Abstract

The deposits of at least four distinct deep-lake cycles of Pleistocene Lake Bonneville are recognized on the basis of isoleucine epimerization in fossil gastropod shells. Deposits of the earliest recognized cycle predate the Lava Creek ash (600,000 yr B.P.). Three younger cycles of Lake Bonneville are represented by lacustrine deposits of the Pokes Point Alloformation (∼200,000 yr B.P.), the Little Valley Alloformation (∼140,000 yr B.P.), and the Bonneville Alloformation (∼30,000 to 11,000 yr B.P.). Analyses of fossil shells from the Main Canyon Formation in the Gentile Valley of southern Idaho support the contention that those deposits span an interval of time from before 2 m.y. B.P. to sometime between the last two cycles of Lake Bonneville, perhaps as recent as 30,000 yr B.P. It is concluded that isoleucine epimerization in non-marine gastropods can provide a useful stratigraphic tool for the study of Quaternary lacustrine deposits.

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