Odale-Articles-Skeleton

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

SKELETON LAKE

Skeleton Lake, highlighted in the Landsat image, is a generally circular lake provisionally classified as a simple meteorite crater. It is located in the Muskoka District of Ontario on the Canadian Shield slightly east of Georgian Bay. It is the largest open body of water within the Muskoka Lakes.
Skeleton Lake (possible) impact structure). Skeleton Lake is believed to have been given its name when the bones of an Ojibwa woman and her son were found on an island in the lake.
  • AGE (ma): ~800 Ma (from geological analysis - (Eyles 2010).
  • Diameter: ~3.5 km.
  • Location: Ontario, Canada N 45° 16’ W 79° 27’.
  • Shock Metamorphism: Intrinsic breccia outcrops that are associated with the formation of the structure occur at Tomelin Bluffs and Opal Island on the north shoreline.
  • Dating Method: No reliable age, the age estimate is based on the high degree of erosion (Elyes 2002).

I initiated my study of the Skeleton Lake structure based on an article in the February 1975 issue of the Journal of the Royal Astronomical Society of Canada. "Skeleton Lake - although the general outline is somewhat irregular, the semi-circular northern shore partly outlines a circular region, 3.2 km in diameter, where water depths average 50m. Breccias occur in shoreline outcrops of Precambrian rocks but have not been deformed to the level of shock metamorphism. The degree of fracturing and brecciation of the country rock resembles that around the Brent crater and is consistent with the position of the breccias at the margin of an eroded crater, beyond the distance where clearly defined shock effects occur. Ordovician limestone float recovered from the southern shore does not occur in outcrop within approximately 65 km of the lake, and may represent a portion of the Palaeozoic sedimentary cover which has been preserved from erosion in the crater basin, as is the case at several other Canadian impact sites. A 3 milligal negative gravity anomaly over the lake supports an impact origin (E. Waddington and M.R. Dence, personal communication)." (Grieve 1975).

The following features imply that the basin which Skeleton Lake now fills is an impact structure (meteorite crater remnant):
Odale-skeleton-03 skeleton lake aeromagnetics.jpg
The type of magnetic field change, a negative magnetic anomaly of ~80nT centered over the crater, is consistent with that found at other impact sites. The model suggests it is 3.4 km wide and the undisturbed bedrock is at ~750 m (Unpublished geomagnetic and electrical survey report Energy Mines and Resources, Earth Physics Branch (GSC) by J.F. Clark). Courtesy of Dr. James Whitehead, Planetary and Space Science Centre, University of New Brunswick. http://www.unb.ca/passc/ImpactDatabase/


This Aeromagnetic Chart was researched and forwarded to me by fellow RASC member Eric Briggs.

SKELETON LAKE is an approximately circular depression of four kilometres diameter in central Ontario. Note: On the left of the image is Opal Island. It is positioned on the lip of the “crater” wall at the North West corner of the lake. This island was the destination of my ground exploration of the structure. I hoped to find a breccia outcrop there.
The Skeleton Lake structure appears to truncate the regional geological and physiographic trends. The country rocks are Precambrian gneisses and migmatites. There are no volcanic or sedimentary rocks in the region, yet outcrops of breccia are found on the lake shore, and erratic boulders of Ordovician limestone occur along the south shore, evidently scoured from the lake bottom by glaciation. Aeromagnetic data indicate low uniform magnetization intensity under the lake, in contrast to the variable regional pattern. A detailed gravity survey has revealed a circular 3.3 milligal negative gravity anomaly which has been modeled by bowl-shaped distributions of low density brecciated rocks similar to those found at other meteor impact sites. These observations all imply that the Skeleton Lake depression is the eroded remnant of an impact crater from Paleozoic time, which has been preserved until the tertiary by a covering of Ordovician sediments. (E.D. Waddington, M.R. Dence; 1979)
Bathymetry documentation of Skeleton Lake illustrates a suggestive “crater like” form on the lake bottom.
The shape of the bottom of Skeleton Lake is similar in shape to the Brent impact structure as it has been highly modified by glacial action.

Personal correspondence from Dr. Michael Dence – to clarify for me the causal process that created the shape of the Skeleton Lake bottom] "The presence of a hill in the centre is not necessarily evidence for a central uplift as Brent has much the same topography and, as our abundant drilling has shown, is a simple bowl shape filled with post-impact sediments. Glaciation has partly excavated the Ordovician sediments and the moving ice has been deflected at the margin of the crater so that it has scooped out more of the relatively soft sediments there than in the middle. Thus the consolidated sediments are more deeply excavated at the edges forming kidney shaped depressions and leaving a central hill. This does not prove that a central uplift is not present. It could be under the Ordovician sediments. We are not likely to have the answer until we drill it, just as we lack evidence of shock metamorphism in the absence of material from the centre as the breccias at the edges, such as your sample from Opal Island, are too far from the centre to show evidence of shock effects.


