Stacked Parahaentzschelinia ichnofabrics from the Lower Permian of the southern Sydney Basin, southeastern Australia: Palaeoecologic and palaeoenvironmental significance

Abstract

Trace fossils are sensitive indicators for environmental parameters (e.g., hydrodynamics, oxygen content, water depth, depositional rates, salinity), providing invaluable information to understand past depositional regimes not attainable from other sources of approaches. Here we report a new trace fossil assemblage dominated by a potential bivalve trace Parahaentzschelinia from the Lower Permian Snapper Point Formation, which records deposition in a storm-influenced delta front environment. Dense populations of Parahaentzschelinia occurred in two sections, forming stacked Parahaentzschelinia ichnofabrics and representing multiple burrow adjustments of a single trace maker (tellinid bivalve) subsequent to rapid sedimentation events caused by possible seasonal/annual runoffs. Statistical analysis of burrow lengths and numbers of adjustments suggests up to seven sequences of sediment accumulation events, supplying 3–13 cm thick clastic deposits at each time. A conservative sedimentation rate of 0.24 cm/year was estimated on the basis of two stacked Parahaentzschelinia ichnofabrics and the probable maximum lifespan of the modern tellinid genus. This rate is generally comparable to those derived from modern deltaic environments, lending further support to the interpretation that the ichnofabrics were formed in and characteristic of a deltaic setting. Our research highlights the utility of stacked equilibrium structures as a sensitive indicator of certain depositional regimes, and its applicability for assessing sedimentation rates as well as depositional conditions in the geological past.