Near the base of the Mesozoic oceanic crust of the western North Atlantic we recognize a discontinuous, diffractive layer, Horizon R. Moho is a distinct, uniform, and more continuous horizon beneath. Variations in “seismic thickness” of the crust occur primarily through changes in the R-to-Moho layer, the maximum thickness occurring midway between fracture zones. We propose that Horizon R may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber. The crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super-R unit. We further propose that this implies the sustained accretion of normal oceanic crust at the Mesozoic Mid-Atlantic Ridge required the presence of a magma chamber.