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10 k.y. depositional cyclicity in the early Eocene: Stratigraphic and ⁴⁰Ar/³⁹Ar evidence from the lacustrine Green River Formation

¹ Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA

⁴⁰Ar/³⁹Ar dating of sanidine from two interbedded tuffs reveals that the maximum average duration of depositional cycles in the Wilkins Peak Member, Green River Formation, was 10 k.y., marking the first time that subprecessional cycles have been recognized in lacustrine strata. The origin of these cycles is uncertain, but may relate to a nonlinear climatic response to orbital forcing of insolation. Alternatively, regional tectonic and geomorphic controls on drainage stability may have promoted autocyclic delivery of sediment to the lake. Owing to an interaction between basin-floor relief and varying amplitudes of lake expansion, only one-third of the cycles identified near the basin center are present near the basin margin. This spatial variability in the temporal completeness of the stratigraphic record is not apparent from examination of individual localities, indicating that studies based on time-series analysis from other lacustrine systems may need reevaluation.

Key Words: lacustrine • cyclicity • Eocene • Green River Formation

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