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The Florissant Formation: A Virtual Tour
Fossil Preservation in the Paper Shales of Florissant


Cross-Section of Paper Shale from Lower Shale Unit (1.8 cm thick)

Fossil Preservation in the Lake Shales


The lacustrine or lake paper shales are made of alternating layers of diatoms and ash-clay. O’Brien et al (2002) used scanning electron microscopy and energy dispersive X-ray analysis to examine these alternating layers. The diatomite layer is lighter in color and consists of the frustules of diatoms. Diatoms are single-celled protists that make their protective shells or frustules of silica. Fossils are almost exclusively found imbedded within the diatomite layers. Furthermore, these diatomite layers are covered by a mucous layer, which was secreted by diatoms and bacteria. Ash-clay layers appear darker in color. The diatomite and ash-clay layers form what is known as a couplet 0.1 to 1.0 mm in thickness. Typically, layers of 3 to 10 couplets are sandwiched between pumice layers, which are several millimeters to centimeters in thickness (pp. 3 & 4). O’Brien et al (2002), proposed the following events for the deposition of the Florissant Fossiliferous shale:


1. Volcanic ash weathering into clay washed into Lake Florissant from the surrounding terrain.
2. The volcanic sediment was deposited as a thin layer of ash-clay, enriching the lake water with silicon.
3. Diatoms bloomed as a result of the added silicon and formed polysaccharide mucus mats.
4. It is hypothesized that the diatom and bacterial mucous film sealed and protected the organisms from decomposition.
5. Mucus mats sank to the lake bottom and formed thin, diatom-rich laminae.
6. The organisms were subsequently fossilized (p. 6).


The paper shale is made of many couplet layers suggesting seasonality to the cycle above. The couplets may be varves representing annual layers. The pumice layers interbedded at irregular intervals within the paper shales record sporadic volcanic eruptions that produced volumes of ash and pumice. These layers were formed rapidly and are much thicker than the couplets.


Both insects and leaves preserved in the Florissant beds are often carbonized Insects and leaves entangled in the diatom mucus mats were incorporated into layers of sediments and volcanic ash at the bottom of Lake Florissant. Many of these insects and leaves decomposed leaving imprints. As the sediments compacted and hardened into shale the imprints became impression fossils. Some organisms only partially decayed retaining a dark colored carbon residue to become compression fossils (carbonization). Many insects have their wings preserved as impressions (no organic residue) while their bodies retain organic residue forming dark compressions. Compressions are often flattened, having a two-dimensional appearance. However, the preservation in diatom layers allows some organisms to retain their three-dimensional character. Some insects are found with organs and appendages. Some leaves can be found with internal structures (Meyer, 2003, pp. 35-37).

The paper shales act as nature’s “plant and insect press” and make Florissant a fossil lagerstätten. Florissant has produced roughly 1700 described species of plants and animals (Meyer, Veatch & Cook, 2004, p. 151). Impressions of leaves, fruits, seeds and flowers account for about 120 species (Manchester, 2001, p. p. 137). Palynology, the study of microscopic plant fossils, such as pollen and spores, adds another 25 genera to the fossil flora of Florissant (Leopold & Clay-Poole, 2001, p. 17). Over 1500 of the 1700 described species at Florissant are insects and spiders (Meyer et al., 2004, p. 158). The most common vertebrate found in the fossil shale are fish, with the majority representing bottom dwellers. Four genera representing catfish, suckers, a bowfin and a pirate perch have been described. Although rare, birds are represented by a small plover, roller, and cuckoo. Only one mammal, a small opossum, has been found within the shale beds. Interestingly, no amphibians or reptiles have been found at Florissant (Nudds & Shelden, 2008, pp. 227 & 228).

It is clear that the fossil bearing shale at Florissant has received a lot of attention from paleontologists over the years. However, within the shale units different depositional environments are also represented by layers of mudstone and siltstone. A recent study comparing shale, mudstone, and siltstone within the middle shale unit at Florissant found that the abundance and preservation quality of the most commonly found insect orders in lacustrine settings did not differ across these different sedimentary environments (Henning, Smith, Nufio, and Meyer, 2012, p. 481).

The findings of this study are surprising for a couple of reasons. Diatoms have been thought to play a major role in the preservation quality of insects found within the shale. However, mudstone is not typically associated with diatom layers indicating that the presence of diatoms may not enhance preservation (Henning et al., 2012, p. 487). Siltstone represents a higher energy depositional environment with larger grain sizes than mudstone or shale. Interestingly, the preservation quality and abundance of fossils found within the siltstone was equal to mudstone and shale. Perhaps the paper shale has been favored over the years because it is easier to split open. The surprising results of this study suggest paleontologists broaden their search for fossils by including the mudstone and siltstone layers found within the shale units at Florissant.

 


Fagopsis Lower Shale Unit


Crane Fly Lower Shale Unit

Bibliography

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.

O’Brien, N.R., Meyer, H.W., Reilly, K., Ross, A.M., and Maguire, S., (2002). Microbial taphonomic processes in the fossilization of insects and plants in the late Eocene Florissant Formation, Colorado: Rocky Mountain Geology, v. 17, pp. 1-11).

Leopold, E.B. and Clay-Poole, S.T. (2001). Fossil leaf and pollen floras of Colorado compared: climatic implications. In Evanoff, E., Gregory-Wodzicki K.M. and Johnson, K.R. [Eds.] Fossil Flora and Stratigraphy of the Florissant Formation, Colorado. (pp. 17-55). Proceedings of the Denver Museum of Nature and Science, series 4, number 1.

Meyer, H.W. (2003). The Fossils of Florissant. Washington: Smithsonian Books.

Meyer, H.W., Veatch, S.W. and Cook, A. (2004). Field guide to the paleontology and volcanic setting of Florissant fossil beds, Colorado (pp. 151-166). In Nelson, E.P. and Erslev, E.A. [Eds.] Field Trips in the Southern Rocky Mountains, USA. Geological Society of America Field Guide 5.

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.


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