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

Evolution of an ancient (Lower Cretaceous) marginal‐marine system from tide‐dominated to wave‐dominated deposition, McMurray Formation

“…The paucity of preserved sedimentary structures is interpreted to result from low sedimentation rates in a wave‐sheltered and stressed marine environment, which promoted infaunal colonization of the substrate (Wheatcroft, ; Dashtgard, ). The suite of trace fossils in Sub‐Facies 4A comprises relatively simple traces that are diminutive in size, are of low diversity, and are relatively abundant; characteristics typical of a brackish‐water suite (Pemberton et al ., ; Pemberton & Wightman, ; Gingras et al ., , 2012a, ; Buatois et al ., ; Gingras & MacEachern, ).…”

“…As discussed in Gingras et al . () it is expected that in fluvial environments bioturbation would only occur subtending from the top of mud beds as marine organisms can only colonize the mud following deposition during the waning‐flood stage and re‐incursion of saline waters up channels (Sisulak & Dashtgard, ). This is in contrast with what is observed in FA3 deposits of the GPV, where both sand and mud beds are bioturbated, even in intervals where there is episodic bioturbation punctuated by non‐bioturbated intervals.…”

“…As outlined in Gingras et al . (), features characteristic of brackish conditions include: (i) low diversity trace suites reflecting stressed environmental conditions; (ii) localized intervals of high bioturbation intensity reflecting high infaunal biomass as a result of low sedimentation rates; (iii) ichnogenera reflecting impoverished marine communities instead of a commingling of freshwater and marine communities; (iv) reduction in trace size reflecting a decrease in organism size as a result of salinity stress; (v) relatively simple traces reflecting ethology of trophic generalists; and (vi) sporadically distributed traces in time and space reflecting opportunistic ( r ‐selected) organism populations. Pemberton et al .…”

“…Gingras et al, 2011;Sisulak & Dashtgard, 2012;Jablonski & Dalrymple, 2016). Colonization of mud beds is interpreted to occur through larval recruitment, which is dependent on basin circulation patterns and tidal exchange (Moser & Macintosh, 2001;Gingras et al, 2002Gingras et al, , 2016. In the mud-dominated upper portion of FA3 (F7 deposits), relatively high intensities of bioturbation suggest preferential colonization and slower deposition rates relative to F5 and F6.…”

“…The ichnological characteristics observed within the PS B and PS G successions are the best evidence that brackish-water conditions prevailed during deposition, and are inferred to have occurred in an overall tide-dominated sheltered bay setting that was notably influenced by wave and fluvial processes. As outlined in Gingras et al (2016), features characteristic of brackish conditions include: (i) low diversity trace suites reflecting stressed environmental conditions; (ii) localized intervals of high bioturbation intensity reflecting high infaunal biomass as a result of low sedimentation rates; (iii) ichnogenera reflecting impoverished marine communities instead of a commingling of freshwater and marine communities; (iv) reduction in trace size reflecting a decrease in organism size as a result of salinity stress; (v) relatively simple traces reflecting ethology of trophic generalists; and (vi) sporadically distributed traces in time and space reflecting opportunistic (r-selected) organism populations. Pemberton et al (1982) were the first to document brackish ichnofossil suites within channel deposits of the McMurray Formation that were likely affected by a proximal marine source.…”