Along-Strike Variation in Displacement and Structural Style Across Mechanical Stratigraphy: An Example from Northwest Alberta, Canada

Gregory S. Soule and Glenn Larson
Devon Canada Corporation, Calgary, AB

Three-dimensional seismic data over the Narraway structure in NW Alberta are used to evaluate the change in the dip displacement in the strike direction, or the displacement gradient, of a single thrust structure. The Narraway structure is a relatively simple thrust sheet at the leading edge of foothills deformation with a basal detachment in the Paleozoic carbonate strata. The high quality, large area three-dimensional seismic survey combined with numerous seismic marker horizons and good well control make this a unique opportunity to observe the displacement gradient. The displacement gradient is the difference in dip displacement divided by the strike distance between section measurements. This gradient results in a “bow and arrow” geometry, and is a useful concept introduced by Elliot (1976) who suggested dip displacement changed by 7% of the strike distance. This study found the value to be approximately 2%.

The three-dimensional seismic data are used to determine the shortening in the dip direction of the thrust sheet and the related folds along strike in a series of dip sections. The displacement gradient is then calculated from the central part of the structure to the tip line of the Paleozoic thrust offset where the basal detachment ramps laterally up to a position in the shallower Cretaceous strata. The less competent Cretaceous strata are folded into tight box folds disharmonically above the Paleozoic. The displacement gradient is also calculated for the Cretaceous section and compared across the lateral ramp for any departure from the gradient established for the Paleozoic and the Cretaceous strata.

Elliott, D., 1976, The energy balance and deformation mechanisms of thrust sheets: Royal Society of London Philosophical Transactions, ser. A, v. 283, p. 289-312.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005