EVIDENCE OF COLLISION AND THE IDENTITY OF THE METEOR,
LAKE ST. MARTIN METEORITE CRATER, MANITOBA, CANADA
BUNTEP, Brandon University
ABSTRACT
Field work in and around the Lake St. Martin’s meteorite crater has found evidence of
shatter cones from this meteorite crater as well as two other structures not described previously from other
meteorite craters that must be characteristic of a violent explosion going through the rocks. Evidence of
“expulsion cones” or narrow, cone-shaped channels cut through the rock from small holes were found in
dolomite just outside the crater. Loose, weathered rocks of impact breccias and melt rocks within the crater
show a consistent inverted bowl structure with the proposed name of umbrella structure. These impact
rocks are dominated by carbonate and chert and the structure could be the result of gas generated by partial
gasification of the rock with the gas moving explosively through the rock and bending the rock around it.
Thin section work clearly demonstrates the volcanic character of the melt rocks and the altered nature of
the impact breccias. Finally, limited chemical data on the melt rocks show some similarities and some
differences as compared to data of previous investigators.
PREVIOUS WORK
The first geological report on the area (Hunter, 1951) described the igneous
rocks of the St. Martin area. The author found the granite to be similar to any other
Precambrian granite from the eastern side of Lake Winnipeg, but he thought the lavas
were fresher than normal Precambrian lavas. More detailed work by McCabe and
Bannatyne in 1970 proposed that the now established crater contained lavas that could be
either volcanic or meteoritic-volcanic in origin. Drilling done by the Manitoba Geological
Survey over many years prior to 1983 revealed evidence of a meteorite impact structure
and described the lavas as “melt rocks” the result of an explosion generated by the meteor
colliding with the earth at a tremendous speed (Bannatyne and McCabe, 1984). The
dominant rocks of the crater were described as impact breccias. Recent work by Reimold
et al (1990) on the geochemistry of the melt rocks concluded that the structure is
meteoritic in origin. The last investigators suggested that additional sampling and
analysis could shed more light into the amazing story of this meteor collision.
FIELD INVESTIGATIONS
Field investigation has identified two locations worthy of further study. These are sites 1
and 2 in Fig.1 in the western part of the crater. These sites were not sampled or studied by
previous investigators who concentrated their study in the eastern part of the crater.
In site 1, impact breccias are exposed with minor amount of melt rocks in between. This
site is an abandoned quarry where gravel was hauled away exposing the bedrock on the
quarry floor. Gypsum forms part of the matrix in bedrock, so with time it dissolved away
and the bedrock weathered into loose fragments.
Fig. 1 Location map of site 1 (Impact Breccia) and site 2 (Dolomite quarry). Capital
letters in red denote geological units as mapped by the Manitoba Geological
Survey. Map from Manitoba Department of Natural Resources, Water Resources
Branch, Map 62O. Legend: Silurian dolomites (S), Ordovician
dolomite/limestone (O), Precambrian granite (PE), St. Martin Complex (P),
Jurassic sandstone, siltstone & gypsum (J).
Geographical coordinates of site 1:
51degrees 45.711 minutes N, 98 degrees 37.743 minutes W
Geographical coordinates of site 2:
51degrees 40.760 minutes N, 98 degrees 43.937 minutes W
The peculiar “inverted bowl” or umbrella structure (see Figures 2 to 4) was observed on
fragments of impact breccia and melt rock that have become loose after interstitial
gypsum was dissolved away. Such a structure has not been described from this or any
other impact structure previously. This structure must be indicative of a violent explosion
going through the rocks. The umbrella shape of the rock suggests that some of the
material within these calcareous rocks was vaporized by the power of the explosion; that
gas (probably, carbon dioxide) was propelled through the rock with such a great force
that it pushed the surrounding rock away gouging a cavity in the shape of an umbrella. In
the case of the melt rock the vesicles are lined up parallel to the umbrella structure
indicating that the rock was cooling down when this arching was taking place.
Site 2 is just outside the crater. This location is an abandoned dolomite quarry. Numerous
examples of shatter cones were collected there (see Fig. 5). The shatter cones have their
apexes towards the northeast, in other words, towards the centre of the present crater.
Examples of another feature characteristic of a major explosion were also found that can
be described as “expulsion cones” (see Fig. 6). The latter are tiny holes with parallel
conical grooves originating from these holes. The holes were probably created by part of
the rock turning into gas (carbon dioxide) and the gas literally cut a narrow channel
through the rock away from the explosion. That is why all expulsion cones show an
approximate parallel orientation.
Fig 2. Example of the umbrella or inverted bowl structure in melt rock. Note
the elliptical cavities that bend around the raised centre. The rock on
the right was broken off the bigger piece
Fig. 3 Side view of the umbrella or inverted –bowl structure in impact breccia
Fig. 4 The underside of the umbrella structure on the same rocks as in Fig. 3.
Note the development of white clay as an alteration product below the
raised centre.
