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The Florissant Formation: A Virtual Tour
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


ScudderPitTop
Top section of Scudder Pit
Caprock Conglomerate overlies Middle Shale Unit

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.

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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