Fossil Preservation in the Lower Mudstone Unit

Very different volcanic deposits preserved Eocene age life at Florissant. A destructive lahar provided a geologic environment in which wood became permineralized with silica. The petrified wood at Florissant is found in the lahar at the top of the mudstone. Although rare, fossil leaf impressions and compressions can be found in some parts of the mudstone. The delicate structures of leaves and insects are more commonly found in the shale units deposited within a lake environment.

During the late Eocene a lahar or volcanic mudflow from the Guffey volcano entombed redwood trees growing in the lower Florissant valley. The volcanic material that ended the life of these trees would also help to preserve them in stone. Portions of trees encased within the mudflow were permeated with groundwater carrying dissolved silica from the volcano, eventually forming petrified wood. Mustoe (2008) concluded that petrification at Florissant occurred in several stages. First, amorphous silica precipitated on cell wall surfaces of the wood. Second, opal-CT and chalcedony filled cell lumina (cell spaces). Finally, chalcedony filled fractures that crosscut permineralized tissues in some specimens. Spaces between adjacent tracheids in the Sequoioxylon were often unmineralized, making the fossil wood permeable to water and susceptible to cleaving radially, tangentially, and transversely from freeze-thaw weathering. This finding has important implications for the preservation of specimens at Florissant Fossil Beds National Monument (p. 127).

Fossil trees in the main Petrified Forest represent Sequoioxylon, which is a name for fossil wood closely related to the Sequoia growing along the coast of present day California. The largest stump has a diameter of 4.1 m when measured at breast height (1.5 m) above the ground. This size suggests a canopy height of 60 m. We can infer from the preserved annual rings that these redwoods were fast growing reaching diameters of 3 m within 500 to 700 years (MacGinitie, 1953, p. 21). Fossil Sequoioxylon pearsallii from Florissant has a higher mean ring width when compared with the modern coast redwood (Sequoia sempervirens) and the giant sequoia (Sequoiadendron giganteum), indicating more favorable growing conditions for the fossil trees. Two of the fossil stumps have been cross-dated, which demonstrates they grew in a single forest (Gregory-Wodzicki, 2001, p. 163). Three interconnected stumps (known as the “Redwood Trio”) share a root system and represent a clone. These characteristics are very much like what is seen in present day forests of Sequoia trees (Nudds & Selden, 2008, p. 214).

Chadronoxylon, an angiosperm dicot, is also present among the Sequoioxylon stumps. Chadronoxylon florissantensis, the most abundant angiosperm wood at Florsissant, is a diffuse porous wood with affinities to the families Salicaceae (willows) and Phyllanthaceae (Wheeler and Meyer, 2012, p.9). Four additional angiosperm woods occur in the lower mudstone unit, but not in the main Petrified Forest. Interestingly, these woods are ring porous, indicative of seasonal environments. Two of the ring porous woods share characteristics with the elm family Ulmaceae and resemble Zelkova. A fourth resembles Koelreuteria of the soapberry family Sapindaceae. A fifth specimen is the first reported occurrence of a Hovenia like wood, from the buckthorn family (Rhamanaceae), in North America (Wheeler and Meyer, 2012, p.1). Zelkova, Koelreuteria, and Hovenia genera are restricted to East Asia today. The occurrence of these fossil woods at Florissant is evidence of Tertiary exchange between East Asia and North America. A sixth, Robinia-like wood (black locust) of the family Fabaceae was found in the caprock conglomerate (Wheeler, 2001, p. 187). One wonders what other wood types may have been present before the area was subjected to scavenging by souvenir collectors between the 1870s and 1969.

Gregory-Wodzicki, K. M. (2001). Paleoclimatic Implications of Tree-Ring Growth Characteristics of 34.1 Ma Sequoioxylon pearsallii from Florissant, Colorado. In Evanoff, E., Gregory-Wodzicki K.M. and Johnson, K.R. [Eds.] Fossil Flora and Stratigraphy of the Florissant Formation, Colorado. (pp. 163-186). Proceedings of the Denver Museum of Nature and Science, series 4, number 1.

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Lloyd, K. J., Worley-Georg, M.P., and Eberle J.J. (2008). The Chadronian mammalian fauna of the Florissant Formation, Florissant Fossil Beds National Monument, Colorado. In Meyer H.W. and Smith, D.M. [Eds.] Paleontology of the Upper Eocene Florissant Formation, Colorado. (pp. 117-126). The Geological Society of America, Special Paper 435.

MacGinitie, H.D. (1953). Fossil Plants of the Florissant Beds, Colorado. Washington, D.C.: Carnegie Institution of Washington Publication 599.

Mustoe, G.E. (2008). Mineralogy and geochemistry of late Eocene silicified wood from Florissant Fossil Beds National Monument, Colorado. In Meyer, H.W., and Smith, D.M., [Eds.], Paleontology of the Upper Eocene Florissant Formation, Colorado (pp. 127-140). Geological Society of America Special Paper 435.

Nudds, J.R. & Selden, P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: The University of Chicago Press.

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.

Wheeler E.A. (2001). Fossil Dicotyledonous Woods from Florissant Fossil Beds National Monument, 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.

Wheeler E.A. and Meyer, H.W. (2012). A new (Hovenia) and an old (Chadronoxylon) fossil wood from the late Eocene Florissant Formation, Colorado, U.S.A. IAWA Journal, 33(3) –001-010.