--> Unbioturbated Carbonaceous Shales in the Cretaceous Western Interior Seaway Record Oxic Bottom–Water Conditions

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Unbioturbated Carbonaceous Shales in the Cretaceous Western Interior Seaway Record Oxic Bottom–Water Conditions

Abstract

Dark gray to black shales are prevalent in the Cretaceous Western Interior Seaway (WIS). Many of these shales are unburrowed suggesting that macrofauna were unable to survive in the paleo-environment, and this has led to interpretations that deeper parts of the WIS may have varied from suboxic (0.2 – 2.0 mg l-1) to anoxic (< 0.2 mg l-1). A comparison of micropaleontological, geochemical, and ichnological datasets from the Joli Fou, Viking, and Westgate formations, Alberta, Canada (Albian to Cenomanian) is undertaken to determine the oxygenation of bottom waters at the time of shale deposition. These shales occur in environments interpreted as upper offshore through to shelf, and the shales are sedimentologically and ichnologically similar to carbonaceous shales preserved throughout the WIS from the Late Albian to the Santonian. Geochemical proxies (e.g., Fe/Al and Mo/Al ratios, Re concentration) and foraminiferal data indicate that the under- and unbioturbated, carbonaceous shales tested in this study were actually deposited under oxic bottom water conditions. Based on this and the results of recent work from the Gulf of Mexico, we conclude that the paucity of burrowing is a manifestation of reduced oxygenation (low oxic: 2.0 < DO < 5.0 mg l-1), which was sufficient to support diverse benthic foraminiferal communities but not burrowing macrofauna. From these data, we propose a hierarchy of datasets for recognizing decreasing oxygen saturation. 1) Highly burrowed sediments (BI ≥ 3) indicate the presence of macrofauna on the seafloor and dissolved oxygen (DO) contents likely exceeding 80% saturation (> 5.0 mg l-1). 2) Under- and unbioturbated carbonaceous shales (BI 0-2) with diverse foraminiferal contents suggest low-oxic conditions (< 80% saturation, but > 2 mg l-1 DO). 3) Sub-oxic and anoxic conditions can be determined from geochemical proxies (e.g., Fe/Al > 0.5, Mo/Al > 0.001). These sediments will show low diversities of foraminifera and be unbioturbated (BI 0). 4) Euxinic bottom water (H2S present) is best determined from Mo (> 15 ppb) and Re (> 15 ppb) enrichment, coupled with a complete lack of benthic foraminifera and no bioturbation (BI 0).