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The Florissant Formation: A Virtual Tour
Overview of the Florissant Formation Stratigraphy

 
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.



Bibliography

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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