HOLLEFORD IMPACT STRUCTURE*
by: Charles O’Dale
* The term structure is used to define an impact crater that is severely altered by erosion.
- Type: Simple
- AGE(ma): 550 ±100 a
- Diameter: 2.35 km
- Location: Ontario, Canada N 44° 28′ W 76° 38′
- Shock Metamorphism: Coesite, PDF in quartz grains (Robertson, Bunch, 1963)
- Country Rock: Biotite hornblende gneiss. The rock is dark-coloured, foliated, unbrecciated, and with interbedded mafics and salics. Biotite, hornblende quartz, and plagioclase have been identified in it (Dawson, 1961).
- Overlying Palaeozoic rocks: mainly well bedded, fine- to very fine-grained, buff to grey limestones and siltstones (Dawson, 1961).
a Dating Method: Geological – estimated to be in the latest Proterozoic or earliest Paleozoic times (Grieve 2006).
VIDEO: OVERFLIGHT OF THE HOLLEFORD IMPACT STRUCTURE in my Cessna C177B – C-GOZM (GOZooM)
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. Impact confirmation of the Holleford structure dates only to the mid-1950’s and is not surprising as the depth of the structure is only ~ 30 m (Beals 1960).
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).
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.
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.
DRILL CORES from the Holleford crater(Dawson, 1961)
|Lithic Unit||Dominant rock type||H-1||H-2||H-3|
|7||Calcareous and dolomitic carbonates (H-3 has 1 ft basal conglomeraete,)||0-220 (220)||0-158 (158)||0-65 (65)|
|6||Green shale and polymictic conglomerate.||220-245 (25)||158-183 (25)||absent|
|5||White calcareous quartz arenite||245-470 (225)||183-366 (183)||absent|
|4||Gray calcareous quartz arenite||470-645 (175)||366-415 (49)||absent|
|3||Calcareous shales and argillaceous limestones and sandstones||645-745 (100)||absent||absent|
|2||Polymictic sedimentary breccia||745-750 (5)||415-440 (25)||absent|
|1||Polymictic breccia||750-?||440-600 (160)||absent|
The breccia is polymict with a variety of rock fragments including lime silicate rocks, micaceous gneiss, and amphibolite contained in a fine-grained matrix. No fragments of the Palaeozoic sedimentary rocks are present and the rock fragments seem to be randomly distributed. The angular fragments vary in size up to a few inches. The matrix, which is dark green, is medium-grained and poorly sorted. Its weak cement allows some samples to be crushed with the fingers. It contains identifiable grains of the various rock varieties as well as quartz, calcite, and earthy white grains. One specimen of the 24 examined showed evidence of crude bedding.
Examination of the breccia in thin sections confirmed the identity of the rock species present as fragments and as bedrock from the surrounding terrain. Plagioclase, potash feldspar, calcite, quartz, biotite, clinopyroxene are usually present in the matrix. The coarse mineral grains
show no evidence of rounding or frosting but are either cleavage or angular fragments. The clinopyroxene varies from colourless to white cleavage fragments with hackly ends. No evidence of secondary growth has been observed on any of the minerals identified. For example neither
the quartz nor the plagioclase fragments exhibit peripheral zones. Hornblende, chlorite, sphene, and hematite are present as accessory constituents
Summary of lithic unit relations (Drill Hole and unit thicknesses in feet.)
1. The Holleford crater was formed in latest Proterozoic or earliest Paleozoic times in a continental, warm-temperate environment.
2. The crater filled with low salinity water and the lake so formed became meromictic. Loose talus from the crater rim covered much of the crater walls.
3. Early in the lake’s history, much of the sediment in the deep basin was deposited by density currents.
4. Later sedimentation in the lake was cyclic and reflected the annual variations in the climate of the area.
5. The monimolimnion was destroyed when the crater was about one-half full of sediment and the lake became holomictic.
6. Later Mid-Ordovician marine sedimentation covered the filled crater with shales and limestones. Compaction induced in the breccia under the crater and in the lacustrine arenites by the overlying load formed a shallow structural basin in the carbonate sequence over the buried crater. (Brian E. StJohn 1968)
Drilling Results quoted from: Geological Survey of Canada, 1945
The first hole was located at a distance of 1400 feet from the estimated centre of the crater. Serious difficulties were encountered including a gas blowout that caught fire and did much damage to the drilling equipment. The hole reached 1128 feet, at which point the rods became stuck and the hole was abandoned. The blowout caused the well on the property went dry for several days; eventually some water returned but was not sufficient for the needs of the farm. A new well was dug 700 feet away from the old one for the farm use. This hole penetrated a sequence of Paleozoic sediments to a depth of 750 feet and then found the expected breccia.The second hole was at a distance of 2500 feet from the centre of the crater to a depth of 1486 feet. This hole passed through a thinner layer of the sediments, 440 feet, and through 160 feet of breccia. Below this, numerous cracks were suggested by fluid loss in the drilling and by core examination. The fragmented material which made up the breccia was all precambrian rock of the type found in the immediate vicinity.The third hole was 3750 feet from the crater centre, on the rim, to a depth of 443 feet. It contacted the thin layer of the sedimentary cover and a thin layer of the breccia.A large amount of core was recovered from all three holes. The contour of the crater floor defined by the drilling, comes close to the mean position of the New Quebec (Pingualuit) and Brent craters. This, and the presence of the breccia and underlying fractured rocks, are strong evidence for an impact origin. No meteorite material was found.
