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TitleGeochemical variation in the Jurassic-Cretaceous strata of the Baccalieu I-78 well, Flemish Pass Basin, Canada: chemostratigraphic and paleoenvironmental implications
AuthorBingham-Koslowski, NORCID logo; Azmy, K
SourceGeoscience Canada; Geoscience Canada vol. 49, no. 2, 2022 p. 75, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20210643
PublisherGeological Association of Canada
Mediapaper; on-line; digital
File formatpdf
ProvinceEastern offshore region; Newfoundland and Labrador
AreaFlemish Pass
Lat/Long WENS -60.0000 -45.0000 47.0000 42.0000
Subjectsgeochemistry; stratigraphy; paleontology; fossil distribution, strata; paleoenvironment; isotopes; Jurassic; Cretaceous
ProgramGEM2: Geo-mapping for Energy and Minerals GEM Synthesis
Released2022 07 19
AbstractThe Jurassic-Cretaceous boundary is notoriously difficult to detect in the absence of fossils as it is not associated with any major glacio-eustatic fluctuations or remarkable global change in lithology. It remains the only Phanerozoic period-level boundary that is missing a golden spike on the geological time scale. Previous palynological analyses have suggested that the Jurassic-Cretaceous boundary resides within the Esso Parex et al Baccalieu I-78 well, with one study reporting that the boundary occurs in core 3. Profiles of paleoenvironmental proxies for core 3, generated from stable isotope and trace element analyses, exhibit a geochemical anomaly between 3288.5 m and 3289 m. Variations in these profiles, corresponding to the anomaly, provide insights into changes in the paleoenvironment during this interval. Anoxic conditions are interpreted to have occurred during deposition of the strata associated with the anomaly based on the d34S, Fe, Mn, Th/U, and Ce/Ce* trends. This is further supported by a peak in total organic carbon (TOC) within the interval, which implies that paleoredox conditions were conducive to the preservation of organic matter. A reversion to background levels above the anomaly for these paleoredox proxies, suggests a change to more oxygenated settings. Low primary productivity across the anomaly is indicated by consistent shifts in the d13C, d15N, P, Ni, Zn, and Cu profiles, which was followed by an increase in primary productivity above the anomaly associated with the onset of upwelling. Paleo-salinity peaks within the anomaly, as indicated by Sr/Ba, B/Ga, and %S/TOC, suggesting a highly restricted environment, but declines above the anomaly implying an uptick in circulation. The various geochemical analyses indicate that the anomaly is associated with a fall in relative sea level, which led to restricted circulation, a stratified ocean, and widespread anoxia. The fall in sea level coupled with an arid climate further resulted in diminished terrestrial input and limited nutrient supply that led to a reduction in primary productivity during deposition of the strata associated with the anomaly. Our results suggest that the Jurassic-Cretaceous boundary in Baccalieu I-78 may fall within the geochemical anomaly (3288.5-3289 m). This is consistent with global trends related to the boundary that indicate a late Tithonian fall in relative sea level, typically linked to tectonism (as opposed to eustasy) in a semi-arid to arid climate. The current investigation demonstrates the usefulness of geochemical analyses in the identification of stratigraphic boundaries (chemostratigraphy) and in paleoenvironmental interpretation.
Summary(Plain Language Summary, not published)
The Jurassic-Cretaceous boundary is difficult to detect in the rock record as it is not associated with a global drop in sea level or with a major global change in lithology. Geochemical analyses can be used to detect subtle changes in rock chemistry that reflect changes in the paleoenvironment which tend to be associated with major geological boundaries. A previous palynological study suggested that the Jurassic-Cretaceous boundary occurred within core 3 of the Baccalieu I-78 well, located offshore Newfoundland. This study employed stable isotope and trace element analyses throughout core 3 to see whether the boundary could be located more accurately and to provide insights into the changes in the paleoenvironment that were occurring around the boundary. A geochemical anomaly was detected in the various geochemical profiles between 3288.5 and 3289 m, possibly indicating the location of the Jurassic-Cretaceous boundary. Interpretation of the geochemical profiles around the anomaly indicates that the anomaly likely reflects a sea level fall, which restricted circulation, leading to a stratified ocean and widespread anoxia. This drop in sea level occurred in an arid climate which led to the cut off of terrestrial inputs, such as nutrients, which lowered primary productivity across the anomaly. Above the anomaly, there is a change back to more oxygenated environments, with evidence of nutrient input from upwelling, and an increase in primary productivity. The results of this study are consistent with global observations for the Jurassic-Cretaceous boundary that indicate a fall in sea level across the boundary and arid to semi-arid environments. Our study demonstrates the usefulness of geochemical analyses for identifying significant stratigraphic boundaries and for interpreting paleoenvironments.

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