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:

 AGE / SIGNIFICANCE  ROCK TYPE
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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