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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Chesapeake Bay impact structure (2)
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United States
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Virginia
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Northampton County Virginia (2)
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geochronology methods
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paleomagnetism (1)
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geologic age
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Cenozoic
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Quaternary
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Pleistocene (1)
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Tertiary
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Neogene
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Miocene
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Calvert Formation (1)
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middle Miocene
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Choptank Formation (1)
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Saint Marys Formation (1)
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upper Miocene
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Eastover Formation (1)
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Pliocene
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upper Pliocene
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Chowan River Formation (1)
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Yorktown Formation (1)
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igneous rocks
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igneous rocks
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plutonic rocks
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granites (1)
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Primary terms
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Cenozoic
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Quaternary
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Pleistocene (1)
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Tertiary
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Neogene
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Miocene
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Calvert Formation (1)
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middle Miocene
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Choptank Formation (1)
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Saint Marys Formation (1)
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upper Miocene
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Eastover Formation (1)
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Pliocene
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upper Pliocene
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Chowan River Formation (1)
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Yorktown Formation (1)
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igneous rocks
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plutonic rocks
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granites (1)
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paleomagnetism (1)
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sedimentary rocks (1)
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United States
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Virginia
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Northampton County Virginia (2)
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sedimentary rocks
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sedimentary rocks (1)
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A 443.9-m-thick, virtually undisturbed section of postimpact deposits in the Chesapeake Bay impact structure was recovered in the Eyreville A and C cores, Northampton County, Virginia, within the “moat” of the structure's central crater. Recovered sediments are mainly fine-grained marine siliciclastics, with the exception of Pleistocene sand, clay, and gravel. The lowest postimpact unit is the upper Eocene Chickahominy Formation (443.9–350.1 m). At 93.8 m, this is the maximum thickness yet recovered for deposits that represent the return to “normal marine” sedimentation. The Drummonds Corner beds (informal) and the Old Church Formation are thin Oligocene units present between 350.1 and 344.7 m. Above the Oligocene, there is a more typical Virginia coastal plain succession. The Calvert Formation (344.7–225.4 m) includes a thin lower Miocene part overlain by a much thicker middle Mio-cene part. From 225.4 to 206.0 m, sediments of the middle Miocene Choptank Formation, rarely reported in the Virginia coastal plain, are present. The thick upper Miocene St. Marys and Eastover Formations (206.0–57.8 m) appear to represent a more complete succession than in the type localities. Correlation with the nearby Kiptopeke core indicates that two Pliocene units are present: Yorktown (57.8–32.2 m) and Chowan River Formations (32.2–18.3 m). Sediments at the top of the section represent an upper Pleistocene channel-fill and are assigned to the Butlers Bluff and Occohannock Members of the Nassawadox Formation (18.3–0.6 m).
Rock-magnetic properties of the ICDP-USGS Eyreville core, Chesapeake Bay impact structure, Virginia, USA
Chesapeake is a 35-Ma-old shallow-marine, complex impact structure with a diameter of ~85 km. The structure is completely buried beneath several hundreds of meters of postimpact sediments. Therefore, subsurface information can be obtained only from geophysical surveys and drill holes. Recently, deep drilling into the inner crater zone, at Eyreville near Cape Charles, was carried out in order to provide constraints on geophysical modeling and cratering processes in a multilayered marine target. We analyzed samples of the Eyreville core including postimpact, impact- produced, and basement-derived units in order to clarify the magneto-mineralogy, to provide physical parameters for better understanding the influence of the impact on the petrophysical and rock-magnetic properties, and to provide rock-magnetic data for magnetic modeling. Results show a complex behavior of physical properties of the lithologies in the Eyreville core due to different lithologies having been affected by shock-induced changes. Our data suggest that pyrrhotite and magnetite carry the magnetic properties in most of the core samples, whereas hematite is present in oxidized clays from the uppermost impact-generated unit (Exmore beds) and related sediment megablocks. The granitic megablock appears to be undeformed based on lack of brittle deformation in magnetite and petrophysically appears as a single block. In contrast, the impactite sequence below the megablock shows brittle deformation and magnetic fabric randomization, and the pyrrhotite in the associated schist fragments is strongly fractured. Thus, the Chesapeake Bay deep core provides an extraordinary opportunity to study the effect of impact on magnetite and pyrrhotite, the two main magnetic minerals creating crustal magnetic anomalies.