ICDP-USGS Workshop on Deep Drilling in the Central Crater
of the Chesapeake Bay Impact Structure, Virginia, USA
September 22-24, 2003
Herndon, Virginia
PROCEEDINGS VOLUME

Lucy E. Edwards, J. Wright Horton, Jr., and Gregory S. Gohn, compilers
Prepared in cooperation with the International Continental Scientific
Drilling Program (ICDP)
2004
U.S. Geological Survey Open-File Report 2004-1016

Foreword
This volume contains the proceedings of the ICDP-USGS
Workshop on Deep Drilling in the Central Crater of the
Chesapeake Bay Impact Structure, Virginia, USA, which was
held September 22-24, 2003, in Herndon, Virginia. This workshop
was jointly sponsored by the International Continental Scientific
Drilling Program (ICDP) and the U.S. Geological Survey (USGS).
The proceedings begin with a brief summary of the workshop
followed by the list of participants and a copy of the agenda. 
The proceedings volume contains two sets of abstracts. The first 
set of 17 abstracts is based on 29 poster presentations that were 
displayed at the workshop. The second set of 19 abstracts consists 
of research proposals that were submitted by members of the scientific 
community following the workshop. The abstracts are given here as 
submitted by the authors without further edit, except for the 
correction of some obvious typographical errors.

Disclaimer
This report is preliminary and has not been reviewed for
conformity with the North American Stratigraphic Code. Any use
of trade, product, or firm names is for descriptive purposes only
and does not imply endorsement by the U.S. Government.
Although all data and software released on this CD-ROM have
been used by the USGS, no warranty, expressed or implied, is
made by the USGS as to the accuracy of the data and related
materials and (or) the functioning of the software.

TABLE OF CONTENTS
WORKSHOP SUMMARY
LIST OF PARTICIPANTS
WORKSHOP PROGRAM WITH LINKS TO SLIDE PRESENTATIONS
POSTER PRESENTATIONS WITH LINKS TO ABSTRACTS
PROPOSALS FOR FUTURE WORK (ABSTRACTS)

WORKSHOP SUMMARY.
Gregory S. Gohn1, J. Wright Horton, Jr.1, and Lucy E. Edwards1, 1U.S. Geological Survey, 926A National Center,
Reston, VA 20192, USA (ggohn@usgs.gov).

Introduction. The International Continental
Scientific Drilling Program (ICDP) and the U.S.
Geological Survey (USGS) sponsored a scientific
workshop on Deep Drilling in the Central Crater of
the Chesapeake Bay Impact Structure. The
workshop was held September 22-24, 2003, in
Herndon, Virginia. The purpose of the workshop
was to review the results of previous investigations of
the Chesapeake Bay impact structure and to provide a
forum for creating scientific and operational plans for
deep drilling in the structures central crater. Over 60
scientists represented 10 countries at the workshop.

The late Eocene Chesapeake Bay impact structure
is among the largest and best preserved of the known
marine impact craters on Earth. This complex crater
lies buried at shallow to moderate depths beneath
postimpact Cenozoic sediments of the Virginia
Coastal Plain and adjacent Continental Shelf on the
U.S. Atlantic continental margin. The diameter of
the impact structure typically is cited as about 85 km.
Principal subdivisions are a 38-km-wide central
crater, which may approximate the location of the
impacts transient crater, and a surrounding 24-kmwide
annular trough that primarily records late-stage
gravitational collapse.

The coreholes and geophysical studies that
ultimately led to recognition and characterization of
the Chesapeake Bay impact structure began in the
late 1980s and continued into the 1990s. Since
2000, the USGS, the Hampton Roads Planning
District Commission, the Virginia Department of
Environmental Quality, and collaborating institutions
have conducted a second phase of multidisciplinary
geophysical, corehole, and hydrologic investigations
of the impact structure (see summaries in Poag and
others, 2004, and Horton and others, in press).
Previous investigations have defined the location,
size, structure, and inferred origin of the major
architectural elements of the Chesapeake Bay impact
structure (Powars and Bruce, 1999; Powars, 2000;
Poag and others, 2004; Horton and others, in press).
Collectively, over 2,000 km of seismic-reflection
data have been analyzed (Poag and others, 2004) and
8 new coreholes have been drilled, geophysically
logged, and analyzed. However, none of the
coreholes were drilled in the central crater.

Workshop Agenda. The workshop began on
September 22 with welcoming remarks by Charles G.
Groat, Director of the USGS, and P. Patrick Leahy,
USGS Associate Director for Geology. The first day
of the program featured reviews of previous corehole
and geophysical investigations of the Chesapeake
Bay impact structure, summaries of the capabilities
of the ICDP Support Group and drilling capabilities
of DOSECC, Inc. (Drilling, Observation, and
Sampling of the Earths Continental Crust), and
discussion of first-order scientific questions that
could be addressed by deep drilling in the central
crater. The first day concluded with an evening
poster session, where voluntary posters by workshop
participants and USGS drill cores were examined and
discussed.

The second day began with a review of
sedimentary, climatic, and tectonic studies of middle
Tertiary to Quaternary (postimpact) sediments of the
U.S. Mid-Atlantic continental margin. The morning
session continued with a lengthy panel and audience
discussion of the scientific objectives and
experiments, drilling strategy, site selection criteria,
funding sources, and logistics of a deep drilling
program in the Chesapeake Bay central crater. In the
afternoon, participants divided into three working
groups: (1) impact processes and products, (2)
postimpact geology, and (3) hydrology. The second
day concluded with reports from the working groups.
The third day consisted of moderated audience
discussions of a variety of issues, including the
identification of standing science teams, drilling and
logging operations, drill-site and sampling protocols,
core storage, and publications.

Identification of Scientific Issues. A significant
outcome of the workshop was the identification of
scientific goals for the proposed drilling program.
The goals were limited to research topics that could
be addressed by core drilling in the central crater.
Crater Structure and Morphology
 Determine the crater depth
 Determine the structural character of the cored
segment of the central crater
Crater Materials
 Determine target composition and stratigraphy
beneath crater
 Determine petrophysical properties of target
materials
 Determine target chemistry and mineralogy for
comparison with North American tektites
 Search for meteorite component in crater materials
to identify projectile type
 Determine isotopic ages for all suitable types of
material
 Determine fracture depth and distribution
 Determine the amount and distribution of melt
 Characterize the types of crater breccias and infer
their formative processes
 Quantify the volumes of breccia types and melts
 Determine the character of resurge and tsunami
sediments
 Determine stratigraphy of the crater fill
 Conduct paleomagnetic studies of shocked rocks
and melt
 Document levels and gradients of shock
deformation
 Document impact damage in fossils
Borehole Geophysical Studies
 Collect a full suite of borehole geophysical logs for
determining petrophysical properties
 Directly measure petrophysical properties of core
samples
 Integrate core and log petrophysical data with
regional gravity, magnetic, seismic, and electrical
conductivity surveys and with numerical models
Impact - Postimpact Transition and Postimpact
Events
 Document the impact-produced local biotic crisis
and recovery
 Document the physical transition from the highenergy
impact environment to the normal shelf
environment
 Document the physical stratigraphy,
biostratigraphy, and sequence stratigraphy of the
postimpact sediments
 Document impact effects on long-term climate
 Determine the postimpact thermal and
hydrothermal history and processes
Hydrologic Resources
 Determine the salinity and other chemical
attributes of ground water in core samples
 Determine the postimpact hydrogeologic history of
the crater area
Modern Deep Biosphere
 Determine the character of deep biota
Workshop participants recommended the creation
of standing scientific working groups (science teams)
to conduct the analyses of the proposed core and
corehole. Seven science teams were defined on the
bases of broad research topics and related
methodologies: (1) crater materials, (2) regional and
borehole geophysics, (3) hydrothermal systems and
hydrologic resources, (4) cratering mechanics and
modeling, (5) environmental effects of impact and
impact-postimpact transition, (6) postimpact
sedimentary, climatic, and tectonic history, and (7)
deep biosphere.

Deep-drilling Site Selection and Proposal. The
workshop participants agreed that a full drilling
proposal for the Chesapeake Bay impact structure
should be submitted to ICDP in January 2004, and
that additional geophysical studies by the USGS were
needed to provide adequate site characterization.
Drill-site selection was addressed in several
plenary and breakout sessions. These discussions
were guided by interpretations of the regional gravity
map and the seismic-reflection profiles that cross the
central crater. Consideration was given to three
general locations characterized by distinctive gravity
anomalies. The three locations are (1) the uplifted
central peak, (2) the moat or deepest part of the
central crater that surrounds the central uplift, and (3)
the rim of the central crater. Each location addresses
a different set of scientific issues with some overlap.
After considering the relative merits of each location
for addressing the major scientific questions listed
above, the moat was the consensus first choice for
a proposed drill site. The other two locations were
consensus scientific choices for a second and (or)
third corehole, if possible, to understand the crater.

References Cited.
Horton, J.W., Jr., Powars, D.S., and Gohn, G.S., in
press, Studies of the Chesapeake Bay impact
structureIntroduction and discussion, chap. A
of Horton, J.W., Jr., Powars, D.S., and Gohn,
G.S., eds., Studies of the Chesapeake Bay impact
structureThe USGS-NASA Langley corehole,
Hampton, Virginia, and related coreholes and
geophysical surveys: U.S. Geological Survey
Professional Paper 1688.
Poag, C.W., Koeberl, Christian, and Reimold, W.U.,
2004, The Chesapeake Bay Crater: Springer,
New York, 522 p. and CD-ROM.
Powars, D.S., 2000, The effects of the Chesapeake
Bay impact crater on the geologic framework
and the correlation of hydrogeologic units of
southeastern Virginia, south of the James River:
U.S. Geological Survey Professional Paper 1622,
53 p.
Powars, D.S., and Bruce, T.S., 1999, The effects of
the Chesapeake Bay impact crater on the
geologic framework and the correlation of
hydrogeologic units of the lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p.

LIST OF PARTICIPANTS
Huirong-Anita Ai (California Institute of Technology)
C.R. "Rick" Berquist (Virginia Division of Mineral Resources)
John M. Brozena (Naval Research Laboratory)
T. Scott Bruce (Virginia Department of Environmental Quality)
Rufus D. Catchings (USGS-Menlo Park)
Gareth S. Collins (University of Arizona)
David Crawford (Sandia National Laboratories)
David L. Daniels (USGS-Reston)
Alexander Deutsch (University of Muenster)
Henning Dypvik (University of Oslo)
Lucy E. Edwards (USGS-Reston)
Scott R. Emry (Hampton Roads Planning District Comission)
Robert B. Fraser (USGS-Reston)
Bevan M. French (Smithsonian Institution)
Billy P. Glass (University of Delaware)
Gregory S. Gohn (USGS-Reston)
Samuel Harvey (Virginia Polytechnic Institute and State University)
Charles E. Heywood (USGS-Richmond)
John A. Hole (Virginia Polytechnic Institute and State University)
J. Wright Horton, Jr. (USGS-Reston)
David Jutson (RWE Dea AG-Wietze)
Kunio Kaiho (Tohoku University)
Thomas Kenkmann (Humboldt-Universitt zu Berlin)
David T. King, Jr. (Auburn University)
Christian Koeberl (University of Vienna)
Michael J. Kunk (USGS-Denver)
Daniel Larsen (University of Memphis)
Steven M. Lev (Towson University)
E. Randolph McFarland (USGS-Richmond)
Susan McGeary (University of Delaware)
Peter P. McLaughlin (Delaware Geological Survey)
Sandra Martinka (U.S. Naval Research Laboratory)
H. Jay Melosh (University of Arizona)
Kenneth G. Miller (Rutgers University)
Daniel J. Milton (USGS-Reston)
Donald H. Monteverde (Rutgers University)
Roger H. Morin (USGS-Denver)
Gregory S. Mountain (Rutgers University)
Dennis L.Nielson (DOSECC)
Jens Orm (Centro de Astrobiologia, INTA/CSIC)
Gordon Osinski (University of Arizona)
Amanda Palmer-Julson (Blinn CollegeBryan)
Lauri J. Pesonen (University of Helsinki)
Lucille W. Petruny (Astra-Terra Research)
Elisabetta Pierazzo (Planetary Science Institute)
Jeffrey B. Plescia (USGS-Flagstaff)
C. Wylie Poag (USGS-Woods Hole)
Jason P. Pope (USGS-Richmond)
David S. Powars (USGS-Reston)
Donald G. Queen (USGS-Reston)
James E. Quick (USGS-Reston)
Wolf Uwe Reimold (University of Witwatersrand)
David P. Russ (USGS-Reston)
Ward E. Sanford (USGS-Reston)
Jean M. Self-Trail (USGS-Reston)
Anjana K. Shah (U.S. Naval Research Laboratory)
Jan Smit (Vrije Universiteit-Amsterdam)
Peter J. Sugarman (New Jersey Geological Survey)
Ryuji Tada (University of Tokyo)
Donald M. Thomas (University of Hawai'i / DOSECC)
David A. Vanko (Towson University)
Suzanne D. Weedman (USGS-Reston)
James Whitehead (University of New Brunswick)

Workshop Program
Monday, September 22nd -- Morning
Introduction and Review of Previous Investigations
8:30-8:40 Welcome: Workshop Conveners
8:40-8:50 Opening Remarks
Charles Groat (Director, USGS)
8:50-9:00 Opening Remarks
Patrick Leahy (Associate Director for Geology, USGS)
9:00-9:15 Meeting Logistics and Agenda Overview
Gregory Gohn (USGS)
9:15-9:45 Unresolved Questions about Earths Marine Impact Craters
Jay Melosh (University of Arizona)
9:45-10:15 Review of Marine Seismic-reflection Surveys: Chesapeake Bay Crater
Wylie Poag (USGS)
10:15-10:40 BREAK
10:40-11:10 Review of Drilling Programs: Chesapeake Bay Crater
David Powars (USGS)
11:10-11:40 Review of Petrography, Geochemistry, and Geochronology: CB Crater
Wright Horton (USGS)
Christian Koeberl (University of Vienna)
11:40-12:10 Review of Hydrologic Issues and Research: Chesapeake Bay Crater
Randy McFarland (USGS)
12:10-1:15 LUNCH

Monday, September 22nd -- Afternoon
Site Characterization: Geophysical Surveys of the Central Crater
1:15-1:45 Gravity and Magnetic Surveys
David Daniels (USGS)
Anji Shah (U.S. Naval Research Laboratory)
1:45-2:15 On-land Seismic-reflection Surveys
Rufus Catchings (USGS)
Operational Issues
2:15-2:45 ICDP Support Group Capabilities
Christian Koeberl (ICDP Science Advisory Group)
2:45-3:15 DOSECC Drilling Capabilities
Donald Thomas (DOSECC)
3:15-3:40 BREAK
Scientific Issues
3:40-4:00 Drilling at the Chicxulub crater
Jan Smit (Vrije Universiteit-Amsterdam)
4:00-5:00 Open Discussion: What are the first-order scientific questions about the
inner crater of the Chesapeake Bay impact structure? Will deep drilling
provide the answers to these questions?
Moderator: Bevan French (Smithsonian Institution)
5:00-5:30 OPEN
5:30-7:30 Poster and Core Examination Session

Tuesday, September 23, 2003
8:30-8:40 Announcements
8:40-9:10 Review of Postimpact Sedimentary, Climatic, and Tectonic History:
Kenneth Miller (Rutgers University)
9:10-10:15 A Scientific Drilling Program for the Central Crater
Panel and Audience Discussion
Moderator: James Quick (USGS)
Panel: Christian Koeberl (University of Vienna)
Jay Melosh (University of Arizona)
Kenneth Miller (Rutgers University)
Jens Orm (Centro de Astrobiologa)
Issues: Scientific objectives
Scientific experiments
Drilling strategy
Site selection
Funding
Logistics
10:15-10:45 BREAK
10:45-11:45 Continue Panel and Audience Discussion
11:45-12:00 Organize Working Group Sessions
12:00-1:15 LUNCH
Scientific Working Groups
1:15-3:00 Working Group Sessions
3:00-3:30 BREAK
3:30-4:45 Reports from Working Groups
4:45-5:00 Announcements
Review Wednesday Agenda

Wednesday, September 24, 2003
Discussion Topics
8:30-8:40 Announcements
8:40-10:15 Identification of Science Teams
Drilling and Logging - Operational Issues and Strategies
10:15-10:45 BREAK
10:45-12:00 Core Storage
Publication Policy
Drill-site Protocols
Open Discussion
12:00 Adjourned

Committees, ICDP Chesapeake Bay Impact Crater Workshop
Convenors: Gregory S. Gohn (USGS-Reston)
Kenneth G. Miller (Rutgers University)
James E. Quick (USGS-Reston)
Steering Committee: Alan R. Hildebrand (University of Calgary)
Christian Koeberl (University of Vienna)
H. Jay Melosh (University of Arizona)
Gregory S. Mountain (Rutgers University)
C. Wylie Poag (USGS-Woods Hole)
Wolf Uwe Reimold (University of Witwatersrand)
Organizing Committee: Lucy E. Edwards (USGS-Reston)
J. Christine Flynt (USGS-Reston)
Gregory S. Gohn (USGS-Reston)
J. Wright Horton, Jr. (USGS-Reston)
Donna M. Johnstone (USGS-Reston)
Colleen T. McCartan (USGS-Reston)
Loretta Morris (USGS-Reston)
David S. Powars (USGS-Reston)
Ellen L. Seefelt (USGS-Reston)
Jean M. Self-Trail (USGS-Reston)

Poster Session

POSTER SESSION

HYDROCODE SIMULATIONS OF THE CHESAPEAKE BAY IMPACT
G. S. Collins and H. J. Melosh
POTENTIAL FIELD GEOPHYSICS OF THE CHESAPEAKE BAY IMPACT SITE
David L. Daniels, Seth D. Tanner, Wilma B. Aleman Gonzalez, Colleen T. McCartan, and
James B. Murray
THREE THINGS TO KNOW ABOUT DAMAGED DINOCYSTS, CHESAPEAKE
BAY IMPACT STRUCTURE, VIRGINIA
Lucy E. Edwards
STRATIGRAPHIC PALEONTOLOGY OF THE CRATER-FILL DEPOSITS,
LANGLEY CORE, CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA
Lucy E. Edwards, Norman O. Frederiksen, and Jean M. Self-Trail
CHESAPEAKE BAY IMPACT STRUCTURE - 35 MILLION YEARS AFTER - AND
STILL MAKING AN IMPACT.
Scott R. Emry and Brian Miller
DISTAL IMPACT EJECTA FROM THE CHESAPEAKE BAY IMPACT CRATER.
Billy P. Glass
POSSIBILITY OF OCEAN WATER INVASION INTO THE CHICXULUB CRATER
AT THE CRETACEOUS/TERTIARY BOUNDARY
Kazuhisa Goto, Ryuji Tada, Eiichi Tajika, Timothy J. Bralower, Takashi Hasegawa, and
Takafumi Matsui
INFLUENCE OF THE CHESAPEAKE BAY IMPACT STRUCTURE ON GROUNDWATER
FLOW AND SALINITY IN THE ATLANTIC COASTAL PLAIN AQUIFER
SYSTEM OF VIRGINIA
Charles E. Heywood
PROPOSED SEISMIC IMAGING AND NUMERICAL MODELING OF THE 35 MA
IMPACT EVENT AT CHESAPEAKE BAY
John A. Hole, Susan McGeary, H. Jay Melosh, and Susan C. Eriksson
PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF
CRYSTALLINE BASEMENT AND IMPACT-DERIVED CLASTS FROM
COREHOLES IN THE WESTERN ANNULAR TROUGH, CHESAPEAKE BAY
IMPACT STRUCTURE, VIRGINIA, USA
J. Wright Horton, Jr., Michael J. Kunk, Charles W. Naeser, Nancy D. Naeser, John N.
Aleinikoff, and Glen A. Izett

CATASTROPHIC EVENTS AT THE END OF THE PERMIAN
Kunio Kaiho, Yoshimichi Kajiwara, Ken Sawada, Chen Zhong-Qiang, Hodaka
Kawahata, Tetsuya Arinobu, Tatsuro Mochinaga and Hisao Sato
A STRUCTURAL COMPARISON BETWEEN THE CHESAPEAKE BAY IMPACT
CRATER, USA, AND THE RIES CRATER, GERMANY: HOW DID THE CENTRAL
CRATER BASIN FORM?
T. Kenkmann
STRUCTURE-FILLING STRATIGRAPHY OF THE MARINE-TARGET
WETUMPKA IMPACT STRUCTURE, ALABAMA, USA.
D. T. King, Jr., L. W. Petruny, and T. L. Neathery
PROPOSED SCIENTIFIC DRILLING INTO THE BOSUMTWI IMPACT
STRUCTURE, GHANA
Christian Koeberl, Bernd Milkrereit, Jonathan T Overpeck, and Christopher A. Scholz
DISTRIBUTION, ORIGIN, AND RESOURCE-MANAGEMENT IMPLICATIONS OF
GROUND-WATER SALINITY ALONG THE WESTERN MARGIN OF THE
CHESAPEAKE BAY IMPACT STRUCTURE.
E. Randolph McFarland and T. Scott Bruce
SPATIAL ANALYSIS OF THE HYDROGEOLOGIC FRAMEWORK OF THE
VIRGINIA COASTAL PLAIN
E. Randolph McFarland and Jason P. Pope
NEOGENE SEQUENCES FROM THE EAST-CENTRAL DELMARVA PENINSULA:
RESULTS FROM DRILLING AT BETHANY BEACH, DELAWARE
P.P. McLaughlin, K.G Miller, J.V. Browning, and R.N Benson.
THE INFLUENCE OF A DEEP SHELF SEA ON THE EXCAVATION AND
MODIFICATION OF A MARINE-TARGET CRATER, THE LOCKNE CRATER,
CENTRAL SWEDEN
Jens Orm and Maurits Lindstrm
PHYSICAL PROPERTIES AND PALAEOMAGNETISM OF THE DEEP DRILL
CORE FROM THE CHESAPEAKE BAY IMPACT STRUCTURE - A RESEARCH
PROPOSAL
Lauri J. Pesonen, Tiiu Elbra, Martti Lehtinen, Johanna M. Salminen, and Fabio
Donadini [see proposals]
AUDIO-MAGNETOTELLURIC DATA FROM THE OUTER MARGIN OF THE
CHESAPEAKE BAY IMPACT STRUCTURE
Herbert A., Pierce [see proposals]

A REVIEW OF MARINE SEISMIC REFLECTION STUDIES OF THE CHESAPEAKE
BAY IMPACT CRATER
C. Wylie Poag
'WET TARGET' EFFECTS ON THE SYNIMPACT PARTIAL FILLING AND
POSTIMPACT BURIAL OF THE CHESAPEAKE BAY IMPACT CRATER,
SOUTHEASTERN VIRGINIA
D.P. Powars, L.E. Edwards, T.S. Bruce, and G.H. Johnson
1:24,000 SCALE VISUALIZATION (?MODEL) OF SEISMIC REFLECTION AND
GRAVITY DATA INNER BASIN CHESAPEAKE BAY IMPACT CRATER
D.P. Powars, D.L. Daniels, J.W Horton, Jr., L.E. Edwards, and G.S. Gohn
LANGLEY COREHOLE STRATIGRAPHY AND GEOPHYSICAL LOGS: GUIDE TO
SEISMIC INTERPRETATIONS
D.P. Powars, G.S. Gohn, L.E., Edwards, J.W. Horton, Jr., and R.D. Catchings
HYDROTHERMAL RESPONSE TO THE CHESAPEAKE BAY BOLIDE IMPACT
Ward E. Sanford
CALCAREOUS NANNOFOSSIL DISTRIBUTION PATTERNS AND SHOCKINDUCED
TAPHONOMY IN THE CHESAPEAKE BAY IMPACT CRATER
Jean M. Self-Trail
SHIPBOARD GRAVITY AND MAGNETIC FIELD OVER THE CHESAPEAKE BAY
IMPACT CRATER
Anjana K. Shah, John Brozena, and Peter Vogt
THE YAXCOPOIL-1 DRILLING IN THE CHICXULUB CRATER:
STRATIGRAPHICAL AND GEOCHEMICAL DATA
J Smit., B. Dressler, S. van der Gaastand, and W. Lustenhouwer
FLUID INCLUSION AND MINERALOGICAL EVIDENCE FOR POST-IMPACT
CIRCULATION OF SEAWATER-DERIVED HYDROTHERMAL FLUIDS AT THE
CHESAPEAKE BAY IMPACT CRATER: A CRITICAL TEST OF THE PHASE
SEPARATION MODEL FOR BRINE GENERATION
David A. Vanko [see proposals]
a laptop simulation was presented by David Crawford

HYDROCODE SIMULATIONS OF THE CHESAPEAKE BAY IMPACT. G. S. Collins and H. J. Melosh,
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. (e-mail: gareth@lpl.arizona.edu).

Introduction. The Chesapeake Bay Impact Crater
(CBIC) formed about 35 million years ago, in a shallow
marine environment (400-600 m water depth).
The crater is complex and developed in a multi-layer,
rheologically-variable target that comprised 0.4-1 km
of soft, water-saturated sediments overlying crystalline
basement (Poag and others, 1994).

Seismic reflection data illustrates that the Chesapeake
Bay crater morphologyoften described as an
inverted sombrerois similar to other marine-target
impact craters. It consists of a ~1-1.5-km deep, highly
disturbed central crater, surrounded by a shallower,
less deformed basin (Poag, 1996). The inner crater has
a diameter of ~40 km; the edge of the outer basin extends
to ~85-km diameter. The morphological divide
between the inner and outer crater is termed the inner
ring or peak ring. Little is known about the nature of
the inner ring. Seismic reflection data show that the
underlying basement is modestly uplifted (Powars and
Bruce, 1999); however, it is unclear whether the pristine
surface expression of the inner ring was elevated
above the floor of the outer crater.

The characteristic structure of CBIC and other marine
impact craters raises a fundamental question: what
is the size of the crater? In other words, which (if either)
of the two concentric crater rings relates to the
initial cavity (transient crater) formed during the impact
process. Answering this question is critical for
any assessment of the environmental consequences of
the Chesapeake Bay impact, because impact energy can
only be reliably estimated from the size of the transient
crater.

Three possible models for the formation of the
CBIC are proposed (Poag, 1996; Kenkmann, 2003;
Hole and others, 2003); each of these implies drastically
different impact energies. The first supposes that
the inner ring is a peak ring, that CBIC is analogous to
lunar peak ring craters such as Schrodinger and that the
outer crater ring is the final crater rim. The formation
of peak rings is currently believed to involve the collapse
of a substantial, transient central uplift (Collins
and others, 2002). Therefore, this model implies a
large (>40-km diameter) transient crater and zone of
deformation. The second model supposes that the inner
ring represents the uplifted and overturned flap of
brecciated target, which forms the temporary rim of the
crater prior to collapse. In this model, the outer ring
corresponds to the distal limit of deformation induced
by accentuated inward collapse of the weak sediments.
The diameter of the transient crater in this model is 30-
40 km; however, the amount of collapse must be significantly
less than in the first model. The third model
supposes that the inner ring marks the boundary between
heavily- and weakly-fractured basement material.
It defines a rheologic boundary between the damaged
basement material, which collapses inwards into
the inner basin, and the relatively undamaged basement,
which remains essentially undeformed during the
impact. As with the second model, the outer ring in
this scenario is the most distal evidence for deformation
in the weaker sediments above the basement. In
this model, the transient crater may be only 20-30 km
in diameter. Both the second and the third scenarios
rely on the sedimentary cover at Chesapeake Bay being
significantly weaker than the underlying basement.

Hydrocode Modeling. We performed some preliminary
hydrocode simulations to test the plausibility
of the proposed formation kinematics of the CBIC. We
used the SALEB hydrocode (Ivanov and Deutsch,
1999), which is a multi-material, multi-rheology extension
to the SALE hydrocode (Amsden and others,
1980). We approximated the target lithology at the
Chesapeake Bay impact site with a weak, upper layer
of water saturated sediments 1-1.5 km in thickness,
overlying granite basement. We used the ANEOS
equation of state for granite and calcite, and typical
rock-strength parameters for granite from experimental
work (Lunborg, 1968; Stesky and others 1974). These
typical strengths were modified by damage, according
to a combined shear and tensile failure algorithm
(Collins and others, 2003, in press); by temperature,
according to thermal softening relations (Ohnaka,
1995); and by high-frequency pressure vibrations, according
to the acoustic fluidization model (Ivanov and
Kostuchenko, 1997; Melosh and Ivanov, 1999). To
simulate the rheologic difference between the weak
sediments and the strong basement, we lowered the
strength parameters for the sedimentary unit to values
around 0.1-10 MPa, with friction coefficients of 0.0
(Bingham fluid) to 0.5. The collapse of the basement
rock was controlled primarily by the acoustic fluidization
parameters.

Preliminary Results. The most promising kinetmatic
model appears to be the third scenario described
above, which supposes that the impact produced a ~25
km diameter transient crater. The more energetic scenarios
tend to significantly disturb and uplift the basement
rocks beneath the outer basin, which is contradictory
to seismic evidence (Poag, 1996; Powars and
Bruce, 1999). However, many more simulations spanning
the available parameter space must be conducted
before firm conclusions can be drawn, and the numerical
simulation results must be validated with geologic
and geophysical observation.

Figure 1: Final crater cross-section derived from numerical
simulations. This simulation tested the third kinematic model
described in the text. (a) Final position of the different rock
units (red material denotes melt); (b) Damage contours
(lighter shading implies more damage); (c) Plastic strain
contours (warmer colors imply greater total plastic strain).
See text for discussion.

Figure 1 illustrates the final crater structure from a
simulation in which the growing cavity reached a
maximum depth of ~6 km and collapse began when the
cavity was ~25-km in diameter. The final crater morphology
is shallow (<1.5 km). The outermost deformation
of significance (see Fig. 1c) defines the outer ring;
it is difficult to define surface features from the calculation
because of the low resolution used in these preliminary
calculations (cell dimension = 150 m). We
define the inner ring in this simulation to be the basement
material that marks the boundary between the
inner, heavily-disturbed crater basin and the outer, lessdeformed
basin. The basement material within the inner
crater is heavily damaged and disturbed; total plastic
strains in this region exceed 1 (100 %; total plastic
strain is the accumulated plastic strain where the sense
of shear is ignored). There is modest uplift of the central
crater floor (1-2 km) and a thin melt sheet (<200
m). The only sedimentary material in the inner crater is
the collapse deposit, which flanks the inner ring. The
sedimentary unit in the outer basin is moderately disturbed
(strains >0.1); however, the underlying basement
is only weakly deformed (total plastic strain
<0.1).

Acknowledgements and References. This work
was supported by NASA Grant NAG5-11493.
Poag C. W. et al. (1994) Meteoroid mayhem in Ole Virginny:
Source of the North American tektite strewn
field. Geology, 22 No. 8: 691-694.
Poag C. W. (1996) Structural outer rim of Chesapeake Bay
impact crater: Seismic and borehole evidence Meteoritics
Planet. Sci. 31: 218-226.
Powars D. S. and Bruce T. S. (1999) The effects of the
Chesapeake Bay impact crater on the geologic framework
and the correlation of hydrogeologic units of
southeastern Virgina, south of the James river. U.S.G.S.
Professional Paper 1612, 82 p.
Kenkmann T. (2003) A structural comparison of the Chesapeake
Bay impact crater, USA, with the Ries crater,
Germany: How did the central crater basin form? This
publication.
Hole J. A. et al. (2003) Proposed seismic imaging and numerical
modeling of the impact event at Chesapeake
Bay. This publication.
Collins G. S. et al. (2002) Hydrocode simulations of Chicxulub
crater collapse and peak-ring formation. Icarus 157:
24-33.
Ivanov B. A. and Deutsch A. (1999) Sudbury impact event:
cratering mechanics and thermal history. GSA Special
Paper 339, GSA, Boulder, CO, 389-397.
Amsden et al. (1980) SALE: A simplified ALE computer
program for fluid flows at all speeds. Technical Report,
LA-8095, Los Alamos National Laboratory, Los Alamos,
NM.
Lundborg N. (1968) Strength of rock-like material Int. J.
Rock Mech. Mining Sci. 5: 427-454.
Stesky et al. (1974) Friction in faulted rock at high temperature
and pressure. Tectonophysics, 23: 177-203.
Ohnaka M. (1995) A shear failure strength law of rock in the
brittle-plastic transition regime. Geophys. Res. Lett. 22:
25-28.
Collins et al. (2002) Damage and deformation in numerical
impact simulations. Meteoritics Planet. Sci., In press.
Ivanov B. A. and Kostuchenko V. N. (1997) Block oscillation
model for impact crater collapse. LPSC XXVII Abs
#1655.
Melosh H. J and Ivanov B. A. (1999) Impact crater collapse
Ann. Rev. Earth Planet. Sci. 27: 385-415.

POTENTIAL FIELD GEOPHYSICS OF THE CHESAPEAKE BAY IMPACT SITE
David L. Daniels, Seth D. Tanner, Wilma B. Aleman Gonzalez, Colleen T. McCartan, James B. Murray
U.S. Geological Survey, 954 National Center, Reston, VA 20192, USA, (dave@usgs.gov)

Introduction. Potential field data has been
used to aid investigations of impact sites around
the world (for example, Pilkington and Grieve,
1992; Plescia, 1999). The Chesapeake Bay
impact site (Poag, and others, 1999) has a gravity
low associated with the inner crater.

Gravity Data. The original gravity data sets
for this investigation come from the National
Imaging and Mapping Agency (S. Spaunhorst,
personal communication, 2001) and the National
Oceanic and Atmospheric Administration
database (Dater and others, 1999). Data points
are mostly on land, collected between 1956 and
1984. The USGS goal was to improve this
gravity coverage, mostly in the island area east
of the lower Delmarva Peninsula where data
points were sparse.

