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| Titel |
Shock metamorphic studies on suevite from the ICDP-USGS Eyreville drill core, Chesapeake Bay impact structure, Virginia, USA. |
| VerfasserIn |
K. Bartosova, Ludovic Ferrière, C. Koeberl  , W. U. Reimold |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021956
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| Zusammenfassung |
| The Chesapeake Bay impact structure (85 km in diameter and 35 Ma old) belongs to the
largest and best preserved impact structures on Earth [1]. In 2005–2006, three cores were
drilled as part of the International ICDP-USGS Chesapeake Bay impact structure drilling
project, at Eyreville farm, in Northampton County (Virginia). The lithologies cored from top
to bottom comprise sedimentary-clast breccia and sedimentary blocks of the Exmore beds, a
granitic and an amphibolitic megablock, gravelly sand, impact breccia, and granite/pegmatite
and mica schist [1]. A stratigraphic column for the impact breccia section was established by
[2].
Shock metamorphic effects in rock and mineral clasts were studied in the impact breccia
section of the drill core (1397–1551 m; e.g., [3]). Rare shocked quartz grains and relatively
abundant melt particles, mostly concentrated in specific depth intervals, were found also
in the Exmore breccia [4]. The shock and related features in the minerals of the
impact breccia interval comprise abundant planar deformation features (PDFs) and
rare planar fractures (PFs) in quartz, ballen quartz, rare PDFs in feldspar, and (not
shock diagnostic) kink banding in mica. Also melt particles are very abundant,
especially in the upper part of the impact breccia section. Previous investigations
of the proportions of shocked quartz grains (i.e., grains with PFs and/or PDFs)
did not reveal any trends with depth [3], but noted that the average proportion of
shocked grains in sedimentary clasts was higher compared to the crystalline clasts [3].
Here we present a more detailed study investigating the relative proportions of
shocked quartz grains in clasts (of different lithologies) in suevite samples. The clasts
show different proportions of shocked quartz grains, from 0 to about 60 rel% of all
quartz grains. Generally, clasts with higher proportion of shocked quartz grains
become less abundant with increasing depth. Clasts with more abundant shocked
quartz grains were noted especially in the upper part of the impact breccia section
(above ~1450 m). Polycrystalline quartz clasts are relatively more shocked. Slightly
metamorphosed clasts (i.e., metasandstones) and schist/gneiss clasts are relatively less
shocked. The quartz grains display (under the optical microscope) mostly one or two,
rarely three or more, sets of PDFs per grain. First results of the universal stage
measurements show that the most common crystallographic orientation of PDFs is the
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