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The
Florissant Formation: A Virtual Tour
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Fossil
Preservation in the Lower Mudstone Unit
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Sequoioxylon pearsalli “Trio” |
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.
The
lower mudstone unit has also produced evidence of mammals
including rodents, lagomorphs, insectivores, and ungulates.
Oreotalpa florissantensis is the oldest known occurrence
of a fossil mole (Family Talpidae) in North America (Lloyd
and Eberle, 2008, p. 539). Several even-toed ungulates
including the oreodont Merycoidodon and the deer-like Leptomeryx are represented at Florissant. Two odd-toed ungulates Mesohippus,
a three-toed collie-sized horse, and a species of Megacerops,
a large brontothere were also a part of the Florissant
fauna. The Florissant mammalian fauna indicates a Chadronian
or latest Eocene age for the Florissant Formation (Lloyd,
Worley-Georg & Eberle, 2008, pp. 122 & 123 and
Prothero and Sanchez, 2004, p. 146).
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Bibliography
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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.
Lloyd, K.J., and Eberle, J.J. (2008). A new talpid from the
late Eocene of North America. Acta Palaeontologica Polonica 53 (3): 539-543.
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. |
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