To that end, the USGS team acquired 65
new land gravity stations in the eastern half of
the inner crater. Figure 1 shows the gravity
anomaly map of the inner crater combining the
new and original data. The Bouguer anomaly is
used for the land stations corrected to a reduction
density of 2.0 gm/cc. The free air anomaly is
used for two key surveys that cover Chesapeake
Bay and the Atlantic Ocean.

A USGS pontoon boat was used to navigate
the very shallow water behind the barrier islands.
A conventional v-hull powerboat was used for
the surf zone of the barrier islands and the
eastern shore of Chesapeake Bay. At low tide,
many small beaches with hard-packed sand
emerge suitable for placement of a gravity meter.
Sand bars and oyster shell bars also emerge at
low water to provide additional measurement
sites.

In the inner basin part of the gravity map, a
30 km x 35 km gravity low, with significant
gradients on three sides, generally coincides with
the inner crater rim as interpreted by Powars and
Bruce (1999). The missing gradient on the SE
edge of the low makes an asymmetric feature
and may reflect density distributions existing
before the impact. The low may reflect a
reduction in density resulting from mixing of the
basement impact debris with lower density
sedimentary rock debris and seawater. The
central area of the low is lumpy with four
small positive gravity anomalies having 1-2
milligals amplitude. The largest of these occurs
at Cape Charles, VA and coincides with the
inferred central peak (Poag, and others, 1999).
These anomalies may arise from lateral density
variations within a layer or from undulations in a
surface separating two layers of differing
density.

Fig. 1. Gravity anomaly map of the Inner Chesapeake
Bay Crater. Bouguer anomaly is used on land and free
air anomaly over water. CC=Cape Charles, VA.
Contour interval=1 milligal. Red dots are the new
gravity stations; coastline=blue; red line=inner
impact crater rim from Powars and Bruce (1999).

Aeromagnetic Data. In contrast with the
gravity data, little correspondence between crater
structures and aeromagnetic contours is
immediately apparent. This may be, in part, due
to lack of resolution of the surveys. The map was
constructed from two regional aeromagnetic
surveys that nearly cover the lower Chesapeake
Bay area. The surveys were flown in 1972 and
1975. Widely spaced flight lines are 3.2 km (2
miles) apart over land and 4.8 km (3 miles) apart
over the ocean. The map (Fig. 2) shows the
aeromagnetic data in color-shaded relief with an
overlay of gravity contour lines. Some magnetic
anomalies show a close correspondence with the
gravity anomalies; the best example is in the
southwest corner of the map, where gravity and
magnetic lows result from the subsurface
Portsmouth granite pluton. The northern part of
the aeromagnetic map shows distinct northwest
trending linear anomalies. Two magnetic
anomalies in the southern half of the inner basin
also show a hint of northwest trends in general
agreement with trends in the gravity contours.
This suggests that part of the basement rocks
may be intact.

Fig. 2. Regional aeromagnetic anomaly map of lower
Chesapeake Bay. Aeromagnetic data are displayed as
color shaded-relief; contour overlay is gravity
anomaly. Red line=Impact crater rims;
blue=Coastline.

References Cited.
Dater, D., Metzger, D., and Hittleman, A., 1999,
Land and Marine Gravity CD-ROM's:
National Oceanic and Atmospheric
Administration, National Geophysical Data
Center, two CD-ROM discs.
Pilkington, M. and Grieve, R.A.F., 1992, The
geophysical signature of terrestrial impact
craters: Reviews of Geophysics, v. 30, no. 2,
p. 161-181.
Plescia, J.B., 1999, Gravity signature of the
Teague Ring impact structure, Western
Australia, in Dressler, B.O., and Sharpton,
V.L., eds., Large Meteorite Impacts and
Planetary Evolution II: Boulder Colorado,
Geological Society of America Special Paper
339, p. 165-175.
Poag, C.W., Hutchinson, D.R., Colman, S.M.,
and Lee, M.W., 1999, Seismic expression of
the Chesapeake Bay impact crater: Structural
and morphologic refinements based on new
seismic data, in Dressler, B.O., and Sharpton,
V.L., eds., Large Meteorite Impacts and
Planetary Evolution II: Boulder Colorado,
Geological Society of America Special Paper
339, p. 149-164.
Powars, D.S., and Bruce, T.S., 1999, The effects
of the Chesapeake Bay impact crater on the
geological framework and correlation of
hydro-geologic units of the Lower York-
James Peninsula, Virginia: U.S. Geological
Survey Professional Paper 1612, 82 p.

THREE THINGS TO KNOW ABOUT DAMAGED DINOCYSTS, CHESAPEAKE BAY IMPACT
STRUCTURE, VIRGINIA, USA. Lucy E. Edwards, U.S. Geological Survey, 926A National Center,
Reston, VA 20192

Fossil dinoflagellates in impact deposits show
impact damage.
Fossil dinoflagellates in impact deposits show
a wide variety of kinds of impact-related
damage.
Fossil dinoflagellates tell age. Because of the
pre-impact shelf setting, age can tell where --
both vertically and laterally -- an individual
particle originated.

CHESAPEAKE BAY IMPACT STRUCTURE - 35 MILLION YEARS AFTER - AND STILL
MAKING AN IMPACT. Scott R. Emry, P.G., Regional Senior Geologist; and Brian Miller, Graphics
Technician II, Hampton Roads Planning District Commission, 723 Woodlake Drive, Chesapeake, VA
23320, USA (semry@hrpdc.org).

Ground water plays an important role in the
economy and quality of life in the Coastal Plain
of Virginia. In 2000, the aquifers in the Coastal
Plain supplied over 100 million gallons of water
per day to the citizens, businesses, and industries
in Virginia (DEQ, 2001). Ground water is the
primary source of useable water in rural areas of
the Coastal Plain and increasingly is being used
to support a growing urban population. Ground
water levels in the multiple aquifer system are
declining due to the increased withdrawals. As a
result of the ground water level decline and the
potential for saltwater intrusion resulting from
the increased uses, the Commonwealth of
Virginia created two ground water management
areas. These two areas cover more than half of
the Coastal Plain Physiographic Province and
more than one-third of the states population.

Fig. 1. Ground Water Management areas in Virginia
and permitted ground water withdrawal locations.

The Hampton Roads area is situated in the
Eastern Virginia Ground Water Management
Area. The thirteen public water utilities in
Hampton Roads serve approximately 1.6 million
people. Due to the relatively low relief of the
Coastal Plain Physiographic Province and the
increased regulatory obstacles for obtaining a
permit to build new reservoirs, the potential for
identifying new surface water sources of water to
meet the needs of a growing population and
expanding economy, the role of ground water
resources in sustaining the Hampton Roads area
is more critical than ever.

Fig. 2.. Hampton Roads, the 31sr largest Metropolitan
Service Area in the United States.

A zone of salty ground water, referred to as the
inland saltwater wedge, is well known to
ground water resource planners and scientists,
but until recently the phenomenon has not been
satisfactorily explained (Sanford, 1913;
Cederstrom, 1943). In 1996, the Directors of
Utilities in Hampton Roads were introduced to
the most dramatic geological event that ever took
place in the Chesapeake Bay region of Virginia.
Geologists for the U.S. Geological Survey
(USGS) and the Virginia Department of
Environmental Quality (DEQ) provided evidence
of a meteor impact that formed a crater over 35
million years ago (Emry, 1999). The contours of
the inland saltwater wedge conform well to the
shape of the craters outer rim. Of the five
public water utilities in Hampton Roads that
currently withdraw and treat brackish ground
water, three are located within the inland
saltwater wedge. A new brackish ground water
development project within the wedge is
underway and another project is being proposed.

Fig. 3. Isocontours of inland saltwater wedge and
associated municipal brackish ground water systems
along crater boundary.

Prior to the discovery of the impact structure,
it was presumed that the ground water flow in
the Coastal Plain aquifer system was relatively
simple and consistent. The typical description of
the aquifer system was alternating layers of
aquifers and confining units gradually dipping
and thickening from the west to the east. With
the discovery of the impact crater, the rules have
changed. Ground water flow can no longer be
described in simple terms.

The 1996 briefing led to local government
financial support of the USGS research of the
impact structure due to implications to the water
utilities in Hampton Roads. The cooperative
effort became a part of the Hampton Roads
Planning District Commissions (HRPDC)
comprehensive Regional Water Program. Phase
I was entitled The effects of the Chesapeake Bay
Impact Crater on the geologic framework and
correlation of hydrogeologic units of the Lower
York James Peninsula, Virginia (Powars and
Bruce, 1999). Phase II was entitled The effects
of the Chesapeake Bay impact crater on the
geologic framework and the correlation of
hydrogeologic units of Southeastern Virginia,
south of the James River (Powars, 2000). Phase
III entails study of deep coreholes within the rim
of the crater and is scheduled for completion in
2004.

The USGS, HRPDC and DEQ are also
working together to replace an aging ground
water flow model with a new regional ground
water model using the recent knowledge of the
Chesapeake Bay Impact Structures effect on
ground water flow. Better modeling tools and a
clearer understanding of how the Chesapeake
Bay Impact Structure affects ground water
movement and quality will assist water resource
planners and scientists estimate more accurately
the long-term sustainability of the ground water
resources of the Coastal Plain aquifer system.

References Cited:
Cederstom, D.J., 1943, Chloride Water in the
Coastal Plain of Virginia: Virginia Geological
Survey, Bulletin 58, 36 p.
DEQ, 2001, Water Use Database Report:
Virginia Department of Environmental
Quality, Richmond, Virginia, public computer
media.
Emry, S.R., 1999, How Important is Ground
Water: Hampton Roads Planning District
Commission Environmental Planning Special
Report, Issue 2, p.1.
http://www.hrpdc.org/publications/specialrepo
rts/groundwater.pdf
Powars, D.S., and Bruce, T.S., 1999, The effects
of the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the Lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p.
http://geology.er.usgs.gov/eespteam/crater/we
brepts.html
Powars, D.S., 2000, The effects of the
Chesapeake Bay impact crater on the geologic
framework and the correlation of
hydrogeologic units of Southeastern Virginia,
south of the James River: U.S. Geological
Survey Professional Paper 1622, 53 p.
http://geology.er.usgs.gov/eespteam/crater/we
brepts.html
Sanford, S., 1913, Underground Water
Resources of the Coastal Plain of Virginia:
Virginia Geological Survey Bulletin No. V,
361 p.

DISTAL IMPACT EJECTA FROM THE CHESAPEAKE BAY IMPACT CRATER.
Billy P. Glass, Geology Department, University of Delaware, Newark, DE 19716, USA (bglass@udel.edu).

Introduction. North American microtektites have
been found on Barbados and in sediment cores from the
Gulf of Mexico, Caribbean Sea, and western North
Atlantic Ocean (Fig. 1) (e.g., Sanfilippo and others,
1985; Glass and others, 1998). Shocked
quartz and coesite have been found at most of these
sites (e.g.,Glass and Wu, 1993; Glass and others,
1998). On Barbados and sites in the Caribbean Sea
and Gulf of Mexico there is no discrete layer, but the
microtektites and shocked grains are found scattered
through 20 cm or more of sediment. At two of the sites
off New Jersey there is a discrete ejecta layer of tektite
glass and shock metamorphosed grains (Sites 612 and
904). At two other sites the ejecta layer consists of
shocked grains, but with no glass (Sites 903 and 1073).
At the sites off New Jersey, the ejecta layer contains
shocked quartz and feldspar with multiple sets of
planar deformation features and large, up to millimeter
size grains, containing mixtures of quartz and coesite.
Fine-grained >rock= fragments are also present. They
contain mica, coesite, quartz, stishovite, garnet, Kfeldspar,
and Na-rich feldspar in approximate
decreasing order of abundance (Fernandes, 1993).
These >rock= fragments may be shock-lithified sediment
(i.e., instant rock).

Figure 1. Map of the North American tektite strewn
field showing North American tektite locations and
sites containing North American microtektites and/or
unmelted impact ejecta believed to be from the
Chesapeake Bay structure.

The ejecta layer at the sites off New Jersey contains
a similar suite of heavy minerals including zircon
(Glass and others, 1998). Most of the zircons exhibit
evidence of shock metamorphism including granular
textures, planar features, and X-ray asterism. None
were found to have broken down to baddeleyite plus
silica, but some of the more heavily shocked zircons
have been partially to almost completely converted into
a high-pressure ZrSiO4 polymorph with a scheelite-like
structure. This phase had earlier been produced in
high-pressure laboratory studies by Reid and Ringwood
(1969). This is the first time that this phase has been
found in naturally-occurring samples. It has been
named reidite after Alan F. Reid who first produced
this phase in the laboratory (Glass and others, 2002).
The ejecta layer is upper Eocene in age and the
tektite fragments have an 40Ar/39Ar age of ~35 Ma
(Obradovich and others, 1989). These ejecta,
including the North American tektites, are believed to
be derived from the Chesapeake Bay structure based
primarily on geographic location and age (Poag and
others, 1994; Koeberl and others, 1996).

Geographic Variations in Ejecta. Assuming that
the North American microtektites and associated ejecta
are from the Chesapeake Bay impact crater, some
general statements can be made about variations in the
ejecta with distance from the source crater. As would
be expected, the number of microtektites per unit area
(or thickness of the ejecta layer) decreases away from
the source crater. The percent glass that is in the form
of fragments, rather than whole splash forms, increases
towards the source crater. The percent glass
(microtektites and tektite fragments) increases away
from the crater, but there are some exceptions, which
may be the result of a ray-like distribution of the glassy
portion of the ejecta. Ejecta have been found in four
sites NE of the Chesapeake Bay crater: Sites 612, 903,
904, and 1073 (Fig. 1). Site 612 is the closest to the
Chesapeake Bay crater. Sites 904 and 903 are ~4 km
NW and ~13 km NNW of Site 612, respectively. Site
1073 is ~62 km NNE of Site 612. The ejecta layer is
thickest at Site 612 (~8 cm) and thins towards Site 904
(~5 cm) and from Site 904 towards Site 903 (~2 cm).
The decrease is due primarily to a decrease in percent
glass in the ejecta layer. The thickness of the layer at
Site 1073 is not known, but there is not a discrete layer
at this site. Like Site 903, there is no glass present.
Thus, Sites 612 and 904 may lie along a glass-bearing
ray and Sites 903 and 1073 are NW of the ray.

Crater Size. Previous workers have derived
equations that relate the thickness of an ejecta blanket
to size of, and distance from, the source crater (e.g.,
Stffler and others, 1975). These equations can be
rearranged and used to calculate the size of the source
crater based on the thickness of the ejecta layer and
estimated distance from the source crater. This method
was used to calculate the size of the source crater for
the Ivory Coast microtektite layer assuming that the
source crater is located at the Bosumtwi crater site.
The size estimated by this method is close to the actual
crater size (Glass and Pizzuto, 1994). The thickness of
the North American microtektite/ejecta layer and
distance from the Chesapeake Bay crater were used to
calculate the crater size (Table 1). The estimated crater
diameter is 46 " 18 km. This is a little more than half the
reported crater size of 85 km (Poag and others, 1994).
Thus, the Chesapeake Bay crater may not be the source
crater for the North American microtektite/ejecta layer,
or the reported crater size is too large, or the equations
do not work when the impact occurs in water.

Table 1. Estimated Crater Size
SITE	 Dist. From CB Crater (km)	Layer Thickness (cm)	Crater Diameter (km)*
612 	322 	8 	36
904 	334 	5 	33
903 	335 	2 	27
94 	1888 	0.11 	52
RC9-58 	2569	 0.16 	77
BARBADOS 	3145	 0.02 	50
Average 46  18
*Calculated from t = 0.06 R (r/R) 3.3 (Stffler and
others, 1975)

Acknowledgments. I thank the Ocean Drilling
Program for samples used in this study. This research
was supported by NSF Grant EAR-9903811.

References Cited.
Fernandes, A., 1993, An investigation of impact ejecta
in upper Eocene sediments on the continental slope
off New Jersey: M. S. Thesis, University of
Delaware, Newark, Delaware, U.S.A., 141 p.
Glass, B.P. and Wu, J., 1993, Coesite and shocked
quartz discovered in the Australasian and North
American microtektite layers: Geology, v. 21, p.
435-438.
Glass, B.P., Koeberl C., Blum J.B., and McHugh
C.M.G., 1998, Upper Eocene tektite and impact
ejecta layer on the continental slope off New Jersey.
Meteoritics and Planetary Science, v. 33, p. 229-
242.
Glass, B.P., Liu, S., and Leavens, P.B., 2002, Reidite:
An impact-produced high-pressure polymorph of
zircon found in marine sediments: American
Mineralogist, v. 87, p. 562-565.
Koeberl, C., Poag, C.W., Reimold, W.U., and Brandt,
D., 1996, Impact origin for the Chesapeake Bay
structure and the source of the North American
tektites: Science, v. 271, p. 1263-1266.
Obradovich, J.D., Snee, L.W., and Izett, G.A., 1989, Is
there more than one glassy impact layer in the Late
Eocene?: Geological Society of America Annual
Meeting, Abstracts with Programs, v. 21, no. 6, p.
A134.
Poag, C.W., Powars, D.S., Poppe, L.J., and Mixon,
R.B., 1994, Meteoroid mayhem in Ole Virginny:
Source of the North American tektite strewn field:
Geology, v. 22, p. 93-104.
Reid, A.F. and Ringwood, A.E., 1969, Newly observed
high pressure transformation in Mn3O4, CaAl2O4,
ZrSiO4: Earth and Planetary Science Letters, v. 6,
p. 205-208.
Sanfilippo, A., Reidel, W.R., Glass, B.P., and Kyte,
F.T., 1985, Late Eocene microtektites and
radiolarian extinctions on Barbados: Nature, v. 314,
p. 613-615.
Stffler, D., Gault, D.E., Wedekind, J., and Polkowski,
G., 1975, Experimental hypervelocity impact into
quartz sand: Distribution and shock metamorphism
of ejecta: Journal of Geophysical Research, v. 80, p.
4062-4077.

POSSIBILITY OF OCEAN WATER INVASION INTO THE CHICXULUB CRATER AT THE
CRETACEOUS/TERTIARY BOUNDARY. Kazuhisa Goto1, Ryuji Tada1, Eiichi Tajika1, Timothy J. Bralower2,
Takashi Hasegawa3, Takafumi Matsui4, 
1Department of Earth and Planetary Sciences, The University of Tokyo (goto@eps.s.u-tokyo.ac.jp); 
2Department of Geosciences, The Pennsylvania State University;
 3Department of Earth Sciences, Kanazawa University; 
4Graduate School of Frontier Sciences, The University of Tokyo.

Introduction. The movement of water rushing into
and receding from the Chicxulub crater is considered
to have a potential to generate the largest tsunamis
caused by the impact at the Cretaceous/Tertiary (K/T)
boundary (Matsui and others, 2002). However, no
strong evidence for operation of this mechanism has
been presented. In this study, samples of the suevite
from the YAX-1 site drilled by the Chicxulub Scientific
Drilling Program (CSDP), were investigated to
test the possibility of the ocean water invasion into the
crater and consequent generation of tsunamis immediately
after the impact.

Lithology of the YAX-1 core. The YAX-1 core is
located approximately 65 km to the south of the center
of the Chicxulub crater, on the southern slope the inside
the crater. The impactite occurred within the interval
between 794.63 m and 894.94 m depth, and is
divided into two lithological units; the Impact Melt
Breccia Unit (Melt breccia to Polymict melt breccia :
822.86 m to 894.94 m) and the Suevite Unit (Redeposited
Suevite to Suevite: 794.63 m to 822.86 m).
The Suevite Unit is subdivided into the Lower
(Suevite) and Upper (Redeposited Suevite) Subunits
based on the grain size of melt fragments.
The Suevite Unit overlies the Impact Melt Breccia
Unit with an irregular contact. The Lower Subunit
is approximately 15 m thick, poorly-sorted, and shows
grain-supported fabric. This subunit is composed of
two normally graded beds. This subunit is composed
of pebble- to cobble-sized, subangular to rounded,
green, black, dark red and brown melt fragments with
small amount of limestone, dolostone and basement
rock fragments. Large intraclast-like grayish melt
fragments of up to 10 cm in diameter occur only in the
basal part of this subunit.

Two types of greenish melt fragments are recognized
in this subunit: one is vesicular melts, which are
common constituent of the underlying impact melt
breccia unit, and the other is melt-coated recrystallized
calcite, which are similar to the cored inclusion type
melt that is interpreted as fall-back ejecta (French,
1998). All melt fragments are totally altered and is
replaced by clay minerals. Matrix in the suevite is
composed of minute carbonate grains, coccolith and
partly recrystallized calcite. Poor sorting, grainsupported
fabric, and presence of intraclast-like large
fragments imply that this subunit is a gravity flow deposit.
The Upper Subunit is approximately 13 m thick,
relatively well-sorted, and shows repetition of grainsupported
and matrix-supported fabric. Composition of
the Upper Subunit is almost the same as of the Lower
Subunit but the grain size is much finer (medium sand
to pebble size), and the cored inclusion type melt
becomes abundant. Cross lamination is observed in the
uppermost several tens centimeter interval of the Upper
Subunit, suggesting the influence of strong current.

Possibility of ocean water invasion. Presence of
mono-directional cross lamination in the uppermost
part of the Upper Subunit suggests the influence of
currents possibly generated by ocean water invasion at
least during the deposition of this interval.
Reworked nannofossils of late Campanian to
early Maastrichtian age are abundant in the matrix of
the Suevite Unit. It is interesting to note that this age
range is considerably narrower than the range of the
sediments excavated by crater formation. This suggests
that nannofossils of late Campanian to early Maastrichtian
age were selectively incorporated in the
Suevite Unit. Carbonate sediments including late
Campanian to early Maastrichtian age deposited in the
area of the later crater formation, should have been
ejected out and/or vaporized during the crater formation
by the impact. Consequently, these reworked
nannofossils should have been derived either from the
unconsolidated sediments deposited outside the crater
or exposed on the rim wall, or the ejecta curtain deposits
deposited around the crater.

According to the study of deep-sea impact crater
in Sweden, resurge deposit formed by the ocean invasion
immediately after the impact was found in the
upper part of the impactite (e. g., Orm & Lindstrm,
2000). These resurge deposit tends to show the reversed
chronology (Orm & Lindstrm, 2000).
Namely, the lower part of the deposit consists of the
sediment with younger age fossils and upper part consists
of sediment with older age fossils (Orm, 1994).
In case of the suevite in YAX-1 core, nannofossil assemblage
of the major part of the Suevite Unit gives
late Campanian to early Maastrichtian age, whereas
latest Maastrichtian nannofossil occurs only in the
basal part of the Lower Subunit. Consequently, in
analogy with the sedimentary process of the resurge
deposits in the deep-sea impact crater in Sweden, most
probable mechanism Suevite unit deposition is the
ocean water invasion.

Sedimentary process of the Upper Subunit. The
maximum size of limestone lithics in the Upper Subunit
shows oscillatory variation representing repetition
of normal grading, whereas the maximum size of
greenish vesicular melt fragments shows inverse grading
in the lower 3 cycles and normal grading in the
upper five cycles. Inverse grading of porous grains
such as pumice fragments is formed by the time lag of
sedimentation between large and less dense, and small
and more dense pumice fragments (Fisher and
Schmincke, 1984). Considering the porous nature of
greenish vesicular melt fragments in the YAX-1 core
that is similar to the pumice grains, formation of inverse
grading of greenish vesicular melt fragments in
cycles 1 to 3 can be explained by the similar mechanism.
On the other hand, normal grading of greenish
vesicular melt fragments in the cycles 4 to 8 could be
explained by water infiltration into pores of the large
greenish vesicular melt fragments and consequent increase
in their bulk densities. The switch from reverse
to normal grading of greenish melt fragments between
cycles 3 and 4 indicate that water infiltrated into the
pores of large greenish melt fragments and increase in
their bulk densities enough to settle earlier than small
fragments between sedimentation of cycle 3. Oscillations
in the bulk chemical composition is also recognized
in association with cycles 1 to 8. Oscillations in
bulk chemical composition are regarded as representing
variation in the ratio of the two components; one is
characterized by higher content of Ca and the other is
characterized by higher contents of Si, Fe, Ti and Al.
The upward fining character of limestone lithics
and upward changes from grain-supported fabric to
matrix-supported fabric in each cycle in the Upper
Subunit are consistent with the characters of a gravity
flow (e. g., Midlleton and Hampton, 1976). Possible
trigger was repeated ocean water invasions and/or impact
seismic waves, because influence of these processes
could have lasted for a few days after the impact
and could have enough magnitude to generate gravity
flows (Matsui and others, 2002; Boslough and others,
1996). Alternatively, similar lithological features can
be formed by reworking of crater-filled sediments by
repeated tsunamis.

Conclusion. 1) Presence of cross lamination in
the uppermost part of the Suevite Unit suggests the
influence of strong current. Abundant occurrence of
nannofossils of late Campanian to early Maastrichtian
age in the matrix of the Suevite Unit may suggest that
the carbonate sediments deposited on the inner rim
margin and outside around the crater were eroded and
transported into the crater by the ocean water invasion.
2) The maximum grain size of limestone lithics and
vesicular melt fragments as well as grain and bulk
chemical compositions varied in cyclic manner by
more than 8 times in the upper part of the Suevite Unit.
The upward fining in grain size and upward changes in
fabric from grain-supported to matrix-supported in
each cycle are consistent with their gravity flow origin
that was triggered either by repeated ocean water invasion
or by impact seismic wave. Alternatively, similar
lithological features can be formed by reworking of
crater-filled sediments by repeated tsunamis.

References Cited.
Boslough, M. B., Cheal, E. P., Trucano, T. G., Crawford,
D. A. and Campbell, D. I., 1996, Axial focusing
of impact energy in the earths interior: a possible
link to flood basalts and hotspots, in Ryder, G.,
Fastovsky, D., and Gartner, S., eds., Cretaceous-
Tertiary event and other catastrophes in earth history,
Geological Society of America Special Paper v. 307,
p. 541-550.
Fisher, R. V., and Schmincke, H. U., 1984, Pyroclastic
Rocks. Berlin Heidelberg: Springer-Verlag. 472p.
French, B. M. 1998. Traces of Catastrophe: A handbook
of shock-metamorphic effects in terrestrial meteorite
impact structures, LPI Contribution No. 954.
Houston: Lunar and Planetary Institute. 120p.
Matsui, T., Imamura, F., Tajika, E., Nakano, Y. and
Fujisawa, Y., 2002, Generation and propagation of a
tsunami from the Cretaceous/Tertiary impact event,
in Koeberl, C., and Macleod, G., eds., Catastrophic
Events and Mass Extinctions: Impact and Beyond,
Geological Society of America Special Paper v. 356,
p. 69-77.
Middleton, G. V., and Hampton, M. A., 1976,
Subaqueous sediment transport and deposition by
sediment gravity flows, in: Marine Sediment Transport
and Environmental Management, edited by
Stanley D. J. and Swift D. J. P. Wiley, New York, p.
197-218.
Orm, J., 1994, The pre-impact Ordovician stratigraphy
of the Tvren Bay impact structure, SE Sweden.
GFF v. 116, p. 139-144.
Orm, J., & Lindstrm, M., 2000, When a cosmic impact
strikes the sea bed. The Geological Magazine v.
137, p. 67-80.

PROPOSED SEISMIC IMAGING AND NUMERICAL MODELING OF THE 35 MA IMPACT EVENT AT
CHESAPEAKE BAY
John A. Hole1, Susan McGeary2, H. Jay Melosh3, and Susan C. Eriksson1, 
1Department of Geosciences, MC 0420, Virginia Tech, Blacksburg, VA 24061-0420, USA (hole@vt.edu); 
2Department of Geology, University of Delaware, Newark, DE 19716, USA; 
3Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.

This proposal addresses the important geologic
question of how large impact craters are formed
through a case study of the 85-km wide Chesapeake
Bay, Virginia, impact crater. Fifty years of study of
the impact process have failed to produce a
predictive, quantitative model of large impact crater
formation. A primary reason is the scarcity of
subsurface structural and rheologic information  the
third dimension. This information is largely
unavailable from other planets, and terrestrial impact
craters are rapidly changed by erosion, viscous
relaxation, metamorphism, and tectonism. Recent
work on the K/T-boundary Chicxulub crater in
Mexico has shown how deep seismic reflection and
seismic velocity images can vastly improve our
knowledge of the architecture of a large crater and
can prompt improved numerical modeling of the
impact process. We propose seismic velocity and
reflection imaging of the Chesapeake Bay impact
structure and numerical modeling based upon the
seismic images. Our primary goal is to investigate
the mechanics of and rheologic controls on impact
crater formation.

Chesapeake Impact Structure. After Chicxulub,
the Chesapeake Bay impact structure is the largest
pristinely preserved impact crater on Earth. The
impact occurred 35 Ma in what is now the coastal
plain of Virginia (Poag and others, 1994; 1999; Poag,
1997). At the time of impact, Late Proterozoic to
Paleozoic crystalline rocks at the target site were
covered by up to a kilometer of late Jurassic to
Eocene unconsolidated, mostly clastic sediments and
several hundred meters of seawater. The impact and
collapse crater was mostly filled by an impacttriggered
tsunami breccia. Subsequent burial in a
passive margin setting by up to 500 m of clastic
sediment provides excellent preservation. Excellent
stratigraphic dating is available for a direct tie to
global climate and paleontologic records.

The Chesapeake Bay impact structure has an
unusual inverted-sombrero morphology (Poag and
others, 1994, 1999). The crater consists of a ~38-km
diameter inner crater that disrupts basement inside a
~85-km diameter shallower crater that only disrupts
the overlying sediments. The outer crater wall is
marked by a slump fault with several hundred meters
of relief. The overlying tsunami breccia thickens
towards the inner crater to replace the entire preimpact
sedimentary sequence. A strong seismic
reflection marking crystalline basement is intact
beneath the outer crater, is disrupted upwards at the
margins of the inner crater, and is missing within the
inner crater. Depth to crystalline rocks (disrupted
basement or impact melt) within the inner crater is
not well constrained, but is much deeper than
surrounding basement.

The unusual crater morphology is presumably due
to impact on a layer of weak unconsolidated
sediments over a hard crystalline target. This
provides an unparalleled opportunity to study the
effects of target rheology upon crater formation.

Impact Processes. Impact craters are formed by
excavation and subsequent collapse (Melosh, 1989;
Melosh and Ivanov, 1999). High-speed bolide
impact initially creates a bowl-shaped transient crater
by explosive compression and excavation. The
transient crater has a diameter:depth ratio of 3:1 to
4:1, and its size is related to impact energy.
Subsequent modification by gravity-driven central
uplift and marginal collapse creates a broader and
shallower crater. The final crater is 1.5 to 2.0 times
wider than the transient crater, but much shallower.
The target material beneath the crater floor must have
been significantly but temporarily weakened to create
observed crater collapse structures. Numerical
models exist that fairly well explain the
hydrodynamics of the compression and excavation
stages. However, the collapse stage is more
dependent upon target rheology and crater size, and is
not as well understood.

We propose three hypotheses to explain the
inverted-sombrero morphology of the Chesapeake
Bay impact structure. In Hypothesis A, the 38-km
wide inner (crystalline) crater represents the transient
(explosive) crater and the full 85-km crater is the
resulting collapse structure. This implies that the
unconsolidated sediments are similar in strength
during collapse to the impact-disrupted crystalline
basement. In Hypothesis C, the transient crater is
much smaller (~20 km) and the inner crater is the
collapse crater. The larger outer crater was created
by tsunami scour, not collapse of the transient crater.
This implies that the sediment is much weaker than
the impact-disrupted crystalline basement and plays
little role in crater formation. Hypothesis B lies
between the above end-members, where the transient
crater was 25-32 km wide and both inner and outer
crater were created by collapse.

Differentiation between these models requires
improved knowledge of the deep structure of the
crater, and in particular of the inner, which is poorly
constrained at this time. Subsequent numerical
modeling can determine the relative contributions of
unconsolidated sediments and impact-disrupted
crystalline rocks on the collapse process, thus
quantifying the rheology of impact collapse.

ICDP Drilling. A team led by Greg Gohn (U. S.
Geological Survey, Reston) is currently preparing a
proposal to the International Continental Drilling
Program (ICDP) to drill a deep borehole into the
inner crater at Chesapeake Bay. This follows from a
decade of USGS-led studies of the crater, including
several recent holes drilled into the crater margin
(Powers and Gohn, 2003). The primary goal of the
ICDP proposal will be to understand the processes
and products of a marine impact through direct
sampling of the target rocks, melt lens or melt breccia
(if any), tsunami breccia containing impact-derived
clasts, and overlying stratigraphy.

In September 2003, the USGS hosted an ICDPfunded
workshop to develop a full ICDP proposal.
One result of the workshop was strong consensus that
the proposed drill site must be characterized by
seismic imaging prior to drilling. Participants
strongly endorsed the concept that such imaging is
required to properly site the drill hole and to properly
interpret the drilling results in terms of local and
regional geology. Currently, the inner crater is
poorly mapped, as existing seismic data either did not
penetrate to the base of the Cenozoic sediments or
were cut off at basement prior to release from
industry. For example, first-order information such
as the depth to melt and/or target rocks in the inner
crater is poorly known. In addition, there are no deep
data on land, where the hole will be located.

Proposed Work. We propose an integrated study
of the deep structure and rheology of the Chesapeake
Bay impact crater. Our primary goal is a better
understanding of the impact process. Our second
goal is support of the USGS-ICDP drilling effort
through site characterization and local and regional
geologic context. Our proposal consists of three
components: seismic imaging, numerical modeling,
and education outreach.

We propose to illuminate the impact crater
through seismic reflection and velocity imaging. A
deep, but high-frequency, seismic reflection line
across a diameter of the crater on land will map
structures within and beneath the sediments, focusing
on the inner crater. A cross line (or lines) will be
acquired at the proposed drill site(s). Coincident
seismic refraction lines will map lithology through
seismic velocity. Sub-surface models will be derived
from integrated reflectivity structure, seismic
velocity, gravity, and drilling contraints. The targets
are to identify collapse faulting, pervasive fracturing
of crystalline rocks, melt production, transient crater
size, and large-scale crater morphology such as a
peak ring and central peak.

