I guess we can speculate. So far though I have seen no reasons to rule it out as a possible leftover fount of the deep that impacted upwards from below. If that was ruled out your scenario would be the default.Or an actual meteor impact after the flood, which is why no fossils are found in the layers leading up to the KT boundary. The flood had already happened and the meteor impact did little harm at all except spread some dust around......
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