Personal correspondence from Dr. Michael Dence - Two further points:

(i) One is that the same general topography is found in other craters where there has been erosion by continental glaciation. The ones that come to mind include Deep Bay and East Clearwater Lake. In both cases the lake bottom has been carved out of post- impact sedimentary rocks leaving a more or less central hill with kidney shaped depressions on each side. The long axis of the "kidneys" is aligned with the direction of glaciation and one kidney tends to be deeper than the other, as is also the case at Brent. However Deep Bay is about 9km across and East Clearwater twice that size and both have central uplifts as demonstrated by drilling. In the case of Deep Bay the drill holes suggest that the central uplift has almost no topographic expression and the crater profile when the sediments are removed is virtually flat, i.e. a flat floored crater. At East Clearwater the central uplift rises (from memory) about 100m above the floor of the crater but this is masked by the later sedimentary rocks, though the lowest points in the northern "kidney" depression are lower than the top of the central peak.

(ii)The second point is that in all these cases and others not mentioned there are thin deposits of glacial debris covering the more consolidated sedimentary rocks. These are typically 5-10m thick but can be >20m in some cases. They tend to be thickest where the ice sheets entered the crater depression and thinnest on the opposite side, though this may not always be the case. They modify the topography carved in the consolidated sedimentary rocks to some extent and can make drilling difficult."


There is also a large piece of Ordovician limestone glacial float on the south area outside of the structure. These large pieces of sedimentary rock were deposited on the lake bottom and then scooped out by the passing glacier.

The following images document the structure from approximately 2000 feet above ground. The superimposed circles on the images represent the approximate position of the proposed impact structure. I took these images of the structure during some of my aerial explorations of the area.


Skeleton Lake from the north east. The “>” indicates the position of Opal Island, my destination for my ground exploration trip. The report that “intrinsic breccia outcrops that are associated with the formation of the crater occur at Tomelin Bluffs and Opal Island on the north shoreline”, gave me a ground destination point of interest. I wanted to get a sample of the breccia rock from that island that might be impact related.
Skeleton Lake from the north west. The “X” on the right of the above image indicates where I was standing on the ice when I took the following ground image.


In (on) Skeleton Lake looking north. After about a 5 kilometer ski, here I am just entering the structure from the south (the “X” in the previous image). Opal Island, my destination, is indicated in the distance. This picture illustrates the immense dimension of the structure. It’s a good thing I love cross country skiing!
A very pleasant ski that followed the crater rim from the south brought me to my destination at Opal Island. Standing here on the ice I am just inside the lip of the Skeleton Lake crater wall (at depth under the ice).


Odale-skeleton-10 breccia 1.jpg
Odale-skeleton-11 breccia 2.jpg
Odale-skeleton-12 breccia 3.jpg


A cut and polished breccia sample that recovered from Opal Island within the Skeleton Lake structure. Breccias are composed entirely of rock and mineral fragments in a matrix of smaller, but similar, fragments. The fragments are derived from local target rocks. The breccia deposit on Opal Island stands in stark contrast to the surrounding target rock in the Central Gneiss Belt. The country rocks comprise of parautochthonous gneisses overlain by northwest-transported allochthonous terraines originating from the pre-Grenvillian Laurentian margin or from farther outboard as inferred for the Parry Sound allochthon.
Impactite breccias that form by the shattering and pulverizing of target rock, typically form irregular bodies tens to hundreds of meters in size. The rocks at the target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock - impact breccia - is the result. These images illustrate a breccia deposit on Opal Island in the Skeleton Lake structure that may have been formed as the result of this process.


Whenever I fly near the area of Skeleton Lake I divert to have a closer look to see the effect of the different sun angles on the cliffs that circle the lake. Future geologic research in the area, which would include drilling through the “crater” floor, might give specific confirmation that the Skeleton Lake structure is in fact the result of an impact event.


References

Eyles N. - [Personal correspondence from Dr. Nick Eyles 2010] "Very little is known of Skeleton Lake or even if it is an impact crater. I can't claim special knowledge of Skeleton Lake but it sits on Grenville rocks that are no older than 1.5 to 1.3 billion [years] and the whole area was peneplained and eroded to low relief (when any crater would have been lost) by c. 800 million years ago. What is seen now could be either the deep uneroded portions of a structure or the more shallow part of a crater formed after 800 Ma and not subjected to much erosion."

Elyes N., Ontario Rocks, Three billion years of environmental change. University of Toronto, 2002.

Grieve R.A.F., Robertson,P.B., IMPACT STRUCTURES IN CANADA. Journal of the Royal Astronomical Society of Canada, Feb. 1975.

Waddington E.D., Dence M.R. ;SKELETON LAKE – A PALEOZOIC IMPACT CRATER IN CENTRAL CANADA; Canadian Journal of Earth Sciences, 1979.



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