We propose numerical modeling of the impact
process, directly incorporating constraints derived
from the seismic data. The models will include a
layered rheology  water over sediment over
crystalline rocks  to reproduce the observed
inverted-sombrero morphology. The results will
better quantify the rheology of impact-disrupted
rocks and will provide new constraints on marine
impacts. In addition, the models will quantify the
effects on global climate through vaporization of
target rocks.

Finally, we propose an education and outreach
effort aimed at secondary-school teachers and the
local community. Teachers will participate in the
field data acquisition and related outreach lectures.
Teachers will then partner with graduate students
involved in data analysis and interpretation. They
will produce exercises that can be used in the
classroom. Web pages and a follow-up workshop
will disseminate the scientific results and classroom
materials.

It is anticipated that this multi-disciplinary
investigation of the Chesapeake Bay structure will
greatly improve current understanding of the
processes involved in marine impacts and in the
formation of large impact craters.

References Cited.
Melosh, H. J., 1989, Impact Cratering: A Geologic
Process: Oxford University Press, 245 p.
Melosh, H. J., and Ivanov, B. A., 1999, Impact crater
collapse: Ann. Rev. Earth Planet. Sci., v. 27, p.
385-415.
Poag, C. W., 1997, The Chesapeake Bay bolide
impact: A convulsive event in Atlantic Coastal
Plain evolution: Sedimentary Geol., v. 108, p.
45-90.
Poag, C. W., Hutchinson, D. R., Colman, S. M., and
Lee, M. W., 1999, Seismic expression of the
Chesapeake Bay impact crater: Structural and
morphologic refinements based on new seismic
data: Geol. Soc. Amer. Special Paper 339, p.
149-164.
Poag, C. W., Powars, D. S., Poppe, L. J., and Mixon,
R. B., 1994, Meteoroid mayhem in ole Virginny:
Source of the North American tektite strewn field:
Geology, v. 22, p. 691-694.
Powars, D. S., and Gohn, G. S., 2003,
http://geology.er.usgs.gov/eespteam/crater/

PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF CRYSTALLINE BASEMENT AND
IMPACT-DERIVED CLASTS FROM COREHOLES IN THE WESTERN ANNULAR TROUGH,
CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA, USA. J. Wright Horton, Jr.1, Michael J. Kunk2,
Charles W. Naeser1, Nancy D. Naeser1, John N. Aleinikoff2, and Glen A. Izett3, 
1U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA (whorton@usgs.gov); 
2U.S. Geological Survey, MS 963, Denver Federal Center, Denver, CO 80225, USA; 
3Dept. of Geology, College of William and Mary and Emeritus, USGS, 3012 East Whittaker Close, Williamsburg, VA 23285, USA.

Introduction. The Chesapeake Bay impact
structure in the Atlantic Coastal Plain of Virginia is
an upper Eocene complex crater formed in a
continental-shelf environment. The target materials
consisted of seawater (~300 m) underlain by lower
Tertiary and Cretaceous, poorly consolidated, watersaturated
sediments (400 to >750 m) and crystalline
basement rocks. An excavated central crater (30-38
km diameter) is surrounded by a flat-floored annular
trough that has an outer margin (80-95 km diameter)
of collapsed fault blocks. These features are
surrounded by concentric faults and underlie 150-400
m of postimpact sediments (Powars and Bruce, 1999;
Powars, 2000; Horton and others, 2003). Impactdisrupted
sediments in the annular trough were
scoured and covered by seawater-resurge deposits of
the Exmore beds (Gohn and others, 2002). Recent
drill cores from the western annular trough at
Bayside, Va. (728.5 m deep), the USGS-NASA
Langley site at Hampton, Va. (635.1 m deep), and
North, Va. (435.1 m deep), and a core from the outer
rim at Watkins School in Newport News (300.3 m
deep), are 8, 19, 24, and 27 km, respectively, outside
the central crater. All four cores penetrated the
Exmore beds. Only the Bayside and USGS-NASA
Langley cores sampled complete postimpact and
crater sections that include crystalline basement.

Basement Granites and U-Pb Zircon Dates.
Crystalline basement below the sedimentary section
has been recovered in the USGS-NASA Langley core
from 626.3 m (top) to 635.1 m (total depth) and in the
Bayside core from 708.9 m (top) to 728.5 m (total
depth). Basement rocks in both cores are pale red,
medium-grained monzogranites that are non-foliated
and pervasively chloritized (Horton, Aleinikoff, and
others, 2002). Chlorite is the principal mafic mineral
in the granites, and at Bayside traces of relict biotite
remain. The top of the granite in each core is
weathered but not saprolitized. Features diagnostic
of shock metamorphism were not found in the
granites. Preliminary SHRIMP U-Pb zircon ages (
2?) for the basement granites indicate
Neoproterozoic crystallization (625  11 Ma at
Bayside and 612  10 Ma at Langley). These dated
granites resemble Neoproterozoic granites of peri-
Gondwanan volcanic-arc terranes to the south
(Horton, Aleinikoff, and others, 2002). This
similarity raises doubts about the Chesapeake Bay
suture shown on earlier tectonic maps (Horton and
others, 1991, and references therein), and suggests an
unmapped suture between the terrane sampled in
these cores and Mesoproterozoic Laurentian
basement beneath the New Jersey Coastal Plain
(Horton, Aleinikoff, and others, 2002).

Shocked Quartz Grains. The Exmore beds
consist of mixed Lower Cretaceous to upper Eocene
sediment clasts (up to boulder size) and minor
crystalline-rock clasts in a matrix of glauconitic,
quartz-rich, muddy sand that contains Cretaceous,
Paleocene, and Eocene fossils (Edwards and Powars,
2003). Planar deformation features (up to 5
intersecting sets) characteristic of shock
metamorphism in quartz occur in rare grains or rock
fragments from the Exmore of all four cores,
confirming earlier evidence that the Exmore beds are
of impact origin (Horton, Aleinikoff, and others,
2002; Horton and others, 2003). The proportion of
shocked to unshocked quartz grains in the Exmore
matrix is very low, indicating that the shocked grains
are diluted by an enormous volume of other sediment
(Horton and others, 2003). This proportion is
consistent with the character of the Exmore beds as a
mixed sedimentary deposit that contains ejecta,
although a distinct ejecta blanket is not intact in the
cores.

Clasts of Impact-derived Crystalline Rock.
Most crystalline-rock clasts in the Exmore beds and
underlying sediments are rounded, detrital, and
essentially undeformed, but a few have angular
shapes and cataclastic fabrics. Shocked quartz is an
integral part of the cataclastic fabric in some clasts,
indicating that this fabric also was produced by the
impact event (Horton, Kunk, and others, 2002;
Horton and others, 2003). Crystalline rock fragments
interpreted to be ejecta include a variety of felsic to
mafic plutonic rocks and felsite (Horton, Kunk, and
others, 2002; Horton and others, 2003). In the
USGS-NASA Langley core, these fragments consist
of a single rock type (felsite) in contrast to more
diverse assemblages in other cores, indicating that
ejecta were distributed unevenly, perhaps in rays
(Horton and others, 2003). Microspherulitic matrix
in some felsite clasts is evidence of high-temperature
devitrification of either impact melt or older volcanic
rock (Horton and others, 2001, 2003). The largest
impact-derived rock fragments are in the lower part
of the Exmore beds, suggesting that, in some areas,
the Exmore is crudely size graded (Horton and
others, 2003). Shocked quartz in a felsite clast
several meters below the Exmore supports other
evidence for fluidization and injection of sediment of
the Exmore beds into underlying, impact-disrupted
material (Horton and others, 2003).

Rock Chemistry. Preliminary chemical analyses
of the Langley core indicate that the granite basement
rock is slightly peraluminous, and a felsite clast from
the Exmore is peraluminous rhyolite. Trace-element
discrimination diagrams indicate a volcanic-arc
origin for both rocks. The granite and rhyolite are
chemically distinct from North American tektites in
trace elements as well as major elements.

Argon Geochronology. Colorless mica in a
cataclastic leucogranite clast from the Exmore beds
(Bayside core) has 40Ar/39Ar age-spectrum data
consistent with Neoproterozoic U-Pb ages of the
other granites. The age spectrum shows no evidence
for Paleozoic metamorphism > about 350oC.
Potassium feldspar from that clast and from basement
granite (USGS-NASA Langley core) have 40Ar/39Ar
age spectra indicating temperatures > about 250oC
until about 346 Ma and 324 Ma, respectively, and
final cooling through feldspar closure (~150oC) at
about 255 Ma and 245 Ma, respectively, without
discernible impact heating (Horton, Kunk, and others,
2002).

Fission-track Dates and Thermal Modeling.
Fission-track dates ( 2?) of zircon (362  49 Ma and
332  44 Ma at Bayside, 375  44 Ma at Langley)
and apatite (239  79 Ma and 198  40 Ma at
Bayside, 184  32 Ma at Langley) from the basement
granites show no impact-related thermal disturbance
(Horton, Kunk, and others, 2002). Modeling of the
apatite fission-track data indicates that impact-related
heating at Langley, if any, probably did not exceed
100C (Horton, Aleinikoff, and others, 2002).

References Cited.
Edwards, L.E., and Powars, D.S., 2003, Impact
damage to dinocysts from the late Eocene
Chesapeake Bay event: Palaios, v. 49, no. 3, p.
275-285.
Gohn, G.S., Powars, D.S., Quick, J.E., Horton, J.W.,
Jr., and Catchings, R.D., 2002, Variation of
impact response with depth and lithology, outer
annular trough of the Chesapeake Bay impact
structure, Virginia Coastal Plain [abs.]:
Geological Society of America Abstracts with
Programs, v. 34, no. 6, p. 465.
http://gsa.confex.com/gsa/2002AM/finalprogram
/abstract_45483.htm
Horton, J.W., Jr., Aleinikoff, J.N., Izett, G.A.,
Naeser, C.W., and Naeser, N.D., 2001,
Crystalline rocks from the first corehole to
basement in the Chesapeake Bay impact
structure, Hampton, Virginia [abs.]: Geological
Society of America Abstracts with Programs, v.
33, no. 6, p. A-448.
Horton, J.W., Jr., Aleinikoff, J.N., Izett, G.A.,
Naeser, N.D., Naeser, C.W., and Kunk, M.J.,
2002, Crystalline basement and impact-derived
clasts from three coreholes in the Chesapeake
Bay impact structure, southeastern Virginia
[abs.]: EOS Transactions, American
Geophysical Union, v. 83, no. 19, Spring
Meeting Supplement, Abstract T21A-03, p.
S351.
Horton, J.W., Jr., Drake, A.A., Jr., Rankin, D.W., and
Dallmeyer, R.D., 1991, Preliminary
tectonostratigraphic terrane map of the central
and southern Appalachians: U.S. Geological
Survey Miscellaneous Investigations Series Map
I-2163, scale 1:2,000,000.
Horton, J.W., Jr., Gohn, G.S., Edwards, L.E., Self-
Trail, J.M., Powars, D.S., Kunk, M.J., and Izett,
G.A., 2003, Recent research in the Chesapeake
Bay impact crater, USAPart 2. Reworked
impact ejecta and impact debris [abs.]: Third
International Conference on Large Meteorite
Impacts, August 5-7, 2003, Noerdlingen,
Germany, Abstract 4051.
http://www.lpi.usra.edu/meetings/largeimpacts20
03/pdf/4051.pdf
Horton, J.W., Jr., Kunk, M.J., Naeser, C.W., Naeser,
N.D., Aleinikoff, J.N., and Izett, G.A., 2002,
Petrography, geochronology, and significance of
crystalline basement rocks and impact-derived
clasts in the Chesapeake Bay impact structure,
southeastern Virginia [abs.]: Geological Society
of America Abstracts with Programs, v. 34, no.
6, p. 466.
http://gsa.confex.com/gsa/2002AM/finalprogram
/abstract_44052.htm
Powars, D.S., 2000, The effects of the Chesapeake
Bay impact crater on the geologic framework
and the correlation of hydrogeologic units of
southeastern Virginia, south of the James River:
U.S. Geological Survey Professional Paper 1622,
53 p.
http://pubs.usgs.gov/prof/p1622/
Powars, D.S., and Bruce, T.S., 1999, The effects of
the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p.
http://pubs.usgs.gov/prof/p1612/

A STRUCTURAL COMPARISON BETWEEN THE CHESAPEAKE BAY IMPACT CRATER, USA, AND
THE RIES CRATER, GERMANY: HOW DID THE CENTRAL CRATER BASIN FORM?
T. Kenkmann1, Institut fr Mineralogie, Museum fr Naturkunde, Humboldt-Universitt zu Berlin, Invalidenstr. 43,
D-10115 Berlin, Germany, thomas.kenkmann@rz.hu-berlin.de

Ries and Chesapeake Baytwo inverted sombrero
craters. The Chesapeake Bay Impact Crater of
85 km diameter has a shape of an inverted sombrero. A
deep and highly disturbed central crater of 35 km diameter
which is formed in crystalline basement is surrounded
by a shallower annular trough in which
unconsolidated sediments have been removed and distorted
to a suite of megablocks. The crystalline basement
is not strongly deformed in this part of the crater.
The inner crater is surrounded by a peak ring, the
outer rim is defined by a 300-1000 m escarpment. The
floor of the inner crater basin is about 500-1000 m
deeper than the peak ring.

The smaller Ries crater, Germany (28 km diameter)
shows many similarities to the Chesapeake Bay crater.
At the Ries, the central crater basin has 12 km diameter
and is also formed in crystalline basement. The gravity
anomly pattern and the thickness of post impact lake
deposits clearly outline the deep central crater basin,
which is about 600 m deep, when subtracting the post
impact infill and the fallback suevite. This inner crater
basin is surrounded by a crystalline inner ring which
forms some 50 m hills in the landscape. Between the
inner ring and the crater rim there is a shallow annular
trough, called the megablock zone.

Kinematic considerations. The inverted sombrero
shape is probably a consequence of a sharp rheological competence
contrast between the crystalline basement and the
unconsolidated, water saturated sedimentary cover. However,
the formation of this type of multi-layer crater is insufficiently
understood with respect to dynamics and kinematics.
A key-question appears to be whether (Scenario A, Fig. 1)
the peak ring represents the remnant of a large collapsing
central uplift that flowed radially outward as a consequence
of a gravitational instability and which eventually produces a
large central crater basin or whether (Scenario B, Fig.1) the
transient cavity was only moderately modified and merely
produced the small central uplift that is presently documented
in the seismic sections. The latter case is in conflict
with crater scaling laws. Weak adjustment movements during
crater modification are expected for craters near the simpleto-
complex transition diameter, that is for crater of 3-5 km
diameter. The peak ring in scenario B may be interpreted as
the remnant of an overturned flap of the crystalline crater
cavity or a step in the crater profile and the inner crater basin
represents the moderately modified transient cavity. In this
scenario, the unconsolidated sedimentary cover was stripped
away from the crystalline basement during crater growth.
Due to the different mechanical response, the transient cavity
itself may had an inverted sombrero shape instead of the
commonly suggested parable-like shape. The solution which
of both scenarios is more realistic is of crucial importance to
determine energy and size of the impact. In scenario A the
crater, in fact, is 85 km in diameter, in scenario B the crater
size is about 35 km.

Suggestion for a borehole locality: A borehole
located along the rim of the inner crater which is penetrating
the crystalline peak ring is likely to resolve
the questions outlined above. If scenario A (Fig. 1)
holds, one should expect heavily deformed crystalline
basement of shock stage Ia and Ib recording pressures
of up to 25 GPa at the proposed locality. The analysis
of shear zone kinematics (oriented drilling is required)
would dominantly indicate a shallowly dipping and
outward directed flow. In scenario A a decrease in
shock metamorphism with increasing depth is suggested.
If scenario B holds, the bore hole would probably
drill through an overturned flap. This, in turn,
would mean that an inverted stratigraphy including
relics of the sedimentary cover and a weaker shock
metamorphic overprint is found. A bore hole located in
the center of the structure near Cape Charles or in the
moat is suited to investigate many scientific questions
such as the occurrence of a coherent melt rock sheet, a
complete section through the tsunami backwash deposits,
hydrothermal alteration, etc., but it is not a suited
locality to resolve the questions outlined above. A
compromise locality along the inner flank of the peak
ring/periphery of the central basin (Fig. 1) would
probably be able to solve both sets of scientific purposes.

Fig. 1. Sketch showing three potential time steps in the evolution of the crater. The main difference between both models regards the degree
of crater floor rebound after the formation of the transient cavity. The sketch is partly based on simulation results of unpublished numerical
models by B.A. Ivanov.

Structure-filling stratigraphy of the marine-target Wetumpka impact structure, Alabama, USA. D. T. King,
Jr.1, L. W. Petruny2, and T. L. Neathery3, 
1Department of Geology, Auburn University, Auburn, AL 36849, USA (kingdat@auburn.edu) , 
2Department of Curriculum and Teaching, Auburn University, Auburn, AL 36849, USAand Astra-Terra Research, Auburn, AL 36831-3323, USA, 
3Neathery and Associates, 1212-H Veterans Parkway, Tuscaloosa, AL 35404, USA.

Introduction. Wetumpka impact structure is a
deeply eroded, arcuate, 7.6-km diameter Late Cretaceous
feature located within the inner Coastal Plain,
Alabama (Fig. 1). This structure was produced by an
impact in shallow marine water estimate to have been
between 35 and 100 m deep (King and others, 2003).
Target stratigraphy included 120 m of unconsolidated
Upper Cretaceous sediment (including three formations)
and underlying pre-Cretaceous metamorphic
crystalline basement rock.

Wetumpka has distinctive exposed, geologic terrains
produced by impact-related processes. These
exposed terrains include Wetumpkas crystalline rim
(crt) and two sedimentary terrains: (a) an interior unit
(resurge unit, isu), and (b) adjacent extra-structure unit
(deformed unit, est) located outside the rim on the
structures southern side (King and others, 2003; Fig.
2).

Fig. 1. Location of Wetumpka impact crater.

Core drilling. Core drilling near the structures
geographic center revealed that Wetumpkas impactrelated
fill has two distinctive units: (1) an upper, resurge-
deposited unit (~ 100 m thick; same as the interior
unit above) and (2) a lower, structure-filling
breccia unit comprised of fall-back ejecta layers,
slumped target-rock blocks, and impact-related sandy
breccias and sands (> 130 m thick; same as breccia b
on Fig. 2; King and others, 2003).

Fig. 2. Geologic map of Wetumpka (from King and others,
2003). Units not noted in text: m = Mooreville Chalk in grabens;
pK = pre-Cretaceous metamorphic rocks.

Core drilling, which was accomplished using a
truck-mounted drilling rig capable of extracting successive
NX cores of ~ 3 m in length at depths of as
much as ~ 200 m, took place in July-August 1998.
Two boreholes, the Schroeder and Reeves wells, respectively
located at N 32o 31.368; W 086o 10.369
and N 32o 31.303; W 086o 10.379 (at * on Fig. 2;
separation distance = 122 m), penetrated nearly 200 m
of weakly consolidated impact-related materials.
Drill-site locations were selected primarily based
on two considerations: (1) we wanted proximity to the
presumed central region of maximum shock pressure
and (2) we needed site availability based on landowner
agreement to access. In addition, we reviewed
the limited available geophysical data, a single gravity
transect profile that showed considerable relief (10
mGal) within the structure and a central peak at the
center of the structure. The Schroeder and Reeves drill
sites were within a few 100 m of the central peak as
defined by the gravity profile. A substantial water
source was also a practical consideration, and both
selected sites were near a lake of size needed for continual
water extraction during drilling.

Both drill-site locations were within a few 100 m
of a local road-cut outcrop of breccia and were also
near exposures of two exotic blocks of target schist,
each measuring ~ 10-15 m across. The breccia in outcrop
(unit b, Fig. 2) was thought to be impact breccia
at the time of drilling, and was later confirmed to have
shocked quartz in its matrix (Nelson, 2000).

Stratigraphy. As mentioned previously, the upper
part of the structure-filling sequence drilled by us
is composed of an interior unit that is a broken formation
or impact mlange. By this, we mean the matrix
of the unit is composed of sediments derived from
target units mixed together, but blocks and megablocks
of intact target units are recognizable within the
body of the broken formation. This interior unit was
mapped originally according to the recognizable target
mega-blocks within it (Neathery and others, 1976), but
has been subsequently recognized as an impact mlange
and mapped as a contiguous unit of impact origin
(Nelson, 2000).

Fig. 3. Stratigraphy drilled at Wetumpka (from King and
others, 2003).

Below the interior unit (~ 60 m thick), lies an interval,
~ 140 m thick, composed of impact breccia layers
intercalated with impactite sands and sandy breccias.
This interval also contains target rock blocks up
to several m in diamter. The impact breccia is polymict,
whereas the impactite sands and sandy breccias
are monomict (but clearly of impact origin). Substantial
evidence of shocked quartz in the fine matrix of
impact breccia was identified from this rock type (see
references in King and others, 2003).

Fig. 4. Inferred cross-section of Wetumpka (from King and
others, 2003).

Stratigraphic Analysis. Using a simple statistical
technique (upward facies transition analysis; see
Selley, 1970), the events in the ideal vertical sequence
of impact breccias, impactite sands and sandy breccias,
and target rock blocks have been interpreted as (1) fall
back of ejecta; (2) slump and related comminution of
target-rock blocks (derived from an unstable rim morphology
(?); and (3) centrifugal fluid flows within the
structure (see King and others, 2003).

References.
King, D.T., Jr., Neathery, T.L., and Petruny, L.W.,
2003, Crater-filling sediments of the Wetumpka
marine-target impact crater (Alabama, USA), in
Dypvik, H., Burchell, M.J. and Claeys, P., eds.,
Submarine craters and ejecta-crater correlation:
Berlin, Springer-Verlag.
Neathery, T.L., Bentley, R.D., and Lines, G.C., 1976,
Cryptoexplosive structure near Wetumpka, Alabama:
Geological Society of America Bulletin, v.
87, p. 567-573.
Nelson, A.I., 2000, Geological mapping of Wetumpka
impact crater area, Elmore County, Alabama [unpublished
M.S. thesis]. Auburn University, Auburn,
Alabama, 187p.
Selley, R.C., 1970, Studies of sequence in sediments
using a simple mathematical device: Geological
Society of London Quarterly Journal, v. 125, p.
557-581.

DISTRIBUTION, ORIGIN, AND RESOURCE-MANAGEMENT IMPLICATIONS OF GROUND-WATER
SALINITY ALONG THE WESTERN MARGIN OF THE CHESAPEAKE BAY IMPACT STRUCTURE.
E. Randolph McFarland1 and T. Scott Bruce2, 
1U.S. Geological Survey, 1730 East Parham Road, Richmond, VA 23228, USA (ermcfarl@usgs.gov), 
2Virginia Department of Environmental Quality, 629 East Main Street, Richmond, VA 23219, USA

Distribution. Stratified unconsolidated sediments that
comprise a regionally extensive system of aquifers and
confining units across the Coastal Plain of Virginia were
severely disrupted within the Chesapeake Bay impact
structure and contain salt water approximately 50 km (30
mi) landward of its normally expected position along the
coast (Fig. 1).

Fig. 1. Locations of sediment-core (triangles) and well (dots) sites
along the western margin of the Chesapeake Bay impact structure
in the Virginia Coastal Plain. Symbols are color referenced to
Figs. 2 and 3. Contours represent specific conductance of ground
water in microseimens ( S) near the top of the Exmore beds that
fill the crater, and are dotted where approximate.

Fig. 2. Relation of the ratios of concentrations of bromide and
chloride (Br/Cl) of sediment-core water and well water to depth
below land surface. Data points are color referenced to sample
locations on Fig. 1.

Ground-water stable hydrogen and oxygen isotopic
ratios (Fig. 3) also indicate mixing of fresh water with
seawater along a trend of increasing specific conductance
approximating the meteoric water line. A second, less
steep trend among some of the highest salinity samples
possibly results from evaporation. The overall lighterthan-
modern isotopic composition indicates the original
seawater to predate the Pleistocene epoch of
approximately 2 million years ago.

Origin. The impact structure contains seawater
emplaced during regional inundation approximately 2
million years ago, along with much older seawater and
evaporative brine emplaced potentially as far back as the
impact event 35 million years ago. Bromide-to-chloride
ground-water concentration ratios (Fig. 2) largely indicate
the source of salinity to be from seawater rather than
dissolution of evaporite minerals. (Salinity resulting from
membrane filtration is precluded by hydraulic characteristics
of the ground-water system.) Enrichment of bromide along
with iodide likely resulted from decay of sedimentary
organic matter. Some enrichment could also have resulted
from evaporation and halite precipitation, possibly
associated with hydrothermal activity following the impact
event, to form brine which has subsequently been partly rediluted.

Fig. 3. Relation between hydrogen and oxygen isotopic ratios
and specific conductances of sediment-core and well water. Data
points are color referenced to sample locations on Fig. 1.
Symbol diameter is proportional to specific conductance
(modern seawater 45,000 S).

In addition to the above, carbon 14 concentrations
indicate ages of fresh ground water outside of the impact
structure as great as 40,000 years. Ratios of chlorine 36 to
total chloride in ground water extracted from the USGSNASA
Langley core (59E 31 on Fig. 1) are below the
detection limit of 10-15 and are consistent with a seawater
source. A single value of 12.1x 10-15 from well 63F 52,
however, suggests a ground-water age toward the center of
the impact structure of several million years or more.
With emergence and resumption of ground-water
recharge during the past 2 million years, fresh-water
flushing displaced residual seawater across the region but
was impeded across the impact structure by lowpermeability
crater-fill sediments and the overlying clayey
Chickahominy Formation. Flushing took place laterally
along the crater outer rim, followed by upward leakage
and surface discharge to areas outside of the crater. Salt
water persisted within the impact structure, even as
flushing outside of the impact structure extended in places
nearly to the edge of the continental shelf during the
Pleistocene glacial maximum of 18,000 years ago. Sea
level has since risen to its present position, and the
residual seawater has merged with the modern ocean. A
convoluted and hydrodynamically unstable transition zone
along the western margin of the impact structure now
separates fresh ground water to the west from salt water to
the east (Fig. 4).

Resource-Management Implications. During much of
the past century, hydraulic gradients have been greatly
increased and flow redirected landward across regional
cones of depression centered on industrial pumping centers
located outside of the impact structure. Salt-water intrusion
across regional distances from the impact structure has not
taken place, however, because most of the ground now
present was emplaced prior to the onset of heavy pumping.
Considering the millennia of fresh-water flushing prior to
pumping during which salt water within the impact structure
maintained its present position, a potentially very long
timeframe could be required for regional salt-water intrusion
to occur even under present gradients. By contrast, localized
intrusion along the western margin of the impact structure
possibly could take place across relatively short distances to
municipal withdrawals being made from within the saltwater
transition zone. Major increases in withdrawal and
desalinization of brackish ground water from the transition
zone are being projected to address rapidly growing
demands for public supplies during the coming several
decades. The feasibility of desalinization is partly dependent,
however, on future increases in ground-water salinity that
are difficult to estimate because of complex hydrogeologic
controls and withdrawal-induced effects within the transition
zone. A detailed local-scale characterization of hydrologic
conditions along the western margin will be critical to
assessment of the potential for salt-water intrusion.

Fig. 4. Simplified preliminary composite section showing the configuration of the salt-water transition zone. Section location is shown on
Fig. 1. USGS-NASA Langley core (59E 31) is projected onto section line.

SPATIAL ANALYSIS OF THE HYDROGEOLOGIC FRAMEWORK OF THE VIRGINIA COASTAL PLAIN.
E. Randolph McFarland1 and Jason P. Pope1, 
1U.S. Geological Survey, 1730 East Parham Road, Richmond, VA 23228, USA (ermcfarl@usgs.gov)

Introduction. The spatial configurations of 8
confined aquifers and 10 intervening confining units are
being delineated across the Virginia Coastal Plain.
Geophysical logs and related lithostratigraphic
information from a network of several hundred
boreholes are being interpreted to estimate the elevations
and thicknesses of aquifers and confining units. Borehole
data are being used largely from historical site files
compiled over a period of several decades, and vary
widely in quality. This information is being significantly
enhanced, however, by more recent, high quality data
obtained from drilling operations associated with the
construction of major water-supply wells, and with
continuous sediment coring to investigate the
Chesapeake Bay impact structure. Published geologic
maps also have been drawn on to estimate the elevations
of unit surface exposures, and to infer the presence and
effects of faults.

Delineation. Different phases have been undertaken
beginning with the Northern Neck and Middle Peninsula
during 2001, following with the Fall Zone during 2002,
and the York-James Peninsula and southeastern Virginia
during 2003. Additional refinement is planned during
2004 and beyond, incorporating additional historical
information along with new data from on-going drilling
operations, to achieve greater spatial resolution of the
hydrogeologic-unit configuration. Further refinement in
the area of the Chesapeake Bay impact structure is
expected to be based on pending results of surface
seismic profiling to delineate a complex assemblage of
faults that is theorized to encompass the structure
margin.

Analyses have been undertaken to synthesize
geophysical-log interpretations into a spatially
consistent, three-dimensional representation of the
confined aquifers and intervening confining units. A
series of 13 vertical sections were constructed as part of
log interpretation (Fig. 1) to correlate hydrogeologic
units across log locations. Hydrogeologic-unit topsurface
elevation point data were then used to construct a
series of structural-contour maps to represent the
configuration of each unit-top surface. The positions of
the lateral margins of the units also were estimated.

GIS Analysis. Point-elevation, contour, and margin
information were processed using a Geographic
Information System (GIS) to generate a series of raster
grids representing essentially spatially continuous unittop
surfaces (Fig. 2). Grids initially generated from the
contours and other information underwent a series of
algebraic operations to further refine the surfaces. Areas
of proximity between adjacent surfaces were identified
and modified as necessary to ensure that no negative unit
thicknesses were generated.

Fig. 1. Hydrogeologic section illustrating correlation of confined aquifers from borehole geophysical logs. Section location
approximately across middle of area shown on Fig. 2. Confining units and surficial unconfined aquifer are not shown.

In addition, land-surface digital-elevation model
(DEM) data were incorporated to modify unit-top
surfaces and margins in proximity to land surface within
areas of incision of the units along stream valleys. Unittop
surface elevations were modified where streams have
incised only partly through a unit. Where streams have
incised entirely through a unit, the margin of the unit
was modified. As a result, major stream valleys exhibit a
complex array of incised hydrogeologic-unit margins
and sequential, valley-wise subcrop belts along which
the units are in close hydraulic connection with the
overlying unconfined aquifer and surface-water system.

Application. The hydrogeologic framework is being
used in conjunction with information on ground-water
levels, sediment hydraulic properties, and ground-water
withdrawals to support revision of a regional groundwater
flow model of the Virginia Coastal Plain. First
developed by USGS during the early 1980's under the
Regional Aquifer System Analysis (RASA) program, the
model has since been adopted by the Virginia
Department of Environmental Quality and the Hampton
Roads Planning District Commission as a means to
manage the ground-water resource. Revision of the
model to reflect current understanding of the aquifer
system will enable the most viable approach toward
evaluating potential regional effects of large and
widespread ground-water withdrawals.

Fig. 2. Geographic information system (GIS) representation of confined-aquifer and confining-unit top surfaces beneath the surficial
unconfined aquifer and channel and bay fill deposits.

THE INFLUENCE OF A DEEP SHELF SEA ON THE EXCAVATION AND MODIFICATION OF A MARINE-TARGET CRATER,
THE LOCKNE CRATER, CENTRAL SWEDEN.. Jens Orm1 and Maurits Lindstrm2, 
1Centro de Astrobiologia (CAB), Instituto Nacional de Tcnica Aeroespacial, Ctra de Torrejn a Ajalvir, km 4, 28850 Torrejn de Ardoz,
Madrid, Spain. (ormo@inta.es); 
2Dept. of Geology and Geochemistry Stockholm University 10691 Stockholm, Sweden (maurits.lindstrom@geo.su.se)

Target environment. Comparisons between craters
formed at different target water depth and in different
strength and configuration of the rocks below the seafloor
is a prerequisite for the understanding of the marine
impact process. The Lockne crater is a wellpreserved
and well-exposed example of a crater that
was strongly affected by the target water. Geologically
well-constrained numerical modeling has shown that
the water depth exceeded the impactor diameter: Approximately
800 m water, 600 m impactor diameter
(Orm and others, 2002). Below the water, about 80 m
of limestone and loose, bituminous mud rested on the
weathered Precambrian crystalline peneplain. Recently,
improved outcrop has shown that its ejecta deposits
and morphological features are better preserved than
hitherto assumed. These new data have improved the
interpretation of the formation of the crater.

Crater excavation. The thick layer of target water
came to incorporate much of the zones where vaporization
and melting normally occur during crater excavation.
The difference in strength between the water and
the rocks generated a concentric shape of the transient
cavity with an at least 14 km wide crater in the water
mass, and a 7.5 km wide crater in the basement. A
brim with ejected crystalline rock surrounds the crater.
There is no obvious uplift of the basement below the
ejecta. The brim is about 2.5 km wide on the northern
and western sides of the crater, but much smaller on
the eastern side. This was previously thought to be due
to irregular preservation from erosion, but outcrops
exposed by a new forest road extending radially outwards
through the eastern brim zone show that the
irregularity is due to the obliquity of impact rather
than erosion. Shuvalov and others (2003) and Lindstrm
and others (2003) show that the obliquity of
impact caused more extensive ejecta downrange (western
side) than uprange (eastern side). Interestingly,
much of the crystalline ejecta of the brim appear to
have been deposited in semi-coherent state as an
anomalously wide flap. It rests on top of a surface
stripped from much of the sediments by the excavation
flow during formation of the water cavity. The target
sediments are progressively more complete below the
flap outwards from the crater. A combination of weak
spallation along the seafloor during passage of the
shock wave, the water cavity excavation, and the crushing
and shearing during flap deposition have caused a
brecciation of the sediments below the flap.

