Deborah A. Spratt

Tectonic wedging beneath the Rocky Mountain foreland basin, Alberta, Canada

High-quality seismic reflection data reveal the geometry of a blind, thinly tapered wedge of allochthonous rocks inserted into autochthonous foreland basin strata for >8 km east of the previously recognized deformation front (triangle zone) of the Canadian Rocky Mountain foothills west of Calgary, Alberta. Upper and lower detachment surfaces have been identified as boundaries between continuous and discontinuous reflection patterns over the length of the wedge. Coherent reflections above the upper and below the lower detachment show that strata outside the wedge are essentially undeformed. The upper detachment is parallel to bedding for at least 7 km, with a dip that decreases gradually from west to east. At its distal limit, the upper detachment lies 350 m above the lower detachment and does not merge with it. The internal reflection geometry of the wedge changes with position. We interpret the thickest part of the wedge to be dominated by thrust slices up to 500 m thick, whereas the toe of the wedge is folded and faulted internally.

Implied basement-tectonic control on deposition of Lower . Carboniferous carbonate ramp, southern Cordillera, Canada

cambrian (Kanasevich et al., 1969) and preThe Mount Head embayment is a regional downwarp along the west coast of Lower Middle Devonian (Norris and Price, 1966; Carboniferous western Canada that developed by differential subsidence, which we recog- Benvenuto and Price, 1979). nize from lithostratigraphic and biostratigraphic patterns. Palinspastic restoration of the Mount Head embayment illustrates that subsidence domains are geometrically coincident with previously identified tectonic elements of the autochthonous basement. Thus, it ap- METHODS pears that basement-tectonic elements controlled sediment accommodation and accumu- Geologic observations were recorded on lation within the Mount Head embayment. The overall shape of this embayment is probably palinspastically restored maps and cross setinherited from the arcuate shape of the Late Proterozoic cratonic rift margin. Within the tions and to basement-tectonic Mount Head embayment, several northeast-southwest-trending Archean and Proterozoic domains defined on aeromagnetic and gravbasement-tectonic elements intersect the autochthonous cratonic margin at high angles. ity maps (Brandley, 1993). The palinspastic Carboniferous piano-key-like structural reactivation of these tectonic elements produced base map is modified from Gibson,s (1985) oriented trends of differential subsidence that partitioned the Mount Head embayment into map of displacement vectors for Jurassic two subbasins (Crowsnest and Kananaskis depocenters) separated by a more positive area rocks. The magnitudes of the vectors were (Highwood high). Moreover, our data imply that Precambrian basement structure noted by others under the Plains and Rocky Mountain fold and thrust belt continued across the altered to reflect displacements at the MisRocky Mountain trench to the west, and that tectonic control on sedimentation noted by sissippian level documented by Bally et al. others for Precambrian and pre-Middle Devonian rocks extends clearly into overlying (1966), Price (1981), and Price and Fermor Carboniferous deposits. (1985), university theses, and Geological Survey of Canada maps and sections across

Stratigraphy, Structure and Tectonic History of the Pink Mountain Anticline, Trutch (94G) and Halfway River (94B) Map Areas, Northeastern British Columbia

Abstract Pink Mountain Anticline stands out in front of the foothills of northeastern British Columbia (57°N, 123°W). Geologic mapping and prestack depth-migrated seismic sections show that it is localized above and west of a northwest-trending subsurface normal fault. Along with isopach maps these data demonstrate episodic normal movement on the fault during deposition of the Carboniferous Stoddart Group, Triassic Montney Formation and possibly the Jurassic–Cretaceous Monteith–Gething formations. West of this fault, during Laramide compression, a pair of backthrusts nucleated on either side of a minor east–west trending Carboniferous fault and propagated across it in an en echelon pattern. One backthrust ramped laterally across the area and separated the Pink Mountain and Spruce Mountain structures, both of which are contained within a 30+ km long pop-up structure above the Besa River Formation decollement. Glomerspirella fossils confirm the existence of the Upper Jurassic Upper Fernie Formation and Upper Jurassic to Lower Cretaceous Monteith Formation at Pink Mountain.

Analysis of High Resolution Aeromagnetic Anomalies From the South-central Alberta Foothills, Canada.

Summary High resolution aeromagnetic (HRAM) data from the south-central Alberta Foothills were processed to enhance near-surface sources of magnetic anomalies and suppress regional gradients. The processed HRAM anomalies are not related to the topography and are induced by the magnetic properties of the rock units underlying the survey area. Siliciclastic strata dominate the surface geology; they have low magnetic susceptibility (10 -5 – 10 -2 SI), and therefore induce small magnetic anomalies (ranging between 9.8 and –10.8 nT). A remarkable correlation can be observed between lithostratigraphic units and HRAM anomalies. Short ground magnetic profiles show good correlations between the magnetic anomalies, lithology, structure and the measured magnetic susceptibilities of the outcropping sedimentary rocks. The magnetization model constructed to reproduce the HRAM data generates anomalies that closely matched the observed values, and reflects the structural and lithological complexity of the study area. HRAM data show the different magnetic signatures of the Middle Blaimore, Brazeau and Lower Coalspur strata and can be used to effectively map near-surface lithology and structure. Methods The surface geology, topographic and well data from the study area were compiled using MapInfo GIS software. To determine the magnetic properties of the sedimentary strata from the area, field and laboratory magnetic susceptibility measurements were carried out. The HRAM data (Figure 1) were processed using different filtering techniques to enhance the shallow, high frequency features, and to suppress the regional gradients. The best results were obtained using a spatial band-pass filter 0.3125– 1.25 km -1 (800-3200 m wavelength), with the output shown in Figure 2. The filtered magnetic anomalies were contoured and displayed over the surface geology data. The information obtained was utilized to constrain a 55.35 km long cross-section that was used to model the magnetic anomalies. A ground magnetic survey was used to delineate magnetic features along the profile, and to compare the results with the HRAM data and magnetic susceptibility measurements.

Seismic Velocity Anisotropy Analysis At Pike¿s Peak, Saskatchewan, Canada

A numerical model with layers of 25 m thickness was developed. Numerical data were computed using an anisotropic raytracer (Figure 1) developed by Leslie and Lawton (1999a). Note that Figure 1 illustrates a change in seismic group velocity with angle through bedding due to anisotropy. An increase in velocity along the raypath is indicated by a change from cool to warm colours. Figure 1: Survey layout with sample rays. Ray colour indicates the velocity along the raypath, as discussed in the text.

Seismic Imaging of Complex Fault-Fold Structures in a Mountainous Setting

A series of numerical seismic modelling experiments was carried out to investigate the performance of different prestack migration algorithms in complex structural areas, particularly in the presence of rugged topography and thrust faults that result in severe lateral and vertical velocity changes.