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| Titel |
Microstructural TEM-investigations on shock-metamorphic chert from the Jebel Waqf as Suwwan impact structure, Jordan |
| VerfasserIn |
Ulrich Bläß, Martin Schmieder, Elmar Buchner |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052331
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| Zusammenfassung |
| The so-called “Jebel Waqf as Suwwan” structure (Arabic for “Mountain of steeply inclined
chert”) located in the Hamada desert of Northeast Jordan, close to the Saudi Arabian border
(centered at N 31Ë 03’, E 36Ë 48’), has been recently reconsidered as the first complex
impact structure in the Middle East [1]. These inferences are based on its morphology
showing a ~5.5 km wide near-circular uplifted crater rim and a prominent central uplift, the
occurrence of shatter cones as macroscopic shock evidence [2], and planar deformation
features (PDFs) observed optically in quartz grains from the Cretaceous “Kurnub sandstone”
[1] and shocked chert from the central uplift [3]. The eroded impact structure is
emplaced in a nearly horizontal sequence of upper Cretaceous to Paleogene sandstones,
limestones, marls, and cherts [1,4]. A Miocene-Pliocene impact age has been suggested
[5].
In order to decipher the true nature of PDF-like features in quartz grains from formerly
re-crystallised chalcedony-quartzine-veinlets in shocked chert from the central
uplift, several conspicuous grains were drilled out from seven thin sections taken
from the study of [3], mounted on TEM grids, thinned to electron transparency by
ion thinning, and subsequently examined by transmission electron microscopy
(TEM).
Our investigations clearly revealed the existence of amorphous PDF lamellae
running parallel crystallographic {10¯11} and {10¯13} planes of quartz. The borders of
both types of PDFs are usually scalloped by tiny dislocation loops, which also
appeared sporadically in some of the adjacent quartz grains. We suggest that these
dislocations nucleated around tiny impact-induced exsolution of bubbles during
slight post-impact deformation. Whereas PDFs along {10¯13} are usually 20-50 nm
wide, decorated by ~60 nm large bubbles, and running straight through the crystal,
PDFs along {10¯11} are not decorated by large bubbles and are appreciably shorter,
sometimes only ~ 0.5 μm long and then offset by ~ 20 nm. Some of {10¯11} PDFs
are partially re-crystallised and, therefore, only visible by their adjoining bands of
dislocations.
However, only a few of the observed quartz grains do show amorphous PDF lamellae.
More frequently, narrow Brazil twins of about 10 nm width are observed in the (0001) basal
plane of quartz, sometimes decorated by large bubbles. Brazil twins in the basal plane of
quartz can be generated by shock-induced shear deformation at high differential stresses and
are considered as a shock indicator for the lower shock pressure regime [2]. Non-shock
related deformation features of these quartz grains are relatively rare, only a few individual
dislocations have been observed and are usually re-equilibrated to small-angle grain
boundaries.
It is remarkable, however, that beside quartz crystals showing clear shock metamorphic
evidence, such as basal Brazil twins or amorphous PDFs, there are still many quartz
crystals, which do not exhibit any deformation features at all. We therefore, suggest
that the formation of shock metamorphic features in these quartz grains strongly
depends on their crystallographic orientation with respect to the shock wave and the
orientation to adjacent cryptocrystalline silica varieties of fibrous chalcedony and
quartzine, which might deform more softly during shock deformation than larger quartz
crystals.
The presence of many Brazil twin PDFs in the basal plane and PDFs along {10¯11} and
{10¯13} lattice planes of quartz, however, provides additional TEM evidence that the Jebel
Waqf as Suwwan structure is of impact origin and suggests that the rocks of the central
uplift have been deformed by low to intermediate shock pressures exceeding 10
GPa.
References:
[1] Salameh E. et al. (2008), Meteoritics & Planetary Science 43, 1681-1690. [2] French
B.M. and Koeberl C. (2010), Earth-Science Reviews 98, 123-170. [3] Schmieder M. et al.
(2011), Meteoritics & Planetary Science, in press. [4] Heimbach W. (1969), Beihefte zum
Geologischen Jahrbuch 81, 149-160. [5] Kenkmann T. et al. (2009), Field trip guidebook for
the first AICAC, 16p. |
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