ABSTRACT 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.

Abstract Diatoms are unicellular, photosynthetic, eukaryotic organisms often classified as among the algae. There are around 15 000 known species, but many more have yet to be described. Their uniqueness lies in the siliceous covering of the cell, each being encased in a pair of silica valves. Silica is virtually inert and indestructible, so after the organism’s death the silica parts remain. The silica parts provide information for the classification of these diverse organisms. Diatoms have been used in forensic science in a variety of ways, the most frequent being the diagnosis of death by drowning. When a person drowns, water will enter the lungs and then enter the bloodstream through ruptures in the peripheral alveoli before being carried to the other organs such as the liver and heart. Naturally, the microscopic contents of the water, which will include diatoms, will pass into the blood as well. The detection of diatoms in the organs can contribute to a diagnosis of death by drowning, a process referred to as the ‘diatom test’ We will discuss this test in more detail, illustrating our discussion with real examples.