3. About 35 million years ago, late Eocene Epoch.
Sea level was high, shoreline of Virginia region was near where
Richmond is today.
Bolide greater than one mile wide, traveling ~70,000 mph
crashed off coast of VA.
Bolide: an extraterrestrial body of unknown compostion, 1-10-km in size
traveling faster than a speeding bullet. Explodes upon impact creating
large crater.
4.
5. Billions of tons of ocean water thrust up to 30 miles into
atmosphere and vaporized.
Millions of tons of debris and rocks propelled into atmosphere.
Immediate regional effects: super-hot blast wave, base surge of
hot debris, gigantic tsunami waves, vaporization of water column
and target rocks, giant earthquakes.
Global effects: short term include fallout of ejected particles and
raging wildfires. Long term include prolonged darkness due to
atmospheric debris, acid rain, and greenhouse warming.
10. Over time, impact crater buried more than 1000ft beneath
sand, silt and clay.
First evidence discovered in 1983, confirmed by seismic
reflection profiles 10 years later.
Largest known impact crater in U.S., 7th largest known on
Earth.
Seismic profiles and core samples used to study crater.
11.
12.
13. Hypothesis: Core samples from the Chesapeake
Bay Impact Structure (CBIS) will reveal the
geologic, hydrologic, and biologic consequences of
large terrestrial bolide impacts.
Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Browning, J.V.,
Cockell, C.S., Horton Jr., J.W., Kenkmann, T., Kulpecz, A.A.,
Powars, D.S., Sanford, W.E., Voytek, M.A., 2008. Deep drilling
into the Chesapeake Bay impact structure. Science 320, 17401745.
14. Three coreholes drilled to composite
depth of 1.76 km at Eyreville farm in
Northampton County, VA.
Geologic: Core samples examined for
shock metamorphism/impact melt.
Hydrologic: Chemical analysis of pore
waters from within the core samples.
Biologic: Analysis of biological remnants
within the core samples.
15. Impact Processes
~40s after impact: Beginning of inward slumping.
Fractures open, fill with lithic and suevitic breccias to
form dikes.
~1.5m after impact: Ejecta plume blowing into
atmosphere, oceanic water column forced away from
impact site.
~6-8m after impact: Ejecta material returns, mixes with
breccias.
16. Impact Processes
~7-15m after impact: Ocean water resurges back into
impact cavity. Water carries with it a 275-m-thick
granitic block. The block was carried a distance of at
least 5 km.
~10m after impact: Granitic rock likely slid into crater
as sand under it was carried in with resurging water.
Avalanche ensued from rim of crater.
17.
18. Post-Impact Processes
Sediment accumulates over time.
Sedimentary beds created loading pressure on
unconsolidated, water-saturated breccia.
Differential Subsidence: Breccia subsides more
quickly than semiconsolidated sediments
surrounding it.
Bowl-shaped depression forms over crater.
19.
20.
21. Pore waters become salty between 100 and
400 meters w/i post-impact section.
300-1000 meters, core samples contain brine.
Hypersaline aquifer, salinities up to twice that
of seawater.
Study hypothesizes that current groundwater
in CBIS survived impact, was never flushed out.
22. Most recent USGS study published Nov. 14,
2013 suggests aquifer is actually remnant
water from the Early Cretaceous North Atlantic
Sea.
Likely 100-145 million years old.
Oldest sizeable body of seawater to be
identified worldwide.
23. Decline in microbial abundance with depth steeper than that of
many typical deep marine sediments. Could be due to high salinity.
Microbial abundance increases below granite rock.
Lowest part (867-1096m), peak salinity 65%.
No cells detected.
Low permeability, water at this depth likely reflects post-impact
environment.
Sterilization by heat likely. Salinity inhibits microbial repopulation.
Microbial abundance increases below granite block.
Sterilization occurred in this area, so organisms must have been
re-introduced at some point, perhaps through fractures.
24.
25. Core samples do reveal some of the geologic,
hydrologic, and biologic consequences of large
terrestrial bolide impacts.
Seismic profiles are also incredibly beneficial to this
research.
The study was published in 2008, but researchers
are still studying core samples from the CBIS and
publishing new findings, most recently in the middle of
this past month.
26. Studies are important to the security of our fresh
water resources.
Hypersaline waters within the CBIS threaten our
freshwater aquifers.
Accurate depth profiles are needed for the drilling
operations in the area.
Also important to understand the dynamics of local
land subsidence, especially during a time of rising sea
levels due to climate change.
27. Campbell, J., Sanford, W., 2013. Oldest large body of ancient seawater identified under Chesapeake Bay. USGS.
http://www.usgs.gov/newsroom/article.asp?ID=3725
Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Browning, J.V., Cockell, C.S., Horton Jr., J.W., Kenkmann, T., Kulpecz,
A.A., Powars, D.S., Sanford, W.E., Voytek, M.A., 2008. Deep drilling into the Chesapeake Bay impact
structure. Science 320, 1740-1745.
Kenkmann, T., Hörz, F., Deutsch, A., 2005. Large Meteorite Impacts III. The Geological Society of America, Boulder, CO.
Mayell, H., 2001. Chesapeake Bay Crater Offers Clues to Ancient Cataclysm. National Geographic News.
http://news.nationalgeographic.com/news/2001/11/1113_chesapeakcrater.html.
Poag, C. W., 1999. Chesapeake Invader, Princeton University Press, Princeton, NJ.
USGS. The Chesapeake Bay Bolide: Modern Consequences of an Ancient Cataclysm.
http://woodshole.er.usgs.gov/epubs/bolide/.
Image Credit: http://mreza.tv/wp-content/uploads/2012/12/asteroid.jpg
Image Credit: http://www.nsf.gov/news/mmg/media/images/ches_impact1_h.jpg
