Florissant Fossil Beds National Monument

The Florissant Fossil Beds National Monument is a national monument located in Teller County, Colorado. The location is famous for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales of the Florissant Formation. 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 an 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.Fossilized Redwood stump with tree rings visible.Excavated Redwood stump with wayside plaque.Redwood stump with young pine. The Florissant Fossil Beds National Monument is a national monument located in Teller County, Colorado. The location is famous for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales of the Florissant Formation. 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 an 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 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, that is now 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. Paleontologists followed soon after to collect fossils for research.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. 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. The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the Laramide Orogeny. 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: 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, 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. Most of the rocks that were deposited after the Oligocene and before the Pleistocene have been eroded away. 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. 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 would have eruptions that included domes, lava flows, and pyroclastic events. Ash from these events would be deposited throughout the area and lahars would flow down the valleys. The ash that settled would create the tuff, and the lahars would form the mudstones and the conglomerates that are found in the Florissant formation.The Florissant was deposited in a paleovalley after one of the lahars dammed the paleovalley. The resulting lake would become as large as 36 km2. There were two cycles of lake environments. The first one created the lower shale unit, while the second lake would create 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 remnants of the volcanoes that permeated the area. Ironically, 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. 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. It is estimated that around 30 stumps are preserved. They are among the largest petrified stumps in the world. The majority of the stumps have been identified as similar to modern sequoias, or redwoods. These trees could have been as tall as 60 meters (= 198 feet) until they were killed by lahars suffocating the oxygen supply to their roots. By looking at the tree rings, some of the trees have been estimated to be 500–700 years old when they died. Some of the stumps belong to angiosperms.