PLEASE NOTE:


*

CCNet 120/2000 - 20 November 2000
---------------------------------


"Cosmologists are always in error, though never in doubt."
   -- Lev Landau (Russian physicist)



(1) IMPACT CRATER DYNAMICS
    Michael Paine <mpaine@tpgi.com.au>

(2) IS MEXICO'S IMPACT CRATER LINKED TO MASS EXTINCTIONS?
    UniSci, 17 November 2000

(3) COMPUTER MODELLING OFFERS NEW INSIGHT INTO FORMATION OF CHICXULUB IMPACT
CRATER
    Ron Baalke <baalke@jpl.nasa.gov>

(4) COMPUTER MODEL: MASSIVE LAVA FLOWS CAUSED K/T MASS EXTINCTION
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(5) IT'S GIGGLE TIME AGAIN: TAKING THE MICKEY OUT OF 2000 SG344
    Exite News, 17 November 2000

(6) LA TIMES LETTER BY DON YOEMANS
    Michael Paine <mpaine@tpgi.com.au>

(7) WHAT'S IN A NAME - WHAT'S A PLANET?
    Larry Klaes <lklaes@bbn.com>

(8) ANALYSIS OF IMPACT GLASSES FROM THE ZHAMANSHIN IMPACT CRATER, KAZAKHSTAN
    M. Zbik et al.

(9) THREE-DIMENSIONAL MAGNETIC IMAGING OF THE CHICXULUB CRATER
    M. Pilkington & A.R. Hildebrand

(10) DISTANT EJECTA FROM THE LOCKNE IMPACT CRATER, SWEDEN
     E. Sturkell et al.

(11) IMPACT EVENTS IN THE STATIGRAPHICAL RECORD
     Y. Su

====================
(1) IMPACT CRATER DYNAMICS

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny

Please see the article 'Is Mexico's Impact Crater Linked To Mass
Extinctions?' at http://unisci.com/stories/20004/1114001.htm

I am not sure of the basis of the headline but the article is interesting,
as is the linked page 'Impact Crater Collapse'
http://www.huxley.ic.ac.uk/geophysics/projects/gareth/crater.html
The researcher is Gareth Collins (gareth@lpl.arizona.edu )

regards
Michael Paine

============
(2) IS MEXICO'S IMPACT CRATER LINKED TO MASS EXTINCTIONS?

From UniSci, 17 November 2000
http://unisci.com/stories/20004/1114001.htm

Mexico's Chicxulub crater is thought to have resulted from an impact with an
asteroid or comet 65 million years ago. Many scientists believe that there
is a link between the impact and the environmental change and mass
extinction, including perhaps that of the dinosaurs, at the end of the
Cretaceous period.

Now computer simulations have revealed that a vast region of the Yucatan
Peninsula may have behaved like a fluid during the formation of the
Chicxulub impact crater.

Gareth Collins, a postgraduate student in the TH Huxley School of the
Environment, Earth Sciences and Engineering, Imperial College, UK, will
announce the results of his computer modeling at the Geological Society of
America annual meeting in Nevada today.

He used innovative computer modeling based on seismic data to analyze the
distinctive complex crater formation at Chicxulub.

Collins explained, "Our understanding of complex crater formation and lack
of a definitive model was limited by the absence of large pristine impact
craters on Earth. This situation changed following the discovery of the
buried Chicxulub impact structure in Mexico in 1990 which is the largest
pristine crater known on Earth."

(Impact craters found on the surface of all terrestrial bodies in the Solar
System can be divided into two morphologically distinct classes: Simple and
Complex.

(Simple craters possess a circular "bowl" shape with an uplifted rim.
Complex craters exhibit somewhat unintuitive structures such as central
peaks or an inner "peaked" ring, terraced rim walls and outer concentric
faulted zones.)

Collins used the results from the Chicxulub Seismic Experiment, which
provided new insight into the kinematics of complex crater formation, to
design novel computer simulations of the Chicxulub crater.

The group at the Imperial College TH Huxley School hope that their study
will further the understanding of crater collapse, the final stage in the
impact crater formation process that produces the strange internal
structures that characterize complex craters.

