VIEWFIELD IMPACT STRUCTURE
by: Charles O’Dale
- Type: Simple
- Age Ma: <190 ± 20 a – JURASSIC
- Diameter: 2.5 km
- Location: N 49° 35’ W 103° 4’
- Shock Metamorphism: PDF
a Based on Jurassic-Triasic lower Watrous red beds being post-impact deposit (Grieve 2006).
The Viewfield structure (103°04’W; 49°35’N) is considered to be a 2.4 km simple crater formed in Jura-Triassic time (~ 200 Ma), based on geophysical and drill hole data. Target rocks consist of Mississippian strata, of which at least 200 m are missing in the center of the structure and replaced by young Watrous Red Beds (WRB). In the rim, Mississippian carbonates are found sandwiched between Lower WRB. There are 50 producing hydrocarbon wells, with estimated primary reserves of 10.5 MMBO and 4.5 BCFG. An impact origin for the structure was suggested by Sawatzky . Detrital quartz grains in the WRB display decorated PDFs. The specific nature of these grains and their very limited occurrence in materials overlying and filling the crater suggest they were washed into the Lower WRB. (Grieve 1998)
Viewfield Impact Structure is located south of the town of Stoughton near the unincorporated area of Viewfield. It is located within the hydrocarbon-producing Williston Basin. It was discovered in 1972 during oilfield drilling activity. It is 2.4 km in diameter, and 100 m deep from the top of the rim to the bottom of the structure. The impact event occurred in the Early Jurassic period, about 190 million years ago. None of the features of the structure is visible on the surface, as the crater is buried under 1,000 m of overburden.
Recent examination of samples from cores of the Lower Watrous red beds has recorded the presence of PDF’s in quartz (Grieve et al 1998).
The Viewfield Structure forms a trap for hydrocarbons, and is an important source of oil. With over fifty active wells in its rim and a reserve of 27 million barrels, it accumulates 400 barrels per day. To date there are 23 oil wells completed in Mississippian strata along the periphery of the crater (Sawatzky 1972).
The rim facies isopach, reveals an irregular lobelike pattern radiating outward from the central cavity, with highest isopach values generally found midway between the inner and outer 0-foot contour intervals. It is interpreted to slump towards the central cavity, and in the 13-33 and 14-20 wells, lies between the same red beds filling the central depression near the 15-29 location.
Isopach – a line on a map connecting points below which a particular rock stratum has the same thickness.
Sawatzky (1972) proposed an impact origin for Viewfield and showed that the dimensions could be explained by employing empirically-derived explosion-crater equations which indicate that almost two-thirds of the original rim has been eroded. The calculated depth of the structure is similar to the profile. He also presented seismic isochron and well-control data which precluded any possibility of explaining the entire Viewfield anomaly by means of multi-stage solution and collapse of the underlying Middle Devonian salt horizon. Post-impact solution and collapse did, however, appear to have contributed to the present-day configuration.
Sawatzky H.B. Buried impact craters in the Williston Basin and adjacent area Lunar and Planetary Institute 1977
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Donofrio, R.R.: Impact Craters: Implications for Basement Hydrocarbon Production. Journal of Petroleum Geology, 1981.
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.B.: Viewfield – A Producing Fossil Crater. CSEG Journal V8 No1, Dec. 1972.
H. B. Sawatzky, Buried impact craters in the Williston Basin and adjacent area, Impact and explosion cratering – Lunar and Planetary Institute 1977