CHARITY SHOAL

CHARITY SHOAL, Lake Ontario, Canada

by: Charles O’Dale

  • Type: Simple
  • Location: N 44° 02’ W 76° 29’
  • Diameter: ~1 Km
The circular feature in the Google map of the Lake Ontario floor represents the position of the Charity Shoal (possible crater).
Charity Shoal is indicated by the small circle on this aeronautical chart. Note the magnetic anomaly just to the east of the structure.
Charity Shoal from GOZooM over Lake Ontario at about 2000′ over the water. The northern part of the shoal was barely visible.
A. Total magnetic intensity (TMI) B. Residual magnetic field map produced by subtraction of 200 m upward continued grid.
A. 200 kHz single-beam bathymetry] B. Residual magnetic field. C. N-S chirp seismic profile (Suttaket al, 2013)

Update 2015

Geological structure of Charity Shoal crater, Lake Ontario, revealed by multibeam bathymetry

Troy L. Holcombe , Scott Youngblut, Niall Slowey

Abstract Acoustic images of Charity Shoal in Lake Ontario, derived from a 1 × 1 m grid model of bathymetry assembled by the Canadian Hydrographic Service in 2010–2011, confirm the existence of a crater, as revealed by its surface morphology. With these higher quality data, it is possible to describe the crater in much greater detail, and arrive at a better interpretation of the geology than was possible using the earlier bathymetry of Lake Ontario. This new bathymetry of Charity Shoal reveals a continuous rim encircling an ovoid-shaped crater floor 1,200–1,500 m in diameter, with the crater floor being largely devoid of relief. Extending 3–4 km southwest of the crater is a ridge capped by a linear zone of unstratified debris that resembles a medial moraine. NE–SW erosional valleys cut across the crater rim in its southwestern sector. Apparently, glacial erosion has stripped the soil zone off stratified bedrock beneath the crater rim, exposing an intricate pattern of micro-ridges and grooves that bear the record of differential resistance to erosion of successive beds within the sequence of rock strata. Mapping of the shallow structure of the bedrock reveals a continuous ring anticline coinciding with the crater rim, with rock strata dipping gently in both directions away from the rim axis. In combination with existing evidence on the regional stratigraphy, these observations and interpretations are consistent with the Charity Shoal crater having formed in a shallow marine environment by an extraterrestrial impact event in the Middle Ordovician, followed by post-impact sedimentation, and much later, erosion during Pleistocene glaciations. Apparently, post-impact sediments infilled the crater and eventually covered the crater rim, leaving only a diminished structural expression of a crater having no more than 20 m of surface relief. Further details of crater history and origin, and a test of the hypothesis of impact, will likely come from acoustic reflection profiling and direct sampling. (Holcombe et al, 2013)

Simulation of Charity Shoal structure from Discovery Channel program “Drain the Great Lakes”.
 ABSTRACT. – 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 SW-trending 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 lime-stones 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 lime-stones, 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 analy-ses of samples from in and around the structure.

I took this image of the north rim of Charity Shoal from about 1000 feet above Lake Ontario in my bird “GOZooM”. This was the only part of the structure visible from the air.

The Carity Shoal Structure is defined by a ring-like magnetic high and central magnetic low. The total field magnetic anomaly is large (> 1500 nT) and cannot be accounted for by the 19.5 m deep basin within the Paleozoic bedrock. The anomalies large magnitude indicates a deep basin and/or demagnetization effects in the Precambrian basement rocks below the structure. De-magnetization effects are associated with meteorite impacts due to shock melting of target rocks but can also be produced by intrusive bodies with remanence directions that oppose the modern main field.

The impact crater model best reproduces the observed anomaly when the basin is modeled as a 450 m deep parabolic impact crater in the Precambrian basement. The modeling results exclude the origin of the Charity Shoal Structure as a shallow erosional feature and are most consistent with a meteorite impact or structural depression (syncline) within the Precambrian basement.(Suttak et al, 2013)

The superimposed circle on the aeronautical chart indicates the position of Charity Shoal. Note the magnetic anomaly indicated to the east of the shoal “magnetic disturbance of as much as 9° exists at lake level in this area”. Could this magnetic anomaly be the result of an impact by an iron meteorite? Verification as an impact crater will require detailed geophysical surveys and collection and analyses of samples from in and around the structure. My aerial exploration of the crater was rather disappointing. Nothing of the bottom of the lake was visible when I was directly over the feature (confirmed with GPS). The peak of the shoal directly to the north, shown in this image, was the only part of the shoal I could see from about 2000 feet over the water (so I could remain within gliding distance of land on engine failure). An underwater exploration trip is in order!