Complex crater collapse has long been an enigmatic issue in planetary
science.

Collins explained, "In order for the complicated internal structures to be
produced that are observed at Chicxulub and many extra-terrestrial complex
craters, the target material must behave as though it were a fluid.

"Of course the collapse process cannot be entirely hydrodynamic, as the end
result would inevitably be a flat surface. Evidently, the fluid collapse
must be frozen or suspended in some way to produce the observed complex
crater morphologies.

"The mechanism driving this transient weakening, however, still remains a
mystery -- this phenomenon appears to violate current understanding of rock
and debris mechanics."

The group at the TH Huxley School and their colleagues at the University of
Arizona believe that one potential material weakening mechanism called
Acoustic Fluidization could come into action as the impact-generated shock
wave transforms the target into a sea of jostling granular material.

Gareth Collins said, "We model the collapse stage of the cratering process
which begins after the initial excavation of the cavity. Our simulations
show that temporary weakening of the target by Acoustic Fluidization allows
the formation of internal peak and ring structures similar to those observed
in terrestrial and extra-terrestrial craters. Our dynamic simulations of
peak-ring formation at Chicxulub are remarkably consistent with observations
from the seismic data."

His research group hopes to use this model for the generation of the
peak-ring at Chicxulub to further their understandings of the geology of
other cratered planets and satellites, such as Mercury, Venus and the Moon.

Dr. Jo Morgan, Collins' supervisor in the Geophysics Research Group, TH
Huxley School, explained, "Improved understanding of large-impact crater
formation will enable us to assess the environmental effects of such impacts
and to determine whether this impact was the dominant force driving the mass
extinction at the end of the Cretaceous period."

(Editor's Note: An intriguing animation based on Collins' computer
simulations is on his group's website at this URL:
http://www.huxley.ic.ac.uk/geophysics/projects/gareth/crater.html)

=============
(3) COMPUTER MODELLING OFFERS NEW INSIGHT INTO FORMATION OF CHICXULUB IMPACT
CRATER

From Ron Baalke <baalke@jpl.nasa.gov>

http://www.ic.ac.uk/templates/news_text_3.asp?P=2259

Computer Modelling Offers New Insight Into The Formation Of the Chicxulub
Crater in Mexico

14 November 2000
Imperial College News Release (United Kingdom)

Computer simulations have revealed that a vast region of the Yucatan
Peninsula, Mexico, may have behaved like a fluid during the formation of the
Chicxulub impact crater.

Gareth Collins, a postgraduate student in the TH Huxley School of the
Environment, Earth Sciences and Engineering, Imperial College, will announce
the results of his computer modelling at the Geological Society of America
annual meeting in Nevada on 14 November. Mr Collins has used innovative
computer modelling based on seismic data to analyse the distinctive complex
crater formation at Chicxulub (see notes to editor 3).

The Chicxulub crater is thought to have resulted from an impact with an
asteroid or comet 65 million years ago. Many scientists believe that there
is a link between the impact and the environmental change and mass
extinction, including perhaps that of the dinosaurs, at the end of the
Cretaceous period.

Gareth Collins explained, "Our understanding of complex crater formation and
lack of a definitive model was limited by the absence of large pristine
impact craters on Earth. This situation changed following the discovery of
the buried Chicxulub impact structure in Mexico in 1990 which is the largest
pristine crater known on Earth."

Mr Collins used the results from the Chicxulub Seismic Experiment (see notes
to editor 2), which provided new insight into the kinematics of complex
crater formation, to design novel computer simulations of the Chicxulub
crater.

The group at the Imperial College TH Huxley School hope that their study
will further the understanding of crater collapse; the final stage in the
impact crater formation process that produces the strange internal
structures that characterize complex craters. Complex crater collapse has
long been an enigmatic issue in planetary science.

Gareth Collins explained, "In order for the complicated internal structures
to be produced that are observed at Chicxulub and many extra-terrestrial
complex craters, the target material must behave as though it were a fluid."

