New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, pea... more New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A ...
Geological Society of America Special Papers, 2009
ABSTRACT A 443.9-m-thick, virtually undisturbed section of postimpact deposits in the Chesapeake ... more ABSTRACT 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 ...
Geological Society of America Special Papers, 2009
The Eyreville and Exmore, Virginia, core holes were drilled in the inner basin and annular trough... more The Eyreville and Exmore, Virginia, core holes were drilled in the inner basin and annular trough, respectively, of the Chesapeake Bay impact structure, and they allow us to evaluate sequence deposition in an impact crater. We provide new high-resolution geochronologic (<1 Ma) and sequence-stratigraphic interpretations of the Exmore core, identify 12 defi nite (and four possible) postimpact depositional sequences, and present comparisons with similar results from Eyreville and other mid-Atlantic core holes. The concurrence of increases in δ 18 O with Chesapeake Bay impact structure sequence boundaries indicates a primary glacioeustatic control on deposition. However, regional comparisons show the differential preservation of sequences across the mid-Atlantic margin. We explain this distribution by the compaction of impactites, regional sediment-supply changes, and the differential movement
Eos, Transactions American Geophysical Union, 2004
... Gregory S. Gohn. USGS, Reston, Virginia,. David S. Powars. USGS, Reston, Virginia,. J. Wright... more ... Gregory S. Gohn. USGS, Reston, Virginia,. David S. Powars. USGS, Reston, Virginia,. J. Wright Horton, Jr. USGS, Reston, Virginia,. Lucy E. Edwards. USGS, Reston, Virginia,. Jean M. Self-Trail. USGS, Reston, Virginia,. Roger H. Morin. USGS, Lakewood, Colorado, USA. Abstract ...
Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake... more Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.
1] On the mid-Atlantic Coastal Plain of the United States, Paleocene sands and silts are replaced... more 1] On the mid-Atlantic Coastal Plain of the United States, Paleocene sands and silts are replaced during the Paleocene-Eocene Thermal Maximum (PETM) by the kaolinite-rich Marlboro Clay. The clay preserves abundant magnetite produced by magnetotactic bacteria and novel, presumptively eukaryotic, ironbiomineralizing microorganisms. Using ferromagnetic resonance spectroscopy and electron microscopy, we map the magnetofossil distribution in the context of stratigraphy and carbon isotope data and identify three magnetic facies in the clay: one characterized by a mix of detrital particles and magnetofossils, a second with a higher magnetofossil-to-detrital ratio, and a third with only transient magnetofossils. The distribution of these facies suggests that suboxic conditions promoting magnetofossil production and preservation occurred throughout inner middle neritic sediments of the Salisbury Embayment but extended only transiently to outer neritic sediments and the flanks of the embayment. Such a distribution is consistent with the development of a system resembling a modern tropical river-dominated shelf.
available online at http://meteoritics.org 1613 Abstract-The late Eocene Chesapeake Bay impact st... more available online at http://meteoritics.org 1613 Abstract-The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best-preserved "wet-target" craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile-rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ~85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the "inverted sombrero" morphology observed at some craters in layered targets.
1] A 30-km-long, radial seismic reflection and refraction survey completed across the northern pa... more 1] A 30-km-long, radial seismic reflection and refraction survey completed across the northern part of the late Eocene Chesapeake Bay impact structure (CBIS) on the Delmarva Peninsula, Virginia, USA, confirms that the CBIS is a complex central-peak crater. We used a tomographic P wave velocity model and low-fold reflection images, constrained by data from two deep boreholes located on the profile, to interpret the structure and composition of the upper 5 km of crust. The seismic images exhibit well-defined structural features, including (with increasing radial distance) a collapsed central uplift, a breccia-filled moat, and a collapsed transient-crater margin (which collectively constitute a $40-km-wide collapsed transient crater), and a shallowly deformed annular trough. These seismic images are the first to resolve the deep structure of the crater (>1 km) and the boundaries between the central uplift, moat, and annular trough. Several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift, moat, and annular trough. Within the moat, breccia extends to a minimum depth of 1.5 km or a maximum of 3.5 km, depending upon the interpretation of the deepest layered materials. The images show $350 to 500 m of postimpact sediments above the impactites. The imaged structure of the CBIS indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling.
