Regional Geologic Setting

The overall crustal structure in southwest Washington consists, on the west, of the thick mafic igneous Siletzia terrain composed of Paleocene-early Eocene submarine basalts and underlying intrusions (Duncan 1982), whereas on the east, shortly north of Mount Adams, is the large north-trending White Pass anticlinorium that exposes Mesozoic tectonite mélange (Miller 1989). Both basement types are overlain by Eocene continental-interior-derived sandstones and shales of the Puget Group (Buckovic 1979) that, in turn, are widely covered by thick, mainly Oligocene and Miocene arc igneous rocks punctured by isolated Miocene plutons. Seismic velocity profiles show that the concealed east margin of Siletzia underlies and probably creates the St Helens and Rainier seismic zones due to strain localization (Stanley et al. 1995; Parsons et al. 1998). Projection south along the St Helens seismic zone would place Siletzia’s eastern margin as passing beneath Mount St Helens. Anticlines on the northwest and northeast margins of the study area expose the Eocene sedimentary rocks that dip steeply beneath the Tertiary volcanic section, and seismic studies between Mounts St Helens and Rainier indicate that the sedimentary section may attain a deformed thickness >10 km in the mid-lower crust (Stanley et al. 1994) eastward of Siletzia’s eastern margin. Thus, the region between Mounts St Helens and Adams is expected to consist of Miocene and Oligocene volcanic rocks and shallow intrusions, underlain by Eocene sandstones and shales, possibly floored by Mesozoic mélange. The region to the west of Mount St Helens consists of a thick section of gabbroic and basaltic Siletzia rocks, perhaps within 10 km of the surface, overlain by a relatively thin section of Eocene sandstones and shales concealed beneath Tertiary volcanic rocks.