PURPLE SPRINGS STRUCTURE

PURPLE SPRINGS STRUCTURE

by: Charles O’Dale

  • Type: Central peak
  • Age Ma: 360 – 325 (Geological dating)a
  • Diameter: 4 km
  • Location: N 49° 52’ 30″ W 111° 52’ 00″

a The Purple Springs structure occurred primarily at Mississippian time (roughly 360 to 325 million years ago).

The Purple Springs Structure is indicated by the red circle. (courtesy Google)
The Purple Springs structure from the north from approximately 4000 feet above ground level. “Ground zero” is image centre.

The Purple Springs structure is located in southcentral Alberta. Seismic lines acquired during the course of oil and gas exploration passed completely over the structure and gave initial evidence that the feature was somewhat circular and fairly extensive in area and depth. The structure is a slightly elliptical depression in Mississippian and Devonian carbonates. The seismic data showed the approximately bowl-shaped with an elliptical expression structure to dominate the carbonates of the Mississippian and Devonian with less structure occuring in the clastic portion of the section. All the deeper horizons show structural deformation near the rim of the structure in the form of normal faulting. These faults dip into the center of the structure and delineate terraces along the rim which step down into the structure.

The meteorite impact theory of the Purple Springs structure is based simply on the lack of evidence for an endogenic process in the area that could create such a large structure. The initial interpretation of a meteorite impact origin hinged largely on a stratigraphic cross section made over the feature which suggested a simple crater morphology at the Mississippian level. The Purple Springs structure is similar to a simple meteorite impact crater. It is bowl shaped with a chaotic core and an elliptical plan – all characteristics of an impact structure.

There are key differences in morphology between this structure and typical impact craters:

  • simple craters are not characterized by normal faulting and terracing along the rim;
  • simple crater typical depth to diameter ratios = 1/5 ;
  • Purple Springs structure ratio = 1/20 ;
  • Purple Springs is much shallower than typical simple craters;
  • simple crater depth-to-breccia lens thickness ~2 ;
  • Purple Springs’ ratio ~1/3

The morphological characteristics of the Purple Springs structure is more likely associated with complex craters having a central peak. The data at Purple Strings give no indication that such a structurally uplifted central peak is present. This might be explained that a fairly oblique impact caused the elongated shape of the structure (and the other anomalies). Another possible explanation is that the carbonate rocks into which the impact occurred, behave uniquely enough during crater formation as to be poorly represented by the scaling criteria. The Purple Springs structure may therefore be closer to the transitional diameter representative of a structure at the onset of complex crater development, having properties of both morphological end members. (Westbroek 1997)

There is a very complex sub-surface structural feature roughly 3.4 km in diameter located in the north-west portion of 10-14W4 that has been interpreted to be of impact origin, although other possible origins have been suggested. The impact would have occurred sometime after the deposition of the Mississippian Livingston formation and prior to deposition of the Jurassic Sawtooth formation. The well 100/04-32-010-14W4 sits almost exactly in the center of this feature.

This (2014) current work has access to an additional seismic line that does pass through the apparent centre of the feature and this data do show the presence of a central uplift under the center of the feature. The additional seismic line shows the presence of a central uplift under the center of the feature and this appears to be sufficient evidence to conclude that the Purple Spring feature is indeed of impact origin. (Allen 2014)

References

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

Kenneth V Allen, Purple Springs, Alberta (10-14W4): Paleozoic Impact Crater GeoConvention 2014: FOCUS

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.