Carbon isotope stratigraphy and geochemical studies/investigation for changes across bentonites in the Upper Cretaceous (Cenomanian-Turonian) successions of WCSB Canada.

摘要

A high-resolution study of isotopic and stratigraphic trends that characterized the aftermath of volcanic events in Cenomanian-Turonian sections from the Western Canada Sedimentary Basin (WCSB) is presented. The research is based on delta18O, delta13C, delta 13Corg, TOC analyses of about 140 fossil (foraminifera, inoceramid) and whole rock samples combined with previously archived data.;Planktic foraminifera of the genera Heterohelix, and Hedbergella, and inoceramid shells used in the analysis were precipitated in isotopic equilibrium with sea water and show no significant diagenetic or vital effects; delta13C values range between -8.86‰ and 3.72‰ delta18O value range from -13.93‰ to -3.77‰, and delta13Corg value range between -28.68‰ to - 23.94‰. The delta13C values shift positively by about ∼5‰, the delta18O values shift negatively by about ∼ 5‰ and delta13C org values shift negatively by ∼1‰ all within the late Cenomanian. These shifts all coincided at the onset of the Ocean Anoxic Event 2 (OAE2), similar to other CTB sections within the WIS/WCSB and worldwide.;The results are similar to those obtained from other sections in the WIS/WCSB and generally reflect two major trends/events; the mid Cenomanian cool period and the warm late Cenomanian/early Turonian period, which incorporates the OAE2.;The TOC as well as delta13Corg results indicate that the low organic content that characterized the mid Cenomanian Belle Fourche Formation, became relatively high by time of the deposition of the late Cenomanian Keld member/Second White Specks Formation, but again become low by the early Turonian. The C/N ratio, delta13Corg and delta 15N results indicate that the main source of organic matter is from C3 plants of terrestrial origin.;Stable isotope (O, C) data were acquired from three sections in the WCSB; Vermilion River VR1 & VR2 and the drill-core from well 6-34-30-deltaW4 (Youngstown), to (1) investigate the trends in the sedimentary sequences deposited in the aftermath of Cretaceous volcanic eruptions and (2) for chemostratigraphic correlation within the Western Interior Seaways (WIS) of North America.;Across the bentonites, delta18O, delta13C org and TOC generally increases while delta13C decreases. The delta18O increases by 0.57-2.14‰ across the bentonites ≥5cm thickness, indicating cooling of ∼2.28°C -- 8.56°C. The environmental recovery time from the effects of volcanic events was found to be 8 to 18 kyr;. The observed delta13C and delta13C org trends from the three sections from WCSB indicate that a correlation exists with the North American stratotype section of the Cenomanian-Turonian OAE2 boundary at Pueblo, Colorado.