Here's a question right back at you. Ash is formed by superheating and pulveriszing rock which drives the volatiles out of it. Doesn't matter whether it's formed by volcanos or impacts? Given that the noachian Martian surface is still visible, ~4Gya+ old, there is plenty of time for impact plume ash to build up in an old impact basin. Not so much from the original impact mind you, but the more easily transported aerosolized ash.

As for why blueberries are round. Iron concretions here on Earth are round due to how they grow in the right chemical environment. They nucleate around some small seed object. Iron bearing fossils from dinosaur bearing formations are my experience, iron not dinosaur being the crucial ingredient. Perhaps the ablation particles from FeNi meteors could be the culprit in this case. Then the periodicity of the layering would probably correspond to chemically favorable formation conditions and not M-asteroid swarms.