ABSTRACT Abrupt sedimentary changes mark the Paleocene-Eocene Thermal Maximum (PETM) in the mid-A... more ABSTRACT Abrupt sedimentary changes mark the Paleocene-Eocene Thermal Maximum (PETM) in the mid-Atlantic Coastal Plain. Across the tectonic downwarp known as the Salisbury Embayment (southeastern Virginia to central New Jersey), kaolinite-rich clay (the Marlboro Clay) abruptly replaces glauconite-rich quartz sand, silt, and illite-smectite clay of the late Paleocene. Dozens of cored boreholes obtained by the USGS and other Federal and State agencies, together with geophysical logs from more than 100 boreholes, allow us to document the widespread distribution and depositional, biostratigraphic, paleoecologic, taphonomic, and tectonic characteristics of the Marlboro Clay. In Maryland and Virginia, the Marlboro Clay is a silvery-gray to pale-red plastic clay and locally includes thin laminations and thicker beds of silt, concretions, pyrite nodules, and rare shell fragments (in downdip cores). The clay is unconformity bounded with highly burrowed contacts. The lower contact is locally so bioturbated that it has been reported as gradational. The upper contact in Virginia has glauconite-rich, sand-filled burrows from the overlying Nanjemoy Formation (lower Eocene) that penetrate as much as 8.5 m down into the Marlboro Clay (total thickness only 9 m). Biostratigraphically, the Marlboro Clay is placed in calcareous nannofossil Zone NP9 (and NP10 in downdip cores) and includes the PETM marker dinocyst species Apectodinium augustum (in downdip cores). In New Jersey, a similar clay deposit is described from the subsurface; and based on consistency in lithology, biostratigraphy, stratigraphic position, isotopic composition and its magnetic properties, we apply the name Marlboro Clay to this unique clay unit throughout the Salisbury Embayment. Structure-contour and isopach maps of the present configuration of the Marlboro Clay reveal a widespread deposit with localized truncation or thinning. Tectonic folding and faulting along the inner Coastal Plain and along the James River structural zone produced syn- and post-depositional erosional thinning on uplifted sides of faults and thickening in downdropped areas. The Chesapeake Bay Impact Structure (CBIS) removed the Marlboro Clay across a large area of southeastern Virginia. Faults and folds associated with the CBIS appear to deform the clay in an outer-fracture-zone that extends up to 25-km outside the crater&#39;s outer rim. The Marlboro Clay thickens and is slightly finer-grained in the deeper part of the Marlboro basin in where structure contours on the top-of-basement generally mimic the tectonic downwarp. The apparent deepest (&gt;200 m below sea level) and thickest (&gt;14 m) part of the Marlboro depositional basin appears to extend from just south of the mouth of the Potomac River northward to the lower Choptank River valley, Maryland, then turns sharply northeastward to beneath the central part of Delaware Bay. The Marlboro Clay of the mid-Atlantic United States thus provides a widespread deposit that in its regional and tectonic setting records, in its unusual sedimentary properties, the severe regional environmental changes associated with global climate change during the PETM.
In 2004-2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refr... more In 2004-2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refraction profiles (5-m source and geophone spacing) across the northern half of the central crater of the Chesapeake Bay impact structure (CBIS). The resulting seismic images provide details of the upper 1.0 km of the moat that surrounds the central uplift within the central crater and serve locally as a complement to 38 km of deep seismic data acquired across the CBIS by the USGS and Virginia Tech in 2004. Two profiles that cross at the 1.76-km-deep, ICDP- USGS Eyreville corehole (EC) provide semi-3D views of the stratigraphy and structures. Preliminary data processing and analysis indicate excellent correlation of the images with the EC, including individual block and impact-debris patterns in the upper 300 to 400 m of the CBIS. Variations in reflection patterns correspond to variations in conditions and processes within the central crater during impact. The seismic images show strong, m...
New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, pea... more New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A ...
