An anoxic event at the Albian-Cenomanian boundary: the Fish Scale Marker Bed, northern Alberta, Canada

Abstract

The Fish Scale Marker Bed (FSMB) of the Shaftesbury Formation, which marks the Albian-Cenomanian boundary, is a regional stratigraphic marker in the Western Interior of Canada. At the outcrop studied on the Smoky River in northwestern Alberta, three major shale units can be distinguished in the FSMB and contiguous strata. The lowermost shale (Unit 1) is bioturbated and contains high-diversity dinoflagellate and moderate-diversity foraminiferal assemblages. It has dominantly Type III (terrestrial) organic matter (OM) and low total organic carbon content (TOC). The unit was deposited in an open-marine, neritic environment of normal salinity. The FSMB (Unit 2) represents a zone of condensed bioclastic accumulation composed of abundant fish remains. The dinoflagellate species diversity is drastically reduced in this unit and it lacks benthonic foraminifera and bioturbation. Unit 2 is characterized by mainly Type II (marine) OM and high TOC values. Unit 2A contains rippled sandstone related to either a shallowing or to deeper water currents. A fish-hash conglomerate making up Unit 2B can alternatively be interpreted as a bioclastic, condensed and winnowed deposit or as a transgressive lag. Unit 2C consists of black, platy shale with abundant fish remains and represents a minor marine trangression during the deposition of FSMB, when the bottom waters were dominantly anoxic. Collectively, the features of Unit 2 suggest deposition under a stratified water column with moderate productivity of planktonic and nektonic organisms in the upper oxygenated layers but with anoxic bottom waters. Unit 3, overlying the FSMB, consists of blocky shale with reduced concentration of fish remains. Due to increased rate sedimentation during its deposition, the organic-rich sediment of Unit 3 was progressively diluted by clastic material and there was an increase in the dissolved oxygen content of the bottom waters. The anoxic event at the FSMB is related to a relative rise in sea level and possibly to the mixing of waters different salinities and temperatures from the Arctic Ocean and the Gulf of Mexico in the Western Interior seaway.