Which geological time boundary marked the extinction of dinosaurs?

Non-avian dinosaurs went extinct at the very end of the Maastrichtian age of the Cretaceous period, around 65.5 million years ago, in a proposed extinction event called the "K-T event", or "K-Pg event" (in more modern scientific parlance) . The specific geological marker of this event is the "K-T boundary", which is usually a thin band of sediment famous for being rich in iridium, which many believe was deposited there during a massive meteorite impact that initiated the K-T event and directly or indirectly caused the extinction of many animal species, including all of the non-avian dinosaurs.