Which modern ocean basin started forming through the separation of two continents at the beginning of pangaea breakup?

The Atlantic ocean was created (and is getting bigger!) as Pangaea drifted apart. The best way to remember which way the continents are drifting is to look at how south america and africa could almost fit together like a puzzle, and in fact they were once touching together. Gradually, we'll see the Atlantic ocean grow and the pacific ocean shrink over the next few million years.