DIFFERENT LAKE TYPES, SAME ORGANIC MATTER ACCUMULATION RATE: IMPLICATIONS FOR CARBON BURIAL IN THE EOCENE GREEN RIVER FORMATION

Fischer assay analysis of retort oil yield is based on continuous, homogenized samples, and thus is ideally suited for estimating overall organic richness of a given stratigraphic interval. Analysis of the ERDA White Mountain #1 core in the central Green River basin indicates that balanced-fill lake deposits of the Tipton and Laney Members contain on average 2-4 times more organic matter than underfilled lake deposits of the Wilkins Peak Member. Because bulk organic matter type (based on Rock Eval HI) is similar in all three members, these differences in Fischer assay gallons/ton are inferred to reflect proportional differences in total organic carbon content. However, recent high-resolution ⁴⁰Ar/³⁹Ar dating of interbedded tuff horizons reveals that despite the large differences in organic richness of these members, long term organic matter accumulation rates remained essentially unchanged throughout their deposition. Higher rates of inorganic (carbonate, evaporite, and siliciclastic) sedimentation during deposition of the Wilkins Peak Member are chiefly responsible for this observation. Although maximum %TOC and Rock Eval S2 are comparable for profundal facies in the Tipton, Wilkins Peak, and Laney members, oil shale beds in the Wilkins Peak Member are thinner and are separated by relatively thick, rapidly deposited intervals of organic carbon-poor littoral and lake plain (palustrine) facies. Wilkins Peak Member deposition was thus characterized by a rapidly changing succession of environments in which lakes frequently recurred but were short-lived, in contrast to the more stable, longer-lived lakes of the Tipton and Laney Members. Organic matter production was promoted by evaporative concentration of nutrients in relatively shallow, well-lit lakes, and preservation may have been enhanced by rapid burial. Another consequence of rapid environmental change is that the relatively coarse vertical resolution of Fischer assay analyses (~30 cm) is insufficient to fully record all of the lacustrine depositional cycles present in the Wilkins Peak Member (thickness 0.1-2.0 m).