No larger melt bodies have been detected despite
intensive studies with drillings, geochemistry (Sturkell
and others, 1998), and geophysics (Sturkell and Orm,
1998). The melt occurs as small fragments incorporated
in the upper, arenitic part of the resurge deposit.
It is also in this unit that quartz with PDFs has been
found so far. The autochthonous breccia within the
basement crater, and the coarse clastic ejecta appear to
have a remarkably low shock level. This may be due to
the circumstance that only a small portion of the
basement was within the zone of sufficiently high
shock pressures. It is indicated in the models by Shuvalov
and others (2003) that most of the basement
crater excavation was driven by a high velocity water
stream generated by the shock wave propagation
through the water. Decimeter-size fragments of granitic
ejecta and beds of resurge-affected material with quartz
with PDFs are known from up to 45 km from the
crater centre (Sturkell and others, 2000).

Crater modification. The water rushing back towards
the basement crater during water cavity collapse
caused additional strong vibrations and easily eroded
the brecciated sediments where they were not protected
by the crystalline flap. On the eastern side, where the
near absence of a flap gave no protection from the
resurge erosion, resurge deposits rest directly on the
crystalline peneplain. Preserved Cambro-Ordovician
sediments below the small flap near the basement crater
rim show that the removal of sediments on this
side of the crater most likely was due to the resurge
flow rather than the excavation flow. The western side
of the crater, on the contrary, has almost no preserved
sediments in the same proximal position below the
flap. This indicates a stronger excavation flow in this
direction prior to flap deposition. The modeling of
oblique impact by Shuvalov and others (2003) shows
an offset of the water cavity relative to the basement
crater supporting the interpretation of a stronger shallow
excavation flow downrange. It also shows that a
stronger resurge flow can be expected on the uprange
side, which appears to correlate well with the indications
for a strong erosion, but thin resurge deposits
are observed on the eastern side.

The semi-coherent behavior of the ejecta flaps may
have caused tangential stresses with resulting wedgeshaped
openings in the brim zone. These openings
were canalizing the resurge flow so that kilometer long
and hundreds of meters wide resurge gullies were
formed. There are 4 known resurge gullies at Lockne
extending radially out from the crater. The floors of the
gullies are covered by resurge deposits and post impact
sediments.

Collapse of the basement crater rim during crater
modification generated terraces with a few tens of meters
width and 5-10 m subsidence. At some locations
in the brim zone, ring faults have likewise generated a
few tens of meters wide grabens. On the eastern side of
the crater, a small graben is filled with resurge breccia.
On its sides, resurge arenites rest directly on the Precambrian
basement. This may give information on the
timing between the faulting and the resurge flow. The
relation between the resurge breccia and the resurge
arenites may also have been affected by the oscillations
in the water movements indicated in the numerical
modelings of the water cavity collapse. Evidence for
such oscillations are found in repeated beds of coarser
and finer resurge deposits on the northern side of the
crater.

Summary. Lockne offers the possibility to, in the
field, follow the spatial relations between different
lithologies. This is valuable in the studies of the influence
of water on the crater formation and resurge dynamics
at other well-preserved, but less exposed marine-
target craters such as Chesapeake Bay crater.

References.
Lindstrm, M., Orm, J., Sturkell, E. and von Dalwigk,
I., 2003, The Lockne crater: Revision and
reassessment of structure and impact stratigraphy:
Advances in Impact Studies, In press.
Orm, J., Shuvalov, V., and Lindstrm, M., 2002,
Numerical modeling for target water depth estimation
of marine-target impact craters: Journal of
Geophysical Research 107, E11.
Shuvalov, V., Orm, J. and Lindstrm, M., 2003,
Hydrocode simulation of the Lockne marine-target
impact event: Advances in Impact Studies, In
press.
Sturkell, E.F.F., Broman, C., Forsberg, P., and Torssander,
P., 1998, Impact-related hydrothermal activity
in the Lockne impact structure, Jmtland,
Sweden: Eur. J. Mineralogy 10, 589-606.
Sturkell, E.F.F. and Orm, J., 1998, Magnetometry of
the marine, Ordovician, Lockne impact structure,
Jmtland, Sweden: Journal of Applied Geophysics
38, 195-207.
Sturkell, E., Orm, J., Nolvak, J. and Wallin, .,
2000, Distant ejecta from the Lockne marine-target
impact crater, Sweden: Meteoritics & Planetary
Science 35, 929-936.

Fig. 1. Geological map of the Lockne crater. Note that the patches of Cambrian shale visible on the south-western
flap occur at "windows" through the flap.

A REVIEW OF MARINE SEISMIC REFLECTION STUDIES OF THE CHESAPEAKE BAY
IMPACT CRATER. C. Wylie Poag, US Geological Survey, 384 Woods Hole Road, Woods Hole, MA
02543-1598, USA (wpoag@usgs.gov).

Introduction. The structure and
morphology of the Chesapeake Bay impact
crater are known almost entirely from
seismostratigraphic analysis of >2000 km of
marine seismic reflection profiles (Poag and
others, 1994; 2004). The seismic data base
includes multichannel, two-channel, and singlechannel
data, collected by the US Geological
Survey (USGS), the National Geographic
Society, the US Minerals Management Service,
Lamont-Doherty Earth Observatory, and Texaco,
Inc. Depth conversion of two-way traveltime
was based primarily on root-mean-square values
derived from multichannel data, calibrated with
nearby corehole stratigraphy and downhole
logging. The principal morphological features
identified are the outer rim, annular trough, peak
ring, inner basin, and central peak. Important
structural and stratigraphic features include faults
and compression ridges in the crystalline
basement, stratified preimpact nonmarine
sediments, displaced sedimentary megablocks,
crater-fill breccia, stratified marine postimpact
sediments, and postimpact growth faults. All of
these features are illustrated in the accompanying
PowerPoint files.

Outer Rim. The outer rim is a steep,
irregularly circular fault scarp formed by the
inward collapse of the sedimentary walls of the
crater. The outer rim is clearly expressed on
seismic profiles as significant disruption of the
horizontal, parallel, discontinuous reflections
that normally typify the preimpact sedimentary
section. Because the basement surface and
preimpact strata slope down to the east, the lip of
the outer rim is at -200 m elevation on the west
and -400 m on the east. The outer rim averages
85 km in diameter; vertical relief ranges from
300 m on the west side to 1250 m on the east
side.

Annular Trough. The annular trough is
that part of the crater between the outer rim and
the peak ring. The annular trough of the
Chesapeake Bay crater varies in width from 15
km to 28 km. The floor of the trough is the
surface of crystalline basement, which has been
imaged only on the western side of the crater,
where multichannel seismic data are available.
The trough floor is distinguished by numerous
extensional faults and a few compressional faults
and ridges. The throw on most faults is 25 m or
less, and the compressional ridges are 30 m to 80
m high. If one assumes that each extensional
fault with >25 m throw is an individual part of a
more extensive fault system, then a pattern of
three major radial fault systems emerges in the
western part of the annular trough.

Peak Ring. The peak ring of the
Chesapeake Bay crater is a raised subcircular
ridge of crystalline basement rocks located inside
the outer rim. The crest of the peak ring
averages 40 km in diameter, and the elevation of
its crest varies from -500 m to -950 m.
Structural relief is 40-300 m, and width of the
ring varies from 4.25 km to 22 km. The eastern
side of the peak ring is not imaged by any
seismic profiles. The morphology there is
extrapolated, using Bouguer gravity data and
assuming approximate symmetry with the
western side (Poag and others, 2004). The peak
ring is quite rugged, and several individual
subpeaks and small ridges can be identified on
seismic profiles.

Inner Basin. The inner basin is the deepest
part of the Chesapeake Bay crater, located inside
the peak ring. Its outer wall is indicated on
seismic profiles by abrupt truncation of the highamplitude
basement reflection. Approximate
diameter of the basin is 28 km. The floor of the
basin is not clearly imaged on any seismic
profile, but scaling calculations (Grieve and
Robertson, 1979) suggest a depth of about 1.3
km from outer rim to inner-basin floor.

Central Peak. The central peak was
originally identified on a single-channel profile
collected in 1996 at the mouth of Cape Charles
Harbor, on the eastern side of Chesapeake Bay
(Poag and others, 1999). There, faint diagonal
reflections indicate the presence of a poorly
defined peak-like feature protruding upward
through crater-fill breccia. Subsequent (1998)
two-channel surveys and reexamination of
several multichannel profiles reveal a series of
smaller subpeaks surrounding the main central
peak (Poag and others, 2004). The central peak
appears to have maximum vertical relief of at
least 1 km, and maximum width across the base
of 12 km.

Comparison Between Seismic and
Gravity Data. Poag and others (2004) found a
good match between seismic interpretations of
crystalline features of the crater (peak ring, inner
basin, and central peak) and residual Bouguer
gravity anomalies. A ring of gravity highs
distinguishes the peak ring, whereas the inner
basin displays a broad gravity low. The central
peak also is located over a gravity high. The
outer rim, a sedimentary feature, is not
distinguished by the gravity data.

Displaced Megablocks. The seismic
profiles reveal two types (sedimentary and
crystalline) of displaced, km-scale megablocks.
The most frequently imaged megablocks consist
of sedimentary target rocks that have slumped,
slid, and collapsed vertically from the outer
crater wall. These megablocks are detached
along their bases from the crystalline floor of the
annular trough. Some megablocks have rotated
and dropped one end as much as 300 m below
the other end. These detached sedimentary
megablocks appear to be restricted to the annular
trough. Downfaulted crystalline megablocks, in
contrast, appear to be confined to the walls of the
inner basin.

Crater-Fill Breccia. Above the displaced
megablocks and inside the inner basin, crater-fill
breccia (the Exmore breccia) typically creates a
pattern of chaotic, incoherent, or repetitive
hyperbolic seismic reflections. The upper
surface, however, is manifest as a distinctive,
high-amplitude, nearly continuous reflection.
The breccia forms a continuous thick blanket
over the entire crater. In the annular trough, the
breccia is ~200 m thick on the west side of the
crater, and ~400 m thick on the east side. The
breccia thickens to >1 km in the inner basin, and
thins to a feather-edge in a breccia apron (5-50
km across) that encircles the outer rim. The
breccia also thins over the principal basement
elevations (peak ring, central peak). Due to
long-term compaction, the morphology of the
upper surface of the breccia mimics the
morphology of the basement, but with much
subdued relief.

Postimpact Deposits. Most of the
postimpact deposits display continuous,
horizontal, parallel, medium- to high-amplitude
reflections typical of marine sediments. The
initial seismically identifiable postimpact deposit
is the Chickahominy Formation, a dense marine
clay. The impedance contrast at its upper and
lower boundaries produces easily recognizable
high-amplitude reflections, which can be traced
across the entire crater. The Chickahominy
interval thickens abruptly at the outer rim and
sags into the crater. The sag-and-thicken
characteristic allows identification of the outer
rim even in parts of the crater where other
structural changes are obscured. The
Chickahominy Formation also sags and thickens
as it crosses from the peak ring or the central
peak into the inner basin.

A series of distinct, postimpact normal faults
extend from the Chickahominy Formation up as
high as the Miocene, Pliocene, and Quaternary
sections inside the crater. Some of the faults can
be traced to within a few meters of the bay floor.
Most of these faults appear to be the result of
differential compaction of the underlying
breccia.

Future Seismic Surveys. Despite the
wealth of seismic data on hand, significant
questions remain to be answered by additional
seismic reflection surveys. Chief among these is
the question of what the basement and outer rim
morphology is like under the Delmarva
Peninsula and the adjacent continental shelf. It is
particularly important to constrain the depth and
detailed geometry of these central features in
order to choose the optimum location for any
future corehole designed to sample the peak ring,
inner basin, or central peak.

References Cited.
Grieve, R.A.F., and Robertson, P.B., 1979, The
terrestrial cratering record, 1, Current status
of observations: Icarus, v. 38, p. 212-229.
Poag, C.W., Koeberl, C., and Reimold, W.U.,
2004, The Chesapeake Bay Crater  Geology
and Geophysics of a Late Eocene Submarine
Impact Structure: Springer, Berlin, 522 p,
with CD-ROM.
Poag, C.W., Powars, D.S., Poppe, L.J., and
Mixon, R.B., 1994, Meteoroid mayhem in
Ole Virginny: Source of the North American
tektite strewn field: Geology, v. 22, p. 691-
694.
Poag, C.W., Hutchinson, D.R., Colman, S.M.,
and Lee, M.W., 1999, Seismic expression of
the Chesapeake Bay impact crater: Structural
and morphological refinements based on new
seismic data, in Dressler, B.O., and Sharpton,
V.L. (eds.) Impact Cratering and Planetary
Evolution II, Geological Society of America
Special Paper 339, p. 149-164.

HYDROTHERMAL RESPONSE TO THE CHESAPEAKE BAY BOLIDE IMPACT
Ward E. Sanford, U.S. Geological Survey, 431 National Center, Reston, VA 20192, USA (wsanford@usgs.gov)

Introduction. The recently discovered buried
impact crater beneath the Chesapeake Bay has been
shown to coincide with a previously known inland
saltwater wedge (Powars and Bruce, 1999). In
addition to an apparently extensive region of nearseawater
salinity being co-located with the crater, the
only well completed within the inner crater in the
tsunami (Exmore ) breccia yielded water with a
salinity greater than seawater (Cl=23 g/L, McFarland,
2002). Theories to the origin of this water have
included osmosis, dissolution of halite, subsequent
evaporation in restricted bays during arid climates,
and heating during impact (Poag, 1999; Poag and
others, 2003). The first three of these theories have
been demonstrated to be improbable from other
known parameters of the water and sediment
(McFarland, 2002). Sanford (2003) argued, based on
simple scoping calculations, that the ground water
and solutes within the deep inner crater has likely not
been flushed out since the time of impact, and the
brine may be a residual of hydrothermal boiling that
resulted from the heat dissipation that followed the
impact. Numerical simulations, which are briefly
described here, have been carried out with the USGS
code HYDROTHERM (Hayba and Ingebritsen,
1994) to investigate the extent to which a
hydrothermal system would have developed in the
Exmore breccia following the impact.

Boundary and Initial Conditions. A radial
slice of the impact crater extending 40 km out from
the center and 8 km downward from the post-impact
(mid-Eocene) seawater-sediment interface was
considered. Pressure was prescribed at the top
boundary to represent 300 m of standing seawater,
and temperature was prescribed at 25 C. The
bottom and side boundaries were no-flow conditions.
The side boundaries were insulated, and the bottom
boundary was assigned a background heat flow of 60
mW/m2. The inner crater was assumed to be filled
with 1.2 km of tsunami breccia and have a central
peak rising 500 m from the crater floor.

Initial conditions were assigned to be those
estimated from temperature and pressure conditions
that would have existed immediately after the crater
filled with the Exmore breccia. The initial pressures
within the breccia were given a lithostatic gradient to
reflect the sudden filling of the crater and lack of
ample time for pressures to equilibrate to hydrostatic.
The initial temperature of the breccia for the
simulation was 25 C. The initial temperatures for
the bedrock were estimated from other high-pressure
bolide impact simulations (e.g., Pierazzo and others,
1997) and ranged from background temperatures at 6
km depth up to 1,200 C at the crater floor. The
permeability of the breccia was varied over the
ranges estimated from cores and sediment analyses
(10-14 to 10-16 m2), whereas the permeability of the
bedrock was assigned a value significantly lower (10-
20 m2). Although extensive fracturing of the bedrock
during impact would increase the permeability of the
bedrock, subsequent heating and melting would result
in healing and closing of fractures in the region
beneath the crater floor.

Results. The simulations with HYDROTHERM
revealed that an extensive hydrothermal system likely
existed in the crater breccia for tens of millennia
following the impact. The evolution of this system
was controlled predominantly by the permeability of
the breccia. The range of permeabilities that were
simulated represented two different styles of thermo -
pneumatic response of the system. For a breccia
permeability of 10-16 m2, fluid cannot escape quickly
enough to prevent thermal pressurization from
developing. Superlithostatic pressures are the result
during early times and would have led to large-scale
quick conditions within the breccia and possibly
convections cells of sediment-water slurry and/or
preferential sinking of large, dense bedrock clasts.
Following the dissipation of this high pressure, a
super-heated steam-phase develops deep in the
breccia but remains relatively distinct from an
overlying water phase. For a breccia permeability of
10-14 m2, fluid can move relatively easily and the
cooling is characterized by the predominance of hotwater
convection cells. For a breccia permeability of
10-15 m2, multiphase convection predominated with
the formation of heat pipes (rising steam with
descending water) that result in the efficient and
rapid cooling of the system.

Certain conditions within the crater could
not be simulated using the assumptions currently
inherent in HYDROTHERM. First, freshwater
conditions are assumed for the phase separation
conditions, whereas seawater would have been
present in the crater. The presence of dissolved salt
in water raises the critical point of water to higher
temperatures and pressures (Bischoff and Pitzer,
1989), which would expand the P-T region in the
breccia under which multiphase conditions would
develop. Second, static permeability conditions are
assumed, whereas the permeability of the breccia
would have evolved simultaneously with the thermal
conditions, responding to changes in pressures and
effective stressed that would have changed porosity
and permeability over time. Although the
hydrothermal conditions under these more realistic
conditions would be slightly different, it is believed
the current simulations capture the likely features of
the hydrothermal system following the impact.

Planned Drilling. In order to test the
hydrothermal model of the post-impact conditions, a
deep drill hole is currently being planned near the
crater center. A 20-cm-diameter hole between 600
and 1000 meters deep has been funded, and drilling is
being planned for the spring of 2004 near the town of
Cape Charles, Virginia. The plan is to install two
6.35-cm diameter observations wells in the hole--one
well near the bottom of the hole and the second well
approximately 300 meters above the first one. Water
quality samples will be collected from these wells to
help determine the extent of the brine within the inner
crater, and to look for additional evidence of
hydrothermal activity associated with the impact. A
variety of environmental isotopes will be analyzed
from these samples in addition to dissolved gases and
major and minor solute constituents. In-situ
permeability tests are being planned for both wells,
which will be constructed with 100-ft screens. In
addition to drill cuttings being collected over the
entire hole depth, a few spot cores are also being
planned. Drilling will continue until near refusal, at
which point a final core will be taken at the bottom.
A wide range of geophysical logs are also being
planned for the completed hole. It is possible that the
final core may intercept impact lithologies and melt
material at the top of the central peak with, but this
will depend on the actual depth of the central peak
and the difficulty of drilling through the material just
above it. Temperature indicators in collected
minerals and fluid inclusions should provide
information on maximum temperature as a function
of depth, which in turn can be used to help constrain
future hydrothermal modeling.

References Cited.
Bischoff, J. L. and Pitzer, K. S., 1989, Liquid-vapor
relations for the system NaCl-H2O: summary of
the P-T-x surface from 300 to 500: American
Journal of Science, v. 289, no. 3, p. 217-248.
Hayba, D. O. and Ingebritsen, S. E., 1994, The
computer model HYDROTHERM, a threedimensional
finite-difference model to simulate
ground-water flow and heat transport in the
temperature range 0 to 1200 C: U. S.
Geological Survey Water-Resources
Investigations Report 94-4045, 85 p.
McFarland, E. T., 2002, Hydrochemical evidence for
the origin of elevated ground-water salinity in
the Chesapeake Bay impact structure,
southeastern Virginia: Geological Society of
America Abstracts, v. 34, no. 6, p. 466.
Pierazzo, E., Vickery, A. M. and Melosh, H. J., 1997,
A re-evaluation of impact melt production:
Icarus v. 127, p. 408-423.
Poag, C. W., 1999, Chesapeake Invader--Discovering
Americas Giant Meteorite Crater: Princeton
University Press, Princeton, 183 p.
Poag, C. W., Koeberl, C. and Reimold, W. U., 2003,
The Chesapeake Bay craterGeology and
geophysics of a late Eocene submarine impact
structure, Springer-Verlag, Berlin, 522 p.
Powers D. S. and Bruce, T. S., 1999, The effects of
the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the Lower York-James
Peninsula, Virginia: U. S. Geological Survey
Professional Paper 1612, 82 p.
Sanford, W. E., 2003, Heat flow and brine generation
following the Chesapeake Bay bolide impact:
Journal of Geochemical Exploration, v. 78-79, p.
243-247.

CALCAREOUS NANNOFOSSIL DISTRIBUTION PATTERNS AND SHOCK-INDUCED
TAPHONOMY IN THE CHESAPEAKE BAY IMPACT CRATER. Jean M. Self-Trail, U.S.
Geological Survey, 926A National Center, Reston, Va 20192 (jstrail@usgs.gov)

The USGS-NASA Langley and Bayside
corehole sites are located between the excavated
central crater and the slumped outer margin of
the western side of the Chesapeake Bay impact
crater. Sedimentary samples of both clast and
matrix material were collected from the impactgenerated
Exmore beds and examined for
calcareous nannofossil content in these cores.
The samples from the Exmore beds contain a
mixed assemblage of calcareous nannofossils
that range from Santonian to late Eocene in age.
The presence of non-reworked Isthmolithus
recurvus and Discoaster saipanensis in both the
impact-generated sediments and the post impact
Chickahominy Formation constrain the age of
impact to approximately 36 Ma using the
timescale of Berggren and others (1995).

Matrix. Comparison between depth and
the number of Cretaceous versus Tertiary
specimens per slide in both cores shows that
Cretaceous specimens are common in matrix at
the top of the Exmore beds and then rapidly
decrease in abundance downcore. They become
absent from the Exmore matrix material at
~244.0 m in the Langley core and 310.0 m in the
Bayside core. Additionally, the mean size of
Cretaceous nannofossils in the Exmore beds
decreases downcore at both Bayside and
Langley, whereas the mean size of Tertiary
calcareous nannofossils remains the same
(~4.3m) throughout. Size sorting of Cretaceous
nannofossils in the Exmore beds may indicate
the influence of gravity on sediment particles in
submarine debris flows that accompanied
collapse of the water column and seawater
resurge back into the crater immediately
following impact.

Clasts. Clast size and composition are
contributing factors used to evaluate the amount
of contamination by impact-driven allochthonous
particles that has occurred in clasts from the
Exmore beds. Of 25 clasts examined, 15 were
predominantly sandy, 15 were predominantly
silty, and the majority (30) consisted of clay.
Contamination occurred most commonly in
clayey clasts. Comparison of clast width to
degree of contamination shows that clast size,
not composition, ultimately controlled the
contamination. Contamination occurred in all
clasts <0.24m in width, regardless of
composition. Clasts >0.34m in width were not
contaminated, except possibly along rims that
were not sampled. Therefore, the forces
generated during impact were powerful enough
to drive allochthonous particles and microfossils
into cohesive sediment clasts of small size, but
were unable to penetrate to the center of clasts
greater than .30m in diameter.

Fractured Calcareous Nannofossils.
Fractured calcareous nannofossils of the genus
Discoaster are recorded from synimpact
sediments within the Chesapeake Bay impact
crater (Self-Trail, 2003). Evidence for shockinduced
taphonomy includes marginal fracturing
of rosette-shaped Discoaster species into
pentagonal shapes and pressure- and
temperature-induced dissolution of ray tips and
edges of discoasters. Rotational deformation of
individual crystallites may be the mechanism
that produces the fracture pattern. Shock wave
fractured calcareous nannofossils were recovered
from synimpact matrix material in or derived
from the Exmore beds. Samples taken from
cohesive clasts within the crater rubble show no
evidence of shock-induced fracturing.
Fracturing of calcareous nannofossils has been
recorded from the USGS-NASA Langley core
(Self-Trail, 2003), the Bayside core and the
Watkins Elementary School core (Self-Trail,
unpublished data).

References Cited.
Berggren, W.A., Kent, D.V., Swisher, C.C.III,
and Aubry, M.-P., 1995, A revised Cenozoic
geochronology and chronostratigraphy, in
Berggren, W.A., and others (eds.),
Geochronology, timescales and global
stratigraphic correlations: SEPM (Society for
Sedimentary Geology) Special Publication
54, p. 129-212.
Self-Trail, J.M., 2003, Shock-wave induced
fracturing of calcareous nannofossils from
the Chesapeake Bay impact crater: Geology,
v. 31, n. 8, p. 697-700.

SHIPBOARD GRAVITY AND MAGNETIC FIELD OVER THE CHESAPEAKE BAY IMPACT
CRATER. Anjana K. Shah, John Brozena, and Peter Vogt, Naval Research Laboratory, Marine Physics
Branch, Code 7420, 4555 Overlook Ave. SW; Washington, DC 20375, USA (ashah@qur.nrl.navy.mil)

Introduction. We present new shipboard
gravity and magnetic field data collected over the
Chesapeake Bay Impact Crater. Previous gravity
coverage is sparse over the Bay, while magnetic
data is limited to older airborne studies with
limited navigation and resolution. The new data
are used to refine structural characteristics of the
crater.

Surveys: Gravity and magnetic field data
were collected on board the R/V Kerhin during
the summer and fall of 2002-2003, yielding maps
covering an area of nearly ~800 km2 over the
Chesapeake Bay Impact Crater. USGS workers
(e.g. Poag and others, 1994) have previously
described the feature as a complex peak-ring
crater whose outer edge is marked by the
boundary of seismically imaged faults. They also
identified an inner ring, within which the
crystalline basement was excavated as part of the
impact process and later filled with sediment and
breccia. The contrast in rock types creates a
gravity anomaly low over the excavated region
(e.g. Koeberl and others, 1996). R/V Kerhin ship
tracks were concentrated over the inner ring with
track spacing varying from 400 m to 4 km.
Previous station data were typically spaced ~9 km
apart.

Gravity. The free air anomaly (FAA) over the
crater (Fig. 1) shows a low of 12-20 mgal over the
inner basin, with sinuous edges following the
inner ring. The variation in the gravity anomaly
is greatest to the southwest, up to 20 mgal, vs. 12-
15 mgal to the northwest and 10-12 mgal to the
northeast on the Delmarva peninsula. The gravity
contrast to the southwest coincides with larger
scale lineations observable in both the gravity and
magnetic field, suggesting that this part of the
inner rim coincides with orogenic features in the
crystalline basement.

The gravity data do not exhibit sharp slopes
within the central basin, but instead show a
gradual decrease toward the center of the basin
over a radial distance of 10-15 km. This gentle
slope contrasts the sharper steps observed in
seismic reflection data (Poag, 1997; Powars and
Bruce, 1999).

Near Cape Charles, the center of the crater,
there is a slight increase in the FAA of ~ 4
mgal, suggesting central uplift.

Figure 1. Free Air Anomaly over the Chesapeake
Bay Impact Crater. Thin black lines delineate R/V
Kerhin shiptracks, crosses mark individual stations
(NIMA, USGS). Heavy lines mark the inner and
outer rim; squares mark crossings of seismic lines
(from Poag and others, 1997). White line extending
radially from the town of Cape Charles shows
location of modeled profile in Figure 2.

Models. The actual depth to the basement
within the inner ring is poorly constrained by
currently available data. Seismic lines show
rough reflections in some places, but it is
difficult to distinguish whether these indicate
varying types of breccia or rock which melted
during the impact process (Powars and Bruce,
1999; Poag, 1997). The nature of the
basement rock is also poorly constrained,
allowing only best guesses for density
contrasts. We thus consider end-member
structure scenarios which are consistent with
the observed gravity anomalies.
For example, a 1-km thick layer with
density contrast 400 kg/m3 (assuming the
sediments and breccia infill have a bulk density of
say ~1900 kg/m3 and the crystalline basement
thus has a bulk density of ~2300 kg/m3) could
produce an ~18 mgal anomaly (Fig 2). Similarly,
2-km thick layer with a density contrast of 200
kg/m3 could also produce such an anomaly. Such
layers, when placed at a depth of 1 km,
comparable to the depth to basement outside the
inner basin, must have a gradual slope in order to
match the observed gravity anomaly. This
suggests that either the walls of the inner rim
slope gently, or there is a gradual decrease in the
density of the basement rock, perhaps due to
increases in shock, deformation, brecciation,
and/or alteration in the direction of the crater
center.

Figure 2. Top: profile of gravity data (green) and
model prediction (black) assuming a density contrast of
400 kg/m3 and the geometry shown below. Bottom:
Dimensions and depth of model higher density layer.

The slightly lower gravity slopes to the north
can be modeled assuming a 1-km thick layer with
density contrast of ~350 kg/m3, producing a ~14
mgal anomaly. A difference in density of the
crystalline rocks to the north could be due to
orogenic variation in the basement rock, as larger
scale gravity and magnetic field trends suggest.
Alternatively, the sedimentary infill layer may be
thinner to the north.

The ~4 mgal increase toward the center of the
crater can be modeled assuming a 400 kg/m3
density anomaly reaching a height of ~250 m.
Alternatively, the anomaly could be produced
assuming 1 km uplift with a density difference of
~95 kg/m3, a different end-member case of
uplifted crystalline rock which is extremely
broken, porous, or altered.

Magnetic Field. Shipboard magnetic field
data exhibit similar features to previous
aeromagnetic data, including a broad 400-nT low
over much of the inner basin, surrounded by
highly positive anomalies to the east and west.
The new data delineate several small (1-2 km
wide) patches of 100-nT highs within the inner
basin. The low may be due to shock and
brecciation reducing the magnetization of the
crystalline basement. The smaller highs may
indicate large regions of undisturbed
basement, or perhaps pockets of impact melt.

Acknowledgments. We are grateful to
Captain Rick Younger and Jake Hollinger of
the Maryland Geologic Survey and R/V Kerhin
for their thoughtful and thorough assistance
during the surveys. We thank David Daniels
of the US Geological Survey for providing
gravity station data from NIMA and the
USGS. Sandra Martinka of the Naval
Research Laboratory (NRL) and David Ball of
ITT Industries contributed significantly to the
data collection and processing. We also thank
Mike Czarnecki, Jim Jarvis, Skip Kovacs,
Robert Liang, Clyde Nishimura, Brian
Parsons, Barbara Vermillion, and Richard
Wilkerson (NRL) for their assistance.

References Cited.
Koeberl, C., Poag, C.W., Reimold, W.U. and
Brandt, D., 1996, Impact Origin of the
Chesapeake Bay Structure and the Source
of the North American Tektites, Science,
V. 271, p. 1263-1266.
Poag, C.W., 1997, The Chesapeake Bay bolide
impact  A convulsive envent in Atlantic
Coastal Plain evolution, in Seagall, M.P.,
Colquhoun, D.J., and Siron, D., eds.,
Evolution of the Atlantic Coastal Plain,
sedimentology, stratigraphy, and
hydrogeography: Sedimentary Geology, v.
108, no. 1-4, p. 45-90.
Poag, C.W., Powars, D.S., Poppe, LJ, and
Mixon, R.B., 1994, Meteoroid mayhem in
Ole Virginny- source of the North
American tektite strewn field: Geology, v.
22, p. 691-694.
Powars, D.S., Bruce, S., 1999, The effects of
the Chesapeake Bay Impact Crater on the
Geological Framework and Correlation of
Hydrogeologic units of the lower York-
James Peninsula, Virginia,: U.S.
Geological Survey Professional Paper
1612, 82 p. , 7 pl.