"Of course the collapse process cannot be entirely hydrodynamic, as the end
result would inevitably be a flat surface. Evidently, the fluid collapse
must be frozen or suspended in some way to produce the observed complex
crater morphologies. The mechanism driving this transient weakening,
however, still remains a mystery - this phenomenon appears to violate
current understanding of rock and debris mechanics."

The group at the TH Huxley School and their colleagues at the University of
Arizona believe that one potential material weakening mechanism called
Acoustic Fluidisation could come into action as the impact generated shock
wave transforms the target into a sea of jostling granular material.

Gareth Collins said, "We model the collapse stage of the cratering process
which begins after the initial excavation of the cavity. Our simulations
show that temporary weakening of the target by Acoustic Fluidisation allows
the formation of internal peak and ring structures similar to those observed
in terrestrial and extra-terrestrial craters. Our dynamic simulations of
peak-ring formation at Chicxulub areremarkably consistent with observations
from the seismic data."

His research group hopes to use this model for the generation of the
peak-ring at Chicxulub to further their understandings of the geology of
other cratered planets and satellites, such as Mercury, Venus and the Moon.

Dr Jo Morgan, Mr Collins' supervisor in the Geophysics Research Group, TH
Huxley School, explained, "Improved understanding of large-impact crater
formation will enable us to assess the environmental effects of such impacts
and to determine whether this impact was the dominant force driving the mass
extinction at the end of the Cretaceous period."

An animation based on Mr Collins' computer simulations is on his group's web
site at: http://www.huxley.ic.ac.uk/geophysics/projects/gareth/crater.html

Mr Collins will be giving his presentation at the annual meeting of the
Geological Society of America on Tuesday 14 November at 9.30am. The
presentation is part of the GSA?s Planetary Geology Division session which
starts at 8.00am in Ballroom D. See the GSA?s web site for more information
on the meeting: http://www.geosociety.org/meetings/2000/

For further information contact:

Susie Renshaw
Press Office
Imperial College
Tel: +44 (0)20 7594 6701
Email: s.renshaw@ic.ac.uk

Notes to editors:

1) Gareth Collins is currently working on a PhD project entitled "Modelling
Large Impacts on Earth and Venus".

2) Professor Mike Warner and Dr Jo Morgan, TH Huxley School, Imperial
College, are part of an international research group that investigated the
Chicxulub crater. They published a paper in Nature (Vol 390, 472-476 (1997))
- Size and morphology of the Chicxulub impact crater - detailing the results
of their investigations into the size and shape of the Chicxulub crater.
From seismic data collected across the offshore portion of the impact
crater, they determined that the diameter of the transient cavity to be
about 200km. This parameter is critical for constraining impact-related
effects on the Cretaceous environment. The offshore seismic data indicated
that the Chicxulub crater has a multi-ring basin morphology, similar to
large impact structures observed on other planets, such as Venus. For
further information see the Geophysics Research Group web site at:
http://www.huxley.ic.ac.uk/research/Comp&Geophys/Geophys/geophys_main.htm

3) Impact craters found on the surface of all terrestrial bodies in the
Solar System can be divided into two morphologically distinct classes:
Simple craters and Complex craters. Simple craters possess a circular 'bowl'
shape with an uplifted rim. Complex craters exhibit somewhat unintuitive
structures such as central peaks, or an inner 'peaked' ring, terraced rim
walls and outer concentric faulted zones.

4) Imperial College of Science, Technology and Medicine is an independent
constituent part of the University of London. Founded in 1907, the College
teaches a full range of science, engineering, medical and management
disciplines at the highest level. The College is the largest applied science
and technology university institution in the UK, with one of the largest
annual turnovers (330 million in 1998-99) and research incomes (173
million in 1998-99). Web site at http://www.ic.ac.uk

=============
(4) COMPUTER MODEL: MASSIVE LAVA FLOWS CAUSED K/T MASS EXTINCTION

From Andrew Yee <ayee@nova.astro.utoronto.ca>

State University of New York-Buffalo
Buffaol, New York

Contact:
Ellen Goldbaum, goldbaum@buffalo.edu
Phone: 716-645-5000 ext 1415
Fax: 716-645-3765

Release date: Thursday, November 16, 2000

UB Research Offers First Evidence that Massive Lava Flows Triggered
Apocalyptic Climate Changes

RENO, Nev. -- Scientists long have believed that mass extinctions are
triggered by sudden global changes in climate.

