[meteorite-list] Slate Islands Impact Structure
JoshuaTreeMuseum
joshuatreemuseum at embarqmail.com
Sun Jun 12 09:54:35 EDT 2011
I hadn't heard about this Lake Superior crater. Interesting that the islands
are the central uplift formation of the crater.
Click the link for the rest of the pdf with maps and pics.
Phil Whitmer
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970028016_1997050774.pdf
14. New Observations at the Slate Islands Impact ,
Structure, Lake Superior
B.O. Dressier 1, V.L. Sharpton 1, B. Schnieders 2 and J. Scott 2
1 Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas,
77058
2 Northwestern Ontario Field Services Section, Ontario Geological Survey,
Thunder Bay
INTRODUCTION
Slate Islands, a group of 2 large and several small
islands, is located in northern Lake Superior, approximately
I0 km south of Terrace Bay. Shatter cones,
breccias and shock metamorphic features provide evidence
that the Slate Islands Structure was formed as a
result of asteroid or comet impact (Halls and Grieve
1976, Grieve and Robertson 1976). Most of the island
group is believed to represent the central uplift of a
complex impact crater. The structure possibly has a
diameter of about 32 km. For Sage ( 1978, ! 991) shock
metamorphic features, shatter cones and pervasive
rock brecciation are the results of diatreme activity.
The present investigations represent the second
year of a co-operative study of the Lunar and Planetary
Institute, Houston, Texas and the Field Services Section
(Northwest) of the Ontario Geological Survey.
The objective of this investigation is to come to a better
understanding of the formation of mid-size impact
structures on Earth and the planets of the solar system.
Impact processes played a fundamental role in the
formation of the planets and the evolution of life on
Earth. Meteorite and comet impacts are not a phenomenon
of the past. Last year, more than 20 pieces of the
Shoemaker-Levy 9 impacted on Jupiter and the
Tunguska comet impacted in Siberia in the early years
of this century. The study of impact processes is a
relatively young part of geoscience and much is still to
be learnt by detailed field and laboratory investigations.
The Slate Islands Structure has been selected for
the present detailed investigations because of the excellent
shoreline outcrops of rock units related to the
impact. The structure is a complex impact crater that
has been eroded so that important lithoiogical and
structural elements are exposed. We know of no other
mid-size terrestrial impact structure with equal or better
exposures.
In this publication we present preliminary results
of our 1994 and 1995 field and laboratory investigations.
We have tentatively identified a few impact melt
and a considerable number of suevite occurrences.
"Bunte Breccia" and "suevite" (for definitions see
Ontario Geological
Engelhardt 1990 and references therein) and other
clastic matrix breccias occur on the islands. (For names
of specific locations mentioned in this publication
please see Figure 14.1 .)
GENERAL GEOLOGY
OF SLATE ISLANDS
A wide variety of Archean and Proterozoic rocks
underlie the islands. Archean rocks make up the bulk of
the Slate Islands bedrock (Sage 1991). They are composed
of greenschist facies, felsic to mafic pyroclastic
rocks, pillowed and variolitic mafic flows, feldspar
porphyry flows interbedded with mudstones, siltstones
and ironstones. Archean gabbros and quartz-feldspar
porphyries intrude the supracrustal rocks (Sage 1991 ).
Laminated argillite and chert-carbonate-hematite
ironstone of the Gunflint Formation and argillite of the
Rove Formation, both of the Animikie Group, as well
as, mafic metavolcanic rocks, intraflow sandstone and
siltstone, and diabase dikes of the Osier Group,
Keweenawan Supergroup, occur on the islands but
spatially are of limited extent (Sage 1991).
Lamprophyres occur on the islands and one dike at the
southeast coast of Patterson Island has been dated by
the U-Pb method on perovskite at about !.1 Ga (oral
communication L.Heaman, University of Alberta,
Edmonton, Alberta, 1994). This dike is cut by breccias
(R.Sage, Ontario Geological Survey, Sudbury, oral
communication 1994) believed to be related to the
Slate Islands impact event. This date provides a maximum
radiometrie age for the impact. However, we
have observed breccias on the islands containing
sandstone and siltstone clasts that strongly resemble
units of the Jacobsville Formation, suggesting a maximum
age of about 800 ma, based on assignment of the
Jaeobsville Formation as Hadrynian (Card et al. 1994).
We did not attempt to reinterpret the distribution of
the various Archean and Proterozoic rock units that
underlie the island group. It is, however, worth noting
that all rocks on the islands are brecciated to various
degrees. Large rock masses on Mortimer and Delaute
islands are monomict breccias and we have observed
granitic rocks and diabase on Patterson Island that
easily break into centimetre-sized angular fragments
Survey, R.P. 164, 53-61 (1995)
53
II
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