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The
Florissant Formation: A Virtual Tour
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Overview of the Florissant Formation Stratigraphy
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The
Florissant Formation Stratigraphy
Two million years after the formation of the Wall Mountain Tuff,
volcanic activity in an area known as the Thirtynine Mile volcanic
field, located a mere 25 to 30 km southwest of the ancient Florissant
basin, would help to form the famous fossil beds at Florissant.
The Guffey volcano was situated among a cluster of towering stratovolcanoes
within the Thirtynine Mile area. Eruptions from the Guffey volcano
produced pyroclastic flows, ash falls, and lahars (volcanic mudflows).
Periodically, the lahars acted as dams to the Florissant valley,
creating ancient Florissant lakes (Evanoff, McIntosh & Murphy,
2001, p. 8). The rock units of the Florissant Formation record
the existence of rivers, volcanic activity, and lakes within Florissant
valley. Fossils from some of these rock units faithfully record
portions of Florissant ecosystems of the Late Eocene.
The
Florissant Formation was deposited upon the eroded surfaces
of the Pikes
Peak
Granite and the Wall Mountain Tuff. Six informal
units make up the Florissant Formation and include from bottom
to top: the lower shale, the lower mudstone, the middle shale,
the caprock conglomerate, the upper shale, and the upper pumice
conglomerate (Evanoff, McIntosh & Murphey, 2001, p. 8).
Multiple
lines of evidence indicate a late Eocene age for the Florissant
Formation,
which accords with the current placement
of the Eocene-Oligocene boundary at 33.7 Ma. The Florissant
Formation is dated at 34.07 Ma based on volcanic minerals in
the formation (Evanoff, McIntosh & Murphey, 2001, p. 14).
The overlapping range of brotontheres and Mesohippus indicates
a Chadronian age (37-34 Ma) for the mammalian fauna of Florissant.
The 70 meter Florissant Formation is almost entirely reversed
in polarity and is most logically correlated with Chron C13,
which spans 33.7-34.7 Ma (Prothero & Sanchez, 2004, p.
145). Thus, radiometric dating, mammalian fossil fauna, and
the magnetic stratigraphy of Florissant corroborate a latest
Eocene age for the Florissant Formation.
The
Florissant Formation represents both fluvial (river) and
lacustrine
(lake) environments.
An initial lake in the paleovalley
became filled with sediments over many years, after which a
stream valley developed. Evidence suggests lahars traveling
down side tributary valleys eventually formed a natural dam,
re-establishing lake conditions. Renewed volcanic activity
resulted in a lahar deposit that entered parts of the second
reservoir-like lake. Eventually, the deposition of a pumice
conglomerate marks the end of lake conditions. Let’s
take a closer look at the major units in the Florissant Formation.
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Bibliography
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Evanoff,
E., McIntosh, W.C. and Murphey, P.C. (2001). Stratigraphic
Summary and 40Ar/39Ar Geocrhonology of the Florissant Formation,
Colorado. In Evanoff, E., Gregory-Wodzicki K.M. and Johnson,
K.R. [Eds.] Fossil Flora and Stratigraphy of the Florissant
Formation, Colorado. (pp. 1-16). Proceedings of the
Denver Museum of Nature and Science, series 4, number 1.
Prothero, D.R. and Sanchez, F., (2004). Magnetic stratigraphy of
the upper Eocene Florissant Formation, Teller County, Colorado.
In Lucas, S.G., Zeigler, K.E., and Kondrashov, P.E. [Eds.]. Paleogene
Mammals. (pp. 129-135). New Mexico Museum of Natural History and
Science Bulletin 26. |
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