Some of these cataclysmic events, like the one originally assumed to have
wiped out the dinosaurs, occurred at about the same time as tremendous
volcanic eruptions called flood basalt eruptions that produced massive flows
of lava exiting the earth's crust.

Scientists have speculated that mass extinctions might be precipitated by
these volcanic eruptions. For those eruptions to have had a sufficient
effect on climate to cause cooling significant enough to lead to the
collapse of ecosystems and the extinction of many species, scientists have
said that they would have had to have been capable of thrusting gases and
particles up into the stratosphere where they would block out sunlight.

Computer models suggested that flood basalt eruptions could cause this kind
of effect on climate, but recent changes in scientists' ideas of the speed
at which flood lavas are erupted has drawn into question these original
findings.

Now, a University at Buffalo geologist has reexamined the issue and shown
that it is indeed very likely that huge flood basalt eruptions caused
dramatic global-scale climate shifts and mass extinctions, even if lava is
erupted relatively slowly.

Elisabeth Parfitt, Ph.D., UB assistant professor of geology, described
results of her research here today (Nov. 16, 2000) at the annual meeting of
the Geological Society of America.

Parfitt's computer models are the first to show that massive sheets of lava
produced by flood basalt eruptions millions of years ago generated such
intense heat that they functioned as a secondary heat event, causing gases
and fine ash to be carried into the upper atmosphere, even in the case where
an eruption might not generate sufficient heat to cause clouds of gases and
particles to reach high enough into the atmosphere to effect the global
climate.

Some of these massive sheets of lava measured as large as 200 kilometers, or
130 miles, long.

"According to our models, these lava flows, which could be as hot as 1,200
degrees Centigrade when they are first erupted, could push ash and gas up to
heights of 30 kilometers above the volcanic vent," said Parfitt.

"Sometimes volcanic eruptions don't form a mountain," she explained.
"Instead, the magma shoots straight up through the earth's crust and is
erupted from a crack, which might be as much as 100 kilometers long. These
flood basalt eruptions often produced these massive sheets of lava, which
can be as much as 100 or 200 kilometers (65 miles or 130 miles) long and
they gave off a huge amount of heat."

Parfitt explained that these huge eruptions are like much bigger versions of
basaltic eruptions, such as those going on now in Hawaii. She added that a
large part of Washington state is covered by flood basalt lava, which
erupted there about 16 million years ago.

===========
(5) IT'S GIGGLE TIME AGAIN: TAKING THE MICKEY OUT OF 2000 SG344

From Exite News, 17 November 2000
http://news.excite.com/news/uw/001117/university-124

AVOIDING ASTEROIDS AND MINING THE MOON

By Marcelo Duran
Rocky Mountain Collegian
Colorado State University

(U-WIRE) FORT COLLINS, Colo. -- The world dodged an extraterrestrial bullet
earlier this month with the announcement that an asteroid will not have a
chance of hitting the Earth in 2030.

The floating rock in space, affectionately named 2000 SG344, initially had
every astronomer in the world running around like Chicken Little saying the
sky is falling. NASA and the International Astronomical Union said the
object had a one in 500 chance of colliding with Earth.

Those may sound like good odds of not being hit by 2000 SG344, but that may
be a little too close for comfort. If the statistic were true, the Earth
would have a better chance of getting hit by the asteroid than one person
picking four out of six Colorado Lotto numbers correctly (which
by the way is 1 in 566.)

Thankfully enough, the initial data was off the mark (sic) and there is now
only a one in a million chance (sic) of turning a small corner of our planet
into a large pothole.

Turn off the doomsday whistles and discard the Y2K emergency rations again
because we are safe from the intergalactic shooting -- for the time being.

Even if that asteroid hit the planet, it wouldn't be close to the projected
disaster scenarios that Hollywood portrayed in "Armageddon" and "Deep
Impact." It would be more like the collision that occurred near Siberia in
1908. That impact turned a forest into a bunch of tooth picks with the power
that equaled 100 (sic) Hiroshima nuclear bombs.

