Implications of Paleo-Redox Proxy Data from Shell Beds in the Pardonet Formation (Norian, Triassic) at Williston Lake, British Columbia

The Pardonet Formation (Norian) tops the Triassic succession of Western Canada. This intercalated fine-grained limestone and shale unit is notable for the occurrence of thick, monospecific accumulations of pteriomorph bivalves colloquially known as ‘flat-clams’ due to their flattened morphology. The depositional system has yet to be documented in detail but the agreement is that the Pardonet Formation represents condensed, pelagic deposition in deep water with reduced oxygen levels. These interpretations, however, have not previously been experimentally demonstrated. It is difficult to reconcile the persistence of flat clams with the interpretation of low-oxygen bottom-waters.

Pyrite framboid and U/Mo paleo-redox proxies were applied to assess the relationship between paleo-redox conditions and the presence of autochthonous bivalves. The results confirmed that the upper Pardonet Formation was deposited during a prolonged period of oxygen depletion in the deep ocean off the Northwest coast of Pangaea. While this has clear implications for the paleoecology of the flat clams, it also relates to the paleoclimate during the late Norian.

The Pardonet Formation may represent an important interval to study to understand how modern global warming might impact the oxygenation of the deep ocean and the resulting impact to deep water ecosystems. Dysoxic to euxinic conditions are interpreted to be a result of a warming climate during the Norian stage combined with possible upwelling offshore of arid lands. The studied interval also coincided with global anoxic episodes that led into the Late Triassic extinction event. This understanding could aid in planning for impacts to relevant economies.