PROVENANCE OF VOLCANIC ASH AT FLORISSANT FOSSIL BEDS NATIONAL MONUMENT

Florissant Fossil Beds National Monument is a well-known locality for insect and plant fossils and well-preserved stumps of redwood trees. The spectacular preservation is a result of the volcanic activity in the area. A lahar (volcanic mudflow) blocked a drainage creating Lake Florissant. Erupting volcanoes and weathering of existing ash deposited silica into the lake, resulting in increased mortality rates of plants and animals. The remains of these organisms fell to the bottom of the lake and were buried, forming the Florissant Formation.

The Florissant formation consists of six units, the Lower Shale, the Lower Mudstone, the Middle Shale, the Upper Shale, the Caprock Conglomerate, and the Upper Pumice Conglomerate. Five of the six units of the Florissant Formation are volcaniclastic; the lower mudstone is not volcaniclastic but is overlain by a lahar. The shale units of the Florissant Formation consist of thin, fossiliferous shales interbedded with tuffaceous material.

The source of the volcanic material at Florissant has been a topic of discussion for many years. Notable contenders include: Grizzly Peak, a volcano or volcanoes in the San Juan field, Mt. Antero, Mt. Princeton Batholith, and a volcano or volcanoes in the Guffey volcanic field. Located approximately ten miles to the west of Florissant Fossil Beds National Monument, the Guffey Volcanic Field is an obvious source of volcanic material.

Samples of the shales and the Caprock Conglomerate have been collected and are being analyzed in order to determine the source of the volcaniclastic material. Volcanic rocks from the Guffey Volcanic Center have been sampled in order to characterize the rocks and use as a comparison for the volcaniclastic material found within the Florissant Formation.