T167 The Chesapeake Bay Impact Structure: Results from the 2005–2006 ICDP-USGS Deep Drilling Project I

Tuesday, 30 October 2007
1:30 PM - 5:30 PM

INITIAL STUDIES OF BRECCIAS, BLOCKS, AND CRYSTALLINE ROCKS IN THE ICDP-USGS EYREVILLE-B CORE, CHESAPEAKE BAY IMPACT STRUCTURE, 1,095-1,766 M DEPTH

Paper number: T167-6
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

The Eyreville-B core from the Chesapeake Bay impact structure (CBIS) has four lithotectonic assemblages (A1-A4) below 1,095 m depth. A1 (1,766-1,560 m) consists of polydeformed, sillimanite- and graphite-bearing mica schists, gneisses, local mylonitic rocks, and granite pegmatite grading into coarse granite. 40Ar/39Ar plateau ages of two muscovites from the pegmatite indicate cooling through ca. 350°C at 244.2 and 243.9 Ma without disturbance by the Eocene impact. However, pegmatitic muscovites have fine striations and microcrenulations that may be impact-related. Cataclastic deformation is widespread in A1, but studies show no shock features away from breccia dikes that decrease with depth in size and abundance.

A2 (1,560-1,397 m) is suevite having altered melt clasts most abundant in the upper part, and blocks <20 m of cataclastic gneiss mostly in the lower part. Graphite-rich cataclastic breccia marks a syn-impact thrust detachment at the base of A2. Melt glasses are partly altered to well-crystallized, Fe-rich smectite. Lithic clasts include cataclastic, greenschist- to lower amphibolite-facies metasedimentary, metavolcanic, and metaplutonic rocks, and black shale previously unknown in the target. The first discovery of coesite and reidite in the CBIS (and in A2) is confirmed by EMP, XRD, and Raman analysis, and consistant with other shock features including maskelynite, PDFs (1-3 sets) in quartz, and ballen quartz. Baddeleyite and corundum are present in trace amounts. The suevite has an incomplete suite of known target materials, deficient in Cretaceous and Tertiary sediment clasts, suggesting that it formed from debris within the crater rather than fallback ejecta.

A3 (1,397-1,371 m) is a mixture of gravelly quartz sand and lithic blocks including amphibolite and a probable rip-up clast of A2 suevite. The lowest sand in A3 contains altered melt clasts and is a reworked suevite. A3 may have served as a low-friction substrate during emplacement of a 275-m granitic slab.

A4 (1,371-1,096 m), is a heterogeneous granitic slab and the rocks have SHRIMP U-Pb zircon ages from Neoproterozoic (615 ± 7 Ma, gneissic granite) to Permian (254 ± 3 Ma, massive granite). It is allochthonous, little deformed, unshocked, and may have detached and slid along with wet sediments during collapse of the transient-crater rim.