Location: USA
Age: Eocene/ Oligocene (33-34 Mya)
| 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 |