IT WAS THE BEST OF TIMES. IT WAS THE WORST OF TIMES. TWO VIEWS OF NORTH AMERICAN CONTINENTAL DIATOM BIOCHRONOLOGY

In one of his many monographs on the economic geology of the western U.S., Waldmar Lindgren stated that “Infusorial earth, rich in the remains of diatoms, has been found at numerous localities . . . . These vegetable remains have no value in determining the age of the strata in which they occur, and hence it seems unnecessary to devote further space to this subject.” Despite his less than favorable opinion of the applicability of diatoms to questions of biochronology, Tertiary deposits of diatomaceous sediments record lacustrine histories that extend thousands, and in some cases, millions of years. Geologists have long been interested in establishing a biochronology for Neogene freshwater diatoms in order to correlate the interbasinal events in the western U.S., and to place those events into a global context.

The geologically ephemeral and spatially limited nature of lacustrine systems leaves a record, except in rare cases (e.g., Lake Baikal), of insufficient longevity to construct a biostratigraphic scheme. Early attempts to construct a Neogene biochronology were based on the Lyellian concept of percentages of extant taxa within a given assemblage. This technique has provided less than satisfactory results due to: 1) a lack of understanding of the diversity and distribution of modern freshwater diatoms; and 2) the fluidity of diatom taxonomic concepts.

Over the past 30 years, two schemes have been developed in an attempt to provide a chronologic framework for the deposition of diatomaceous lacustrine sediments in the western U.S. The first is based largely on benthic diatoms. This method utilizes a global taxonomic database derived from the literature and has been validated through extensive collecting and study of museum collections. The second method utilizes relatively few planktic centric taxa whose evolutionary history has been calibrated by radiometric or vertebrate paleontological techniques. Each method has its limits, and a combination of the two methods can provide the highest resolution biochronology for the Neogene basins of the western U.S., but a great deal of work has yet to be done.