IDENTIFYING THE EARLY EOCENE CLIMATIC OPTIMUM AT RAVEN RIDGE, NORTHEASTERN UINTA BASIN, COLORADO-UTAH, USING TOTAL ORGANIC CARBON DATA FROM SEDIMENT FOR CHEMOSTRATIGRAPHIC CORRELATION

The intertonguing fluvial Colton and lacustrian Green River Formations at Raven Ridge, on the northeastern edge of the Uinta Basin, Colorado/Utah, have produced an extensive mammalian fauna representing 64 genera from 34 families, including index taxa such as Omomys, Smilodectes, Homogalax, and Haplomylus. The fauna suggests an age range of early Eocene (mid-Wasatchian or Wa5, ~53.5mya) through early middle Eocene (mid-Bridgerian or Br2, ~48.5mya), an interval hypothesized to coincide with the onset, peak, and subsequent decline of the Early Eocene Climatic Optimum (EECO).

In order to stratigraphically localize the EECO at Raven Ridge, more than 300 sediment samples were collected through several ~900m stratigraphic sections and analyzed for Total Organic Carbon (TOC) content in order to produce a carbon curve for correlation with established marine carbon and oxygen datasets. Resulting δ¹³C values vary between -30‰ and -20‰, values that are within the expected range for a typical Eocene C₃ plant dominated ecosystem, and are comparable to TOC data from the Paleocene-Eocene Thermal Maximum in Wyoming’s Bighorn Basin. The Raven Ridge TOC data show a more negative δ¹³C excursion lower in the section (Wa5-Wa7 biochrons based on the mammalian fauna) with a minimum value of -29.67‰, followed by a more positive excursion higher in the section (Wa7-Br1 biochrons) with a maximum value of -20.51‰. These δ¹³C results are consistent in order of occurrence and amplitude with δ¹³C excursions observed in marine carbon curves associated with the EECO, and suggest the EECO started at Raven Ridge during Wa6, peaked during Wa7, and then temperature declined during Br1 and into Br2.

The biostratigraphic and geochemical data that estimate the timing of the EECO at Raven Ridge are consistent with the results from previous biostratigraphic and paleobotanical studies of the EECO in the Wind River and Green River basins. Further, the Raven Ridge δ¹³C data provide an additional independent means of inter-basin correlation for North American late early and early middle Eocene strata, and indicates the Raven Ridge paleofauna is ideally situated for investigations into patterns of faunal change throughout the onset, peak, and decline of an extended interval of global warming.