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Charity Shoal feature

Title: Small Rimmed Depression in Lake Ontario: An Impact Crater?
Authors: Troy L. Holcombe, John S. Warren, David F. Reid, William T. Virden, David L. Divins

Detailed bathymetry of Lake Ontario reveals a small circular feature and adjoining SWtrending ridge associated with a small topographic high identified as Charity Shoal on nautical charts. The feature consists of a circular basin 1,000 m in diameter and 19+ m deep, completely surrounded by a low-relief rim that rises to within 5 m of the water surface over much of its extent. A N53E tapering ridge is contiguous with the feature and extends southwestward. Bedrock consists of middle Ordovician limestones 100-150 m thick overlying rocks of Precambrian age. The limited information available suggests that the feature may be an extraterrestrial impact crater, but other origins such as sinkhole, volcanic cone, or kettle, are not ruled out. Time of formation is not known, but likely times include the Pleistocene when the area was exposed by glacial erosion, the middle Ordovician near the time of deposition of limestones, or the Cambro-Ordovician or Precambrian when erosion surfaces of this age were exposed. A subtle negative magnetic anomaly coincides with the feature and is consistent with an impact origin, though not positively diagnostic. Relief of the feature is low compared to that typical of an impact crater of this size. Glaciation may have diminished relief by eroding the rim and filling the central basin with drift. Verification as an impact crater will require detailed geophysical surveys and collection and analyses of samples from in and around the structure.

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A small equidimensional circular depression 1000 meters in diameter, with a continuous encircling rim, coincides with the feature referred to as Charity Shoal on nautical charts. An elongated ridge extends southwest from the feature, resembling the tail of a crag-and-tail feature common to some drumlin fields. The basin is slightly deeper than 18 meters and the rim rises to depths of 2-6 meters. The origin of the feature remains unknown. Although a sinkhole in the limestone terrane is a possibility, an origin related to a meteor crater, that was subsequently glaciated, seems more likely. Aeromagnetic mapping by the Geological Survey of Canada revealed a negative magnetic anomaly over Charity Shoal, which is a characteristic feature of simple impact craters (Pilkington and Grieve, 1992).
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Highest point is approx 2m below the surface of Lake Ontario - thought to be a glaciated meteorite impact crater or collapsed karst sinkhole. - Diameter is about 2 km.
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