PROPOSALS FOR FUTURE WORK
Participants were encouraged to submit proposals for future work. The following
proposals were received [click on blue name(s) to view proposal]:
RELATING IMPACT DEBRIS IN THE STRATIGRAPHIC RECORD TO THE SOURCE
CRATER  THE CHESAPEAKE CASE
A. Deutsch
POSTIMPACT DINOFLAGELLATE AND CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY,
LITHOLOGY, SEDIMENTATION ACCUMULATION RATES, AND PATTERNS IN THE
FILLING AND REFILLING OF THE CHESAPEAKE BAY IMPACT CRATER
Lucy E. Edwards, David S. Powars, and Jean M. Self-Trail
PALEONTOLOGY OF IMPACT-DERIVED MATERIAL, CHESAPEAKE BAY IMPACT
CRATER
Lucy E. Edwards, Jean M. Self-Trail, and U.S. Geological Survey Crater Project team members
THE CHESAPEAKE BAY CRATER CORE  CLAY MINERAL INVESTIGATIONS
Ray E. Ferrell and Henning Dypvik
PROGRESS REPORT AND CONTINUING PROPOSAL FOR COLLABORATIVE RESEARCH
ON LITHIC EJECTA, SHOCKED MINERALS, IMPACT MELT, AND CRYSTALLINE
BASEMENT IN THE CHESAPEAKE BAY IMPACT STRUCTURE
J. Wright Horton, Jr., Michael J. Kunk, John N. Aleinikoff, Charles W. Naeser, Nancy D. Naeser, and Glen
A. Izett
COSMIC IMPACT AND SEQUENCE STRATIGRAPHY
D. T. King, Jr. and L. W. Petruny
IMPACTITE SEQUENCE AND POST-IMPACT ALTERATION/HYDROTHERMAL ACTIVITY:
POTENTIAL SCIENTIFIC TARGETS OF AN ICDP BOREHOLE INTO THE CHESAPEAKE
BAY IMPACT CRATER
David A. Kring
HOT TIME IN THE CRATER: POST-IMPACT PORE WATERS, HYDROTHERMAL
CIRCULATION, AND BIOLOGICAL ACTIVITY IN THE EARLY LATE EOCENE
CHESAPEAKE BAY IMPACT CRATER
Dan Larsen and Laura J. Crossey
AN ISOTOPIC AND TRACE ELEMENT INVESTIGATION OF MELT-ROCK AND IMPACT
BRECCIA FROM THE CHESAPEAKE BAY IMPACT CRATER TO ESTABLISH THE SOURCE
OF THE NORTH AMERICAN TEKTITE STREWN FIELD
Steven M. Lev
CHARACTERIZING THE HYDROGEOLOGIC AND STRUCTURAL PROPERTIES OF AN
IMPACT CRATER FROM ANALYSIS OF GEOPHYSICAL LOGS  THE CHESAPEAKE BAY
DEEP COREHOLE
Roger H. Morin
PROPOSAL FOR A COMPARATIVE STUDY OF EARLY CRATER MODIFICATION,
INCLUDING WATER RESURGE AND HYDROTHERMAL ACTIVITY IN A LARGE MARINETARGET
IMPACT CRATER, CHESAPEAKE BAY, USA
Jens Orm, Fernando Aylln Quevedo, Maurits Lindstrm, Erik Sturkell, Enrique Daz Martnez, Jess
Martnez Fras, David S. Powars, and J. Wright Horton, Jr.
PRE- AND POST-IMPACT PALEOCEANOGRAPHIC CONDITIONS ON THE EOCENE
MIDATLANTIC CONTINENTAL MARGIN: EVIDENCE FROM RADIOLARIANS
Amanda Palmer-Julson
PHYSICAL PROPERTIES AND PALAEOMAGNETISM OF THE DEEP DRILL CORE FROM
THE CHESAPEAKE BAY IMPACT STRUCTURE - A RESEARCH PROPOSAL
Lauri J. Pesonen, Tiiu Elbra, Martti Lehtinen, Johanna M. Salminen and Fabio Donadini
A PROPOSAL TO COLLECT TENSOR MAGNETO-TELLURIC SOUNDINGS ACROSS THE
CENTRAL CRATER OF THE CHESAPEAKE BAY IMPACT STRUCTURE
Herbert A. Pierce
A PROPOSAL TO ANALYZE THE STRATIGRAPHIC AND PALEOECOLOGICAL RECORD
OF SYNIMPACT TO POSTIMPACT TRANSITION IN THE USGS-ICDP CENTRAL-CRATER
CORE, CHESAPEAKE BAY IMPACT CRATER
C. Wylie Poag
PROGRESS REPORT AND CONTINUING PROPOSAL FOR COLLABORATIVE RESEARCH
ON LITHOSTRATIGRAPHY, SEISMOSTRATIGRAPHY, AND STRUCTURE OF THE
CHESAPEAKE BAY IMPACT STRUCTURE
David S. Powars, Gregory S. Gohn, J. Wright Horton, Jr., and Rufus D. Catchings
ARGUMENT FOR ICDP SCIENTIFIC DRILLING OF THE CHESAPEAKE BAY IMPACT
STRUCTURE, EASTERN SEABOARD, UNITED STATES, AND MOTIVATION FOR
PARTICIPATION IN THIS PROJECT BY THIS CONSORTIUM
W.U. Reimold, C. Koeberl, I. McDonald, R.L. Gibson, P.J. Hancox, and T.C. Partridge
FLUID INCLUSION AND MINERALOGICAL EVIDENCE FOR POST-IMPACT CIRCULATION
OF SEAWATER-DERIVED HYDROTHERMAL FLUIDS AT THE CHESAPEAKE BAY IMPACT
CRATER: A CRITICAL TEST OF THE PHASE SEPARATION MODEL FOR BRINE
GENERATION
David A. Vanko
SHOCKED BASEMENT ROCKS FROM WITHIN THE PROPOSED CHESAPEAKE BAY
IMPACT DRILL CORE
James Whitehead and Richard A. F. Grieve

RELATING IMPACT DEBRIS IN THE STRATIGRAPHIC RECORD TO THE SOURCE CRATER  THE
CHESAPEAKE CASE. A. Deutsch1, 1Institut fr Planetologie, Universitt Mnster, Wilhelm-Klemm-Str.10, D-
48149 Mnster, Germany (deutsca@uni-muenster.de).

Distant ejecta. Distant ejecta comprise variably
shocked mineral and rock fragments, impact melt glass,
high-pressure phases, as well as spherules of widely
varying chemical and textural composition and morphology,
and the so-called microkrystites. Both the
latter, have been discussed to represent high temperature
condensates from the vapor plume, expanding over
the growing crater. Distant ejecta material may be
heavily contaminated by constituents of the impactor,
resulting in enhanced abundances of the so-called meteoritic
component (mostly PGEs). Exotic chemical
components such as, for example, extraterrestrial
amino acids have been reported only from the K/T
boundary ejecta layer and are still the subject of discussion.
Geochemical signals like sharp excursions in
the stable isotope record may characterize marine
sediments on top of distant ejecta deposits. However,
these "anomalies" are secondary consequences of an
impact event. They only can be related to the short and
long term corollaries of such an event if unambiguous
mineralogical evidence for impact metamorphism is
documented (e.g., planar deformation features in
quartz).

The ejected material having suffered rapid quenching,
is either ejected ballistically or suspended and
transported by large scale motions in the stratosphere.
Distant ejecta can occur up to thousands of kilometers
away from the crater; at present roughly 20 horizons of
such ejecta have been discovered, their age range from
3.47 Ga to nearly recent (cf. Table 1).

Tektites. Tektites (from [Greek] = molten) a subgroup
of impact glasses, are characterized by their
chemical relatively homogeneous composition and
H2O contents below 0.02 wt-%. The tektite-melt is
considered to originate extremely early in a cratering
event from near-surface materials, followed by jet ejection
of this melt.

Tektites and microtektites with a diameter generally
less than 1 mm, are ideal objects for Ar-Ar dating.
They occur in four well-constrained major strewn
fields, the Australasian, the Ivory Coast, the Central
European (Moldavite), and the North American one.
Additional discoveries  some of those isolated finds -
are listed in Table 1.

Impact debris in the Upper Eocene. Offshore
drilling revealed that sediments spanning the Eocene -
Oligocene boundary, contain microtektites and microkrystites,
for example, in the Caribbean Sea or off New
Jersey. At least two layers exist; the impact melt particles
display a wide range in chemical composition.
Based on Sr-Nd isotope systematics, one group of
this distant ejecta was related to the North American
tektite strewn field (e.g., Shaw and Wasserburg, 1982;
Ngo and others, 1985; Stecher and others, 1989). The
other materials were considered to represent ejecta of
the 100-km-sized Popigai impact crater, Siberia (e.g.,
Langenhorst, 1996; Whitehead and others, 2000; Liu
and others, 2001).

Table 1. Traces of impact events in the geologic record;
selected occurrences in the Cenozoic

known ejecta horizons ? 20 
occurrences of tektites
and tektite-like objects

Age
[Ma] 

known craters >165
source crater - diameter

tektite-like glass?; Argentinean pampas	0.48	unknown
Australasian tektite strewn field	0.784 	unknown
Darwin glass?, Tasmania	 0.816 	unknown
tektite-like glass, Tikal, Belize	0.8 	unknown
Ivory Coast tektite strewn field	1.3 	Bosumtwi (phi 10.5) Ghana
S-Ural tektite specimen, Russia	6.4	unknown
Moldavite tektite strewn field	14.34 	Ries (phi 24 km), Germany
Urengoites - 3 tektite specimen, Russia	~24 	unknown
Libyan Desert Glass - Great Sand Sea, W Egypt	28.5	unknown
North American tektite strewn field	35.5 	Chesapeake Bay(phi 90 km), U.S.A.
L. Eocene microtektites, microkrystites, global?	35.7 Popigai 	(<=100 km), Russia
K/T boundary, global 	65 Chicxulub (phi<=200km), Mexico

The Popigai  L. Eocene ejecta connection. Only
a detailed Sr, Nd isotope study of various target and
impact melt lithologies of the Popigai crater (Kettrup
and others, 2003) provided the final proof for the relation
of the stratigraphic older microkrystites and associated
microtektites to this Siberian crater. The, in
comparison to tektites from other strewn fields (e.g.,
Moldavites; Lange, 1995), surprisingly large geochemical
variations, and the unusual spread in isotope
compositions and isotope model parameters of these
microkrystites and microtektites reflect an origin from
the uppermost layers at lithologically different parts of
the Popigai target area (Figure 1). The leucocratic microkrystites
and microtektites have a high affinity to
the post-Proterozoic volcanics and sedimentary rocks
that were exposed in the northern part of the target
area. The melanocratic microkrystites, in contrast,
originated mostly from crystalline basement. The
ejecta, which is related to Popigai, is probably distributed
globally (Vonhof and Smit, 1999).

Tracking down Chesapeake as the source crater
of the North American tektite (NAT) strewn field.
On the basis of Nd model ages, it was argued already
in the 80ies of the last century that the parent crater of
the NATs should be located somewhere at the U.S. east
coast (Shaw and Wasserburg, 1982). Hence, the discovery
of the impact nature of the Chesapeake structure
immediately led to the suggestion that the NATs
represent the distant ejecta of the Chesapeake impact
event (Koeberl and others, 1996). This proposal seems
to be well constrained, although ultimate proof is not
available so far due to the lack of isotope data for target
rocks at the Chesapeake impact site.

It should be one major objective of a new core hole
into the Chesapeake crater to drill various lithologies
that may match precursor rocks to the NATs. Such
rocks may occur either as lithic clasts in breccias, or in
the sub-crater basement.
Fig. 1. Time-corrected epsilontUR(Sr)-epsilontCHUR(Nd) diagram (t = 35.7
Ma) for Popigai impactites, glass coatings of gneiss bombs
and target rocks, Upper Eocene microkrystites and microtektites
(yellow; Whitehead and others, 2000; Liu and others,
2001) and the North American tektite layer (red; Shaw and
Wasserburg, 1982; Ngo and others, 1985; Stecher and others,
1989). a = including data of Permo-Triassic basalts
from the Putorama region; b = the field is defined by data of
NAT (Bediasites, Georgiates, Marthas vineyard, Barbados)
and DSDP site 612 tektites; c = the marked area (diagonal
lines) is defined by data of leucocratic microkrystites and
microtektites. Circles show data points of target lithologies,
squares of impact breccias (modified from Kettrup and others,
2003).

References Cited.
Deutsch A., Ostermann M. and Masaitis V. L. (1997)
Geochemistry and neodymium-strontium isotope
signature of tektite-like objects from Siberia (urengoites,
South-Ural glass). Meteoritics & Planetary
Science, v. 32, p. 679-686.
Kettrup, B., Deutsch, A. and Masaitis, V.L. (2003)
Homogeneous impact melts produced by a heterogeneous
target? Sr-Nd isotopic evidence from the
Popigai crater, Russia. Geochimica Cosmochimica
Acta, v. 67, p. 733-750.
Koeberl, C., Poag, W.C., Reimold, W.U. and Brandt,
D. (1996) Impact origin of the Chesapeake Bay
structure and the source of the North American tektites:
Science, v. 271, p. 1263-1266.
Lange J.-M. (1995) Lausitzer Moldavite und ihre
Fundschichten. Schriftenreihe fr Geowissenschaften,
v. 3, p. 1-138 Berlin (in German).
Langenhorst, F. (1996) Shocked quartz in Late Eocene
impact ejecta from Massignano (Ancona, Italy):
Clues to shock conditions and source crater. Geology,
v. 24, p. 487-490.
Liu S., Glass B. P., Ngo H. H., Papanastassiou D. A.
and Wasserburg G. J. (2001) Sr and Nd data for upper
Eocene spherule layers. Lunar & Planetary Science
XXXII. Lunar Planetary Institute, Houston.
#1819 (abstr. CD-ROM).
Ngo H. H., Wasserburg G. J. and Glass B. P. (1985)
Nd and Sr isotopic composition of tektite material
from Barbados and their relationship to North
America tektites. Geochimica Cosmochimica Acta,
v. 49, p. 1479-1485.
Shaw H. F. and Wasserburg G. J. (1982) Age and
provenance of the target material for tektites and
possible impactites as inferred from Sm-Nd and Rb-
Sr systematics. Earth and Planetary Science Letters,
v. 60, p. 155-177.
Stecher O., Ngo H. H., Papanastassiou D. A. and
Wasserburg G. J. (1989) Nd and Sr isotopic evidence
for the origin of tektite material from DSDP
Site 612 off the New Jersey coast. Meteoritics, v.
24, p. 89-98.
Vonhof H. B. and Smit J. (1999) Late Eocene microkrystites
and microtektites at Maud Rise (Ocean
Drilling Project Hole 689B; Southern Ocean) suggest
a global extension of the approximately 35.5
Ma Pacific impact ejecta strewn field. Meteoritics &
Planetary Science, v. 34, p. 747-755.
Whitehead J., Papanastassiou D. A., Spray J. G.,
Grieve R. A. F. and Wasserburg G. J. (2000) Late
Eocene impact ejecta: Geochemical and isotopic
connections with the Popigai impact structure. Earth
and Planetary Science Letters, v. 181, p. 473-487.

POSTIMPACT DINOFLAGELLATE AND CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY,
LITHOLOGY, SEDIMENTATION ACCUMULATION RATES, AND PATTERNS IN THE FILLING AND
REFILLING OF THE CHESAPEAKE BAY IMPACT CRATER. Lucy E. Edwards, David S. Powars, and Jean
M. Self-Trail1, 
1U.S. Geological Survey, 926A National Center, Reston, VA 20192, U.S.A. (leedward@usgs.gov)

Introduction. The 85-km diameter Chesapeake
Bay impact crater is now buried beneath 400-500 m of
upper Eocene to Holocene sediments. The upper
Eocene to lower Miocene units, unknown or quite
poorly known outside the impact structure, are well
preserved as a result of the crater setting. Middle
Miocene to Holocene units are relatively well known
outside the crater but, as a result of continued
compaction and structural adjustments, are often more
complete stratigraphically where they have been
studied inside the crater. The proposed ICDP deep
corehole, to be located where the postimpact
sedimentation should be the greatest, offers a unique
opportunity to study these Eocene and younger deposits.

Fig. 1. Index map showing location of four cores in Virginia.

Previous work. Studies of sediment accumulation
rates in the USGS-NASA Langley corehole (L, on Fig.
1) show an initial rapid filling of the crater excavation,
an unconformity-punctuated series of small
sedimentary packages, and a second episode of rapid
infilling (Fig. 2). The Exmore core (EX on Fig. 1) was
one of several cores used in the Miocene dinocyst
zonation of de Verteuil and Norris (1996). The
expanded section of the proposed deep corehole should
allow refinement of individual species ranges.
Postimpact sediments thicken and dip (sag) into the
crater across the inner and outer rims and show
variations in thickness and facies of the postimpact
units that appear related to directional sources and
amounts of sediment supply. (Powars and Bruce,
1999). Within the annular trough faulting and
thickening of postimpact units also occurs over four
collapse structural rings (Powars and others, 2003).

Proposed study. Here we present a brief summary
of the postimpact stratigraphy and some of the
questions remaining to be answered.
The Chickahominy Formation (upper Eocene),
represents nearly continuous deposition over
approximately 2 m.y. as the crater filled initially. In
the annular trough, this unit shows at least two fining
upward sequences. Conspicuous reworking of older
material has been found in several cores (L, WS on
Fig. 1). Can this reworking be tied to faulting? to
patterns of sea-level change? How do thickness and
lithology of the unit relate to sediment supply?
The Delmarva beds (lower lower Oligocene) were
recognized by Powars and others (1992) but, since that
time, they were combined with what we now know to
be the Drummond Corner beds (Powars and others, in
press) by Powars and Bruce (1999) and Powars (2000).
The planned deep corehole offers the best chance of
recovery of both units and detailed study of their
lithostratigraphic and biostratigraphic differences.
The Drummonds Corner beds (upper lower
Oligocene) were first recognized in the Langley core
(Powars and others, in press) and show significant
differences both lithostratigraphically and
biostratigraphically from the older Delmarva beds,
which may be confined to the inner parts of the crater.
Again, the planned deep corehole should clarify
matters.

The Old Church Formation (upper Oligocene) is
only 1 m thick at its type section. A much thicker
section is found within the crater. Numerous questions
remain about its range of age, the presence or absence
of significant unconformities within it, and its
correlation with other coastal plain Oligocene units.
The Calvert Formation (lower and middle Miocene)
has several formal and informal members. In the SW
part of the annular trough, three very thin members are
present, separated by unconformities. The uppermost,
the Plum Point, may show paleontological aspects of a
confined basin. The Calvert has the widest variation in
thickness of all the postimpact units. An expanded
section is most likely present in the inner crater.
The Choptank Formation (upper middle Miocene)
is absent in both the Langley and Exmore cores. Is its
absence due to nondeposition or erosion? or to
inaccurate correlations?

The St. Marys Formation (upper Miocene) is the
first postimpact unit that is preserved across the entire
region and is finer grained and thicker in the southern
part of the annular trough than to the north. An
expanded section in the inner crater is also expected.
The position of the St. Marys-Eastover boundary
relative to the dinocyst DN9-10 boundary will be
explored.

The Eastover Formation (upper Miocene also may
be present in an expanded section. At the Langley core
site, named members were difficult to delineate due to
stratigraphic completeness.

Fig. 2. Sediment accumulation rates for the postimpact
section in the USGS-NASA Langley corehole. From Edwards
and others (in press).

The Yorktown Formation (lower and upper
Pliocene) has four named members onshore. At the
Langley core site, the presence of the lower member is
equivocal. The thickest and most complete Yorktown
section (including the lower member) was documented
in the Kiptopeke core and the deep core should allow
an even more complete section.
Various Quaternary formations, where cored, are
often cut out by subsequent units.

References Cited.
de Verteuil, Laurent and Norris, Geoffrey, 1996,
Miocene dinoflagellate stratigraphy and systematics
of Maryland and Virginia: Micropaleontology, v.
42, Supplement, 172 p.
Edwards, L.E., Barron, J.A., Brewster-Wingard, G.L.,
Bukry, David, Bybell, L.M., Cronin, T.M., Poag,
C.W., and Weems, R.E., Paleontology of the upper
Eocene to Quaternary stratigraphic section in the
USGS-NASA Langley core, Hampton, Virginia, in
Horton, J.W., Jr., Powars, D.S., and Gohn, G.S.,
eds., Studies of the Chesapeake Bay impact
structureThe USGS-NASA Langley corehole,
Hampton, Virginia, and related coreholes and
geophysical surveys: U.S. Geological Survey
Professional Paper, chapter H (in press).
Powars, D.S., 2000, The effects of the Chesapeake Bay
impact crater on the geologic framework and the
correlation of hydrogeologic units of Southeastern
Virginia, south of the James River: U.S.
Geological Survey Professional Paper 1622, 53 p.
Powars, D.S., and Bruce, T.S., 1999, The effects of the
Chesapeake Bay impact crater on the geological
framework and correlation of hydrogeologic units
of the Lower York-James Peninsula, Virginia: U.S.
Geological Survey Professional Paper 1612, 82 p.
Powars, D.S., Bruce, T.S., Edwards, L.E., Gohn, G.S.,
Self-Trail, J.M., Weems, R.E., Johnson, G.H.,
Smith, M.J., and McCartan, C.T., Physical
stratigraphy of the upper Eocene to Quaternary
postimpact section in the USGS-NASA Langley
core, Hampton, Virginia, in Horton, J.W., Jr.,
Powars, D.S., and Gohn, G.S., eds., Studies of the
Chesapeake Bay impact structureThe USGSNASA
Langley corehole, Hampton, Virginia, and
related coreholes and geophysical surveys: U.S.
Geological Survey Professional Paper, chapter G
(in press).
Powars, D.S., Gohn, G.S., Catchings, R.D., Horton,
J.W., Jr., and Edwards, L.E., 2003, Recent research
in the Chesapeake Bay impact crater, USAPart 1.
Structure of the western annular trough and interpretation
of multiple collapse structures [abs.]:
Third International Conference on Large Meteorite
Impacts, Aug. 5-7, 2003, Noerdlingen, Abs. 4053.
Powars, D.S., Mixon, R.B., and Bruce, Scott, 1992,
Uppermost Mesozoic and Cenozoic geological
cross section, outer coastal plain of Virginia: in
Gohn, G.S., ed., Proceedings of the 1988 U.S.
Geological Survey workshop on the Geology and
geohydrology of the Atlantic Coastal Plain: U.S.
Geological Survey Circular, Report: C 1059, p. 85-

PALEONTOLOGY OF IMPACT-DERIVED MATERIAL, CHESAPEAKE BAY IMPACT CRATER
Lucy E. Edwards, Jean M. Self-Trail, and U.S. Geological Survey Crater Project team members, 926A National
Center, Reston, VA 20192, U.S.A. (leedward@usgs.gov)

Introduction. The Chesapeake Bay impact crater is
one of the Earth's largest and best preserved records
of an impact into a primarily siliciclastic, largely
unconsolidated, wet, sedimentary target. At the USGSICDP
Chesapeake Bay workshop in September,
2002, a clear consensus emerged that a deep corehole
should be sited in the central crater area away from
the central peak, where the impact-derived materials
are predicted to be the thickest. Fossils in the
proposed core are expected to show the distinctive
characteristics seen in previous, shallower cores in
terms of mixed stratigraphic ages and a variety of
impact-induced damage. We know that as a result of
the impact, material from an area over 85-km wide
was redistributed. The patterns of sediment
redistribution as told by the constituent fossils will be
analysed at a variety of scales from the placement of
megablocks and boulders to micrometer-sized
pockmarks on individual particles. Additionally, the
patterns of damage to microfossils will be
documented and related to experimental shock and
(or) temperature conditions.

Previous Work. The complex mixture of clasts and
sediments of mixed ages was noted in the Exmore
beds by Poag and others (1992). Partially melted
fossil dinoflagellates were noted by Powars and
others (2002) and damaged dinocysts and shocked
nannofossils were reported by Edwards and Self-
Trail (2002), Edwards and Powars (2003), and Self-
Trail (2003). Experimental shocking of modern
bacterial spores was reported in Horneck and others
(2001). Frederiksen and others (in press) report the
distribution of clasts and matrix material in the
impact related material in the USGS-NASA Langley
core. Distinctive patterns in the presence of
nonmarine Cretaceous clasts (rare high in the section;
exclusive presence low in the section) and select
marine Cretaceous and younger microfossils in the
matrix (matrix contains younger ages than
neighboring clasts; Cretaceous species present only
in upper part of the section; ages found that are not
known from onshore studies) place constraints on
crater history.

Proposed Study.
(1) Biostratigraphic dating and
paleoenvironmental interpretations of individual
clasts in the impact-derived material, with emphasis
on the patterns of clast distribution.
(2) Biostratigraphic dating of the variety of ages in
the impact-derived matrix material, with emphasis on
the patterns that reveal source location or other
aspects crater history.
(3) Biostratigraphic inventories of specific
locations (rinds on clasts, suspected injections) to
determine or verify infiltration.
(4) Description and documentation of the various
kinds of impact-related damage to fossils.
(5) Vertical distribution of microfossils in matrix
material, with emphasis on patterns produced by size
variation between Cretaceous and Tertiary
assemblages.
(6) Documentation of patterns of damage both
vertically within a core, and laterally among cores
taken within the crater.
(7) Recognition and documentation of reworked
damaged specimens in post-impact sediments.
(8) Experimental studies under controlled
laboratory conditions to try to reproduce the various
kinds of damage observed.
(9) Comparative studies of sediments from outside
the crater that are similar to inferred target materials.

Fig. 1. Partially melted dinoflagellate cyst (left) and
shocked calcareous nannofossil (right) from the
Chesapeake Bay impact.

References Cited.
Edwards, L.E., and Powars, D.S., 2003, Impact
damage to dinocysts from the Late Eocene
Chesapeake Bay event: Palaios, v. 49, no. 3, p.
275-285.
Edwards, L.E., and Self-Trail, J.M., 2002, Shocking
news -- Impact effects on marine microfossils,
Chesapeake Bay impact structure, Virginia: AGU
Spring Meeting, Washington, D.C.
Frederiksen, N.O., Edwards, L.E., Self-Trail, J.M.,
Bybell, L.M., and Cronin, T.M., Paleontology of
the impact-modified and impact-generated
sediments in the USGS-NASA Langley core,
Hampton, Virginia, in Horton, J.W., Jr., Powars,
D.S., and Gohn, G.S., eds., Studies of the
Chesapeake Bay impact structureThe USGSNASA
Langley corehole, Hampton, Virginia, and
related coreholes and geophysical surveys: U.S.
Geological Survey Professional Paper, chapter D
(in press).
Horneck, G., Stffler, D., Eschweiler, U., and
Hornemann, U., 2001, Bacterial Spores Survive
Simulated Meteorite Impact: Icarus, v. 149, no.
1, p. 285-290.
Poag, C.W., Poppe, L.J., Folger, D.W., Powars, D.S.,
Mixon, R.B., Edwards, L.E., and Bruce, Scott,
1992, Deep Sea Drilling Project Site 612 bolide
event: New evidence of a late Eocene impactwave
deposit and a possible impact site, U.S. east
coast: Geology, v. 20, p. 771-774.
Powars, D.S. , Johnson, G.H. , Edwards, L.E. ,
Horton, J.W. Jr., Gohn G.S., Catchings, R.D.,
McFarland, E.R., Izett, G.A., Bruce. T.S., Levine,
J.S. and Pierce, H.A., 2002. An expanded
Chesapeake Bay impact structure, eastern
Virginia: new corehole and geophysical data:
Lunar and Planetary Science XXXIII (2002),
abstract 1034.pdf
Self-Trail, J.M., 2003, Shock-wave-induced
fracturing of calcareous nannofossils from the
Chesapeake Bay impact crater: Geology, v. 31,
no. 8, p. 697-700.

THE CHESAPEAKE BAY CRATER CORE  CLAY MINERAL INVESTIGATIONS. Ray E. Ferrell1,
and Henning Dypvik2, 
1 Department of Geosciences, Louisiana State University, Louisiana 70803 4101, USA,
2Department of Geosciences, University of Oslo, P.O.Box 1047 Blindern, N-0316 Oslo, Norway

Introduction. In this study we suggest
detailed analyses of the clay mineral composition of
syn- and early- post-impact sediments in order
recognize aspects of impact crater formation and
development, such as; sediment-sources, the
presence and alteration of possible melt and melt
particles, the degree of weathering in the source
area and its general climatic conditions, post impact
hydrothermal alteration and diagenetic
transformations in the sedimentary succession. In
this project we would like to study samples from
the Exmore Breccia, combined with possible
analyses of weathering crusts from the basement
below and analyses of the immediate late postimpact
sediments above. The study should be done
on core samples and will include sample and core
descriptions, grain size separations, and quantitative
X-ray diffraction analyses based on simulation of
clay mineral diffraction patterns and reference
intensity ratios (RIR) for other minerals in the
sediments.

Samples. These mineralogical analyses
should preferably be done after the first
sedimentological and stratigraphical logging of the
core has taken place. In this study we need pieces
of core-samples of about 10 grams size. The
samples will be grain-size separated. The less than
2 m fraction will be destroyed in the process,
while material larger than 2 m normally will be
unharmed, and in part available for other studies.
We will need matrix and clast samples. The number
of samples and their distribution is difficult to state
at the present stage, but up to about 50 samples are
anticipated, for the few hundred meters from just
below, through and just above the impactite
(Exmore Breccia). The number of samples needed
is highly dependent on the results of the coring and
the nature of the sediments encountered and
whether new questions appear. It would be an
advantage for this study, of course, if bentonite
additives in the drilling mud could be omitted.

Methods. The samples will be separated in
two size fractions; particles larger than, and smaller
than 2 m. Each will be examined by quantitative
X-ray diffraction methods. The clay fraction will be
consumed in this study, while a portion of the
coarse fraction can be used by others. The X-ray
diffraction analyses will mainly be done in LSU
Baton Rouge (LA) and some in UiO (Oslo,
Norway) on Siemens (Bruker, D5000) and Philips
X-ray (XPert) diffraction instruments, respectively.
Possible additional electron microscope analyses
(TEM, SEM) can also be performed, if necessary.
The clay mineralogical simulations will be
according to Clay++ (Huang and others, 1993;
Aparicio and Ferrell, 2001) and will supply the
standard clay mineralogical runs. Some special
chemical treatments may be required to further
differentiate the clay mineral assemblages (Dypvik
and Ferrell, 1998; Dypvik and others, 2003). The
initial processing and peak decomposition for
qualitative analysis will be accomplished with the
latest version of the MacDiff program (Petschick,
2003).

The main goals.
Melt/tektites. In both the Chicxulub and the Mjlnir
impactites, altered glasses are commonly found as
clay minerals, normally as various modifications of
smectite. Such alteration of glass is a fast and
common process in nature. Clay mineralogical
analyses may be one way of detecting if any glass
has been formed in the crater. The clay
mineralogical determinations may also be used in
correlation between crater deposits and tektites, e.g.
from the possibly related North American strewn
field.

Tracking hydrothermal and groundwater processes.
In relation to the impact event and after,
hydrothermal processes may have been active in the
crater. During hydrothermal activity clay mineral
formation and mineral transformation takes place. It
is possible that the clay mineralogical studies may
shed some new light on such alteration in the
Chesapeake Bay core. Normal circulating
groundwater may also produce a distinct
mineralogical signature.

Impact Mechanisms. It is possible that the timing of
the possible glass to clay minerals alterations and
synsedimentary glauconite formation can help in
timing the impact, in the case pure glass phases are
missing. Any possible dating?

Clay minerals in the search for source area
characteristics / paleoclimate. In some cases clay
minerals can be used to recognize specific source
rocks/areas, may be that is possible here?
Basement, older sediment units and weathering
horizons could all carry their special clay
mineralogical fingerprint. Consequently the clay
mineralogical composition may give additional
information on the mechanisms of crater filling, as
well as the paleoclimatic conditions in the
surrounding region.

Diagenetic alteration, burial history. The clay
minerals can be transformed and altered during
increasing burial. They could in this case be used to
give information about the burial history of the
Chesapeake Bay crater.

Comparison with other craters. Clay mineralogical
studies have been performed in the Chicxulub and
the Mjlnir core studies (Dypvik and Ferrell, 1998;
Dypvik and others, 2003; Oretga-Huertas and
others, 2002; Pollastro and Bohor, 1993). In the
Chicxulub Crater clay minerals dominate inside the
crater, while glass is found outside (Smit,
pers.comm., 2003). In the Mjlnir Crater no
smectite or glass so far have been found inside the
crater, while ejecta 30 km outside the rim have beds
highly enriched in smectite, probably derived from
altered impact glass (Dypvik and Ferrell, 1998).

Conclusion. This study may contribute to
understanding of target composition and
paleoclimatic conditions, target stratigraphy, melt
recognition and alteration, tektite-melt correlation,
hydrothermal evolution as well as the burial history
of the structure.

References cited.
Aparicio, P., and Ferrell, R.E., 2001, An application
of profile fitting and CLAY++ for the quantitative
representation (QR) of mixed-layer clay minerals:
Clay Minerals, v.36, p. 510-514.
Dypvik, H., and Ferrell, R.E., 1998, Clay mineral
alteration associated with a meteorite impact in
the marine environment (Barents Sea): Clay
Minerals, v.33, p. 51- 64.
Dypvik, H., Ferrell, R.E., and Sandbakken, P.T.,
2003, The clay mineralogy of sediments related to
the marine Mjlnir impact crater: Meteoritics and
Planetary Sciences, September issue, in press.
Huang, K., Ferrell, R.E., and LeBlanc, W.S., 1993,
Computer assisted interpretation of clay mineral
XRD patterns: Abstracts with Program Clay
Mineral Society 30th Annual Meeting, San Diego,
CA, P.51.
Ortega-Huertas, M., Martinez-Ruiz, F., Palomo I.,
and Chamley, H., 2002, Review of the
mineralogy of the Cretaceous-Tertiary boundary
clay: evidence supporting a major extraterrestrial
catastrophic event: Clay Minerals, v. 37, p.395 
411.
Pollastro, R.M., and Bohor, B.F., 1993, Origin and
clay mineral genesis of the Cretaceous Tertiary
boundary unit, western interior of North America.
Clays and Clay Minerals, v.41, p.7  25.
Petschick, R., 2003, MacDiff 4.0.
http://www.geol.uni- erlangen/de/
macsoftware/macdiff/macdiff4d.html

PROGRESS REPORT AND CONTINUING PROPOSAL FOR COLLABORATIVE RESEARCH ON
LITHIC EJECTA, SHOCKED MINERALS, IMPACT MELT, AND CRYSTALLINE BASEMENT IN THE
CHESAPEAKE BAY IMPACT STRUCTURE. J. Wright Horton, Jr.1, Michael J. Kunk2, John N. Aleinikoff2,
Charles W. Naeser1, Nancy D. Naeser1, and Glen A. Izett3, 
1U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA (whorton@usgs.gov); 
2U.S. Geological Survey, MS 963, Denver Federal Center, Denver, CO 80225, USA; 
3Dept. of Geology, College of  William and Mary and Emeritus, USGS, 3012 East Whittaker Close,
Williamsburg, VA 23285, USA.

Introduction. The Chesapeake Bay crater is one
of the largest and best preserved examples on Earth
of a marine impact crater that had a target of thick,
poorly consolidated siliciclastic sediments between
the water column and the underlying crystalline
basement (Earth Impact Database, 2003). The
structure has an excavated central crater about 38 km
wide surrounded by a flat-floored annular trough
about 24 km wide, and the slumped outer margin is
about 85 km in diameter (Powars and Bruce, 1999).
It is preserved beneath a blanket of postimpact
sediments about 150 to 400 m thick and can be
sampled only by drilling.

The purpose of current and planned research on
lithic ejecta, shocked minerals, impact melt, and
crystalline basement from the Chesapeake Bay crater
is to understand the character and distribution of
materials and their significance to the formative
processes of the Chesapeake Bay crater and other
marine impact craters. These investigations are part
of the U.S. Geological Survey (USGS) Chesapeake
Bay Impact Crater Project, which is supported by the
National Cooperative Geologic Mapping Program in
cooperation with other organizations. Recent studies
have focused on samples from four new coreholes in
the annular trough on the western side of the structure
as summarized below. A deep corehole in the central
crater of the Chesapeake Bay impact structure is
being proposed to the International Continental
Scientific Drilling Program (ICDP) in order to extend
this research into other parts of the crater.

Scientific Issues. Current studies are focused on
understanding the composition, lithology, age,
stratigraphy, and thermochronology of target rocks
and crater-fill units in the annular trough. The value
of these studies would be enhanced by expanding
them to encompass the central crater as well.
Expansion of this research into the central crater will
allow these investigations to address additional
problems such as the amount and location of impact
melt, the age of the crater as determined from
samples of melt and associated materials, shock-wave
attenuation in target rocks, hydrothermal activity,
thermal models of the crater, the search for meteorite
components for projectile identification,
characterization of the target, and process-oriented
comparison to other craters.

