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Interpretation of airborne and ground magnetic data from the south-central Alberta Foothills shows a remarkable correlation between surface geology and residual magnetic anomalies. The near-surface magnetic 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 (1025 to 1022 SI) and therefore induce small magnetic anomalies ranging between 9.8 and 210.8 nano Telsa. Most of the magnetic anomalies occur in uppermost Cretaceous sandstones (Brazeau and Lower Coalspur strata) and appear to increase in intensity at the contact with the Tertiary Upper Coalspur Formation and with the underlying marine shale of Alberta Group strata. The Albian Beaver Mines sandstone also exhibits higher magnetic anomalies, contrasting with the underlying lower Blairmore strata and the overlying shale of the Blackstone Formation.
Ground magnetic data show good correlation with highresolution aeromagnetic (HRAM) anomalies and the magnetic susceptibilities measured from the surface geology. The depth estimates to the magnetic sources that generate the magnetic anomalies indicate values ranging from 20 to 800 meters.
The occurrence of HRAM anomalies in the Beaver Mines, Brazeau and Lower Coalspur strata appears be related to their depositional history and petrographic compositional stages of the Middle and Upper Cretaceous sandstones from the southern and central Alberta Foothills. Cretaceous nonmarine sandstone from the study area contains up to 17% detrital opaque heavy minerals, which consist of magnetite, ilmenite and rare grains of chromite and pyrite.
The magnetization models constructed to reproduce the magnetic anomalies closely match both ground and airborne observed values. Seismic data interpretation constrains the magnetic mapping results and suggests that HRAM data may be used in the early stages of exploration to assist in mapping lithology and structure between 2D seismic lines.
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