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A discussion of Folds in the Southern Canadian Rockies
Speaker: Paul MacKay | GeoConsultants Ltd. & Shale Petroleum Ltd.
Date: March 9, 2017
Location: Schlumberger, Second Floor of the Palliser One Building, 125 9th Ave. Calgary T2G 0P6
The Southern Canadian Rockies are one of the best studied thrust belts in the world due to their easy accessibility, good general access to data, excellent exposure, and a long history of observations. Folds in the Rockies have also been described but are generally discounted in importance to the regional structural deformation due to the fact that the amount of strain represented in the folding is minor compared to the exceedingly large displacement strain on the thrust faults. However, the folding gives great insight into the kinematics of the system. Close observations of the fold bring up issues with palinspastic restorations, timing of thrusts, thrust linkage, and accepted models of fold propagation.
The classic folds observed at Mt Kidd and other areas are related to thrust failure. These folds can be considered as the creep prior to reaching the yield point and catastrophic failure in mechanics. These types of folds form prior to fault failure. In general the failure does not migrate from depth to surface but is a series of minor tears that coalesce into one continuous failure surface. Thus, the concept of fault propagation fold model representing a fault migrating to surface is likely kinematically problematic although it will fit some geometry scenarios. Reliance on seismic data to map the broad structure is useful but seismic is unable to image the vertical surfaces that are characteristic of these folds so that the folds are often over looked when relying solely on subsurface data.
Folds associated with faulting have a remarkably consistent fold style. There is a tight hanging wall anticline (interlimb angle less than 90 degrees), which has an over-thickened hinge and a thin forelimb. There is also a corresponding tight footwall syncline with and over-thickened hinge and a thinned backlimb. The cut-offs angles are high averaging approximately 90 degrees. The fundamental anisotropy of the bedding contributes substantially to the fold style and is likely one of the controlling influences on the fold. This style of fold is observed at different scales from hand samples to the sides of mountains. These folds appear to be fixed hinge model, which will have implication to fracture distribution and style.
Paul MacKay received a B.Sc.(honours, geological sciences) from Queen’s University in 1980 and a Ph.D. from the University of Calgary (1991). He initially worked for Amoco Canada then moved to Morrison Petroleums, Northstar Energy, and Devon Canada before beginning an international consulting practice. He is currently President of Shale Petroleum Ltd. a private oil company based in Calgary, Alberta. His expertise is in fracture systems, petroleum exploration and development in structurally complex reservoirs. He teaches field courses in Structural Geology/Geophysics in the Canadian Rockies and field seminars on Fractured Reservoirs in Wyoming. He is an Adjunct Professor in the Department of Geology and Geophysics at the University of Calgary and is the Past-President of the Canadian Society of Petroleum Geologists. He is a member or the CSPG, CSEG, AAPG, and APEGA.