The Florissant Formation is a sedimentary geologic formation outcropping around Florissant, Teller County, Colorado. The formation is noted for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales. Based on argon radiometric dating, the formation is Eocene (approximately 34 million years old ) in age and has been interpreted as a lake environment. The fossils have been preserved because of the interaction of the volcanic ash from the nearby Thirtynine Mile volcanic field with diatoms in the lake, causing a diatom bloom. As the diatoms fell to the bottom of the lake, any plants or animals that had recently died were preserved by the diatom falls. Fine layers of clays and muds interspersed with layers of ash form "paper shales" holding beautifully-preserved fossils. The Florissant Fossil Beds National Monument is a national monument established to preserve and study the geology and history of the area.
History
The name Florissant comes from the French word for flowering. In the late 19th century tourist and excavators came to this location to observe the wildlife and collect samples for collections and study. The Petrified Forest, which is one of the main attractions at the monument today, lost much of its mass due to collectors removing large amounts of petrified wood from the site.
During the 1860s and 1870s the area was mapped by geologists for the first time. Geologists of the Hayden Survey visited the area in the early 1870s, and fossil plants from the beds were described by Leo Lesquereux, fossil insects by Samuel Hubbard Scudder, and vertebrate fossils by Edward Drinker Cope. The formation was first formally named as the Florissant Lake Beds by Charles Whitman Cross in 1894.
In 1969, the Florissant Fossil Bed National Monument was established after a long legal battle between local land owners and the federal government. Today, the park receives approximately 60,000 visitors a year, and is the site of ongoing paleontological investigations.Meyer, H. W. & Weber, L. (1995). Florissant Fossil Beds National Monument: Preservation of an Ancient Ecosystem. Rocks and Minerals, v. 70, p. 232-239 The formation itself was renamed the Florissant Formation in 2001 to conform with the requirements of the North American Commission on Stratigraphic Nomenclature.
Geology
In the late Eocene to early Oligocene, approximately 34 million years ago, the area was a lake environment with redwood trees. The basement is the Proterozoic aged Pikes Peak Granite. There is an unconformity from the Pikes Peak Granite to the next unit, the Wall Mountain tuff.Foos, A. & Hannibal, J. (1999). Geology of the Florissant Fossil Beds National Monument. Field Guide, 9 pages The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the Laramide Orogeny.Steven, T. A. (1997). Middle and Late Cenozoic Tectonic and Geomorphic Development of the Front Range of Colorado. Rocky Mountain Association of Geologists, 1997, Colorado Front Range Guide Book, p. 115-125 The Wall Mountain Tuff was deposited as a result of a large eruption from a distant caldera. The Florissant Formation itself is composed of alternating units of shale, mudstone, conglomerate, and volcanic deposits. There are six described units within the Florissant Formation. In order from bottom to top: the lower shale unit, lower mudstone unit, middle shale unit, caprock conglomerate unit, upper shale unit, and the upper pumice unit. Each of the shale units represents lacustrine environments, composed of very thin shales that are abundant in fossils, which alternate with tuffs from eruptions. The lower mudstone has been interpreted as a stream environment with the top of the unit being a lahar deposit. The mudstones were deposited on a valley floor, but not in a lake. The separation of the shale units by non-lake deposits could mean that there were two generations of lake deposits. Lahars that went through the valley could have dammed up the valley, allowing for the creation of a lake. The middle and upper shale units were then deposited in this second generation of the lake. The caprock conglomerate was deposited as a large lahar went through the valley and accumulated down on the lake floor.
The Laramide Orogeny, which created the modern Rocky Mountains, had been uplifting the area to the west since the end of the Cretaceous,Dickinson, W. R. et al (1988). Paleogeographic and Paleotectonic Setting of the Laramide Sedimentary Basins in the Central Rocky Mountain Region, Geological Society of America Bulletin, v. 100, p. 1023-1039 although the exact timing of the orogeny is debated In the late Eocene to the Early Oligocene, volcanic episodes began to occur to the southwest of the Florissant area. These episodes of eruption would deposit ash and other volcanic debris on the Florissant location, and the volcanic material would be one of the most important factors in the fossilization of the plants and animals that are so abundant in the formation. The fossil bearing paper shales are intercalated with larger deposits of volcanic material.Harding, I. C., & Chant, L. S. (2000). Self-Sedimented Diatom Mats as Agents of Exceptional Fossil Preservation in the Oligocene Florissant Lake Beds, Colorado, United States. Geology (Boulder), 28(3), 195-198.
