SEDIMENTARY SULFUR GEOCHEMISTRY OF THE PALEOGENE GREEN RIVER FORMATION, WESTERN USA - IMPLICATIONS FOR INTERPRETING DEPOSITIONAL AND DIAGENETIC PROCESSES IN SALINE ALKALINE LAKES

The sulfur geochemistry of the lacustrine Paleogene Green River Format ion (Colorado, Utah, and Wyoming, USA) is unlike that of most marine a nd other lacustrine rocks. Distinctive chemical, isotopic, and mineral ogical characteristics of the formation are pyrrhotite and marcasite, high contents of iron mineral sulfides strikingly enriched in S-34, cy clical trends in sulfur abundance and deltaS-34 values, and long-term evolutionary trends in deltaS-34 values. Analyses that identified and quantified these characteristics include carbonate-free abundance of o rganic carbon (0. 1 3-47 wt%), total iron (0.31-13 wt%), reactive iron (>70% of total iron), total sulfur (0.02-16 wt%), acid-volatile monos ulfide (S(Av)), disulfide (S(Di)>70% of total sulfur), sulfate (S(SO4) ), and organosulfur (S(Org)); isotopic composition of separated sulfur phases (delta S-34(Di,Av) up to +49 parts-per-thousand); and mineralo gy, morphology, and paragenesis of sulfide minerals. Mineralogy, morph ology, deltaS-34(Di,Av) and deltaS-34(Org) have a distinctive relation , reflecting variable and unique depositional and early diagenetic con ditions in the Green River lakes. When the lakes were brackish, dissim ilatory sulfate-reducing bacteria in the sediment produced H2S, which initially reacted with labile iron to form pyrite framboids and more g radually with organic matter to form organosulfur compounds. During a long-lived stage of saline lake water, the amount of sulfate supplied by inflow decreased and alkalinity and pH of lake waters increased sub stantially. Extensive bacterial sulfate reduction in the water column kept lake waters undersaturated with sulfate minerals. A very high H2S :SO4 ratio developed in stagnant bottom water aided by the high pH tha t kinetically inhibited iron sulfidization. Progressive removal Of H2S by coeval formation of iron sulfides and organosulfur compounds cause d the isotopic composition of the entire dissolved sulfur reservoir to evolve to deltaS-34 values much greater than that of inflow sulfate, which is estimated to have been +20-parts-per-thousand. A six-million- year interval within Lake Uinta cores records this evolution as well a s smaller systematic changes in deltaS-34, interpreted to reflect appr oximately; 100,000-year lake-level cycles. When porewater was exceptio nally reducing, unstable FeS phases eventually recrystallized to pyrrh otite during diagenesis. A much later reaction related to weathering a ltered pyrrhotite to marcasite.