Current Methods. Current methods include
thin-section petrography supplemented by electron
microprobe, scanning electron microscope, and X-ray
diffraction; universal-stage and spindle-stage studies
of shocked minerals; structural analysis of
deformational microfabrics as well as macroscopic
faults, fractures and veins; geochemistry of major and
trace elements (including rare earths and platinum
group) by X-ray fluorescence spectroscopy (XRF)
and instrumental neutron activation analysis (INAA);
ion-microprobe (SHRIMP) U-Pb dating of minerals
such as zircon; and thermochronology by the
40Ar/39Ar and fission-track methods.

Preliminary Results in the Annular Trough.
Our preliminary results from the recent USGS-NASA
Langley, Bayside, and North coreholes in the annular
trough on the western side of the crater (respectively
19, 8, and 24 km outside the central crater), and from
the Watkins School corehole on the outer margin (27
km outside the central crater), are summarized in
recent abstracts (Horton, Aleinikoff, and others,
2001, 2002; Horton, Kunk, and others, 2002; Horton,
Gohn, and others, 2003; Horton, Kunk, and others,
this volume). A lack of evidence for shock
metamorphism or discernible impact heating >100C
in crystalline basement from coreholes ?8 km outside
of the central crater implies that these rocks were
outside the transient cavity and provides boundary
conditions for modeling. An intact ejecta layer is not
preserved in the cores. Shocked quartz is found in
rock fragments or sand grains from the Exmore beds
in all four cores, where it is diluted by a much larger
volume of unshocked material in these mixed
siliciclastic sediments interpreted as seawater resurge
deposits. Rare clasts of possible impact melt from
the Exmore beds are being investigated chemically
and isotopically, and the search continues for highpressure
minerals such as those reported by Glass
(2002) from proposed distal ejecta.

Proposed Collaboration in the Central Crater.
In addition to being a logical extension of current
research, deep coring in the central crater is likely to
recover impact breccias, melt, hydrothermal
minerals, and shocked target rocks unlike those
studied in the annular trough. New research
collaborations are sought to enhance and expand
capabilities in areas such as the measurement of
physical properties (for example, density, magnetic
susceptibility and remanent magnetization, and
strength properties), sensitive trace-element analyses
by radiochemical neutron activation analysis
(RNAA) or inductively coupled plasma mass
spectrometry (ICPMS), Nd and Sr isotopes,
cosmogenic isotopes, high-pressure mineralogy, fluid
inclusions, and thermochronology techniques that
complement those described above. Much can be
learned about marine craters and processes by
comparative studies of impactites from the
Chesapeake Bay crater, well studied land-target
craters such as the Miocene Ries crater in Germany,
and other marine-target craters such as the
Ordovician Lockne crater in Sweden. We hope to
broaden scientific collaboration with a variety of
international crater researchers.

References Cited.
Earth Impact Database, 2003,
<http://www.unb.ca/passc/ImpactDatabase/>
(Accessed: 10/27/03)
Glass, B.P., 2002, Distal impact ejecta from the
Chesapeake Bay impact structure [abs.]:
Geological Society of America Abstracts with
Programs, v. 34, no. 6, p. 465.
http://gsa.confex.com/gsa/2002AM/finalprogram
/abstract_37135.htm
Horton, J.W., Jr., Aleinikoff, J.N., Izett, G.A.,
Naeser, C.W., and Naeser, N.D., 2001,
Crystalline rocks from the first corehole to
basement in the Chesapeake Bay impact
structure, Hampton, Virginia [abs.]: Geological
Society of America Abstracts with Programs, v.
33, no. 6, p. A-448.
Horton, J.W., Jr., Aleinikoff, J.N., Izett, G.A.,
Naeser, N.D., Naeser, C.W., and Kunk, M.J.,
2002, Crystalline basement and impact-derived
clasts from three coreholes in the Chesapeake
Bay impact structure, southeastern Virginia
[abs.]: EOS Transactions, American
Geophysical Union, v. 83, no. 19, Spring
Meeting Supplement, Abstract T21A-03, p.
S351.
Horton, J.W., Jr., Gohn, G.S., Edwards, L.E., Self-
Trail, J.M., Powars, D.S., Kunk, M.J., and Izett,
G.A., 2003, Recent research in the Chesapeake
Bay impact crater, USAPart 2. Reworked
impact ejecta and impact debris [abs.]: Third
International Conference on Large Meteorite
Impacts, August 5-7, 2003, Noerdlingen,
Germany, Abstract 4051.
http://www.lpi.usra.edu/meetings/largeimpacts20
03/pdf/4051.pdf
Horton, J.W., Jr., Kunk, M.J., Naeser, C.W., Naeser,
N.D., Aleinikoff, J.N., and Izett, G.A., 2002,
Petrography, geochronology, and significance of
crystalline basement rocks and impact-derived
clasts in the Chesapeake Bay impact structure,
southeastern Virginia [abs.]: Geological Society
of America Abstracts with Programs, v. 34, no.
6, p. 466.
http://gsa.confex.com/gsa/2002AM/finalprogram
/abstract_44052.htm
Powars, D.S., and Bruce, T.S., 1999, The effects of
the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p., available online
at http://pubs.usgs.gov/prof/p1612/

COSMIC IMPACT AND SEQUENCE STRATIGRAPHY. D. T. King, Jr.1, and L. W. Petruny2, 
1Dept. of Geology, Auburn University, Auburn, AL 36849, USA (kingdat@auburn.edu) , 
2Department of Curriculum and Teaching, Auburn University, Auburn, AL 36849, USA and Astra-Terra Research, Auburn, AL 36831-3323, USA.

Introduction. Scientific revolutions of thought
concerning cosmic impacts on Earth and about sequence
stratigraphy of terrestrial sediments have both
come forth as key theories in Earth science during the
past 30 years (see Melosh, 1989, for a review of impact
geology and Miall, 1997, for a review of sequence
stratigraphy). However, not much thought has gone
into the joint implications of these theories. One of the
axioms of sequence stratigraphy is that sedimentation
in general, and its stratal patterns in particular, respond
to changes in accommodation space, that is, the space
available for sediment accumulation within the depositional
realm (Vail and others, 1977).

Fig. 1. Nearshore impact effects.DL = downlapping
into crater accommodation space. I-R H = impact-related
hiatus in downlapping deep marine sediments.
Impactites stippled; marine slope sediments = \\\.
S.L.= sea level. 

Where cosmic impacts of size occur, accommodation
space is instantly created. In the marine realm,
where most sequence stratigraphic studies are focused,
this accommodation space would be differentially
filled depending upon water depth in the crater area
(see Orm and Lindstrm, 2000, regarding water
depth). The three scenarios considered here are: nearshore
impacts, shelfal impacts, and deep marine impacts.
We will consider only impacts of large size,
such as craters ~ 100 km or more in diameter, in this
discussion. We will neglect discussing the variables
associtated with impact tectonism, e.g., shelf collapse,
and center on sedimentation response to impact-related
changes in topography and bathymetry on a passive
margin. We will also consider the effect of relative
sea-level rise upon impact-related sequence stratigraphy.
Implicit in our assumptions is that the target is
mainly sedimentary material.

Nearshore impacts. Impacts in which the outer
rim of the structure intersects shoreline are considered
here as nearshore impacts. After nearshore impacts,
previously established drainage and longshore drift
patterns must re-establish themselves. In a transgressive
phase, this may not be as important as in a regressive
phase. Changes in paleogeography of shoreline
configuration are to be expected and, on the down-dip
side of the impact structure, there is considerable new
accommodation space. This space may act as a sediment
trap, particularly for clastics, which may otherwise
have moved into deeper realms. Down-dip from
the structure, a hypothetical impact-related hiatus
(i.e., condensed section or surface of starvation) may
develop due to up-dip trapping of materials (Fig. 1). In
carbonate shoreline systems, which may be more sensitive
to changes in water depth, temperature, and circulation
patterns, local carbonate buildup (reefal)
sedimentation may attend the structural rim.

In addition, nearshore impacts of size will likely
have devastating effects upon the adjacent shore and
may be related to hemispheric or global catastropies.
In these instances, devastated ecosystems, especially
those within the local drainage system, may produce
copious amounts of organic rich clastic sediment,
which could temporarily increase sedimentation rates
in the nearshore realm. This effect will rapidly diminish
with time, but may be a factor in early-formed sequence
stratigraphy within a nearshore impact structure
(i.e., development of sequence or parasequence
boundaries; terminology of Van Wagoner and others,
1990).

Shelfal impacts. Shelfal impacts produce circular
structures of size that do not intersect the shoreline
and are formed entirely or almost entirely upon the
continental shelf, e.g., Chesapeake Bay crater. Shelfal
impacts act more like tectonic features (e.g., strikeoriented
grabens) in the way that they potentially disrupt
shelfal sedimention patterns. The bowl of the impact
structure acts temporarily like a depressed part of
the shelf wherein sediments, especially dip-fed clastic
sediments, commence filling the depression in a classic
downlapping pattern like that seen in deeper water
(i.e., along the continental shelf). In deeper shelf settings,
it is possible that the equivalent of deep-sea fan
systems may develop along the up-dip rim of the structure
as dip-fed sediments enter the depression. Coveal
sedimentation on the down-dip side of the structure
would likely be characterized by slumps and pelagic
sedimentation. A similar impact-realted hiatus is to
be expected within deeper water facies, which are now
deprived of dip-fed sediments that would have come to
the continental margin were it not for the new accommodation
space higher on the shelf. This pattern
should continue until the accommodation space is
filled, and only at that time will normal sedimentation
resume on shelf areas deeper than the structure and on
the continental margin within the dip-fed sedimentation
shadow of the structure. In transgressive phases,
filling of crater accommodation space will proceed
more slowly than in regressive phases, especially with
clastics. With shelfal carbonates, great depth inside the
structural bowl may preclude carbonate accumulation
at any significant rate.

Fig. 2. Shelfal impact effects. Same symbols and abbreviations
as in Fig. 1.

In addition, shelfal impact structures of size may
be affected to a significant degree by subsequent
isostatic adjustment, which is significant because
changes in water depth on a shelf can have a profound
effect upon sedimentation. This is especially true in
carbonate depositional systems, which are quite sensitive
to light, temperature, circulation patterns, salinity,
etc. (Wilson, 1975).

In order to test the hypotheses presented here,
newly proposed ICDP drilling at Chesapeake Bay crater
should occur within the up-dip reaches of the crater
area. A second well (an ICDP or other well) in deeper
water beyond the Atlantic margin shelf break would
encounter the envisioned impact-related hiatus.

Deep marine impacts. Deep marine impacts may
produce structures of size on the sea floor, provided
that the water is not too deep versus the size of impactor
(Orm and Lindstrm, 2000; in some scenarios,
very deep ocean water may itself form the crater and
thus not much of an effect is to be found upon the sea
floor itself; Gersonde and others, 1997). Deep marine
impacts, as discussed here, do not have any intra-rim
area that intersects the continental shelf, but may peripherially
involve the continental slope or rise (Fig.
3). The effects of these impacts are (1) to disrupt ongoing
sedimentation patterns in the deeper realm, e.g.,
turbitide flow systems, especially if the impact is adjacent
to a significant source of terrestrial clastic sediment
and (2) to provide a fresh area for pelagic sedimentation.
Sedimentation within such impact structures
may be much greater during regressive phases
wherein sediment quantities feeding into the deeper
realms of the ocean are expected to be greater. Pelagic
sedimentation will resume shortly after impact, and
thus these early pelagic layers may become a surface
upon which downlapping may occur with subsequent
progradation, i.e., a sequence boundary.

Fig. 3. Deep marine impact effects. Same symbols and
abbreviations as in Fig. 1. Pelagic sediments in crater
= parallel lines.

References Cited.
Gersonde, R., Kite, F.T., Bleil, U., Diekmann, B.,
Flores, J.A., Gohl, K., Grahl, G., Hagen, R., Kuhn,
G., Seirro F.J., Vlker, D., Abelmann, A.. and
Bostwick, J.A., 1997, Geological record and reconstructions
of the late Pliocene impact of the Eltanin
asteroid in th southern Ocean, Nature, v. 390, P.
357-63.
Melosh, H.J., 1989, Impact cratering, a geologic process:
New York, Oxford University Press, 245p.
Miall, A.D., 1997, The geology of stratigraphic sequences:
Berlin, Springer, 433p.
Orm, J., and Lindstrm, M., 2000, When cosmic impact
strikes the sea bed: Geological Magazine, v.
137, p. 67-80.
Vail, P.R., Mitchum, R.M, Jr., Todd, R.G., Widmeir,
J.M., Thompson, S., III, Sangree, J.B., Bubb, J.N.,
and Hatlelid, W.G., 1977, Seismic stratigraphy and
global changes of sea level, in Payton, C.E., ed.,
Seismic stratigraphy  applications to hydrocarbon
exploration: Tulsa, America Association of Petroleum
Geologists Memoir 26, p. 49-212.
Van Wagoner, J.C., Mitchum, R.M, Jr., Campion,
K.M., Rahmanian, V.D., 1990, Siliciclastic sequence
stratigraphy in well logs, cores, and outcrops:
Tulsa, American Association of Petroleum
Geologists Methods in Exploration Series, v. 7,
55p.
Wilson, J.L.,1975, Carbonate facies in geologic history:
Berlin, Springer, 471p.

IMPACTITE SEQUENCE AND POST-IMPACT ALTERATION/HYDROTHERMAL ACTIVITY:
POTENTIAL SCIENTIFIC TARGETS OF AN ICDP BOREHOLE INTO THE CHESAPEAKE BAY
IMPACT CRATER. David A. Kring, Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721 USA (kring@LPL.arizona.edu).

Introduction. The Chesapeake Bay impact crater
(Poag and others, 1994; Poag, 1997) is one of only a
few surviving complex craters to have been formed
on a continental shelf. The crater is also relatively
easy to access, despite being buried beneath 300 to
500 m of post-impact sedimentation. This makes the
structure a good candidate for a deep borehole project
and the recovery of a continuous core through the
impact structure.

Science Potential. A deep borehole through the
Chesapeake Bay crater would nicely complement a
previous ICDP-sponsored borehole project that
targeted the Chicxulub impact crater (e.g., Dressler
and others, 2003; Kring and others, 2003). Like the
Chesapeake Bay crater, Chicxulub was produced on a
continental shelf. However, differences in some of
the parameters involved may have produced different
structures and impact lithologies. The water depth at
the Chesapeake Bay impact site was 200 to 500 m,
while it was only ~100 m at the Chicxulub site. Both
impacts involved a layer of sediments over a granitic
continental crust, but the sediment horizon was
thinner in the case of Chesapeake Bay (~1 km) than
Chicxulub (~3 km). The Chesapeake Bay crater is
also much smaller than the Chicxulub crater. These
three factors may have produced significant
differences in shock-metamorphic conditions and the
excavation, transport, and deposition of impact
lithologies. In addition, the relative proportions of
water depths to rim heights may have influenced the
way in which agitated seawater was able to erode the
rim and/or rework breccias within each crater and
their surrounding impact ejecta blankets.

In the case of the ICDP-sponsored Chicxulub
Scientific Drilling Project, continuous core was
obtained in the structural trough between the peak
ring and final crater rim, augmenting discontinuous
borehole samples collected during previous oil
exploration projects and several shallow research
cores. The project recovered an impactite sequence
composed of several melt-rich polymict breccias and
a basal melt unit (e.g., Dressler and others, 2003;
Kring and others, 2003). The impactite sequence was
also altered by an impact-generated hydrothermal
system (Zurcher and Kring, 2003; Zurcher and others,
2003).

Chesapeake Bay. To better understand the
excavation, transport, and depositional processes
involved in the formation of the Chesapeake Bay
crater, a similar core through the impactite sequence
is needed. Analyses of lithic clasts, their abundances
relative to melt fragments and matrix as a function of
depth, the range of compositions and textures among
melt fragments, etc. would reveal depths of
excavation, mixing phenomena during transport, and
sorting phenomena that may have occurred during
deposition through a water column.

As illustrated at Chicxulub, hydrothermal systems
can be generated in large impact craters. A similar
system was probably generated in the Chesapeake
Bay structure, because such systems can span the
entire diameter of a crater and affect material to
depths in excess of a kilometer (Kring, 2000). To
study the chemical and thermal evolution of the
hydrothermal system in the Chesapeake Bay
structure, a core sample within the crater is needed,
ideally one that penetrates the entire impactite
sequence and a portion of the underlying floor of the
crater. Analyses of the mineralogy and fabric of the
impactite sequence and crater floor, in addition to
stable isotope analyses, can be used to infer
temperatures in the system, fluid chemistry, and how
both evolved with time. Coordinated fluid inclusion
studies would further constrain temperatures and fluid
chemistry.

The same core sample can conceivably be used to
address all of the above issues: (a) shock
metamorphism of target materials, (b) excavation,
transport, and deposition of impact lithologies, and
(c) post-impact hydrothermal activity.

Proposed Borehole Location. The best type of
core sample to resolve these scientific issues is from a
borehole that penetrates the entire sequence of
breccia units, including any melt horizons that may be
present (even if discontinuous), and into the fractured
basement beneath the impact breccia sequence, in the
trough between the central uplift and the outermost
modification zone within the final crater rim. This
would be located ~10 km radially from the center of
the crater. The borehole would need to penetrate 300
to 500 m of post-impact sediments and then at least
another 1.5 km to penetrate the previously described
sedimentary breccia, crystalline breccia, and a
significant amount of the underlying crystalline target
rocks. This borehole would be modestly deeper than
that in the Chicxulub Scientific Drilling Project,
which produced continuous core from 404 m to a
final depth of 1511 m for a cost of ~$1.5M. A
relatively deep borehole of this type would best
augment existing data from shallower core samples at
larger crater radii around the Chesapeake Bay
structure.

A good secondary target would be the central
uplifted peak. As studies of the Puchezh-Katunki
crater have shown (Pevzner and others, 1992;
Naumov, 1992; 1993), extensive hydrothermal
activity can alter the central uplift. Studies of shock
deformation as a function of depth would also be
feasible if a borehole penetrated the central uplift.
High numbers of structural faults in the central uplift
might, however, decrease the probability for
successful core recovery.

Ideally, boreholes in both locations would be
recovered. This would facilitate a comparison of the
impactite sequence deposited between the central
peak and crater rim with that deposited above the
central peak, which would further constrain the
excavation, transport, and depositional processes in
an impact event in a water-covered continental shelf
environment. Two boreholes would also allow the
spatial extent of any hydrothermal system to be
mapped and a better evaluation of the respective roles
the central uplift and impactites may have had as heat
sources driving post-impact fluid circulation.

References Cited.
Dressler, B.O., Sharpton, V.L., Morgan, J., Buffler,
R., Moran, D., Smit, J., Stffler, D., and Urrutia,
J., 2003, Investigating a 65-Ma-old smoking gun:
Deep drilling of the Chicxulub impact structure:
EOS, Trans. Am. Geophy. Union, v. 84, no. 14, p.
125 and 130.
Kring, D.A., 2000, Impact-induced hydrothermal
activity and potential habitats for thermophilic and
hyperthermophilic life: Catastrophic Events &
Mass Extinctions - Impacts and Beyond, Lunar
and Planetary Institute Contrib. 1053, p. 106-107.
Kring, D.A., Horz, F., and Zurcher, L., 2003, Initial
assessment of the excavation and deposition of
impact lithologies exposed by the Chicxulub
Scientific Drilling Project, Yaxcopoil, Mexico:
Lunar and Planetary Science XXXIV, Abstract
#1641, Lunar and Planetary Institute, Houston
(CD-ROM).
Naumov, M.V., 1992, Zeolitization of impactites and
breccia in Pechezh-Katunki astrobleme: Lunar and
Planetary Science XXIII, p. 967-968.
Naumov, M.V., 1993, Zonation of hydrothermal
alteration in the central uplift of the Puchezh-
Katunki astrobleme: Meteoritics, v. 28, p. 408-
409.
Pevzner, L.A., Kirjakov, A.F., Vorontsov, A.K.,
Masaitis, V.L., Mashchak, M.S., and Ivanov,
B.A., 1992, Vorotilovskaya drillhole: First deep
drilling in the central uplift of large terrestrial
impact crater: Lunar and Planetary Science XXIII,
p. 1063-1064.
Poag, C.W., Powars, D.S., Poppe, L.J., and Mixon,
R.B., 1994, Meteoroid mayhem in Ole Virginny:
Source of the North American tektite strewn field:
Geology, v. 22, p. 691-694.
Poag, C.W., 1997, The Chesapeake Bay bolide
impact: a convulsive event in Atlantic Coastal
Plain evolution: Sedimentary Geology, v. 108, p.
45-90.
Zurcher, L. and Kring, D.A., 2003, Preliminary
results on the post-impact hydrothermal alteration
in the Yaxcopoil-1 hole, Chicxulub impact
structure, Mexico: Lunar and Planetary Science
XXXIV, Abstract #1735, Lunar and Planetary
Institute, Houston (CD-ROM).
Zurcher, L., Kring, D.A., Dettman, D., and Rollog,
M., 2003, Stable isotope record of post-impact
fluid activity in the Chicxulub crater as exposed
by the Yaxcopoil-1 borehole: Lunar and Planetary
Science XXXIV, Abstract #1728, Lunar and
Planetary Institute, Houston (CD-ROM).

HOT TIME IN THE CRATER: POST-IMPACT PORE WATERS, HYDROTHERMAL
CIRCULATION, AND BIOLOGICAL ACTIVITY IN THE EARLY LATE EOCENE
CHESAPEAKE BAY IMPACT CRATER.
Dan Larsen1 and Laura J. Crossey2, 
1Department of Earth Sciences, University of Memphis, Memphis, Tennessee, 38152 USA (dlarsen@memphis.edu), 
2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 USA (lcrossey@unm.edu).

Introduction. The early late Eocene-age
Chesapeake Bay impact was a first-order
convulsive event in Earth history (Poag and
others, 1994; Poag and others, 2004). Along
with sister Popigai impact in northern Siberia,
these events produced a one-two cosmic punch
that caused dispersal of widespread ejecta
plumes, and potentially enhanced? or
accelerated? Late Eocene global cooling (Vonhof
and others, 2000). The effects of the Chesapeake
Bay event are still evident today in the inland
distribution of a saline ground-water wedge,
which affects ground water resources to as many
1.8 million people, and localized subsidence in
the southern Chesapeake Bay area.

Many aspects of the Chesapeake Bay impact
and its effects have become clear through
extensive seismic surveys and ground-water
related coring studies in the southern Chesapeake
Bay area (Poag and others, 2004; Powars, 2000).
However, many questions regarding the size and
characteristics of the inner crater, degree of meltrock
formation, and post-impact modification
remain unanswered. A particularly interesting
series of questions pertains to the degree to
which hydrothermal circulation and associated
thermophilic ecosystems occurred following
impact, and whether the current pore waters in
the inner crater fill are remnant from the impact
event and post-impact hydrothermal activity
(Sanford, 2003). Although hydrothermal
alteration has been described to some degree in
several impact sites (McCarville and Crossey,
1996; Sturkell and others, 1998; Osinski and
others, 2001), it has not been described for a
geologically young, well-preserved, shallowmarine
impact site, such as Chesapeake Bay
impact. Furthermore, the potential that the inner
crater pore waters are remnant from the impact
event provides a unique opportunity to describe
and model water-rock interactions in an impactderived
hydrothermal system. Such
environments could have been the crucibles of
early life on Earth (Farmer, 2000; Kring, 2000);
however, no aqueous remnant of an impactdriven
hydrothermal system has been sampled.

Post-Impact Hydrothermal Activity. As
proposed by Poag and others (2004), the initial
impact at the target site vaporized and shattered
the marine sediment cover, vaporized an
immense quantity of seawater, and potentially
melted basement rock. Resurge breccias that
were subsequently deposited the crater would
thus have immediately encountered hot brine and
begun reacting immediately. Assuming only
thermal conduction, Sanford (2003) estimated
peak temperatures of over 450C in the breccias
after 10,000 yrs. Convective circulation would
likely result in higher temperatures, but lower
time duration at peak temperatures. Existing
core studies from outside the inner crater indicate
extensive oxidation in some parts of the breccia
and pyrite precipitation in adjacent rock, and
common reaction (fusion?) rims on sedimentary
clasts (Poag and others, 2004). However, the
sedimentary matrix of the breccias is described
as containing glauconite, quartz, mica, and
calcite, suggesting minimal degrees of postimpact
alteration outside the inner crater. Landbased
impact structures commonly show
evidence of propyllitic alteration at temperatures
of as much as 300C (Koeberl and others, 1989;
McCarville and Crossey, 1996; Osinski and
others, 2001); however, alteration temperatures
might be expected to be proportionally lower for
marine impacts because greater ejecta dispersal
(Orm and Lindstrm, 2000) and vaporization of
water. No evidence for post-impact
microbiological communities is presently
observed in the breccias; however, they might
expect to exist based on the presence of organic
carbon and likely oxidation-reduction gradients
(suggested by iron oxides or hydroxides
oxidation and pyrite in close proximity).

Proposed Study. We propose a threepronged
sampling approach to investigate postimpact
water-rock-biological interaction system
at the Chesapeake Bay impact site: (1) sampling
and analysis of pore waters from a proposed
inner crater well (Ward Sanford, pers. comm.,
2003), (2) sampling of outer and inner crater fill
and sub-crater materials for petrological and
mineralogical analysis, and (3) sampling of
crater-fill materials for evidence of post-impact
biological activity. Pore waters will be analyzed
for major, minor, and trace elements, as well as a
suite of isotopic tracers. The principal objectives
of the water analysis are to determine if the pore
fluids show evidence of initial impact conditions,
the probable mechanisms for attaining high
salinity, and the minimum residence time of pore
waters in the crater. Outer and inner crater
materials will be analyzed to identify secondary
mineralogy by light microscopy, scanning
electron microscopy (SEM), and X-ray
diffraction. Special emphasis will be placed on
changes in silica and clay mineralogy, which
may be particularly sensitive indicators of lowtemperature
alteration. Stable carbon and
oxygen isotopes of secondary carbonate minerals
will also be completed to better constrain fluid
composition, temperature conditions, and
potential microbial activity. Mineralogical,
isotopic, and textural data will be used to
establish the sequence of alteration effects and
ranges of alteration temperatures, and the
distribution of alteration in the crater. The
results will be compared with conductive and
convective heat-flow/circulation model results in
the crater. Pore-water chemistry and
mineralogical data will be modeled to determine
the probable reaction paths that occurred
following impact and evaluate whether the
current pore waters are related to the initial
impact. Crater-fill materials will be examined in
SEM after acid-etching to determine evidence
for post-impact microbial activity. In addition,
organic fractions from the breccia matrix and
post-impact sediments will be separated and
analyzed for biomarkers indicative of
hydrophilic biological communities. These data
along with chemistry and stable oxygen,
hydrogen, carbon and nitrogen compositions of
pore waters will be used to attempt to reconstruct
post-impact biological communities and their
relationship to hydrothermal alteration.
The results from our proposed study will
coordinate well with hydrologic studies (Ward
Sanford), fluid inclusion studies (David Vanko),
and other alteration and geochemical studies.
The combined results will test several
fundamental questions related to impact
processes, especially in marine settings: (1) Are
impact pore waters persistent in marine impacts?
(2) To what degree does hydrothermal alteration
occur in marine impacts and how well does heat
dissipate by this mechanism? and (3) Is there
evidence for post-impact biological activity
related to hydrothermal processes?

References Cited.
Farmer, J.D., 2000, Hydrothermal systems:
Doorways to early biosphere evolution: GSA
Today, v. 10, no. 7, p. 1-9.
Koeberl, C., Fredriksson, K., Gtzinger, M., and
Reimold, W.U., 1989, Anomalous quartz
from the Roter Kamm impact crater,
Namibia: Evidence for post-impact
hydrothermal activity?: Geochemica et
Cosmochimica Acta, v. 53, p. 2113-2118.
Kring, D.A., 2000, Impact events and their effect
on the origin, evolution, and distribution of
life: GSA Today, v. 10, no. 8, p. 1-7.
McCarville, P.M., and Crossey, L.J., 1996, Postimpact
hydrothermal alteration of the
Manson impact structure, in Koeberl, C., and
Anderson, R.R., eds., The Manson Impact
Structure, Iowa: Anatomy of an Impact
Structure: Boulder, Colorado, Geological
Society of America Special Paper 302, p.
347-376.
Orm, J., and Lindstrm, M., 2000, When a
cosmic impact strikes the sea bed: Geological
Magazine: v. 137, p. 67-80.
Osinski, G.R., Spray, J.G., and Lee, P., 2001,
Impact-induced hydrothermal activity within
the Haughton impact structure, arctic
Canada: Generation of a transient, warm, wet
oasis: Meteoritics and Planetary Science, v.
36, p. 731-745.
Poag, C.W., Powars, D.S., Poppe, L.J., Mixon,
R.B., 1994, Meteoroid mayhem in Ole
Virginny: Source of the North American
tektite strewn field: Geology, v. 22, p. 691-
694.
Poag, C.W., Koeberl, C., and Reimold, W.U.,
2004, The Chesapeake Bay Crater: Geology
and Geophysics of a Late Eocene Submarine
Impact Structure: Springer-Verlag, Berlin,
522 p.
Powars, D.S., 2000, The effects of the
Chesapeake Bay impact crater on the
geological framework and the correlation of
hydrologic units in southeastern Virginia,
south of the James River: U.S. Geological
Survey Professional Paper 1622, 53 p.
Sanford, W.E., 2003, Heat flow and brine
generation following the Chesapeake Bay
bolide impact: Journal of Geochemical
Exploration, v. 78-79, p. 243-247.
Sturkell, E.F.F., Broman, C., Forsberg, P., and
Torssander, P., 1998, Impact-related
hydrothermal activity in the Lockne impact
structure, Jmtland, Sweden: European
Journal of Mineralogy, v. 10, p. 589-606.
Vonhof, H.B., Smit, J., Brinkhuis, H.,
Montanari, A., and Nederbragt, A.J., 2000,
Global cooling accelerated by early late
Eocene impacts: Geology, v. 28, p. 687-690.

An Isotopic and Trace Element Investigation of Melt-rock and Impact Breccia from the Chesapeake
Bay Impact Crater to Establish the Source of the North American Tektite Strewn Field. Steven M.
Lev, Department of Physics, Astronomy & Geosciences, Towson University, Towson, MD 21252, USA
(slev@towson.edu)

The Chesapeake Bay impact structure is
thought to be the source of the North American
Tektite (NAT) strewn field (Poag et al., 1994;
Koeberl et al., 1996; McHugh et al., 1998 and
Glass et al., 1998). This correlation is primarily
based on the geochemistry of late Eocene tektite
samples from as far south as Barbados and north
to the New Jersey continental margin. The most
compelling geochemical evidence is the isotopic
fingerprint derived from the Sr and Nd isotopic
composition of the NAT strewn field samples
(Shaw and Wasserburg, 1982; Ngo et al., 1985;
Stecher et al., 1989 and Glass et al., 1998). The
Chesapeake Bay Drilling Project will provide an
opportunity to directly test this model. If the
Chesapeake Bay crater is truly the source of the
NAT then there should be a favorable
comparison between the isotopic composition of
the NAT samples and melt-rock, impact breccia
and the target bedrock, all of which are likely to
be encountered during the proposed drilling of
the central crater.

The majority of the NAT samples exhibit a
narrow range in their Sr and Nd isotopic
composition and are thought to be similar to
moldavites in that these tektites were derived
from melting of homogeneous surface deposits
during the initial impact (Stecher et al., 1989).
However, there is some heterogeneity when late
Eocene tektite samples from DSDP site 612 and
ODP site 904A are included (Shaw and
Wasserburg, 1982; Stecher et al., 1989 and Glass
et al., 1998). The 612 and 904A tektite deposits
are geographically closer to the target area than
other NAT samples and exhibit evidence of less
severe shock (Stecher et al., 1989; McHugh et
al., 1998 and Glass et al., 1998). This coupled
with the isotopic heterogeneity suggests that
these tektites, while still derived from the late
Eocene Chesapeake Bay impact, may represent
deeper parts of the target area stratigraphy.
Recent work by Kettrup et al. (2003)
compared the Sr and Nd isotopic composition of
impactites from the Popigai crater in Siberia to
the target rocks and previously published data
from microkrysites and associated Upper Eocene
microtektites. Based on this isotopic comparison
the Popigai crater is the likely source of these
ejecta. This is a significant result since prior to
this investigation the Upper Eocene ejecta, now
linked isotopically to the Popigai crater, were
thought to be too diverse geochemically to have
only one source. The isotopic range, as defined
by Kettrup et al. (2003), recorded by the target
area lithologies can account for the range in
Upper Eocene ejecta.

The range of lithologies present in the
Popigai target area are somewhat similar to the
range of lithologies in the Chesapeake Bay target
area in that the lithology changes with depth
from sands, shales and carbonates to crystalline
basement rocks. The heterogeneity present in
the Popigai target area led to a fairly large range
in the Sr and Nd isotopic composition of related
ejecta which may also be the case for the NATtype
ejecta including the DSDP 612 and ODP
904A sites. By correlating the geographic
distribution of ejecta derived from shallow target
area lithologies versus deeper lithologies we may
be able to gain insight into such questions as the
trajectory of the Chesapeake Bay projectile as
well as the extent of melting and mixing that
occurred post impact.

