I always thought the telltale would be a thin layer of mixed metals in the geological record. Assuming the civilisation lasts a few thousand years then that's a

I always thought the telltale would be a thin layer of mixed metals in the geological record. Assuming the civilisation lasts a few thousand years then that's a very thin layer 60m years later. And it would contain a lot of iron, copper, tin, lead, etc all mixed together in that thin layer. It would look very weird from a geological point of view.