WEST HAWK IMPACT CRATER
by: Charles O’Dale
- Type: Simple
- Age (ma): 100 (geological analysis)a
- Diameter: 2.44 km
- Location: Manitoba, Canada. N 49° 46′ W 95° 11′
- Shock Metamorphism: PDF in quartz grains, a large amount of glassy and melted fragments, isotropization of tectosilicates, and shock-induced twinning in target carbonate rocks grains.
a Dating Method: The presence of a breccia lens, which is usually eroded or not present in very old craters implies a Mesozoic age of 250 to 65 MA. (see the Presqu’ile Impact Structure for an illustration of a crater with the breccia lens fully eroded away). The impact crater is devoid of rocks whose presence would be detected if the crater existed 180 million years ago. The sediments that have accumulated in the crater do not contain fossil remains but resemble 60 million year old Manitoba sediments. No reliable age has been determined by isotopic age dating (Ogilvie et al., 1984). The best estimate for the West Hawk impact event is 100 million years ago.
General Area: Subdued relief near the southwestern edge of the Canadian Shield. The area is heavily forested and has been glaciated. The target rocks are crystalline.
Specific Features: Structure lies within the polygonal 3.6 km diameter West Hawk Lake. The shape of the lake contrasts with others in the immediate area. There is a partial ring of hills rising -40 m around the lake. The crater is clearly superimposed upon the structural fabric of the rocks in the area. A major fold can be seen northwest of the crater.
The West Hawk Lake Impact Crater, a simple crater, was excavated in metavolcanic and metasedimentary rocks of the Precambrian Superior Province. It lies within Whiteshell Provincial Park in eastern Manitoba, about 100 km east of Winnipeg.
Severe erosion of the crater’s original fractured rim has expanded the shoreline to a roughly circular lake with an average diameter of 3.6 km. The present erosion plane is approximately 66 m below the original surface leaving some inconspicuous hills around the circumference of the lake that may be the remains of part of the rim. Numerous cliffs showing fractured rock are found around the shoreline. The southern boundary of the lake is underlain by volcanic greenstones of Archean age while the northern area consists of Keewatin meta-clastics sediments and tuffs underlain by granite and gneiss. (Halliday and Griffin, 1963)
In the fall of 2001 by a team of researchers led by Drs. J. Teller and M. Boyd studied the unique characteristics of the sediments in the basin of West Hawk Lake.
Four cores were extracted from the deepest part of the lake and analyzed to relate changes in the sediments to:
Moisture content, bulk mineralogy, clay mineralogy, and particle size distributions were determined from regularly spaced samples in the sequence as well as from specifically targeted samples. Microscopic charcoal analysis was done at regularly spaced intervals to construct a regional fire record. The charcoal record indicates that the occurrence of fires in the region surrounding West Hawk Lake may be related to climate.
Boyd, M., Kling, H., McMillan, K. and Teller, J.T. 2003. “A varved paleoecological record from West Hawk Lake meteorite impact crater, southeastern Manitoba, Canada”. Abstracts, 2003 CANQUA meeting, Halifax, Nova Scotia, (CONCLUSION).
- The target rock type is Precambrian crystalline and metasedimentary;
- The impact crater is completely enclosed in the 3.6 km diameter 110 m deep West Hawk Lake;
- A 94 m thick accumulation of unconsolidated sediments resting on top of bedrock covers the crater (Boyd 2008);
- The bedrock below the lake and the unconsolidated sediments consist of a badly broken up and disturbed zone of 330 m thick fall back breccia lens. The impact crater was discovered when a drill core taken from West Hawk Lake was studied and found to possess impact breccias. Further examination of these breccias has revealed numerous signs of shock metamorphism. These include secondary minerals such as zeolite, zoisite, calcite, chlorite, montmorillonite and biotite. These minerals are common in impact melt and indicate that hydrothermal alteration was widespread after the impact event. Hot springs on earth resulting from hydrothermal vents are a source of exotic forms of life, and;
- Beneath the breccias is a 200 m thick zone of fractured bedrock of metamorphosed sedimentary and volcanic rocks. The broken bedrock is characterized by a number of mineralogic, crystallographic and textural changes that are produced by instantaneous application of high stress. These are called shock metamorphic features and are characteristic of known meteorite impact craters.
A magnetic study of the crater found a ground magnetic low of about 250 nT (Nanotelsa – total intensity magnetic anomaly). Two factors contribute to the magnetic low over this crater. (Grieve et al: Magnetic Properties of Three Impact Structures in Canada):
I took these early morning images of West Hawk Lake flying from north east to south west. I was on my way west to explore more meteorite craters and the mountains. Although the original 2.44 km diameter meteorite crater is enclosed within the 3.6 km diameter lake and not visible, the destruction that occurred during impact is revealed in the “bowl shape” of the lake and its environments. The lake is famous for its clarity to its 110 m depth. The environment is typical of the shallow vegetation on top of the Precambrian rock found in northern Ontario.
WEST HAWK LAKE, METEORITE IMPACT STRUCTURE West Hawk Lake is almost 4 km wide and nearly circular in shape. It was drilled in the 1960s by the Dominion Observatory and found to be approximately 100 m deep and contain approximately 100 m of sediment overlying the Precambrian basement. The circular shape and great depth of the lake, as well as the presence of shock-metamorphosed quartz and other features indicates the lake was formed by a meteor impact. The oldest sediments in the crater are of Cretaceous age indicating the impact occurred prior to that time, possibly in the Paleozoic. Because the lake is so deep compared to its diameter, it has been believed for many years that sediment deposited in the lake would be protected from erosion during glaciation. Glacier ice moving over the lake would shear over the top of the lake and not penetrate to the bottom. Previously deposited sediment would therefore be unaffected by glaciation. In addition, the great depth and the fact that there are no major rivers draining the lake means that sediments entering the lake would not be eroded or flushed through the lake. Consequently, the possibility exists that a complete Holocene, glacial and pre-glacial record spanning perhaps millions of years 29 might be preserved in the sediment in-fill at the bottom of the lake. Jim Teller from the Department of Geology, University of Manitoba, has undertaken coring of the upper 15 m of the sediment in-fill and is planning to core the remaining 75-85 m to elucidate the glacial history, the history of glacial Lake Agassiz and climate variability of the mid continent over possibly the last million years or more. (48th Annual Meeting Institute on Lake Superior Geology)
Boyd M., James T. Teller, Zhirong Yang, Matthew Boyd, William M. Buhay, Kyle McMillan, Hedy J. Kling, Alice M. Telka, Postglacial sedimentary record and history of West Hawk Lake crater, Manitoba, Springer Science+Business Media, 2008
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Halliday, I., Griffin, A. A., Evidence in support of a meteoritic origin for West Hawk Lake, Manitoba, Canada. Journal of Geophysical Research, v. 68, pp. 5297-5305. 1963.
Halliday, I., Griffin, A. A., Summary of drilling at the West Hawk Lake crater. Journal of the Royal Astronomical Society of Canada, v. 61, pp. 1-8. 1967.
Ogilvie, B. Y., Robertson, B. and Grieve,R.A.F., Meteorite impact features in Canada: An inventory and an evaluation. Unpublished Report, 180 p. 1984.