Alan R. Carroll¹, Jeffrey T. Pietras¹, Meredith K. Rhodes¹, Clark M. Johnson¹, Brian L. Beard¹

(1) UW-Madison, Madison, WI

ABSTRACT: Application of Advanced Provenance Tools to Lacustrine Stratigraphy: Strontium Isotopes in the Eocene Green River Formation, Wyoming

Strontium isotopic analysis of carbonate and evaporite lithologies represents a powerful technique for tracing the provenance of lake waters, and therefore provides a crucial linkage between lacustrine stratigraphy and processes shaping the surrounding landscape. Within the Green River Formation, changes in 87Sr/86Sr occur on at least two stratigraphic scales. On the larger scale, 87Sr/86Sr decreases upward from the Tipton Member (0.7146-0.7150) to the Wilkins Peak Member (0.7126-0.7142). The sequence boundary separating these members marks a shift from balanced-filled to underfilled basin conditions. We attribute the decrease in 87Sr/86Sr to the approximately coeval onset of Absaroka volcanism to the north. Runoff from this area would have had lower 87Sr/86Sr ratios than runoff from the much more radiogenic Precambrian rocks that surround the greater Green River basin. On a smaller scale, 87Sr/86Sr varies greatly within individual 1-3 m lake expansion-contraction cycles in the Laney Member. The highest 87Sr/86Sr ratios occur at the base of each cycle, in trangsressive ooid and ostracod grainstone and stromatolite facies. We hypothesize that reworking of older, exposed lake-plain deposits resulted in the release of radiogenic Sr through cation exchange with clays. The lowest 87Sr/86Sr ratios occur within laminated calcimicrite deposited during maximum lake expansion, when the lake plain was submerged and runoff from the Absaroka volcanic rocks was at a maximum. 87Sr/86Sr ratios are slightly higher again in mud-cracked dolomicrite recording lake desiccation, but appear to be primarily controlled by concentration of residual Sr from the previous highstand. These results indicate that the interpretation of long-terms trends in lakewater isotopic composition from cyclic lacustrine carbonates must be based on comparison of similar facies.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado