The western Snake River basin of Idaho and Oregon contains a thick sequence of continental sediments that range in age from at least middle Miocene to Holocene. Some of these sediments are diatomaceous and contain distinctive forms that are biostratigraphically useful. The Sucker Creek Formation (Barstovian-middle Miocene) can be characterized by the occurrence of Coscinodiscus(?) miocaenicus. The Poison Creek Formation of Barstovian(?)-Clarendonian age (middle to early late Miocene) contains Coscinodiscus(?) sp. cf. C. gorbunovii v. gorbunovii. The upper Miocene (Hemphillian) Chalk Hills Formation is highly diatomaceous and is characterized by the presence of primitive forms of Stephanodiscus at its base and by extinct forms of Cyclotella in its upper part. Ash correlations within the Chalk Hills Formation demonstrate that fossil freshwater diatoms can be used to time-correlate lacustrine sediments. The Glenns Ferry Formation of Pliocene (Blancan) age is dominated at its base by species of Cyclotella and Stephanodiscus. Benthic and epiphytic diatoms are most common in the upper Glenns Ferry Formation. The lower to middle Pleistocene (Irvingtonian) Bruneau Formation contains modern species of Stephanodiscus, Cyclotella, and Melosira. Careful morphologic comparisons of these and other diatoms of the western Snake River basin with similar forms in other regions may shed hght on the feasibility of a model of worldwide Neogene lacustrine diatom biochronology.

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