The "Cambrian Explosion" is an oft quoted troublesome event of creationists and IDers. Explosions sound like exciting events don't they. But it doesn't seem to bother professional biologists so much. The "explosion" did last for tens of millions of years, which is actually a very large number of generations, and there were pre-cambrian organisms, but their fossils were just so small we didn't have the techniques to spot them until relatively recently.
The range of dates I've seen for the Cambrian explosion from 5 million to 30 million years, which is hugely problematic for the theory of evolution. So, in literally no time at all geologically, all of the phyla of the world appear literally at once. Their fossil precursors were all microbes.
BTW, the term "explosion" has no meretricious appeal for me. I find that "explosions" are used by theorists to get themselves out of the corners they've painted themselves into.