An example of basalt invasion into wet sediments:
neat basalt
The article is a quick read with great examples. I learned about "peperite."
A good quote "Although liquid, the basaltic lava has a higher density than the wet sediment. Thus,a basaltic flow on the surface of the earth will tend to burrow into an accumulation of wet sediment rather than flow above it." This brings up another, intermediate possibility for the altered zones that I had not considered. I still think that these rocka are so highly altered by water, that some type of subaqueous intrusion/extrusion is involved.
The article "Lave and water" is here