MAPLE CREEK (AKA – WHITE VALLEY) IMPACT STRUCTURE
by: Charles O’Dale
- Type: Central peak
- Age Ma: <75 a b c – CRETACEOUS
- Diameter: 6 km
- Location: N 49° 48’ W 109° 6’
- Shock Metamorphism: PDF in quartz
a The youngest units involve in the Maple Creek structure are the lowermost Eastend Formation. They constrain the age to only <75 Ma (Grieve 2006).
b pre-1977 K-Ar, Ar-Ar and Rb-Sr ages recalculated using the decay constants of Steiger and Jager (1977) Ages in millions of years (Ma) before present.
c The structure disrupts mainly Late Cretaceous rocks giving an age of impact of less than 75Ma (Westbroek et al 1995)
The Maple Creek structure (109°20’W; 49°51’N) is barely visible at the surface, where repeated sections of the Upper Cretaceous Bearpaw Formation are very poorly exposed. A structural anomaly was noticed by Whitaker  and subsequent Vibroseis seismic profiles indicated structural disturbance to ~ 1 km depth and a slightly elliptical zone ~ 6 km in diameter with an annular trough and a central core ~ 2 km in diameter. Drilling indicates structural complexity in the core, with brecciation and repetition of beds. Interpretation of seismic data suggests a structural uplift of ~ 500 m. A number of possible origins had been suggested, including impact, but all were considered equivocal. Although the bulk of the lithologies in the central uplift are shales, there are a few sandy lenses in the Upper Cretaceous Lea Park Formation with rare quartz displaying decorated PDFs (Grieve 1998)
The Maple Creek (also known as “White Valley”) structural anomaly is located approximately 23 km east-southeast of the town of Maple Creek. Whitaker (1976) appears to have been the first to identify structural disturbance in this area. He noted that sediments of the Upper Cretaceous Bearpaw Formation, outcropping in this area, are deformed and highly slickensided (Gent 1992).
The White Valley (Maple Creek) structure is most likely explained as a complex meteorite impact structure. It is some 7.5 km in diameter, with a nearly 2 km central uplift and is characterized by a terraced rim, a downdropped trough, and an uplifted center. The structure extends to a depth of 1300 m but the trough of the crater itself is only 100 to 220 m deep based on seismic data and scaling relationships. Structural uplift is estimated at 620 m, in good agreement with the scaling results. the gravity anomaly over the structure is consistent with the complex impact crater model. The structure is between about 55 and 60 Ma old (Westbroek, 1997).
The slickenside identification MIGHT have been mistaken for a shatter cone (author).
In 1989, 2D reflection seismic lines across the possible complex impact crater was brought to the attention of the Geological Survey of Canada. A more detailed examination of the silty units in available cores from the structure produced evidence of shock metamorphism in the form of PDFs in rare quartz grains and confirmed its impact origin (Grieve 2006). There is no evidence in previous geological work done in the area that suggests any volcanic activity (Kent, 1968).
The structure has many of the morphological characteristics of a complex impact crater. It has an outer raised rim, annular synform and a raised central uplift. The rim of the structure shows many normal faults indicative of extension during the uplift phase of crater formation. The maximum depth of the structure is estimated at 1300m. The structure disrupts mainly Late Cretaceous rocks giving an age of impact of less than 75Ma. The seismic shows the feature to be about 6km wide with a 4km diameter inner trough and a 3km circular uplift.
From the seismic data, some general morphological dimensions can be approximated. The outer raised rim measures about 6km based on the topographic expression of the structure. The ring trough measures about 4km in diameter and the central uplift is about 3km wide. The maximum depth of the chaotic portion of the structure appears to be about 1.3km as this is the depth to the Mississippian. However, it is thought that the transient crater was much shallower than this, perhaps only 600m deep. The effect of the rebounding process, though, may have been to pull up underlying strata to the depth of disruption now seen in the seismic data.
The structure is fairly asymmetric on both lines. This may be because the impact did not result in a pure explosion as is believed to occur in the case of complex cratering but instead there was a substantial amount of momentum transfer to the target rocks as well as explosive energy. The force of the impact was not equal in all directions resulting in asymmetric deformation of the target rocks. This structure may also represent a more transitional form of crater between the purely simple and complex forms. Heterogeneities within the target rocks might also have contributed to this asymmetry as they would have reacted differently to the compressional and extensional forces acting on them during crater formation.
Apparently, after the transient crater formed, the bottom began to rebound which in effect drew in material from below and the sides of the transient crater. This not only resulted in the normal faults along the rim but also caused structural pull-up along the lower horizons of about 42.8m. Additional pull-up, perhaps achieved by an increase in velocity, is more difficult to explain as one would expect brecciation of the target rocks to decrease the average velocity. The well data is currently inconclusive on this point.
However, shock metamorphic effects and the rotation of higher velocity rock units into the region of uplift may contribute to the overall average velocity increase in the central uplift region (Westbroek et al 1995).
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Gent, M. R., Kreis, L. K. and D. Gendzwill., The Maple Creek structure, southwestern Saskatchewan. Summary of Investigations 1992, Saskatchewan Geological Survey, Rep. 92-4, p 204-208. 1992.
Grieve R.A.F., Impact structures in Canada, Geological Association of Canada, 2006.
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.
Kent, D.M., The geology of the Upper Devonian Saskatchewan Group and equivalent rocks in western Saskatchewan and adjacent areas: Prov. of Sask. Dept. of Min. Res., Report No. 99, 24 maps, 224p, 1968
Hans-Henrik Westbroek and Robert R. Stewart Seismic interpretation of the White Valley structure: A possible meteorite impact crater CREWES Research Report – Volume 7 (1995)
Hans-Henrik Westbroek Seismic interpretation of two possible meteorite impact craters: White Valley, Saskatchewan and Purple Springs, Alberta. University of Calgary, Department of Geology and Geophysics 1997.
Whitaker, S.H.: Geology and groundwater resources of the Cypress area (72F) Saskatchewan Research Counsel, Geological Division, 1976.