Ichnofaunal Variability in an Early Cambrian Sand-Rich Shallow-Marine Complex: The Gog Group of the Southern Canadian Rocky Mountains  

The trace fossils of the Gog Group record animal-sediment interactions on a broad continental shelf of early Cambrian age.  This poorly fossiliferous unit has been largely overlooked from an evolutionary standpoint, despite the insights it provides to the understanding of early Phanerozoic ecosystems.  Our study focuses on the lower portion of the Gog Group, which reaches some 320 m in thickness.  It integrates sedimentological and ichnological datasets with the main objective of characterizing the palaeoecological factors controlling the distribution of trace fossils. 

Five facies associations with distinctive assemblages are recognized: (1) Non-bioturbated tabular bodies of planar and trough cross-stratified sandstone, comprising the Fort Mountain Formation.  (2) Packages of sandstone interbedded with shale and wavy-and lenticular-bedded sandstone.  This facies characterizes the Lake Louise Formation which contains the highest ichnodiversity (Arthrophycus, Conostichus, Cruziana, Diplichnites, Diplocraterion, Halopoa, Monomorphichnus, Palaeophycus, Phycodes, Planolites, Rhizocorallium, Rosselia, Rusophycus, Skolithos, Teichichnus, Trichophycus). (3) Tabular to lenticular bodies of cross-stratified and parallel-laminated sandstone intercalated with wavy to lenticular thin-bedded sandstone, which are included in the lower St.  Piran Formation.  Robust pipe rock ichnofabrics occur within the sandy packages, while Planolites and Teichichnus are associated with the thinly bedded heterolithic intervals.  (4) Compound cross-stratified sandstone packages locally bioturbated with Skolithos and Rosselia. (5) Uncomformably overlying the latter facies, the lowermost deposits of the upper St.  Piran Formation are dominated by heterolithic intervals showing wavy, lenticular and flaser bedding containing an impoverished trace-fossil assemblage composed of Diplichnites, Helminthopsis, Helminthoidichnites, Monomorphichnus, Planolites, Rusophycus and Skolithos.

Facies associations 1 and 4 reflect strong unidirectional currents and a rapidly shifting substrate in a shallow subtidal environment.  The Skolithos-bearing intervals reflect the activity of suspension-feeders that colonized stable dunes.  By contrast, the highly-bioturbated facies association 2 records the benthic activity of mainly deposit-feeding faunas in muddy areas of the shelf characterized by low-energy conditions and discontinuous sand sedimentation.  Prograding over this unit, facies association 3 records the growth of a sand-sheet complex.  Pipe rocks dominated by dwelling trace fossils of suspension-feeders occur at the margins and fronts of individual components of the complex, reflecting a moderate-energy environment, discontinuous sand sedimentation, and little or no mud deposition.  Finally, facies association 5 reflects deposition on a marginal-marine setting in which progradation of a tidal-flat complex over the shallow shelf is demonstrated by an impoverished suite dominated by grazers and shallow-tier trace fossils of detritus-feeders. 

Overall, the ichnofauna of the Gog Group provides a remarkably actualistic picture of Lower Cambrian shallow-marine benthic communities.  Tiering and intense bioturbation (mostly by suspension-feeders), record the establishment of a mixground ecology early in the Phanerozoic.