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.