That fits everything you mentioned except it was not a "previously unknown extinction event."
IDK... I thought that's what I saw or read. But what you are describing seems more like the story. Rocks discovered from the day the dinosaurs died 66 million years ago when an asteroid hit the Earth
I can't find it now. Sucks.
(edit: this story on it is better written: Rocks at Asteroid Impact Site Record First Day of Dinosaur Extinction - UT News
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