Name: Florissant Formation
Location: USA
Age: Eocene/ Oligocene (33-34 Mya)
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2. Geological Setting and Age

The Florissant fossil beds sit on the border between the Oligocene (25-34 million years ago) and the Eocene (34-55 million years ago). The Florissant region contains five different rock units, the oldest being Pikes Peak Granite (1.02 billion years old, Precambrian), forming a large intrusive batholith of pink granite. The Florissant formation (33-34 million years ago) was formed when a complex of volcanoes erupted. The initial eruptions buried a forest containing many giant redwoods. Much is still preserved today. Later eruptions changed drainage systems to change and lead to the formation of a large lake. The beds then formed in the lake partly due to subsequent eruptions.

The rocks of the Florissant Formation can be divided into the following layers:

YOUNGEST Upper tuff
  Upper tuff and interfingering lake shales
  Lake shales
  Lower tuff and interfingering lake shales
  Lower tuff
  Basal arkosic breccia
LAKE SHALES Coarse, conglomeritic sandstone
  Fine-grained sandstone, argillaceous bands
  Coarse grey and yellow sandstone
  Dark brown clay shales
OLDEST White clay shale

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