Effects of climate and sea-level change on sedimentation and benthic oxygen in the Cretaceous Greenhorn Sea, North America

This dissertation reports the effects of high-frequency climate change and of lower frequency, third-order sea-level change on sedimentation and depletion of benthic oxygen in the prodeltaic environment of the epeiric Cretaceous Western Interior Seaway. While several workers have studied strata belonging to the pelagic parts of the seaway and have proposed a variety of models explaining depletion of benthic oxygen, little is known about changes in benthic oxygen levels over time in the shallower, prodeltaic environment. This study attempts to remedy this situation by investigating the signatures of benthic oxygen levels preserved in strata belonging to primarily the prodeltaic facies of the basin.;Previous studies have documented rhythmically alternating beds of limestone and shale in pelagic deposits of Cenomanian-Turonian age that have been widely attributed to climatic cyclicity in the Milankovitch band. Whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial. The first part of this study tests the dilution model by investigating deposits along a 110 km, roughly shore-normal transect, within 130 km of the paleoshoreline. In each section, quartz sand and coarse silt content varies antithetically with calcium-carbonate content, suggesting that the limestone/marlstone beds accumulated during times when rates of terrigenous sediment transport to offshore sites were relatively slow. Conversely, the shaly beds were deposited during times of enhanced sediment flux. Further, trends in bioturbation and fecal-pellet preservation suggest that the more shaly beds were deposited under less oxic conditions than were the more marly beds. Thus, results suggest that climate cyclicity did modulate coarse sediment input and depletion of benthic oxygen in the prodeltaic environments of the seaway.;The second part of this study tests the hypothesis that the level of benthic oxygenation in the prodeltaic environment of the seaway was affected by oxygen-poor water mass(es) at times of maximum transgression and development of condensed sections on third-order time scales. A total of three such third-order, sea-level cycles were studied. Results essentially support the hypothesis, in that they provide evidence for depletion of benthic oxygen at the time of maximum flooding. Depletion in benthic oxygen is evidenced by a simultaneous decrease in the intensity of bioturbation, increase in the content of labile organic carbon, decrease in the Mn/Fe ratio and an increase in the Ni/Al, Cr/Al, and Zn/Al ratios.;Results indicate that condensed sections associated with third-order maximum transgressions in the prodeltaic environments of the seaway are especially prone to enhanced burial of labile organic carbon and production of source rocks on a regional scale.