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Petrographic and shock metamorphic studies of the impact breccia section (1397–1551 m depth) of the Eyreville drill core, Chesapeake Bay impact structure, USA

By
Katerina Bartosova
Katerina Bartosova
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Ludovic Ferrière
Ludovic Ferrière
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, and Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada
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Christian Koeberl
Christian Koeberl
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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W. Uwe Reimold
W. Uwe Reimold
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany
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Susanne Gier
Susanne Gier
Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Published:
January 01, 2009

The moat of the 85-km-diameter and 35.3-Ma-old Chesapeake Bay impact structure (USA) was drilled at Eyreville Farm in 2005–2006 as part of an International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) drilling project. The Eyreville drilling penetrated postimpact sediments and impactites, as well as crystalline basement-derived material, to a total depth of 1766 m. We present petrographic observations on 43 samples of suevite, impact melt rock, polymict lithic impact breccia, cataclastic gneiss, and clasts in suevite, from the impact breccia section from 1397 to 1551 m depth in the Eyreville B drill core. Suevite samples have a fine-grained clastic matrix and contain a variety of mineral and rock clasts, including sedimentary, metamorphic, and igneous lithologies.

Six subunits (U1–U6, from top to bottom) are distinguished in the impact breccia section based on abundance of different clasts, melt particles, and matrix; the boundaries between the subunits are generally gradational. Sedimentary clasts are dominant in most subunits (especially in U1, but also in U3, U4, and U6). There are two melt-rich subunits (U1 and U3), and there are two melt-poor subunits with predominantly crystalline clasts (U2 and U5). The lower part (subunits U5 and U6), which has large blocks of cataclastic gneiss and rare melt particles, probably represents ground-surge material. Subunit U1 possibly represents fallback material, since it contains shard-like melt particles that were solidified before incorporation into the breccia. The melt-poor, crystalline clast–rich subunit U2 could have been formed by slumping of material, probably from the central uplift or from the margin of the transient crater.

Melt particles are most abundant near the top of the impact breccia section (above 1409 m) and around 1450 m, where the suevite grades into impact melt rock. Five different types of melt particles have been recognized: (1) clear colorless to brownish glass; (2) melt altered to fine-grained phyllosilicate minerals; (3) recrystallized silica melt; (4) melt with microlites; and (5) dark-brown melt. Proportions of matrix and melt in the suevite are highly variable (~2–67 vol% and 1–67 vol%, respectively; the remainder consists of lithic clasts).

Quartz grains in suevite commonly show planar fractures (PFs) and/or planar deformation features (PDFs; 1 or 2 sets, rarely more); some PDFs are decorated. On average, ~16 rel% of quartz grains in suevite samples are shocked (i.e., show PFs and/or PDFs). Sedimentary clasts (e.g., graywacke or sandstone) and polycrystalline quartz clasts have relatively higher proportions of shocked quartz grains, whereas quartz grains in schist and gneiss clasts rarely show shock effects. Rare feldspar grains with PDFs and mica with kink banding were observed. Ballen quartz was noted in melt-rich samples. Evidence of hydrothermal alteration, namely, the presence of smectite and secondary carbonate veins, was found especially in the lower parts of the impact breccia section.

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GSA Special Papers

The ICDP-USGS Deep Drilling Project in the Chesapeake Bay impact structure: Results from the Eyreville Core Holes

Gregory S. Gohn
Gregory S. Gohn
U.S. Geological Survey, Reston, Virginia, USA
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Christian Koeberl
Christian Koeberl
Department of Earth & Planetary Sciences, Rutgers University, USA
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Kenneth G. Miller
Kenneth G. Miller
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Germany
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Wolf Uwe Reimold
Wolf Uwe Reimold
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Germany
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Geological Society of America
Volume
458
ISBN print:
9780813724584
Publication date:
January 01, 2009

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