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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association


Geology of Yellowstone Park Area; 33rd Annual Field Conference Guidebook, 1982
Pages 73-101

Geology of the Lamar River Formation, Northeast Yellowstone National Park

William J. Fritz


The Eocene Lamar River Formation exposed in the Lamar River Valley of Yellowstone National Park consists chiefly of volcaniclastic sediments deposited between two sub-parallel chains of andesitic stratovolcanoes. Recognizable facies include mudflows, braided streams, meandering streams, and lakes. Less than 10% of the formation consists of basaltic dikes, sills, and flows, trachyandesite flows, and ignimbrites. The depositional environment was, thus, very similar to that of the Mount St. Helens, Washington area.

A very diverse flora of temperate to tropical logs, stumps, leaves, needles, cones, and pollen is preserved throughout the formation. The mixture of habitat indicators was caused by transportation of plant parts, including some upright stumps, from various elevations within the depositional basin.

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