Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, U.S.A.

Analyses of 21 samples collected from a core of the 52.8-cm-thick Stark Shale Member of the Dennis Limestone in Wabaunsee County, Kansas, demonstrate four cycles with two-orders-of-magnitude variations in contents of Cd, Mo, P, V and Zn, and order-of-magnitude variations in contents of organic carbon, Cr, Ni, Se and U. The observed variability in amounts and/or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water-column stratification and the resulting absence/presence of dissolved O2 or H2S. High Cd, Mo, U, V, Zn and S contents, a high degree of pyritization (DOP) (0.75–0.88), and high high V(V + Ni) (0.84–0.89) indicate the presence of H2S in a strongly stratified water column. Intermediate contents of metals and S, intermediate DOP (0.67–0.75) and intermediate V(V + Ni) (054–0.82) indicate a less strongly stratified anoxic water column. Whereas, low metal contents and low V(V + Ni) (0.46–0.60) indicate a weakly stratified, dysoxic water column. High P contents at the top of the organic-matter-rich intervals within the Stark Shale Member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions. Relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified H2S-bearing water column indicates a combined halocline-thermocline with the fresher near-surface water the transport mode for the terrestrial organic matter. The predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline. Relatively low amounts of degraded, hydrogen-poor organic matter characterize intervals deposited in a weakly stratified, dysoxic water column. The inferred variability in chemistry of the depositional environments may be related to climate variations and/or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member.