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Chubb Crater - Part 12 of 12

Hello Listees and Listoids,

Part 12 sums up some of the important steps in the recognition and
confirmation of Chubb Crater as an impact scar as culled from Mrs.
Marvin's and Mr. Kring's article below. Have fun :-)


MARVIN U.B. and KRING D.A. (1992) Authentication controversies and
impactite petrography of the New Quebec Crater (Meteoritics 27-5, 1992,
585-595):

01) The Inuit word "Ungava" means far away, and, indeed, the site is so
remote that few field parties have visited the crater since it was first
noted on U.S. Army Air Force photographs taken in 1943.

02) A meteoritic origin was proposed for the New Quebec Crater in 1949
on the basis of an aerial photograph showing its unique circularity and
raised rim amid Precambrian gneisses of the Canadian Shield.

03) In an unpublished report prepared in 1949 for the Arctic Institute
of North America, the astronomers, G.V. Douglas and Mary C.V. Douglas
wrote that if the crater were of impact origin the meteorite must have
fallen nearly vertically because the rim elevations on all sides were
about the same.

04) Early in 1950, a prospector, Fred W. Chubb, proposed, again from a
photograph, that the crater in Ungava might be the surface expression of
a diamond pipe.

05) Newspaper sponsorship generated widespread publicity. "Discovery in
the Tundra," appeared in the 1950 August 14 issue of Time magazine
describing what was "... almost certainly a meteorite crater (there was
no lava or other sign of volcanic activity), and the biggest yet
discovered.

06) Time also stated that in the absence of Indian or Eskimo legends
about a huge explosion, "Dr. Meen estimated that the meteorite must have
fallen at least 3000 years ago."

07) In his report, Meen (1950) ... confirmed that his party found no
traces of diamonds, volcanic rock, or meteoritic materials. However, he
had observed several features suggestive of explosive impact.

08) Meen found arcs of concentric ridges, ... From this Meen postulated
that the impacting meteorite had followed a trajectory from WSW to ENE.
He was well aware that a circular crater does not demand a vertical
impact.

09) In July and August of 1951, Meen led a second expedition to Chubb
Crater co-sponsored by the Royal Ontario Museum and The National
Geographic Society.
This group continued geological studies, sounded the lake, conducted a
magnetometer survey of the area, and collected sand-sized soils in which
to look for meteoritic fragments of spheroids. They found none but did
measure a positive magnetic anomaly on the east side of the crater rim,
and Meen (1951, 1952) offered this as positive evidence of a buried
meteorite.

10) The New Quebec Crater was re-examined by J.M. Harrison (1954), of
the Geological Survey of Canada, who found persuasive evidence that the
crater was a preglacial feature.

11) In a later paper, Meen (1957) concurred with Harrison's argument for
glaciation at the crater.

12) Reasoning from what he called inferential and negative evidence,
Harrison concluded that the crater was formed by the explosion of a
large meteorite. No other process seemed adequate to account for this
unique, circular, rimmed bowl in polished granite in a Precambrian
shield area with no vestige of recent volcanism. Harrison added that
glaciation had destroyed much of the evidence, however.

13) An impact origin gained additional support from E.M. Shoemaker
(1962) who found sheeting in the gneisses that dipped outward by as much
as 70° on the rim crest and flattened to low inclinations on the
surrounding plains.

14) Baldwin (1963) positively accepted the New Quebec Crater as a
modern, terrestrial meteorite crater - although no shock products had
been discovered there. Baldwin based his judgement entirely on the
crater's form.

15) In 1963, Kenneth L. Currie and Michael R. Dence concluded that the
upheaval and outward tilting of the rocks in the rim supported an impact
origin. However, on a later survey Currie (1964, 1965) began to doubt a
meteoritic origin.

16) While searching the inner rim for possible fallback materials,
Currie (1964) collected "... two ovoid bombs of black glassy  material
containing abundant fragments of partially decomposed granite" ...

17) Currie concluded that the crater had been formed by some exceptional
kind of volcanism related to the explosive variety that formed the
hydrovolcanic maar craters of the EifeI district in Germany.

18) M.J.S. Innes (1964), of the Dominion Observatory, conducted a
gravity survey in an east-west direction across the New Quebec Crater
and found a 6 mgal negative anomaly centered on the crater. At that
time, Innes' gravity data constituted the best field evidence of an
impact origin for the New Quebec structure.

19) In 1966, two teams of investigators re-examined a sample of Currie's
vesicular rock from the New Quebec Crater and found multiple sets of
shock-induced planar deformation features in quartz grains. For most
investigators, this petrographic evidence of shock effects, together
with Innes' gravity data, confirmed an impact origin for the New Quebec
Crater.

20) The first discovery of New Quebec impactite after that of Currie
(1964) was made in August, 1986, when J.D. Boulger picked up a rounded,
vesicular pebble, 1.75 cm across, on the south shore of Lac Laflamme (=
Museum Lake). A thin section proved to be rich in quartz grains with
multiple sets of planar features.

21) Boulger returned to the New Quebec Crater in 1988 August and found
one more pebble of impactite, about 2.5 cm across; the University of
Montreal party discovered 21 additional samples.

22) This ample supply of material allowed Grieve et al. (1989, 1991) to
determine a reliable date of 1.4 ± 0.1 Ma for the cratering event, to
identify the impacting body as a chondrite, and to match the bulk
compositions of the impactite glasses with those of the gneissic country
rocks.

That's all folks. I hope you enjoyed it as much as I did !


Best regards,

Bernd

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