Three-dimensional seismic reflection investigation of the upper crustal Winagami sill complex of northwestern Alberta, Canada

Abstract

Using a 3-D seismic reflection data set recorded by the Canadian petroleum exploration industry to 5.1 s two-way traveltime (∼15 km depth) in northwestern Alberta, the Winagami reflection sequence (WRS), an interpreted sill complex previously identified on Lithoprobe 2-D multichannel seismic reflection lines, is investigated to determine its 3-D geometry and reflective characteristics. Clear evidence of the top of the WRS emerges from the data set. Data sections outline a 3-D reflective sheet dipping to the southeast. From polarity comparisons with reflections from the sedimentary sequence, the reflective signature of the deep body is inferred to result from higher impedance material than the surrounding host rocks, thus reinforcing earlier interpretations that the deep reflections result from doleritic sills intruded into gneissic crystalline basement. Simple 1-D forward modelling reveals that the thickness of the sheet is on the order of 100 m throughout the survey region. From 3-D Kirchhoff forward modelling, a model of the reflective sheet is constrained between 11 and 16 km depth along the northwestern edge of the survey area. The absence of the Winagami reflections, and basement reflectivity in general, in the southeast of the survey region coincides with a positive aeromagnetic anomaly inferred to be caused by magmatic rocks. The presence of the magmatic rocks may have either influenced the geometry of the intrusion of the sills, overprinted their reflective signature or been co-magmatic, dependent upon the relative timing of emplacement of the two features. The Winagami sill complex appears to have been intruded horizontally into the crust during a period of tectonic compression. The emplacement of such deep Precambrian sills represents an under-appreciated mode of magmatic addition into the crystalline crust. © 2006 The Authors Journal compilation © 2006 RAS.