But who knows if NASA's projection of an asteroid-free 2030 is correct?
Those brainiacs from Houston (sic) might know how to keep the space station
in orbit, but judging from other calculations, I would heavily invest in
asteroid-proof clothing in the near future.

However, we are not out of harm's way yet, since scientists expect (sic)
collisions in the 2071-73 time frame. If an asteroid were heading toward
anyone from this generation at that time, they would be too old and slow to
run away from the impact area. All we could do then would be to yell and
point, "It's coming right for us!"

Another issue buzzing around intergalactic circles is the possibility of
mining other celestial bodies like the moon or even Mars. The potential for
profit and a new source of essential resources is important, but mining in
space must have some consequences.

I may not be a rocket scientist or expert in physics, but I think that
stripping the moon and taking a large amount of moon rocks will affect a
little something called the tides in our oceans. Sure, we would get more ore
and material to make more hubcaps, but if the tides were screwed up people
in Colorado would have good beach front real estate.

If we did have mining on other planets, that action would lead to
colonization. Humans having a permanent presence on Mars would not be good
news for that planet, since we have a habit of destroying any environment we
inhabit.

I think living on another planet would be great and summer vacations would
take on a whole new meaning, but having our DNA floating around in other
parts of the solar system is dangerous for the Milky Way galaxy. Just look
at our track record with Earth.

We shouldn't be in the business of destroying other planets, which is how
bad reputations start.

Instead of putting more people on the moon, we need to figure out the
ingredients of moon pies, which is more baffling than the Electoral College
system.

(C) 2000 Rocky Mountain Collegian via U-WIRE

=============
(6) LA TIMES LETTER BY DON YOEMANS

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,
See http://www.latimes.com:80/news/comment/20001119/t000111050.html
Mike

Near-Earth Objects
LA times, Nov 19 2000

Your Nov. 11 editorial correctly pointed out that there should be an
appropriate concern within the U.S. over the issue of space objects on
Earth-threatening orbits. A near-Earth object larger than one kilometer
would be expected to cause a global rather than a local disaster if it were
to strike the Earth. Your editorial noted that the British task force report
was critical of U.S. efforts in this area, but the report notes that "the
United States is doing far more about near-Earth objects than the rest of
the world put together." It further points out that our work to discover 90%
of objects larger than one kilometer within 10 years "is progressing well."
NASA is funding the vast majority of current efforts to find these objects.
Quite apart from the money spent on six flight projects to nine comets and
asteroids in the next decade, NASA currently spends about $3.5 million per
year on ground-based observations of near-Earth objects. Five U.S.
observatories are operating full-time to discover them. It is now estimated
that the total population of near-Earth asteroids larger than one kilometer
numbers about 1,000, and some 435 have been discovered already. Forty
percent of these discoveries have been made in the last two years.

DON YEOMANS Manager,
NASA Near-Earth Object Program Office
Jet Propulsion Laboratory Pasadena

=============
(7) WHAT'S IN A NAME - WHAT'S IN A PLANET?

From Larry Klaes <lklaes@bbn.com>

Hi Benny

Here's a good paper on what constitutes a planet:

http://www.boulder.swri.edu/~alan/papers.html


Regarding the Criteria for Planethood And Proposed Planetary Classication
Schemes

S. Alan Stern and Harold F. Levison
Department of Space Studies, Southwest Research Institute, 1050 Walnut
Street, No. 426
Boulder, CO 80302

Submitted to the Transactions of IAU 2000: 04 October 2000
Revised: 11 November 2000

Abstract

We examine the question of planetary classication, making recommendations both for the
criteria by which planethood should be evaluated, as well as for more
detailed physical and dynamical subtype classication schemes.

Preamble

The 1995 film entitled "The Englishman Who Went Up a Hill But Came Down a Mountain"
is set in a small South Wales village during the first world war. The inhabitants of this village
were very proud of their local mountain. Then two English cartographers appeared and decided
that the mountain was no mountain at all, but only a hill. Indeed, the object in question was
slightly too short to be a mountain. The villagers' solution involved moving a lot of dirt.