Map view close-up of the detailed bathymetry of Charity Shoal area. Depths are in meters and the contour interval is 2 m (image courtesy of Virden et al. 1999).
Three-dimensional perspective of Charity Shoal area showing distinct bowl-shaped depression and rim (image courtesy of Virden et al. 1999).

ABSTRACT (Bathymetry of Lake Ontario. National Geophysical Data Center)

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. (Virden et al. 1999).

 

In this article, the 3-D renditions of Charity Shoal are courtesy of the National Oceanic & Atmospheric Administration (NOAA), U.S. Department of Commerce. A side-scan sonar survey has identified a 19+m deep, 1000m diameter circular feature in Lake Ontario. It is completely surrounded by a low relief rim that rises to within 5m of the water surface. It consists of bedrock of middle Ordovician limestones 100-150 m thick overlying rocks of Precambrian age. 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.A sediment sample collected from the crater floor was stiff, varved lake clays covered with a thin layer of coarse sand. The time of the structure’s formation cannot be determined, 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. The structure is imbedded in the limestone formation typical of that around the Kingston area and may be a sinkhole, volcanic cone or kettle (Edsall 1992).

Private Correspondence from Mr. John Warren, Canadian Hydrographic Service (CHS) – Charity Shoal is a 19+m deep, 1000m diameter circular feature in northeastern Lake Ontario. Bathymetric analysis (Holcombe 2001) suggested that it may be an impact crater (see images left and right). As a result, the Geological Survey of Canada collected samples of the rocks from the site in the hopes of discovering the origin of this feature, but the results were inconclusive and more detailed sampling, including drilling, would be required.

An earlier side-scan sonar survey provided some information about the surrounding terrain and substrate, but failed to identify it as a possible impact structure. It is completely surrounded by a low relief rim that rises to within 5m of the water surface. It consists of bedrock of middle Ordovician limestones 100-150 m thick overlying rocks of Precambrian age. 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 (Edsall 1992).

 

References

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Edsall, T.A., Brown, C.L., Kennedy, G.W., and French, J.R.P. III. 19912. Surficial substrates and bathymetry of five lake trout spawning reefs in nearshore waters of the Great Lakes. Great Lakes Fishery Commission Technical Report 58, Ann Artbor, Michigan, 1992.

Troy L. Holcombe, Scott Youngblut, Niall Slowey, Charity Shoal crater, Lake Ontario, revealed by multibeam bathymetry Geo-Marine Letters August 2013, Volume 33, Issue 4, pp 245-252

T.L. Holcombe, J.S. Warren, D.F. Reid, W.T. Virden, and D.L. Divins, 2001, Small Rimmed Depression in Lake Ontario: An Impact Crater?, Journal of Great Lakes Research., 27(4): 510-517, 2001.

Troy L. Holcombe , Scott Youngblut, Niall Slowey. Geological structure of Charity Shoal crater, Lake Ontario, revealed by multibeam bathymetry Geo-Marine Letters August 2013, Volume 33, Issue 4, pp 245-252

Ian SPOONER, George STEVENS, Jared MORROW, Peir PUFAHL, Richard GRIEVE, Rob RAESIDE, Jean PILON, Cliff STANLEY, Sandra BARR, and David MCMULLIN, Identification of the Bloody Creek structure, a possible impact crater in southwestern Nova Scotia, Canada. Meteoritics & Planetary Science 44, Nr 8, 1193–1202 (2009)

Haynes, C. V., Younger Dryas ‘‘black mats’’ and the Rancholabrean termination in North America Departments of Anthropology and Geosciences, Arizona, January 23, 2008.

Isabel Israde-Alcántara, James L. Bischoff, Gabriela Domínguez-Vázquez, Hong-Chun Li, Paul S. DeCarli, Ted E. Bunch, James H. Wittke, James C. Weaver, Richard B. Firestone, Allen West, James P. Kennett, Chris Mercer, Sujing Xie, Eric K. Richman, Charles R. Kinzie, and Wendy S. Wolbach, Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis, University of Hawaii, Honolulu, HI, January 31, 2012

P. A. Suttak, J. I. Boyce and D. Hrvoic, GEOPHYSICAL MAPPING AND MODELLING OF THE CHARITY SHOAL STRUCTURE, LAKE ONTARIO, CANADA. Large Meteorite Impacts and Planetary Evolution V (2013)

Virden, W.T., Warren, J.S., Holcombe, T.L., Reid, D. F., and Berggren, T.L. 1999. Bathymetry of Lake Ontario. National Geophysical Data Center, World Data Center A for Marine Geology & Geophysics, Boulder CO. Rept. MGG–14.

Younger Dryas Extinction Impact Related? (Bloody Creek @ 29:30).