[meteorite-list] Slate Islands Impact Structure

From: JoshuaTreeMuseum <joshuatreemuseum_at_meteoritecentral.com>
Date: Sun, 12 Jun 2011 09:54:35 -0400
Message-ID: <261A7528FCFA469C8EBEBEE321AD39CA_at_ET>

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


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,

2 Northwestern Ontario Field Services Section, Ontario Geological Survey,
Thunder Bay


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


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 .)



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)



Received on Sun 12 Jun 2011 09:54:35 AM PDT

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