LIPID ANALYSIS OF THE MIDDLE TO LATE JURASSIC (BATHONIAN TO OXFORDIAN) SUNDANCE GROUP, SHELL, WY

The Sundance Group at Red Canyon Ranch outside of Shell, WY is a 92 m stack of epicontinental shales, limestone, and mixed system carbonate-siliciclastic high energy sequences. Despite the abundance of fossils and unique sedimentology, little chemostratigraphic analysis has been conducted on these Middle Jurassic epicontinental deposits. In 2005, meter interval samples were taken for stable isotope analysis of the bulk organic matter. As expected for this time period, carbon isotope values showed a strong C3 plant signature (d13C = -23.8 to -29.1‰). The evidence of organic carbon found in the previous analysis led to an analysis of the lipid content within these shale units. The limestones and mixed system units were omitted from the lipid analysis because no evidence of organic carbon was found in the original analysis.

In 2006 - 2007, a 50 g sub-sample was obtained from each of 78 shale samples. These sub-samples were ground individually and lipid extracted using a 2:1 chloroform:methanol solution. Extract colors ranged from transparent to a dark translucent yellow with darker colors indicative of high lipid content. These samples were dried down using a rotovap and centrivap system. The final lipid products were various shades of yellow, orange, purple and black. Total lipid content of these samples ranged from 0.0002 wt % to 0.103 wt %. The lipid samples show a stable isotope range of -27.00 to -37.59‰.

The greater range in lipid versus bulk organic carbon d13C values may result from mixing of multiple isotopically-distinct carbon sources (i.e., aquatic and terrestrial primary producers) in these sediments. Future work will focus on examining the unique lipid composition of different carbon sources (i.e., lipid fingerprinting) to test this interpretation. The carbon isotope values from the extracted lipids demonstrate the potential for using stable isotope analysis of multiple classes of organic molecules to complement paleoenvironmental interpretations from bulk organic carbon isotope values.