Upper crustal structure of southern Queen Charlotte Basin from sonobuoy refraction studies

Abstract

The Queen Charlotte Basin, which lies on the west coast of Canada, was the location of a petroleum exploration program in the 1960's. Recently published tectonic models indicate that estimates of hydrocarbon potential require re-evaluation, and a renewed exploration interest has been expressed. In 1981, a seismic refraction survey of the upper crust using radio-telemetering sonobuoys and a 27 L air-gun source was carried out. A particular objective of the study was to determine the existence and depth extent of any sedimentary layer, hypothesized on the basis of other studies, beneath or within the Tertiary Masset volcanics in which some of the exploration wells had terminated. Three reversed profiles and one unreversed profile up to 40 km long were recorded. Interpretation of the data made use of the travel-time and amplitude information of the seismic sections by comparison with theoretical sections computed by two-dimensional ray tracing and a new asymptotic ray theory synthetic seismogram algorithm.Consistent with the earlier industry results, sediment thicknesses vary considerably throughout the southern Queen Charlotte Basin. The Tertiary Masset volcanics appear to be pervasive throughout the study area, with thicknesses varying from less than 1 km to greater than 3 km. On three of the four profiles a low-velocity layer, interpreted as Mesozoic sediments or sediments interbedded with volcanics, was found to lie beneath the volcanics. Thicknesses ranged from about 1 km to zero at a pinchout. The lowermost layer of all models is considered to be crustal rocks and is identified with the top of Wrangellia, an allochthonous terrane proposed to underlie the southern Queen Charlotte Basin. Along the profile for which no low-velocity layer sediments were interpreted, the Wrangellia terrane forms a dome rising to within 2 km of the surface. Other recent studies suggest that hydrocarbon sources could be associated with the Mesozoic rocks of Wrangellia and with any sediments underlying the Tertiary lavas, as well as with Tertiary marine sediments above the volcanics. Thus further exploration is warranted.