Recurrent depletion of benthic oxygen with 4th-order transgressive maxima in the Cretaceous Western Interior Seaway

A study was designed to investigate the relationship between depletion of benthic oxygen and 4th-order sea-level change within the Cretaceous Western Interior Seaway of North America. Three 4th-order sea-level change cycles were studied in strata belonging to the Tropic Shale and the Tununk Shale Member of the Mancos Shale in Utah. These sedimentary deposits accumulated within prodeltaic environments. h, strata belonging to all three 4th-order cycles exhibit evidence for depletion of benthic oxygen at times of peak 4th-order transgression and development of condensed sections. The evidence includes decreased intensities of bioturbation, increased CrAl, NiAl, and ZnAl ratios, and decreased MnFe ratios. The data also suggest that depletion of benthic oxygen in the distal section preceeded that in the more proximal one and thus may reflect episodic expansion of an oxygen-depleted water mass(es) from the pelagic to the prodeltaic environment of the seaway. Evidence for subsequent increases in benthic oxygen level with the onset of 4th-order regressions is interpreted to reflect either aggradation of the seafloor above the oxygen-poor water due to sediment accumulation or contraction of the oxygen-depleted water mass from the prodeltaic part of the seaway, or a combination of the two. nic C ratios of strata interpreted to have been deposited under conditions of depleted benthic oxygen are distinctly below 0.4 and thus were not useful for recognition of paleo-oxygen levels in the prodeltaic environment. This study highlights the importance of depletion of benthic oxygen as a mechanism for enhancing preservation of organic carbon in condensed sections on a hundred thousand year time scale. Our results underline the potential for source rock formation in similar prodeltaic deposits of other epicontinental seaways during periods of relatively high sea level and global depletion of benthic oxygen.