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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

SKGS-AAPG

Eighth International Williston Basin Symposium, October 19, 20, and 21, 1998 (SP13)

Pages 177 - 178

MAPPING STRUCTURES AND THE BASEMENT IN THE CANADIAN HALF OF THE WILLISTON BASIN USING HIGH RESOLUTION AEROMAGNETIC AND GRAVITY DATA

T.S. HAMILTON, Wichita State University, Department of Geology, Wichita, Kansas, U.S.A.
A.S. HEDINGER, Petrel Robertson Consulting Ltd., Calgary, Alberta, Canada
J.E. KLOVAN, Petrel Robertson Consulting Ltd., Calgary, Alberta, Canada
S. CHRISTENSEN, Petrel Robertson Consulting Ltd., Calgary, Alberta, Canada

ABSTRACT

Potential field data have recently been used for basement mapping and structural control beneath sedimentary successions (Ross et al., 1993). Basement-related structural trends may be of importance in controlling the deposition and subsequent evolution of the sedimentary succession and, therefore, hydrocarbon maturation, migration and accumulation (Flawn, 1965; Hein and Breakey, 1993) in the Williston Basin (Gibson, 1995). To this end, digital high resolution aeromagnetic and gravity data were analyzed for a 70,000 km2 area in southeastern Saskatchewan and southwestern Manitoba covering much of the Canadian half of the Williston Basin (49.0° N, 100° W to 50.5° N, 105.5° W). Data sets were gridded at 400 m and analyzed for the aeromagnetics and at 2000 m for the gravity to produce a series of 1:500,000 maps that include: a background (IGRF) corrected magnetic anomaly map, a Bouguer gravity anomaly map and GRIDEPTH maps (Robertson, 1994) with structural solutions for Euler deconvolution of the two total field anomaly maps and their derivatives. The domains and structures inferred from the potential field maps were compared to regional geology and tectonics described in previous studies (Redley and Hajnal, 1995; Zhu and Hajnal, 1993; Kent, 1987; Thomas, 1974) or known from well control and seismic data.

The aeromagnetic anomaly map was produced as a histogram-equalized false-colour display with illumination from 45 degrees azimuth and 45 degrees elevation. The data range from -150 to +575 nT. This map is sensitive to lithologies with contrasting magnetization. The magnetic map is dominated by a series of north to northwesterly-trending highs with local amplitudes up to about 500 nT, lows of up to about 200 nT and spatial wavelengths of 20 to 30 km or about 2 to 3 townships across. The Bouguer anomaly map was gridded at 2 km and processed and displayed as a linearly apportioned colour contour map with 2.5 mgal contour lines and an overall range of values from -5 to -80 mgal. Owing to the coarseness of the grid, it is not possible to resolve features smaller than about 1 township in lateral extent. The Bouguer anomaly map is sensitive to contrasts in the density of underlying rock units. Most of this contrast resides below the level of the sedimentary basin, although structures within the basin and strongly contrasting lithologic types may contribute to the signal mapped. This map is dominated by a series of northwest-trending highs and lows 3 to 6 km across with amplitudes of about 20 mgal.

The Canadian portion of the Williston Basin is underlain by two fundamentally different Precambrian basement terranes: the Archean Superior Craton in the east and the Proterozoic Trans-Hudson Orogen in the west, the latter comprising several north-trending tectonic belts (Green et al., 1985). In the southwest corner of the map, the Roncott High, known to be associated with a high-standing block of preserved (Middle Devonian) Prairie Evaporite (deMille et al., 1964) and the Minton Field, is manifested as a magnetic high and a gravity low. It may represent a small block of Archean, like other pieces of the Churchill previously recognized to lie within the Trans-Hudson Orogen, or like the Wyoming Craton just to the southwest. The region of salt collapse and the western edge of the pinnacle reef basin appears to be controlled by a magnetic low and a gravity high, coincident with the Central Plains Conductivity Anomaly (Alabi et al., 1975; Handa and Camfield, 1984). This geophysically distinctive feature lies along the buried extension of the Reindeer-South Indian Lakes Belt of magnetically subdued tonalites and migmatites interpreted to have been a Proterozoic back arc basin (Green et al., 1985). Farther east, on both maps, paired north-trending belts of highs and lows mark the buried extension of the Flin Flon-Snow Lake volcanic area of the Trans-Hudson. Intense magnetic and gravity highs along the eastern edge of the map mark the edge of the Superior Craton (Gibb et al., 1978).

Euler deconvolution maps, calculated from the potential field maps and their gradients at a variety of structural factors and depth limits (Reid et al., 1990; Robertson, 1994), delineate northwest- and, to a lesser extent, northeast-trending faults and contacts that may control different domains and whole tracts of the overlying geology of the Williston Basin. Use of short window spacings and maximum offsets of about 4 km for the aeromagnetic deconvolution emphasizes structures that cut the sedimentary section and yields more than 160,000 solutions over the map, with many linear features of 15 to more than 50 km in length which have northwesterly through easterly orientations. Basement tectonic zones as mapped by the potential field anomalies and their inversions appear to correlate with structurally complex zones on structure contour maps at several different levels in the basin and with important geologic domains such as the edge of the middle Devonian pinnacle reef basin and the solution-collapse edges of the Prairie Evaporite. Mapping out faults and buried contacts by means of high resolution potential fields provides a high level of detail and a hitherto unrecognized structural framework for a fresh look at the geological development and hydrocarbon plays in the Canadian portion of the Williston Basin.

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