EARLY TRIASSIC ICHNOFOSSIL ASSEMBLAGES IN NORTHWEST PANGEA: CHARACTERIZING THE SHALLOW-MARINE HABITABLE ZONE

Early Triassic ichnofossil assemblages from the northwestern margin of Pangea were studied in order to characterize them with respect to diversity, abundance, bioturbation intensity, and ethology as a function of depositional environment. Comparative analysis of trace fossil assemblages from different parts of the study area reveals that shoreface architecture rather than relative geologic age (and thus recovery from the end-Permian extinction event) is the main factor controlling infaunal colonization of the benthic realm.

Lower shoreface environments preserve the greatest amount of benthic colonization during this Lower Triassic recovery interval. Environments immediately distal of this (i.e. offshore transition / proximal offshore) exhibit minimal evidence of significant infaunal colonization regardless of relative position after the end-Permian extinction. Where preserved, upper shoreface environments typically lack trace fossils. This is likely attributable to the low preservation potential of trace fossils in these areas. When comparing trace fossil assemblages in lower shoreface environments, those developed along large shoreline embayments, such as parts of the Peace River Basin in Western Canada, display a greater degree of bioturbation than those developed on shorelines in direct connection with the Panthalassan Ocean.

Trace fossil evidence supports the hypothesis that a colonization window existed in Early Triassic shoreface environments. The lower limits of this window were controlled by the presence of a chemocline between the offshore transition and lower shoreface environments. The chemocline represents the boundary between waters aerated by diffusion and normal and storm wave action and stagnant, anoxic to euxinic waters below with chlorobiaceae biomarkers. This chemocline thereby defined a narrow habitable zone with limited carrying capacity from which a modern fauna could emerge. Not until this chemocline broke down could organisms from the oxygenated colonization window expand and produce innovative ways to exploit newly available niches.