Isotopic investigation of oxygen from carbonate sediments, Laney Member, Green River Formation, Wyoming

Amalia Doebbert, Department of Geology and Geophysics, University of Wisconsin, Madison, Madison, Wisconsin, USA, [email protected]

The Laney Member of the Eocene Green River Formation of Wyoming contains two distinct lacustrine phases.  Lithological change clearly denotes a rapid transition from balance-filled to overfilled conditions between the Lower LaClede bed and Upper LaClede bed of the Laney.  Across the Lower-Upper LaClede transition, preliminary oxygen isotope data from carbonates in two cores has shown a drop of approximately 6‰ in less than 30 meters.  Based on estimated sedimentation rates, this represents only 200-300 kyr. 

The rapidity of this shift suggests that it is unlikely to be a product of uplift adjacent to the basin, the interpretation of similar-magnitude isotopic shifts that has been made in several recent studies of intermontane basins.  The goal of this study is to generate additional oxygen isotope data from the section of interest and place it in a stratigraphic context to develop an understanding of the event that caused the isotopic shift.  New understanding of influences on oxygen isotopes gained by this study may allow better interpretation of isotopic datasets in the future.

During summer 2005, four stratigraphic sections and one core section were measured in detail and sampled.  These sections are stratigraphically immediately above, encompassing, or immediately below the Lower-Upper LaClede transition.  Further samples were also taken from one previously measured section and the two original cores.  Locations were chosen with a goal of broad areal distribution in the basin.  Oxygen isotope data obtained from these samples will clarify the behavior throughout the basin of isotopes before, during, and after the transition.