The main purpose of classication schemes is to allow the organization of thoughts. This
is particularly true of reductionist classication schemes. Classication schemes have played
and continue to play a useful role in astronomy because they allow us to put
bodies into a structure with recognizable correlations of physical parameters.

There has been much recent discussion inside the planetary community, and in
the public at large, over the status of Pluto. Despite the fact that many members of
our community believed that such a debate was a matter of semantics, at times this
discussion became very heated. Indeed, occasionally, the debate looked more like one between
religious fanatics than scientists. While the discussion that has gone on may at times have centered
on semantics, we believe that classication schemes play an important role in the scientic
process.

One reason for developing a new classication scheme is to provide a framework for the
evaluation of new ideas. We in planetary science have reached just such a
time when new facts and new understandings motivate classication schemes for the bodies we
study.

FULL PAPER at http://www.boulder.swri.edu/~alan/papers.html

===========
(8) ANALYSIS OF IMPACT GLASSES FROM THE ZHAMANSHIN IMPACT CRATER, KAZAKHSTAN

Organosilane occurrence in irghizite samples from the Zhamanshin impact
crater, Kazakhstan
Zbik M, Jasieniak M, Smart RS
METEORITICS & PLANETARY SCIENCE 35: (5) 943-947 SEP 2000

The composition of surface deposits on vesicle walls in irghizites (i.e.,
impact glasses at site) from the Zhamanshin meteorite crater were studied
using time-of-flight secondary ion mass spectrometry (TOFSIMS). The cavity
walls are unique interfaces for condensation of gases from the superheated,
high-silica melt during the impact. Initially, signals from the cavity wall
are dominated by hydrocarbon fragments whereas the glass fracture face
surrounding the cavity gave only signals corresponding to glass components.
After 12 h in ultra high vacuum (UHV), signals from the cavity wall are
dominated by peaks corresponding to fragments normally measured from
organosilanes and organosiloxanes with the majority of the hydrocarbon
signals markedly reduced. Characteristic hydrocarbon fragments are now
observed on the glass fracture surface next to the cavity in an annulus
around the cavity perimeter. There are also minor signals in this region
from organosilanes and organosiloxanes. In contrast, four tektites
(Australites) (i.e., glassy distal ejecta) gave no organosilane or
organosiloxane signals after the same preparation and vacuum evaporation
procedure. These species appear to be formed only at the impact site where
higher levels of organic material are likely to be present in soil and are
trapped before evaporation. This appears to be the first report of naturally
occurring silicon-organic compounds. Copyright 2000 Institute for
Scientific Information

Addresses:
Zbik M, Univ S Australia, Ian Wark Res Inst, Mawson Lakes, SA 5095,
Australia.
Univ S Australia, Ian Wark Res Inst, Mawson Lakes, SA 5095, Australia.

=================
(9) THREE-DIMENSIONAL MAGNETIC IMAGING OF THE CHICXULUB CRATER

Pilkington M, Hildebrand AR
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH  105: (B10) 23479-23491 OCT 10
2000

Although few magnetization measurements are available for the structural
elements of the Chicxulub impact crater, the magnetization intensities of
the melt sheet, upper breccia unit, and central uplift are 3-4 orders of
magnitude greater than the 3- to 4-km-thick carbonate and evaporite
stratigraphy covering the Yucatan block. This allows three-dimensional
modeling of the crater's structure by inversion using a two-layer model. Two
layers are separately inverted by dividing the crater's magnetic field
expression into <40- and >40-km wavelength components. The upper layer
(average depth 2 km) models the distribution of highly-magnetized zones in
the crater's melt sheet. The lower layer (average depth 5 km) represents
relief on the Yucatan block's basement surface and effectively maps the
crater's similar to 50-km-diameter central uplift and possibly the
expression of the surrounding collapsed disruption cavity fill. The
shallower magnetized zones consist of two generally concentric
distributions, at radii of similar to 20 and similar to 45 km. These highly
magnetized zones are thought to result from hydrothermal systems, localized
at the edge of the central uplift and the collapsed disruption cavity,
having produced magnetic phases during alteration of the melt sheet.
Copyright 2000 Institute for Scientific Information

Addresses:
Pilkington M, Geol Survey Canada, Continental Geosci Div, 615 Booth St,
Ottawa, ON K1A 0E9, Canada.
Geol Survey Canada, Continental Geosci Div, Ottawa, ON K1A 0E9, Canada.
Univ Calgary, Dept Geol & Geophys, Calgary, AB T2N 1N4, Canada.