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).
Coesite and Shocked Quartz from Holleford Crater, Ontario, Canada.
The Holleford Crater, a circular depression in southern Ontario, is filled with Paleozoic sediments and underlain by brecciated Precambrian igneous and metamorphic rocks. The presence of coesite in two core samples of this breccia has been established by petrographic and x-ray diflraction methods. Shattered quartz in the coesite-bearing samples exhibits planar fractures. The shocked quartz is the result of great shock pressures and the association of coesite with the shocked quartz strongly suggests that Holleford Crater originated from a hypervelocity impact. (Science. 1963 Oct)
The origin of the Holleford Crater brecciaK. R. Dawson
This paper reports the results of a petrographic and geochemical study undertaken to evaluate the possibility that the polymict breccia discovered in drill core from Holleford, Ontario, is the product of meteorite impact. The breccia lies in a bowl-shaped depression on the Proterozoic (Grenville) land surface and is overlain by Paleozoic rocks. The coarse fragmental constituents have been derived from the Proterozoic rather than the Paleozoic rocks. The arenaceous fraction of the breccia exhibits a very limited variety of mineral constituents, wholly of local derivation, and grain forms which indicate little or no transportation. Clay minerals from the matrix of the breccia are typical of normal sediments. No meteoritic materials have been identified microscopically, and the Ni content of the breccia is only slightly above average for sedimentary rocks. The bowl-shaped form of the breccia, as indicated by diamond drilling and geophysical data reported by Beals, is the principal criterion favoring genesis by meteorite impact. Other data neither refute nor confirm this hypothesis. (1961 Mineralogical Association of Canada)
The Holleford meteorite crater is about 20 miles northwest of Kingston, Ontario. It forms a circular structural depression 1.46 miles in diameter and about 950 ft deep. It is iilled by sedimentary rocks of Late Proterozoic and Early Paleozoic age with a maximum thickness of 850 ft. The rocks of this section and the polymictic breccia underlying them have been divided into seven lithic units. The bulk of the sequence consists of microlaminated calcareous quartz arenites with lesser amounts of argillaceous and carbonaceous material. These arenites are unlike any rocks found locally outside the crater. They are overlain by carbonates and shales of the Black River Group with apparent unconformity. The origin postulated for the arenite sequence in the crater is a Late Proterozoic or Early Paleozoic fresh-water lake. This lake existed in a subtropical to temperate climate and only rarely froze over. It is probable that it was meromictic for part of its existence, but later became holomictic. Later compaction of the underlying breccias and the paleolacustrine arenites formed a shallow structural basin in the Black River carbonates. (Ontario Department of Geology, Queen’s University, Kingston, Ontario Received January 26, 1968 Accepted for publication April 17, 1968)
Aerial Exploration of the Holleford Impact Structure
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 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.
Ground Exploration of the Holleford Impact Structure
The Holleford Impact Structure is a short afternoon drive from Ottawa/Kingston.To find it, follow highway 38 to Hartington.
These images illustrate the amount of erosion and sedimentary deposition that has occurred in the 500+ million years since the impact.
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.This is a diagram of the sedimentary rock over the south rim of the Holleford impact crater. Note the tension (tensile) fractures over the southern rim anticline of the Holleford crater. Development of extensional fractures and normal faults on the crest and the upper axial zone of an anticline.
Beals, C. S., A probable meteorite crater of Precambrian age at Holleford, Ontario. Ottawa Dominion Observatory Publications, v. 24, pp. 117-142. 1960.
Beals, C. S., Ferguson, G. M., & Landau, A., [Scientists Report II.] A Search for Analogies Between Lunar and Terrestrial Topography on Photographs of the Canadian Shield, Part II,Journal of the Royal Astronomical Society of Canada, Vol. 50, p.257-258
Bunch TE, Cohen AJ., Coesite and Shocked Quartz from Holleford Crater, Ontario, Canada. Science. 1963 Oct