ELBOW IMPACT STRUCTURE

ELBOW IMPACT STRUCTURE

by: Charles O’Dale

  • Type: Complex
  • Age ma: 395a ± 25
  • Diameter: 8 km
  • Location: N 49° 51’ W 106° 45’
  • Firmly identified as an impact structure with the discovery of PDFs in quartz (Grieve et al 1998).

a Dating Method: Devonian strata are disturbed and brecciated with unaffected Jurassic strata over the structure (Grieve 2006).

Saskatchewan, Canada – Gow Lake and Deep Bay craters,Carswell, Dumas, Elbow, Maple Creek and Viewfield impact structures. Yellow = ATHABASCA FORMATION, Pink = PRECAMBRIAN BASEMENT COMPLEX, Grey = INTERIOR PLAINS. Courtesy Breaking Science News at Sci-News.com Science Website.
Elbow structure – Google image. The superimposed circle illustrates the position of the 8 km diameter buried crater.
Ground zero of the Elbow impact structure is imaged under the wing of GO ZooM. The superimposed circle illustrates the position of the 8 km diameter buried crater. Lake Diefenbaker is in the background.
Elbow structure – Saskatchewan. The nature of the structural disturbance, a central structurally uplifted area surrounded by a down-faulted annular trough, with no disturbance of “deep” horizons describe the characteristic morphology of a complex impact structure (Grieve 2006).

The Elbow structure (106°45’W; 50°58’N) is 8 km in diameter, circular in plan, with a ring depression and an uplifted core of fractured and brecciated target rocks [1]. The structural form is that of a complex impact structure. It has been drilled for hydrocarbons although there is no current production. Originally, the Elbow structure was thought to results from salt tectonics or cryptovolcanism. Sampling of drill hole material indicates the presence of quartz grains with decorated planar deformation features (PDFs). (Grieve 1998)

Planar deformation features in quartz. This sample is from the Manicouagan impact crater.

Possible link to the Late Devonian Extinction

The red dot represents the approximate area of the Elbow impact 395 million years ago in the Devonian Period.

The Elbow impact structure in south-central Saskatchewan, is just north of the village of Riverhurst and south of the town of Elbow (north of Diefenbaker lake). It is 8 kilometers (5.0 mi) in diameter and the impact occurred between 370 to 420 million years ago during the Devonian Period.

The position of the Elbow impact structure is illustrated by the superimposed circle in the background. The Gardiner Dam, under the wing of GO ZooM, is the largest embankment dam in Canada and one of the largest embankment dams in the world. Lake Diefenbaker is upstream. The dam rises 64 metres (209 feet) in height, is almost 5 km (3.1 mi) long and has a width of 1.5 km (0.93 mi) at its base with a volume of 65,000,000 cubic meters.

The Elbow structure was discovered during a search for oil. It consists of a symmetrical dome in Mesozoic rocks beneath which reposes a cylindrical mass of broken Paleozoic rock. There is evidence of some broken rock having been displaced stratigraphically upward by violent movement, possibly explosive in nature. The structure is marked by a positive gravity anomaly which reflects the upward displacement of the disturbed mass and the dome formed in Mesozoic strata.

The structure apparently experienced two periods of activity. The first was post-Mississippian in age and explosive in nature; the other, probably Tertiary in age and non-violent, resulted in the uplift of the Mesozoic and Tertiary strata to form a dome. The nature of the second movement is unknown but it is believed to be related to the Laramide orogeny (DeMille 1960).

The Elbow complex structure is centered at 106°45’ W and 49°51’N in west Saskatchewan. The rim-to-rim diameter is approximately 8 km with a central uplift surrounded by an annular depression on the seismic profiles [Grieve et al., 1998; Sawatzky, 1977]. These structural anomalies were first noticed by DeMille [1960] from the seismic images, and the impact origin was not emphasized until 1998 when the PDFs were detected in the well cuttings. However, only limited materials are currently available to discuss the impact origin of the structure. (Wei Xie, 2014 ).


References

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

Demille, G., The Elbow structure of south-central Saskatchewan. Journal of the Alberta Society of Petroleum Geologists, v 8, n 5, p 154-162. 1960.

Grieve, R. A. F., Kreis, K., Therriault,A.M.and P.B.Robertson., Impact structures in the Williston Basin. Meteoritics and Planetary Science, v 33, n 4, p A63-A64. 1998.

Grieve R.A.F., Impact structures in Canada, Geological Association of Canada, 2006.

Sawatzky, H. (1976), Two Probable Late Cretaceous Astroblemes in Western Canada-Eagle Butte, Alberta and Dumas, Saskatchewan, Geophysics, 41(6), 1261-1271.

Sawatzky, H. (1977), Buried impact craters in the Williston Basin and adjacent area, paper presented at Impact and Explosion Cratering: Planetary and Terrestrial Implications.

Xie W. Seismic Characterization of A Possible Buried Impact Structure near Bow City in Southern Alberta Department of Physics University of Alberta 2014

Earth Impact Database