a The structure appears to be created in the glacial remains of the Late Wisconsin North American glacial episode, 25,000–12,000 years ago.
Geological Features of Jocko Rivers Provincial Park
The bedrock of Jocko Rivers Provincial Park is within the Tomiko Terrane of the Central Gneiss Belt, in the northwest part of the Ontario Grenville Province. The Mesoproterozoic Tomiko Terrane consists mainly of metamorphosed quartzites and granitic plutons. Most of the park contains clastic siliceous metasediments in the form of arkosic quartz-feldspar gneisses, biotite and hornblende-rich quartz-feldspar gneisses and minor muscovite-quartzfeldspar gneisses. There is gneissic quartz monzonite of the Late Precambrian granitic suite also present within the park. The representation of clastic siliceous metasedimentary gneisses of the Tomiko Terrane is of regional significance (Frey and Duba 2002).
The Quaternary geology of the park is Late Wisconsinan in age. The small areas of bedrock uplands along the shorelines have a discontinuous till cover of thin sandy-silty ground moraine (Frey and Duba 2002). (Jocko River, Ontario Parks 2006)
An initial reconnaissance of the area established that immediately surrounding the structure is a very old dense forest with no trails to follow. In order to reach the clear lake within the structure we would have to haul our canoe kilometres through very dense bush and then down a very steep wall (and back again). Instead we decided to canoe to the crater starting from a crossing of the Jocko River over 6 kilometres to the north. The starting point (extreme left of the Google image) is >1 metre higher in altitude than our “crater” destination (extreme right in the image) measured by GPS.
The positions of the sounding points A – D are illustrated on the Google image above. The plot on the right documents the structure profile from lake bottom to the top edge of the rim.
We also measured a pH of 7.5 (normal) at the centre of the lake (point A in the above image)
Formation of the Structure (2 hypotheses)
After exploring this structure on foot and by canoe, I put forward these two hypotheses, one of which may be the explanation for the formation of this bowl shaped geological structure:
Erosion of the surrounding glacial regolith by the output of fresh-water springs on base of the crater south wall forming a circular gully; or
An impact from a cosmic-velocity bolide. Proof would entail either discovery of meteorite fragments around the structure from the original bolide and/or discovery of impact related geology. The Whitecourt Crater is an example of geological evidence for impact confirmation of a small crater of similar geology.
A metal detector search for meteorites around the south rim of the “crater” concluded with a negative result.
Area Geologic Description:
A water parting extending eastward from near the northern boundary of the area separates the Jocko River and Mattawa River drainage basins. Elevations along the water partings generally range from 1,000 to 1,400 feet with local elevations as low as 700 feet and as high as 1,600 feet. Lake Nipissing has an average elevation of 644 feet. Some waterways such as Trout Lake and the Mattawa River probably cover deep, pre-Pleistocene valleys in fault zones. Drainage within the area is controlled by three drainage systems: the Mattawa River system, the Jocko River system, and the Lake Nipissing – French River system. The Mattawa River and Jocko River systems are part of the Ottawa River drainage basin. (Lumbers 1971)