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Book Chapter

Perspectives on the paleolimnology of the late Eocene Florissant lake from diatom and sedimentary evidence at Clare’s Quarry, Teller County, Colorado, USA

Mary Ellen Benson
Mary Ellen Benson
Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225, USA
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Dena M. Smith
Dena M. Smith
National Science Foundation, Alexandria, Virginia 22314, USA
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Sarah A. Spaulding
Sarah A. Spaulding
U.S. Geological Survey and Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80309, USA
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Publication history
29 May 201827 December 2018


The late Eocene Florissant Formation in central Colorado is a rich and diverse continental Lagerstätte yielding well-preserved fossil assemblages from lacustrine and fluvial facies. This investigation focused on the lacustrine facies at Clare’s Quarry and used biotic and abiotic evidence to characterize aspects of the lake and processes that resulted in the accumulation and preservation of the host rock and its fossils. Autecology of modern analogs representing the fossil diatom taxa was used to augment sedimentary data in characterizing the lake, propose peripheral habitats within the catchment area, and suggest a terrestrial source for mudstone units.

The sedimentary and stratigraphic record at the study site reveals a lake with sufficient depth to allow bottom waters to remain isolated and anoxic for long periods. Sediments that accumulated in the lake produced distinct lacustrine lithofacies that are interpreted as representing at least three modes of origin: stable lake, pyroclastic, and mud turbidite sedimentation. Slow, suspension settling of fine clays and volcanic ash into a moderately deep, stable lake resulted in laminated shales. These laminated shales contain frustules of diatoms from planktic and benthic lake habitats; diatoms transported into the lake from streams and wetlands; fish, mollusks, ostracods, and insects; and plants from marginal and upslope environments. Intermittent volcanic eruptions produced air-fall ash and granular tuff that accumulated as interbeds within the lake shales. Periods of stable lake sedimentation were frequently interrupted by rapid influxes of suspended fine clays, perhaps as mud-dominated turbidites that prograded into the lake at intervals of high runoff triggered by climatic, volcanic, or tectonic events.

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GSA Special Papers

From Saline to Freshwater: The Diversity of Western Lakes in Space and Time

Geological Society of America
ISBN electronic:




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