DIVERSE TRACE FOSSIL ASSEMBLAGES FROM THE MIDDLE TRIASSIC STRATA IN NORTHEASTERN BRITISH COLUMBIA AND THEIR ECOLOGICAL IMPLICATION

Highly diverse ichnofossil assemblages occur within Middle Triassic (Ladinian) strata in northeastern British Columbia. These trace fossils occur within heterolithic shale, siltstone, and very fine- to medium-grained sandstone dominated successions of the upper Toad and lower Liard formations in outcrops along Williston Lake in northeastern British Columbia. These trace fossils occur within a wide range of depositional environments, from offshore through offshore transition, lower and upper shoreface, to intertidal and supratidal flat. However trace fossil diversity and abundance are particularly high in the proximal offshore, offshore transition and distal lower shoreface. In contrast, trace fossil assemblages from Lower Triassic successions in western Canada occur within a much narrower range of depositional environments and are largely excluded from most offshore successions.

Trace fossils in the study interval are interpreted to represent the feeding, dwelling, crawling and grazing activities of a diverse suite of marine invertebrates including mollusks, anemones, arthropods, echinoderms and vermiform organisms. Middle Triassic trace fossil assemblages from Williston Lake are much higher in overall diversity than older Mesozoic successions in western North America and are in fact amongst the most diverse assemblages reported in the literature regardless of age or geographic location.

The high diversity and abundance of trace fossils in Middle Triassic successions at Williston Lake indicate that environmental constraints, which resulted in depressed diversity and restricted marine colonization to narrow environmental belts during the Lower Triassic, were alleviated. Thus, in marked contrast to Lower Triassic successions, the Middle Triassic northwest Pangaea shelf and shoreline were characterized by well-oxygenated bottom waters. Previously uninhabitable environmental niches, precluded due to anoxic / dysoxic bottom water conditions, were reinvaded by Middle Triassic faunas. Abundant new ecospace resulted in preservation of diverse suites of trace fossils which include numerous composite and compound forms, retention of several forms typically considered diagnostic of Paleozoic deposition as well as introduction of new forms restricted to Mesozoic and later deposition.