Most of the rocks that were deposited after the Oligocene and before the Pleistocene have been eroded away.Chapin, C. E., & Cather, S. M. (1983). Eocene Tectonics and Sedimentation in the Colorado Plateau, Rocky mountain Association of Geologists, p. 33-54Gregory, K. M., & Chase, C. G. (1992). Tectonic significance of paleobotanically estimated climate and altitude of the late eocene erosion surface, Colorado. Geology (Boulder), 20(7), 581-585 Most of the remaining units are composed of clasts of weathered Pikes Peak Granite, volcanics, and mud that were transported by streams that flowed through the area. Some mammoth bones have been found within these units and have been dated to around 50,000 years old.
Thirtynine Mile Volcanic Field
Around 25–30 kilometers to the southwest, a series of stratovolcanoes, similar to modern day volcanoes like Mt. St Helens, developed and erupted periodically. Called the Guffey volcanic center, within the larger Thirtynine Mile volcanic field, the volcano had eruptions that included domes, lava flows, and pyroclastic events.Chapin, C. E., & Epis, R. C. (1964). Some Stratigraphic and Structural Features of the Thirtynine Mile Volcanic Field, Central Colorado. The Mountain Geologist, V. 1, p. 145-159Wobus, R. A. et al. (1990). Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado, Geology, V. 18, No 7, p. 642-445 Ash from these events settled throughout the area and lahars flowed down the valleys. The ash that settled created the tuff, and the lahars formed the mudstones and the conglomerates of the Florissant formation.
The Florissant was deposited in a paleovalley after one of the lahars dammed it up. The resulting lake became as large as 36 km2. There were two cycles of lake environments. The first one created the lower shale unit, while the second lake created the middle and upper shale units. Eventually, the volcanoes became dormant and started to erode away. Over time, the volcanoes became so eroded that there are no more obvious signs of them on the surface. Instead, the Eocene erosional surface is the only remnant of the area's volcanoes.
Fossils
The volcanic material that caused so much destruction led to the preservation of the fossils within the Florissant Formation's shales and mudstones. As the ash was deposited on the landscape, it was carried by water through streams and lahars to the lake. The lahars then covered the base of the redwoods that were living at the time, and the trunks of the trees became harder and fossilized. Through permineralization, the precipitates that were in the ground water flowed through the tree trunks, replacing the original matter with siliceous minerals, replacing the organic matter with silica. This process of mineralization led to the preservation of the stumps of the trees.
Within the lake itself, the volcanic deposits were periodically deposited into the lake either through direct deposition or though weathering. The volcanic material was rich in silica. Living in the lake at the time were diatoms, whose shells were also composed of silica. The influx of more silica caused the diatoms to bloom in the lake. As the population of the diatoms massively increased, the stress from the volcanic episodes at the same time caused large die-offs of the local biota. As the plants and animals died off, their leaves and bodies fell into the lake and eventually large amounts of organic matter accumulated at the bottom of the lake. This process was repeated often, possibly yearly, as the runoff from rain collected in the lake, causing cyclical diatom blooms and die-offs. The diatom blooms and die-offs created layers of ash-clay deposits, called couplets, resulting in many thin layers. Each microlayer of ash and clay was compacted by overlaying sediments to create "paper shales" (usually between 0.1 and 1.0 mm thick). Within these paper shales the best preserved fossils can be found in the formation. Based on this information, it has been estimated that the lake could have lasted 2,500 to 5,000 years, if the diatom couplets represent annual cycles.
Paleoflora There is a large diversity of plants in the beds of the Florissant Formation, ranging from large redwoods to microscopic pollen. The petrified forest is one of the main tourist attractions at the monument with an estimated 30 preserved stumps. They are among the largest petrified stumps in the world. The majority of the stumps have been identified as belonging to Sequoia affinis, a close relative of the modern coast redwood (S. sempervirens). These trees could have been as tall as until they were killed by lahars suffocating the oxygen supply to their roots. Dendrochronological examination of the tree rings has resulted in estimated ages of 500–700 years old when the trees were killed and buried. Some of the stumps belong to angiosperms.
The Florissant Formation is also known for its fossilized leaves and the impressions they made. There are also specimens of fruits, seeds, cones, and flowers, which are all preserved in the paper shales of the formation. Most of the leaves come from trees and shrubs. Angiosperms are the dominant paleoflora, but there are also conifers present.
Some of the cones, foliage, and pollen have been identified as coming from the sequoias. There are distinct differences in the fossilized Sequoia affinis material to the modern redwoods of California. The foliage in the fossils is thinner and the female cones are smaller than the modern examples. There have been more than 130 species of pollen that have been identified in the shale beds at Florissant. These species represent samples from many different habitats that were located near and around the lake, as well as farther up the valley.
Benthic diatoms are the dominant type of siliceous algae found in the beds. These are easy to fossilize due to their silica shells. During periods of volcanism, the influxes of silica from volcanic ash lead to blooms of algae, which lead to algal mats and the exceptional preservation of the fossils. Florissant is significant in that it is one of the earliest known examples of freshwater diatoms.
Fauna