Distinguishing tectonic versus eustatic controls in Turonian strata of the Western Interior Seaway

It is hypothesized that high-frequency, low-amplitude sea-level changes during the Turonian ultra-greenhouse period were caused by Milankovitch-driven freeze/thaw cycles in ephemeral ice sheets in Antarctica or aquifer eustasy. Here, we use two-dimensional airy back-stripping of five Turonian formations distributed along 1500 km of depositional strike from Alberta, Canada, to New Mexico, USA, within the Cretaceous Western Interior Seaway of North America, to isolate eustatic and tectonic effects from accommodation produced by sediment compaction and subsidence. The timing of eustatic cycles is constrained by newly acquired ⁴⁰Ar/³⁹Ar sanidine ages and regional biostratigraphy from the Ferron Notom deltaic complex and Gallup Sandstone, as well as previously reported radiometric ages based on regional biostratigraphic correlations from the Ferron Last Chance delta and Cardium Formation. Backstripped sea-level changes show frequencies that match Milankovitch cycles within error. We show primarily symmetrical sea-level changes, with falls and rises that average from +20 to 30 m in amplitude. The symmetry and magnitudes of sea-level changes are more consistent with glacio-eustatic than tectonic mechanisms. They are probably too large to be related to aquifer eustasy or steric effects. Several units show local tectonic rebound that likely enhanced the magnitude of sea-level drops. If Antarctic ice sheets had been present during the Turonian stage, they would have likely nucleated around the Gamburtsev Mountains, which would have been sufficiently high and had low erosion rates that are inconsistent with an ice-free environment.