Seismic interpretation of two possible meteorite impact craters: White Valley, Saskatchewan and Purple Springs, Alberta

Hans-Henrik Westbroek

ABSTRACT

High-velocity impact events have played a major role in the formation of the Solar System, including the Earth. The terrestrial record of impact structures shows over 150 examples, most of which are on the surface. This work uses the seismic reflection method to analyse sub-surface structures as possible meteorite impacts. I investigate two structures of possible meteorite impact origin: White Valley, Saskatchewan, and Purple Springs, Alberta. The impact hypothesis is evaluated by comparing structure geometry as seen on seismic reflection images with models (i.e., scaling criteria) derived from other known structures, laboratory experiments and numerical modelling.

The White Valley 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.

The Purple Springs structure is also reasonably well described by scaling criteria for a complex impact crater. It is elliptical in nature, about 4 km long and 3 km wide. The structure is apparent in some 780 m of rock extending from the Mississippian to the Devonian Elk Point Group and is about 320 Ma old. The Mississippian shows about 180 m of downdrop into the structure in excellent agreement with scaling criteria. The rim of the structure is characterized by listric normal rim faults delineating terraces which drop to a relatively flat central plain. The lack of a prominant central uplift suggests post-impact processes may have substantially altered the structure to its current form.

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