SKELETON LAKE STRUCTURE

by: Charles O’Dale

• Type: Simple
• AGE (ma): ~800 Ma (geological analysis – Eyles 2010)^a – PROTEROZOIC
• 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.

^a Dating Method: No reliable age, the age estimate is based on the high degree of erosion (Elyes 2002).

2023 UPDATE:  The following Skeleton Lake north shore breccia images are courtesy of Carol  Alanko.

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).

Skeleton Lake, Ontario—evidence for a Paleozoic impact crater

E. D. Waddington, M. R. Dence Canadian Journal of Earth Sciences, 1979, 16(2): 256-263, 10.1139/e79-025

ABSTRACT – Skeleton Lake, Ontario (lat. 45°15′N, long. 70°27′W), contains an approximately circular depression, 3.6 km in diameter and 65 m maximum depth, which appears to truncate the regional geological and physiographic trends of the local Precambrian gneisses and migmatites. Outcrops of breccia are present on the northern lake shore, and erratics of Ordovician limestone, evidently scoured from the lake bottom by glaciation, occur along the south shore. Aeromagnetic data indicate a field of low uniform magnetic intensity across the lake, in contrast to the variable regional magnetic pattern. A detailed gravity survey has revealed a circular gravity anomaly of −3.3 ± 0.7 mGal (−33 ± 7 μm∙s−2) over the depression. The anomaly has been modelled by bowl-shaped distributions of low density brecciated rocks. These observations suggest that the Skeleton Lake depression is the eroded remnant of an impact crater of Paleozoic age which has been preserved by a covering of Ordovician sediments until recently exhumed.

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

Skeleton Lake, Ontario 45°15’N, 79°26’W. This crater has been examined with geophysical methods over the past decade. Rock specimens analysed by the Earth Physics Branch have not shown clear evidence of shock metamorphism and further work is required. Aeromagnetic and surface magnetic data have been correlated by Clark (1981). The crater is about 3.5 km in diameter. It is in the Muskoka District of Ontario, near the southerly edge of Grenville Province. A clearly recognizable magnetic signature on the maps shows a slightly asymmetrical, circular anomaly of -300 nT. Map numbers are ll2G, 126G, 127G and 148G.(Clark)

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.

AERIAL AND GROUND EXPLORATION OF SKELETON LAKE

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 brecca rock from that island that might be impact related.

Geology:

Skeleton Lake lies on the rocks of the Precambrian Shield, a formation covering half of Canada.  These rocks are the oldest on earth and originated in the Precambrian Era, a period that had such simple organisms that only the faintest of fossils can be found. The original igneous or volcanic rocks formed a billion years ago.  As the continental plates were merging, resultant pressures created a massive range of jagged mountains.

Under these mountains, a massive dome of granite pushed upward. The granite bedrock around Skeleton lake metamorphosed primarily into gneiss, striped with distinctive bands of colour, that has been glaciated, weather and slowly reduced to a rolling landscape over time. (The Bones of Skeleton Lake, Chapter 1: The Geology of Skeleton Lake.)

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.

Epilogue: Origin of the name Skeleton Lake

The lake had been given a name by the First Nations familiar with this region in the Muskoka district of Ontario. Early surveyors (1872) heard of it as ‘Paqua-sa-ca-cun’ or ‘Pasqua-sa-ca-tun’. Ojibeway dialect scholars translate this to mean ‘Waters-of-the-rolling hills’ or Lake-in-the green lands’. There is no connection with any fossilized skeletal remains of archeological or modern biological life. Local inhabitants, over many years, have corrupted part of the original name to a somewhat ominous sounding ‘ska-le-tun’.

Surprisingly, the Canadian Geographical Board officially approved the place-name only in 1951 having confirmed it was not in use elsewhere on any major geographical feature. Although derivation remains a conundrum it is perhaps unimportant compared to the (probable) cosmic event which created this ancient astrobleme which is presently a tranquil and picturesque summer resort area.