STRATIGRAPHY OF THE UPPER EOCENE FLORISSANT FORMATION, COLORADO

The Florissant Formation is a 74 m thick sequence of shale, tuffaceous mudstone and siltstone, tuff, arkosic and volcaniclastic sandstone and conglomerate, that contains the most diverse late Eocene leaf and insect biotas in North America. The formation was deposited in an upland paleovalley cut into the Proterozoic Pikes Peak Granite and the upper Eocene Wall Mountain Tuff (age of 36.7 Ma). The formation includes a lower shale, a lower mudstone, the middle shale, the “caprock” conglomerate, an upper shale, and the upper pumice conglomerate. These units represent a combination of two lacustrine events, two episodes of fluvial deposition and an episode of lahar deposition into the Florissant lake. Single crystal 40Ar/39Ar laser fusion dating of sanidine crystals in the upper part of the formation have yielded an age of 34.07+/-0.10 Ma. Fossil mammals from the fluvial parts of the Formation include the horse Mesohippus and large brontotheres, indicating the Chadronian Land Mammal Age.