References Cited.
Glass, B. P., Koeberl, C., Blum, J. D.
and McHugh, C. M. G., 1998, Upper Eocene
tektite and impact ejecta layer on the
continental slope off New Jersey.
Meteoritics and Planetary Science: 33, p.
229-241.
Kettrup, B., Deutsch, A. and Masaitis,
V. L., 2003, Homogeneous impact melts
produced by a heterogeneous target? Sr-Nd
isotopic evidence from the Popigai crater,
Russia. Geochimica et Cosmochimica Acta:
67 (4), p. 733-750.
Koeberl, C., Poag, C. W., Reimold, W.
U. and Brandt, D., 1996, Impact origin of
the Chesapeake Bay structure and the
source of the North American tektites.
Science: 271, p. 1263- 1266.
McHugh, C. M. G., Snyder, S. W. and
Miller, K. G., 1998, Upper Eocene ejecta of
the New Jersey continental margin reveal
dynamics of Chesapeake Bay impact. Earth
and Planetary Science Letters: 160, p. 353-
367.
Ngo, H. H., Wasserburg, G. J. and Glass
B. P., 1985, Nd and Sr isotopic compositions
of tektite material from Barbados and their
relationship to North American Tektites.
Geochimica et Cosmochimica Acta: 49, 1479-
1485.
Poag, C. W., Powars, D. S., Poppe, L. J.
and Mixon, R. B., 1994, Meteroid mayhem in
Ole Virginny: Source of the North American
tektite strewn field. Geology: 22, p. 691-694.
Shaw, H. F. and Wasserburg, G. J.,
1982, Age and Provenance of the target for
tektites and possible impactites as inferred
from Sm-Nd and Rb-Sr systematics. Earth
and Planetary Science Letters: 60, p.155-177.
Stecher, O., Ngo, H. H., Papanastassiou,
D. A., Wasserburg, G. J., 1989, Nd as Sr
isotopic evidence for the origin of tektite
material from DSDP site 612 off the New
Jersey coast. Meteoritics: 24, p. 89-98.

CHARACTERIZING THE HYDROGEOLOGIC AND STRUCTURAL PROPERTIES OF AN
IMPACT CRATER FROM ANALYSIS OF GEOPHYSICAL LOGS  THE CHESAPEAKE BAY
DEEP COREHOLE. Roger H. Morin, U.S. Geological Survey, MS 403, Denver Federal Center, Denver,
Colorado 80225, USA.

Geophysical logging operations are planned
in conjunction with continuous coring of the
Chesapeake Bay Impact Crater Corehole. A
fairly comprehensive suite of logs would consist
of caliper, induction, long-short normal
resistivity, natural gamma activity, gamma
spectral, temperature, fluid conductivity, fullwaveform
sonic, acoustic televiewer, optical
televiewer, neutron porosity, gamma-gamma
density, flowmeter, magnetic susceptibility, and
formation micro-scanner. Geophysical logs have
been an important source of information for
lithologic interpretation at various drill sites in
the Virginia Coastal Plain (i.e., Powars and
Bruce, 1999).

However, it is anticipated that unstable hole
conditions, a heavy drilling mud, and possible
budget limitations may preclude some of these
measurements. Nevertheless, acquisition of data
from several of these logging tools should be
relatively routine and it is anticipated that some
good-quality logs will be successfully recovered.
Some of these measurements, such as resistivity
and gamma activity, may be directly related to
lithology. Others, such as porosity, temperature,
and fracture distribution, may infer hydrologic
properties and processes. In addition, the
combination of sonic and density logs may
provide information on the elastic properties of
the rocks and on their response to local stress
conditions; data derived from these two logs may
also be used to refine seismic and gravity
surveys, respectively, collected for site
characterization (Catchings and others, 2001).
It will be important to plan and integrate
complementary laboratory measurements made
on core samples with the interpretation of these
downhole measurements. Lab results will serve
as reference calibration points to improve
quantitative estimates of porosity, density, and
compressibility computed from logs.
Analysis and interpretation of log data
generated from a variety of geophysical tools
will help characterize the rheological properties
of the rocks cored in this impact structure and
provide a continuous vertical profile of changes
in these properties as the hole penetrates surficial
sediments, breccia, and basement rocks. This
general information will be of broad interest to
numerous other investigators who will use these
data within the context of their own studies.
Finally, logs obtained from this deep corehole
will be compared to other logs collected in a
nearby pilot hole drilled over the central peak of
the crater to examine spatial variability of
physical properties and spatial correlation among
lithologic sequences.

References Cited.
Catchings, R.D., Saulter, D.E., Powars, D.S.,
Goldman, M.R., Dingler, J.A., Gohn, G.S.,
Schindler, J.S., and Johnson, G.H., 2001,
High-resolution seismic reflection/refraction
images near the outer margin of the
Chesapeake Bay impact crater, York-James
Peninsula, southeastern Virginia: U.S.
Geological Survey Open-File Report 01-407,
18 p.
Powars, D.S., and Bruce, T.S., 1999, The effects
of the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p.

PROPOSAL FOR A COMPARATIVE STUDY OF EARLY CRATER MODIFICATION, INCLUDING
WATER RESURGE AND HYDROTHERMAL ACTIVITY IN A LARGE MARINE-TARGET IMPACT
CRATER, CHESAPEAKE BAY, USA. Jens Orm1, Fernando Aylln Quevedo1, Maurits Lindstrm2, Erik
Sturkell3, Enrique Daz Martnez1, Jess Martnez Fras1, David S. Powars4, and J. Wright Horton, Jr.4, 
1Centro de Astrobiologa, INTA/CSIC, Madrid, Spain (ormo@inta.es); 
2Stockholm University, Stockholm, Sweden; 
3Icelandic Meteorological Office, Reykjavk, Iceland, 
4U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA.

Introduction. Drilling a deep corehole in the
central part of the Chesapeake Bay impact crater
(CBIC) will provide information on several aspects
of marine impact cratering. Experience by the team
members from other marine-target craters, notably
the Lockne crater in central Sweden, indicates that
the most complete sequence of impact-generated and
postimpact deposits can be found in the inner crater
(fig. 1). It is also where most, if not all, of the effects
of impact generated heat can be studied. This
research proposal is based on comparison of
materials from the proposed drilling in the central
part of the CBIC, from existing coreholes in outer
parts of the crater, and from the well studied Lockne
crater. The team is composed of experts on marinetarget
cratering and modification, the sedimentology
of impact craters, and hydrothermal mineralogy.

Crater Modification. The CBIC is a large impact
structure that formed in relatively shallow water
when compared to other marine impacts such as
Eltanin and Lockne (Orm and Lindstrm, 2000). At
the size and water depth of CBIC, a crater rim too
high to allow forceful resurge is expected (Orm and
others, 2002). However, drill cores in the outer
slumped zone of the CBIC show that sediments of the
Exmore beds resemble the resurge deposit at the
Lockne crater. The Lockne resurge probably
contained relatively more water than at CBIC. At
Lockne, a nearly 14 km wide cavity formed in the
about 1 km deep target sea (Orm and others, 2002).
The excavation stripped most of the 80 m thick
sedimentary part of the target from a 2-3 km wide
zone surrounding a 7.5 km wide, nested, basement
crater (fig. 1). Extensive flaps from the crystalline
basement crater covered the stripped surface, but
were not thick enough to be an obstacle for resurge
flow. There is an apparent increase in the relative
content of fragments from the deepest part of the
target the towards the top of the resurge deposit (i.e.
in the coarse clastic Lockne Breccia towards the
arenitic Loftarstone). The Loftarstone is also the only
unit where melt fragments and shocked quartz have
been found. The interpretation is that the lower part
of the resurge unit was deposited from a debris flow
carrying material ripped up from the disturbed
sedimentary part of the target surrounding the crater.
Meanwhile, the upper part of the resurge unit carries
more high-energy ejecta from deeper parts of the
target, laid down from suspension. These beds seem
also to have been partly reworked, possibly from
oscillation due to collapse of a central peak of water
(Fig. 1). The situation at CBIC, with shallower water
deepening seaward and underlying target layers of
poorly consolidated sediments, may have generated
an even more debris-loaded flow than at lockne, with
less deposition out of suspension. Comparative
studies of the resurge deposits at CBIC and Lockne
can give valuable information on the nature of the
resurge flows in each crater and of the resurge
processes that produced them. Orm (1994) studied
the provenance of the fragments in the resurge
breccia of the marine Tvren crater, Sweden, and
found a relation between the resurge erosion and clast
distribution in the breccia. This technique can be used
at CBIC. It will also be possible at CBIC to assess the
relative timing of the block slumping and the resurge
flow. A deep drill core from the central part of the
CBIC can be compared to more distal drill cores,
revealing any spatial variations in clast distribution of
the resurge deposit. Investigations may also
determine if the water was deep enough to oscillate
due to collapse of a central peak of water, and how
far out in the megablock zone the effects of these
oscillations can be traced in the sediments.

Hydrothermal Activity. Studies at Lockne
indicate that even a relatively large marine-target
crater may lack the melt bodies expected at a similarsized
land-target crater in crystalline rock. At
Lockne, 1 km of the upper part of the transient cavity
was formed in the water column. This upper part
normally includes much of the zones of vaporization
and melting in a land-target crater of this size.
Impact-generated heat at Lockne only caused a
relatively short-lived, low temperature hydrothermal
system (Sturkell and others, 1998). The CBIC may be
sufficiently large, deep enough into crystalline
basement, and formed in sufficiently shallow water,
to have got a hot crater floor and melt bodies. Such
melt bodies could have had short-term violent
interaction with the resurging water and more
extended hydrothermal activity after the crater filled
with debris. The fluids in these hydrothermal cells
may have come both from the resurging seawater
(early stage) and from intruding groundwater (later
stage with different fluid composition). A deep
corehole in the central crater would provide insights
into post-impact hydrothermal activity, including
possible mineralization and alteration zones,
temperature and composition of the mineralizing
fluids, timing between seawater and ground-water
phases, and duration of the heat flow in both
basement and infill.

Inner and Outer Crater Comparisons. In the
CBIC, the thin lower Tertiary marine target
sediments found outside the crater are missing
throughout the crater except as disaggregated,
reworked particles (Powars and Bruce, 1999). These
thin, easily eroded sediments were either stripped off
by the initial, outward excavation flow or by the
subsequent inward water resurge. To distinguish
between excavation flow and water resurge, we
propose to compare materials and structures in the
inner and outer parts of the CBIC to those of Lockne
and other marine craters by integrating data from
coreholes or outcrops and seismic surveys. In the
Lockne crater, ejecta flaps were deposited before the
resurge (fig. 1), so missing strata below the flaps
must be related to the initial excavation flow.
However, on the eastern side of the Lockne crater,
where a smaller flap suggests less excavation flow,
missing sediments were possibly eroded by the water
resurge.

Investigations are needed at the CBIC to
determine if a flap is present and detectable by
seismic surveys or drilling. It is important to compare
data from drill core and seismic surveys in the central
crater with data from the outer part of the crater,
where slumping probably occurred before or during
the resurge. Numerical modeling could be used to
estimate the amount of excavation that occurred
outside the central crater before the water resurge and
block slumping.

Suggested Deep Corehole Positions. If possible,
the central crater should be drilled in at least two
locations. The maximum gain for the hydrothermal
study will, most likely, be from a corehole in the
deepest moat zone around the putative central peak.
The largest melt bodies in a crater of this size are
expected to be located in the moat. The moat is also
likely to have the most complete resurge sequence.
This drilling must reach at least a few hundred meters
into the fractured, and possible heated crystalline
rocks below the crater infill. The second drill site
should be located on top of the crystalline basement
crater rim to give information about the excavation
and resurge erosion at this position, as well as the
formation of the crystalline rim. It is important to test
for the existence and possible amount of overturned
crystalline material and (or) sediment layers at the
rim, and determine if sediments exist under an
overturned flap. This information would provide data
on excavation flow at a critical location for numerical
models.

References Cited.
Orm, J., 1994, The pre-impact Ordovician
stratigraphy of the Tvren Bay impact structure, SE
Sweden: Geologiska Freningens I Stockholm
Frhandlingar, v.116 (Pt. 3), p.139-144.
Orm, J., and Lindstrm, M., 2000, When a cosmic
impact strikes the seabed: Geological Magazine, v.
137, p. 67-80.
Orm, J., Shuvalov, V.V., and Lindstrm, M., 2002,
Numerical modeling for target water depth
estimation of marine-target impact craters: Journal
of Geophysical Research, v. 107, no. E12, p. 1-9.,
5120, doi:10.1029/2002JE001865.
Powars, D.S., and Bruce, T.S., 1999, The effects of
the Chesapeake Bay impact crater on the
geological framework and correlation of
hydrogeologic units of the lower York-James
Peninsula, Virginia: U.S. Geological Survey
Professional Paper 1612, 82 p.
Sturkell, E.F.F., Broman, C., Forsberg, P., and
Torssander, P., 1998, Impact-related hydrothermal
activity in the Lockne impact structure, Jamtland,
Sweden: European Journal of Mineralogy, v. 10,
no. 3, p. 589-606.

Fig.1. Schematic illustration of the sequence of excavation
and resurge at Lockne. Note the concentric
shape with a wide water cavity. Blue is water, yellow
is sedimentary rock, and red is crystalline basement.

PRE- AND POST-IMPACT PALEOCEANOGRAPHIC CONDITIONS ON THE EOCENE MIDATLANTIC
CONTINENTAL MARGIN: EVIDENCE FROM RADIOLARIANS.
Amanda Palmer-Julson, Natural Sciences Division, Blinn College, Bryan, TX 77805-6030, USA
(ajulson@blinn.edu).

Introduction. Biostratigraphic and
paleoenvironmental analysis of radiolarian
assemblages is proposed for material recovered
from the planned Chesapeake Bay impact
structure central crater corehole and related onand
offshore drill sites.

Previous Studies. Palmer (1987) studied
radiolarians from DSDP Leg 95 (Sites 612 and
613, New Jersey Transect) and Atlantic Slope
Project Site ASP 15. While less diverse than
tropical assemblages, the fauna provided
valuable biostratigraphic information. Later work
focused on re-investigating the timing of the
microtektite layer recovered at DSDP Site 612
(Miller and others, 1991), where once again
radiolarians proved valuable in supplementing
biostratigraphic results from calcareous
microfossils in establishing the nature and timing
of the impact event responsible for the
Chesapeake Bay crater.

Poag (1997) reported radiolarians in the
Chickahominy Formation recovered from the
Kiptopeke core (Virginia coastal plain). Based
on the presence of radiolarians, Poag interpreted
the depositional environment for this post-impact
unit as representing high biological productivity
in the surface waters.

Palmer (1986) demonstrated in a study of
Miocene radiolarians from the mid-Atlantic
coastal plain and continental margin that distinct
faunal differences allowed recognition of neritic
vs. oceanic assemblages. The Eocene
radiolarians of the region remain unstudied from
this perspective.

Proposed Study. I propose to conduct a
thorough analysis of radiolarian assemblages
from the Chesapeake Bay central crater corehole
and related coastal plain and offshore sites. The
biostratigraphic and paleonvironmental data
would be a valuable complement to the work of
other investigators.

References Cited.
Miller, K.G., Berggren, W.A., Zhang, and
Palmer-Julson, A.A., 1991, Biostratigraphy
and isotope stratigraphy of upper Eocene
microtektites at Site 612: How many
impacts? Palaios, v. 6, p. 17-38.
Palmer, A. A., 1986, Cenozoic radiolarians as
indicators of neritic versus oceanic influence
in continental margin deposits: U.S. mid-
Atlantic Coastal Plain, Palaios, v. 1, p. 122-
132.
Palmer, A.A., 1987, Cenozoic radiolarians from
Deep Sea Drilling Project Sites 612 and 613
(Leg 95, New Jersey Transect) and Atlantic
Slope Project Site ASP 15: Initial Reports
DSDP, v. 95, p. 339-357.
Poag, C.W., 1997, The Chesapeake Bay bolide
impact  A convulsive event in Atlantic
Coastal Plain evolution, in Seagall, M.P.,
Colquhoun, D.J., and Siron, D., eds.,
Evolution of the Atlantic Coastal Plain 
sedimentology, stratigraphy, and
hydrogeology: Sedimentary Geology, v. 108,
no. 1-4, p. 45-90.

PHYSICAL PROPERTIES AND PALAEOMAGNETISM OF THE DEEP DRILL CORE FROM THE
CHESAPEAKE BAY IMPACT STRUCTURE - A RESEARCH PROPOSAL. Lauri J. Pesonen1, Tiiu Elbra1, Martti
Lehtinen2, Johanna M. Salminen1 and Fabio Donadini1, 
1Division of Geophysics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki. (lauri.pesonen@helsinki.fi), 2Department of Geology, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki.

Introduction. Deep drilling has become a powerful
tool to test the validities of the geological and
geophysical models of large impact structures (Dressler
and others, 2003). Drilling has also become important in
proving that some of the circular structures are impact
structures (Pesonen and others, 1999a,b; Tsikalas and
others, 2002). The programme provided by the ICDP
(International Continental Scientific Drilling Program)
has opened new challenges for drilling through large
impact structures, such as the 65 Ma old Chicxulub
structure in Yucatan, Mexico (Dressler and others, 2003)
and the 1.07 Ma Bosumtwi structure in Ghana (Plado and
others, 2000). We describe a project plan where highresolution
paleomagnetic and petrophysical
measurements will be carried out of the future deep drill
cores through the Chesapeake meteorite impact structure,
Virginia, USA. In our project we will use an ultra
sensitive DC SQUID magnetometer for paleomagnetic
determinations coupled with novel petrophysical
techniques recently developed for impact research at the
Division of Geophysics and Department of Geology of
the University of Helsinki, Finland. Our proposal
includes microscopic and X-ray analysis of the samples
in order to understand the shock-induced changes in the
physical properties of the target rocks. The results will be
used for calibrating the geophysical loggings, for dating
the post-impact, impact and target rocks and for
providing constraints for the 4D geophysical modelings
of the Chesapeake structure.

Fig. 1. A view of the new paleomagnetism and petrophysical
laboratory of the Geophysics Division the University of
Helsinki.

Sampling. Chesapeake is a 35 Ma old shallow
marine, complex impact structure with a diameter of ca.
85 km. The structure has been mapped with shallow
drillings and geophysical data but its horizontal width
and vertical depth are poorly known (Poag, 2003). A
series of deep drill cores is planned by the joint efforts of
ICDP and USGS and other partners. If the drilling will
be realized, our aim is to analyze small chips of the core
including post-impact, impact and pre-impact (basement)
units of the structure. The sampling interval will be dense
enough to allow high-resolution paleomagnetic,
magnetostratigraphic and paleosecular variation (PSV)
data to be extracted from the core. A dense sampling
extending through the fractured bedrock down to
unfractured bedrock will help to delineate the
progressive downward damping of the shock effects on
the physical properties of rocks. Petrographic studies
(thin section studies with optical microscopy and X-ray
analysis; Pesonen and others, 1999a) of the samples will
be carried out to determine their degree of shock.
Microscopic studies will also provide constraints in
estimating the hydrothermal changes in remanent
magnetization and in other physical properties. If the
drill core is azimuthally oriented, the samples will allow
true vector paleomagnetic data. In the case of an
unoriented core, the magnetic declination will be
measured with respect to a common fiducial line along
the core, which provides relative orientation. The
possibility to use the viscous remanent magnetization
(VRM) technique to reorientate the core will be studied
(Jrvel and others, 1996). 

Paleomagnetic measurements. Paleomagnetic measurements,
including magnetic polarity determinations,
will be done using an automated ultra sensitive DCSQUID
magnetometer. Both alternating field (up to 160
mT) and thermal demagnetization (up to 700 C)
treatments will be done for each specimen to allow the
various remanence components to be isolated. One of
them may be a drilling induced remanence, which will be
studied with care. Magnetic hysteresis properties will be
measured with a VSM in order to determine the relative
amount of low coercive force grains, which are relevant
to understand the shock remanent magnetization possible
present in the impactite units. Hysteresis properties are
also used to determine the magnetic domain sizes of the
remanence carriers. We will also measure the coercivity
spectra of the remanence carriers using new IRM- and
ARM-instruments installed in the laboratory of the
University of Helsinki (Fig. 1).

Petrophysical measurements. The following physical
properties of the drill core samples will be measured.
The instruments used are mostly described in Pesonen et
al. (1999a).
 dry and wet bulk densities and the grain density using
the Archimedean principle
 porosities (effective, total)
 seismic velocities (P and S) and their attenuation using
ultrasonic techniques
 magnetic susceptibility and its anisotropy
 natural remanent magnetization (NRM) and its demagnetization
characteristics, its nature and origin
 electrical conductivity and inductive conductivity
 thermal conductivity and thermal diffusivity
These measurements are used to calibrate the drill hole
geophysical logging data. The knowledge of the physical
properties of various rocks are crucial for modeling the
gravity, magnetic, electromagnetic and seismic profiles
since the geophysical models strongly depend on the
contrasts of physical properties between the rocks of the
impact structure and those beyond. The physical property
data are relevant to study the depth extent of shock on
the drill core (Pesonen and others, 1992; Langenhorst
and others, 1999; Plado and others, 2000). Measurements
of the thermal conductivity and heat capacity will be
carried out for understanding the cooling history of the
structure.

Other magnetic measurements. To understand the
paleomagnetic data and the effects of shock on magnetic
properties of the rocks we will carry out:
 magnetic mineral determinations with KLY-3 using
both low and high temperature treatments
 magnetic susceptibility anisotropy determinations with
KLY-3 device to interprete the paleomagnetic data and
to look correlations with shock degree and magnetic
anisotropy
 magnetic hysteresis and grain size determinations using
VSM magnetometer and IRM and ARM instruments in
order to interpret the paleomagnetic data

Summary. We have a plan for a geophysical project
where high-resolution paleomagnetic and petrophysical
measurements of the future drill cores from the
Chesapeake meteorite impact structure will be carried
out. Our proposal includes microscopic and X-ray
analysis of the samples in order to understand the shockinduced
changes in the physical properties of the rocks.
The results will be used for calibrating the geophysical
loggings, for constructing the magnetostratigraphy of the
sequences to get estimates of the ages of the post-impact,
impact and fractured target rocks. The data will provide
constraints for the 4D geophysical modelings.

References
Dressler, B.O., Sharpton, V.L., Morgan, J., Buffler, R.,
Moran, D., Smit, J., Stffler, D., Urrutia, J., 2003,
Investigating a 65-Ma-old smoking gun: Deep
drilling of the Chicxulub impact structure. EOS
Trans. AGU, 84 -125.
Jrvel, J., Pesonen, L.J., Sarap ,O. and Pietarinen, H.
1996. The Iso-Naakkima metorite impact structure:
physical properties and palaeomagnetism. Open File
Report Q19/29.1/3232/95/1, Geological Survey of
Finland, 43 p.
Langenhorst, F., Pesonen, L.J., Deutsch, A. and
Hornemann, U., 1999. Shock experiments on
diabase: microstructural and magnetic properties. In:
30th Lunar Plan. Sci., Abstract No. 1241 (CDROM).
Pesonen, L.J., Marcos, N. and Pipping, F., 1992.
Paleomagnetism of the Lake Lappajrvi impact
structure, western Finland. In: L.J. Pesonen & H.
Henkel (eds.), Terrestrial Impact Craters and
Craterform Structures, with a Special Focus on
Fennoscandia. Tectonophysics, 216, 123-142.
Pesonen, L.J., Elo, S., Lehtinen, M., Jokinen, T.,
Puranen, R. and Kiveks, L., 1999a. The Lake
Karikkoselk impact structure, central Finland - new
geophysical and petrographic results. In: B. Dressler
and V.L. Sharpton (eds.), Geological Society of
America Special Volume, 392, 131-147.
Pesonen L.J., Kuivasaari, T., Lehtinen, M., Elo, S.,
1999b. Paasselk - a new meteorite impact structure
in Eastern Finland, Meteoritics & Planetary Science,
34, no.4, Suppl., 91-92
Plado, J., Pesonen, L.J., Koeberl, C. and Elo, S., 2000.
The Bosumtwi meteorite impact structure, Ghana: A
magnetic model. Meteoritics & Planetary Science
35, 723-732.
Poag, C.W., Koeberl, C., Reimold, W.U., 2003. The
Chesapeake Bay Crater. Springer, Berlin, Germany,
522 p.
Tsikalas, F., Faleide, J.I., Eldholm, O., Dypvik, H., 2002.
Seismic correlation of the Mjolnir marine impact
crater to shallow boreholes. In: J. Plado, L. Pesonen,
Impacts in Precambrian shields. Springer, Berlin,
Germany, 307-321

A PROPOSAL TO COLLECT TENSOR MAGNETO-TELLURIC SOUNDINGS ACROSS THE
CENTRAL CRATER OF THE CHESAPEAKE BAY IMPACT STRUCTURE. Herbert A. Pierce,
U.S. Geological Survey, National Center, MS 954, Reston, VA 20192, USA (hpierce@usgs.gov).

Introduction. During 2000 and 2001 two
audio-magnetotelluric (AMT) electrical sections
were constructed across the outer margin of the
Chesapeake impact crater consisting of a total of
17 stations. The tensor audio-magnetotelluric
soundings on the York-James and Middle-Neck
Peninsulas demonstrated that electromagnetic
soundings could provide electrical data to a
depth of 800 meters important to understanding
the overall structure of the crater and map the
location of the outer margin. The electrical
response of the sediments and bedrock coupled
with borehole resistivity information allowed
interpretations to be extended to the James River
from the two coreholes drilled on the NASA
Langley Research Center property during 1975
and 2000 (Gohn and others, 2001). AMT stations
collected near Mathews and to the northeast
allowed an electrical section to be constructed
that provided information to a depth of 250-800
meters.

Fig. 1. Location of proposed MT transect indicated by
the white dashed line originating northeast of Cape
Charles.

Proposal. To map the electrical structure
within the central crater I propose an experiment
to collect 20-30 of the lower frequency tensor
magneto-telluric (MT) soundings and use them
to identify depth to basement and geometry from
Cape Charles northeast along the axis of the
Delmarva Peninsula (figure 1). The location of
the soundings would be spaced approximately
one kilometer apart starting from Cape Charles
and proceeding to the northeast across the central
crater of the Chesapeake Bay impact structure.
These MT soundings and the electrical sections
made from them will have a greater depth of
exploration than the AMT soundings because of
the much lower frequencies employed. The MT
electrical sections will miss the upper 100 meters
but will cover depths from 100 meters to more
than five kilometers. The technique can collect
information routinely to a depth 40 kilometers
with a limit of about 100 kilometers for long
duration soundings. Stations will use a remote
reference to remove noise associated with
cultural activity. Interpretations of the electrical
sections should provide insight on the structure,
geometry, and map depth to bedrock within the
central crater of the Chesapeake Bay impact
structure.

References Cited.
Gohn, G.S., Clark, A.C., Queen, D.G., Levine,
J.S., McFarland, E.R., and Powars, D.S., 2001,
Operational summary for the USGS-NASA
Langley corehole, Hampton, Virginia: U.S.
Geological Survey Open-File Report 01-87-A,
21 p.
http://geology.er.usgs.gov/eespteam/crater/OF
R_01_87.pdf

A Proposal to Analyze the Stratigraphic and Paleoecological Record of Synimpact to Postimpact
Transition in the USGS-ICDP Central-Crater Core, Chesapeake Bay Impact Crater.
C. Wylie Poag, US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543-1598, USA
(wpoag@usgs.gov)

Introduction. Poag (2002), Poag and others
(2004), and Poag and Norris (in press) have
documented the transition from synimpact
deposition to postimpact deposition in four cores
taken from inside the Chesapeake Bay impact
crater (Kiptopeke core on southern tip of
Delmarva Peninsula; USGS-NASA Langley core
on James-York Peninsula; North core and
Bayside core on Middle Neck). These authors
describe the transition as a complex succession
of litho- and biofacies that separates the
synimpact Exmore impact breccia (sensu lato)
from the overlying postimpact Chickahominy
Formation. Three stratigraphic units have been
identified in the transitional interval.

Flowin Facies. The stratigraphically lowest
transitional unit consists of dark, greenish-gray,
clayey silt, with centimeter-scale laminae of fine
to very fine sand. Nodular concentrations of
framboidal pyrite are numerous, and
foraminifera reworked from deeper in the
Exmore breccia (chalky, leached specimens) are
concentrated in the thin, white, horizontal sand
laminae, along with concentrations of muscovite
flakes. Indigenous microfossils appear to be
lacking, however.

At NASA Langley, this silt-rich layer is in
sharp contact with the underlying glauconitic
quartz sand of the Exmore breccia, but at
Bayside and North, the basal contact is
transitional. Also, the upper part of the silt-rich
layer at Bayside and North is more obviously
stratified with white, sandy, micaceous laminae
and burrow casts, whose spatial orientations
change markedly along the core. Some laminae
are horizontal, but others are inclined, with
variable angles and directions. The
multidirectionality of laminae in this unit has
been interpreted to result from successive
turbidity currents triggered by impact-generated
storms (possibly hypercanes). This is the flowin
lithofacies of Poag and others (2004).

Fallout Layer. The upper ~3 cm of the
laminated, silt-rich interval at NASA Langley,
contains numerous millimeter-sized, porous,
lattices of framboidal pyrite. The key impactrelated
features of the pyrite lattices are the pore
structures. Each pore is nearly perfectly
spherical, of uniform ~1-mm diameter, and
spatially arranged as if the lattice originally had
enveloped a layer of microspherules ~34 mm
thick. These properties are similar to those of
impact-derived layers of glass microspherules
reported from other fallout ejecta deposits. Poag
(2002), Poag and others (2004), and Poag and
Norris (in press) inferred that the pores in the
pyrite lattices originally contained glass
microspherules ejected from the Chesapeake Bay
crater. Over time, the microspherule glass
dissolved, or altered to clay. Though a
stratigraphically equivalent silt-rich interval is
present above the Exmore breccia at the other
core sites (where it is inferred to be the final
impact-generated deposit), the pyrite lattices
have been found only at NASA Langley.

Dead Zone. The initial postimpact deposit
is 19-49 cm thick, and composed mainly of fine,
horizontal, parallel laminae of fine to very-fine
sand, silt, and clay. The clay and silt laminae are
disturbed in places by burrows, which are filled
with medium to coarse sand and microfossils
reworked from the underlying Exmore breccia.
Additional reworked microfossils comprise
much of the micaceous white sand concentrated
in horizontal laminae and lenses. The lack of
indigenous microbiota in this interval led Poag
(2002) and Poag and others (2004) to interpret
this as a dead zone, representing
paleoenvironments hostile to marine life. The
dead zone differs from the silt-rich layer below
the fallout layer primarily in having more
uniform, horizontal, parallel distribution of
laminae, and lacking nodular pyrite. In the
Bayside core, the dead zone reaches its
maximum known thickness (~49 cm). Since no
pyrite-lattice layer has been identified at
Bayside, however, the base of the dead zone
there can only be approximated by the upward
change from multidirectionally inclined,
moderately thick laminae, to horizontal, thin
laminae.

Chickahominy Formation. The dead zone
is succeeded conformably by as much as 220 m
of silty marine clay assigned to the
Chickahominy Formation. Chickahominy
deposition continued for ~2.1 m.y. to the end of
the Eocene. The exceptional Chickahominy
sedimentary record is a product of relatively
deep-water, fine-grained, microfossiliferous
deposition within a slowly subsiding, closed
basin, which underwent no syndepositional
tectonism or synchronous major eustatic sea-
level changes. Subsequent Cenozoic sea-level
falls and marine transgressions have eroded the
top of the Chickahominy Formation, but it has
never been subjected to significant tectonic
activity.

Planktonic foraminifera and bolboformids
document biochronozones P15 and P16-17 of the
late Eocene in the Chickahominy Formation
(Poag and Aubry, 1995). Coeval benthic
foraminferal assemblages constitute a single
biozone (Cibicidoides pippeni Zone), which is
represented by 150 calcareous and agglutinated
species. The Cibicidoides pippeni Zone can be
further divided into five subbiozones. The
species represented in the Cibicidoides pippeni
assemblage indicate a paleodepth of ~300 m for
the Chickahominy seafloor, which exhibited
oxygen deprivation and high flux rates of organic
carbon.

Clearly, understanding the nature and
history of the flowin facies, fallout layer, dead
zone, and Chickahominy Formation can yield
crucial information regarding the age of the
impact, the late Eocene paleoenvironments, the
resultant marine deposits, and their coeval biota.

Proposed Research. I propose to analyze
the mineralogical and micropaleontological
record preserved in sediments between the base
of the flowin facies and the top of the
Chickahominy formation in the new USGSICDP
core. I will interpret the record in the
context of previous studies, and test hypotheses
regarding the stratigraphic succession and
paleoecological implications of the sediments
(and their biota) and determine the genesis of the
respective depositional units. The analysis will
apply mainly standard micropaleontological
techniques, using a binocular stereomicroscope,
augmented with scanning electron microscopy.
Analyses will determine biozonation, species
richness and diversity, predominance,
equitability, and microhabitats.

Sample Requirements. Proposed analyses
require two different sets of samples: (1) The
first set should be quarter-core samples, 2.5 cm
long, taken in continuous succession from the
top of the sandy Exmore section to the base of
the Chickahominy Formation. Such dense
sample distribution is required because the
section is relatively thin, and some important
features (such and the fallout layer) could easily
be missed in any sampling gaps. (2) The second
sample set is restricted to the Chickahominy
Formation. Previous studies cited above show
that quarter-core samples, 2.5 cm long, spaced
one-meter apart, are sufficient to interpret the
biostratigraphic and paleoecological record of
the Chickahominy.

Fig. 1. Segment of USGS-NASA Langley core,
showing lithic transition from Exmore breccia (sensu
lato) through dead zone. From Poag and others
(2004).

References Cited.
Poag, C.W., 2002, Synimpact-postimpact
transition inside Chesapeake Bay impact
crater: Geology, v. 30, p. 995-998.
Poag, C. W., and Aubry, M.-P., 1995, Upper
Eocene impactites of the U.S. East Coast:
Depositional origins, biostratigraphic
framework, and correlation: PALAIOS, v.
10, p. 16-43.
Poag, C.W., Koeberl, C., and Reimold, W.U.,
2004, The Chesapeake Bay Crater  Geology
and Geophysics of a Late Eocene Submarine
Impact Structure: Springer, Berlin, 522 p,
with CD-ROM.
Poag, C.W., and Norris, R.D., in press,
Stratigraphy and paleoenvironments of Early
Postimpact deposits  USGS-NASA Langley
core, Chesapeake Bay impact crater, in
Horton, J.W., and Powars, D.S., (eds.),
Studies of the Chesapeake Bay impact
structure  The USGS-NASA Langley
corehole, Hampton, Virginia, and related
corehole and geophysical surveys: Chapter F.

