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Trace-Fossil Assemblages in the Lower Triassic Toad Formation, La Biche River Map Area, Southeastern Yukon
Two outcrop sections of the Lower Triassic (Griesbachian = Early Induan) Toad Formation in southeastern Yukon (La Biche River map area; NTS 95 C) contain well-preserved trace fossils. Strata consist of interbedded shale, siltstone, and sandstone in varying proportions and are interpreted to record deposition on a wave-dominated shelf. Trace-fossil diversity and abundance are low in facies interpreted to be of distal-shelf origin but are moderate to high in offshore-transition and lower-shoreface strata. Sandstone packages interpreted as shoreface deposits are effectively barren. The most common ichnofossils are dwelling networks (Spongeliomorpha), infaunal feeding traces (most commonly Planolites), and arthropod-produced furrowing traces (Cruziana). Ichnotaxonomic diversity of the Toad Formation is high relative to many well-documented Griesbachian successions. This supports suggestions that infauna recovered more rapidly in high-paleolatitude regions than in low-paleolatitude regions following the end-Permian extinction. The studied strata are an outcrop analogue for correlative, gas-producing strata (Montney Formation) in the Ring/Border Field of northeastern British Columbia and the Pedigree Field of northwestern Alberta. Similarities in sedimentology and ichnology between the two successions suggest that prospective reservoir strata may be present regionally in the Lower Triassic.
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