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Environmental Implications of Ichnofossils from the Lower Cretaceous Grand Rapids Formation, Cold Lake Oil Sands Deposit
The Lower Cretaceous Grand Rapids Formation within the southern portion of the Cold Lake oil sands deposit is characterized by a well developed brackish water ichnofossil assemblage. Tenichnogenera have been vecognized: Gyrolithes, Monocraterion, Palaeophycus, Planolites, Rosselia, Skolithos, Teichichnus, and Cylindrichnus. In addition, nondescript, passively-filled burrows and fugichnia (escape structures) were observed.
This ichnofossil assemblage is characterized by a mixture of simple, horizontal and vertical structures common to both the Skolithos and Cruziana ichnofacies. The simple nature of these ichnofossils is a reflection of the non-specialized feeding strategies employed by the trace-making organisms. Such animals are opportunistic in nature and display an r-strategy in population dynamics. Typically, the individual ichnofossils of this assemblage are smaller in size than their respective fully marine analogues. This relative size reduction is an adaptive response to the high salinity-related, physiochemical stresses imposed on the osmoregulatory apparatus of the trace-making community. Furthermore, although bioturbation may be intense in argillaceous sediments, ichnotaxonomic diversity is characteristically very low. The aforementioned characteristics suggest that salinity was a dominant limiting factor on bioturbation and ichnofossil distribution.
The presence of a well developed, brackish water, ichnofossil assemblage has important local and regional paleoenvironmental implications. On a local scale its presence implies that, although salinity levels fluctuated, brackish water conditions prevailed throughout much of the deposition of the Grand Rapids Formation. On a regional scale this may imply that fully marine conditions may not have been attained during deposition of the Mannville Group within this portion of the Alberta Basin but that brackish water conditions were more eposition of the Mannville Group than presently interpreted.
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