Geological Society of America Special Papers, 2009
ABSTRACT A 443.9-m-thick, virtually undisturbed section of postimpact deposits in the Chesapeake ... more ABSTRACT 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&amp;amp;amp;#x27;s central crater. Recovered ...
Geological Society of America Special Papers, 2009
The Eyreville and Exmore, Virginia, core holes were drilled in the inner basin and annular trough... more The Eyreville and Exmore, Virginia, core holes were drilled in the inner basin and annular trough, respectively, of the Chesapeake Bay impact structure, and they allow us to evaluate sequence deposition in an impact crater. We provide new high-resolution geochronologic (<1 Ma) and sequence-stratigraphic interpretations of the Exmore core, identify 12 defi nite (and four possible) postimpact depositional sequences, and present comparisons with similar results from Eyreville and other mid-Atlantic core holes. The concurrence of increases in δ 18 O with Chesapeake Bay impact structure sequence boundaries indicates a primary glacioeustatic control on deposition. However, regional comparisons show the differential preservation of sequences across the mid-Atlantic margin. We explain this distribution by the compaction of impactites, regional sediment-supply changes, and the differential movement
Eos, Transactions American Geophysical Union, 2004
... Gregory S. Gohn. USGS, Reston, Virginia,. David S. Powars. USGS, Reston, Virginia,. J. Wright... more ... Gregory S. Gohn. USGS, Reston, Virginia,. David S. Powars. USGS, Reston, Virginia,. J. Wright Horton, Jr. USGS, Reston, Virginia,. Lucy E. Edwards. USGS, Reston, Virginia,. Jean M. Self-Trail. USGS, Reston, Virginia,. Roger H. Morin. USGS, Lakewood, Colorado, USA. Abstract ...
Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake... more Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.
1] On the mid-Atlantic Coastal Plain of the United States, Paleocene sands and silts are replaced... more 1] On the mid-Atlantic Coastal Plain of the United States, Paleocene sands and silts are replaced during the Paleocene-Eocene Thermal Maximum (PETM) by the kaolinite-rich Marlboro Clay. The clay preserves abundant magnetite produced by magnetotactic bacteria and novel, presumptively eukaryotic, ironbiomineralizing microorganisms. Using ferromagnetic resonance spectroscopy and electron microscopy, we map the magnetofossil distribution in the context of stratigraphy and carbon isotope data and identify three magnetic facies in the clay: one characterized by a mix of detrital particles and magnetofossils, a second with a higher magnetofossil-to-detrital ratio, and a third with only transient magnetofossils. The distribution of these facies suggests that suboxic conditions promoting magnetofossil production and preservation occurred throughout inner middle neritic sediments of the Salisbury Embayment but extended only transiently to outer neritic sediments and the flanks of the embayment. Such a distribution is consistent with the development of a system resembling a modern tropical river-dominated shelf.
available online at http://meteoritics.org 1613 Abstract-The late Eocene Chesapeake Bay impact st... more available online at http://meteoritics.org 1613 Abstract-The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best-preserved "wet-target" craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile-rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ~85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the "inverted sombrero" morphology observed at some craters in layered targets.
1] A 30-km-long, radial seismic reflection and refraction survey completed across the northern pa... more 1] A 30-km-long, radial seismic reflection and refraction survey completed across the northern part of the late Eocene Chesapeake Bay impact structure (CBIS) on the Delmarva Peninsula, Virginia, USA, confirms that the CBIS is a complex central-peak crater. We used a tomographic P wave velocity model and low-fold reflection images, constrained by data from two deep boreholes located on the profile, to interpret the structure and composition of the upper 5 km of crust. The seismic images exhibit well-defined structural features, including (with increasing radial distance) a collapsed central uplift, a breccia-filled moat, and a collapsed transient-crater margin (which collectively constitute a $40-km-wide collapsed transient crater), and a shallowly deformed annular trough. These seismic images are the first to resolve the deep structure of the crater (>1 km) and the boundaries between the central uplift, moat, and annular trough. Several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift, moat, and annular trough. Within the moat, breccia extends to a minimum depth of 1.5 km or a maximum of 3.5 km, depending upon the interpretation of the deepest layered materials. The images show $350 to 500 m of postimpact sediments above the impactites. The imaged structure of the CBIS indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling.