Fig. 5 Shatter cones in dolomite from site 2
emanate from tiny holes in the rock and point towards the coin in the
picture
SAMPLING
Samples of melt rocks and impact breccias were collected from site 1. Ten
thin sections were cut and a detailed report on three of them was prepared by Vancouver
Petrographics Ltd.
Six samples of melt rocks were analysed for major elements by ICP-ES and minor
elements by neutron activation (method BQ-NAA-1) . The results are included in
Appendix I.
PETROGRAPHY
SUMMARY
Ten thin sections were prepared for petrographic study. Seven of these
were from melt rocks and three from impact breccias. Three representative samples (two
melt rocks and one breccia) were submitted for a detailed petrographic study.
The results show that the melt rocks have definite volcanic features with some important
differences. The breccia on the other hand shows unique features akin to a hot spring
deposit.
MELT ROCKS
These are fine grained and vesicular with a variable amount of red stain;
from faint red to deep red. There is a variable amount of vesicles and some are filled with
gypsum. There are fragments of all sizes and shapes. The angular fragments suggest that
these are no ordinary lava flows. Most fragments show strong alteration, so the original
composition would be hard to establish. The vesicles are elliptical and the rock in places
shows a flow texture. Minerals with bright colours, probably clays, fill up parts of
vesicles or are replacing parts of the fragments. The rocks exhibit low magnetism. The
thin sections show fragments that have been cracked and split with angular pieces having
moved some distance away. This suggests the rock was under tremendous pressure from
an explosion.
BRECCIAS
These rocks show no obvious volcanic features and under the microscope
are a mixture of fine -grained chert and carbonate. They contain numerous vugs partly
filled with crystals of quartz. Interestingly, the petrographer describes this rock as being
similar to a siliceous sinter that formed in a hot-spring environment.
GEOCHEMISTRY
Reimold et al (1990) performed analytical work on samples of melt rocks
taken from the eastern part of the St. Martin crater. The results of the present study show
similar results with some minor, maybe important, differences.
With regards to major oxides, the current results have slightly lower SiO2, MgO, CaO
and Na2O concentrations. On the other hand, slightly higher contents were obtained in
Al2O3 and Fe2O3. Finally, the K2O contents were much higher. These results suggest
that melting in a different part of the crater involved different formations that resulted in
producing a slightly different melt rock. Part of the meteor was also incorporated into the
melt rock and this contribution could have produced the variation in melt rock
composition.
With regards to the minor elements there is very good agreement in almost all of the 30
or so analysed elements. However, all of the samples of the present study show much
higher As and U contents (24 X and 7X respectively). This suggests that either the
original host rocks of the area contained higher As and U or these metals were enhanced
in the meteor itself. Nevertheless, it is an important difference from previous data and
needs further study.
CONCLUSIONS
This study has described the first evidence of shatter cones in the St.
Martin meteorite crater area. Another evidence of its explosive origin is provided by the
discovery of the, so-called, “umbrella structure” in rocks within the crater from violent
gasification within the calcareous target rocks. Another new feature of this explosion is
provided by the formation of “expulsion cones” generated by explosive gases within the
rock. All the features above provide more information about the way this violent event
unfolded through the target rocks.
The petrographic work attested to the volcanic features of the melt rocks. As for the
impact breccias they show evidence of a catastrophic explosion that broke up even small
fragments.
The chemical results are comparable to those of previous investigators that sampled
different parts of the crater. The higher potassium, arsenic and uranium may reflect the
composition of the meteor or the targeted rocks or both.
A P P E N D I X I
MAJOR ELEMENT CONCENTRATIONS OF MELT ROCKS FROM THE ST. MARTIN CRATER
| ||||||||
sample
|
SiO2
|
Al2O3
|
Fe2O3
|
MgO
|
CaO
|
Na2O
|
K2O
|
TiO2
|
%
|
%
|
%
|
%
|
%
|
%
|
%
|
%
| |
SM-1
|
57.68
|
16.28
|
5.16
|
3.75
|
2.91
|
2.61
|
5.32
|
0.41
|
SM-2
|
58.48
|
16.61
|
4.05
|
3.16
|
2.75
|
2.56
|
6.19
|
0.43
|
SM-3
|
60.25
|
16.24
|
4.33
|
1.7
|
2.91
|
3.17
|
5.59
|
0.43
|
SM-4
|
62.49
|
16.7
|
4.24
|
1.15
|
2.33
|
3.22
|
5.79
|
0.44
|
SM-5
|
59.32
|
15.67
|
5.27
|
2.43
|
1.49
|
2.3
|
6.26
|
0.44
|
SM-6
|
59.58
|
15.89
|
4.96
|
2.35
|
1.47
|
2.43
|
6.16
|
0.43
|
P2O5
|
MnO
|
Cr2O3
|
L.O.I.
