[meteorite-list] Slate Islands Impact Structure

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Sun, 12 Jun 2011 10:54:29 -0400
Message-ID: <BANLkTinXUhGLsJ7p5GzMY2YT18mSrktBYA_at_mail.gmail.com>

Hi Phil and List,

This is a general observation about impact structures, and not so much
about Slate Islands - please forgive the topic drift.

I am always impressed with the people finding new craters using
widely-available resources like Google Earth. In my amateur zeal, I
tried looking over the land in a several mile radius around my home in
south central Florida. To my stunned eyes, there were craters every
where I looked. Florida looked like the Moon in some spots. My
excitement only lasted about a nanosecond because each time I could
identify the feature as something more mundane - sinkhole, cattle
pond, retention pond, subsidence depression, etc. Florida is littered
with sinkholes and not all are filled with water.

So, what impresses me when people find these new impact structures is
this - how the heck can one distinguish an impact crater from a
satellite photo? Is there a quick and dirty list of criteria to look
for? I'm not a geologist, nor do I have any formal training, but I do
have a PC and an armchair. ;)

Best regards,

MikeG

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I know Florida is a bad place to look, but it's good training course
to identify all of the circular and ellipsoid things that are not a
crater.
On 6/12/11, JoshuaTreeMuseum <joshuatreemuseum at embarqmail.com> wrote:
> 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
>
> t
>
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Received on Sun 12 Jun 2011 10:54:29 AM PDT