The Cenomanian–Turonian boundary interval in the Western Canada Foreland Basin: Stratigraphy, geochemistry, geochronology and sea‐level changes recorded in expanded and condensed clastic successions

Abstract A 300 m thick section at Nini Hill in the proximal foredeep of the Western Canada Foreland Basin is dominated by shallow‐marine mudstone that spans the Cenomanian–Turonian boundary (CTB). The section preserves a 185 m thick record of OAE2, characterised by an ornate positive excursion in the organic carbon‐isotope profile. Osmium‐isotopes show the characteristic shift to unradiogenic ratios 21 m below the onset of OAE2. Carbon‐isotope events (CIE) at Nini Hill are correlated with both the Chalk reference section at Eastbourne, UK and the well‐dated SH#1 core in Utah, the latter permitting correlation to other sections in the southern USA. However, only the ~450 m thick, deep‐water CTB section in the Saku Formation, Japan, appears to match the CIE detail at Nini Hill. High‐resolution correlation utilising CIEs allows, for the first time, sea‐level changes, mapped in the poorly fossiliferous strata of Western Canada, to be correlated with coeval events in the USA and Europe. The globally‐recognised sub‐plenus unconformity that underlies OAE2 in many passive‐margin sections spanning the North Atlantic region is correlative with up to six high‐frequency sequences preserved in the highly expanded foredeep. Various studies have inferred sea‐level change of 10–40 m for this event, suggesting that thermo‐ and aquifer‐eustasy may have been supplemented by glacio‐eustasy. Other sea‐level changes of ~10–30 m recognised in Canada correlate with coeval events in the USA and Europe. Lower‐amplitude sea‐level cycles of ~5–10 m, form a persistent signal throughout the Canadian CTB interval. Strata thin dramatically from foredeep to forebulge due to condensation and lap out, hiatuses being represented by cryptic mud‐on‐mud disconformities. Without knowledge of physical stratigraphy, interpretation of carbon‐ and osmium‐isotope profiles in attenuated successions is prone to misinterpretation. Osmium data show that the influence of a large igneous province diminished markedly northward within the Western Interior Seaway.