PLEASE NOTE:
*
Date sent: Wed, 14 Jan
1998 13:14:44 -0500 (EST)
From:
Benny J Peiser B.J.PEISER@livjm.ac.uk
Subject:
CC-DIGEST, 14/01/98
To:
cambridge-conference@livjm.ac.uk
Priority: NORMAL
CAMBRIDGE-CONFERENCE DIGEST, 14 January 1998
--------------------------------------------
(1) RICHARD HUGGETT'S BOOK "CATASTROPHISM" RE-ISSUED
(2) NEW RESEARCH ON SLATE ISLAND IMPACT CRATER
(3) HOW COSMIC CATASTROPHE PRODUCED WOMEN'S BEST FRIENDS: 65
MILLION
YEAR OLD DIAMONDS ARE FOREVER
(4) WHY SIZE DOES MATTER, OR HOW BIG IS THE CHICXULUB IMPACT CRATER?
(5) SEARCHING FOR THE PHYSICAL NATURE OF IMPACTING BODIES
======================================================================
(1) RICHARD HUGGETT'S BOOK "CATASTROPHISM" RE-ISSUED
From: Phil Burns pib@nwu.edu
A second edition of Richard Huggett's book
"Catastrophism" has just
been published by Verso Books (ISBN 1-85984-129-5). Most of the
text
remains unchanged from 1990 edition except for a few minor
corrections.
Huggett has added a new prologue, a revised final chapter, and
some useful
1990s references. The prologue mentions, among other things,
Clube et al's
work on the Taurid complex and the Tollmann's scenario for an
impact induced
series of tsunamis as the event behind the Biblical deluge story.
The
updated final chapter includes a discussion of Herb Shaw's ideas
about the
role of non-linear dynamics in the solar system.
===================================================================
(2) NEW RESEARCH ON SLATE ISLAND IMPACT CRATER
B. O. Dressler*) & V. L. Sharpton: Breccia formation at a
complex
impact crater: Slate Islands, Lake Superior, Ontario, Canada.
TECTONOPHYSICS, 1997, Vol.275, No.4, pp.285-311
*) LUNAR & PLANETARY INSTITUTE, 3600 BAY AREA BLVD, HOUSTON, TX, 77058
The Slate Islands impact structure is the eroded remnant of a
similar
30-32 km-diameter complex impact structure located in northern
Lake
Superior, Ontario, Canada. Target rocks are Archean supracrustal
and
igneous rocks and Proterozoic metavolcanics, metasediments, and
diabase. A wide variety of breccias occurs on the islands, many
of
which contain fragments exhibiting shock metamorphic features.
Aphanitic, narrow and inclusion-poor pseudotachylite veins,
commonly
with more or less parallel boundaries and apophyses branching off
them,
represent the earliest breccias formed during the compression
stage of the
impact process. Coarse-grained, polymictic elastic matrix
breccias form
small to very large, inclusion-rich dikes and irregularly shaped
bodies that
may contain altered glass fragments. These breccias have sharp
contacts with
their host rocks and include a wide range of fragment types some
of which
were transported over minimum distances of similar to 2 km away
from the
center of the structure. They cut across pseudotachylite veins
and contain
inclusions of them. Field and petrographic evidence indicate that
these
polymictic breccias formed predominantly during the excavation
and central
uplift stages of the impact process. Monomictic breccias,
characterized by
angular fragments and transitional contacts with their host
rocks, occur in
parautochthonous target rocks, mainly on the outlying islands of
the Slate
Islands archipelago. A few contain fragmented and disrupted,
coarse-grained,
polymictic elastic matrix breccia dikes. This is an indication
that at least
some of these monomictic breccias formed late in the impact
process and that
they are probably related to a late crater modification stage. A
small
number of relatively large occurrences of glass-poor, suevitic
breccias
occur at the flanks of the central uplift and along the inner
flank of the
outer ring of the Slate Islands complex crater. A coarse,
glass-free,
allogenic breccia, containing shatter-coned fragments derived
from
Proterozoic target rocks (upper target strata), observed at two
locations
may be analogous to the 'Bunte Breccia' of the Ries crater in
Germany. At
one of these locations, this breccia lies close to a crater
suevite deposit.
At the other, it overlies parautochthonous, monomictic breccia.
The Slate
Islands impact breccias are superbly exposed, much better than
breccias in
most other terrestrial impact structures. Observations, including
those
indicative of multiple and sequential processes, provide insight
on how
impact breccias form and how they relate to the various
phases of the
impact process. Eventually they will lead to an improved
understanding of
planetary impact processes.
