JOHN-PAUL ZONNEVELD, MURRAY K. GINGRAS, TYLER W. BEATTY
PALAIOS 25 (6), 368-392, (1 June 2010) https://doi.org/10.2110/palo.2009.p09-135r
Diverse and locally abundant Lowermost Triassic (lower Induan, Griesbachian) trace-fossil assemblages are described and their significance for the location and characteristics of western Pangean environmental refugia are assessed. Trace fossils within the Montney Formation in northwestern Alberta and northeastern British Columbia record the activities of a wide variety of marine invertebrates. Many forms represent the dwelling and feeding traces of allochthonous storm-transported colonizers. Anachronistic forms—more typical of Paleozoic than Mesozoic successions—including Cruziana, Diplichnites, Monomorphichnus, and Trichophycus, are common. Notably these Paleozoic holdovers, as well as Rhizocorallium, Thalassinoides, and Spongeliomorpha, were likely constructed by marine arthropods. Trace fossils are rare in both shallow water (upper shoreface and foreshore) and offshore depositional settings, but are abundant in offshore transition to distal lower shoreface depositional settings. Low diversity and low ichnofabric indices characterize autochthonous infauna in Montney offshore transition settings, whereas high diversity, low ichnofabric indices distinguish allochthonous infauna in the same settings. High diversity and high ichnofabric indices typify distal lower shoreface successions. Several lines of evidence, including diminutive trace fossils and low diversity of resident infauna in proximal-offshore settings, support the hypothesis of shallow marine anoxic to dysoxic conditions in the study area during the Griesbachian. This trace-fossil distribution, and the abundance of allochthonous faunas in the study interval, reflect an infauna whose distribution was limited by both wave-stressed proximal settings and oxygen-stressed distal settings, resulting in colonization of a very narrow habitable zone. High diversity of trace-fossil assemblages in the study interval suggests the presence of shallow marine refugia wherein organisms survived the extinction interval and weathered the adverse conditions that dominated the world's oceans during the lowermost Mesozoic. Mid- to high paleolatitude refugia, such as the Pedigree-Ring-Kahntah area, played a crucial role in both extinction survival as well as post-event recolonization of the world's oceans.