PROGRESS REPORT AND CONTINUING PROPOSAL FOR COLLABORATIVE RESEARCH ON
LITHOSTRATIGRAPHY, SEISMOSTRATIGRAPHY, AND STRUCTURE OF THE CHESAPEAKE BAY
IMPACT STRUCTURE. David S. Powars1, Gregory S. Gohn1, J. Wright Horton, Jr.1, and Rufus D. Catchings2,
1U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA (dspowars@usgs.gov); 
2U.S. Geological Survey, 345 Middlefield Rd, MS 977, Menlo Park, CA 94025-3591, USA.

Introduction. Geologic and geophysical field
investigations of the marine, late Eocene Chesapeake
Bay impact structure have revealed a buried 85-kmwide
complex crater surrounded by a ~35-km-wide
outer fracture zone. The impact into an eastward
dipping, multi-layered target (crystalline basement
overlain by water-saturated, unconsolidated sediments,
and seawater) produced a 38-km-wide central crater
surrounded by a 21 to 31 km-wide annular trough (see
fig 1). The deepest part of the central crater or moat
excavated crystalline basement to a depth of >2-km,
and it encircles a central uplift (fig. 1). The central
crater is surrounded by a raised and faulted crystalline
basement. The western part of the annular trough
consists of locally faulted crystalline basement overlain
by impact-modified and impact-generated sediments,
and it has an outer margin of slumped, unconsolidated
sediment blocks.

Fig.1 Interpretive cross section of western side of the
Chesapeake Bay impact crater.

Most of the past studies primarily have focused on
the western half of the structures annular trough and
outer margin. The current and planned integration of
data and interpretations from coreholes and
geophysical surveys in the Chesapeake Bay impact
structure is designed to develop a three-dimensional
understanding of the craters lithology, stratigraphy,
structural geometry, and evolution. These
investigations are part of the U.S. Geological Survey
(USGS) Chesapeake Bay Impact Crater Project, which
is supported by the National Cooperative Geologic
Mapping Program in cooperation with other
organizations. Extension of this research into the
central crater is facilitated by a proposal to the
International Continental Scientific Drilling Program
(ICDP) for a deep corehole in the central crater of the
Chesapeake Bay impact structure.

Scientific Issues. Understanding the crater
materials and structure will provide a framework for
regional ground-water models as well as constraints for
modeling the crater formation and mechanics,
including the size of the transient cavity, the energy of
impact, morphologic evolution of the crater during its
formation, effects of bathymetric and sediment depths
and asymmetries, and general implications for
terrestrial marine craters. Some questions include
structure and composition of the crystalline basement
and its influence on crater formation; the nature of the
craters inner ring, moat, and central peak; and the
mechanics of shock compression, gravitational collapse
of the transient cavity, seawater resurge, and
postimpact settling and slumping.

Current Methods. Results and interpretations
based on direct observations, measurements, and
analysis of samples from coreholes and water wells are
extended laterally by correlation with seismic
reflection and refraction surveys. They also provide
observed constraints on the geometry and physical
properties of rock volumes at depth for more realistic
models and interpretations of regional gravity and
magnetic surveys.

Preliminary Results. Preliminary results from
integrating data from four recent coreholes in the
western annular trough, new high-resolution seismic
reflection and refraction surveys, and gravity and
magnetic surveys in progress are summarized in recent
abstracts (Catchings and others, 2002; Gohn and
others, 2002; Horton and others, 2002, 2003; Poag and
others, 2002; Powars and others, 2002, 2003).
Highlights include evidence (from lack of shock and
thermal features) that granites from two coreholes to
basement in the annular trough were outside the
transient cavity, that the outer margin of the annular
trough formed by inward extensional collapse, and that
the outer to central annular trough consists of a variable
pile of highly fractured and fault-bound
parautochthonous mega-slump blocks overlain and
injected by resurge deposits while the inner 8-km of the
annular trough consist mainly (?) of resurge deposits
overlying the crystalline basement. The outer to central
annular trough appears to have four main concentric
zones of extensional collapse structures that are formed
by numerous small-offset faults rather than a few major
faults. Most of these collapse structures appear to be
associated with normal and compressional basement
faults and are sites of the greatest postimpact
subsidence in the trough. Gravity surveys in progress in
the central crater support the existence of a central peak
surrounded by an irregularly shaped structurally
complex moat. Improved hydrogeologic models are a
current and future societal benefit for a region that is
increasingly dependent on limited supplies of fresh
ground water.

Proposed Collaboration in the Central Crater.
Understanding the crater materials and structure
require expanding beyond current studies of the
annular trough to include the central crater. One or
more deep corehole(s) in the central crater will provide
constraints for interpreting and modeling seismic
reflection and refraction, gravity, and magnetic data to
address stratigraphic and structural questions away
from the corehole itself.

Current and planned investigations of lithology and
stratigraphy of synimpact crater-fill deposits are based
on samples from coreholes and correlations of the
corehole data with seismic reflection and other
geophysical surveys in order to understand the three-dimensional
distribution, stratigraphy, and structural
relations of crater materials. Planned investigations
include sedimentological comparisons of the Exmore
beds in the annular trough and central crater to
determine the character of seawater resurge and
tsunamis. Plans for future research are to extend these
studies into the central crater to include presently
unknown impact breccias, impact melt rocks, and (or)
suevites that may underlie the resurge deposits. Plans
for structural analysis includes the study of faults,
fractures, and veins, and contrasting features that
distinguish contractional deformation due to the initial
compression and excavation from extensional
structures related to subsequent collapse of the
transient cavity.

We propose to build on current collaboration with
geophysicists involved in seismic reflection and
refraction, gravity, magnetic, and MT surveys, and
with numerical modelers. We also invite collaborate
with international experts on marine craters in
comparative studies to understand the influence of
unconsolidated versus consolidated target layers and
water depths on formation of resurge deposits, and
contrasting processes of fluidization in preimpact target
rocks.

References Cited.
Catchings, R.D., Powars, D.S., Gohn, G.S., and
Goldman, M.R., 2002, High resolution seismic
reflection survey of the southwestern margin of the
Chesapeake Bay impact structure, Virginia [abs.]:
EOS Transactions, American Geophysical Union,
v. 83, no. 19, Spring Meeting Supplement,
Abstract T21A-05.
Gohn, G.S., Powars, D.S., Quick, J.E., Horton, J.W.,
Jr., and Catchings, R.D., 2002, Variation of impact
response with depth and lithology, outer annular
trough of the Chesapeake Bay impact structure,
Virginia Coastal Plain [abs.]: Geological Society
of America Abstracts with Programs, v. 34, no. 6,
p. 465.
http://gsa.confex.com/gsa/2002AM/finalprogram/a
bstract_45483.htm
Horton, J.W., Jr., Gohn, G.S., Edwards, L.E., Self-
Trail, J.M., Powars, D.S., Kunk, M.J., and Izett,
G.A., 2003, Recent research in the Chesapeake
Bay impact crater, USAPart 2. Reworked
impact ejecta and impact debris [abs.]: Third
International Conference on Large Meteorite
Impacts, August 5-7, 2003, Noerdlingen,
Germany, Abstract 4051.
http://www.lpi.usra.edu/meetings/largeimpacts200
3/pdf/4051.pdf
Horton, J.W., Jr., Kunk, M.J., Naeser, C.W., Naeser,
N.D., Aleinikoff, J.N., and Izett, G.A., 2002,
Petrography, geochronology, and significance of
crystalline basement rocks and impact-derived
clasts in the Chesapeake Bay impact structure,
southeastern Virginia [abs.]: Geological Society
of America Abstracts with Programs, v. 34, no. 6,
p. 466.
http://gsa.confex.com/gsa/2002AM/finalprogram/a
bstract_44052.htm
Poag, C.W., Gohn, G.S., and Powars, D.S., 2002,
Chesapeake Bay impact crater: Seismostratigraphy
and lithostratigraphy of displaced sedimentary
megablocks [abs.]: 33rd Lunar and Planetary
Science Conference, League City, Texas, March
2002, Abstract No. 1019.
Powars, D.S., Gohn, G.S., Catchings, R.D., Horton,
J.W., Jr., and Edwards, L.E., 2003, Recent
research in the Chesapeake Bay impact crater,
USAPart 1. Structure of the western annular
trough and interpretation of multiple collapse
structures [abs.]: Third International Conference
on Large Meteorite Impacts, August 5-7, 2003,
Noerdlingen, Germany, Abstract 4053,
http://www.lpi.usra.edu/meetings/largeimpacts200
3/pdf/4053.pdf
Powars, D.S., Gohn, G.S., Edwards, L.E., Catchings,
R.O., Bruce, T.S., Johnson, G.H., and Poag, C.W.,
2002, Lithostratigraphic framework of the craterfill
deposits: Western annular trough, Chesapeake
bay impact crater [abs.]: Geological Society of
America Abstracts with Programs, v. 34, no. 6, p.
465.

ARGUMENT FOR ICDP SCIENTIFIC DRILLING OF THE CHESAPEAKE BAY IMPACT
STRUCTURE, EASTERN SEABOARD, UNITED STATES, AND MOTIVATION FOR
PARTICIPATION IN THIS PROJECT BY THIS CONSORTIUM. W.U. Reimold1, C. Koeberl2, I.
McDonald3, R.L. Gibson1, P.J. Hancox1, and T.C. Partridge4, 
1 Impact Cratering Research Group (ICRG), School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa (reimoldw@geosciences.wits.ac.za), 
2 Department of Geological Sciences, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria (Christian.koeberl@univie.ac.at), 
3 School of Earth, Ocean and Planetary Sciences, Cardiff University, P.O. Box 914, CrdiffCF10 3YE, United Kingdom
(iain@earth.cf.ac.uk), 
4 School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, private Bag 3, P.O. Wits 2050, Johannesburg, South Africa (tcp@iafrica.com)

Why ICDP drilling of Chesapeake Bay?
Amongst the approximately 170 impact
structures known on Earth, all formed in
continental (including the continental shelf)
environments. A recent compilation by Dypvik
and Jansa (2003) lists 7 impact structures that
formed in the submarine environment (but on the
continental shelf) and have remained located in
the marine realm. Further 11 impact structures
are known to have formed in the marine realm
but are now located on land. Chesapeake Bay, at
an assumed diameter of approximately 80 km,
represents the second largest of these 18 marine
impact structures, only surpassed by the about
200 km Chicxulub impact structure in Mexico
that already has been drilled by ICDP earlier this
year. The Yaxcopoil-1 drill core is currently the
subject of a worldwide investigation and has the
potential to make a major contribution to our
understanding of the Chicxulub impact event
and, consequently, the global environmental
effects that this impact at K/T boundary times
must have had. However, the single ca. 1500 m
long drill core from Chicxulub did not explore
the entire crater fill nor did it extend into the
crater floor. While this borehole has certainly
provided extensive new information about the
Chicxulub impact, this project can not solve all
remaining problems about this particular impact,
per se, and impact cratering in the marine
environment, in general.

Chesapeake Bay already has been studied
quite extensively, especially by shallow drilling
and geophysical analysis (e.g., Poag et al., 2003
 and literature reviewed therein). Thus, much of
the groundwork required for successful
identification of a drilling location has already
been completed. But what are the reasons why
Chesapeake Bay should be investigated by
comprehensive drilling? The structure represents
a relatively young (35 Ma) impact structure that
is nearly completely preserved. The sedimentary
cover strata are not very thick, so that drilling of
the crater interior would not be curtailed by
expensive drilling of cover strata. However, the
cover strata themselves represent a formidable
argument in favor of drilling into Chesapeake
Bay, as they represent a complete cross-section
through the eastern Coastal Plains of the US,
drilling of which would complement the
extensive former drilling and sequence
stratigraphic investigations of these plains that
have been conducted further to the north. The
age of the impact crater, as constrained by
biostratigraphy and paleomagnetism, seems to
coincide with a strong paleo-environmental event
that affected much of this planet at that time.
Near coincident with the Chesapeake Bay impact
is also the Popigai impact into northeastern
Siberia, and much effort has been made in
identifying the respective global effects of these
two impact events. Chesapeake Bay is the most
likely source crater for the North American
tektite strewnfield, but the currently available
materials from this crater have so far not allowed
to unambiguously confirm this hypothesis.

Furthermore, Chesapeake Bay is of
significant hydrological importance as a water
source for some 1.6 million people living in the
environs of the structure, with much of the
reservoir being brackish. Thus, drilling of the
crater could make a major contribution to the
knowledge about and hydrogeological modeling
of this important water reservoir.

Above all, drilling of this impact structure
has the potential to strongly supplement the
currently very limited understanding of
catastrophic impact into the shallow marine
environment and the subsequent evolution of
such a large, complex impact structure. In detail,
drilling would contribute to the following
aspects:
* General understanding of the impact
process for cases of impact into shallow
marine environment.
* Provide a ground truth basis for
improvement of numerical modeling of
this process.
* Obtain a full succession of crater fill
materials (impactites and post-impact
sediment), study of which will
contribute to the previous aspects, and
will be used to investigate the possible
association with the North American
tektite strewn-field.
* Hands-on analysis of crater and subcrater
materials is required to provide a
basis for geophysical modeling (e.g.,
gravity modeling).
* Without a full understanding of the
crater fill, crater modeling is not
possible and the actual crater size can
not be scaled at sufficient precision.
Once the size of the crater will be better
constrained, the impact energy can be
scaled, and, thus, inference made
regarding the environmental effect that
this large-scale event would likely have
caused.
* What is the nature of the crater fill  in
comparison to the drilling information
from Chicxulub?
* What is the nature and deformation of
the crater floor and basement below the
crater?
* Investigation of the post-impact
hydrothermal overprint on the crater
and basement below could be
investigated.
* What is the nature of the central uplift
(mega-breccia?) that has been modeled
(see Poag et al., 2003)? How did it form
and collapse?
* What is the macro- and microdeformation
of the crater floor and
basement rocks? Can shock and thermal
zoning be identified?
* The nature of the Exmore Breccia 
considered by Poag et al. (2003) the
result of post-impact wave and wind
action  must be clarified.
* Search for development of life in
impactite and deformed impact
basement settings could be carried out.
* Is there significant impact melt? In
which form (coherent or disseminated)
does it occur?
* Is it possible to utilize such melt to
obtain absolute age data for the impact
event?
* In comparison to Chicxulub and the 10
km Bosumtwi crater, Ghana, that will
hopefully be drilled with ICDP support
in 2004, the intermediate size
Chesapeake Bay structure will provide a
medium sized benchmark for
comprehensive impact modeling.

In conclusion, drilling of Chesapeake Bay
would provide information regarding the preimpact
geological setting, the impact process
itself, post-impact sedimentation and biogenic
development, and present-day hydrology of the
crater.

Participation of this research consortium in
the Chesapeake Bay ICDP drilling project.
The members of this team all have
extensive expertise in the field of impact
cratering studies, and some of them have
participated in the investigation of post-impact
crater sediment, from a paleo-climatic/-
environmental point of view (PJH, TCP and D.
Brandt of the ICDP). WUR will concentrate on
the mineralogical and, in collaboration with CK
and IMcD, geochemical study of impactites of
the crater fill and impactite and pseudotachylitic
breccia injections into the crater floor. Aims of
these studies will include thorough
characterization of the target rocks, comparison
of these with impactite compositions  also for
the purpose of comparison with the North
American tektites, possible identification of a
meteoritic (projectile) component in impact melt
rock (by instrumental neutron activation analysis
[CK] and ICP-MS analysis of the platinum group
elements [IMcD]), and investigation of
hydrothermal overprint on crater fill and
basement materials. In addition, detailed study of
the mesoscopic (structural) deformation of the
crater floor and thermal and shock deformation
of the crater floor rocks will be carried out
(WUR, Roger Gibson of the ICRG) in order to
contribute to our understanding of shock and
thermal energy distribution across the central
parts of large impact structures (depending on
where with respect to a Chesapeake Bay central
uplift feature would be drilled, one could
compare with results obtained on the central
uplift of the Vredefort impact structure  Gibson
and Reimold, 2001), also in comparison against
the results of numerical modeling of these
aspects of impact structures.

Where should be drilled?
It is proposed to drill close to the center, but
possibly not directly into the central area, of the
impact structure, in order to obtain a
comprehensive profile through the entire crater
fill, as well as information about the possible
central uplift feature. As the nature of the
possibly existing central uplift in this crater is
entirely unconstrained, it may not be possible to
obtain much information about this structural
feature from a single borehole. In any case,
drilling must extend into the crater floor, in order
to constrain the nature of the lower target as well
as impact deformation below the crater and
hydrothermal alteration on the basement to the
crater
References Cited.
Dypvik, H., 2003, Sedimentary signatures and
processes during marine bolide impacts: a
review. Sedimentary Geology 161, 309-337.
Gibson, R.L. and Reimold, W.U., 2001, The
Vredefort Impact Structure, South Africa:
The Scientific Evidence and a Two-Day
Excursion Guide. Council for Geoscience,
Pretoria, Memoir 92, 111 pp.
Poag, C.W., Koeberl, C., and Reimold, W.U.,
2003, The Chesapeake Bay Crater: Geology
and Geophysics of a Late Eocene Submarine
Impact Structure. Impact Studies Series,
Springer-Verlag, Berlin-Heidelberg-New
York, 522 pp.

FLUID INCLUSION AND MINERALOGICAL EVIDENCE FOR POST-IMPACT CIRCULATION
OF SEAWATER-DERIVED HYDROTHERMAL FLUIDS AT THE CHESAPEAKE BAY IMPACT
CRATER: A CRITICAL TEST OF THE PHASE SEPARATION MODEL FOR BRINE
GENERATION.
David A. Vanko, Department of Physics, Astronomy & Geosciences, Towson University, Towson, MD
21252, USA (dvanko@towson.edu)

Introduction. A significant amount of thermal
energy is deposited as a result of an impact
event. Post-impact heat sources may include
impact melt sheets and a potentially significant
volume of basement rock beneath the target area.
Dissipation of this heat is likely to involve
hydrothermal circulation through the permeable
crater fill material, particularly when the impact
is on a continental shelf. The hydrothermal
setting envisioned for the Chesapeake Bay
impact crater therefore shares several features of
current models for mid-ocean ridge and
seamount hydrothermal systems (and has
numerous differences as well). Deep core
samples from the impact crater can potentially
provide information about post-impact
hydrothermal conditions recorded in secondary
or alteration mineralogy and mineral chemistry
and in fluid inclusions.

Background. Post-impact hydrothermal
systems have been recognized at several impact
sites. In the Chicxulub crater, hydrothermal
alteration of crystalline basement materials in the
ejection breccias is dominated by pyroxenequartz-
anhydrite, and fluid inclusions in the
anhydrite may contain evidence of hot, boiling
seawater (Gonzalez-Partida et al., 2000). The
Kardla crater in Estonia formed in a shallow
Ordovician sea, and hydrothermal convection
within crystalline breccia material caused
chloritization and secondary quartz, the latter
containing 150-300C fluid inclusions (Kirsimae
et al., 2002). Impact breccias of the Haughton
structure, Canada, are mineralized with quartz,
sulfide, sulfate and carbonate minerals, with
presumed formation temperatures from <100C
to >200C (Osinski et al., 2001). The Lockne
marine impact structure in Sweden contains
hydrothermal calcite, quartz and sulfides  fluid
inclusions in quartz contain either hydrocarbons
(chiefly methane and ethane) or saline brines,
with maximum temperatures of 210C (Sturkell
et al., 1998a). One geophysical consequence of
the large degree of secondary mineralization
within the altered Lockne impact breccias is that
they exhibit an unusually small density contrast
and the structure overall exhibits a rather small
negative gravity anomaly relative to other
impacts (Sturkell, 1998; Sturkell et al., 1998b).

Brine generation. High fluid salinities,
elevated temperatures, and the occurrence of
both liquid-dominated and vapor-dominated
fluid inclusions in some secondary minerals all
suggest that post-impact hydrothermal fluid flow
may frequently involve phase separation, or
boiling. At the Chesapeake Bay impact site,
Sanford (2003) hypothesizes that hydrothermal
activity accompanied by phase separation
generated the brine that is still thought to exist
today within the Exmore breccia. Sanford
(2003) calculates that such a brine, generated 35
Ma ago, would still be present because of low
groundwater velocities and minor molecular
diffusion effects. Drilling the impact crater will
provide specimens from deep within the impact
breccia and in the basement that should contain
secondary minerals and fluid inclusions. Fluid
inclusion investigations can test the hypothesis
of hydrothermal brine generation, and provide
firm chemical and temperature constraints for the
hydrothermal system.

Deep-sea example. Fluid inclusions can
provide excellent records of subseafloor boiling
in deep-sea hydrothermal systems. One example
is the deep-sea PACMANUS system in the
eastern Manus Basin back-arc spreading system
offshore Papua New Guinea (Vanko et al., in
press). An Ocean Drilling Program leg devoted
to this location (Leg 193, http://wwwodp.
tamu.edu/publications/pubs.htm) succeeded
in coring directly into the hydrothermal system
at two different sites. Core samples were
recovered from depths of up to almost 400 m
below the seafloor (in over 1600 m of water).
The cores contain numerous veins of anhydrite, a
secondary mineral that hosts aqueous fluid
inclusions. Microthermometric studies of the
inclusions shows how fluid temperatures
increase with depth, and how the hightemperature
fluids apparently intersected the
seawater boiling curve and generated both lowsalinity
vapor and high-salinity brine (Figure 1).
Similar investigations of core material from the
proposed deep drilling within the Chesapeake
Bay impact crater have the potential to define the
critical parameters of post-impact hydrothermal
activity: fluid temperatures and temperature
profiles; fluid chemistry; fluid-rock interaction
and secondary mineralogy and mineral
chemistry; nature of the heat source driving
circulation; and possibly even timing and
longevity of the hydrothermal circulation.

Fig. 1. This is a plot of fluid inclusion trapping
temperatures from anhydrite veins beneath the
Snowcap area of the PACMANUS deep-sea
hydrothermal field (from Vanko et al., in press).
Depths are meters below the seafloor (mbsf). The
solid line is the boiling curve for seawater-salinity
NaCl-H2O solution, separating the liquid field at
lower temperatures from the field of phase separation
into liquid plus vapor at higher temperatures. Most of
the inclusions are in the liquid field, but some
inclusions appear to have been trapped very near or
at boiling conditions. The arrows indicate samples
that contain additional independent evidence for
boiling: vapor-rich low-salinity inclusions, dense
saline brine inclusions, or both.

Facilities. The fluid inclusion laboratory at
Towson University contains a USGS-type gasflow
heating and freezing stage capable of
temperature control between -196 and +700C.
The stage is mounted on a Leitz petrographic
microscope with an adjustable coverslipcompensated
40X LWD objective and a Spot
Insight digital camera. Sample preparation
equipment includes a precision low-speed saw
and a polishing unit. Mineral chemical analysis
is available using microprobes at the University
of Maryland-College Park or the USGS-Reston,
and laser-Raman microprobes at the USGS or
Virginia Tech can be used to characterize
Raman-active fluid inclusion contents such as
hydrocarbons and many types of daughter
minerals.

Undergraduate Research. Towson
Universitys geology program requires
undergraduate majors to carry out a senior
research project culminating in a paper and a
poster presentation. Sample material from the
Chesapeake Bay impact crater would potentially
support the research projects for several
undergraduate students under the authors
direction. Because of the proximity of Towson
(near Baltimore) to the Chesapeake Bay, student
interest is likely to be very high.

References Cited.
Gonzalez-Partida, E., Carillo-Chavez, A., and
Martinez-Ibarra, R., 2000, Fluid inclusions
from anhydrite related to the Chicxulub crater
impact breccias, Yucatan, Mexico: Preliminary
report: International Geology Review, v. 42,
no. 3, p. 279-288.
Kirsimae, K., Suuroja, S., Kirs, J., Karki, A.,
Polikarpus, M., Puura, V., and Suuroja, K.
2002, Hornblende alteration and fluid
inclusions in Kardla impact crater, Estonia:
Evidence for impact-induced hydrothermal
activity: Meteoritics and Planetary Science, v.
37, no. 3, p. 449-457.
Osinski, G.R., Spray, J.G., and Lee, P., 2001,
Impact-induced hydrothermal activity within
the Haughton impact structure, arctic Canada:
Generation of a transient, warm, wet oasis:
Meteoritics and Planetary Science, v. 36, no. 5,
p. 731-745.
Sanford, W., 2003, Heat flow and brine
generation following the Chesapeake Bay
bolide impact: Journal of Geochemical
Exploration, v. 78-79, no. 9, p. 243-247.
Sturkell, E.F.F., 1998, The marine Lockne
impact structure, Jamtland, Sweden: a review:
Geologische Rundschau, v. 87, p. 253-267.
Sturkell, E.F.F., Broman, C., Forsberg, P., and
Torssander, P., 1998a, Impact-related
hydrothermal activity in the Lockne impact
structure, Jamtland, Sweden: European Journal
of Mineralogy, v. 10, p. 589-606.
Sturkell, E.F.F., Ekelund, A., and Tornberg, R.,
1998b, Gravity modelling of Lockne, a marine
impact structure in Jamtland, central Sweden:
Tectonophysics, v. 296, p. 421-435.
Vanko, D.A., Bach, W., Roberts, S., Yeats, C.J.,
and Scott, S.D., In press, Fluid inclusion
evidence for subsurface phase separation and
variable fluid mixing regimes beneath the
deep-sea PACMANUS hydrothermal field,
Manus Basin back-arc rift, Papua New
Guinea: Journal of Geophysical Research.

SHOCKED BASEMENT ROCKS FROM WITHIN THE PROPOSED CHESAPEAKE BAY IMPACT
DRILL CORE. James Whitehead1, and Richard A. F. Grieve2. 
1Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, NB. E3B 5A3 Canada (jwhitehe@unb.ca);
2Earth Sciences Sector, Natural Resources Canada, Ottawa, ON, K1A OE4, Canada.

Introduction. Despite its 85 km diameter, many
facets of the Chesapeake impact structure are poorly
understood. The location and depth of the intended
ICDP drill core is of paramount importance in
determining the types of questions that can be
addressed by the drilling programme. Three potential
drilling locations within the impact structure were
selected by the participants of the ICDP-USGS
Workshop. These are: 1) the central uplift; 2) the
annular trough, an; 3) the inner rim/overturned flap.
Our primary interests are in better understanding the
deformation and shock metamorphic features present in
the crystalline basement rocks of the structure. The
origin of the inner rim/overturned flap is also of interest
and, as we outline below, can be indirectly assessed by
drilling in the annular trough.

Potential drill sites and goals. The mode in which
bulk deformation of the crystalline targets are
accommodated during the compression and
modification stage of impact is poorly understood, yet
has significant ramifications for the formation process
of central peaks. The modification stage deformation
structures (bulk intracrystalline deformation,
intercrystalline slip, microfaults, pseudotachylyte slip
horizons, breccia zones, etc.) may be best developed in
the region of the basement that has been most affected
by modification stage uplift, i.e., the central uplift. A
drill site at this location within the Chesapeake Bay
impact structure (CBIS) would also afford the greatest
thickness of crystalline basement rocks.
The inner nested crater present at the CBIS, may
reflect a target in which a weak layer overlies a stronger
layer (Melosh, 1989). As such, the edge of the 38 km
wide inner crater at Chesapeake, which is located on a
mixed sedimentary/crystalline basement target, may be
either the margin of the transient cavity, or a peak ring.
These two different interpretations have significant
implications for the final calculated diameter of the
Chesapeake Bay structure. The shock levels present in
the target rocks at various inferred transient craternormalised
depths/distances from the centre yield
information on the ultimate diameter of the structure
and may, indirectly, constrain the diameter of the
Chesapeake Bay structure. Although drilling at the
central uplift may yield a greater section through the
basement lithologies, this region is also the most likely
region of structural disturbance, that may thwart
attempts to establish the transient crater-normalised
shock attenuation rate. Structural disturbance may
include bulk rotation of near-horizontal isobars at the
impact centre to steeper or vertical dips owing to
rotation during upward flow of the central uplift. In
addition, structural repetition of marker horizons are
evident in central peaks on sedimentary targets (e.g.,
Red Wing (Brenan et al., 1975); Gosses Bluff (Milton
et al., 1996); Cloud Creek (Stone and Therriault,
2003)), and may be apparent impact structures in
crystalline targets (e.g., West Clearwater) though more
detailed studies are needed. Although establishing if
such repetition is present is a worthy goal in itself, the
presence of such disturbance could cause difficulties in
establishing the shock attenuation rate, the likely
diameter of the structure, and thus preclude us from
commenting on the most likely origin of the inner
crater.

Proposal. Drill core in the annular trough, which
appeared to be the site of preference of the bulk of the
participants at the ICDP-USGS meeting. We argue that
a site in the annular trough, but close to the peak or on
the flank of the peak, would be the optimum site for
establishing shock attenuation rates in the target,
assuming a significant length of drill core in the
basement target can be acquired. Such a site would also
fulfill the objectives of drilling the annular trough: a
large section of crater-fill and post-crater deposits, with
low-risk of failing to drill less well-defined features
such as the central uplift or the rim of the inner crater.
Shock attenuation rates have been established for
only a few other complex craters (Charlevoix and Slate
Islands, Canada (Robertson and Grieve, 1977),
Puchezh-Katunky, Russia (Ivanov, 1994), Kara, Russia
(Basilevsky et al. 1983) and Woodleigh (Whitehead et
al., 2003). However, the exposure is poor at
Charlevoix, precluding an analysis of small- to mediumscale
changes in shock with distance, the rate at
Woodleigh is poorly constrained, and at Puchezh-
Katunky the rate is constrained by five measurement
over ~ 5km of core and, thus, would not resolve the
effects of structural disturbances in the target. In
addition to providing constraints on the size of the
Chesapeake Bay impact structure, the characterisation
of the shock attenuation in a continuous section of drill
core on the edge of the central uplift in the annular
trough will provide valuable information on how shock
levels diminish with depth in a contiguous section of
core, and in the presence of potential structural
disturbances.

Samples from such a site should also contain
features that may help elucidate the mode in which the
flow of the target occurred in response to the impact.

Comparisons. We propose that direct comparisons
between the deformation features present in the
basement rocks of the Chesapeake Bay drill core with
core that is available to us from the central uplift/near
central uplift regions of several Canadian impact
structures, be performed. These comparisons would
contribute to our knowledge of the mode in which the
central uplift processes occur, as well as highlight
possible differences between the uplift process in
craters on crystalline versus mixed targets.

References Cited.
Basilevsky A.T., Ivanov B.A., Florensky K.P.,
Yakovlev O.I., Fel'dman V.I., Granovsky L.V.,
and Sadosky M.A., 1983, Impact craters on the
moon and planets (in Russian). Moscow: Nauk
Press.
Brenan R.L., Peterson B.L., and Smith H.J., 1975,
The origin of Red Wing Creek structure:
McKenzie County, North Dakota: Wyoming
Geological Association of Earth Sciences
Bulletin, v.8, p.1-41.
Ivanov B.A., 1994, Geomechanical models of impact
cratering: Puchezh-Katunki structure, in,
Dressler B.O., Grieve, R.A.F. and Sharpton V.L.,
eds., Large Meteorite Impacts and Planetary
Evolution: Geological Society of America
Special Paper v.293, p.81-92.
Melosh, H.J., 1989, Impact cratering: A geologic
process: Oxford University Press, New York,
pp.245.
Milton D.J., Glikson A.Y., and Brett R., 1996,
Gosses Bluff - a latest Jurassic impact structure
central Australia. Part 1: Geological structure,
stratigraphy and origin: AGSO Journal of
Australian Geology and Geophysics v.16, no.4,
p.453-486.
Robertson P.B., and Grieve R.A.F., 1977, Shock
attenuation at terrestrial impact structures, in
Roddy D.J., Pepin R.O. and Merrill R.B., eds.,
Impact and Explosion Cratering, edited by New
York: Pergamon Press. pp. 687-702.
Whitehead J., Grieve R.A.F., Spray J.G. and Glikson
A.W., 2003, Planar deformation features in the
Woodleigh impact structure, Western Australia,
and their bearing on the diameter and degree of
structural uplift in the structure: Annual Meeting
of the NE Section of the Geological Society of
America, Halifax, Nova Scotia, March 27-29th,
2003.

SLIDE PRESENTATIONS   
PowerPoint (R) presentations are available on CD-ROM only.    
REVIEW OF MARINE SEISMIC-REFLECTION SURVEYS:  CHESAPEAKE BAY CRATER  [3 Mb] C. Wylie Poag   REVIEW OF PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGYCHESAPEAKE BAY CRATER [3 Mb] J. Wright Horton, Jr., Christian Koeberl, and W. Uwe Reimold (presenters), and John N. Aleinikoff, Gregory S. Gohn, Glen A. Izett, Michael J. Kunk, Charles W. Naeser, Nancy D. Naeser, C. Wylie Poag, and David S. Powars (contributors)  
U.S. GEOLOGICAL SURVEY GROUND WATER STUDIES RELATED TO THE CHESAPEAKE BAY IMPACT STRUCTURE [22 Mb] Randy McFarland  
SHIPBOARD GRAVITY AND MAGNETIC FIELD OVER THE CHESAPEAKE IMPACT CRATER [1 Mb] Anjana Shah, John Brozena, Sandra Martinka, Peter Vogt, David Ball, Mike Czarnecki, Jim Jarvis, Skip Kovacs, Robert Liang, Barbara Martin, Brian Parsons, Richard Wilkerson, Rick Younger, and Jake Hollinger  
POST-IMPACT STRATIGRAPHY: LATE EOCENE-RECENT [14 mb] Ken Miller  
THE CHICXULUB CRATER SCIENTIFIC DRILLING PROJECT (CSDP) [24 Mb] Jan Smit