Since this happened after the Flood, you should be able to dig anywhere in the world where the soil hasn't been excavated or eroded down to bedrock and consistently find a layer of volcanic ash.
Assuming a Biblical timeline, there were no oceans pre-deluge. The so-called “fountains of the great deep” would not contain salt water, therefore it came from outside the system.
Also under these assumptions, derived from Biblical descriptions, the climate was stable and verging upon tropical, so icebergs were not part of the equation due to that alone. Not to mention, there were no large bodies of surface water or even rain/snow required to form icebergs.
So, if this is to be taken as some sort of effect from the Biblical deluge, it occurred many years afterward. A very significant geological disturbance of some kind would be necessary to break off huge sheets of the polar icecap and send them scraping across the ocean floor and eventually adrift, I’d imagine.