page_title: Florissant Fossil Beds National Monument
section_title: Geology
hierarchical_section_title: Florissant Fossil Beds National Monument / Geology
caption_attribution_description: The Florissant Fossil Bed 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. Based on argon radiometric dating, the formation is Eocene (approximately 34 million years old [1]) 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 algal bloom. As the algae fell to the bottom of the lake, any plants or animals that had recently died were preserved by the algal mats. en.wikipedia.org/wiki/Florissant_Fossil_Beds_National_Mon... en.wikipedia.org/wiki/Wikipedia:Text_of_Creative_Commons_...
context_page_description: 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 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.
context_section_description: 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.
caption: 2013 aerial photo
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