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Process Ichnology of the Anisian Wedge, Montney Formation: Size-Diversity Trends and Feeding-Strategy Distributions of Ichnofossils Within a Post-Extinction Environment

Abstract

Process Ichnology is an approach to ichnological analysis that focuses on the interpretation of trace fossils as sedimentary structures, as opposed to paleontological objects. The aim of process ichnology is to better assess and interpret physical and chemical depositional stresses in ancient sedimentary environments. One aspect of process ichnology is the Size Diversity Index (SDI), which is calculated by the diameter of the largest burrow observed multiplied by the diversity of ichnogenera observed. Coupled with Bioturbation Intensity (BI), this function can provide proxies for environmental stresses including salinity, oxygen levels and sedimentation. Such stresses are of great interest in understanding how biota recovered after the Permian mass extinction. Here, we present and interpret SDI trends from the highly bioturbated Anisian Wedge within the Lower Triassic Montney Formation in the Groundbirch, British Columbia area. SDI assessments were binned into 50 cm core length, where ichnological diversity was determined and the largest burrow cross section was measured. Trace fossil assemblage of the Anisian Wedge was dominated by a dense Phycosiphon fabric, with robust Rosselia and subordinate Teichichnus, Palaeophycus, Planolites, and Zoophycos. The ichnological assemblage suggests that deposit feeding was the main feeding strategy, with grazing forms being present within isolated zones. Traces indicate a fully marine paleoenvironment. Fluctuations in BI and SDI within the Anisian Wedge suggest both stable (low variability in BI within a bin) and unstable conditions (high variability in BI within a bin). High variability in BI, coupled with low SDI is interpreted to indicate flux in oxygen levels, whereas high SDI and high variability in BI suggests fluctuation in energy/sedimentation. General trends show that oxygen fluctuation was the dominant process influencing biotic/trace distribution at the top and base of the interval, and between those intervals was a succession associated with energy/sedimentation variability. Patterns in BI and SDI can be used as a proxy to stratigraphically correlate local paleoenvironmental conditions between cores within the Groundbirch area. Compared to older strata of the Montney Formation, the Anisian Wedge displays higher SDI and more persistent BI. However, the infaunal community still shows evidence of environmental stress and the environmental stability is compromised both by wavering oxygenation and sedimentation.