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The
Florissant Formation: A Virtual Tour
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Middle Shale Unit |
Middle Shale Unit and Caprock Conglomerate |
The Middle Shale Unit
Additional lahars flowed down the valley eventually damming the
Florissant drainage. Water filled the valley and its tributaries
forming a second lake Florissant, which was 1.5 kilometers wide
and 20 km long (Meyer, 2003, p. 29). The middle shale unit consists
of paper shales, pumice conglomerate, and volcanic siltstone beds
(Evanoff, McIntosh & Murphey, 2001, p. 7). Paper shales found
within the Florissant Formation consist of alternating layers of
diatomite and volcanic ash-clay. Repeated deposition of volcanic
ash within the lake triggered abundant diatom growth. Fossils are
found within the layers formed by the deposition of diatoms (O’brien,
Meyer, Reilly, Ross, and Maguire, 2002). A recent study demonstrated
that layers of mudstone and siltstone within the middle shale
unit also contain insect
fossils (Henning et. al, 2012, p. 481). Within
the monument, it is the middle shale unit that provides the wealth
of fossil
insects
and leaves. Fossils of fish, mollusks, and ostracods are rare within
the middle shale unit. |
Scudder
Pit
Middle Shale Unit and Caprock Conglomerate
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Top section of Scudder Pit
Caprock Conglomerate overlies Middle Shale Unit
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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.
Henning, J.T., Smith, D.M., Nufio, C.R. and Meyer, H.W.
(2012). Depositional setting and fossil insect preservation:
a study of the late Eocene Florissant Formation, Colorado.
Palaios 27: 481-488.
Meyer,
H.W. (2003). The Fossils of Florissant. Washington:
Smithsonian Books. |
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