Oceanic Anoxic Event 3 (Coniacian-Santonian, Late Cretaceous) in the Western Interior Seaway

The late Turonian-earliest Campanian Niobrara Cycle records a second-order sea level transgression and prolonged highstand in the Western Interior Seaway (WIS) of North America. Evidence for third-order sea-level cycles superimposed on the Niobrara includes strandline migrations of the western shore and the widespread distribution of carbonate-rich strata across much of the seaway, suggesting that these cycles are eustatic in origin.

Oceanic Anoxic Event 3 was a prolonged interval of organic carbon burial during Coniacian-Santonian time. OAE 3 differs from other Cretaceous anoxic events because it is not represented by a single, short-lived black shale event, and it is regionally restricted to the equatorial Atlantic (ODP Leg 159), southern Caribbean (e.g., La Luna Formation and ODP Leg 207), and the WIS (Smoky Hill Member, Niobrara Formation).

Analysis of the molecular and isotopic composition of the Coniacian-Santonian Smoky Hill Member of the Mancos Shale in southwestern Colorado (Mesa Verde) reveals a strong correlation between percent total carbonate and inferred sea level changes. Numerous first occurrences of benthic and nektonic mollusks are associated with highstand, while numerous last occurrences are associated with third-order regression. Total organic carbon and elevated bulk C/N ratios lead carbonate abundance, suggesting a decoupling between peak transgression and maximum OM burial. Relative abundances of presumed low-oxygen tolerant planktic (Heterohelix) and benthic (Neobulimina) foraminifers also track TOC. Molluscan diversity is very low through the two most TOC-rich intervals in the upper Coniacian and middle Santonian. Bulk geochemical and molecular characteristics do not indicate large changes in OM source associated with transgression/regression, but rather support denitrification during transgression, and oxygenation and enhanced degradation of terrestrially-derived OM during regression. Thus, the eustatic transgressive third-order cycles expressed in the WIS were likely associated with an expanded oxygen minimum zone related to conditions of higher productivity and a deeper, more stably stratified water column.