Continuous seismic reflection profiling of the deep basement, Hardeman County, Texas

Abstract

Our understanding of the crust and upper mantle would be enhanced if geophysical studies of the deep basement rocks provided information of resolution and character more nearly like that of geological observations of basement rocks at and near the surface. A test of the continuous seismic reflection profiling technique, the geophysical method with by far the highest resolution and the best potential in this regard, at a site in the midcontinent provided abundant information on intrabasement diffractors and reflectors to depths as great as about 45 km. Conventional equipment and techniques, including nonexplosive vibratory sources, were used with minor modification. In the upper part of the section below the sediments, there are reflectors continuous over the entire length of a profile that give evidence for warping, faulting, unconformities, and other structural features. An age of 1,265 ± 40 m.y. for a sample from a nearby hole indicates that these are Precambrian rocks and not part of the Cambrian basement rocks of the Wichita Province. Detailed correlation with the Precambrian section is inhibited by scarcity of geological information. In the lower part of the section, reflections are not, in general, continuous over more than a few kilometres, but zones and discontinuities within the basement may be distinguished on the basis of spatial density, length, and dip of reflectors. Zones of low reflector density may be plutons; curvature of reflections may indicate deep folded structures. The scale of such features is a few kilometres, and it contrasts with the markedly larger scale of the smallest features of the deep basement that can be resolved by other methods. The method appears to have outstanding potential.