|
TOTAL
| ||||
%
|
%
|
%
|
%
|
%
| ||||
SM-1
|
0.2
|
0.02
|
0.004
|
5.5
|
100.01
| |||
SM-2
|
0.19
|
0.01
|
0.005
|
5.3
|
99.88
| |||
SM-3
|
0.24
|
0.01
|
0.004
|
4.3
|
99.82
| |||
SM-4
|
0.22
|
0.01
|
0.004
|
3.1
|
99.89
| |||
SM-5
|
0.22
|
0.02
|
0.004
|
6.5
|
100.03
| |||
SM-6
|
0.22
|
0.02
|
0.005
|
6.4
|
100
| |||
MINOR ELEMENT CONCENTRATIONS IN MELT ROCKS FROM THE ST. MARTIN CRATER
| ||||||||
method
|
all ppm
| |||||||
sample :
|
SM-1
|
SM-2
|
SM-3
|
SM-4
|
SM-5
|
SM-6
| ||
ICP-ES
|
Ba
|
717
|
626
|
899
|
1012
|
629
|
673
| |
Ni
|
12
|
10
|
12
|
6
|
8
|
14
| ||
Sr
|
543
|
449
|
4696
|
548
|
141
|
145
| ||
Zr
|
126
|
126
|
128
|
124
|
128
|
126
| ||
Y
|
8
|
8
|
9
|
8
|
10
|
10
| ||
Nb
|
< 5
|
< 5
|
< 5
|
7
|
5
|
7
| ||
Sc
|
7
|
6
|
7
|
7
|
6
|
6
| ||
BQ-NAA-1
|
Ca %
|
2
|
2
|
2
|
2
|
<1
|
<1
| |
Fe %
|
3.63
|
2.92
|
3.03
|
2.99
|
3.65
|
3.43
| ||
Na %
|
1.94
|
1.93
|
2.39
|
2.44
|
1.74
|
1.82
| ||
Co
|
9
|
7
|
5
|
4
|
5
|
4
| ||
Ni
|
<100
|
<100
|
<100
|
<100
|
<100
|
<100
| ||
Cr
|
31
|
39
|
34
|
34
|
33
|
33
| ||
Au
|
< 2
|
< 2
|
< 2
|
< 2
|
< 2
|
< 2
| ||
La
|
41
|
43
|
40
|
38
|
37
|
37
| ||
Ce
|
83
|
88
|
80
|
74
|
76
|
76
| ||
Nd
|
34
|
35
|
33
|
32
|
30
|
31
| ||
Sm
|
5.3
|
5.4
|
5.2
|
4.7
|
5.2
|
5.1
| ||
Eu
|
1.4
|
1
|
1.2
|
1
|
0.8
|
1
| ||
Tb
|
<0.5
|
<0.5
|
<0.5
|
0.5
|
0.6
|
<0.5
| ||
Yb
|
0.6
|
0.5
|
0.5
|
0.6
|
0.7
|
0.7
| ||
Lu
|
0.06
|
0.06
|
0.08
|
0.06
|
0.08
|
0.07
| ||
Hf
|
4
|
4
|
4
|
4
|
4
|
4
| ||
Ta
|
<0.5
|
<0.5
|
0.8
|
0.9
|
<0.5
|
0.5
| ||
Sc
|
6.8
|
6.6
|
7
|
7.6
|
6
|
6.3
| ||
As
|
22
|
10
|
13
|
9.4
|
8.7
|
10
| ||
Rb
|
52
|
67
|
86
|
79
|
110
|
100
| ||
Cs
|
<1
|
<1
|
1
|
1
|
2
|
2
| ||
Ba
|
610
|
620
|
850
|
980
|
590
|
660
| ||
U
|
4.3
|
7
|
4.1
|
4.8
|
6.1
|
5.4
| ||
Th
|
10
|
10
|
9.5
|
9
|
10
|
10
| ||
Ir
|
< 5
|
< 5
|
< 5
|
< 5
|
< 5
|
< 5
| ||
Sr
|
< 500
|
< 500
|
3900
|
< 500
|
< 500
|
< 500
| ||
Zn
|
130
|
100
|
< 50
|
< 50
|
60
|
53
| ||
REFERENCES
Bedrock Geology, Dauphin Lake Area, Manitoba Dept. of Natural Resources, Water Resources Branch, Figure 2, Map 62O
Hunter, HE, 1951, Igneous Rocks in the Lake St. Martin Area, Manitoba, Manitoba Dept. of Mines, publication 50-10
Currie, KL, 1969 Lake St. Martin impact structure, Manitoba (620) (abstract). Geological Survey of Canada Dept. of Energy, Mines & Resources, Paper 70-1, Part A, p.111
McCabe, HR & Bannatyne, BB 1970, Lake St. Martin Crypto-explosion Crater and Geology of the Surrounding Area, Geological Paper 3-70, Manitoba Mines Branch
Bannatyne, BB & McCabe, H. 1984, Manitoba Crater Revealed, GEOS, Vol.13, p.10-13
Reimold, WU Barr, JM, Grieve, RAF, and Durrheim, RJ, 1990, Geochemistry of the melt and country rocks of the Lake St. Martin impact structure, Manitoba, Canada, Geochimica et Cosmochimica Acta, Vol.54, pp. 2093-2111
Vancouver Petrogaphics Ltd., Report on petrography of samples from the Lake St. Martin crater, February 2006
Acme Analytical Lab. Ltd., Report on analysis, March 2006
