In the Purcell anticlinorium of southeastern Canadian Cordillera, processing and interpretation of surface seismic and electromagnetic data suggest that prominent seismic reflectivity and enhanced electrical conductivity are spatially coincident over a wide area. Detailed examination reveals a different story however. The reflective layers are traceable to outcrop and to a drill hole, where they are found to be caused by acoustic impedance contrasts between Meso-Proterozoic gabbroic sills and the surrounding metasedimentary rocks. In contrast, electrical logs from the drill hole show that the layers with high electrical conductivity are magnetic sulphides(probably pyrrhotite) enriched zones within the metasedimentary rocks. The sills are highly resistive. This detailed comparison shows that attempts to interpret observed colocated reflectivity and high conductivity in terms of a single cause, whilst tempting and certainly satisfying by appealing to Occam's Razor, must be done with care. We speculate that some explanations for the causes of crustal reflectors and conductors may be incorrect, and propose an alternative paradigm of interlayered sills and highly conducting mineralrich sediments to explain both.
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