ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 39-61 PALAE0 Recurrent depletion of benthic oxygen with 4th-order transgressive maxima in the Cretaceous Western Interior Seaway Parvinder S. Sethi *, Elana L. Leithold Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Box 8208, Raleigh, NC 27695-8208, USA Received 15 August 1995; revision 11 July 1996; accepted I 1 July 1996 Abstract 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. In Utah, 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 Cr/A1, Ni/A1, and Zn/A1 ratios, and decreased Mn/Fe 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. S/Organic 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. Keywords: Cretaceous; organic carbon; periodicity; Western Interior; metals; sulfur 1. Introduction Level of benthic oxygenation is one of the primary controls that can affect the concentration of total organic carbon preserved in sediments (e.g., Calvert, 1983; Emerson, 1985; Stein, 1991; Ingall et al., 1993). Therefore, recognition of the distribution of oxygen-poor intervals in the strati- graphic record may enhance our ability to predict the distribution of source rocks in time and space. Recently, workers have begun to utilize concepts of sequence stratigraphy in order to probe relation- * Corresponding author. Present address: Department of Geology, Radford University, Box 6939, Radford, VA 24142, USA, 0031-0182/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0031-0182(96)00104-6