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The
Florissant Formation: A Virtual Tour
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Introduction & Pikes Peak Granite
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Introduction
The
Florissant Formation represents an Eocene-aged accumulation
of rock units including tuffs, mudstones, shales, sandstones
and
conglomerates. These different rock types were formed by pyroclastic
flows, lahars, lake and river environments. Fossils embedded
within the lahar mudstones and lake shales give scientists a
window into the latest Eocene ecosystems of Florissant.
Our virtual
tour will explore the six informal units of the Florissant Formation
as described by Evanoff et. al. (2001). It is helpful to study
older rock units to provide a geological context for understanding
the events
that
led to
the
deposition
of the Florissant Formation. We will briefly describe
two older stratigraphic units, the Pikes Peak Granite and the
Wall Mountain Tuff.
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Mary
Klass Resting Comfortably Atop Pikes Peak Granite |
Pikes Peak Granite
Pikes
Peak, at an elevation of 14,110 ft, is just 12 miles South
East of Florissant. Pikes
Peak granite has a radiometric
age of 1,080 Ma (Evanoff, McIntosh & Murphy, 2001, p.
3). The billion year old pink Pikes Peak granite formed as
an intrusive
batholith during the Precambrian and covers 1,150 square
miles (Foos & Hannibal, 1999, p. 1). The mountain building
episode known as the Larimide Orogeny, which started in the
Late Cretaceous
65 to 70 Ma ago, created the current Rocky Mountains and
resulted in the uplift and exposure of the Pikes Peak Granite
(Meyer, 2003, p. 23). During the Late Eocene, rivers
eroded the granite to form a valley. Late Eocene sediments
from volcanic activity, lakes, and rivers, filled the valley
depositing the rock 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.
Foos, A. & Hannibal, J. (1999). Geology of Florissant Fossil
Beds National Monument.
http://www.nature.nps.gov/geology/education/foos/flfo.pdf
Meyer, H.W. (2003). The Fossils of Florissant. Washington: Smithsonian
Books.
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