Palaeoenvironmental implications of trace fossils in estuary deposits, Cretaceous Bluesky Formation, Cadotte region, Alberta, Canada

Estuarine settings are characterised by numerous physical and chemical stresses that can strongly influence the behaviour of burrowing organisms. Although lowered salinity and fluctuating salinity levels normally represent the chief stresses recognised in bays and estuaries, high sedimentation rates, high current energy, turbidity, and low levels of oxygen in bottom and interstitial waters are known to be significant factors that strongly influence the resultant ichnofossil assemblages. This study builds on earlier research and suggests that the effects of each of these parameters can be observed in the rock record through trace fossil analysis. The subsurface lower Albian Bluesky Formation in the Cadotte region of Alberta has been interpreted to represent an estuarine deposit. Examination of the trace fossil assemblages from various facies therein suggests that physicochemical stresses were variable across the ancient estuary and mainly constituted the following: (1) low salinity and fluctuating salinity levels, which are interpreted to have contributed to patterns of low ichnofossil diversity and burrow dimunition proximal to the fluvial point source(s) in the upper and central parts of the depositional system; (2) high sedimentation rates and current energy, evidenced ichnologically by sporadic, penetrative bioturbation and the rare preservation of opportunistic sediment stirring, are most significant in the vicinity of the tidal inlet in the lower estuary and in the bayhead delta of the upper estuary; (3) high turbidity associated with the turbidity maximum in tidal channels of the central reaches of the estuary probably inhibited suspension-feeding behaviours locally; and, (4) low levels of dissolved oxygen in quiescent-water embayments and lagoons, often represented in brackish-water deposits as a Trichichnus, Palaeophycus, and Diplocraterion assemblage.