=========
(10) DISTANT EJECTA FROM THE LOCKNE IMPACT CRATER, SWEDEN

Distant ejecta from the Lockne marine-target impact crater, Sweden
Sturkell E, Ormo J, Nolvak J, Wallin A
METEORITICS & PLANETARY SCIENCE 35: (5) 929-936 SEP 2000

The Lockne impact event took place in a Middle Ordovician (455 Ma)
epicontinental sea. The impact resulted in an at least 13.5 km wide,
concentric crater in the sea floor. Lockne is one of very few locations
where parts of an ejecta layer have been preserved outside the crater
structure. The ejecta from the Lockne impact rests on progressively higher
stratigraphic levels with increasing distance from the crater, hence forming
a slightly inclined discontinuity surface in the pre-impact strata. We
report on a similar to 30 cm thick sandy layer at Hallen, 45 km south of the
crater centre. This layer has a fining upward sequence in its lower part,
followed by low-angle cross-laminations indicating two opposite current
directions. It is rich in quartz grains with planar deformation features and
contains numerous, up to 15 cm large, granite clasts from the crystalline
basement at the Lockne impact site. The layer is within a sequence dated to
the Baltoniodus gerdae conodont subzone. The dating is corroborated by
chitinozoans indicating the latest Kukruse time below and the late Idavere
above the impact layer. According to the chitinozoans biostratigraphy, some
erosion may have occurred because of deposition of the impact layer. The
Hallen outcrop, today 45 km from the centre of the Lockne crater, is at
present the most distant accessible occurrence of ejecta from the Lockne
impact. It is also the most distant location so far found where the resurge
of water towards the crater has affected the bottom sediments. A greater
crater diameter than hitherto assumed, thus representing greater impact
energy, might explain the extent of the ejecta blanket. Fluidisation of
ejecta, to be expected at a marine-target impact, might furthermore have
facilitated the wide distribution of ejecta.
Copyright 2000 Institute for Scientific Information

Addresses:
Sturkell E, Nord Volcanol Inst, Grensasvegur 50, IS-108 Reykjavik, Iceland.
Nord Volcanol Inst, IS-108 Reykjavik, Iceland.
Univ Dannunzio, Dipartimento Sci, Int Res Sch Planetary Sci, I-65127
Pescara, Italy.
Tallinn Tech Univ, Inst Geol, EE-10143 Tallinn, Estonia.
Stockholm Univ, Dept Geol & Geochem, SE-10691 Stockholm, Sweden.

==============
(11) IMPACT EVENTS IN THE STATIGRAPHICAL RECORD

The devastating impact events in the stratigraphic record
Su Y, CHINESE JOURNAL OF GEOPHYSICS-CHINESE EDITION 42: 49-56, Suppl. S 1999

Thirty percent of 160 are known impact crater structures are buried by
postimpact sediment, and there must be much more impact crater structures
that have not be found in the sediment, especially in the oceanic bottom.
Even if the impact craer structures were destroyed by it active geologic
processes causing surface renewal, the impact ejecta and dedris of NEOs had
survived and marked the geohysical and geopchemical anomalies related to
impact in the sediment, providing NEO impact events with new ground
evideces. Though a limited number of stratigraphic evidences discovered, the
discovery of the evidences for impacts in the stratigraphic record has
developed an important research area of NEO impacts. This paper will present
a brief review of studies head in this direction. Copyright 2000 Institute
for Scientific Information

Addresses: Su Y, Chinese Acad Sci, Natl Astron Observ, Beijing Astron
Observ, Beijing 100012, Peoples R China.
Chinese Acad Sci, Natl Astron Observ, Beijing Astron Observ, Beijing 100012,
Peoples R China.

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