Neogene and Quaternary lacustrine diatom biochronology, Western USA

Neogene and Quaternary lacustrine diatomaceous deposits are numerous in the western United States, particularly in the Great Basin. Some of these sediments are interbedded with volcanic rocks that have been dated radiometrically or by the fssion track method. Fossil lacustrine diatom floras can thus be arranged in geochronological order. By this means, a biochronological pattern of lacustrine diatom evolution has emerged: obligate nonmarine Actinocyclus Ehrenberg (Family Hemidiscaceae) appeared in the early Miocene and attained maximum diversity in the middle middle Miocene. A single species of non-marine Actinocyclus persisted to the end of the middle Miocene. Only two species of marine Actinocyclus survive in freshwater today. Genera belonging to the Family Thalassiosiraceae typify the late Miocene to Holocene lacustrine diatom biochronology. Mesodictyon Theriot et Bradbuty nom. prov. is restricted to the late Miocene. Cyclotella (Kutzing) Brebisson probably fist appeared during the middle Miocene and diversified rapidly during the Pliocene. Stephanodiscus Ehrenberg and Cyclostephanos Round appeared during the latest Miocene and species radiated in the Pliocene and Pleistocene. These forms are often abundant, well-preserved, easily identifiable, and widespread, and can be used to correlate and date lacustrine diatomaceous sediment in the western United States.