Sr isotope ratios in carbonate-rich lacustrine strata provide highly resolved and geographically specific records of past changes in weathering and regional drainage patterns. The Eocene Green River Formation is perhaps the best documented pre-Quaternary lacustrine unit in the world; therefore, these strata are ideally suited for studying the behavior of Sr. The 87 Sr/ 86 Sr ratios measured for primary carbonate in lake expansioncontraction cycles of the Laney Member are directly linked to changing lake facies. Four distinct cycles are preserved in a 6.4 m interval of the Arco Washakie Basin No. 1 core, each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts, laminated micrite facies deposited during lake highstands, and lake-marginal dolomicrite facies, which represent subsequent lowstands. Initial 87 Sr/ 86 Sr ratios range from 0.711 99 to 0.713 31. The least radiogenic isotope compositions are associated with laminated micrite that records increased regional runoff from Phanerozoic marine carbonate and Tertiary volcanic rocks during lacustrine highstands. The most radiogenic isotope compositions are associated with dolomicrite, recording a higher proportion of runoff from basin-bounding Precambrian uplifts and the exposed lake plain during lacustrine lowstand. The observed 87 Sr/ 86 Sr variations may also be due in part to changes in differential weathering of the surrounding landscape, where drier climates are associated with decreased differential weathering and, therefore, higher 87 Sr/ 86 Sr in runoff.
40 Ar/ 39 Ar 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 ϳ10 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.
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