Basalt sills cored in lowermost Berriasian sediments on Shatsky Rise in the northwest Pacific yield a mean 40Ar-39Ar incremental heating age of 144.6 ± 0.8 Ma, providing a minimum estimate on the age of the Jurassic-Cretaceous boundary. The largest part of the rise was emplaced at rates similar to those of several continental flood basalts. The rise formed at an oceanic triple junction, and the sills, as well as lavas from two dredge sites, exhibit an ocean ridge–type Nd-Pb-Sr isotope signature. This signature is provisionally consistent with either a perisphere or meteorite-impact model, but is not predicted by the plume-head model of plateau formation. However, no model can yet be rejected, given the very sparse sampling of Shatsky Rise.