Effects of Water-residence Time on the Isotopic Evolution of an Eocene Closed- Basin Lake Complex

Pitman J. K., R. D. Norris, L. S. Jones, and Corfield R. M.

Carbonate-bearing oil shales in the Eocene Green River Formation, Colorado, Utah, and Wyoming, provide a unique opportunity to study the carbon and oxygen isotopic evolution of an ancient organic-productive saline lake complex that was the site of nearly continuous lacustrine deposition over a 10 m.y. time span. The isotopic characteristics of Ca-Mg-Fe carbonate minerals in the formation exhibit distinct relations reflecting variable depositional and hydrologic processes, and the effects of early diagenesis. In Colorado and Wyoming, the saline-mineral facies in the middle part of the formation, deposited during a prolonged period of evaporation and lake shallowing, displays large differences in covariation between carbon and oxygen whereas oil-shale facies in Utah and, ab ve and below the evaporite sequence in Colorado and Wyoming, tend toward good covariance. Intervals displaying overall high degree of covariation (r >.7) indicate periods when the water-residence time of the lake system was long and the lakes may have been hydrologically closed. In contrast, sequences such as the saline facies showing large variations in carbon and oxygen are consistent with a lake regime subject to significant fluctuations in water-residence time whereby closed-lake conditions alternated with short-lived lakes. Short water-residence-time waterbodies may have reduced to playas in parts of the Wyoming sequence. A striking feature of the isotope profiles in Colorado and Wyoming is a long-term trend toward more negative ^dgr¹³C values in the saline-mineral fac es followed by a reversal to more positive ^dgr¹³C values in the overlying oil-shale facies. This suggests a change in the source or degradation rate of organic carbon that supplied the lake dissolved inorganic carbon which could reflect alterations in regional climate.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California