lavas can be subdivided into three stratigraphic units. The first unit contains cinder cones with reverse magnetization and Y/Nb ratios between 0.37 and 0.41. Cinder cones which belong to the second unit show normal magnetization and Y/Nb ratios of c . 0.948–0.779 Ma and 0.323–0.300 Ma. These units define volcanic cycles ending in violent Plinian eruptions. The third and youngest unit possibly marks the beginning of a further volcanic cycle that started c. 0.095 Ma ago.

Dating impact craters: palaeogeographic versus isotopic and stratigraphic methods – a brief case study

Isotopic and stratigraphic ages of the ~ 80 km diameter Puchezh-Katunki (Russia; 220 ± 10 to 167 ± 3 Ma) and the ~ 20 km diameter Obolon (Ukraine; 215 ± 25 to 169 ± 7 Ma) impact structures are associated with significant age uncertainties. As a case study, reconstructions of the palaeogeography at the time of crater formation (Late Triassic to Middle Jurassic) based on recent palaeogeographic maps help further to constrain impact ages. Palaeogeographic studies suggest that Puchezh-Katunki is older than 170 Ma and that Obolon is younger than 185 Ma. This also rules out that Obolon formed during a ~ 214 Ma Late Triassic multiple impact event as recently discussed.

Meteorite traces on a shatter cone surface from the Agoudal impact site, Morocco

The recently discovered Agoudal impact site in Morocco is a small, eroded impact structure with well-developed shatter cones. A scanning electron microscopic study of a shatter cone surface has revealed the presence of schreibersite – a phosphide very rare on Earth but common in iron meteorites – and Fe–Ni oxides. This is the first reported evidence for primary meteoritic matter adherent to shatter cones and suggests that the Agoudal crater was formed by the impact of an iron meteorite, probably the Agoudal IIAB iron. Shatter cones from other terrestrial impact structures might also hold valuable information about the nature of the impacting projectiles.

Rare metals on shatter cone surfaces from the Steinheim Basin (SW Germany) – remnants of the impacting body?

The ~3.8 km Steinheim Basin in SW Germany is a well-preserved complex impact structure characterized by a prominent central uplift and well-developed shatter cones that occur in different shocked target lithologies. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron probe microanalysis have revealed, for the first time, the occurrence of rare metals on the Steinheim shatter cone surfaces. Shatter cones produced from the Middle Jurassic (Aalenian) Opalinus Claystone (‘Opalinuston’), temporarily exposed in the central uplift in spring 2010, and shatter cones in Upper Jurassic (Oxfordian) limestones from the southeastern crater rim domain are commonly covered by faint coatings. The Opalinus Claystone shatter cone surfaces carry coatings dominated by Fe, Ca, P, S and Al, and are covered by abundant small, finely dispersed microparticles and aggregates of native gold, as well as locally elevated concentrations of Pt. On several surfaces of the claystone shatter cones, additional Fe, Ni and Co was detected. The Ca–Mn-rich coatings on the limestone shatter cone surfaces locally include patches of Fe, Ni, Co, Cu and Au in variable amounts and proportions. The intriguing coatings on the Steinheim shatter cones could either stem from the impacted Lower Jurassic to Palaeogene sedimentary target rocks; from the crystalline-metamorphic Variscan crater basement; or, alternatively, these coatings might represent altered meteoritic matter from the Steinheim impactor, possibly an iron meteorite, which may have been remobilized during post-impact hydrothermal activity. We here discuss the most plausible source for the rare metals found adherent to the shatter cone surfaces.

An updated and refined Holocene uplift history of southern Tenerife (Canary Islands) and the possible consequences for future volcanic activity

Various uplift markers suggest asymmetrical uplift of Tenerife Island, with stable conditions in the north but significant uplift of up to 45 m in the south over the past ~42 ka. Fossil shells in beach deposits uplifted by 7.5–9 m were 14 C-dated at a Holocene age of 2460±35 bp (1σ). This confirms earlier results and documents very young, and probably still ongoing, uplift of southern Tenerife potentially caused by ascending magma. This underlines that southern Tenerife is probably undergoing a further cycle of volcanic activity that started ~95 ka ago.