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I am a climate scientist and geologist and think that climate, geology, and geography are incredibly interesting fields that people deserve to know more about. If you have any questions that you’ve sat with for a while, are just curious, want to know more about future or past scenarios, or even have worldbuilding questions, feel free to ask!
(Not a geologist warning)
Not the oldest, but a neat one: Banded iron formations. Probably formed because early life evolved photosynthesis, releasing large amounts of oxygen which precipitating iron out of the early oceans. Also "extinct" because the conditions for it to form probably will never exist on Earth again.
Stromatolites are somewhat similar. Refers to both current day living guys, and their ancient fossil ancestors. Both form layered mats and are some of the earliest fossils. I think there's earlier fossils but they're a good mix of old and decently large.