The significance of trace fossils in the McMurray Formation, Alberta, Canada

Abstract

Abstract This paper considers the paleoecological and paleodepositional significance of bioturbated channel-associated sands of the McMurray Formation, Alberta, Canada. The facies associations described in this paper include: 1) thalweg-associated cross-stratified sand; 2) bar-related Inclined Heterolithic Stratification; and 3) bar-top / tidal-flat deposits. Thalweg-associated cross-stratified sands contain mud-lined Skolithos and Cylindrichnus, with rare Planolites, Palaeophycus, Siphonichnus, Conichnus, and bivalve-generated equilibrichnia and fugichnia. Bar-related inclined heterolithic stratification contains Planolites-Teichichnus-Cylindrichnus associations, Cylindrichnus-Skolithos-Planolites assemblages, and monospecific Gyrolithes-dominated facies, any of which may contain subordinate Siphonichnus, Palaeophycus, Psilonichnus, and Arenicolites. Bar-top/tidal-flat deposits are characterized by gently dipping to horizontal, bioturbated, heterolithic media containing Planolites and Cylindrichnus, with rare Skolithos, Thalassinoides and Arenicolites. The trace-fossil assemblages in the three facies associations show numerous features characteristic of brackish-water environments: 1) suites are of low diversities; 2) suites contain marine-derived ichnogenera; 3) ichnogenera are characterized by size diminution; 4) ethological associations correspond to the activities of trophic generalists; 5) intervals locally indicate high infaunal biomasses; and 6) intervals display evidence of r-selected (opportunistic) colonization strategies. Such trace-fossil assemblages are only consistent with examples of brackish-water ichnocoenoses in modern settings and in high-certainty brackish-water deposits documented from around the world. These ichnological observations are supported by the abundance of tidally generated sedimentary structures (sigmoidal bedding, draped foresets, reactivation surfaces and bidirectionally oriented cross-stratification) as well as marine dinocysts recovered from these facies. The paleontological and physical sedimentological characteristics require the presence of tidal currents and brackish-water to explain middle McMurray Formation deposition. Bioturbation ascribable to fresh-water conditions is present, albeit rarely, in the McMurray Formation. This includes occurrences of irregularly shaped shafts and tunnels displaying variable diameters, as well as Taenidium and Naktodemasis observed in bar-top units. These trace fossils are normally found in association with root traces and pedogenically altered sediments situated near the top of the lower McMurray. These assemblages confirm that during McMurray time, freshwater and brackish-water ichnocoenoses were present and yielded discrete and readily discernible trace fossil suites. Brackish-water thalweg, bar, and bartop units, which are consistently devoid of pedogenic alteration and root traces, are explained by: 1) the presence of brackish-water in the depositional setting; and 2) the presence of tides to facilitate the landward transport of marine-derived larvae and the establishment of a bar-top tidal zone. As such, contrary to some recent interpretations, assertions that the McMurray Formation channels can be broadly interpreted as fluvial channels are not tenable.