CARBON BURIAL AND BOTTOM WATER REDOX IN THE CANADIAN BASIN OF THE WESTERN INTERIOR SEAWAY DURING THE CONIACIAN-SANTONIAN

The sedimentary record of the Western Interior Seaway (WIS) includes episodic deposition of organic carbon (OC)-rich shales. Understanding the spatial variability in OC deposition and water column oxygenation is important for determining the mechanisms of OC burial within the WIS, which are particularly poorly understood during the Coniacian-Santonian (~86 Ma). While studies of geochemical cycling during the Coniacian-Santonian within the Niobrara Fm in the US portion of the WIS exist, the geochemical history of the western Canadian Basin remains undetermined. Core locations in southern Alberta were deposited in shallower conditions (50 m vs. 200 m) and bathed by cooler, more brackish Boreal water compared to the aforementioned US sections. Paleontological data supports the intermittent development of anoxia within the western Canadian WIS.

Here we present bulk geochemistry of Niobrara-aged sediments from southern Alberta well cores 13-20-17-7W4 and 16-04-22- 15W4. A suite of redox sensitive trace metals (TMs; Mo, Re, U), Fe/Al ratios, and pyrite Fe measurements are used to evaluate redox conditions. Redox proxies indicate two intervals of O₂ limitation during the Coniacian Verger and upper Santonian First White Specks mbrs of the Niobrara Fm, with more oxic conditions during the intervening lower Santonian Medicine Hat Mbr. Absence of benthic fauna within the lower Verger Member coincides with a phase of abundant ashfalls (bentonites). Increased marine productivity during the late Santonian is indicated by elevated numbers of planktic foraminifera and calcareous nannofossils within the First White Specks Mbr. These records are compared to US Coniacian-Santonian WIS records from the Berthoud State #3 and Portland #1 cores to elucidate basin wide vs. regional controls on redox conditions and OM burial. Sediments from the US portion of the seaway record consistently elevated but variable TM concentrations (e.g. Mo between 3-100 ppm) during deposition of the Niobrara Fm, whereas the Canadian cores record two distinct intervals of elevated TM concentrations (e.g. Mo up to 33 ppm) with low TM accumulations during the Medicine Hat Mbr (e.g. Mo <3 ppm). Thus geochemical cycling within the seaway reflects the interplay between regional and basin-scale mechanisms.