Miocene tholeiitic basalt flows and intrusions crop out along the Pacific Coast from Seal Rock, Oregon, to Grays Harbor, Washington. Based on extensive mapping of dikes, sills, and lava flows, previous workers proposed that these coastal basalts erupted from local vents. However, based on field associations and petrogenetic considerations, others have suggested that the coastal basalts represent the distal ends of plateau-derived flows of the Columbia River Basalt Group that flowed into estuarine and deltaic environments, invading and deforming soft sediment.

To what depth the coastal basalt dikes extend is a critical test of the alternative hypotheses regarding their origin. Dikes marking former vents would likely be connected at depth with a magma source or conduit, i.e., these dikes would be “rooted” at depth. Invasive flows, on the other hand, would be “rootless.” Gravity data provide geophysical constraints on the depths of coastal basalt intrusions.

Our gravity profiles across coastal basalt dikes near Astoria, Oregon, are consistent with shallow near-surface basalt masses. All of the intrusions investigated (including the linear dikes at Fishhawk Falls and Denver Point, the arcuate segments of the “ring dike” on the Klaskanine River, the U-shaped dike at Voungs River Falls) can be interpreted as extending less than 300 m below sea level; many continue for only about 100 m below the surface. The gravity data do not match the profiles expected from deep vertical dikes.

Additional evidence supporting the plateau origin for the coastal basalts includes stratigraphic correlations and areal distributions. Mapping, geochemical analyses, and magnetic polarity determinations by us and by other workers demonstrate that all the coastal basalt units have Columbia River Basalt Group counterparts in the Willamette Valley.