Hi Sarah,

It depends on how deep the beds are.  A soil texture transition can cause what is known as a "perched water table," preventing water from moving out of the raised bed and into the soil beneath.  If the raised bed is fairly shallow, this means that you will in effect restrict the area in which roots can grow (since they need air as well as water, and will not survive in completely saturated soil).  

A layer of cardboard can cause the same thing, though it can (at least for a time) help prevent perennial weeds like bindweed or thistles from exploring your new raised bed too.

If the raised bed is deep enough (18" minimum), you should still have ample space for roots to proliferate even with a texture transition.  Deeper soil would be better for larger plants like tomatoes; you could get away with a bit shallower than 18" for less deeply rooting plants (like lettuce or radishes, for example).

I hope this is helpful!

Cordially,

John

Good luck and happy gardening!