IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
HOLLEFORD IMPACT STRUCTURE*
The Holleford structure is indicated in this Landsat image by the SMALL circle (inside the larger circle).
The red dot represents the approximate area of the Holleford impact 550 million years ago in the Cambrian Period.
- AGE(ma): 550 ±100
- Diameter: 2.35 km
- Location: Ontario, Canada N 44° 28' W 76° 38'
- Shock Metamorphism: PDF in quartz grains (Robertson, Bunch, 1968)
- Dating Method: Estimated in the latest Proterozoic or earliest Paleozoic times (see text).
* The term structure is used to define an impact crater that is severely altered by erosion.
This image taken North of the Holleford structure looking roughly south from approximately 1500 feet above ground documents how difficult it is to identify the Structure (located in the centre of the image). Kingston is visible to the south east (the upper left corner of the picture).
The Holleford Impact Structure is located 27 kilometres north of Kingston and 132 kilometres south-west of Ottawa in southern Ontario. Confirmation of the structure as an impact site dates only to the mid-1950's. Several farms had been maintained at the site and the in the recent past aircraft have been flying overhead without anyone noticing that this may be an “unusual” structure. This is not surprising as the depth of the structure is only ~ 30 m (Beals 1960).
(Airphoto A1161-43) Holleford is identified as a “structure of interest”.
A systematic study of 200,000 aerial photographs of areas of the Canadian Shield was commenced in the early 1950’s when the Pingualuit Crater (formerly Chubb and then New Quebec) and the Brent Crater were confirmed as impact structures. As a result, Holleford was identified as a “structure of interest”. Over the years since that search, scientists have pieced together much of the Holleford Structure’s geological history (UNB 2003)
Geologic Survey of Canada Research at Holleford
The research program (Beals 1960) that commenced as a result of the aerial discovery included a study of the surface geology by the Geologic Survey of Canada, and a series of four geophysical studies, namely:
- magnetic observations (1);
- seismic studies (2);
- gravity studies (3), and;
- three diamond drill holes (4).
|1. The magnetic studies performed by the Geological Survey of Canada document that there is a minimum magnetic disturbance within the impact structure, here outlined by the circle on the aeromagnetic map (Grieve 2006).
The Holleford structure's magnetic signature
2. The seismic studies, while not definitive, were nevertheless consistent with the presence of a considerable thickness of low-velocity material under the impact structure of the kind associated with a meteorite impact, namely, the broken and shattered layers designated as breccia.
|3. The Bouguer gravity anomaly map produced by the Gravity Division, indicate that the gravity magnitude contours were circular and follow in a general way the outline of the impact structure depression. The central gravity low indicates an anomaly of approximately two milligals. This suggests a circular depression filled to an estimated depth of ~250 metres with material which is on the average of a lower density than the surrounding Precambrian rocks.
The Holleford structure's Bouguer gravity signature.
|4. The three diamond drill holes located 430, 760 and 1140 metres from the impactor contact point revealed an impact structure profile and underlying strata. The rocks encountered in the drill cores include, from top to bottom, grey limestone and limey sandstone, polymict breccia, crystalline limestone, biotite and biotite-pyroxene gneiss, lime silicate rocks and rare amphibolite. The explosive impact of the meteorite was evident in the hundreds of feet of shattered rock detected here at the depths predicted for a meteorite impact site.
Diamond drill holes to document the profile of the Holleford structure.
The studies concluded that the Holleford Meteorite Structure was formed by a body of approximately 100 metres in diameter impacting at a velocity of ~20 km/sec. The resulting explosion had a kinetic energy six times that of the object that formed the Barringer Crater and it blasted a crater 244 metres deep out of the Precambrian, crystalline and metasedimentary late Precambrian or early Cambrian target rock. After the impact, the depression filled with water to become a circular lake. The Palaeozoic seas gradually filled the crater with >30 metres of sediments. Glaciation has removed the rim crest but the crater-fill products are preserved (Beals and Hitchen, 1970). The deposition time of the undisturbed, Potsdam sedimentary (filling complex) has been used to establish the approximate age of the structure (Beals 1960).
|The <30 metres depth of the “bowl” shape of the impact structure becomes evident while flying by the structure at lower altitudes. Over a half-billion years of sediment covers the original crater. I have to admit that the first time I flew to this impact structure I had a difficult time in finding and identifying the feature.
The Holleford structure viewed from the North West.
This image from the south looking north was taken in early spring when the colour contrast of the impact structure is most obvious.
The dark floor of the impact structure is mostly wooded bog. A uniformly flat area at the north end of the impact structure represents the drainage pattern out of the impact structure. It extends outward beyond the structure circumference to the vicinity of nearby Knowlton Lake.
With the magic of the new digital cameras, a fellow RASC member, Joseph Morgan, took this image on one of his flights. (The white triangle on the lower left of the image is the "low" wing of the photo aircraft).
Ground Exploration of the Holleford Impact Structure
The Holleford Impact Structure is a short afternoon drive from Ottawa. To find it, follow highway 38 north from Kingston through Harrowsmith and into Hartington. Turn right/east at the store/gas station. Immediately on the right is a fire station. You are now on the correct road. Follow the road east for a kilometre or two and it will take a bend to the north. Continue for another kilometre or so and it comes to a T branch. Turn right/east and continue up the hill.
You should be roughly where this image was taken, on the west rim of the impact structure looking east into the structure. Note the smooth slope of the internal western impact structure wall descending in front of us.
These images illustrate the amount of erosion and sedimentary deposition that has occurred in the 500+ million years since the impact.
Continue east along the road into the impact structure to the first large barn with a windmill on the north side and you should see a historical society sign inside the fence line. This is ground zero of the meteorite impact that originally formed this crater. The northern rim of the impact structure is visible in the background.
THE HOLLEFORD CRATER - A meteorite travelling 55,000 kilometres per hour smashed into the earth here eons ago, blasting a hole 244 metres deep and 25 kilometres wide. Aerial photogrphs revealed the crater in 1955, and since then scientists have pieced together much of its geological history. Analysis of drill samples suggest that the meteorite struck in the late Precambrian or early Cambrian period (between 450 and 650 million years ago). At first the depression filled with water, becoming a circular lake. Later, Palaeozoic seas swept in sediments, filling the crater to its present depth of about 30 metres. The explosive impact of the meteorite (estimated to have been only 90 metres in diameter) is still evident in the hundreds of feet of shattered rock that drilling has detected beneath the original crater floor.
This is a wide angle image from within the impact structure looking west from the eastern part of the structure.
These ground images illustrate what remains of the Holleford Impact Structure and its rim. There is a section of the eastern paved road that had a “rock cut” of sedimentary rock positioned within the structure. This sedimentary rock was deposited millions of years AFTER the impact that formed the original crater.
Beals, C. S., A probable meteorite crater of Precambrian age at Holleford, Ontario. Ottawa Dominion Observatory Publications, v. 24, pp. 117-142. 1960.
Grieve R.A.F., Impact structures in Canada, Geological Association of Canada, 2006.
UNB, Aerial Radar Courtesy of the Planetary and Space Science Centre at the Image courtesy of Earth Impact Database, UNB, 2003.
University of New Brunswick
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