Firmground Zoophycos in the Lower Cretaceous Viking formation, Alberta: A distal expression of the Glossifungites ichnofacies

The substrate-controlled Glossifungites ichnofacies is a firmground suite of trace fossils that commonly demarcates erosional discontinuities in sedimentary successions. The Glossifungites ichnofacies typically is characterized by biogenic structures that are vertical to subvertical, sharp-walled unlined and commonly passively infilled. The structures hitherto have been restricted to permanent or semi-permanent domiciles, and predominantly to vertical to subvertical burrows of suspension-feeding organisms. Several cores of the late Albian (Lower Cretaceous) Viking Formation from the Hamilton Lake Field of Alberta, Canada contain an. atypical Glossifungites ichnofacies dominated by the foraging, probing and deposit-feeding/ dwelling structures of firmground Thalassinoides, Rhizocorallium, and Zoophycos. The Thalassinoides are passively infilled with coarse grains of sand and chert granules. Rhizocorallium displays active stages of infill, although the final, horizontal U-shaped tube is passively filled. The Zoophycos, however, demonstrates active infill manifest by chert granules and coarse sand distributed in the spreite, and this is a departure from the normal expression of the Glossifungites ichnofacies. The firmground structures subtend from the regional stratigraphic discontinuity BD4, excavated into black, silt and sand-poor offshore and shelf mudstones. BD4 is interpreted to reflect a transgressively modified sequence boundary. Most Zoophycos, Thalassinoides, and Rhizocorallium penetrate no deeper than 3-4 cm below the discontinuity into the underlying mudstones. Where BD4 overlies a sandier substrate, firmground Zoophycos are not present within the suite. The facies directly overlying BD4 consist of fining-upward, pebble-, granule-, and very coarse-grained sandbearing, thoroughly burrowed muddy sandstones to sandy mudstones, typically 5-25 cm thick. This basal transgressive lag contains Teichichnus, Planolites, Palaeophycus, Diplocraterion, Chondrites and rare Helminthopsis, and is the source of the coarse-grained material incorporated into the fill of the firmground ichnogenera. The granule bearing sanely mudstones overlying the transgressive lag contain abundant Teichichnus, Planolites, Helminthopsis, Anconichnus, Chondrites, and Terebellina, and reflect rapid deepening to proximal offshore conditions. Progressive deepening is recorded in the accumulation, of distal off- shore and shelf mudstones of the late Albian Westgate Formation. Although excavation of BD4 occurred above fairweather Leave base, its colonization occurred under much lower energy conditions. Within distal softground settings, ichnogenera capable of deeply penetrating muddy substrates are largely restricted to Zoophycos, Thalassinoides, Rhizocorallium, and Chondrites. These ichnogenera constitute forms capable of being excavated, albeit shallowly, within a firm substrate. This is a low energy, distal expression of the Glossifungites ichnofacies.