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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 (3)
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James River (2)
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United States
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Atlantic Coastal Plain (8)
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Chesapeake Bay (1)
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Delaware (1)
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Maryland (4)
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North Carolina (2)
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Potomac River (1)
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Salisbury Embayment (4)
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Virginia
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Dinwiddie County Virginia (1)
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Henrico County Virginia (1)
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Northampton County Virginia (2)
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Petersburg Virginia (1)
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Rappahannock River (1)
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Richmond Virginia (1)
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Westmoreland County Virginia (1)
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fossils
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Invertebrata
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Mollusca (1)
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Protista
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Foraminifera (4)
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Radiolaria (1)
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microfossils (8)
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palynomorphs (1)
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Plantae
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algae
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diatoms (3)
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nannofossils (1)
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geologic age
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Cenozoic
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Quaternary
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Pleistocene (2)
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Tertiary
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Neogene
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Miocene
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Calvert Formation (7)
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middle Miocene
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Choptank Formation (7)
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Pungo River Formation (2)
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Saint Marys Formation (7)
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upper Miocene
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Eastover Formation (14)
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Pliocene
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upper Pliocene
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Chowan River Formation (4)
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Yorktown Formation (8)
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Paleogene
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Eocene
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Nanjemoy Formation (2)
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upper Eocene
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Piney Point Formation (2)
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Oligocene (2)
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upper Cenozoic
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Chesapeake Group (4)
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Mesozoic
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Cretaceous
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Potomac Group (1)
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Primary terms
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biogeography (1)
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Cenozoic
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Quaternary
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Pleistocene (2)
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Tertiary
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Neogene
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Miocene
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Calvert Formation (7)
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middle Miocene
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Choptank Formation (7)
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Pungo River Formation (2)
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Saint Marys Formation (7)
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upper Miocene
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Eastover Formation (14)
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Pliocene
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upper Pliocene
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Chowan River Formation (4)
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Yorktown Formation (8)
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Paleogene
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Eocene
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Nanjemoy Formation (2)
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upper Eocene
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Piney Point Formation (2)
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Oligocene (2)
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upper Cenozoic
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Chesapeake Group (4)
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data processing (1)
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Invertebrata
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Mollusca (1)
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Protista
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Foraminifera (4)
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Radiolaria (1)
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Mesozoic
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Cretaceous
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Potomac Group (1)
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paleoclimatology (1)
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paleoecology (5)
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palynomorphs (1)
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Plantae
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algae
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diatoms (3)
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nannofossils (1)
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plate tectonics (1)
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sea-level changes (4)
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sedimentation (3)
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stratigraphy (7)
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United States
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Atlantic Coastal Plain (8)
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Chesapeake Bay (1)
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Delaware (1)
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Maryland (4)
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North Carolina (2)
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Potomac River (1)
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Salisbury Embayment (4)
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Virginia
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Dinwiddie County Virginia (1)
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Henrico County Virginia (1)
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Northampton County Virginia (2)
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Petersburg Virginia (1)
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Rappahannock River (1)
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Richmond Virginia (1)
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Westmoreland County Virginia (1)
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Eastover Formation
Geology and biostratigraphy of the Potomac River cliffs at Stratford Hall, Westmoreland County, Virginia
Abstract The cliffs along the Potomac River at Stratford Hall display extensive exposures of Miocene marine strata that belong successively to the Calvert, Choptank, St. Marys, and Eastover Formations. Within the lower part of this sequence, in the Calvert and Choptank Formations, there is well-developed cyclic stratigraphy. Above the Miocene units lies the marginal marine to deltaic Pleistocene Bacons Castle Formation, which is the highest and youngest formation exposed in the cliffs. The goals of this field trip guide are to (1) show the Miocene formations exposed in the cliffs and discuss the paleoenvironments within which they formed, (2) demonstrate the cyclicity in the Miocene marine formations and discuss its origin, (3) compare and contrast the section exposed at the Stratford and Nomini Cliffs with the classic Miocene Calvert Cliffs sequence exposed to the northeast in Calvert County, Maryland, and the Miocene sequence recovered in the Haynesville cores to the southeast in Richmond County, Virginia, (4) discuss and explain why a detailed correlation among these three places has been so difficult to attain, and (5) show typical lithologies of the Bacons Castle Formation and discuss the paleoenvironments in which they formed.
Two cores at the outer margin of the Chesapeake Bay impact structure show significant structural and depositional variations that illuminate its history. Detailed stratigraphy of the Watkins School core reveals that this site is outside the disruption boundary of the crater with respect to its lower part (nonmarine Cretaceous Potomac Formation), but just inside the boundary with respect to its upper part (Exmore Formation and a succession of upper Eocene to Pleistocene postimpact deposits). The site of the U.S. Geological Survey–National Aeronautics and Space Administration Langley core, 6.4 km to the east, lies wholly within the annular trough of the crater. The Potomac Formation in the Watkins School core is not noticeably impact disrupted. The lower part of crater unit A in the Langley core represents stratigraphically lower, but similarly undeformed material. The Exmore Formation is only 7.8 m thick in the Watkins School core, but it is over 200 m thick in the Langley core, where it contains blocks up to 24 m in intersected diameter. The upper part of the Exmore Formation in the two cores is a polymict diamicton with a stratified zone at the top. The postimpact sedimentary units in the two cores have similar late Eocene and late Miocene depositional histories and contrasting Oligocene, early Miocene, and middle Miocene histories. A paleochannel of the James River removed Pliocene deposits at the Watkins School site, to be filled later with thick Pleistocene deposits. At the Langley site, a thick Pliocene and thinner Pleistocene record is preserved.
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).
Asteroid and comet impact events are known to cause profound disruption to surface ecosystems. The aseptic collection of samples throughout a 1.76-km-deep set of cores recovered from the deep subsurface of the Chesapeake Bay impact structure has allowed the study of the subsurface biosphere in a region disrupted by an impactor. Microbiological enumerations suggest the presence of three major microbiological zones. The upper zone (127–867 m) is characterized by a logarithmic decline in microbial abundance from the surface through the postimpact section of Miocene to Upper Eocene marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments and sediment megablocks. In the middle zone (867–1397 m) microbial abundances are below detection. This zone is predominantly quartz sand, primarily composed of boulders and blocks, and it may have been mostly sterilized by the thermal pulse delivered during impact. No samples were collected from the large granite block (1096–1371 m). The lowest zone (below 1397 m) of increasing microbial abundance coincides with a region of heavily impact-fractured, hydraulically conductive suevite and fractured schist. These zones correspond to lithologies influenced by impact processes. Our results yield insights into the influence of impacts on the deep subsurface biosphere.