=========================================================================
(3) HOW COSMIC CATASTROPHE PRODUCED WOMEN'S BEST FRIENDS: 65
MILLION
YEAR OLD DIAMONDS ARE FOREVER
R. M. Hough*), I. Gilmour, C. T. Pillinger, F. Langenhorst, A.
Montanari: Diamonds from the iridium-rich K-T boundary layer at
Arroyo el
Mimbral, Tamaulipas, Mexico. GEOLOGY, 1997, Vol.25, No.11,
pp.1019-1022
OPEN UNIVERSITY*),PLANETARY SCIENCE RESEARCH INSTITUTE, MILTON
KEYNES
MK7 6AA,BUCKS,ENGLAND
Diamonds, up to 30 mu m in Size, were found in the
iridium-rich layer
from the K-T boundary sito at Arroyo El Mimbral and the spherule
bed
from Arroyo Ei Penon, northeastern Mexico. Stepped heating
experiments
indicate two or more isotopically distinct diamond components
with carbon
isotopic Compositions Characteristic of a mixture of Carbon
sources, The
diamonds' crystal form is cubic-not the hexagonal polymorph of
diamond,
lonsdaleite, which has been used previously to infer formation
due to shock
transformation of graphite. The size crystallography, and
mineralogic
associations of K-T diamonds are similar to those of
impact-produced
diamonds from the Ries crater in Germany where both shock
transformation of
graphite and a mode of formation by condensation from a vapor
plume have
been inferred, The discovery of impact-produced diamonds in
association with
high Ir contents for these sediments supports their impact
origin, K-T age,
and the inference that their source was from the buried impact
crater of
Chicxulub on the Yucatan peninsula, Mexico.
======================================================================
(4) WHY SIZE DOES MATTER, OR HOW BIG IS THE CHICXULUB IMPACT
CRATER?
J. Morgan*), M. Warner, J. Brittan, R. Buffler, A. Camargo,
G. Christeson, P. Denton, A. Hildebrand, R. Hobbs, H. Macintyre,
G. Mackenzie, P. Maguire, L. Marin, Y. Nakamura, M. Pilkington,
V. Sharpton, D. Snyder, G. Suarez, A. Trejo: Size and morphology
of the
Chicxulub impact crater. NATURE, 1997, Vol.390, No.6659,
pp.472-476
*) UNIVERSITY OF LONDON, IMPERIAL COLLEGE OF SCIENCE,
TECHNOLOGY &
MEDICINE, DEPARTMENT OF GEOLOGY, LONDON SW7 2BP,ENGLAND
The Chicxulub impact in Mexico has been linked to the mass
extinction
of species at the end of the Cretaceous period. From seismic data
collected across the offshore portion of the Impact crater, the
diameter of the transient cavity is determined to be about 100
km. This
parameter is critical for constraining impact-related effects on
the
Cretaceous environment, with previous estimates of the cavity
diameter
spanning an order of magnitude in impact energy. The offshore
seismic data
indicate that the Chicxulub crater has a multi-ring basin
morphology,
similar to large Impact structures observed on other planets,
such as Venus.
============================================================================
(5) SEARCHING FOR THE PHYSICAL NATURE OF IMPACTING BODIES
G. Schmidt: Clues to the nature of the impacting bodies from
platinum-group elements (rhenium and gold) in borehole samples
from the
Clearwater East crater (Canada) and the Boltysh impact crater
(Ukraine).
METEORITICS & PLANETARY SCIENCE, 1997, Vol.32, No.6,
pp.761-767
UNIVERSITY OF MAINZ, INSTITUT FUER KERNCHEMIE, FRITZ
STRASSMANN
WEG 2,D-55099 MAINZ,GERMANY
Seven large (10 g) impact melt rock samples from boreholes
from the
Boltysh impact crater (Ukraine) and six samples from the East
Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd,
Re and Au
by the nickel sulfide technique in combination with neutron
activation.
Earlier analyses of Clearwater East impact melt rocks have shown
that they
are strongly enriched in Ir, Os, Pd and Re. In this work, I
confirm earlier
findings and demonstrate similarly high enrichments of Rh and Ru.
The
average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt
rock samples
from Clearwater East are CI- chondritic and yield an average Ir
content of
25.2 +/- 6.5 ng/g relative to an average upper crust
concentration of 0.03
+/- 0.02 ng/g Ir. The amount of meteoritic component
corresponds to 4 to 7%
of a nominal CI component for Clearwater East. The impact melt
rock samples
from a bore hole from Boltysh are low in Ir with an average of
0.2 +/- 0.1
ng/g. The CI-normalized abundances increase from the refractory
to the more
volatile siderophile elements (Os < Ir < Ru < Rh similar
to Pd similar to Au
similar to Ni similar to Co). Because of the low Ir anomaly and
uncertainties in making corrections (correlations are weak) for
indigenous
siderophile elements, no clear projectile assignment can be made.