ABSTRACT Abrupt sedimentary changes mark the Paleocene-Eocene Thermal Maximum (PETM) in the mid-A... more ABSTRACT Abrupt sedimentary changes mark the Paleocene-Eocene Thermal Maximum (PETM) in the mid-Atlantic Coastal Plain. Across the tectonic downwarp known as the Salisbury Embayment (southeastern Virginia to central New Jersey), kaolinite-rich clay (the Marlboro Clay) abruptly replaces glauconite-rich quartz sand, silt, and illite-smectite clay of the late Paleocene. Dozens of cored boreholes obtained by the USGS and other Federal and State agencies, together with geophysical logs from more than 100 boreholes, allow us to document the widespread distribution and depositional, biostratigraphic, paleoecologic, taphonomic, and tectonic characteristics of the Marlboro Clay. In Maryland and Virginia, the Marlboro Clay is a silvery-gray to pale-red plastic clay and locally includes thin laminations and thicker beds of silt, concretions, pyrite nodules, and rare shell fragments (in downdip cores). The clay is unconformity bounded with highly burrowed contacts. The lower contact is locally so bioturbated that it has been reported as gradational. The upper contact in Virginia has glauconite-rich, sand-filled burrows from the overlying Nanjemoy Formation (lower Eocene) that penetrate as much as 8.5 m down into the Marlboro Clay (total thickness only 9 m). Biostratigraphically, the Marlboro Clay is placed in calcareous nannofossil Zone NP9 (and NP10 in downdip cores) and includes the PETM marker dinocyst species Apectodinium augustum (in downdip cores). In New Jersey, a similar clay deposit is described from the subsurface; and based on consistency in lithology, biostratigraphy, stratigraphic position, isotopic composition and its magnetic properties, we apply the name Marlboro Clay to this unique clay unit throughout the Salisbury Embayment. Structure-contour and isopach maps of the present configuration of the Marlboro Clay reveal a widespread deposit with localized truncation or thinning. Tectonic folding and faulting along the inner Coastal Plain and along the James River structural zone produced syn- and post-depositional erosional thinning on uplifted sides of faults and thickening in downdropped areas. The Chesapeake Bay Impact Structure (CBIS) removed the Marlboro Clay across a large area of southeastern Virginia. Faults and folds associated with the CBIS appear to deform the clay in an outer-fracture-zone that extends up to 25-km outside the crater&#39;s outer rim. The Marlboro Clay thickens and is slightly finer-grained in the deeper part of the Marlboro basin in where structure contours on the top-of-basement generally mimic the tectonic downwarp. The apparent deepest (&gt;200 m below sea level) and thickest (&gt;14 m) part of the Marlboro depositional basin appears to extend from just south of the mouth of the Potomac River northward to the lower Choptank River valley, Maryland, then turns sharply northeastward to beneath the central part of Delaware Bay. The Marlboro Clay of the mid-Atlantic United States thus provides a widespread deposit that in its regional and tectonic setting records, in its unusual sedimentary properties, the severe regional environmental changes associated with global climate change during the PETM.
In 2004-2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refr... more In 2004-2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refraction profiles (5-m source and geophone spacing) across the northern half of the central crater of the Chesapeake Bay impact structure (CBIS). The resulting seismic images provide details of the upper 1.0 km of the moat that surrounds the central uplift within the central crater and serve locally as a complement to 38 km of deep seismic data acquired across the CBIS by the USGS and Virginia Tech in 2004. Two profiles that cross at the 1.76-km-deep, ICDP- USGS Eyreville corehole (EC) provide semi-3D views of the stratigraphy and structures. Preliminary data processing and analysis indicate excellent correlation of the images with the EC, including individual block and impact-debris patterns in the upper 300 to 400 m of the CBIS. Variations in reflection patterns correspond to variations in conditions and processes within the central crater during impact. The seismic images show strong, m...
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