The abyss doesn't start until cavern layer I believe. So the very first part of the abyss still stays the sulphurous sea, whether this is intended or not, idk.Believe me, you'd be surprised... there's entire threads dedicated to other stuff that's unrelated to Terraria!
and speaking of u n r e l a t e d . . .
...Does anyone experience issues with the first part of the Abyss? Sometimes, there'll just be a completely empty chunk with the abyss gravel and walls, but it'll play wasteland and still have the poisonous water (that's not unrelated though, so why did I say that?)
Also, the empty chunk is supposed to be like that, as that's how the first layer of the abyss is made.