Melt water pulse 1a was likely caused by a meteor strike to the massive North American ice sheet resulting in something like a 100’ foot rise in global sea levels.

The global flooding resulting from such a cataclysmic event likely would’ve created tsunami style floods much larger in places than the amplitude of the total sea level rise after the chaos.

Randall Carlson has a great presentation on JRE (#606) where he touches on a lot of the evidence around this cataclysm. It’s so interesting.