Reconstructing a Miocene pitfall trap: Recognition and interpretation of fossiliferous Cenozoic palaeokarst

Abstract The middle Miocene cave deposit at AL90 Site in the Riversleigh World Heritage Area of Queensland, Australia, is the largest excavated palaeokarst deposit within the extensive Cenozoic freshwater limestones of the region. Stratigraphic relationships between various lithological components of the cave deposit indicate a dynamic, complex depositional history including dissolution, infill and reworking of cave materials. Euhedral spar, shelf-stone, calcite rafts and desiccation cracks on cave sediments suggest standing water was variably present in the cave environment. The AL90 Local Fauna is moderately diverse comprising at least 20 vertebrate families and 32 species, yet is taphonomically biased towards large marsupial herbivores (families Diprotodontidae, Macropodidae, and Balbaridae) and cave-dwelling hipposiderid bats. Fossil material is generally exceptionally well-preserved with, in many cases, articulated skeletons recovered. The cave entrance appears to have acted as a natural pit-fall trap. The complex (unconformable) depositional structure and lithology, vertebrate taphonomy and faunal composition at AL90 Site are typical of deposits and assemblages formed in the inner environment of the upper vadose zone. In the past, some similar unroofed cave deposits have been interpreted as clastic fluvio-lacustrine sedimentary deposits formed by surface processes. Analysis of characteristic palaeokarst features at AL90 Site has enabled the identification of other less well-preserved, poorly-exposed or relict cave deposits at Riversleigh and has facilitated interpretation of the depositional and palaeoenvironmental history of those deposits. The findings presented here confirm earlier interpretations that specific, highly-fossiliferous early Miocene to early Pliocene fossil deposits at Riversleigh represent relict caves whose roofs and walls are no longer intact.