THE ROLE OF CLIMATE AND PALEOCEANOGRAPHY IN DEAD ZONE DEVELOPMENT OF THE JURASSIC SUNDANCE SEA

The Middle to Late Jurassic (168 to 160 Ma) Stockade Beaver (SB) and Redwater Shales (RS) of the Sundance Formation of the western United States record transgressive incursions of an epiric seaway on the North American craton. Both units consist of claystone and siltstone deposited in a shallow, disoxic marine environment. A combined sedimentological, ichnological, stratigraphic, and geochemical analysis of RS and SB sections shows how oxygen content in the Sundance Sea was constrained by changes in paleoceanography and paleoclimate. The SB was deposited in a very shallow marine sea, as evidenced by its relatively thin sections (generally < 10 m) and robust Cruziana Ichnofacies. Shallower siltstones of the SB contain abundant Rhizocorallium and fossils which indicate well oxygenated environments. We conclude that the deeper marine facies of the SB correspond to deep shelf disoxia located in restricted subbasins within the Sundance Sea. In contrast, the RS units are much thicker and generally contain less silt and bioturbation than the SB, implying deposition in a deeper neritic setting. Faunal abundance and bioturbation decrease in more proximal settings in the RS indicative of more anoxic environments in shallower waters, a conclusion confirmed with geochemical analyses. This suggests that, unlike the SB, the RS was characterized by shallow marine anoxia and became more oxygenated in deeper offshore settings. The RS therefore represents a shallow marine dead zone setting. We hypothesize that a change from an arid to a more wet, temperate climate in RS time fostered higher rates of continental weathering and an influx of nutrients into the Sundance Sea. Higher nutrients led to high rates of paleoproductivity and development of shallow marine anoxia much in the same way as modern dead zones. In the upper sections of the RS, the seaway rapidly oxygenated. Eolianites in the overlying Morisson Formation show a shift back to drier conditions by the end of the RS, possibly explaining the rapid oxygenation of the seaway. The rapid shift to oxygenated waters within the RS may indicate recent shelf anoxia could be ameliorated by a similar reduction in nutrients, providing possible mitigation strategies for modern dead zones.